WO2021089505A1

WO2021089505A1 - An offshore installation and a method of tethering a subsea flexible pipe

Info

Publication number: WO2021089505A1
Application number: PCT/EP2020/080749
Authority: WO
Inventors: Bjarke PEDERSEN
Original assignee: National Oilwell Varco Denmark I/S
Priority date: 2019-11-05
Filing date: 2020-11-03
Publication date: 2021-05-14

Abstract

The present invention relates to an offshore installation comprising a flexible pipe, said flexible pipe comprises from the inside and out an internal pressure sheath, one or more armor layers and an outer sheath, and at least one length of the flexible pipe is provided with a stocking surrounding the outer sheath and comprising individual fiber straps braided around the outer sheath in such a way that the stocking will tighten around the flexible pipe when subjected to pull and the stocking is adapted for long-term connection with a tether line, thereby connecting the flexible pipe to the tether line in the offshore installation.

Description

AN OFFSHORE INSTALLATION AND A METHOD OF TETHERING A SUBSEA FLEXIBLE PIPE

TECHNICAL FIELD

The present invention relates to an offshore installation comprising a flexible pipe and one or more stockings connecting the flexible pipe with tether lines. The invention also relates to a method of tethering a subsea flexible pipe.

BACKGROUND

Offshore installations for transferring fluids, such as petrochemical fluids are well known in the art. Such offshore installations in which fluids are transferred from/to wellheads or other structures, such as connecting elements (manifolds) located on the seabed to a higher level, e.g. to a sea surface installation, are also referred to as riser systems. Such offshore installations are usually subjected to very high and varying loads due to wind, water waves, water current, shifting water levels as well as gravity and friction upon engagement with the seabed. In particular in offshore installations with flexible pipe(s) which are adapted to transporting fluids between a seabed station and a floating unit, e.g. a sea surface floating unit such as a vessel, are subjected to high loads including varying pulling forces due to the dynamic movements of the floating unit. Thus, it may be required to tethering at least parts of the flexible pipe to the sea-bed using a riser tethering configuration.

A number of configurations for flexible pipe systems have for example been described in "Recommended Practice for Flexible Pipe", ANSI/API 17B, fifth Edition, May 2014 (hereafter API17B).

When tethering is applied on a flexible pipe it is necessary to mount the flexible pipe with clamps to attach the pipe with the tether-line. The clamps are normally made from steel and may be quite cumbersome to mount on the flexible pipe. Examples of well-known clamps are e.g. described in international patent application WO 2004/090348 A1 and US patent no. US 7100641 B2.

Although the steel clamps function quite well and provide good attachment between the flexible pipe and the tether lines, they are relatively expensive and as mentioned cumbersome to mount on the flexible pipe. Normally these clamps are mounted on the pipe during the installation of the pipe at the offshore field location, i.e. on an offshore installation, which further contributes to make the mounting cumbersome.

DISCLOSURE OF THE INVENTION

The object of the invention is to provide an offshore installation comprising a flexible pipe for transferring fluid at least partly below sea surface, and preferably between a seabed station and a floating unit above seabed, e.g. a sea surface floating unit such as a vessel, which offshore installation is relatively simple thus saving construction cost, simple to install thus saving costly installation time and where the risk of damaging the flexible pipe when subjected to high pulling forces is kept at a low level.

The present invention provides a novel offshore installation, which meets this object. The offshore installation of the invention and embodiments thereof have shown to have a large number of advantages which will be clear from the following description. The invention is defined in the claims.

In one aspect the present invention relates to an offshore installation comprising a flexible pipe, the flexible pipe comprises from the inside and out an internal pressure sheath, one or more armor layers and an outer sheath. The flexible pipe is advantageously of the unbonded type suitable for transportation of aggressive fluids, such as oil and gas. At least one length of the flexible pipe is provided with a stocking surrounding the outer sheath and comprising individual fiber straps braided around the outer sheath in such a way that the stocking tightens around the flexible pipe when subjected to pull. The stocking is adapted for long-term connection with a tether line, thereby connecting the flexible pipe to the tether line in the offshore installation.

Advantageously the installation comprises an unbonded flexible pipe a stocking surrounding the outer sheath in a length section of the flexible pipe, wherein the stocking comprises individual fiber straps braided around the outer sheath providing that the stocking is at least partly tightened around the flexible pipe and wherein the pipe is tethered via at least one tether line to a seabed and/or a subsea installation via the stocking. Thus, the tethering line provides a pulling force in the fiber straps, which ensures that the stocking is tightened around the flexible pipe.

A protection sleeve may advantageously be located between the outer sheath and the stocking to protect the outer sheath.

Advantageously, the stocking is tightened around the flexible pipe by a pulling force applied to said straps, said pulling force advantageously being applied by the tethering line pulling in the stocking.

The tighten principle of the stocking inherently adjusts to variations in the outer pipe diameter thus facilitating a reliable grip on the pipe during its service life when subjected to functional and environmental loads. Such variations in the outer pipe diameter can be caused by e.g. changes in bore pressure/temperature and/or varying pipe tensile loads during the dynamic movements of the pipe system in the offshore installation. Also, the stocking inherently adjusts its grip on the pipe as a function of the pulling force, i.e. the higher the pulling force the higher the squeeze load on the pipe. Therefore, the stocking principle ensures a reliable long-term connection with the pipe under varying load conditions which for a conventional steel clamp otherwise needs be achieved using a more complex and costly design principle. The fiber straps may advantageously form helically wound biaxial braid, preferably braided according to the "Chinese finger" principle.

The fiber straps may conveniently comprise a first set of fiber straps and a second set of fiber straps, wherein the first set of fiber straps is travelling in a first helical direction around the outer sheath and the second set of fiber straps is travelling in an opposite second helical direction around the outer sheath. The first set of fiber straps may be braided by the second set of fiber straps by passing under and over straps of the second set of fiber straps.

Each set of fiber straps comprises for example 1-10 fiber straps, such as 2-5 fiber straps. To ensure a balanced load in the stocking it is desired that the number of fiber straps in the first set of fiber straps and a second set of fiber straps is identical and preferably also the fiber straps as such are identical in the two sets of straps.

In an embodiment, each of the first set of fiber straps and the second set of fiber straps comprises individually from each other 1 to 10 fiber straps.

T fiber straps are advantageously helically wound such that the winding angle to the center axis of the pipe in straight condition and when the stocking is tightened around the flexible pipe, which is 30-85 degrees.

In an embodiment, the fiber straps is helically wound with a substantially constant pitch determined when the pipe is in straight condition and the stocking is tightened around the flexible pipe.

In an embodiment, the fiber straps is helically wound with a varying pitch along the length of the pipe determined with the pipe in straight condition and when the stocking is tightened around the flexible pipe, wherein the winding pitch closer to the tether line is longer than the winding pitch further from the tether line. The fiber straps may for example have a first pitch in a first section adjacent to the tether line, a second pitch shorter than the first pitch in a middle section of the stocking and a third pitch shorter than the second pitch in a further section further from the tether line. The individual fiber straps of the stocking may be braided to form a substantially rhombus like pattern surrounding the flexible pipe. The fiber strapsadvantageously have a certain flexibility and the rhombus like pattern may change as the braided fiber straps tighten up around the flexible pipe, when the stocking is subjected to pulling forces. The stocking may be braided before it is mounted on the flexible pipe or it may be braided directly around the flexible pipe, i.e. in situ.

The individual fiber straps may comprise braided, twisted or knitted fibers, such as carbon fibers, glass fibers, aramid fibers, steel fibers, polyethylene fibers, mineral fibers and/or mixtures comprising at least one of the foregoing fibers. Preferred fibers includes steel fibers and aramid fibers, such as fibers of Kevlar. The fibers may be of stable fibers and/or comprise advantageously continuous filaments. Continuous filaments means fibers of a significant length but should not be understood to mean that the length is perpetual or infinite. Continuous fibers, such as continuous filaments, strands, yarns, or rovings preferably have length of at least about 10 m, preferably at least about 100 m, more preferably at least about 1000 m. The diameter of the individual fibers may for example be between about 5 pm and 25 pm.

The expression "long-term" means a period from about 15 to 25 years, such as about 20 years.

Stockings of braided fiber straps have been used for flexible pipes during handling of the pipes, for example where the pipes are installed or removed from an offshore plant. Thus, the stockings have only been used temporarily for a short term such as hours or days when installing or removing the flexible pipe.

WO 2006/132544 A1 describes a device and method to secure a suspended object, and use thereof. The device is a stocking mounted on a drilling hose, such as a rotary hose/Kelly, and the stocking serves to prevent that the drilling hose falls down on the drilling platform in case of rupture. Such an incident may injure the staff on the drilling platform. Thus, the stocking is mounted on the drilling hose for security reasons when the drilling hose is raised or lowered from a drilling site.

In an embodiment of the offshore installation the stocking is attached to the outer sheath by one or more fastening means. The fastening means may be a collar, e.g. a metal collar which serves to attach at least a part of the stocking to the flexible pipe. The collar will anchor a part of the stocking to the outer surface of the flexible pipe and in this manner attach the stocking to the flexible pipe. The anchoring of the stocking is preferably performed at a location opposite of the pulling end of the stocking, i.e. opposite the end where a tether line is attached. A part of the stocking may also be integrated with the outer sheath of the flexible pipe and thereby attached to the flexible pipe. By attaching a part of the stocking to the outer surface or outer sheath of the flexible pipe an improved squeeze load between the stocking and the pipe can be achieved.

In an embodiment, a protection sleeve is located between the outer sheath and the stocking at at the anchoring site to protect the outer sheath. A further protection sleeve may conveniently located between the outer sheath and the stocking at the end of the stocking, where it is attached to the tether line.

The stocking is intended for connection with a tether line and will be subjected to pulling forces and variations of pulling forces. In an embodiment the stocking can resist a pull (force) from the tether line of at least about 50 kN, such as a pull (force) of at least about 100 kN, such as from about 200 kN to about 900 kN, such as about 300 kN to about 800 kN.

The flexible pipe may be provided with two or more stockings for connection with two or more tether lines. Each stocking may extend from about 1 m to about 10 m, such as 1 m to 5 m, along the length of the flexible pipe.

In an embodiment, the stocking has a first end portion and a second end portion, wherein the second end section is located closer to the seabed and/or subsea installation than the first end portion of the stocking and where the stocking being connected to the tether line at the second end section of the stocking.

Advantageously, the fiber straps of the stocking at the second end thereof are terminated in one or more connecting devices which is/are connecting the stocking to the at least one tether line, preferably the fiber straps of the stocking at the second end thereof are terminated in one single connecting devices which is connected to the tether line.

Advantageously the stocking extends from about 1 m to about 10 m along the length of the flexible pipe. Preferably the stocking is attached to the outer sheath at an end portion of the stocking further away from the tether line. In an embodiment the end portion further away of the stocking is up to about half the stocking extending length, such as up to about 25 % of the stocking extending length.

In an embodiment, the stocking is attached to the outer sheath at a first end portion of the stocking further away from the tether line than a second end portion of the stocking and the first end portion further away of the stocking is up to about 15 % of the stocking extending length.

In an embodiment the stocking is provided with 8 to 20 braided individual fiber straps. The fiber straps are braided to form a pattern of substantially parallel straps forming a rhombus like pattern, which can be stretched when the stocking is subjected to pull. When the pattern is stretched it will tighten up and "lock" around the flexible pipe. The braiding is provided by well- known braiding techniques which have also been used in the prior art stockings.

In an embodiment the stocking is provided on the flexible pipe at a position near the sea-surface. Thus, it is possible to attach the flexible pipe to a sea surface installation by means of a tether line and the stocking. The stocking may , in an embodiment be provided on the flexible pipe at a position near the sea-bed, and the flexible pipe can be attached to a sea bed installation by means of a tether line and the stocking.

The stocking or rather the fibers straps forming the stocking can be made from material selected from a wide range of materials, however, the materials should meet the requirement of long-lasting properties. In an embodiment the stocking is made from polymer material such as polyaramide. Polyaramide is a strong and long lasting material and very suitable for producing the stocking. Thus, the stocking can be braided from straps made from polyaramide fibers.

The diameter of the fiber straps may vary depending on the pull force affecting the straps, however, in unloaded condition the individual fiber strap may have a diameter in the range of about 0.3 cm to about 2 cm, such as in the range of about 0.5 cm to about 1.5 cm.

The outer diameter of a flexible pipe is typically in the range from about 10 cm to about 100 cm.

In an embodiment of the offshore installation the fiber straps forming the stocking are made from a metallic material, and in an embodiment the fiber straps are made from a thermoplastic material or composite material. Advantageously the fibers of the straps comprise aramide fibers, steel fibers or combinations comprising aramid fibers or steel fibers.

The offshore installation comprises a flexible pipe and the pipe may comprise a carcass , preferable constituted by wound metallic strips, and preferably wound around the pipe with a winding angle between 80° to 89° in respect of the axis of the pipe. The carcass serves to support the internal pressure sheath in respect of radial forces and to prevent collapse of the internal pressure sheath in case of sudden pressure drop in the bore of the pipe.

If the flexible pipe comprises a carcass a polymer layer is extruded onto the carcass and forms the internal pressure sheath. If the flexible pipe does not comprise a carcass the polymer layer can be extruded onto a mandrel and the polymer layer, i.e. the internal pressure sheath, constitutes the innermost layer of the pipe.

On the outer side of the polymer layer constituting the internal pressure sheath one or more layers such as armor layers are applied.

The flexible pipe may comprise a pressure armor. One or more pressure armor layers may be present, preferable constituted by wound metallic strips, and preferably wound around the pipe with a winding angle between 55° to 89° in respect of the axis of the pipe. The pressure armor supports the pipe in respect of radial forces exposed to the pipe.

The flexible pipe may further comprise one or more tensile armors. One or more tensile armoring layers, preferable constituted by wound metallic wires, and preferably wound around the pipe with a winding angle between +/- 20° to +/-55⁰ in respect of the longitudinal axis. If two tensile armor layers are applied the layers are preferably cross-wound. The tensile armor serves to protect the pipe against damage caused by axial forces.

The term "cross-wound layers" means that the layers comprise wound elongate elements that are wound in opposite direction relatively to the longitudinal axis of the pipe where the angle to the longitudinal axis can be equal or different from each other.

The unbonded flexible pipe included in the installation may comprise both pressure armor and tensile armor. If this is the case the one or more pressure armor layers are wound around the internal pressure structure and the one or more tensile armor layers are wound around the pressure armor. The unbonded flexible pipe may comprise at least one pressure armor layer and two tensile armor layers. Preferably the two tensile armor layers are cross-wound.

Although the external armor layers may be made from any suitable material such as steel, stainless steel and fiber reinforced polymer, the one or more external armor layers are preferably made from carbon steel. Preferably the external armor layers are located in an annulus of the pipe and protected by the internal pressure sheath and an outer sheath.

Consequently, the flexible pipe preferably comprises an outer polymer layer to form an outer sheath. The outer sheath and the internal pressure sheath form together an annulus in which the armor layers, such as pressure armor and tensile armor can be located and protected from ingress of water.

The present invention also relates to the use of a stocking on at least one length of a flexible pipe, where the flexible pipe comprises from the inside and out an internal pressure sheath, one or more armor layers and an outer sheath where the outer sheath is provided with the stocking such that the stocking is surrounding the outer sheath and comprising individual fiber straps braided around the outer sheath in such a way that the stocking will tighten around the flexible when subjected to pull and the stocking is adapted for long-term connection with a tether line, thereby connecting the flexible pipe to the tether line.

In an embodiment, the flexible pipe is arranged between a seabed installation and a sea surface installation and wherein the stocking is connected to the tether line, which tether line is anchored to the seabed or a seabed installation via an anchoring arrangement. In an embodiment, the riser pipe is connected to the seabed installation, such as an installation at a production site and the tether line is connected to the same seabed installation.

The angle between the tether line and the pipe section surrounded by the stocking is advantageously less than 45 degrees, such as less than about 30 degrees, such as less than about 20 degrees.

The stocking may conveniently be connected to the tether line at a water depth of at least 100m, such as at least 500 m.

The invention also comprises a method of tethering a subsea flexible pipe. The method comprises applying a stocking to surround an outer sheath of the flexible pipe, wherein the stocking comprises individual fiber straps braided around the outer sheath providing that the stocking will tighten around the flexible pipe when subjected to pull and connecting the stocking to at least one tether line.

In an embodiment, the method comprises attaching the stocking to the flexible pipe during production of the pipe and before the pipe is reeled up for transportation.

In an embodiment, the stocking is mounted to the flexible pipe when the flexible pipe is installed offshore.

The method may conveniently comprise attaching the stocking to an outer sheath of the flexible pipe at a first end portion of the stocking, terminating the fiber straps of the stocking in one or more connecting devices at an opposite second end portion of the stocking and connecting the stocking to said at least one tether line via said connecting device(s).

In an embodiment the stocking is adapted with means, such as coupling or rings, allowing connection to a tether line. The coupling or rings are preferably attached to the stocking at one end of the stocking and connected with the stocking by means of the fiber straps. A part of or all of the braided individual fiber straps can be collected and connected to the coupling or ring. A stocking for use according to the invention can be attached to one, two or more couplings or rings. The couplings or rings are preferably made from metallic material such as stainless steel.

DETAILED DESCRIPTION OF THE INVENTION

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

Fig. 1 illustrates an example of an unbonded flexible pipe suitable for an installation of an embodiment of the invention; Fig. 2 shows an installation with a flexible pipe according to an embodiment of the invention;

Fig. 3 shows a part of an embodiment of an installation comprising a flexible pipe with a stocking;

Fig. 4 shows part of an embodiment of an installation comprising a flexible pipe with a stocking; and

Fig. 5 shows part of a further embodiment of an installation comprising a flexible pipe with a stocking.

The drawings are intended to illustrate the principles of the invention and are not dimensionally stable or accurate. The same or corresponding reference numbers are used for the same parts in the drawings.

Figure 1 shows a flexible pipe 1 having a longitudinal centre axis x-x. From the inside to the outer side the flexible pipe comprises a carcass 10 supporting an internal pressure sheath 11. The internal pressure sheath 11 is reinforced with a pressure armor layer 12 and two tensile armor layers 13,

14. The outer surface of the flexible pipe 1 is constituted by an outer sheath

15. The internal pressure sheath 11 and the outer sheath 15 are made from material which is substantially fluid tight, and the annular space between the internal pressure sheath 11 and the outer sheath 15 is the annulus. The annulus houses the armor layers 12, 13 and 14.

In the illustrated flexible pipe 1 the internal pressure sheath 11 is made from polyethylene and the outer sheath 15 is made from polyamide. The carcass 10 is made from stainless steel; the pressure armor 12 and the tensile armor 13, 14 is made from carbon steel. Figure 2 illustrates an installation according to the invention. The flexible pipe 1 is installed between a floating storage facility 2 at sea level 3 and a production facility 4 at the sea bottom 5.

The flexible pipe 1 is mounted with buoys 6 allowing the flexible pipe to form an "S"- shape between the floating storage facility 2 and the production facility 4. For controlling the movements of the flexible pipe 1, the pipe is attached to anchors 8a, 8b and 8c at the sea bed 5 by means of tether lines 7a, 7b and 7c. Moreover, the flexible pipe is attached to the floating storage facility 2 by means of the tether line 7d.

The tether lines 7a, 7b, 7c and 7d are each connected to a stocking 20a, 20b, 20c and 20d on the flexible pipe 1.

The stockings 20a, 20b, 20c and 20d provide and convenient and cost- effective way of attaching the flexible pipe 1 with the tether lines 7a, 7b, 7c and 7d, which allow the pipe to have sufficient freedom of movement, while at the same time being controlled by the tether lines.

Figure 3 shows a flexible pipe 1 with a stocking 20 mounted on the outer surface of the outer sheath 15. The stocking 20 is made from fiber straps or treads 21 of fiber material which are braided into a net like structure forming the stocking 20 around the flexible pipe 1.

At one end 22 the treads 21 are collected in a collar like structure terminating the stocking. The collar like structure may by attached to the outer sheath 15, e.g. by clamping, gluing or other means thereby anchoring the end 22 of the stocking 20 to the flexible pipe 1. At the other end of the stocking 20 the straps 21 are collecting in a connection device 23. The connection device 23 is adapted for connection with a tether line (not shown in figure 3).

Figure 4 illustrates the connection device 23 connected to a tether line 7 via an eyelet piece 24. The straps 21 are collected in the connection device 23 and when subjected to a pull force from the tether line 7 this pull will be transferred to the straps 21 of the stocking 20 which due to the net-like pattern of the braiding will tighten up around the surface of the outer sheath 15 and enhance its grip on the flexible pipe 1.

Consequently, the installation comprising the stocking provides a very efficient attachment of the tether line 7 to the flexible pipe 1, and the installation according to the invention provides a very efficient, reliable and cost-effective way for long-time attachment of a flexible pipe with a tether line.

Figure 5 shows a section of a flexible pipe with a stocking 20 mounted on the outer surface of the outer sheath 15. The stocking is is attached to the outer sheath at an anchoring site at a first end portion of the stocking using a metallic collar 22. A protection sleeve 16a is located between the outer sheath and the stocking at at the anchoring site to protect the outer sheath and/or to provide a distribution of load.

The protection sleeve may e.g. be a wound protection band, such as a woven adhesive tape and/or two half shells of a suitable material.

At the opposite end of the stocking the fiber straps of the stocking are collected in a or forming a connecting device 23 for being connected to a tether line

A further protection sleeve 16b is located between the outer sheath and the connecting device 23, to provide a protection of the outer sheath. This is preferred in particularly where the connection between the tether line and the connection device comprises abrasive or hard elements, such as a metallic lock.

The stocking has a first section 20e closer to the connecting device 23 a second middle section 20f further from the connecting device 23 and a third section adjacent to the anchoring site. As it can be seen the fiber straps are wound with a larger pitch in the first section 20e than in the second section 20f and with a larger pitch in the second section 20f than in the third section 20g. Bands 24 are located to hold the straps at desired locations.

Claims

1. An offshore installation comprising a subsea flexible pipe, said flexible pipe comprises from the inside and out an internal pressure sheath defining a bore, one or more armor layers and an outer sheath, the installation further comprises a stocking surrounding the outer sheath in a length section of the flexible pipe, wherein the stocking comprises individual fiber straps braided around the outer sheath providing that the stocking is at least partly tightened around the flexible pipe and wherein the pipe is tethered via at least one tether line to a seabed and/or a subsea installation via said stocking.

2. The offshore installation according to claim 1, wherein the fiber straps form helically wound biaxial braid, preferably braided according to the "Chinese finger" principle.

3. The offshore installation according to claim 1 or claim 2, wherein the fiber straps comprise a first set of fiber straps and a second set of fiber straps, wherein the first set of fiber straps is travelling in a first helical direction around the outer sheath and the second set of fiber straps is travelling in an opposite second helical direction around the outer sheath, wherein the first set of fiber straps are braided by the second set of fiber straps by passing under and over straps of the second set of fiber straps.

4. The offshore installation according to claim 3, wherein each set of fiber straps comprises 1-10 fiber straps, such as 2-5 fiber straps and wherein the number of fiber straps in the first set of fiber straps and a second set of fiber straps is identical.

5. The offshore installation according to claim 3 or claim 4, wherein the fiber straps is helically wound with a substantially constant pitch.

6. The offshore installation according to claim 3 or claim 4, wherein the fiber straps is helically wound with a varying pitch along the length of the pipe, wherein the winding pitch closer to the tether line is longer than the winding pitch further from the tether line.

7. The offshore installation according to any one of the preceding claims, wherein the stocking is tightened around the flexible pipe by a pulling force applied to said straps, said pulling force advantageously being applied by the tethering line pulling in the stocking.

8. The offshore installation according to any one of the preceding claims, wherein the stocking is attached to the outer sheath by one or more fastening means.

9. The offshore installation according to any one of the preceding claims, wherein the stocking can resist a pull (force) from the tether line of at least about 50 kN, such as a pull (force) of at least about 100 kN, such as from about 200 kN to about 900 kN, such as about 300 kN to about 800 kN.

10. The offshore installation according to any one of the preceding claims, wherein the flexible pipe is provided with two or more stockings

11. The offshore installation according to any one of the preceding claims, wherein the stocking is provided by 8 to 20 braided individual fiber straps, preferably each of the first set of fiber straps and the second set of fiber straps comprises individually from each other 1 to 10 fiber straps.

12. The offshore installation according to any one of the preceding claims, wherein the stocking extends from about 1 m to about 10 m along the length of the flexible pipe, preferably the stocking is attached to the outer sheath at a first end portion of the stocking further away from the tether line than a second end portion of the stocking, preferably the first end portion further away of the stocking is up to about half the stocking extending length, such as up to about 25 % of the stocking extending length.

13. The offshore installation according to any one of the preceding claims, wherein the stocking has a first end portion and a second end portion, wherein the second end section being located closer to the seabed and/or subsea installation than the first end portion of the stocking and wherein the stocking being connected to the tether line at the second end section of the stocking.

14. The offshore installation according to any one of the preceding claims, wherein the fiber straps of the stocking at the second end thereof are terminated in one or more connecting devices which is/are connecting the stocking to the at least one tether line, preferably the fiber straps of the stocking at the second end thereof are terminated in one single connecting devices which is connected to the tether line.

15. The offshore installation according to any one of the preceding claims, wherein the stocking is provided on the flexible pipe near the sea-surface.

16. The offshore installation according to any one of the preceding claims, wherein the stocking is provided on the flexible pipe near the sea-bed.

17. The offshore installation according to any one of the preceding claims, wherein the stocking is at least partly made from polymer material such as polya ramide.

18. The offshore installation according to any one of the preceding claims, wherein the stocking is at least partly made from metallic material.

19. The offshore installation according to any one of the preceding claims, wherein the stocking is at least partly made from thermoplastic material.

20. The offshore installation according to any one of the preceding claims, wherein the flexible pipe is arranged between a seabed installation and a sea surface installation and wherein the stocking is connected to the tether line, which tether line is anchored to the seabed or the seabed installation via an anchoring arrangement.

21. The offshore installation according to any one of the preceding claims, wherein the stocking is connected to the tether line at a water depth of at least 100m, such as at least 500 m.

22. The offshore installation according to any one of the preceding claims, wherein the flexible pipe is of the unbonded type and wherein the pipe is arranged for transporting oil and/or gas.

23. A method of tethering a subsea flexible pipe the method comprising applying a stocking to surround an outer sheath of the flexible pipe, wherein the stocking comprises individual fiber straps braided around the outer sheath providing that the stocking will tighten around the flexible pipe when subjected to pull and connecting the stocking to at least one tether line.

24. The method of claim 23, wherein the method comprises attaching the stocking to the flexible pipe during production of the pipe and before the pipe is reeled up for transportation.

25. The method of claim 23 or claim 24, wherein the stocking is mounted to the flexible pipe when the flexible pipe is installed offshore.

26. The method of any one of claims 23-25, wherein the method comprises attaching the stocking to an outer sheath of the flexible pipe at a first end portion of the stocking, terminating the fiber straps of the stocking in one or more connecting devices at an opposite second end portion of the stocking and connecting the stocking to said at least one tether line via said connecting device(s).