WO2011150945A1

WO2011150945A1 - A flexible pipe system

Info

Publication number: WO2011150945A1
Application number: PCT/DK2011/050198
Authority: WO
Inventors: Tommy Pedersen; Claus Dencker Christensen
Original assignee: Nkt Flexibles I/S
Priority date: 2010-06-04
Filing date: 2011-06-06
Publication date: 2011-12-08
Also published as: AU2011260714B2; EP2576333A4; MY163286A; EP2576333A1; CN103052562B; AU2011260714A1; CN103052562A; BR112012030701B1; BR112012030701A2; EP2576333B1

Abstract

A flexible pipe system for transferring fluid between a seabed installation (7) and a sea surface installation (6). The flexible pipe system comprises a fixed subsea structure (2) with a support section (3) and a flexible transporting unit (1 ) with an axis along a length thereof, a sea surface end (4) and a seabed end (5) arranged with a horizontal distance and a vertical distance. The sea surface end (4) is connected to the sea surface installation (6) and the seabed end (5) is the touchdown point on the seabed of the flexible transporting unit (1 ) or the point along the flexible transporting unit (1) where it is connected to the seabed installation (7) which ever has the shorter horizontal distance to the sea surface end (4). The flexible transporting unit (1 ) is supported by the support section (3) of the fixed subsea structure (2) at a flexible pipe supporting area (1a) defining a flexible transporting unit sea surface length (1b) extending along the flexible transporting unit (1) from its sea surface end (4) to the flexible pipe supporting area (1a). At least a section of the flexible transporting unit (1) is movable along the axis with respect to the support section (3) of the fixed subsea structure (2) such that the flexible transporting unit sea surface length (1b) can very between a minimum and a maximum length.

Description

A FLEXIBLE PIPE SYSTEM

TECHNICAL FIELD

The present invention relates to a flexible pipe system for transferring fluid between a seabed installation and a sea surface installation. The flexible pipe system of the invention is for example useful for transferring hydrocarbon containing fluids from a seabed installation such as a well and to a sea surface installation such as a floating platform or a vessel.

BACKGROUND ART

Flexible pipe systems of the above type are well known in the art and are generally used for transporting fluids, electricity, electromagnetic waves and other media which are transferred to or from a sea surface installation, in particular a floating sea surface installation, such as a floating platform and a vessel.

Flexible pipes hanging from a sea surface installation to a seabed installation are subjected to a lot of forces and movements due to weather conditions, such as wind, water current and waves. In situations where the sea surface installation is a floating sea surface installation, the forces and movements to which such flexible pipes may be subjected can be even higher. In order to avoid damage of a flexible pipe extending between a sea surface installation and a seabed installation e.g. to avoid that the flexible pipe is subjected to a bending with a bending radius below a certain acceptable level for the flexible pipe (usually called the minimum bending radius for the flexible pipe), various flexible pipe systems have been suggested in the prior art.

A number of configurations for flexible pipe systems have for example been described in Recommended Practice for Flexible Pipe, API 17B, third edition, 2002, section 11.4.10, for example the configurations for lifting of oil, known under the name of "LAZY S" or "LAZY WAVE" and "STEEP S" or "STEEP WAVE," respectively. In these configurations the hoses extend in a catenary between the sea surface installation and an intermediate positive buoyancy element capable of imparting to the hose, over a portion of its length, a curved configuration of concavity turned toward the seabed. In order to provide the flexible pipe with sufficient length for allowing the weathervaning vessel to move due to influence by the weather, the prior art shallow water systems usually are structured to arrange the flexible pipes to have a wave configuration, such as an ^XS' configuration or similar wave configuration e.g. as described in US 4,793,737. In such wave configuration the flexible pipes are supported by a supporting unit, such as a seabed structure, for example a mid water arch or a mid water jacket, at a distance from the seabed.

In US 4,906,137 is described a flexible pipe system comprising a lower portion of a subsea oil delivery hose which is held taut between one or more buoyancy devices installed on the hose and a stationary point on the subsea floor.

In order to provide a certain extra length of the flexible pipe to allow movements while simultaneously avoiding that the flexible pipe is subjected to undesired contact with the seabed and/or for example a vessel, a system is described in US 6,857,822 comprising a tensioning mechanism that prevents excess slack. The system of US 6,857,822 comprises a base located on a seabed comprising a rigid arm which can pivot in first and second, opposite directions away from an equilibrium position. The rigid arm comprises a tension-applying element. The flexible pipe arranged to transport a fluid from a seabed installation to a vessel is clamped to the arm such that the arm will pivot in response to tension in the riser for example caused by a movement of the vessel.

The above system has in practice shown to have only limited application in particular because it is difficult to adjust the tensioning mechanism to a level where it appl ies a sufficient tension, while simultaneously avoiding uncontrolled swinging of the pivoting arm and possibly over bending of the flexible pipe.

The object of the invention is to provide a flexible pipe system comprising a flexible transporting unit for transferring fluid between a seabed installation where the flexible transporting unit can accommodate to forces applied to it due to weather conditions as well as due to movement of the sea surface installation and/or the seabed installation, while simultaneously providing a high degree of stability and control over movements of the flexible transporting unit such that the risk of damaging the flexible transporting unit is reduced.

An additional object is to provide a flexible pipe system which is simple to produce and install.

DISCLOSURE OF INVENTION

The object of the invention is solved by the flexible pipe system as defined in the claims. Additional beneficial solutions which may have additional advantages are defined in the sub claims and are described in the following.

The flexible pipe system of the invention is specifically adapted for transferring fluid between a seabed installation and a sea surface installation. The flexible pipe system comprises a fixed subsea structure with a support section and a flexible transporting unit with an axis along its length, a sea surface end and a seabed end arranged with a horizontal distance and a vertical distance. The sea surface end is connected to the sea surface installation. The seabed end is the touchdown point of the flexible transporting unit or the point along the flexible transporting unit where it is connected to the seabed installation which ever has the shorter horizontal distance to the sea surface end. The touchdown point of the flexible transporting unit is the point along the touchdown point of the flexible transporting unit where it meets the seabed. The flexible transporting unit is supported by the support section of the fixed subsea structure at a flexible pipe supporting area defining a flexible transporting unit sea surface length, which is the length of the flexible transporting unit which extends from its sea surface end to the flexible pipe supporting area. At least a section of the flexible transporting unit is movably along its axis with respect to the support section of the fixed subsea structure such that the flexible transporting unit sea surface length can vary between a minimum flexible transporting unit sea surface length and a maximum flexible transporting unit sea surface length.

As described below the flexible pipe system may comprise two or more flexible transporting units, but is in the following described mainly with one flexible transporting unit.

The flexible pipe system of the invention thereby provides a wave configuration of the flexible transporting unit which in the following is referred to as a "sliding-S", which sliding-S configuration provides the flexible transporting unit with a surplus of length while simultaneously ensuring that the risk of damaging the flexible pipe system due to this surplus of length is reduced compared to prior art systems. Simultaneously any risk of the flexible transporting unit to over-bend or touch down and drag across the seabed and/or to be unduly stretched which may result in damaging of the flexible transporting unit, has been reduced to a minimum.

The sea surface installation may in practice be any type of installation arranged at or near the sea surface, where the term "near" is used to mean closer to the sea surface than to the seabed, preferably up to about 20 m below the sea surface. Examples of sea surface installations include platforms and vessels. In one embodiment the sea surface installation is a floating installation, preferably selected from a vessel and a floating platform. In one embodiment the sea surface installation is a moored floating installation.

The flexible pipe system is most beneficial in situations where the sea surface installation is a vessel, such as a weathervaning vessel or a spread-moored vessel.

Weathervaning vessels are often used in shallow waters. An example of a shallow water system comprising a weathervaning vessel which may in one embodiment be used in combination with the present invention is described in co-pending application DK PA 2009 01333. In another embodiment a hang-off system as described in co-pending application DK PA 2009 01376 comprising a weathervaning vessel is used in combination with the present invention. However, it should be observed that any weathervaning vessel as well as any spread-moored vessel in practice can be applied as a sea surface installation in the present invention.

A 'floating weathervaning vessel' or merely a 'weathervaning vessel' is a vessel from which one or more flexible transporting units are leading to a subsea structure. Such weathervaning vessels as well as spread-moored vessels are known to the skilled person and usually comprise an external turret system or an internal turret system. The internal turret system leads the flexible transporting units through the hull bottom whereas the external turret system leads the flexible transporting unit from a topside hang-off structure extending beyond the hull and the rail of the vessel. A weathervaning vessel is usually moored to the seabed but may also in certain situations be moored by line to one or more fixed structures, and is usually moored while still having a large freedom to move to adapt to forces applied to the weathervaning vessel e.g. by wind, water current and waves.

In shallow water the weather conditions have a vast influence on movements of a vessel and even though the weathervaning vessel is moored, the movements of the weathervaning vessel may become quite considerable. The term 'seabed' is generally used to denote the subsea floor and the seabed installation is an installation in or in contact with the seabed.

The seabed installation may for example be a fixed installation, such as a well, an anchoring installation and/or a secondary fixed subsea structure. The skilled person will understand that the type of seabed installation is not important for the present invention in general and accordingly any seabed installation can be applied in the present invention.

In most situations the seabed end is the touchdown point of the flexible transporting unit. However, in certain situations the seabed end will be the point along the flexible transporting unit where it is connected to the seabed installation, namely in the situations where the flexible transporting unit is connected to the seabed installation without having any touch down point between the subsea structure and the seabed installation.

The flexible pipe system may comprise one or more flexible transporting units. The flexible transporting unit(s) may be any kind of flexible transporting units which are used in offshore applications. In one embodiment the one or more flexible transporting units comprise at least one unbonded flexible offshore pipe. In one embodiment the one or more flexible transporting units comprise at least one bonded flexible offshore pipe. The flexible transporting unit may for example be a flexible riser or an umbilical. Preferably the shallow water system comprises a plurality of flexible transporting units e.g. comprising a plurality of rises, such as a plurality of risers and optionally at least one umbilical. Flexible transporting units - sometimes also called jumpers - such as risers and umbilicals are well known in the art. Risers are usually applied for transportation of petrochemical products from the seabed to a sea surface installation such as a weathervaning vessel. Umbilicals are often used for transporting fluids, electricity, signals and other to and/or from installations at or beyond the seabed. In one embodiment the flexible transporting unit comprises at least one flexible pipe. In one embodiment the flexible transporting unit comprises at least one umbilical. In one embodiment the flexible transporting unit comprises at least one flexible riser pipe, such as a jumper. In one embodiment the flexible transporting unit comprises two or more pipes connected to each other and in fluidic connection with each other. It is well known to the skilled person how to connect pipes to each other to obtain a fluidic connection. The two or more connected pipes may be of similar types or of different types, provided that at least one of the pipes is a flexible pipe. In one embodiment the flexible transporting unit comprises a rigid pipe section, which rigid pipe section is substantially rigid, e.g. provided by a substantially rigid cover layer applied to a section of a flexible pipe of the flexible transporting unit.

In one embodiment the flexible transporting unit comprises a rigid pipe and a flexible pipe connected to each other and in fluidic connection with each other.

In one embodiment the flexible pipe system comprises two or more flexible transporting units, preferably arranged in substantially side by side relation to each other. The flexible transporting units may be of similar types or of different types, of similar size or of different sizes. The sea surface end is the position of the flexible transporting unit where it is connected to the sea surface installation. The connection may provide a fluidic communication or merely be a fixation depending on the type and arrangement of sea surface installation. In one embodiment the sea surface end is fluidically connected to the sea surface installation. In one embodiment the seabed end is the touchdown point of the flexible transporting unit. In most situations where the seabed end is the touchdown point of the flexible transporting unit the horizontal distance between the sea surface end and the seabed end can vary from a minimum horizontal distance to a maximum horizontal distance. Due to the structure provided by the present invention, the risk of dragging the flexible transport unit over the seabed at the touchdown point is however highly reduced. The difference between the minimum horizontal distance and the maximum horizontal distance is preferably about 50 m or less, such as about 40 m or less, such as about 30 m or less, such as about 25 m or less, such as about 20 m or less, such as about 15 m or less, such as about 10 m or less. The area along the pipe at and near the touchdown point may be covered with an extra external reinforcement such as it is generally known in the art. In one embodiment the vertical distance between the sea surface end and the seabed end is be a substantially stationary distance. In this situation this substantially stationary distance is also referred to as the vertical minimum distance.

In one embodiment the vertical distance varies from a minimum vertical distance to a maximum vertical distance, where the difference between the minimum vertical distance and the maximum vertical distance preferably is about 10 m or less, such as about 5 m or less.

In one embodiment the seabed end is the point along the flexible transporting unit where it is connected to the seabed installation. The connection may provide a fluidic communication or merely be a fixation depending on the type and arrangement of the seabed installation. In one embodiment the seabed end is fluidically connected to the seabed installation.

In one embodiment the horizontal distance between the sea surface end and the seabed end is a substantially stationary distance. In this situation it is also referred to as the horizontal minimum distance. In one embodiment the vertical distance between the sea surface end and the seabed end is a substantially stationary distance and in this situation it is also referred to as the vertical minimum distance. The subsea structure is preferably a substantially rigid structure fixed to the seabed in a relatively rigid fixation. The subsea structure may be fixed to the seabed by any method, e.g. by being held to the seabed with a heavy anchoring element (a dead weight), by being partly embedded in the seabed or by being provided with one or more piles which are driven into the seabed e.g. in a substantially vertical direction.

In one embodiment the subsea structure is fixed to the seabed in a position which is substantially below the sea surface installation in at least one position of the sea surface installation. In this embodiment the minimum horizontal distance between the sea surface end and the seabed end is relatively short, such as about 30 m or less, such as about 20 m or less, such as about 15 m or less, such as about 10 m or less. If the minimum horizontal distance between the sea surface end and the seabed end is too short, the risk of over bending of the flexible transporting unit may be increased. In such situations the flexible transporting unit may be arranged in a structure to extend in a detour to and from a minimum horizontal distance to the sea surface installation and the seabed installation which is longer than the minimum horizontal distance between the sea surface end and the seabed end. In one embodiment the sea surface installation is a weathervaning vessel with a hull, the subsea structure is arranged at the seabed such that it at least in one position of the weathervaning vessel will be under the hull of the weathervaning vessel.

In one embodiment the subsea structure is fixed to the seabed in a position having a minimum distance to a flexible pipe vertical plane through the sea surface end and the seabed end of the flexible transporting unit which is up to about 100 %, such as up to about 75 %, such as 50 %, such as up to about 40 %, such as up to about 30 %, such as up to about 20 %, such as up to about 10 %, such as up to about 5 % of the minimum horizontal distance between the sea surface end and the seabed end of the flexible transporting unit, preferably the subsea structure is fixed to the seabed in a position substantially lying in the flexible pipe vertical plane. The minimum distance from the subsea structure to the flexible pipe vertical plane is determined substantially perpendicular to the flexible pipe vertical plane. The flexible pipe vertical plane is an imaginary plane used merely for describing the positions of elements of the flexible pipe system in relation to each other.

When the subsea structure is fixed to the seabed in a position substantially lying in the flexible pipe vertical plane, the flexible transporting unit can be applied with a substantially straight minimum horizontal direction between the sea surface end and the seabed end.

In one embodiment the subsea structure is a pile structure, comprising one or more piles, such as a mono pile. Monopiles are preferred because they are simple and cost effective to install. Multipile structures may however be stronger and more stable, and may in certain applications be the preferred choice.

In one embodiment the subsea structure is a dead weight structure, comprising a deadweight which is substantially large to fix the subsea structure to the seabed.

The support section of the fixed subsea structure is the part of the subsea structure adapted to support the flexible transporting unit in a sliding fashion. In most situations the support section is the uppermost part of the subsea structure since such an arrangement provides the simplest installation of the flexible transporting unit onto the support section. However, it should be observed that the support section need not be the uppermost part of the subsea structure and that parts of the subsea structure may protrude vertically above the support section. In one embodiment the support section of the subsea structure is arranged at a distance D-sea surface from the sea surface and at a distance D-seabed from the seabed, preferably D-sea surface is substantially larger than D- seabed. All distances to the sea surface are determined at still water at ebb tide unless otherwise specified.

The minimum horizontal distance between the sea surface end and the seabed end of the flexible transporting unit is determined as the minimal distance possible without damaging the flexible pipe system and with intact mooring of the sea surface installation, if any.

The minimum vertical distance between the sea surface end and the seabed end of the flexible transporting unit is determined at still water at flood tide.

In one embodiment D-seabed is at least about 1 m, such as at least about 2 m, such as at least about 3 m, such as at least about 4 m, such as at least about 5 m, such as at least about 6 m, such as at least about 7 m, such as at least about 8 m, such as at least 10 m, such as at least 25 m, such as at least 50 m, such as at least 50 m, such as at least 100 m. In one embodiment D-seabed is at least about 0.1 %, such as at least about 1 %, such as at least about 2 %, such as at least about 3 %, such as at least about 4 %, such as at least about 5 %, such as at least about 10 % m, such as at least about 15 %, such as at least about 20 %, such as at least 30 %, such as at least 40 %, such as at least 50 %, such as at least 60 %, such as at least 70 %, such as up to about 100 %, such as up to about 90 %, such as up to about 80 % of the distance between the sea surface and the seabed.

It should be understood that the flexible pipe system of the present invention is highly beneficial to apply at shallow waters, such as at water depths up to about 200 m, such as preferably with a depth of up to about 100 m, such as with a depth of up to about 80 m, such as with a depth of up to about 80 m, such as with a depth of up to about 70 m, such as with a depth of up to about 60 m, such as with a depth of up to about 55 m, such as with a depth of up to about 50 m, such as with a depth of up to about 45 m, such as with a depth of up to about 40 m, such as with a depth of up to about 35 m, such as with a depth of up to about 30 m, such as with a depth of up to about 25 m, such as with a depth of up to about 20 m, such as with a depth of up to about 15 m, such as with a depth of up to about 10 m. However, due to the stability and simplicity of the flexible pipe system the flexible pipe system of the invention may be applied at deeper waters, e.g. up to a depth of 3000 m or more, as it will be clear from the following. The support section of the subsea structure should preferably be shaped to provide a good sliding surface for allowing the flexible transporting unit to slide relative to it. In one embodiment the support section of the subsea structure comprises a curved surface, preferably comprising an arch shape for supporting the flexible transporting unit.

In one embodiment the subsea structure has a width and a length perpendicular to the width. The width may preferably be substantially parallel to the flexible pipe vertical plane defined above. The support section may preferably have an arched shape which is substantially uniform along at least a part of its length. The length of the support section should preferably be at least as long as the outer diameter of at least a sliding section of the flexible transporting unit where the sliding section is the length section of the flexible transporting unit which comprises a flexible pipe supporting area as the flexible transporting unit slides relative to the support section from a position where the flexible transporting unit sea surface length is at a maximum to a position where it is at a minimum.

In one embodiment the length of the support section is sufficiently long to support two or more flexible transporting units in a side by side relation. In order to prevent the two or more flexible transporting units from sliding against each other a separating fence may be arranged between the flexible transporting units.

In one embodiment the support section of the subsea structure has a gutter structure, preferably comprising a channel with a channel bottom support surface and a pair of side fences, the flexible transporting unit is supported in the channel. If the flexible pipe system comprises a plurality of flexible transporting unit supported by the same support section the gutter structure may be a multi channel gutter structure comprising a channel for each flexible transporting unit.

The side fences may have equal or different sizes and shape. In one embodiment the fences have, independently of each other, a height - preferably a minimum height - from the channel bottom support surface which is at least about 10 %, such as at least about 20 %, such as at least about 30 %, such as at least about 40 %, such as at least about 50 %, such as at least about 60 %, such as at least about 75 %, such as at least about 100%, such as at least about 500 %, such up to about 1000 % of the outer diameter of the flexible transporting unit at its flexible pipe supporting area. The length of the channel is not important provided that it is sufficiently long to provide a stable support. In one embodiment the channel has a length, and the channel bottom support surface is arch curved along the channel length and the distance of the fences is substantially constant along the channel length.

In one embodiment the channel has a length, the channel bottom support surface is arch curved along the channel length, and the distance of the fences is substantially larger at a first and a third channel sections closer to one of the sea surface end and the seabed end than along a second channel section between the first and third channel sections. Thereby the fences allow a transverse movement of the flexible transporting unit.

In one embodiment the support section of the subsea structure comprises a coating of a low friction material, having a sufficiently low friction to provide that the friction between the support section and the supporting area of the flexible transporting unit is lower than it would have been without the low friction coating. The low friction material may be any low friction material with a sufficient strength and a sufficient durability in sea water. In one embodiment the low friction material is selected from polymers, e.g. a flour polymer such as PTFE (poly tetra fluro ethylene), ceramic coatings e.g. comprising chrome oxide and/or alumina oxide, tungsten carbide coating, metal coatings e.g. comprising zinc, aluminium, stainless steel and/or nickel. The skilled person will understand that other low friction coatings may be applied as well. In order to reduce friction between the support section and the flexible transporting unit, the support section may comprise a mechanical anti friction structure optionally comprising elements which will move or rotate as a result of the relative movement between the support section and the flexible transporting unit.

In one embodiment the support section of the subsea structure comprises one or more rotatable bars for reducing friction between the flexible pipe supporting area of the flexible transporting unit and the support section of the subsea structure.

In one embodiment the support section of the subsea structure comprises a rotatable wheel or a rotatable muff for reducing friction between the flexible pipe supporting area of the flexible transporting unit and the support section of the subsea structure.

As mentioned above the flexible pipe system may be applied both at shallow waters as well as at deep waters. In one embodiment the minimum horizontal distance between the sea surface end and the seabed end of the flexible transporting unit is at least about 2 m, such as at least about 4 m, such as at least about 6 m, such as at least about 8 m, such as at least about 10 m, such as at least about 15 m, such as at least about 20 m, such as at least about 30 m, such as at least about 40 m, such as at least about 50 m, such as at least about 75 m, such as at least about 100 m, such as at least about 200 m, such as at least about 500 m, such as up to about 3000 m, such as up to about 2000 m, such as up to about 1000 m.

In one embodiment the minimum vertical distance between the sea surface end and the seabed end of the flexible transporting unit is about 3000 m or less, such as about 2000 m or less, such as 1000 m or less, such as about 500 m or less, such as about 400 m or less, such as about 300 m or less, such as about 200 m or less, such as about 100 m or less, such as about 60 m or less, such as about 50 m or less, such as about 40 m or less, such as about 30 m or less, such as about 20 m or less, such as about 10 m or less.

The sliding section of the flexible transporting unit may in principle have any length provided that sufficient arrangement is provided to prevent the flexible transporting unit from coming into undesired contact with the seabed. The sliding section of the flexible transporting unit is as defined above. As explained later on one or more buoyancy elements may be arranged along the flexible transporting unit sea surface length to provide a w-waved configuration of the flexible transporting unit whereby the risk of undesired contact between the flexible transporting unit and the seabed can be reduced or eliminated.

The sliding section of the flexible transporting unit is at least about 1 m, such as at least about 2 m, such as at least about 3 m, such as at least about 4 m, such as at least about 5 m, such as at least about 6 m, such as at least about 7 m, such as at least about 8 m, such as at least about 9 m, such as at least about 10 m, such as at least about 11 m, such as at least about 12 m, such as at least about 13 m, such as at least about 14 m, such as at least about 15 m, such as up to about 50 m, such as up to about 40 m, such as up to about 35 m, such as up to about 30 m, such as up to about 25 m, such as up to about 20 m.

In one embodiment the maximum flexible transporting unit sea surface length is up to about twice the sum of the minimum horizontal and the minimum vertical distance between the sea surface end and the seabed end of the flexible transporting unit, preferably from about the sum of the minimum horizontal distance plus 10% of the minimum vertical distance to about the minimum horizontal distance plus 300% of the minimum vertical distance, such as from about the sum of the minimum horizontal distance plus 20% of the minimum vertical distance to about the minimum horizontal distance plus 200% of the minimum vertical distance, such as from about the sum of the minimum horizontal distance plus 50% of the minimum vertical distance to about the minimum horizontal distance plus 150% of the minimum vertical distance, such as from about the sum of the minimum horizontal distance plus 75% of the minimum vertical distance to about the minimum horizontal distance plus 100% of the minimum vertical distance. In one embodiment the minimum flexible transporting unit sea surface length is up to about X m shorter than the maximum flexible transporting unit sea surface length, wherein X is at least about 0.5 m, such as at least about 1 m, such as at least about 2 m, such as at least about 3 m, such as at least about 4 m, such as at least about 5 m, such as at least about 6 m, such as at least about 8 m, such as at least about 10 m, such as at least about 15 m, such as at least about 20 m, such as at least about 25 m, such as at least about 30 m, such as at least about 40 m, such as at least about 50 m, such as at least about 60 m, such as up to about 100 m. Thereby it is provided that a certain pulling or pushing of the unit is allowed without any damages or stretching forces being applied.

In one embodiment the minimum flexible transporting unit sea surface length is at least about the straight line distance between the sea surface end and the seabed end of the flexible transporting unit, such as at least about the sum of the minimum horizontal and the minimum vertical distance between the sea surface end and the seabed end of the flexible transporting unit, such as from about the sum of the minimum horizontal distance plus 50% of the minimum vertical distance to about the minimum horizontal distance plus 400% of the minimum vertical distance, such as from about the sum of the minimum horizontal distance plus 125% of the minimum vertical distance to about the minimum horizontal distance plus 200% of the minimum vertical distance.

As it should be clear from the above the flexible transporting unit is not clamped to the support section of the fixed subsea structure in such a way that it cannot slide. Preferably the flexible transporting unit is not clamped to the support section of the fixed subsea structure. According to an embodiment of the invention the flexible transporting unit can slide relative to the support section of the fixed subsea structure to thereby provide change of the flexible transporting unit sea surface length so that the flexible transporting unit can accommodate to variations in forces applied to it as well as to optional movements of the sea surface installation.

In one embodiment the flexible transporting unit, upon an increase in tension in the flexible transporting unit from the sea surface installation beyond a maximum acceptable tension point, is arranged to slide in relation to the support section of the fixed subsea structure to lengthen the flexible transporting unit sea surface length to a position when there is substantially no tension beyond the maximum acceptable tension point in flexible transporting unit from the sea surface installation.

The maximum acceptable tension point is preferably a tension point which is sufficiently low to ensure that neither the flexible transporting unit, nor the subsea structure is damaged due to such tension.

The flexible pipe system may comprise one or more ballasts attached directly or indirectly to the flexible transporting unit at the flexible transporting unit sea surface length and/or at the flexible transporting unit subsea length to increase control of the sliding of the flexible transporting unit relative to the support section.

The skilled person will be able to calculate useful positions and weight of such ballast based on the teaching herein including the examples below.

In one embodiment the flexible transporting unit comprises one or more ballasts to provide a pulling force to slide the flexible transporting unit relative to the support section of the fixed subsea structure to shorten the flexible transporting unit sea surface length to a position when there is substantially no tension beyond the maximum acceptable tension point in the flexible transporting unit from the sea surface installation. The resulting force of the one or more ballast(s) may preferably be arranged and selected such that they provide substantially no resulting pulling force to the flexible transporting unit when the flexible transporting unit has reached its minimum flexible transporting unit sea surface length position.

In order to control the length of the sliding section of the flexible transporting unit it is in one embodiment desired to provide the flexible transporting unit and/or the subsea structure with sliding stop e.g. in the form of physical blocking barrier.

In one embodiment the flexible transporting unit comprises a sliding stop arranged to stop the flexible transporting unit from sliding beyond the slide stop, the slide stop preferably being provided by a physical barrier.

The sliding stop may for example be provided by a clamp attached to the flexible transporting unit and arranged to slide in a clamp house fixed in relation to the subsea structure, the clamp house preferably has a sliding length of at least about 1 m, such as at least about 2 m, such as at least about 3 m, such as at least about 4 m, such as at least about 5 m, such as at least about 6 m, such as at least about 8 m, such as at least about 10 m, such as at least about 15 m, such as at least about 20 m, such as at least about 25 m, such as at least about 30 m, such as at least about 40 m, such as up to about 50 m. In one embodiment the subsea installation is a secondary fixed subsea structure, the seabed end is clamped to the secondary fixed subsea structure, and is in fluidic communication with a flexible pipe arranged as an extension of the flexible transporting unit. The secondary fixed subsea structure preferably has a height from seabed which is at least about 0.1 %, such as at least about 1 %, such as at least about 2 %, such as at least about 3 %, such as at least about 4 %, such as at least about 5 %, such as at least about 10 % m, such as at least about 15 %, such as at least about 20 %, such as at least 30 %, such as at least 40 %, such as at least 50 %, such as at least 60 %, such as at least 70 %, such as up to about 100 %, such as up to about 90 %, such as up to about 80 % of the distance between the sea surface and the seabed. The secondary fixed subsea structure may have a similar or different height than the subsea structure. In most situations it is desired that the secondary fixed subsea structure has a height which is at least 50 % of the height of the support section position of the subsea structure.

The secondary fixed subsea structure may preferably have a height and be arranged such that the section of the flexible transporting unit between the subsea structure and the secondary fixed subsea structure is substantially longer than the distance between the subsea structure and the secondary fixed subsea structure when the flexible transporting unit sea surface length has its maximum flexible transporting unit sea surface length, and is sufficiently short not to have a touchdown point between the subsea structure and the secondary fixed subsea structure and thereby it will not lie on the seabed when the flexible transporting unit sea surface length has its minimum flexible transporting unit sea surface length.

As indicated above the flexible pipe system may comprise one or more buoyancy elements arranged to support the flexible transporting unit.

Such buoyancy elements are well known in the art, and the skilled person will be able to select a suitable size and shape of a buoyancy element in dependence of which type and weight of pipe it should be adapted to support.

In one embodiment the flexible pipe system comprises at least one buoyancy element arranged to support the flexible transporting unit at its flexible transporting unit sea surface length. As indicated above such buoyancy elements may be used to decrease or eliminate any risk of the flexible transporting unit coming into damaging contact with the seabed.

In one embodiment the flexible transporting unit extends in a catenary configuration, preferably selected from a last-s lazy-s configuration and a steep-s configuration. According to an embodiment of the invention a flexible pipe system has been provided wherein the flexible transporting unit is arranged to slide relative to the support section of the subsea structure such that the flexible transporting unit sea surface length of the flexible transporting unit at any time is sufficiently long to avoid damage due to pull from the weathervaning vessel and simultaneously is sufficiently short to not come into contact with the seabed.

According to an embodiment of the invention a flexible pipe system has been provided wherein the flexible transporting unit is arranged to slide relative to the support section of the subsea structure as a function of the minimum horizontal and minimum vertical distance between the sea surface end and the seabed end of the flexible transporting unit.

One or more parts of the flexible pipe system may be coated with an anti fouling coating to prevent or reduce marine growth; such anti fouling coating optionally comprises an epoxy or an anti fouling paint. Such anti fouling coatings are well known.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. The invention is defined by the features of the independent claim(s). Preferred embodiments are defined in the dependent claims. Any reference numerals in the claims are intended to be non-limiting for their scope. BRIEF DESCRIPTION OF DRAWINGS

The invention will be explained more fully below in connection with a preferred embodiment and with reference to the drawings in which:

FIG. 1 is a schematically drawing of a flexible pipe system of the invention. FIG. la is a drawing of the same flexible pipe system as in FIG. 1, but where the minimum horizontal and minimum vertical distances have been shown.

FIG. 2 is a schematically drawing of a flexible pipe system of the invention which is a variation of the flexible pipe system shown in FIG. 1.

FIG. 3 is a schematically drawing of a flexible pipe system of the invention which is another variation of the flexible pipe system shown in FIG. 1.

FIG. 4 is a side view of a sectional cut through another flexible pipe system.

FIGs. 5 and 6 show a part of the flexible pipe system of FIG. 4 in two different positions.

FIG. 7 shows a closer view of a section of FIG. 6. The figures are schematic and may be simplified for clarity. Throughout, the same reference numerals are used for identical or corresponding parts.

The flexible pipe system shown in FIG. 1 and FIG la comprises a fixed subsea structure 2 with a support section 3 and a flexible transporting unit 1 with a sea surface end 4 and a seabed end 5 arranged with a horizontal distance HD and a vertical distance VD. The sea surface end 4 is connected to a sea surface installation 6, here a hang-off turret system 6a of a vessel which preferably is a spread-moored vessel or a weathervaning vessel. The seabed end 5 is the touchdown point of the flexible transporting unit 1, i.e. the point along the flexible transporting unit 1 that touches down to the seabed. Even though this touchdown point 5 is the seabed end of the flexible transporting unit 1, the flexible transporting units 1 exceed beyond the seabed end 5 in a flexible transport section 5a which is connected to a seabed installation 7, e.g. a well installation. The flexible transporting unit 1 is supported by the support section 3 of the fixed subsea structure 2 at a flexible pipe supporting area la, which defines a flexible transporting unit sea surface length lb extending along the flexible transporting unit from its sea surface end 4 to the flexible pipe supporting area la. At least a section of the flexible transporting unit is movable along its axis with respect to the support section 3 of the fixed subsea structure 2 such that the flexible transporting unit sea surface length lb can vary between a minimum flexible transporting unit sea surface length and a maximum flexible transporting unit sea surface length. Upon movement of the sea surface installation 6 the sea surface installation 6 may provide an increase in tension in the flexible transporting unit 1 beyond a maximum acceptable tensioning point, which results in that the subsea section lc of the flexible transporting unit between the seabed end 5 and the flexible pipe supporting area la is drawn partly upon the support section 3 to a position where there is substantially no tension in the flexible transporting unit 1 from the sea surface installation 6 beyond the maximum acceptable tensioning point, such that the length of the subsea section lc is decreasing, whereas the length of the flexible transporting unit sea surface length lb is increasing. The line 8 indicates the sea surface line. In this embodiment of FIG. 1 the support section 3 has a curved shape provided by having a shape with a substantially circular cross-section. For providing a low friction surface the support section 3 may for example comprises one or more rotatable bars arranged in the part of it where it is in contact with the flexible pipe supporting area la of the flexible transporting unit 1. In another similar embodiment the support section 3 of the subsea structure 2 may be in the form of a rotatable wheel or it may be provided with a rotatable muff for reducing friction between the flexible pipe supporting area la of the flexible transporting unit 1 and the support section 3 of the subsea structure.

The flexible pipe system shown in FIG. 2 comprises a fixed subsea structure 12 with a support section 13 and a flexible transporting unit 11 with a sea surface end 14 connected to a sea surface installation 16 and a seabed end 15, which is the touchdown point of the flexible transporting unit 11, i.e. the point along the flexible transporting unit 11 that touch down to the seabed. The flexible transporting units 11 exceed slightly beyond the seabed end 15 in a flexible transport section which is connected to a seabed installation 17. The flexible transporting unit 11 has flexible transporting unit sea surface length lib and a subsea section 11c. The flexible transporting unit 11 comprises a sliding stop 19, for example provided by a clamp or similar mounted to the flexible transporting unit 11 to ensure that the flexible transporting unit 11 does not slide over the support section 13 beyond a desired point of the pipe. As indicated with the double arrow SL the flexible transporting unit 11 can slide over the support section 13 with a length SL, so that the flexible transporting unit sea surface length lib can vary between a minimum flexible transporting unit sea surface length and a maximum flexible transporting unit sea surface length. When the flexible transporting unit 11 sea surface length lib, the touchdown point 15 of the flexible transporting unit 11 is closer to the sea surface end 14 of the flexible support unit 11 than in any other position and the horizontal distance between the sea surface end 14 and the seabed end 15 of the flexible transporting 11 is the minimum horizontal distance between the sea surface end 14 and the seabed end 15 of the flexible transporting unit 11.

The flexible pipe system shown in FIG. 3 comprises a fixed subsea structure 22 with a support section 23 and a flexible transporting unit 21 with a sea surface end 24 connected to a sea surface installation 26 and a seabed end 25 connected to a seabed installation 27 which here is a secondary fixed subsea structure. The seabed end 25 is clamped to the secondary fixed subsea structure 27, and is in fluidic communication with a flexible pipe 2 Id which may be an extension of the flexible transporting unit 21. In the shown embodiment, the secondary fixed subsea structure 27 has a height which is substantially similar to the height of the subsea structure 22.

The flexible transporting unit 21 has flexible transporting unit sea surface length 21b and a subsea section 21c. The subsea section 21c is provided with ballast 29 to ensure that the sliding of the flexible transporting unit 21 does not result in that the flexible transporting unit sea surface length 21b becomes too long. The flexible transporting unit 21 can slide over the support section 23, so that the flexible transporting unit sea surface length 21b can vary between a minimum flexible transporting unit sea surface length and a maximum flexible transporting unit sea surface length. The secondary fixed subsea structure 27 ensure that the subsea section 21c can have sufficient length for the sliding of the flexible transporting unit 21 to occur without undesired tension is provided in the subsea section 21c. Simultaneously the secondary fixed subsea structure 27 ensures that the subsea section 21c does not come into contact with the seabed. The line 28 indicates the sea surface line.

The flexible pipe system shown in FIGs. 4-7 comprises a fixed subsea structure 32 with a support section 33 and a flexible transporting unit 31 with a sea surface end 34 connected to a sea surface installation 36 and a seabed end 35, which is the touchdown point of the flexible transporting unit 31.. The flexible transporting unit 31exceed beyond the seabed end 35 in a flexible transport section 39 which is connected to a not shown seabed installation. The sea surface installation 36 is here a hang-off turret system of a vessel which preferably is a spread-moored vessel or a weathervaning vessel. The flexible transporting unit 31 has flexible transporting unit sea surface length 31b and a subsea section 31c. The subsea section 31c is provided with ballast 39a, in the form of weight modules to control that the sliding of the flexible transporting unit 31 keeps the flexible transporting unit sea surface length 31b on a desired level.

The subsea section 31c of the flexible transporting unit 31 is provided with a sliding stop provided by a clamp 41 attached to the subsea section 31c of the flexible transporting unit and arranged to slide in a clamp house 42, 43, 44 provided by a lower clamp stopper 42, an upper clamp stopper 43 and a rigid connecting member 44 providing the distance where the clamp 41 can travel from the lower clamp stopper 42 to the upper clamp stopper 43. The clamp house 42, 43, 44 is connected to the subsea structure 32 so that it extend from the support section 33 to a secondary support element 45 connected to the subsea structure 32 via a rigid sidebar 46. Both the support section 33 and the support element 45 are provided with a not shown gutter structure for providing a good support of the flexible transporting unit 31.

In FIG. 5 the clamp 41 is in its low position and the flexible transporting unit 31 is in a position such that the flexible transporting unit sea surface length 31b has a minimum length and in FIG. 6 the clamp 41 is in its high position and the flexible transporting unit 31 is in a position such that the flexible transporting unit sea surface length 31b has a maximum. When the clamp 41 is in its low position the weight modules 39a are lying on the seabed. When a tension is provided in the flexible transporting unit 31 e.g. from the sea surface installation 36, the flexible transporting unit 31 will start sliding over the support section 33. As the flexible transporting unit 31 is sliding over the support section 33 and the clamp 41 is travelling towards its high position, the weight modules 39a are lifted from the seabed, thereby moving the touchdown point of the flexible transporting unit 31 away from the subsea structure 32. When there is no longer tension provided in the flexible transporting unit 31, the weight modules 39a will result in a sliding back force until a point where the tension provided from the sea surface installation 36 increases the tension provided by the weight modules 39 or until the weight modules 39a again are lying on the seabed. When the weight modules 39a again are lying on the seabed, the touchdown point 35 of the flexible transporting unit 31 is closer to the sea surface end 34 of the flexible support unit 31 than in any other position and the horizontal distance between the sea surface end 34 and the seabed end 35 of the flexible transporting 31 is the minimum horizontal distance between the sea surface end 34 and the seabed end 35 of the flexible transporting unit 31.

Some preferred embodiments have been shown in the foregoing, but it should be stressed that the invention is not limited to these, but may be embodied in other ways within the subject-matter defined in the following claims.

Claims

PATENT CLAIMS

1. A flexible pipe system for transferring fluid between a seabed installation and a sea surface installation, the flexible pipe system comprises a fixed subsea structure with a support section and a flexible transporting unit with an axis along a length thereof, a sea surface end and a seabed end arranged with a horizontal distance and a vertical distance, the sea surface end is connected to the sea surface installation and the seabed end is the touchdown point of the flexible transporting unit or the point along the flexible transporting unit where it is connected to the seabed installation which ever has the shorter horizontal distance to the sea surface end, the flexible transporting unit is supported by the support section of the fixed subsea structure at a flexible pipe supporting area defining a flexible transporting unit sea surface length extending along the flexible transporting unit from its sea surface end to the flexible pipe supporting area, at least a section of the flexible transporting unit is movable along the axis with respect to the support section of the fixed subsea structure such that the flexible transporting unit sea surface length can vary between a minimum flexible transporting unit sea surface length and a maximum flexible transporting unit sea surface length.

2. A flexible pipe system as claimed in claim 1 wherein the sea surface installation is a floating installation, preferably selected from a vessel and a platform.

3. A flexible pipe system as claimed in any one of claims 1 and 2, wherein the sea surface installation is a moored floating installation, preferably a weathervaning vessel or a spread-moored vessel.

4. A flexible pipe system as claimed in any one of the preceding claims, wherein the seabed installation is a fixed installation, preferably selected from a well, an anchoring installation and a secondary fixed subsea structure.

5. A flexible pipe system as claimed in any one of the preceding claims, wherein the flexible transporting unit comprises at least one flexible pipe.

6. A flexible pipe system as claimed in any one of the preceding claims, wherein the flexible transporting unit comprises an umbilical or a flexible riser pipe, such as a jumper.

7. A flexible pipe system as claimed in any one of the preceding claims 1-5, wherein the flexible transporting unit comprises two or more pipes connected to each other and in fluidic connection with each other.

8. A flexible pipe system as claimed in any one of the preceding claims, wherein the flexible transporting unit comprises a rigid pipe section, which rigid pipe section is substantially rigid, e.g. provided by a substantially rigid cover layer applied to a section of a flexible pipe of the flexible transporting unit.

9. A flexible pipe system as claimed in any one of the preceding claims, wherein the flexible pipe system comprises two or more flexible transporting units, preferably arranged in substantially side by side relation to each other.

10. A flexible pipe system as claimed in any one of the preceding claims, wherein the subsea structure is a substantially rigid structure fixed to the seabed in a relatively rigid fixation.

11. A flexible pipe system as claimed in any one of the preceding claims, wherein the subsea structure is fixed to the seabed in a position having a minimum distance to a flexible pipe vertical plane through the sea surface end and the seabed end of the flexible transporting unit which is up to about 100 %, such as up to about 75 %, such as 50 %, such as up to about 40 %, such as up to about 30 %, such as up to about 20 %, such as up to about 10 %, such as up to about 5 % of the minimum horizontal distance between the sea surface end and the seabed end of the flexible transporting unit, preferably the subsea structure is fixed to the seabed in a position substantially lying in the flexible pipe vertical plane.

12. A flexible pipe system as claimed in any one of the preceding claims, wherein the subsea structure is a pile structure, comprising one or more piles, such as a mono pile.

13. A flexible pipe system as claimed in any one of the preceding claims, wherein the subsea structure is a dead weight structure, comprising a deadweight which is substantially large to fix the subsea structure to the seabed.

14. A flexible pipe system as claimed in any one of the preceding claims, wherein the support section of the subsea structure is arranged at a distance D-sea surface from the sea surface and at a distance D-seabed from the seabed, preferably D-sea surface is substantially larger than D-seabed.

15. A flexible pipe system as claimed in claim 14, wherein D-seabed is at least about 1 m, such as at least about 2 m, such as at least about 3 m, such as at least about 4 m, such as at least about 5 m, such as at least about 6 m, such as at least about 7 m, such as at least about 8 m, such as at least 10 m, such as at least 25 m, such as at least 50 m, such as at least 50 m, such as at least 100 m.

16. A flexible pipe system as claimed in claim 14, wherein D-seabed is at least about 0.1 %, such as at least about 1 %, such as at least about 2 %, such as at least about 3 %, such as at least about 4 %, such as at least about 5 %, such as at least about 10 % m, such as at least about 15 %, such as at least about 20 %, such as at least 30 %, such as at least 40 %, such as at least 50 %, such as at least 60 %, such as at least 70 %, such as up to about 100 %, such as up to about 90 %, such as up to about 80 % of the distance between the sea surface and the seabed.

17. A flexible pipe system as claimed in any one of the preceding claims, wherein the support section of the subsea structure comprises a curved surface, preferably comprising an arch shape for supporting the flexible transporting unit.

18. A flexible pipe system as claimed in claim 16, wherein the support section of the subsea structure has a width and a length perpendicular to the width, the width is substantially parallel to a vertical plane through the sea surface end and the seabed end of the flexible transporting unit, the support section preferably has an arched shape which is substantially uniform along at least a part of its length.

19. A flexible pipe system as claimed in any one of the preceding claims, wherein the support section of the subsea structure has a gutter structure, preferably comprising a channel with a channel bottom support surface and a pair of side fences, the flexible transporting unit is supported in the channel.

20. A flexible pipe system as claimed in claim 19, wherein each of the side fences independently of each other has a height - preferably a minimum height - from the channel bottom support surface which is at least about 10 %, such as at least about 20 %, such as at least about 30 %, such as at least about 40 %, such as at least about 50 %, such as at least about 60 %, such as at least about 75 %, such as at least about 100%, such as at least about 500 %, such up to about 1000 % of the outer diameter of the flexible transporting unit at its flexible pipe supporting area.

21. A flexible pipe system as claimed in any one of claims 19 and 20, wherein the channel has a length, preferably the channel bottom support surface is arch curved along the channel length, the distance of the fences is substantially constant along the channel length.

22. A flexible pipe system as claimed in any one of claims 19 and 20, wherein the channel has a length, preferably the channel bottom support surface is arch curved along the channel length, the distance of the fences is substantially larger at a first and a third channel sections closer to one of the sea surface end and the seabed end than along a second channel section between the first and third channel sections.

23. A flexible pipe system as claimed in any one of the preceding claims, wherein the support section of the subsea structure comprises a coating of a low friction material, having a sufficiently low friction to provide that the friction between the support section and the supporting area of the flexible transporting unit is lower than it would have been without the low friction coating, preferably the low friction material is selected from polymers, e.g. a flour polymer such as PTFE (poly tetra fluro ethylene), ceramic coatings e.g. comprising chrome oxide and/or alumina oxide, tungsten carbide coating, metal coatings e.g. comprising zinc, aluminium, stainless steel and/or nickel.

24. A flexible pipe system as claimed in any one of the preceding claims, wherein the support section of the subsea structure comprises one or more rotatable bars for reducing friction between the flexible pipe supporting area of the flexible transporting unit and the support section of the subsea structure.

25. A flexible pipe system as claimed in any one of the preceding claims, wherein the support section of the subsea structure comprises a rotatable wheel or a rotatable muff for reducing friction between the flexible pipe supporting area of the flexible transporting unit and the support section of the subsea structure.

26. A flexible pipe system as claimed in any one of the preceding claims, wherein the minimum horizontal distance between the sea surface end and the seabed end of the flexible transporting unit is at least about 2 m, such as at least about 4 m, such as at least about 6 m, such as at least about 8 m, such as at least about 10 m, such as at least about 15 m, such as at least about 20 m, such as at least about 30 m, such as at least about 40 m, such as at least about 50 m, such as at least about 75 m, such as at least about 100 m, such as at least about 200 m, such as at least about 500 m, such as up to about 3000 m, such as up to about 2000 m, such as up to about 1000 m.

27. A flexible pipe system as claimed in any one of the preceding claims, wherein the minimum vertical distance between the sea surface end and the seabed end of the flexible transporting unit is about 3000 m or less, such as about 2000 m or less, such as 1000 m or less, such as about 500 m or less, such as about 400 m or less, such as about 300 m or less, such as about 200 m or less, such as about 100 m or less, such as about 60 m or less, such as about 50 m or less, such as about 40 m or less, such as about 30 m or less, such as about 20 m or less, such as about 10 m or less.

28. A flexible pipe system as claimed in any one of the preceding claims, wherein the maximum flexible transporting unit sea surface length is up to about twice the sum of the minimum horizontal and the minimum vertical distance between the sea surface end and the seabed end of the flexible transporting unit, preferably from about the sum of the minimum horizontal distance plus 10% of the minimum vertical distance to about the minimum horizontal distance plus 300% of the minimum vertical distance, such as from about the sum of the minimum horizontal distance plus 20% of the minimum vertical distance to about the minimum horizontal distance plus 200% of the minimum vertical distance, such as from about the sum of the minimum horizontal distance plus 50% of the minimum vertical distance to about the minimum horizontal distance plus 150% of the minimum vertical distance, such as from about the sum of the minimum horizontal distance plus 75% of the minimum vertical distance to about the minimum horizontal distance plus 100% of the minimum vertical distance.

29. A flexible pipe system as claimed in any one of the preceding claims, wherein the minimum flexible transporting unit sea surface length is up to about X m shorter than the maximum flexible transporting unit sea surface length, wherein X is at least about 0.5 m, such as at least about 1 m, such as at least about 2 m, such as at least about 3 m, such as at least about 4 m, such as at least about 5 m, such as at least about 6 m, such as at least about 8 m, such as at least about 10 m, such as at least about 15 m, such as at least about 20 m, such as at least about 25 m, such as at least about 30 m, such as at least about 40 m, such as at least about 50 m, such as at least about 60 m, such as up to about 100 m.

30. A flexible pipe system as claimed in any one of the preceding claims, wherein the minimum flexible transporting unit sea surface length is at least about the minimum straight line distance between the sea surface end and the seabed end of the flexible transporting unit, such as at least about the sum of the minimum horizontal and the minimum vertical distance between the sea surface end and the seabed end of the flexible transporting unit, such as from about the sum of the minimum horizontal distance plus 50% of the minimum vertical distance to about the minimum horizontal distance plus 400% of the minimum vertical distance, such as from about the sum of the minimum horizontal distance plus 125% of the minimum vertical distance to about the minimum horizontal distance plus 200% of the minimum vertical distance.

31. A flexible pipe system as claimed in any one of the preceding claims, wherein the seabed end is the touchdown point of the flexible transporting unit.

32. A flexible pipe system as claimed in claim 31, wherein the horizontal distance between the sea surface end and the seabed end can vary from a minimum horizontal distance to a maximum horizontal distance, where the difference between the minimum horizontal distance and the maximum horizontal distance preferably is about 50 m or less, such as about 40 m or less, such as about 30 m or less, such as about 25 m or less, such as about 20 m or less, such as about 15 m or less, such as about 10 m or less.

33. A flexible pipe system as claimed in any one of claims 31 and 32, wherein the vertical distance between the sea surface end and the seabed end can be a substantially stationary distance (in which situation it is also referred to as the vertical minimum distance) or it can vary from a minimum vertical distance to a maximum vertical distance, where the difference between the minimum vertical distance and the maximum vertical distance preferably is about 10 m or less, such as about 5 m or less.

34. A flexible pipe system as claimed in any one of the preceding claims 1-30, wherein the seabed end is the point along the flexible transporting unit where it is connected to the seabed installation.

35. A flexible pipe system as claimed in claim 34, wherein the horizontal distance between the sea surface end and the seabed end is a substantially stationary distance (in which situation it is also referred to as the horizontal minimum distance), and the vertical distance between the sea surface end and the seabed end is a substantially stationary distance (in which situation it is also referred to as the vertical minimum distance).

36. A flexible pipe system as claimed in any one of the preceding claims, wherein the flexible transporting unit is not clamped to the support section of the fixed subsea structure.

37. A flexible pipe system as claimed in any one of the preceding claims, wherein the flexible transporting unit can slide along its axis relative to the support section of the fixed subsea structure to thereby provide change of the flexible transporting unit sea surface length.

38. A flexible pipe system as claimed in any one of the preceding claims, wherein the flexible transporting unit, upon an increase in tension in the flexible transporting unit from the sea surface installation beyond a maximum acceptable tension point, is arranged to slide relative to the support section of the fixed subsea structure to lengthen the flexible transporting unit sea surface length to a position when there is substantially no tension beyond the maximum acceptable tension point in flexible transporting unit from the sea surface installation.

39. A flexible pipe system as claimed in any one of the preceding claims, wherein the flexible transporting unit comprises one or more ballasts to provide a pulling force to slide the flexible transporting unit relative to the support section of the fixed subsea structure to shorten the flexible transporting unit sea surface length to a position when there is substantially no tension beyond the maximum acceptable tension point in the flexible transporting unit from the sea surface installation, preferably the resulting force of the one or more ballast(s) provides substantially no pulling force to the flexible transporting unit when the flexible transporting unit has reached its minimum flexible transporting unit sea surface length position.

40. A flexible pipe system as claimed in any one of the preceding claims, wherein the flexible transporting unit comprises a sliding stop arranged to stop the flexible transporting unit from sliding beyond the slide stop, the slide stop preferably being provided by a physical barrier.

41. A flexible pipe system as claimed in claim 40, wherein the sliding stop is provided by a clamp attached to the flexible transporting unit and arranged to slide in a clamp house fixed in relation to the subsea structure, the clamp house preferably has a sliding length of at least about 1 m, such as at least about 2 m, such as at least about 3 m, such as at least about 4 m, such as at least about 5 m, such as at least about 6 m, such as at least about 8 m, such as at least about 10 m, such as at least about 15 m, such as at least about 20 m, such as at least about 25 m, such as at least about 30 m, such as at least about 40 m, such as up to about 50 m.

42 A flexible pipe system as claimed in claim 40, wherein the sliding stop is provided by a clamp with a tether, the clamp is arranged to prevent the flexible transporting unit to slide in its axial direction relatively to the support section of the fixed subsea structure beyond a selected point along the flexible transporting unit.

43. A flexible pipe system as claimed in any one of the preceding claims, wherein the subsea installation is a secondary fixed subsea structure, the seabed end is clamped to the secondary fixed subsea structure, and is in fluidic communication with a flexible pipe arranged as an extension of the flexible transporting unit, the secondary fixed subsea structure preferably has a height from seabed which is at least about 0.1 %, such as at least about 1 %, such as at least about 2 %, such as at least about 3 %, such as at least about 4 %, such as at least about 5 %, such as at least about 10 % m, such as at least about 15 %, such as at least about 20 %, such as at least 30 %, such as at least 40 %, such as at least 50 %, such as at least 60 %, such as at least 70 %, such as up to about 100 %, such as up to about 90 %, such as up to about 80 % of the distance between the sea surface and the seabed.

44. A flexible pipe system as claimed in claim 43 wherein the secondary fixed subsea structure has a height and is arranged such that the section of the flexible transporting unit between the subsea structure and the secondary fixed subsea structure is substantially longer than the distance between the subsea structure and the secondary fixed subsea structure when the flexible transporting unit sea surface length has its maximum flexible transporting unit sea surface length, and is sufficiently short not to lie on the seabed when the flexible transporting unit sea surface length has its minimum flexible transporting unit sea surface length.

45. A flexible pipe system as claimed in any one of the preceding claims, further comprising one or more buoyancy elements arranged to support the flexible transporting unit, the buoyancy elements preferably comprise at least one buoyancy element arranged to support the flexible transporting unit at its flexible transporting unit sea surface length.

46. A flexible pipe system as claimed in any one of the preceding claims, wherein the flexible transporting unit extends in a catenary configuration, preferably selected from a last-s lazy-s configuration and a steep-s configuration.

47. A flexible pipe system as claimed in any one of the preceding claims, wherein the sea surface installation is a weathervaning vessel with a hull, the subsea structure is arranged at the seabed such that it at least in one position of the weathervaning vessel will be under the hull of the weathervaning vessel.

48. A flexible pipe system as claimed in claim 47, wherein the flexible transporting unit is arranged to slide relative to the support section of the subsea structure such that the flexible transporting unit sea surface length of the flexible transporting unit at any time is sufficiently long to avoid damage due to pull from the weathervaning vessel and simultaneously is sufficiently short to not come into contact with the seabed.

49. A flexible pipe system as claimed in any one of the preceding claims, wherein the flexible transporting unit is arranged to slide relative to the support section of the subsea structure as a function of the minimum horizontal distance and the minimum vertical distance between the sea surface end and the seabed end of the flexible transporting unit.

50. A flexible pipe system as claimed in any one of the preceding claims, wherein at least a part of the flexible pipe system is coated with an anti fouling coating to prevent or reduce marine growth, such anti fouling coating optionally comprises an epoxy or an anti fouling paint.