NO20141434A1 - BACK-UP MISSING ARRANGEMENT - Google Patents

Abstract

The invention relates to a positioning system for mooring a first floating structure to a second structure, for example an FAU to an FPSO. The system comprises a main positioning arrangement for maintaining the first floating structure at a predetermined distance (1c) from the second structure. The system further comprises an additional positioning arrangement or a back-up system for mooring the first floating structure to an underlying seabed with, for example, an anchor. This auxiliary positioning arrangement is configured to establish, in the event of failure of the main positioning arrangement, a resultant force (Fres) acting on the first floating structure, which force has a magnitude and direction ensuring a minimum distance (lmin) between the first floating structure and the other structure. The resultant force is the total force acting on the first floating structure and involves, for example, a wave-added mass, a damping force, a horizontal force component from the auxiliary positioning arrangement and a total force from the main positioning arrangement.

Description

Technical field:

The present invention relates to a positioning system for mooring a first floating structure to a second structure as stated in the introductory part of claim 1, as well as an installation that uses the inventive positioning system.

Background and known technique:

There are systems that avoid collisions between two offshore structures as exemplified in patent publication GB2096963 where a solution is described where tension lines run with a maximum length between a vessel and dedicated mooring lines to a platform. Furthermore, patent publication US 5'159'891 shows a solution which enables the towing of a supply vessel away from a platform using a set of mooring lines. The main purpose of the latter is to enable positioning adjustments without the need to reset (Eng. re-set) any anchors.

However, none of the aforementioned known publications discloses additional or back-up positioning arrangement configured to be moored between a floating offshore structure to be positioned and a seabed, and where the positioning arrangement can provide a resultant force in a direction away from the connected second structure in the event failure of the primary/main positioning arrangement, and where the resultant force is sufficient to maintain a minimum distance between the two structures.

It is therefore an object of the present invention to provide a back-up positioning arrangement which ensures a minimum distance between a floating structure and another structure in the event of failure of the floating structure's main positioning system.

Summary of the invention:

The present invention is presented and characterized in the independent claims, while the non-independent claims indicate other characteristics of the invention.

The invention relates in particular to a positioning system suitable for mooring a first floating structure to a second structure. The system comprises a main positioning arrangement suitable for maintaining the first floating structure at a predetermined distance (/c) from the second structure. The system is further characterized in that it comprises an additional positioning arrangement or a back-up system suitable for mooring the first floating structure to an underlying seabed with, for example, an anchor. This additional positioning arrangement is configured to establish, in the event of failure of the main positioning arrangement, a resultant force ( Fres ) acting at least indirectly on the first floating structure, which force has a magnitude and direction ensuring the maintenance of a minimum distance between the first floating structure and the other structure. Furthermore, said minimum distance (/„,,-„) can span (Eng. range) from close to zero, which is close to direct impact, and up to a distance that is assumed to be sufficiently safe for the intended purpose, for example within a range from 10 meters to 100 meters. The first floating structure can be a floating accommodation unit (FAU) and the second structure can be a hydrocarbon production facility such as a floating production, storage and offloading (FPSO) unit.

In a preferred embodiment, the additional positioning arrangement is arranged on the first floating structure, facing away from the second structure during use.

In another preferred embodiment, the positioning system comprises a tensioning means which is mountable on the first floating structure and connected to a first end of the additional positioning arrangement. Said tensioning means, for example one or more winches, is suitable for providing controllable tension along the additional positioning arrangement. Furthermore, said tensioning means is preferably configured to enable control of the minimum distance (lmi„) by regulating the tension along the additional positioning arrangement.

In another preferred embodiment, the additional positioning arrangement further comprises one or more mooring lines such as a chain, a cable, a rope, a cable, etc., or combinations thereof. The mooring line(s) may be connected at a first end to the first floating structure and is connected at a second end to a seabed mooring means. In this embodiment, the largest horizontal component of the mooring line, or mooring lines, is in use in a direction away from the second structure. Accordingly, the mooring line(s) may also be configured non-parallel to a straight line drawn through both the first floating structure and the second structure. However, the resultant horizontal force, in the specific embodiment, should have a net force along this straight line and in a direction away from the other structure. The mooring line, or at least one of the mooring lines, is preferably oriented parallel, or almost parallel, to said straight line.

In another preferred embodiment, the additional positioning arrangement comprises two or more mooring lines, where each of the mooring lines has its largest component oriented parallel to the underlying seabed and in a direction away from the other structure in use. In addition, the horizontal component of the two mooring lines in this embodiment defines a plane parallel to the underlying seabed, which has a mutual angle of less than 120°, preferably less than 90°, even more preferably less than 45°, even more preferably less than 25°, for example parallel or close to parallel.

In another preferred embodiment, the additional positioning arrangement comprises a first elongate part such as a cable connectable at one end to the first floating structure, and a second elongate part such as a chain connected between a seabed mooring means and the other end of the first elongate part, wherein the weight and/or stiffness of the second elongated part is higher than the first elongated part. The length of the second elongated part is preferably less than the length of the first elongated part, more preferably less than 30% of the length of the elongated part, even more preferably less than 20%, for example around 10%.

In another preferred embodiment, the main positioning arrangement comprises a dynamic positioning system, for example a combined positioning arrangement comprising both a dynamic positioning system and a mooring line system.

The invention also includes an installation such as an installation suitable for hydrocarbon production. The installation comprises a first floating structure, a second structure and a positioning system according to any of the aforementioned features.

In a preferred embodiment, the installation further comprises a bridge or walkway which connects the first floating structure and the second structure, preferably a detachable telescopic walkway.

In the following description, a number of specific details are set forth to provide a thorough understanding of embodiments of the claimed positioning system and associated installation. However, one skilled in the relevant art will recognize that these embodiments may be practiced without one or more of the specific details, or with other components, systems, etc. In other examples, well-known structures or operations are not shown, nor described in detail, to avoid unclear aspects of the embodiments shown.

Brief description of the figures:

Fig. la and lb are, respectively, a sketch seen from above and a sketch seen from the side, of an installation that uses a positioning system according to one embodiment of the invention, Fig. 2 shows a mooring line profile of a 2500 m long, 90 mm thick mooring cable that runs from the sea surface to a water depth of 300 m when subjected to a line tension of 4000 kN, Figure 3 shows graphs representing the dynamic behavior in waves at various initial tension levels of the first floating structure as a function of time after a failure of a main positioning arrangement, where the first floating structure is anchored at a depth of 100 m by two 1500 m long cables with a thickness of 90 mm, Figure 4 shows graphs presenting the dynamic behavior in waves of the first floating structure as a function of time after a failure of the main positioning arrangement, where a surge offset is set up by anchoring the first floating structure at a depth of 100 m with two 15 00 m long cables with a thickness of 90 mm, and the second wave displacement is set up at a first floating structure anchored at 100 m depth with two 800 m long chains with a thickness of 84 mm, Figure 5 shows graphs presenting the dynamic the behavior in waves of the first floating structure as a function of time after a failure of the main positioning arrangement, where a first wave displacement is set up by anchoring the first floating structure at 300 m depth with two 1200 m long cables of 90 mm thickness, a second wave displacement is set up by anchoring the first floating structure at 300 m depth with two 2500 m long cables of 90 mm thickness and with an initial tension identical to the first wave displacement, and a third wave displacement is set up by anchoring it the first floating structure at 300 m depth with two 2500 m long chains with a thickness of 90 mm, and with an initial cut that is higher than the first and second wave displacements, and Figure 6 shows gr afer presenting the dynamic behavior in waves of the first floating structure as a function of time after a failure of a main positioning arrangement, where a first wave displacement is set up by anchoring the first floating structure at 300 m depth with two 1200 m long cables with thickness of 90 mm, a second wave displacement is set up by anchoring the first floating structure at a depth of 300 m with two 2500 m long cables with a thickness of 90 mm, and with an initial tension higher than the first wave displacement, and a third wave displacement is set up by anchoring the first floating structure at a depth of 300 m with two mooring lines comprising a bottom chain (Eng. bottom chain) with 200 m length and thickness 180 mm and a

top cables with a length of 450 m and a thickness of 90 mm, and with an initial stretch less than the first and second wave displacements,

Detailed description of the invention

In the present invention, an additional or back-up mooring arrangement 3 is presented. The main purpose is to avoid contact between a first floating structure 2 and a second structure 1 in the event of any kind of failure of a main mooring arrangement 4 to the first floating structure 2, which main mooring arrangement 4 is configured to control the position of the first floating structure 2 relative to the second structure 1. The main mooring arrangement 4 may be a Dynamic Positioning System (DP) comprising one or more propulsion units 4 and/or a substantially symmetrical distribution of mooring lines extending from it first floating structure 2 to an underlying seabed 5.

An example of a first floating structure 2 is a FAU (Eng. floating accommodation unit). Examples of a second structure 1 may be an FPSO (Floating Production, Storage and Offloading) unit, an FPU (Floating Production Unit) or an F SO (Floating, Storage and Offloading) unit.

In what follows, an installation is shown on figures la and lb which will be described and analysed. A FAU 2 is connected to an FPSO 1 via a telescopic bridge 6. The term telescopic bridge will henceforth denote a bridge that can be extended or retracted in the longitudinal direction between the FAU 2 and the FPSO 1 relative to a mean length (Eng. mean length) le. The relative position of the FAU 2 and the FPSO 1 is controlled by a plurality of DP propulsion systems 4 distributed on the hull of the FAU 2. The FAU 2 is further connected to a back-up mooring arrangement 3 comprising two almost parallel oriented mooring lines 3a,3b extending from one or more mooring winches 7 placed on the FAU 2 to an underlying seabed 5. This, or these, winch(s) 7 is in the design placed on the side of the FAU 2 and faces away from the bridge which is connected to the FPSO 1 and enables control of the pretension in each mooring line 3a, 3b.

Based on the above-mentioned installation, two different cases of failure in DP/propulsion units are considered to be most critical: • DP operation-over scenario; At a given time, the DP system 4 provides full propulsion in the direction of the FPSO 1, i.e. in the direction of the bridge. This extreme case will be analyzed below. • Blackout scenario: At a given point in time, a complete failure of propulsion occurs. Consequently, the back-up mooring arrangement 3 shall keep the FAU 2 at a safe distance from the FPSO 2 with zero steering forces from the FAU 2 and possibly very unfavorable weather conditions.

Figure 1 le represents the smallest clearance between the FAU 2 and the FPSO 1 when the telescopic bridge 6 is in its middle position and Im represents the distance from the FAU 2 to an anchor 8.

In a first embodiment, the mooring line(s) comprises a wire-based line (Eng. wire-based line). For example, at a water depth d = 100 m, a completely wire-based line of length lwire= 1500 m can be used. At a depth of 300 m, the cable length can be 2500 m. At least one purpose in choosing the length of the mooring line 3a,b can be to reduce vertical forces at the anchor 8 far below the maximum permissible stretch of the mooring line 3a,b, for example half of the minimum breaking load (Eng. minimum breaking load, MBL). Wires of shorter length can be used. However, this will require the use of anchors that are specially designed for such large vertical forces. Anchors can, as an example, be of the drag embedment type, high holding capacity anchors (Eng. high holding capacity anchors). The mooring line profile (water depth as a function of the horizontal distance from the FAU 2) for a mooring line system 3 with two mooring lines 3a,b, a maximum water depth of 300 m, an average cable diameter of 90 mm and a pretension of 4000 kN, is shown in figure 2 .The pretension of 4000 kN is a typical maximum tension that the mooring line system can be subjected to shortly after a DP drift-over.

In a second embodiment, the mooring line(s) comprises a chain-based line. For example, a water depth of d = 100 m and a completely chain-based line of length lCham = 800 m can be used.

In a third and preferred embodiment, the mooring line(s) comprises a chain-based line and cable-based line, where the former is connected to its anchor 8. In order to reduce the total weight of the mooring line(s) 3a,b, the length of the chain should preferably be substantially shorter than the length to the wire. As an example, such a combined chain and wire back-up mooring arrangement mooring the FAU 2 to the underlying seabed 5 with 300 m of mooring line(s) 3a,b may comprise an anchored chain of grade R2 with a mean diameter of 180 mm and a length of 200 m and a wire connected between the chain and the FAU 2 with a diameter of 90 mm and a length of 450 m (outside the roller

9) (Eng. outside fairlead).

Simulations

Full DP operation-over scenario

With reference to the full DP drift-over scenario described above, a simulation of the forces involved has been carried out for all of the above-mentioned designs. Again, in a DP operation over scenario, the DP system 4 provides full propulsion in the direction of the FPSO, i.e. in the direction of the bridge.

In the simulation, the system shown in figures 1 and 2 has been assumed, i.e. an FPSO 1, a FAU 2, a telescopic bridge 6 that connects the FPSO 1 and the FAU 2, two mooring lines 3,3a,b connected between the FAU 2 and a underlying seabed 5 anchored to a suitably sized anchor 8 on the side of the FAU 2 and facing away from the FPSO 1. As a result of the telescopic design of the bridge 6, the relative distance between the FAU 2 and the FPSO 1 can vary around a mean position FAU mean. This variation will hereafter be referred to as a horizontal FAU displacement or FAU displacement. Similarly, an FAU displacement in the direction of the FPSO 1 will be referred to as an FAU wave displacement.

In the simulations, the DP system 4 is assumed to comprise four 4.4 MB azimuth propulsion units 4 enabling a maximum open-water propulsion of approximately 810 kN each, giving a total of 3240 kN. If one takes into account the interaction between propulsion unit and hull, the total available propulsion is considered to be around 3000 kN. Furthermore, the model used for the simulation of the dynamic displacement of the FAU 2 as a function of time after the DP drift-over scenario is:

ir is the horizontal acceleration of the FAU 2, x is the displacement of the FAU 2, A is a surge added mass (Eng. surge added mass), F is the damping force, FT is the horizontal component of the mooring line tension and FDP is the total force from the propulsion units 4.

The following damping model is assumed:

where Bi and B2 are respectively the linear and quadratic damping coefficients, x is the horizontal speed of the FAU 2. The contribution from the damping of the hull has been established based on fatigue tests (Eng. decay tests) with an FPSO of the type Sevan 300 FPSO. The data has been adjusted (Eng. sculled) to take account of the (small) differences in scaling and effect in different mooring systems. The following values are used for the contribution from the hull:

In addition, the thrust will make a damping contribution due to the assumed speed dependence of the thrust (based on the thrust curve of the above 4.4 MW propulsion units)

Wire-based mooring system water depth d= 100 m

A cable-based mooring system 3 is assumed, comprising two 1500 m mooring lines 3a,b with a mean cable diameter of 90 mm, anchored at a water depth of 100 m and subjected to prestress levels during normal operation of 75 kN and 135 kN, which correspond respectively to horizontal tension of 40 kN and 100 kN. Any contribution from waves is ignored. The results are shown in Figure 3 and present the FAU wave displacement (displacement of the FAU in the direction of the FPSO 1) as a function of time after failure of the DP. The dashed line shows the FAU wave displacement with small bias and the solid line shows the FAU wave displacement with high bias.

A significant dynamic overshoot (Eng. overshoot) can be seen for the FAU displacement in the direction towards the FPSO (FAU wave displacement). For the lowest prestress level (dashed line) it is seen that the maximum displacement in the direction of the FPSO is approximately 42 m approximately 75 seconds after the failure. For the highest pretension level (full tension line), the maximum displacement is reduced to approximately 30 m. A further reduction in maximum displacement can be achieved by further increasing the pretension with control of the mooring winch(s) 7. In this simulation, the maximum tension in the line during dynamic overshoot found to be 3750 kN and 3450 kN for the lowest and highest prestress levels, respectively.

Comparison, chain-based and cable-based mooring system - water depth d = 100 m

To enable a direct comparison between a chain-based mooring system and a wire-based mooring system, similar simulations have been carried out as described above for a back-up mooring arrangement comprising two mooring lines 3a,b of 800 m anchored at a water depth of 100 m, but where each mooring line 3a,b comprise a chain with a diameter of 84 mm and which is subjected to a prestress of 325 kN (200 kN in the horizontal direction). The results from the FAU wave displacement as a function of time after failure of the DP with chain as mooring lines are shown in Figure 4 with a dashed line. The FAU wave displacement of the wire-based system for prestressing avl35kN (100kNi horizontal direction) is added for comparison (solid line). Note that the preload level for the chain-based system (325 kN) is significantly greater than for the above-mentioned cable-based system (135 kN) due to much higher chain weight. The maximum FAU displacement is seen to be almost identical for the two mooring systems.

Consequently, assuming that the maximum FAU wave displacement is the critical parameter, a cable-based system for a water depth of 100 m will generally be more advantageous than a chain system due to its significantly lower horizontal prestress at zero FAU position (Eng. at zero FAU position). It is usually desirable to minimize this force in order to reduce the required thrust to compensate for the bias, thereby reducing fuel consumption.

Wire-based mooring system - water depth d = 300 m

A cable-based mooring system 3 comprising two 2500 m mooring lines 3a,b with an average cable diameter of 90 mm anchored at a water depth of 100 m and exposed to prestress levels during normal operation with horizontal components corresponding to 290 kN and 400 kN is assumed. All contributions from waves are ignored. The results are shown in Figure 5 which presents the FAU wave displacement (the displacement of the FAU in the direction of FPSO 1) as a function of time after failure of the DP. The dashed line shows the FAU wave shift with a small bias and the solid line shows the FAU wave shift with a large bias. In the same way as for the mooring systems described above for water depths of 100 m, significant dynamic overshoot is seen at the FAU wave displacement. Figure 5 also shows the FAU wave displacement when using a cable-based mooring system 3 with 1200 m mooring lines 3a,b (dash-dotted line). At 2500 m, the maximum FAU wave displacement is about 54 m and about 43 m for a prestress level along the horizontal direction of 290 kN and 400 kN, respectively. In this case, a prestress level of 400 kN is considered to be beneficial. Again, a further reduction in maximum FAU displacement requires a further increase in bias. In general, an FAU wave displacement higher than 40 m is considered undesirable due to the limitation in length of the existing bridge design. At 1500 m, the maximum FAU wave displacement was found to be around 42 m, which is significantly less than for the mooring system with 2500 m cable length at an identical prestress level (290 kN). However, in this case the maximum tension peak is significantly higher, almost 4500 kN. With a horizontal component of the prestress of 290 kN and 400 kN, the required thrust to compensate for the prestress would be 580 kN and 800 kN respectively. A reduction of the mean diameter of the wire from 90 mm to 70 mm did not significantly change the maximum FAU wave displacement.

Wire-based and chain-based mooring system - water depth d = 300 m

Similar simulations as for the 300 m cable-based system have been carried out for a back-up mooring system comprising two mooring lines, each having a top cable length of 450 m (excluding roller cleats) with a mean diameter of 90 mm and a bottom chain length of 200 m with a mean diameter of 180 mm. The horizontal component of the prestress level is 125 kN. The result of the FAU wave displacement is shown with a solid line in Figure 6, together with cable-based mooring systems is 2500 m / 400 kN (dashed line) and 1200 m / 290 kN (dash-dotted line). For the combined cable-based/chain-based system, the FAU wave displacement is around 37 m, which is lower than for the two cable-based systems (both above 40 m). In addition, the bias level is significantly lower. Accordingly, based on the results from Figure 6, a back-up mooring system based on a combined bottom chain system and a top wire system is considered to be the preferred embodiment of the invention after failure of the DP causing a DP drift-over scenario, also for water depths up to and above 300 m.

Blackout

With reference to the above-mentioned full scenario, i.e. no propulsion is available in the DP system constituting the main mooring arrangement, simulations of the forces involved have been carried out in the first and third embodiments, i.e. respectively with a fully wire-based mooring system and a combined wire-based and chain-based system. In this case, the most critical conditions are assumed to be waves coming in the direction of the bridge in the direction of the FPSO 2, i.e. waves in the 0 degree direction, see arrow 10 in figure la, and with a co-linear environment ). The following set of environmental conditions is assumed in this analysis:

Hs is the significant wave height, Tp is the peak period of the wave spectrum, Uw is the average velocity over a period of one hour and Uc is the surface flow velocity.

Wire-based mooring system - water depth d = 100 m

A cable-based mooring system 3 comprising two 1500 m mooring lines 3a,b with a mean cable diameter of 90 mm anchored at 100 m water depth and exposed to prestress levels during normal operation of 135 kN, which corresponds to horizontal tension of 100 kN, is assumed.

In this case, the analyzes show that a situation with a significant height Hs of 10.5 m, the maximum FAU wave displacement is approximately 35 m. Note that 10.5 m is significantly higher than the upper wave limit to connect the FAU to an FPSO, which is of the order of 5-7 m. Again, the displacement can be reduced by increasing the preload level during normal operation. The corresponding maximum mooring line tension is found to be 5692 kN.

Chain-based and Wire-based mooring system - water depth d = 300 m

Simulations have been carried out for a back-up mooring system with a water depth of 300 m, comprising two mooring lines, each of which has top cables with a length of 450 m (outside roller cleats) and a mean diameter of 90 mm and a bottom chain length of 200 m and mean diameter of 180 etc. The horizontal component of the prestress level is 125 kN.

In general, the results for this specific back-up mooring system with a water depth of 300 m are quite similar to the results for a water depth of 100 m for the drift-over scenario. It can therefore be concluded that a back-up mooring system will be able to keep FAU 2 away from FPSO 1 in the event of a complete blackout scenario. This conclusion applies to sea conditions where Hs is at least 10 m, which is significantly higher than practical operating limits when FAUen 2 is connected to FPSO 1.

In the foregoing description, various aspects of the assembly according to the present invention have been described with reference to the illustrated embodiment. For the purpose of explaining, specific numbers, systems and configurations, presented to provide an in-depth understanding of the device and its operation. However, this description is not intended to be construed in a limiting manner. Various modifications and variations of the illustrated embodiment shown, in addition to other embodiments of the apparatus which are obvious to a person skilled in the art, shall be considered to lie within the scope of the present invention.

Claims (12)

1. A positioning system for mooring a first floating structure (2) to a second structure (1), comprising - - a main positioning arrangement (4) for keeping the first floating structure (2) at a predetermined distance ( lc ) from the second the structure (1), characterized in that the positioning system further comprises - an additional positioning arrangement (3,3a,3b) for mooring the first floating structure (2) to an underlying seabed (5), the additional positioning arrangement (3) is configured to establish, in the event of failure of the main positioning arrangement (4 ), a resultant force ( Fres ) acting on the first floating structure (2) with a magnitude and direction that ensures the maintenance of a minimum distance ( lmm) between the first floating structure (2) and the second structure (1). 2. The positioning system according to claim 1, characterized in that the additional positioning arrangement (3) is arranged on the first floating structure (2) and faces away from the second structure (1) during use. 3. The positioning system according to claim 1 or 2, characterized in that the positioning system comprises a tensioning means (7) mountable on the first floating structure (2) and connected to a first end of the additional positioning arrangement (3). 4. The positioning system according to claim 3, characterized in that the stretching means (7) enables control of the minimum distance (lmm) by regulating the stretch along the additional positioning arrangement (3). 5. The positioning system according to any of the preceding claims, characterized in that the additional positioning arrangement (3) comprises a mooring line (3a,3b) which is connectable at a first end to the first floating structure (2) and which is connected at a second end to a seabed mooring means (8), where the largest component of the mooring line (3a,3b) is oriented in parallel with the underlying seabed (5) during use in a direction away from the second structure (1). 6. The positioning system according to any of the preceding claims, characterized in that the additional positioning arrangement (3) comprises two mooring lines (3a,3b), each having its largest component oriented parallel to the underlying seabed (5) and in a direction away from the second structure (2) in use, where the component of the two mooring lines (3a,3b) defines a plane parallel to the underlying seabed (5) with a mutual angle of less than 120°. 7. The positioning system according to any of the preceding claims, characterized in that the additional positioning arrangement (3) comprises a first elongated part connectable at one end to the first floating structure (2) and a second elongate part connected between a seabed mooring means (8) and the other end of the first elongate part, wherein at least one of the weight and stiffness of the second elongated part is greater than the first elongated part. 8. The positioning system according to claim 7, characterized in that the length of the second elongated part is less than the length of the first elongated part. 9. The positioning system according to any of the preceding claims, characterized in that the main positioning arrangement (4) comprises a dynamic positioning system. 10. An installation comprehensive a first floating structure (2), a second structure (1) and a positioning system according to any one of claims 1-9. 11. The hydrocarbon production installation according to claim 10, characterized in that the installation further comprises a bridge (6) which connects the first floating structure (2) and the second structure (1). 12. The installation according to claim 11, characterized in that the bridge is a detachable telescopic bridge (6).