WO2024224036A1

WO2024224036A1 - Apparatus, system and method for tethering a subsea well assembly

Info

Publication number: WO2024224036A1
Application number: PCT/GB2024/050820
Authority: WO
Inventors: James Stewart; Charles GOURLEY
Original assignee: Tla Subsea Ltd
Priority date: 2023-04-24
Filing date: 2024-03-27
Publication date: 2024-10-31
Also published as: GB2624264A; GB2624264B; GB202306000D0

Abstract

The present invention relates to an apparatus, system and method for tethering a subsea well control equipment under tension of a subsea well assembly. A tethering member (36) is arrangeable to tether the subsea well control equipment (24) to a subsea mount (34). A subsea tensioning apparatus (38) is arrangeable to tension the tethering member. The subsea tensioning apparatus comprises a chain block (C) that is configured to be coupled to the tethering member and apply a tension to the tethering member.

Description

APPARATUS, SYSTEM AND METHOD FOR TETHERING A SUBSEA WELL ASSEMBLY

TECHNICAL FIELD

The present invention relates to an apparatus, system and method for tethering a subsea well assembly, particularly subsea well control equipment of a subsea well assembly.

INTRODUCTION

During its lifespan, a subsea well assembly can operate in a variety of modes, including a construction, production, intervention, de-construction and/or abandonment mode. A subsea well assembly typically comprises a subsea well control equipment during one or more modes of operation. A subsea well control equipment is provided to safeguard the subsea well assembly during one or more mode of operation. For example, the subsea well control equipment may control pressure in the wellbore, control wellbore access, and isolate a wellbore when necessary. The type of subsea well control equipment may vary according to the type of subsea well assembly and type of mode. Examples of subsea well control equipment include a subsea blowout preventer (BOP), subsea intervention lubricator (SIL) or a lower riser package (LRP).

Figure 1 depicts an example of a conventional subsea well assembly 10 in a construction, intervention, de-construction and/or abandonment mode. As shown, the subsea well assembly 10 comprises a tree 16 releasably connected to a wellhead 18 disposed at an upper end of a primary conductor 20 extending into a wellbore 22, a subsea blowout preventer (BOP) 24 releasably connected to the tree 16, and a lower marine riser package (LMRP) 26 releasably connected to the subsea BOP 24. The tree 16 may not necessarily be present, for example, during drilling operations. The subsea BOP 24 is provided as a subsea well control equipment to control pressure in the wellbore 22, control access to the wellbore 22 and isolate the wellbore 22, when necessary, so as to prevent blowouts caused by an uncontrolled release of crude oil or natural gas from the well. The subsea BOP may be part of the subsea well assembly during one or more mode of operation, for example a construction, intervention, de- construction and/or abandonment mode. The tree 16, the subsea BOP 24, and the LMRP 26 are at least substantially vertically arranged or stacked one-above-the-other, and are generally coaxially aligned with the wellhead 18. The subsea well assembly further comprises a riser 28 to connect the subsea well assembly to a floating vessel 12 at the sea surface 14.

During the lifespan, the subsea well assembly is subject to various loads including cyclical loads due to riser movement (for example, from surface vessel motions, wave actions, vortex induced vibrations, or combinations thereof) and environmental loads such as subsea currents. Together, these loads can induce fatigue in one or more component of the subsea well assembly, such as the tree 16 when present, the wellhead 18 and the primary conductor 20. Over time these loads may compromise the integrity of the subsea well assembly. This may be of particular concern due to the configuration, weight, and vertical arrangement of the subsea well assembly components, which present a relatively large surface area for interacting with the subsea current loads. Additionally, the loads can induce bending moments and associated stresses in one or more components of the subsea well assembly, which may, for example, be increased when the relatively tall and heavy combination of a tree and subsea well control equipment are angled relative to vertical. The bending moments and associated stresses further induce fatigue in the subsea assembly.

The increasing size and weight of subsea BOPs and longer drill times that are now commonly employed also increase the risk of subsea well assembly fatigue. In particular, the risk of fatigue when using newer generation subsea BOPs with legacy subsea well assemblies which have a less robust design.

The present invention relates to a solution to mitigate the risk of subsea assembly fatigue described above.

SUMMARY OF INVENTION

Embodiments of the present invention relate to a tethering solution to improve the strength and fatigue performance of subsea well assemblies comprising a subsea well control equipment. The subsea well control equipment may be, but is not limited to, a blowout preventer (BOP), a Subsea Intervention Lubricator (SIL), a Lower Riser Package (LRP).

A first aspect of the invention provides a subsea tensioning system for tethering a subsea well control equipment to a subsea mount, the system comprising: a tethering member arrangeable to tether the subsea well control equipment to the subsea mount; and a subsea tensioning apparatus arrangeable to tension the tethering member, wherein the tensioning apparatus comprises a chain block, whereby the chain block is arrangeable to be coupled to the tethering member and the chain block is configured to apply a tension to the tethering member.

By tensioning the tethering member under a tensile load, the tensioning apparatus advantageously restricts, preferably inhibits, movement and lateral bending of the subsea well control equipment. It follows that the tensioning apparatus advantageously enhances the stability and fatigue performance of a subsea well assembly by restricting, preferably inhibiting, movement and lateral bending of the subsea well control equipment.

Further, the present invention advantageously utilises the mechanical advantage of the chain block to apply and maintain tensile loads in the tethering member using a relatively low input torque. This may be particularly advantageous in view of the practicalities of applying tensile loads to a tethering member in subsea conditions.

Optionally, the system may comprise the subsea mount arrangeable on a seabed in a spaced relationship to the subsea well control equipment.

Optionally, the chain block may be configured to operate in a tension mode to apply tension to the tethering member.

Optionally, the chain block may comprise a load chain, wherein the chain block is configured to convey (pay-in) the load chain to apply the tension to the tethering member.

Optionally, the chain block may comprise a first coupling and a second coupling. Optionally, the first coupling and second coupling may be axially aligned and arranged on opposite sides of the chain block.

Optionally, the first coupling may be a movable coupling mounted on the conveyable load chain.

Optionally, the second coupling may be a fixed coupling mounted on a fixed portion of the chain block.

Optionally, the first coupling and/or second coupling may be a releasable coupling to allow for attachment and detachment of the chain block. For example, the first coupling and/or second coupling may comprise a hook. The hook may include a safety catch to allow for the chain block to be securely and releasably coupled to the tethering member and the subsea well control equipment or subsea mount during use. The releasable coupling of the chain block allows for the reuse, retrofitting, removal and/or reinstallation of the chain block in one or more subsea well assembly.

Optionally, the first chain block coupling and/or the second chain block coupling may be a rotatable coupling to improve the tethering configuration and arrangement of the chain block.

Optionally, the chain block may be arrangeable to be coupled to the subsea well control equipment and the tethering member:

For example, the chain block may be arrangeable in a first arrangement where the first coupling is coupled to the tethering member and the second coupling is coupled to the subsea well control equipment.

Alternatively, for example, the chain block may be arrangeable in a second arrangement where the first coupling is coupled to the subsea well control equipment and the second coupling is coupled to the tethering member.

Optionally, the chain block may be arrangeable to be coupled to the subsea mount and the tethering member: For example, the chain block may be arrangeable in a third arrangement where the first coupling is coupled the tethering member and the second coupling is coupled to the subsea mount.

Alternatively, for example, the chain block may be arrangeable in a fourth arrangement where the first coupling is coupled to the subsea mount and the second coupling is coupled to the tethering member.

Optionally, the chain block may further comprise: a rotatable chain sheave configured to convey the load chain; a gear assembly, with a gear ratio, configured to rotate the chain sheave; and an actuator configured to actuate the gear assembly in response to receiving a driving input.

In response to the actuator receiving the driving input, the gear assembly is actuated by the actuator to rotate the chain sheave. The rotating of the chain sheave conveys the load chain to pay-in the load chain. The conveying (paying-in) action of the load chain is transmitted to the tethering member to apply the tension to the tethering member.

In an arrangement of the tensioning apparatus where the chain block is coupled to the subsea well control equipment, the tension is applied to the tethering member in a tensioning direction towards the subsea well control equipment.

In an arrangement of the tensioning apparatus where the chain block is coupled to the subsea mount, the tension is applied to the tethering member in a tensioning direction towards the subsea mount.

Optionally, the actuator may be a mechanically driven actuator. Alternatively, the actuator may be a hydraulically driven actuator.

Optionally, the actuator may be operable by a driving tool, whereby the driving action of the driving tool provides a driving input to the actuator. The actuator may comprise an interface configured to engage the driving tool and a drive shaft to be operably driven by the driving tool. For example, the actuator may comprise a Remotely Operated Vehicle (ROV) interface such that the chain block can be operated and controlled by driving tool carried and controlled by a ROV.

Optionally, the actuator may comprise a clutch to control the driving input of the driving tool.

Optionally, the chain block may further comprise a brake for the chain sheave. The brake may be configured to inhibit back rotation of the chain sheave as the tension is being applied to the tethering member. As such, the brake may at least substantially maintain the tension applied to the tethering member by the chain block.

Optionally, the chain block may be configured to operate in a slack mode to reduce tension in the tethering member. In the slack mode, the chain block may be configured to at least reduce, preferably remove, the tension applied to the tethering member during the tension mode.

Optionally, the brake may be releasable to allow for reverse rotation of the load chain sheave. Reverse rotation of the load chain sheave reverse conveys the load chain to pay-out the load chain. The reverse conveying (paying-out) of the load chain causes the tension in the tethering member to be at least reduced. As such, to operate the chain block in the slack mode, the chain block may be configured to release the brake to at least reduce the tension previously applied to the tethering member.

Optionally, the tensioning apparatus may comprise a tension monitoring system configured to monitor tension in the tethering member. For example, the tensioning monitoring system may comprise a tension load cell and/or a tension load pin to monitor tension in the tethering member. The tension monitoring system may, for example, communicate the measured tension in the tethering member to an operator and/or other personnel at the surface (or other remote location) to enable tension monitoring, control and quantification of the external loads on the subsea well control equipment, and allow for rapid identification in the event of a potential tethering member failure. The tensioning monitoring system may, for example, communicate the measured tension in the tethering member to the actuator to provide loop feedback control. Optionally, the chain block may comprise a fail-safe clutch configured to automatically release at least some of the tension in the tethering member under excessive loading. In this way, the fail-safe clutch may minimise the potential risk of damage to the tensioning apparatus, subsea well control equipment and/or other subsea well assembly components under excessive loading.

Optionally, the tensioning apparatus may comprise a frame to rigidly support the chain block. The frame may protect the chain block from damage during use and allow for ease of handling.

Optionally, the tensioning apparatus may comprise a buoyancy aid to aid the buoyancy of the chain block and control the orientation of the chain block during use in subsea conditions. For example, the buoyancy aid may be coupled to the frame. The buoyancy aid may maintain the orientation of the tensioning apparatus while subsea to help avoid twisting of the tethering member. The buoyancy aid may maintain the orientation of the tensioning apparatus so that the actuator interface is in a generally vertical plane relative to the seabed. In this orientation, a driving tool carried by ROV may be able to more easily interface with the chain block.

Optionally, to aid the mechanical advantage of the tensioning apparatus, the chain block may comprise at least one pulley around which the load chain is looped. For example, the chain block may comprise a compound pulley system comprising a fixed pulley and a moveable pulley.

Optionally, the tethering member has a first end portion and a second end portion.

Optionally, when the chain block is arranged to be coupled to the subsea well control equipment and the tethering member, the first end portion of the tethering member may be configured to be coupled to the first coupling or second coupling of the chain block and the second end portion of the tethering member may be configured to be coupled to a third coupling of the subsea mount.

Optionally, when the chain block is arranged to be coupled to the subsea mount and the tethering member, the second end portion of the tethering member may be configured to be coupled to the first coupling or second coupling of the chain block and the first end portion of the tethering member may be configured to be coupled to a fourth coupling of the subsea well control equipment.

Optionally, the chain block of the subsea tensioning apparatus is a primary chain block and the subsea tensioning apparatus may further comprise a secondary chain block, wherein the second chain block is configured to apply a pre-tension to the tethering member.

The pre-tension applied to the tethering member by the secondary chain block may be less than the tension applied to the tethering member by the primary chain block.

Optionally, the gear ratio of the primary chain block may be higher than a gear ratio of the second chain block.

The secondary chain block may be configured to apply the pre-tension prior to the application of the tension to the tethering member by the primary chain block. As such, the tensioning apparatus applies tension to the tethering member in a two-stage process.

Optionally, the primary chain block and secondary chain block may be arrangeable to be coupled to respective ends of the tethering member.

Optionally, when the primary chain block is arranged to be coupled to the subsea well control equipment and the tethering member, the secondary chain block may be configured to be coupled to the subsea mount and the tethering member.

Optionally, where the primary chain block is arranged to be coupled to the subsea mount and the tethering member, the secondary chain block may be configured to be coupled to the subsea well control equipment and the tethering member.

Optionally, when the chain block is arranged to be coupled to the subsea well control equipment and the tethering member, the subsea tensioning apparatus may further comprise a tensioning mechanism arranged on the subsea mount, wherein the tensioning mechanism is configured to apply a pre-tension to the tethering member. The pre-tension applied to the tethering member by the tensioning mechanism may be less than the tension applied to the tethering member by the chain block.

The tensioning mechanism may be configured to apply the pre-tension prior to the application of the tension to the tethering member by the chain block.

Optionally, the chain block and the tensioning mechanism may be arrangeable to be coupled to respective ends of the tethering member.

Optionally, the tensioning mechanism may comprise a fifth coupling to engage an end of the tethering member.

Optionally, the fifth coupling may be a releasable coupling to allow for attachment and detachment. For example, the fifth coupling may comprise a hook. The hook may include a safety catch to allow for the tensioning mechanism to be securely and releasably coupled to the tethering member during use. The releasable coupling of the tensioning mechanism allows for the reuse, retrofitting, removal and/or reinstallation of the tensioning mechanism.

Optionally, the fifth coupling may be a rotatable coupling to improve the tethering configuration with respect to the tensioning mechanism.

Optionally, the tensioning mechanism is coupled to one end of the tethering member and the chain block is coupled to the respective other end portion of the tethering member:

For example, the chain block may be arranged in a first arrangement where the first coupling is coupled to the first end of the tethering member and the second coupling is coupled to the subsea well control equipment, and the tensioning mechanism may be arranged where the fifth coupling is coupled to the second end of the tethering member.

Alternatively, for example, the chain block may be arranged in a second arrangement where the first coupling is coupled to the subsea well control equipment and the second coupling is coupled to the first end of the tethering member, and the tensioning mechanism may be arranged where the fifth coupling is coupled to the second end of the tethering member.

Optionally, the tensioning mechanism may comprise an actuator.

Optionally, the actuator may be operable by a driving tool. The actuator may comprise an interface configured to engage the driving tool, and a drive shaft to be operably driven by the driving tool.

Optionally, the driving tool may be carried and controlled by a Remotely Operated Vehicle (ROV), and the interface may be a ROV interface.

Optionally, the driving tool for the chain block and the driving tool for the tensioning mechanism may be the same driving tool. For example, the driving tool for the chain block and the driving tool for the tensioning mechanism may be a common driving tool carried and controlled by the ROV.

Optionally, the tensioning mechanism may comprise a rotatable drum, wherein the drum is rotatable to wind-in (reel-in) the tethering member to apply the pre-tension to the tethering member.

Optionally, the tensioning mechanism may be configured to operate in a pre-tension mode to apply the pre-tension to the tethering member. When the tensioning mechanism is operating in the pre-tension mode, the rotatable drum uses a rotary action to wind-in the tethering member onto the drum. For example, the rotatable drum rotates to winds-in the tethering member until the tethering member is under the desired pre-tension and the tethering member is suitably taut.

In an arrangement of the tensioning apparatus where the tensioning mechanism is arranged on the subsea mount, the pre-tension is applied to the tethering member in a tensioning direction towards the subsea mount. By applying the pre-tension to the tethering member where the tethering member becomes taut under the pre-tension, the tensioning mechanism allows for the removal of slack in the tethering member before the application of the tension by the chain block. Since, the pre-tension applicable by the tensioning mechanism is less than the tension applicable by the chain block, the tensioning mechanism is able to remove slack in the tethering member with a lower tension than the chain block. The tensioning mechanism may be configured to apply the pre-tension to the tethering member, removing slack, faster than the chain block.

Optionally, the tensioning mechanism may comprise a lock for the tensioning mechanism to at least substantially maintain the pre-tension applied to the tethering member. In the example where the tensioning mechanism comprises the rotatable drum, the rotatable drum may comprise a drum lock configured to prevent back rotation of the rotatable drum as the pre-tension is being applied. As such, the drum lock may at least substantially maintain the pre-tension applied to the tethering member. For example, the drum lock may comprise a ratchet wheel and pawl mechanism.

Optionally, the tensioning mechanism may be configured to operate in a slack mode to reduce tension in the tethering member. In the slack mode, the tensioning mechanism may be configured at least reduce, preferably remove, the pre-tension applied to the tethering member during the pre-tension mode. For example, the rotatable drum may be configured to reverse rotate to wind-out (reel-out) the tethering member and thereby at least reduce the pre-tension in the tethering member. By at least reducing the pre-tension in the slack mode, the tensioning mechanism allows for slack in the tethering member.

Optionally, the lock may be releasable to allow the tensioning mechanism to operate in the slack mode. For example, the drum lock may be releasable to allow the drum to reverse rotate in the opposite direction to wind-out the tethering member from the drum and thereby at least reduce tension in the tethering member. As such, to operate the tensioning mechanism in the slack mode, the tensioning mechanism may be configured to release the lock to at least reduce the pre-tension applied to the tethering member. The rotary action of the rotatable drum allows for a continuous (non-discrete and nonlimiting) winding-in and/or winding-out of the tethering member. Advantageously, the rotatable drum can accommodate tethering members of variable lengths, which can be wound-in or wound-out from the rotatable drum as required. The rotatable drum can rotate to wind-in a tethering member of any length until it is under the pre-tension and the tethering member is suitably taut. The rotatable drum can reverse rotate to wind-out a tethering member of any length until the tension is reduced or removed and the tethering member is suitably slack. Thus, by using the rotary action of the rotatable drum, the tensioning apparatus is not limited to tensioning a tethering member of only a specific pre-cut length. Further, the rotatable drum allows for the tensioning mechanism to be locatable at variable distances from the subsea well control equipment. The rotatable drum also allows for the tensioning apparatus to be suitable for use, and re-use, at different subsea well assembly sites.

Optionally, the tethering member may be pre-mounted on the tensioning mechanism prior to arranging subsea. For example, the tethering member may be pre-coupled to the fifth coupling. As such, one end of the tethering member is fixedly engaged to the tensioning mechanism and so, advantageously, the coupling of the tethering member to the tensioning mechanism does not need to be performed in a subsea environment. Additionally, by being pre-mounted on the tensioning mechanism, the tethering member is controlled in the subsea environment during the subsea arranging of the tensioning apparatus. In direct contrast, a conventional loose/unmounted tethering member may be come tangled and/or caught on subsea objects, including the subsea well control equipment, ROV and/or natural/environmental snag points, and thereby pose a safety and operational concern. In the example where the tensioning mechanism comprises a rotatable drum, the tethering member may be pre-wound on the rotatable drum to pre-mount the tethering member.

Optionally, the tensioning mechanism may be configured to operate in a deployment mode to deploy the pre-mounted tethering member from the tensioning mechanism for tethering. When deployed for tethering, a free end of the tethering member can be coupled to the chain block. For a tethering member pre-wound on the rotatable drum, the tensioning mechanism may be configured to operate in a deployment mode, whereby the rotatable drum is configured to wind-out the pre-wound tethering member so as to deploy the rotatable drum for tethering. In the deployment mode, the tensioning mechanism may be configured to allow the rotatable drum to free-wheel as it winds-out the tethering member for tethering. Advantageously, the rotatable drum can free-wheel without requiring a clutch.

A second aspect of the invention provides a subsea method of tethering a subsea well control equipment under tension, the method comprising: providing a subsea mount; providing a tethering member; providing tensioning apparatus, wherein the tensioning apparatus comprises a chain block configured to apply a tension to the tethering member; arranging the subsea mount on the seabed in spaced relationship to the subsea well control equipment; coupling the chain block to the subsea well control equipment or the subsea mount; coupling the chain block to an end of the tethering member; coupling the respective other end of the tethering member to the respective other of the subsea mount or the subsea well control equipment; wherein the respective other end of the tethering member is directly or indirectly coupled to the respective other of the subsea mount or the subsea well control equipment; and actuating the chain block to apply the tension to the tethering member.

Optionally, tensioning apparatus may be a tensioning apparatus according to the first aspect of the invention.

Optionally, the subsea mount may be a subsea mount according to the first aspect of the invention.

Optionally, the tethering member may be a tethering member according to the first aspect of the invention.

Optionally, when the tensioning apparatus further comprises the secondary chain block configured to apply a pre-tension to the tethering member, the method may further comprise: providing the secondary chain block configured to apply a pre-tension to the tethering member; coupling the secondary chain block to the respective other of the subsea mount or the subsea well control equipment; coupling the secondary chain block to the respective other end of the tethering member; actuating the secondary chain block to apply the pre-tension prior to actuating the chain block to apply the tension to the tethering member, wherein the pre-tension is less than the tension; and wherein coupling the respective other end of the tethering member comprises coupling the respective other end of the tethering member to the second chain block.

Alternatively, when the chain block is coupled to the subsea well control equipment and the tensioning apparatus further comprises the tensioning mechanism configured to apply a pre-tension, the method may further comprise: providing the tensioning mechanism configured to apply a pre-tension to the tethering member; mounting the tensioning mechanism on the subsea mount; coupling the tensioning mechanism to the respective other end of the tethering member; actuating the tensioning mechanism to apply the pre-tension prior to actuating the chain block to apply the tension to the tethering member, wherein the pre-tension is less than the tension; and wherein coupling the respective other end of the tethering member comprises coupling the respective other end of the tethering member to the tensioning mechanism.

A third aspect of the invention provides a subsea tensioning apparatus for tensioning a tethering member, wherein the tethering member is configured to tether a subsea well control equipment to a subsea mount arranged on a seabed, the subsea tensioning apparatus comprising: a first chain block configured to be coupled to the subsea well control equipment or the subsea mount, and to an end of the tethering member, wherein the first chain block is configured to apply a first tension to the tethering member; and a second chain block configured to be coupled to the respective other of the subsea mount or the subsea well control equipment, and to the respective other end of the tethering member, wherein the second chain block is configured to subsequently apply a second tension to the tethering member, and wherein the second tension is greater than the first tension.

The tensioning apparatus is configured to provide tension in a two-stage process. In the first stage, the first chain block is configured to apply a first tension to the tethering member. In the second stage, the second chain block is configured to subsequently apply a second tension to the tethering member, such that the tethering member is under a final tethering tension.

Advantageously, the combined first tension and second tension (final tension) at least restricts, preferably inhibit, movement of the tethering member, and thereby movement and lateral bending of the subsea well control equipment. It follows that the tensioning apparatus advantageously enhances the stability and fatigue performance of a subsea well assembly by restricting, preferably inhibiting, movement and lateral bending of the subsea well control equipment.

A fourth aspect of the invention provides a subsea tensioning apparatus for tensioning a tethering member, wherein the tethering member is configured to tether a subsea well control equipment to a subsea mount arranged on a seabed, the subsea tensioning apparatus comprising: a tensioning mechanism configured to be arranged on the subsea mount and coupled to an end of the tethering member, wherein the tensioning mechanism is configured to apply a first tension to the tethering member; and a chain block configured to be coupled to the subsea well control equipment and coupled to the respective other end of the tethering member, wherein the chain block is configured to subsequently apply a second tension to the tethering member, and wherein the second tension is greater than the first tension.

The tensioning apparatus is configured to provide tension in a two-stage process. In the first stage, the tensioning mechanism is configured to apply a first tension to the tethering member. In the second stage, the chain block is configured to subsequently apply a second tension to the tethering member, such that the tethering member is under a final tethering tension.

Optionally, the tensioning mechanism may be configured to operate in a first tension mode to apply the first tension to the tethering member.

Optionally, the chain block may be configured to operate in a second tension mode to apply the second tension to the tethering member.

Optionally, the chain block may comprise a load chain, wherein the chain block is configured to convey (pay-in) the load chain to apply the second tension to the tethering member.

Optionally, the chain block may comprise a first chain block coupling and a chain block second coupling.

Optionally, the first chain block coupling and second chain block coupling may be axially aligned and arranged on opposite sides of the chain block.

Optionally, the first chain block coupling may be a movable coupling mounted on the conveyable load chain.

Optionally, the second chain block coupling may be a fixed coupling mounted on a fixed portion of the chain block.

Optionally, the first chain block coupling and/or second chain block coupling may be a releasable coupling to allow for attachment and detachment. For example, the first chain block coupling and/or second chain block coupling may comprise a hook. The hook may include a safety catch to allow for the chain block to be securely and releasably coupled to the tethering member and the subsea well control equipment or subsea mount during use. The releasable coupling of the chain block allows for the reuse, retrofitting, removal and/or reinstallation of the chain block in one or more subsea well assembly.

Optionally, the tensioning mechanism may comprise a tensioning mechanism coupling.

Optionally, the tensioning mechanism coupling may be a releasable coupling to allow for attachment and detachment. For example, the tensioning mechanism coupling may comprise a hook. The hook may include a safety catch to allow for the tensioning mechanism to be securely and releasably coupled to the tethering member during use. The releasable coupling of the tensioning mechanism allows for the reuse, retrofitting, removal and/or reinstallation of the tensioning mechanism.

Optionally, the tensioning mechanism coupling may be a rotatable coupling to improve the tethering configuration with respect to the tensioning mechanism.

Optionally, the tensioning mechanism is coupled to a second end of the tethering member and the chain block is coupled to a first end of the tethering member:

For example, the chain block may be arranged in a first arrangement where the first chain block coupling is coupled to a first end portion of the tethering member and the second chain block coupling is coupled to the subsea well control equipment, and the tensioning mechanism may be arranged where the tensioning mechanism coupling is coupled to a second end portion of the tethering member.

Alternatively, for example, the chain block may be arranged in a second arrangement where the first chain block coupling is coupled to the subsea well control equipment and the second chain block coupling is coupled to the first end portion of the tethering member, and the tensioning mechanism may be arranged where the tensioning mechanism coupling is coupled to the second end portion of the tethering member.

Optionally, the tensioning mechanism may comprise an actuator.

Optionally, the actuator may be operable by a driving tool. The actuator may comprise an interface configured to engage the driving tool and a drive shaft to be operably driven by the driving tool.

Optionally, the tensioning mechanism may comprise a rotatable drum, wherein the drum is rotatable to wind-in (reel-in) the tethering member to apply the first tension to the tethering member.

When the tensioning mechanism is operating in the first tension mode, the rotatable drum uses a rotary action to wind-in the tethering member onto the drum. For example, the rotatable drum rotates to wind-in the tethering member until the tethering member is under the desired first tension and the tethering member is suitably taut.

In an arrangement of the tensioning apparatus where the tensioning mechanism is arranged on the subsea mount, the first tension is applied to the tethering member in a tensioning direction towards the subsea mount.

By applying the first tension where the tethering member becomes taut under the first tension, the tensioning mechanism allows for the removal of slack in the tethering member before the application of the second tension by the chain block. Since, the first tension is lower than the second tension, the tensioning mechanism is able to remove slack in the tethering member at a relatively lower tension than the chain block would. Optionally, the tensioning mechanism may be configured to apply the first tension to the tethering member, removing slack, faster than the chain block would. Optionally, the tensioning mechanism may comprise a lock for the tensioning mechanism to at least substantially maintain the first tension applied to the tethering member. In the example where the tensioning mechanism comprises the rotatable drum, the rotatable drum may comprise a drum lock configured to prevent back rotation of the rotatable drum as the first tension is being applied. As such, the drum lock may at least substantially maintain the first tension applied to the tethering member. For example, the drum lock may comprise a ratchet wheel and pawl mechanism.

Optionally, the tensioning mechanism may be configured to operate in a first slack mode to reduce tension in the tethering member. In the first slack mode, the tensioning mechanism may be configured at least reduce, preferably remove, the first tension applied to the tethering member during the first tension mode. For example, the rotatable drum may be configured to reverse rotate to wind-out (reel-out) the tethering member from the drum and thereby at least reduce the first tension in the tethering member. By at least reducing the first tension in the first slack mode, the tensioning mechanism allows for slack in the tethering member.

Optionally, the lock may be releasable to allow the tensioning mechanism to operate in the first slack mode. For example, the drum lock may be releasable to allow the drum to reverse rotate in the opposite direction to wind-out the tethering member from the drum and thereby at least reduce tension in the tethering member. As such, to operate the tensioning mechanism in the first slack mode, the tensioning mechanism may be configured to release the lock to at least reduce the first tension applied to the tethering member.

The rotary action of the rotatable drum allows for a continuous (non-discrete and nonlimiting) winding-in and/or winding-out of the tethering member. Advantageously, the rotatable drum can accommodate tethering members of variable lengths, which can be wound-in or wound-out from the rotatable drum as required. The rotatable drum can rotate to wind-in a tethering member of any length until it is under the first tension and the tethering member is suitably taut. The rotatable drum can reverse rotate to wind-out a tethering member of any length until the first tension is reduced or removed and the tethering member is suitably slack. Thus, by using the rotary action of the rotatable drum, the tensioning apparatus is not limited to tensioning a tethering member of only a specific pre-cut length. Further, the rotatable drum allows for the tensioning mechanism to be locatable at variable distances from the subsea well control equipment. The rotatable drum also allows for the tensioning apparatus to be suitable for use, and re-use, at different subsea well assembly sites.

Optionally, the tethering member may be pre-mounted on the tensioning mechanism prior to arranging subsea. For example, the tethering member may be pre-coupled to the tensioning mechanism coupling. As such, the second end portion of the tethering member is fixedly engaged to the tensioning mechanism and so, advantageously, the coupling of the tethering member to the tensioning mechanism does not need to be performed in a subsea environment. Additionally, by being pre-mounted on the tensioning mechanism, the tethering member is controlled in the subsea environment during the subsea arranging of the tensioning apparatus. In direct contrast, a conventional loose/unmounted tethering member may be come tangled and/or caught on subsea objects, including the subsea stack, ROV and/or natural/environmental snag points, and thereby pose a safety and operational concern. In the example where the tensioning mechanism comprises a rotatable drum, the tethering member may be pre-wound on the rotatable drum to pre-mount the tethering member.

Optionally, the tensioning mechanism may be configured to operate in a deployment mode to deploy the pre-mounted tethering member from the tensioning mechanism for tethering. When deployed for tethering, the free, first end portion of the tethering member can be coupled to the chain block. For a tethering member pre-wound on the rotatable drum, the tensioning mechanism may be configured to operate in a deployment mode, whereby the rotatable drum is configured to wind-out the prewound tethering member so as to deploy the rotatable drum for tethering. In the deployment mode, the tensioning mechanism may be configured to allow the rotatable drum to free-wheel as itwinds-out the tethering member for tethering. Advantageously, the rotatable drum can free-wheel without requiring a clutch.

In response to the actuator receiving the driving input, the gear assembly is actuated by the actuator to rotate the chain sheave. The rotation of the chain sheave conveys the load chain to pay-in the load chain. The conveying (paying-in) action of the load chain is transmitted to the tethering member to apply the second tension to the tethering member.

In an arrangement of the tensioning apparatus where the chain block is coupled to the subsea well control equipment, the second tension is applied to the tethering member in a tensioning direction towards the subsea well control equipment.

Optionally, the actuator may be operable by a driving tool. The actuator may comprise an interface configured to engage the driving tool and a drive shaft to be operably driven by the driving tool. For example, the actuator may comprise a Remotely Operated Vehicle (ROV) interface such that the chain block can be operated and controlled by driving tool carried and controlled by a ROV.

Optionally, the chain block may further comprise a brake for the chain sheave. The brake may be configured to inhibit back rotation of the chain sheave as the second tension is being applied to the tethering member. As such, the brake may at least substantially maintain the second tension applied to the tethering member. Optionally, the chain block may be configured to operate in a second slack mode to reduce tension in the tethering member. In the second slack mode, the chain block may be configured to at least reduce, preferably remove, the second tension applied to the tethering member during the second tension mode.

Optionally, the brake may be releasable to allow for reverse rotation of the load chain sheave. Reverse rotation of the load chain sheave reverse conveys the load chain to pay-out the load chain. The reverse conveying (paying-out) of the load chain causes the second tension in the tethering member to be at least reduced. As such, to operate the chain block in the slack mode, the chain block may be configured to release the brake to at least reduce the second tension previously applied to the tethering member.

Optionally, the tensioning apparatus may comprise a tension monitoring system configured to monitor tension in the tethering member. For example, the tensioning monitoring system may comprise a tension load cell and/or a tension load pin to monitor tension in the tethering member. The tension monitoring system may, for example, communicate the measured tension in the tethering member to an operator and/or other personnel at the surface (or other remote location) to enable tension monitoring, control and quantification of the external loads on the subsea well control equipment, and all for rapid identification of tethering member failure. The tensioning monitoring system may, for example, communicate the measured tension in the tethering member to the actuator to provide loop feedback control.

Optionally, the chain block may comprise a fail-safe clutch configured to automatically release at least some of the tension in the tethering member under excessive loading. In this way, the fail-safe clutch may minimise the potential risk of damage to the tensioning apparatus, subsea well control equipment and/or other subsea assembly components under excessive loading.

Optionally, the tensioning apparatus may comprise a frame to rigidly support the chain block. The frame may protect the chain block from damage during use and allow for ease of handling. Optionally, the tensioning apparatus may comprise a buoyancy aid to aid the buoyancy of the chain block and control the orientation of the chain block during use in subsea conditions. For example, the buoyancy aid may be coupled to the frame. The buoyancy aid may maintain the orientation of the tensioning apparatus while subsea to help avoid twisting of the tethering member. The buoyancy aid may maintain the orientation of the tensioning apparatus so that the actuator interface is in a generally vertical plane relative to the seabed. In this orientation, a driving tool carried by ROV may be able to more easily interface with the chain block.

A fifth aspect of the invention provides a subsea method of tensioning a tethering member, wherein the tethering member is tethering a subsea well control equipment to a subsea mount, the method comprising: providing a subsea tensioning apparatus, wherein the tensioning apparatus comprises a chain block configured to apply a tension to the tethering member; coupling the chain block to the subsea well control equipment or the subsea mount; coupling the chain block to an end of the tethering member; actuating the chain block to apply the tension to the tethering member.

Optionally, wherein the tensioning apparatus is a tensioning apparatus according to the first aspect of the invention.

Optionally, when the subsea tensioning apparatus further comprises a secondary chain block configured to apply a pre-tension to the tethering member, the method may further comprise: providing the secondary chain block; coupling the secondary chain block to the respective other of the subsea mount or the subsea well control equipment; coupling the secondary chain block to the respective other end of the tethering member; actuating the secondary chain block to apply the pre-tension prior to actuating the chain block to apply the tension to the tethering member, wherein the pre-tension is less than the tension.

Optionally, wherein the tensioning apparatus is a tensioning apparatus according to the third aspect of the invention.

Alternatively, when the chain block is coupled to the subsea well control equipment and the tensioning apparatus further comprises a tensioning mechanism configured to apply a pre-tension; the method may further comprises: providing the tensioning mechanism; mounting the tensioning mechanism on the subsea mount; coupling the tensioning mechanism to the respective other end of the tethering member; actuating the tensioning mechanism to apply the pre-tension to the tethering member prior to actuating the chain block to apply the tension to the tethering member, wherein the pre-tension is less than the tension.

Optionally, wherein the tensioning apparatus is a tensioning apparatus according to the fourth aspect of the invention.

It will be appreciated that any feature described herein as being suitable for incorporation into one or more aspects or embodiments of the present invention is intended to be generalisable across any aspect or embodiment of the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings illustrate presently exemplary embodiments of the disclosure, and together with the general description given above and the detailed description of the embodiments given below, serve to explain, by way of example, the principles of the disclosure. Figure 1 shows a schematic view of a conventional example of a subsea well assembly with a subsea blowout preventer (BOP);

Figure 2A shows a schematic view of an example subsea well assembly with a BOP and tensioning system according to the present invention;

Figure 2B shows a schematic view of an example of subsea well assembly with a subsea intervention lubricator (SIL) and tensioning system according to the present invention;

Figure 20 shows a schematic view of an example of a subsea well assembly with a lower riser package (LRP) and tensioning system according to the present invention;

Figure 3A shows a front-on view of an example subsea tensioning apparatus according to the present invention;

Figure 3B shows a side-on view of an example subsea tensioning apparatus according to the present invention;

Figure 30 shows a view from the top of an example subsea tensioning apparatus according to the present invention;

Figure 3D shows a shows a cross-sectional view along A-A of the tensioning apparatus on a mount shown in Figure 30;

Figure 4A shows a front-on view of the subsea tensioning apparatus according to the present invention with the gear assembly cover omitted;

Figure 4B shows an enlarged view of gear assembly of the subsea tensioning apparatus shown in Figure 4A;

Figure 40 shows a perspective view of the subsea tensioning apparatus shown in Figure 4A; Figure 4D shows a bottom view of the subsea tensioning apparatus shown in Figure 4A;

Figure 5 shows an enlarged view of the frame of a subsea tensioning apparatus according to the present invention.

Figure 6A shows a back view of an example subsea tensioning apparatus according to the present invention with the brake assembly cover omitted;

Figure 6B shows a perspective view of the subsea tensioning apparatus shown in Figure 6A;

Figure 7 shows a view of a first example of a tensioning system according to the present invention, where the tensioning system is tethering a subsea BOP under tension;

Figure 8 shows an enlarged view of the subsea tensioning apparatus of the system shown in Figure 7;

Figure 9 shows a view of an alternative example of a tensioning system according to the present invention, where the tensioning system is tethering a subsea BOP under tension;

Figure 10 shows a view of another example of a tensioning system according to the present invention, where the tensioning system is tethering a subsea BOP under tension;

Figure 11 shows a view of another example of a tensioning system according to the present invention, where the tensioning system is tethering a subsea BOP under tension;

Figure 12 shows a perspective view of an example of the subsea mount of the tensioning system, according to the present invention; Figure 13 shows a view of another example of a tensioning system according to the present invention, where the subsea tensioning apparatus comprises a primary chain block and a secondary chain block;

Figure 14 shows a view of a further example of a tensioning system according to the present invention, where the subsea tensioning apparatus comprises a chain block and a tensioning mechanism;

Figure 15 shows a view of an alternative example of a tensioning system according to the present invention, where the subsea tensioning apparatus comprises a chain block and a tensioning mechanism;

Figure 16A shows a side-on view the mount and tensioning mechanism of the tensioning system shown in Figures 14 and 15;

Figure 16B shows an alternative view of the mount and tensioning mechanism of Figure 16A;

Figure 17A shows a view of the tensioning mechanism of the tensioning system shown in Figures 14 and 15 with the brake assembly engaged;

Figure 17B shows a view of the tensioning mechanism of the tensioning system shown in Figures 14 and 15 with the brake assembly disengaged;

Figure 18 shows an example of a method of tethering a subsea well control equipment according to the present invention;

Figure 19 shows an example of a method of tensioning a tethering member, whereby the tethering member is tethering a subsea well control equipment and a subsea mount, according to the present invention;

Figure 20 shows another example of a method of tensioning a tethering member, whereby the tethering member is tethering a subsea well control equipment and a subsea mount, according to the present invention; and Figure 21 shows another example of a method of tensioning a tethering member, whereby the tethering member is tethering a subsea well control equipment and a subsea mount, according to the present invention.

DETAILED DESCRIPTION

The present invention relates to a tensioning apparatus, system and methods to tether under tension a subsea well control equipment of a subsea well assembly. Examples of subsea well control equipment include, but are not limited to, a subsea blowout preventer (BOP), a subsea intervention lubricator (SIL), and a lower riser package (LRP).

Figure 2A depicts a first example of a subsea well assembly 10’ according to the present invention. In the example depicted, the subsea well assembly is arranged for construction, intervention, de-construction and/or abandonment mode, and the subsea well control equipment comprises a subsea blowout preventer (BOP) 24. The subsea well assembly 10’ further comprises a tree 16 releasably connected to a wellhead 18 disposed at an upper end of a primary conductor 20 extending into a wellbore 22. The subsea BOP 24 is releasably connected to the tree 16, and a lower marine riser package (LMRP) 26 releasably connected to the subsea BOP 24. The tree 16, subsea BOP 24, and LMRP 26 are vertically arranged or stacked one-above- the-other, and are generally coaxially aligned with wellhead 18. The subsea well assembly also further comprises a riser 28, wherein the riser 28 connects the subsea well assembly 10’ to a floating vessel 12 at the sea surface 14. The tree 16 may not necessarily be present, for example, during drilling operations. The wellhead 18 has a central axis and extends vertically upward from wellbore 22 above the seabed 30.

A second example of a subsea well assembly 10” according to the present invention is depicted in Figure 2B. In this example, the subsea well assembly is arranged for intervention and/or abandonment mode, and the subsea well control equipment comprises a subsea intervention lubricator (SIL) 24’. The SIL 24’ is releasably connected to the tree 16. A riser 28 extends from the SIL 24’ to a tool catcher 44. A wireline 46 extends between the tool catcher 44 and a floating vessel 12 at the sea surface 14. The purpose of such well assemblies is to allow a downhole tool to be deployed from the wireline 46 into the subsea well, for example, to enhance production or to measure conditions in the well.

A third example a subsea well assembly 10”’ according to the present invention is depicted in Figure 2C. In this example, the subsea well assembly is an open water intervention riser system arranged in construction, intervention and/or abandonment mode, and the subsea well control equipment comprises a lower riser package (LRP) 24” releasably connected to the tree 16. An emergency disconnect package (EDP) 48 is arranged above and releasably connected to the LRP (24”). The subsea well assembly further comprises a riser 28 extending between the EDP 48 and a floating vessel 12 at the sea surface 14. The riser 28 may comprise a dual bore riser as shown, or mono bore riser.

To minimise load fatigue, the subsea well assemblies 10’, 10”, 10’” depicted in Figures 2A to 2C further comprise one or more tethering systems 32 for tethering the subsea well control equipment under tension.

For the sake of clarity and brevity, the tensioning systems and apparatus according to the present invention are described below in connection to a subsea well assembly comprising a subsea BOP, where the subsea BOP is tethered under tension to a subsea mount. However, it will be appreciated that the tensioning systems and apparatus described below may be deployed and used with subsea well assemblies comprising any other suitable subsea well control equipment, including, but not limited to, a SIL or LRP.

As shown in the tethering system examples depicted in Figures 7, 9, 10, 11 , 13, 14 and 15, the tethering system 32 according to the present invention comprises a subsea mount 34 arranged on a seabed 30 in spaced relationship to the subsea BOP 24, a tethering member 36 to tether the BOP to the subsea mount, and a subsea tensioning apparatus 38 to tension the tethering member.

An optimal layout for the tensioning of the subsea BOP may comprise four subsea tensioning systems 32 that are arranged equi-spaced around the well and attached to the subsea BOP at each corner with a tethering member angle of 45 degrees. The radial distance from the well is a function of the tethering member attachment point on the subsea BOPs and is typically around 20m. However, any suitable number and arrangement of tensioning systems may be utilised to provide a desired tethering effect of the subsea BOP.

The subsea tensioning apparatus is configured to tension the tethering member whereby the tethering member is taut (without slack) and under a tensile load that is sufficient to restrict, preferably prevent, undesired movement and lateral bending of the subsea BOP when subjected to external loads during one or more mode of operation of the subsea well assembly. As such, undesired movement and lateral bending of the subsea BOP due to the external loads is impeded, and the overall fatigue performance of the subsea well assembly is improved.

The subsea tensioning apparatus 38 of the present invention comprises a chain block C configured to apply a tension to the tethering member 36.

The chain block C is configured to operate in a tension mode to apply the tension to the tethering member 36 tethering the subsea BOP 24 to the subsea mount 34.

Figures 3A to 3D depict a schematic arrangement of an example of a chain block C according to the present invention. The chain block C comprises a load chain L, a chain sheave 100, a gear assembly 110 and an actuator 120.

In tension mode, the actuator 120 is configured to actuate the gear assembly in response to a driving input. When actuated, the gear assembly is configured to drive the chain sheave. When rotating, the chain sheave is configured to convey the load chain to pay-in the load chain.

As depicted in the Figures, the actuator 120 may comprise an interface 122 and a drive shaft 124.

The interface 122 is configured to receive a driving tool (not shown), whereby the driving action of the driving tool provides the driving input to the actuator. The interface may comprise teeth, apertures, protrusions and/or indents to enhance the engagement of the driving tool with the interface. The interface may be configured to releasably engage the driving tool to allow for the engagement and dis-engagement of the driving tool. If the driving tool is carried by a Remotely Operated Vehicle (ROV), the interface 122 may comprise a ROV interface. An ROV may be employed during the installation, maintenance, deconstruction and abandonment modes of the subsea assembly. The ROV may include multiple arms for manipulating objects, and a subsea camera for viewing the subsea operations. Streaming video and/or images from the cameras may be communicated to the surface or other remote location for viewing on a live or periodic basis.

In the example depicted in the Figures, the interface 122 is an ROV interface comprising an ROV bucket. Such ROV buckets are well known in the art. Alternatively, the ROV interface may comprise any suitable interface known in the art, such as a hydraulic Hot Stab connection, a Manual Connection (e.g. D handle, T-bar or Fishtail), or a QD direct drive coupling.

As shown, the drive shaft 124 may comprise a first end 126 and a second end 128, where the first drive shaft 124 extends from the interface 122, through a central bore of the chain sheave 100 and to the gear assembly 110. The first end 126 of the drive shaft 124 extends into the interface 122 and is coupled thereto. Hence, when the driving tool engages the actuator, the drive shaft 124 is rotatable under the driving action of the driving tool. The second end 128 of the drive shaft 124 is coupled to a pinion gear 112 of the gear assembly 110.

The gear assembly 110 comprises a plurality of gears having any suitable configuration to provide a desired gear ratio. In one example, the gear assembly 110 may have a gear ratio of 43.3: 1.

Figures 4A to 4D depict a schematic arrangement of an example of a gear assembly 100 according to the present invention. As seen in Figures 4A-D, the gear assembly 110 may comprise the pinion gear 112, a first pair of reduction gears 114a, 114b, an associated pair of second reduction gears 116a, 116b and a sheave drive gear 118. The pinion gear 112 meshes with the first reduction gears 114a, 114b. The first reduction gears 114a, 114b are arranged in parallel, on opposing sides of the pinion gear 112, with the pinion gear 112 positioned in between. Each first reduction gear and associated second reduction gear is mounted on a respective reduction drive shaft. The second reduction gears 116a, 116b are arranged on the respective reduction shafts to the rear of the associated first reduction gears. The first reduction gears 114a, 114b and second reduction gears 116a, 116b are arranged in tandem on the respective reduction shafts, where the second reduction gears 116a, 116b are arranged to the rear of the first reduction gears 114a, 114b. The sheave drive gear 118 meshes with the second reduction gears 116a, 116b. The second reduction gears 116a, 116b are arranged in parallel, on opposing sides of the sheave drive gear 118, with the sheave drive gear positioned in between. The sheave drive gear 118 is coupled to the chain sheave 100 so as to rotate the first chain sheave in use.

With this arrangement, rotation of the pinion gear 112 by the drive shaft 124 causes the first reduction gears 114a, 114b to rotate, which in turn cause the associated second reduction gears 116, 116b to rotate, which in turn cause the sheave drive gear 118 to rotate, and thereby cause the chain sheave 100 to rotate.

As depicted in the Figures, the chain sheave 100 may comprise a rotatable wheel over which the load chain L is looped. The chain sheave may comprise teeth, apertures, protrusions and/or indents to engage the load chain.

As the chain sheave 100 rotates, it conveys the load chain L that is looped over the chain sheave 100, paying-in the load chain. The conveying (paying-in) load chain has a conveying force and a conveying rate. The conveying force of the load chain is dependent on the driving input torque and the gear ratio. The conveying rate of the load chain is dependent on the driving input speed and the gear ratio.

In operation, the driving input of the driving tool is transmitted by the chain block C to the load chain L via the drive shaft 124, the gear assembly 110 and the chain sheave 100.

In the present invention, the tensioning apparatus 38 may further comprise a frame F to rigidly support the chain block C. The frame F may comprise a plurality a struts, and have an open structure. The frame may be configured to protect the tensioning apparatus from damage during use. The frame may allow for ease of handling during construction, deconstruction and transportation. An example of a frame F is depicted in Figures 3A, 4A and 5. The tensioning apparatus 38 may comprise a buoyancy aid B to aid the buoyancy and control the orientation of the tensioning apparatus during use in subsea conditions. The buoyancy aid may maintain the orientation of the tensioning apparatus while subsea to help avoid twisting of the tethering member. The buoyancy aid may maintain the orientation of the tensioning apparatus so that the first chain block and the second chain block are accessible. In an example shown in the Figures, the tensioning apparatus may comprise a buoyancy aid B coupled to a side-strut of frame.

To enhance the mechanical advantage of the tensioning apparatus, the chain block C may further comprise one or more pulley about which the load chain L is looped. Each pulley may comprise one or more pulley wheel. As shown in the example depicted in the Figures, the chain block C may comprise a compound pulley comprising a first movable pulley P1 and a second fixed pulley P2. As the load chain is conveyed, the movable pulley P1 moves with the load chain. The second fixed pulley P2 is mounted on the frame F. In the example depicted in the Figures 3A to 3D, each pulley defines three pulley wheels (W).

In the present invention, the chain block C may further comprise a first coupling and a second coupling. As shown in the example depicted in Figures 3A to 3D, the first coupling may comprise a first hook H1 , which is attached to the first movable pulley P1 . In this way, as the load chain L is conveyed under actuation of the actuator, the first hook H1 and the first movable pulley are also conveyed along with the load chain L. As such, the first hook H1 may be referred to as a “movable hook”. The second coupling may comprise a second hook H2, which is attached to the second pulley P2. Since, the second pulley P2 is fixed, the second hook H2 is also static. As such, the second hook H2 may be referred to as a “fixed hook”. In this example, the first hook H1 and the second hook H2 are axially aligned and arranged on opposite sides of the chain block C.

In the example, each hook H1 and H2 includes a safety catch that is biased in the closed position to allow for the chain block to be securely and releasably coupled to the tethering member and one of the subsea BOP or subsea mount during use.

The chain block C may additionally or alternatively comprise any other suitable coupling means. The first coupling and the second coupling allow the chain block C to be coupled to the tethering member and one of the subsea BOP or the subsea mount, depending on the arrangement of the chain block. As such, the chain block provides an interconnection between the tethering member 36 and one of the subsea BOP and the subsea mount, depending on the arrangement of the chain block C.

In operation, when chain block C is coupled to the tethering member 36 and the chain block C is operating in the tension mode, the conveying action of the load chain L is transmitted to the tethering member to apply the tension to the tethering member.

As shown in Figures 7, 8 and 9, the chain block C may be attached to the subsea BOP 24 and the tethering member 36. As such, the tethering member 36 extends from the subsea mount 34 to the subsea BOP 24 via the chain block C.

In the example of the tethering system 32 depicted in Figures 7 and 8, the chain block C may be arranged such that it is attached to the subsea BOP via hook H2. A first end portion 36a of the tethering member 36 is attached to hook H1 of the chain block C and a second end portion 36b of the tethering member 36 is attached to the subsea mount 34. When the chain block C is operating in tension mode, the chain block is actuated by the driving input of the driving tool to convey (pay-in) the load chain L in a direction towards the subsea BOP 24. As the load chain L is conveyed, the conveying action is directly transmitted to the tethering member coupled to hook H1 and tension is thereby applied to the tethering member 36 in a tensioning direction towards the subsea BOP 24.

Conversely, in the example of the tethering system 32 depicted in Figure 9, the chain block C may be arranged such that it is attached to the subsea BOP via hook H1 and the first end portion 36a of the tethering member is coupled to the chain block via hook H2. A second end portion 36b of the tethering member is attached to the subsea mount 34. When the chain block C is operating in tension mode, the chain block is actuated by the driving input of the driving tool to convey (pay-in) the load chain L in a direction towards the subsea mount 34. As the load chain L is conveyed, the conveying action is indirectly transmitted to the tethering member 36 coupled to the hook H2, via the chain block C. As a result, tension is thereby applied to the tethering member in a tensioning direction towards the subsea BOP 24. In an alternative arrangement as shown in Figures 10 and 11 , the chain block C may be attached to the subsea mount 34 and the tethering member 36. As such, the tethering member 36 extends from the subsea BOP 24 to the subsea mount 34, via the chain block C.

In an example of the tethering system 32 depicted in Figure 10, the chain block C may be arranged such that the chain block is attached to the subsea mount 34 via hook H2 and the second end portion of the tethering member 36b is attached to hook H1 of the chain block C. The first end portion 36a of the tethering member is coupled to the subsea BOP 24. When the chain block C is operating in tension mode, the chain block is actuated by the driving input of the driving tool to convey (pay-in) the load chain L in a direction towards the subsea mount 34. As the load chain L is conveyed, the conveying action is directly transmitted to the tethering member coupled to hook H1 , and tension is thereby applied to the tethering member in a tensioning direction towards the subsea mount 34.

Conversely, in an example depicted in Figure 11 , the chain block C may be arranged such that it is attached to the subsea mount via hook H1 and the second end portion 36b of the tethering member is coupled to hook H2 of the chain block C. The first end portion 36a of the tethering member is coupled to the subsea BOP 24. When the chain block C is operating in tension mode, the chain block is actuated by the driving input of the driving tool to convey (pay-in) the load chain L in a direction towards the subsea BOP 24. As the load chain L is conveyed, the conveying action is indirectly transmitted to the tethering member 36 coupled to hook H2 and tension is thereby applied to the tethering member in a tensioning direction towards the subsea mount 34.

Any slack in the tethering member may be removed by the conveying action of the load chain L as the tension is applied by the chain block. In the examples depicted in Figures 7, 8 and 9, the conveying action of the load chain L may displace the first end portion 36a of the tethering member in the tensioning direction towards the subsea BOP 24, removing any slack in the tethering member. In the examples depicted in Figures 10 and 11 , the conveying action of the load chain L may displace the second end 36b of the tethering member in the tensioning direction towards the subsea mount 34, removing any slack in the tethering member. As shown in Figure 12, the subsea mount 34 may include a plurality of anchor points for attachment of the tethering member 36 or the chain block C to the subsea mount. The plurality of anchor points may be positioned at a plurality of locations on the subsea mount to accommodate tethering members of different lengths. The anchor points may be padeye connection points 40 and/or bollard connection points 42 of the type known in the art.

Advantageously the mechanical advantage provided by the gear ratio of gear assembly 110, and any pulley, enables a high output torque to be generated with a relatively low input torque. In this way, to the tensioning apparatus of the present invention can be used to apply and maintain high tensile loads in the tethering member with a relatively low input torque. This is particularly advantageous when operating under subsea conditions.

The tension applied by the tensioning apparatus may be sufficient to at least restrict, preferably prevent, undesired movement of the tethering member when subjected to external loads during one or more mode of operation of the subsea well assembly. As such, undesired movement and lateral bending of the subsea BOP 24 due to the external loads is impeded, and the overall fatigue performance of the subsea well assembly is improved. For example, the tension applied by the tensioning apparatus to the tethering member may fall in the range of approximately 10,000 N to 500,000 N, depending on the subsea well assembly and external loads acting on the subsea assembly and tethering member. Advantageously, the chain block may have a mechanical advantage where a tension of up to 200,000 N can be applied with as little driving input torque of 55 Nm (40.5 ft.lbf).

In this way, the tensioning apparatus is configured to apply a tension to the tethering member so as to reinforce and/or stabilize the subsea BOP 24, tree 16, wellhead 18 and primary conductor 20 by restricting movement of subsea BOP 24. As a result, the tensioning apparatus 38 according to the present invention improves the strength and fatigue resistance, and reduces bending moments, of the subsea BOP 24, the tree 16 (when present), wellhead 18 and primary conductor 20. The chain block C may further comprise a brake 130 configured to prevent back- rotation of the load chain sheave 100. In the illustrated example shown in Figures 6A- B, the chain block may comprise a brake with a ratchet wheel and pawl mechanism disposed adjacent to the load chain sheave 100. A pair of pawls 132 may be provided on opposite sides of the ratchet wheel 134, and arranged to engage one of the tooth valleys of the ratchet wheel 134 and thereby stop the ratchet wheel 134 from rotating backward. This braking effect ensures the tension applied to the tethering member is maintained and the operational safety of a chain block C. Such braking structures are described in GB2501515B, the content of which is incorporated by reference in its entirety. However, the brake may include any suitable braking structure known in the art.

The chain block may be configured to operate in a slack mode to reduce tension in the tethering member. In the slack mode, the chain block may be configured to reduce or at least substantially remove the tension applied to the tethering member during the tension mode. Optionally, the brake 130 may be released to allow the chain block to operate in a slack mode. For example, the chain block may further comprise a clutch system (not shown) to allow the pinion shaft to disengage from the pawl system. This allows for reverse rotation of the load chain sheave. Reverse rotation of the load chain sheave reverse conveys (pays out) the load chain. The reverse conveying of the load chain causes the tethering member to move in a slackening direction, opposite to the tensioning direction, and the tension in the tethering member is at least reduced.

The chain block may act as a primary chain block to apply the tension to the tethering member, and the tensioning apparatus may further comprise a secondary chain block configured to apply a pre-tension to the tethering member. For example, as shown in Figure 13, the subsea tensioning apparatus may comprise a first chain block C1 and a second chain block C2. The first chain block C1 and the second chain block C2 may take the same general construction as chain block C described above and depicted in Figures 3A to 6B.

The first chain block C1 and second chain block C2 may be coupled to respective ends of the tethering member. In an example, the first chain block C1 may be coupled to the subsea BOP 24 and the tethering member 36, and the second chain block C2 may be coupled to the subsea mount 34 and the tethering member 36. For example, as shown in Figure 13, hook H1 of the first chain block C1 may be attached to a first end portion 36a of the tethering member 36 and hook H2 may attached to the subsea BOP 24. Hook H1 of the second chain block C2 may be attached to the subsea mount 34 and hook H2 may be attached to a second end portion 36b of the tethering member.

In an alternative arrangement, the first chain block 01 may be coupled to the subsea mount 34 and tethering member 36, and the second chain block 02 may be coupled to the subsea BOP 24 and the tethering member 36.

The first chain block 01 is configured to apply a first tension to the tethering member 36 and the second chain block 02 is configured to subsequently apply a second tension to the tethering member 36. Thus, the tensioning apparatus applies tension to the tethering member 36 in a sequential, two stage tensioning process.

The first chain block 01 initially applies the first tension to the tethering member 36 during a first tensioning stage. The second chain block 02 then subsequently applies the second tension to the tethering member 36, during a second tensioning stage, such that the tethering member 36 is under a final tethering tension (combined first tension and second tension). In other words, the first chain block 01 applies a pretension to the tethering member 36 prior to the application of the tension to the tethering member 36 by the second chain block 02.

The first chain block 01 and second chain block 02 may be configured whereby the first tension is less than the second tension. Hence, the first chain block 01 may be configured to remove slack at a lower tension that the second chain block 02. For example, the first tension applied to the tethering member by the first chain block 01 may fall within the range of approximately 5,000N to 20,000N.

As described above, the mechanical advantage of each chain block, and thereby the tension applied to the tethering member by each chain block, is dependent on the gear ratio and driving input torque of each chain block. Therefore, when the second gear ratio of the second chain block 02 is higher than the first gear ratio of the first chain block 01 , and the same driving input torque is applied to each chain block, the second tension applied by the second chain block C2 is greater than the first tension applied by the first chain block C1 .

Also, the conveying rate of each chain block is dependent on the gear ratio and the driving input speed. Therefore, when the second gear ratio of the second chain block C2 is higher than the first gear ratio of the first chain block C1 , and the same driving input speed is applied, the first conveying rate of the load chain of the first chain block C1 is faster than the second conveying rate of the load chain of the second chain block C2. As such, the tethering member 36 can be displaced faster by the first chain block C1 than the second chain block C2.

Hence, the first chain block C1 with the lower first gear ratio is advantageously able to remove slack in the tethering member 36 relatively more quickly, and at a relatively lower tension than the second chain block C2. The second chain block C2 with higher second gear ratio is advantageously able to subsequently apply the relatively higher tension to the tethering member 36 with relatively less displacement of the tethering member than the first chain block C1.

Alternatively, the tensioning apparatus may further comprise a tensioning mechanism 140 configured to be arranged on the subsea mount 34, wherein the tensioning mechanism 140 is configured to apply the pre-tension to the tethering member 36.

The tensioning mechanism 140 may operate in a pre-tension mode to apply the pretension to the tethering member 36.

The tensioning mechanism 140 is configured to apply the pre-tension to the tethering member 36 prior to the application of the tension to the tethering member 36 by the chain block C. Hence, the tensioning apparatus with the tensioning mechanism 140 and the chain block C applies tension to the tethering member in a two-stage process.

In the first stage, the tensioning mechanism 140 is configured to operate in the pretension mode to apply the pre-tension to the tethering member 36. In the second stage, the chain block C is configured to operate in the tension mode to apply the tension to the tethering member 36. Given that the tethering member will become taut under the pre-tension, the tensioning mechanism 140 may, for example, be configured to apply the pre-tension to the tethering member 36 to remove slack in the tethering member 36 before applying the tension to the tethering member by the chain block.

The tensioning mechanism is configured to apply the pre-tension to the tethering member, where the pre-tension is less than the tension applied to the tethering member by the chain block. Hence, the tensioning mechanism is able to remove slack at a lower tension than the chain block. The pre-tension may fall within the range of approximately 1000N to 4000N.

In the examples of the subsea tensioning system depicted in Figures 14 and 15, the tensioning mechanism 140 is mounted on the subsea mount 34 and the chain block C is attached to the subsea BOP 24. To tether the subsea mount 34 and the chain block C, the second end portion 36b of the tethering member 36 is attached to the tensioning mechanism 140 and the first end portion 36a of the tethering member is attached to the chain block.

In the examples of the tensioning system shown in Figures 14 and 15, the tensioning mechanism 140 may comprise a winch 142. Figures 16A to 17B depict a schematic arrangement of an example of the winch 142 according to the present invention. In this example, the winch 142 comprises a rotatable drum 144 to which the tethering member (not shown) may be attached. The drum 144 uses a rotatory action to windin (and wind-out) the tethering member 36 around the drum 144. When the tensioning mechanism is operating in the pre-tension mode, the rotatable drum 144 is configured to rotate to wind in the tethering member onto the drum until the tethering member is under the desired pre-tension and the tethering member is taut. The winch 142 may be configured to rotate the drum 144 so as to apply the pre-tension, removing slack, faster than chain block.

As shown in Figures 16A and 16B, the winch 142 may be arranged on the mount 34. For example, the winch 142 may be rotatably coupled to the mount 34 to allow for rotation of the winch about a vertical axis that is substantially perpendicular to the seabed 30. For example, the winch 142 may be mounted to the mount 34 via a subsea rated slew bearing. Optionally, the tensioning mechanism may be configured to operate in a pre-slack mode to reduce tension in the tethering member. When operating in the pre-slack mode, the rotatable drum 144 is configured to rotate to wind-out the tethering member from the drum, and thereby at least reduce, preferably remove, the pre-tension in the tethering member.

The tensioning mechanism 140 may comprise an actuator to drive the rotatable drum to wind-in the tethering member and apply the pre-tension. In the example shown in Figures 14 to 17B, the winch comprises an actuator including a first ROV interface 152a and a first drive shaft 152b. The first ROV interface 152a is configured to receive a driving tool carried by a ROV. The first ROV interface 152a may have a common configuration/design to the ROV interface 122 of the chain block, whereby both the tensioning mechanism and the chain block are drivable by a common driving tool.

The drive shaft 152b extends from the first ROV interface 152a to the rotatable drum 144. As such, the drive shaft 152b, and thereby the drum 144, is rotatable under the driving action of the driving tool to wind-in the tethering member onto the drum and apply the pre-tension.

The tensioning mechanism 140 may comprise a lock to maintain the pre-tension applied to the tethering member 36. As shown in Figures 16A to 17B, the winch 140 may further comprise a drum lock 146 configured to prevent back rotation of the rotatable drum so as to at least substantially retain the pre-tension applied to the tethering member. In the example, the drum lock 146 is a ratchet wheel 148 and pawl 150 mechanism.

The drum lock 146 may comprise an actuator to control the engagement or disengagement of the drum lock. In the example shown in Figures 16 to 17B, the actuator comprises a second ROV interface 154a and second drive shaft 154b. The second ROV interface 154b is configured to receive a driving tool carried by a ROV. The second ROV interface 154a may have a common configuration/design as the first ROV interface 152a of the winch, whereby the rotatable drum 144 and drum lock 146 are drivable by a common driving tool. The second drive shaft 154b extends from the second ROV interface 154a to the pawl 150. As such, the drive shaft 154b, and thereby the pawl 150, are rotatable under the driving action of the driving tool. Depending on the rotating direction of the driving tool, the pawl may be rotated to an engaged position where the pawl 150 engages the ratchet wheel 148 and the drum lock 146 is thereby engaged and provides a locking effect. Alternatively, if the driving tool rotates in the opposite direction, the pawl may be rotated to a disengaged position where the pawl is spaced from the ratchet wheel 147 and the drum lock is thereby disengaged.

As shown in Figure 17A, during the pre-tension mode, the drum lock 146 is engaged whereby the pawl 150 engages the ratchet wheel 148. As such, the drum 144 can be rotated to wind-in the tethering member 36 onto the drum to apply the pre-tension. The drum lock prevents back rotation of the ratchet wheel 148, thereby preventing loss of the pre-tension in the tethering member 36.

As shown in Figures 17B, during the pre-slack mode, the drum lock 146 is disengaged whereby the pawl 150 is spaced from the ratchet wheel 148. As such, the drum 144 can be rotated (free-wheel or driven) to wind-out the tethering member 36 from the drum to remove tension.

The tethering member 36 may be pre-mounted and stored on the tensioning mechanism 140. As such, a second end portion 36b of the tethering member may be fixedly engaged to the tensioning mechanism 140 prior to locating subsea and so attachment does not need to be performed in a subsea environment. In the illustrated examples, where the tensioning mechanism 140 is a winch 142, the tethering member 36 may be pre-wound on the rotatable drum 144 of the winch 142.

The tensioning mechanism 140 may have a deployment mode to deploy the premounted tethering member for tethering to the subsea BOP 24. For example, where the tethering member is pre-wound on the rotatable drum 144 and when the tensioning mechanism is operating in deployment mode, the rotatable drum 144 may be configured to wind-out the tethering member 36 for tethering to the BOP 24. The rotatable drum may be configured to free wheel as it winds-out the tethering member for tethering. For example, in the deployment mode, the pawl 148 may be spaced from the ratchet wheel using an ROV tool to operate the second ROV interface 154 so as to disengage the drum lock 146 and thereby allow the drum 144 to freely rotate. The first end portion 36a of the tethering member 36 may then connected to the chain block C. The first end portion 36a of the tethering member may be coupled to the chain block C using hook H1 or hook H2, depending on the arrangement of the chain block. The chain block C is also attached to the subsea BOP 24.

In the arrangement of the tensioning system shown in Figure 14, a first end portion 36a of the tethering member is attached to the chain block C via hook H1 and the chain block is attached to the subsea BOP 24 via hook H2.

In the arrangement of the tensioning system shown in Figure 15, a first end portion 36a of the tethering member is attached to the chain block C via hook H2 and the chain block is attached to the subsea BOP 24 via hook H1.

The pawl 150 is then brought into engagement with the ratchet wheel 148 using the second ROV interface 154. The pre-tension is then applied by using the first ROV interface 152 to wind the tethering member 36 back onto the rotatable drum 144 on the mount 34. A pre-tension sufficient to remove slack from the tethering member is applied, for example, up to 1300 N (-140 kgf).

Once the tethering member is taut under the pre-tension (slack has been removed from the tethering member), the chain block will be operated in tension mode to subsequently apply the tension.

The tensioning apparatus may further comprise a tensioning monitoring system to monitor the tension of the tethering member. The tensioning monitoring system may comprise communication means to communicate the measured tension to a remote operator and/or location to enable tension monitoring, quantification of external loads on the BOP and/or rapid identification in the event of a potential tethering member failure. The tensioning monitoring system may comprise a tension load cell TLC. The load cell may be an acoustic or visual display load cell. As tension is applied to the tethering member the tension load cell is compressed and the change in tension is identified.

A method M according to the present invention of tethering a subsea well control equipment under tension is shown in Figure 18. In a first step M1 , a tensioning system according to the present invention is provided. The tensioning system comprises the subsea mount, the tethering member and a tensioning apparatus comprising a chain block to apply the tension to the tethering member. In a second step M2, the subsea mount is arranged on the seabed in spaced relationship to the subsea well control equipment. In a series of coupling steps, the tethering member is coupled to tether the chain block and the subsea mount. For example, in a third step M3, the chain block is coupled to the subsea well control equipment or the subsea mount. In a fourth step M4, an end of the tethering member is coupled to the chain block. In a fifth step M5, a respective other end of the tethering member is coupled to the respective subsea well control equipment or the subsea mount. In step M6, the chain block is actuated to apply the tension to the tethering member. The tension is sufficient to restrict, preferably inhibit, movement of the tethering member under subsea loads.

A method N according to the present invention of tensioning a tethering member, where the tethering member is configured to tether the subsea well control equipment to the subsea mount is shown in Figure 19. In a first step N1 , a tensioning apparatus according to the present invention is provided. The tensioning apparatus comprises a chain block configured to apply a tension to the tethering member. In a second step N2, the chain block is coupled to the subsea well control equipment or the subsea mount. In third step N3, the chain block is coupled to one end of the tethering member, where a respective other end of the tethering member is coupled to the subsea mount. In the fourth step N4, the chain block is actuated to apply the tension to the tethering member.

A further method O according to the present invention of tensioning a tethering member tethering the subsea well control equipment to the subsea mount is shown in Figure 20. In a first step 01 , a tensioning apparatus according to the present invention is provided. The tensioning apparatus comprises a tensioning mechanism arranged on the subsea mount and configured to apply a first tension to the tethering member, and a chain block configured to apply a second tension to the tethering member. In an optional second step 02, the tensioning mechanism is coupled to one end of the tethering member. Optional step 02 may not be required, for example, when the tethering member is pre-mounted on the tensioning mechanism prior to arranging subsea. In third step 03, the chain block is coupled to the respective other end of the tethering member. In a fourth step 04, the chain block is coupled to the subsea well control equipment. The third step 03 and the fourth step 04 are interchangeable. In fifth step 05, the tensioning mechanism is actuated to apply the first tension to the tethering member. Applying the first tension removes slack from the tethering member. In a sixth step 06, after the fifth step 05, the chain block is actuated to apply the second tension. The second tension is greater than the first tension. The combined first tension and second tension is sufficient to restrict, preferably inhibit, movement of the tethering member under subsea loads.

A further method Q according to the present invention of tensioning a tethering member that is tethering the subsea well control equipment to the subsea mount is shown in Figure 21. In a first step Q1, a tensioning apparatus according to the present invention is provided. The tensioning apparatus comprises a first chain block configured to apply a first tension to the tethering member, and a second chain block configured to apply a second tension to the tethering member. In a second step Q2, the first chain block is coupled to the subsea well control equipment or the subsea mount and an end of the tethering member. In a third step Q3, the second chain block is coupled to the respective other of the subsea mount or the subsea well control equipment and the respective other end of the tethering member. In a fourth step Q4, the first chain block is actuated to apply the first tension to the tethering member. Applying the first tension removes slack from the tethering member. In a fifth step Q5, after the fourth step 04, the second chain block is actuated to apply the second tension. The second tension is greater than the first tension. The combined first tension and second tension is sufficient to restrict, preferably inhibit, movement of the tethering member under subsea loads.

It will be appreciated that the order of the coupling steps described in methods M, N, O and Q described above can be performed in any suitable order.

In general, the tethering member 36 may comprise or consist of any elongate flexible member suitable for subsea use and capable of withstanding the anticipated tensile loads without deforming or elongating. Non-limiting examples of suitable tethering members include chain(s), wire rope, and Dyneema® rope available from DSM Dyneema LLC of Stanley, North Carolina USA. In the particular embodiments shown, the tethering member comprises Dyneema® rope, which is suitable for subsea use and is sufficiently strong to withstand the anticipated tensions. In the context of the present invention, a seabed 30 may be understood to mean any subsea surface that allows for the arrangement of the subsea mount in spaced relationship to the subsea well control equipment to achieve a tethering effect as the tethering member extends from the subsea well control equipment to the subsea mount.

The tensioning apparatus of the present invention may apply a tension to the tethering member during one or more mode of operation of the subsea well assembly. For example, the tensioning apparatus may apply tension to the tethering member during the installation of the subsea well assembly (construction), during drilling (production), intervention, during deconstruction and/or abandonment of the subsea well assembly.

It should be appreciated that tethering apparatus and tethering members described above may be deployed and installed on an existing subsea mount or subsea well control equipment. Moreover, tensioning apparatus, tethering members and subsea mounts can be independently retrieved and reused at different locations, and different subsea well assemblies, as required.

It will also be appreciated by persons skilled in the art that the above embodiment has been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departing from the scope of the invention as defined by the appended claims.

Claims

1. A subsea tensioning system for tethering a subsea well control equipment and a subsea mount, the system comprising: a tethering member arrangeable to tether the subsea well control equipment to the subsea mount; and a subsea tensioning apparatus arrangeable to tension the tethering member, wherein the tensioning apparatus comprises a chain block, whereby the chain block is arrangeable to be coupled to the tethering member and the chain block is configured to apply a tension to the tethering member.

2. The subsea tensioning system according to claim 1, wherein the chain block comprises a load chain, wherein the chain block is configured to convey the load chain to apply the tension to the tethering member.

3. The subsea tensioning system according to claims 1 or 2, wherein the chain block comprises a first coupling and a second coupling, wherein the first coupling is a movable coupling mounted on the conveyable load chain and the second coupling is a fixed coupling mounted on a fixed portion of the chain block.

4. The subsea tensioning system according to claim 3, wherein: the chain block is arrangeable in a first arrangement where the first coupling is coupled to the tethering member and the second coupling is coupled to the subsea well control equipment; or the chain block is arrangeable in a second arrangement where the first coupling is coupled to the subsea well control equipment and the second coupling is coupled to the tethering member; or the chain block is arrangeable in a third arrangement where the first coupling is coupled the tethering member and the second coupling is coupled to the subsea mount; or the chain block is arrangeable in a fourth arrangement where the first coupling is coupled to the subsea mount and the second coupling is coupled to the tethering member.

5. The subsea tensioning system according to any preceding claim, where the chain block further comprises: a rotatable chain sheave configured to convey the load chain; a gear assembly configured to rotate the chain sheave; and an actuator configured to actuate the gear assembly in response to a driving input.

6. The subsea tensioning system according to claim 5, wherein the chain block further comprises a brake, wherein the brake is configured to inhibit back rotation of the chain sheave as the tension is being applied to the tethering member.

7. The subsea tensioning system according to any preceding claim, the tensioning apparatus further comprising a frame configured to rigidly support the chain block.

8. The subsea tensioning system according to any preceding claim, the tensioning apparatus further comprising a buoyancy aid to aid the buoyancy of the chain block and control the orientation of the chain block during use in subsea conditions.

9. The subsea tensioning system according to any preceding claim, wherein the chain block is a primary chain block, and the subsea tensioning apparatus further comprises: a secondary chain block; wherein the primary chain block and secondary chain block are arrangeable to be coupled to respective ends of the tethering member; wherein the secondary chain block is configured to apply a pre-tension prior to application of the tension to the tethering member by the primary chain block; and wherein the pre-tension is less that the tension.

10. The subsea tensioning system according to any of claims 1 to 8, wherein, when the chain block is coupled to the subsea well control equipment and the tethering member, the subsea tensioning apparatus further comprises: a tensioning mechanism arranged on the subsea mount; wherein the chain block and the tensioning member are arrangeable to be coupled to respective ends of the tethering member; wherein the tensioning member is configured to apply a pre-tension to the tethering member; wherein the tensioning mechanism is configured to apply the pre-tension prior to the application of the tension to the tethering member by the chain block; and wherein the pre-tension is less than the tension.

11. The subsea tensioning system according to claim 10, wherein the tensioning mechanism comprises a rotatable drum, wherein the rotatable drum is configured to reel-in the tethering member to apply the pre-tension to the tethering member.

12. The subsea tensioning system according to claim 11 , wherein the rotatable drum further comprises a drum lock configured to inhibit back rotation of the rotatable drum as the pre-tension is being applied.

13. The subsea tensioning system according to any preceding claim, the tensioning apparatus further comprising a tension monitoring system configured to monitor tension in the tethering member.

14. A subsea method of tethering a subsea well control equipment under tension, the method comprising: providing a subsea mount; providing a tethering member; providing a subsea tensioning apparatus, wherein the tensioning apparatus comprises a chain block configured to apply a tension to the tethering member; arranging the subsea mount on the seabed in spaced relationship to the subsea well control equipment; coupling the chain block to the subsea well control equipment or the subsea mount; coupling the chain block to an end of the tethering member; coupling the respective other end of the tethering member to the respective other of the subsea mount or the subsea well control equipment; wherein the respective other end of the tethering member is directly or indirectly coupled to the respective other of the subsea mount or the subsea well control equipment; and actuating the chain block to apply tension to the tethering member.

15. The method according to claim 14, wherein the subsea tensioning apparatus further comprises a secondary chain block configured to apply a pre-tension to the tethering member, and the method further comprises: providing the secondary chain block; coupling the secondary chain block to the respective other end of the subsea mount or the subsea well control equipment; actuating the secondary chain block to apply the pre-tension prior to actuating the chain block to apply the tension to the tethering member, wherein the pre-tension is less than the tension; and wherein coupling the respective other end of the tethering member comprises coupling the respective other end of the tethering member to the second chain block.

16. The method according to claim 14, wherein, when the chain block is coupled to the subsea well control equipment and the subsea tensioning apparatus further comprises a tensioning mechanism configured to apply a pre-tension to the tethering member, the method further comprises: providing the tensioning mechanism; mounting the tensioning mechanism on the subsea mount; actuating the tensioning mechanism to apply the pre-tension prior to actuating the chain block to apply the tension to the tethering member, wherein the pre-tension is less than the tension; and wherein coupling the respective other end of the tethering member comprises coupling the respective other end of the tethering member to the tensioning mechanism.

17. A subsea tensioning apparatus for tensioning a tethering member, wherein the tethering member is configured to tether a subsea well control equipment to a subsea mount arranged on a seabed, the subsea tensioning apparatus comprising: a first chain block configured to be coupled to the subsea well control equipment or the subsea mount, and coupled to an end of the tethering member, wherein the first chain block is configured to apply a first tension to the tethering member; and a second chain block configured to be coupled to the respective other of the subsea mount or the subsea well control equipment, and coupled to the respective other end of the tethering member, wherein the second chain block is configured to subsequently apply a second tension to the tethering member, and wherein the second tension is greater than the first tension.

18. A subsea tensioning apparatus for tensioning a tethering member, wherein the tethering member is configured to tether a subsea well control equipment to a subsea mount arranged on a seabed, the subsea tensioning apparatus comprising: a tensioning mechanism configured to be arranged on the subsea mount and coupled to an end of the tethering member, wherein the tensioning mechanism is configured to apply a first tension to the tethering member; and a chain block configured to be coupled to the subsea well control equipment and coupled to an other end of the tethering member, wherein the chain block is configured to subsequently apply a second tension to the tethering member, wherein the second tension is greater than the first tension.

19. The subsea tensioning apparatus according to claim 18, wherein the chain block comprises a load chain, and wherein the chain block is configured to convey the load chain to apply the second tension to the tethering member.

20. The subsea tensioning apparatus according to claim 18 or 19, wherein the chain block comprises a first coupling and a second coupling, wherein the first coupling is a movable coupling mounted on the conveyable load chain and the second coupling is a fixed coupling mounted on a fixed portion of the chain block.

21. The subsea tensioning apparatus according to claim 20, wherein: the chain block is arrangeable in a first arrangement where the first coupling is configured to engage the tethering member and the second coupling is configured to engage the subsea well control equipment; or the chain block is arrangeable in a second arrangement where the first coupling is configured to engage the subsea well control equipment and the second coupling is configured to engage the tethering member.

22. The subsea tensioning apparatus according to claims 19 to 22, wherein the chain block further comprises: a rotatable chain sheave configured to convey the load chain; a gear assembly configured to rotate the chain sheave; and an actuator configured to actuate the gear assembly.

23 The subsea tensioning apparatus according to claim 23, wherein the chain block further comprises a brake, wherein the brake is configured to inhibit back rotation of the chain sheave as the second tension is being applied to the tethering member.

24. The subsea tensioning apparatus according to claims 18 to 23, wherein the tensioning mechanism comprises a rotatable drum, wherein the rotatable drum is configured to reel-in the tethering member to apply the first tension to the tethering member.

25 The subsea tensioning apparatus according to claim 24, wherein the rotatable drum comprises a drum lock configured to inhibit back rotation of the rotatable drum as the first tension is being applied.

26. A subsea method of tensioning a tethering member, wherein the tethering member is tethering a subsea well control equipment to a subsea mount, the method comprising: providing a subsea tensioning apparatus, wherein the tensioning apparatus comprises a chain block configured to apply a tension to the tethering member; coupling the chain block to the subsea well control equipment or the subsea mount; coupling the chain block to one end of the tethering member; actuating the chain block to apply the tension to the tethering member.

27. The method according to claim 26, wherein the subsea tensioning apparatus further comprises a secondary chain block configure to apply a pre-tension to the tethering member, and the method further comprises: providing the secondary chain block; coupling the secondary chain block to the respective other of the subsea mount or the subsea well control equipment; coupling the secondary chain block to the respective other end of the tethering member; actuating the secondary chain block to apply the pre-tension prior to actuating the chain block to apply the tension to the tethering member, wherein the pre-tension is less than the tension.

28. The subsea method according to claim 26, wherein the tensioning apparatus further comprises a tensioning mechanism arranged on the subsea mount and configured to apply a pre-tension to the tethering member, and the method further comprises: providing the tensioning mechanism; mounting the tensioning mechanism on the subsea mount; coupling the tensioning mechanism to the respective other end of the tethering member; actuating the tensioning mechanism to apply the pre-tension to the tethering member prior to actuating the chain block to apply the tension to the tethering member, wherein the pre-tension is less than the tension.