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Title:
A SYSTEM AN METHOD FOR STABILIZING A RISER
Document Type and Number:
WIPO Patent Application WO/2021/006743
Kind Code:
A1
Abstract:
A system and a method for stabilizing a drilling pipe and/or production riser (3) extending between a wellhead coupling (311) of a subsea well (1) and a well valve assembly (22) on a surface installation (2) which is floating in a water mass (4) or is supported on a seabed (51) and projects up above the water mass (4), wherein the wellhead coupling (311) forms a fluid-tight connection of a high-pressure barrier (12) of the well (1) and the drilling pipe and/or production riser (3), and wherein the drilling pipe and/or production riser (3) is subjected to a horizontal load component (Lh) that generates a bending moment (Mw) around the wellhead coupling (311), the system being c h a r a c t e r i z e d i n that it comprises: - a foundation (14) to be positioned on the seabed (51); and - a supporting structure (32) arranged to be connected to a lower portion (31) of the drilling pipe and/or production riser (3), the supporting structure (32) projecting outwards from the drilling pipe and/or production riser (3) and cooperating with the foundation (14) to absorb a portion of the bending moment (Mw) around the wellhead coupling (311).

Inventors:
MATHIS WOLFGANG (NO)
Application Number:
PCT/NO2020/050172
Publication Date:
January 14, 2021
Filing Date:
June 22, 2020
Export Citation:
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Assignee:
NEODRILL AS (NO)
International Classes:
E21B17/01
Foreign References:
US4854781A1989-08-08
US4556340A1985-12-03
US4363567A1982-12-14
US3983706A1976-10-05
Attorney, Agent or Firm:
HÅMSØ PATENTBYRÅ AS (NO)
Download PDF:
Claims:
C l a i m s

1. A system for stabilizing a drilling pipe and/or production riser (3) extending between a wellhead coupling (311) for a subsea well (1) and a well valve as sembly (22) on a surface installation (2) which is floating in a water mass (4) or is supported on a seabed (51) and projects up above the water mass (4), the wellhead coupling (311) forming a fluid-tight connection of a high- pressure barrier (12) of the well (1) and the drilling pipe and/or production riser (3), and the drilling pipe and/or production riser (3) being subjected to a horizontal load component (Lh) that generates a bending moment (M ) around the wellhead coupling (311), the system being c h a r a c t e r i z e d i n that it comprises:

- a foundation (14) to be positioned on the seabed (51); and

- a supporting structure (32) to be connected to a lower portion (31) of the drilling pipe and/or production riser (3), the supporting structure (32) pro jecting outwards from the drilling pipe and/or production riser (3) and coop erating with the foundation (14) to absorb a portion of the bending moment (Mw) around the wellhead coupling (311).

2. The system in accordance with claim 1, wherein the supporting structure (32) is provided with abutments (321) abutting supportingly against the foundation (14) at a radial distance from the wellhead (13).

3. The system in accordance with any one of the preceding claims, wherein the connection between the supporting structure (32) and the drilling pipe and/or production riser (3) is formed as a clearance-free connection.

4. The system in accordance with claim 3, wherein the supporting structure (32) comprises a coupling (322) formed as a sleeve for surrounding a por tion of the drilling pipe and/or production riser (3) in a press fit.

5. The system in accordance with claim 3, wherein the supporting structure (32) comprises a coupling (322) formed as a sleeve that is shrunk around a portion of the drilling pipe and/or production riser (3).

6. The system in accordance with any one of the preceding claims, wherein the foundation (14) is a suction foundation which is provided with a housing (141) with an open bottom and a top that projects up above the seabed (51) and is closed with a top cover (142).

7. The system in accordance with claim 6, wherein the suction foundation (14) surrounds the wellhead (13), and the top cover (142) is connected to the lower portion (31) of the drilling pipe and/or production riser (3) and forms said supporting structure.

8. The system in accordance with claim 6 or 7, wherein an upper portion of a low-pressure barrier (11) and an upper portion of a high-pressure barrier (12) of the well (1) are supported in a supporting pipe (143) in the suction foundation (14).

9. A method for stabilizing a drilling pipe and/or production riser (3) that ex tends between a wellhead coupling (311) for a subsea well (1) and a well valve assembly (22) on a surface installation (2) which is floating in a water mass (4) or is supported on a seabed (51) and projects up above the water mass (4), the drilling pipe and/or production riser (3) being subjected to a horizontal load component (Lh) that generates a bending moment (M ) around the wellhead coupling (311), the method comprises the steps of: a) positioning an foundation (14) on the seabed (51);

b) establishing a low-pressure barrier (11) and a high-pressure barrier (12) in the well (1);

c) forming a wellhead (13) that projects up above the seabed (51); d) connecting the high-pressure barrier (12) of the well (1) to a drilling pipe and/or production riser (3) by means of a fluid-sealing wellhead cou pling (311); and

e) connecting the drilling pipe and/or production riser (3) to a well valve assembly (22) on the surface installation (2), c h a r a c t e r i z e d i n that the method comprising the further step of:

f) connecting a supporting structure (32) to a lower portion (31) of the drilling pipe and/or production riser (3), the supporting structure (32) pro jecting outwards from the drilling pipe and/or production riser (3) and coop erating with the foundation (14) in such a way that a portion of the bending moment (Mw) around the wellhead coupling (311) is absorbed by the sup porting structure (32) and the foundation (14).

10. The method in accordance with claim 9, wherein the coupling between the supporting structure (32) and the drilling pipe and/or production riser (3) is formed as a clearance-free connection.

11. The method in accordance with claim 10, wherein the supporting structure (32) comprises a coupling (322) formed as a sleeve surrounding a portion of the drilling pipe and/or production riser (3) in a press fit.

12. The method in accordance with claim 10, wherein the supporting structure (32) surrounds a coupling (322) formed as a sleeve that is shrunk around a portion of the drilling pipe and/or production riser (3).

13. The method in accordance with any one of claims 9-12, wherein the founda tion (14) is a suction foundation which is provided with a housing (141) with an open bottom and a top that projects up above the seabed (51) and is closed with a top cover (142).

14. The method in accordance with claim 13, wherein the suction foundation (14) surrounds the wellhead (13), and the top cover (142) is connected to the lower portion (31) of the drilling pipe and/or production riser (3) and forms said supporting structure. 15. The method in accordance with claim 13, wherein an upper portion of a low- pressure barrier (11) and an upper portion of a high-pressure barrier (12) in the well (1) are supported in a supporting pipe (143) in the suction founda tion (14).

Description:
A SYSTEM AN METHOD FOR STABILIZING A RISER

The invention relates to a system for stabilizing a drilling pipe and/or production riser which extends between a subsea well and a surface installation which is floating in a water mass. In connection with the subsea well, a wellhead will project up above an unconsolidated mass that forms a seabed. A wellhead coupling forms a fluid-tight con nection of the high-pressure barrier of the well and the drilling pipe and/or production riser. In the operative position, the drilling pipe and/or production riser is connected to a well valve assembly on the surface installation.

A method for stabilizing a drilling pipe and/or production riser is described as well. When a subsea well is established, for example a well for recovering hydrocarbons, adjacent wells may become destabilized. This may happen when, initially, a hole is established in the unconsolidated mass in the seabed for the insertion of a conductor, which is to define a passage for casing or some other low-pressure barrier through the unconsolidated mass, and a foundation is formed for a wellhead and a riser extending upwards from the wellhead. The hole in the unconsolidated mass is often formed by unconsolidated mass being flushed loose and removed from a portion ahead of (be low) a lower end of the conductor, so that the conductor can be lowered successively into the hole. The removal of the unconsolidated mass may result in the seabed be coming unstable so that the lateral support of adjacent wells against the surrounding unconsolidated mass is weakened.

This problem is particularly relevant when a group of established and planned wells are connected to a so-called tension-leg platform (TLP). A tension-leg platform is a floating platform that is vertically anchored to the seabed by means of several tension legs that hold the platform in position above the seabed independently of the tide and waves. High-pressure well pipes and the drill string extend through a so-called drilling pipe and/or production riser. For the sake of simplicity, the term "riser" will be used herein for a drilling pipe and/or production riser. The riser extends between a wellhead on the seabed and the platform where, among other things, well valves and blowout preventers are arranged. As a rule, tension-leg platforms are used at great water depths, typically from 300 metres and deeper. The connection between the riser and the wellhead, that is the connection of a low-pressure pipe and a high-pressure pipe (casing and tubing) at the seabed, is subjected to great strains because of water cur rents and horizontal drift of the platform combined with the specific weight of the ris er. There is therefore an expressed need for reducing the risk of fatigue fault at the transition between the riser and the wellhead.

Similar sets of problems are associated with jack-up platforms which rest on the sea bed in shallow waters.

Stabilizing the well valve arrangement, typically a blowout preventer (BOP), which forms a transition between the wellhead and the riser according to the prior art, solves neither the problem with the stability of the conductor itself relative to the surrounding unconsolidated mass in the seabed nor the problem of fatigue failure in the transition between the wellhead and the riser owing to the deflection of the riser caused by hori zontal load components of various origins to which the riser is subjected.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art or at least provide a useful alternative to the prior art.

The object is achieved through the features that are specified in the description below and in the claims that follow.

The invention provides a system and a method for reducing the strain on a wellhead by a riser that extends upwards from the wellhead being stabilized by means of a sup porting structure that projects radially outwards from the drilling pipe and/or produc tion riser and cooperates with the foundation. The foundation of the well may be formed of a concrete or metal structure resting on the seabed and forming a large contact surface with the unconsolidated mass, or one or more suction foundations that have been pushed into the unconsolidated mass under the seabed, for example a con ductor anchor node (CAN) of the kind that is disclosed in NO patent No. 313340.

A suction foundation has a housing with an open bottom and a top that is closed in a fluid-sealing manner with top cover. When one suction foundation is used as the base for said supporting structure, the suction foundation is preferably provided with a sup porting pipe which is open at both ends and extends from an opening in the top cover and at least towards the bottom of the suction foundation, the supporting pipe fitting fluid-sealingly against the top cover of the suction foundation and having a length that is smaller than, equal to or larger than the height of the suction foundation. The suc tion foundation is typically cylinder-shaped with a diameter of about 4-10 metres, whereas the diameter of a conductor typically lies in the range of 0.75-1.20 metres (30-48 inches). The suction foundation exhibits a regular, well defined interface with the surroundings. The base area of the suction foundation is typically circular but may, per se, also be polygonal, and the dimensions are known to the operator of the well.

The well is established by a conductor being moved downwards in the unconsolidated mass in a suitable manner known per se, until a sufficient depth has been achieved. Then low-pressure and high-pressure barriers are established in the form of pipes ex tending from above the seabed into the underground. A wellhead is established above the seabed, comprising necessary elements for hanging casing and tubing, among other things, and elements for connecting the pipes of the well and a marine riser which extends from the wellhead up to a platform on the surface where the riser is connected to the necessary well valves and blowout preventers.

If a suction foundation with a supporting pipe is used, the conductor can be lowered through the supporting pipe, which provides support for the conductor. From the state of the art, solutions are known in which a suction foundation with a low-pressure housing replaces a conventional, long conductor. "Conductor" as used herein is also meant to cover solutions of that kind.

Extending from a portion of the riser, there is a supporting structure which is arranged to transmit, to the seabed, a vertical load component that arises when the riser is de flected by horizontal load components. If the foundation is a suction foundation, the load is typically transmitted to portions of the top cover of the suction foundation, near the periphery of the suction foundation or other braced or reinforced portions of the top cover. The supporting structure is preferably secured or abuts against protrud ing portions of a frame surrounding said riser portion at a distance above the well head. Thereby the riser is stabilized by being supported directly in the foundation at a horizontal distance from the wellhead. The advantage of this is that the strains on the wellhead caused by horizontal forces, including cyclic loads, to which the riser is sub jected are eliminated to a great extent through the load transmission to the founda tion. By the use of a suction foundation, the risk of instability in the wellhead when an adjacent well is established is reduced. This is because of the large contact surface of the suction foundation against the surrounding unconsolidated mass and the ability of the suction foundation to prevent unconsolidated mass from being washed out during the establishing of a new well through the suction foundation. A suction foundation that projects up above the wellhead may be used as well, typical ly by the suction foundation being installed after the well has been established. The top of the suction foundation may then be connected to the riser and, together with the housing of the suction foundation, form said supporting structure.

The invention is defined by the independent claim. The dependent claims define ad vantageous embodiments of the invention.

In a first aspect, the invention relates more specifically to a system for stabilizing a drilling pipe and/or production riser that extends between a wellhead coupling for a subsea well and a well valve assembly on a surface installation which is floating in a water mass or is supported on a seabed and projects up above the water mass, the wellhead coupling forming a fluid-tight connection of a high-pressure barrier in the well and the drilling pipe and/or production riser, and the drilling pipe and/or produc tion riser being subjected to a horizontal load component that generates a bending moment around the wellhead coupling, the system being characterized by comprising :

- a foundation to be positioned on the seabed; and

- a supporting structure to be connected to a lower portion of the drilling pipe and/or production riser, the supporting structure projecting outwards from the drilling pipe and/or production riser and cooperating with the foundation to absorb a portion of the bending moment around the wellhead coupling.

It is a substantial portion of the bending moment that is absorbed by the system spec ified in the first aspect.

The supporting structure may be provided with abutments that abut supportingly against the foundation at a radial distance from the wellhead.

The connection between the supporting structure and the drilling pipe and/or produc tion riser may be formed as a clearance-free connection. To achieve a clearance-free connection, the supporting structure may comprise a coupling formed as a sleeve that surrounds a portion of the drilling pipe and/or production riser in a press fit. Alterna tively, the supporting structure may comprise a coupling formed as a sleeve that has been shrunk around a portion of the drilling pipe and/or production riser.

There are various alternative well foundations. The foundation may be, for example, a template or a mud mat.

The foundation may be a suction foundation which is provided with a housing with an open bottom and a top that projects up above the unconsolidated mass and is closed with a top cover.

The suction foundation may enclose the wellhead, and the top cover may be connect ed to the lower portion of the drilling pipe and/or production riser and form said sup porting structure. An upper portion of a low-pressure barrier and an upper portion of a high-pressure barrier in the well may be supported in a supporting pipe in the suction foundation. Thereby the wellhead is further stabilized .

In a second aspect, the invention relates more specifically to a method for stabilizing a drilling pipe and/or prod uction riser that extends between a wellhead coupling for a subsea well and a well valve assembly on a surface installation which is floating in a water mass or is supported on a seabed and projects up above the water mass, the drilling pipe and/or prod uction riser being subjected to a horizontal load component that generates a bending moment around the wellhead coupling, the method compris ing the steps of:

a) positioning a foundation on the seabed ;

b) establishing a low-pressure barrier and a high-pressure ba rrier in the well ; c) forming a wellhead that projects up above the seabed ;

d) connecting the high-pressure barrier of the well to a d rilling pipe and/or pro duction riser by means of a fluid-sealing wellhead coupling ; and

e) connecting the drilling pipe and/or production riser to a well valve assembly on the surface installation, characterized by the method comprising the further step of: f) connecting a supporting structure to a lower portion of the drilling pipe and/or production riser, the supporting structure projecting outwards from the drilling pipe and/or production riser and cooperating with the foundation in such a way that a por- tion of the bending moment around the wellhead coupling is absorbed by the support ing structure and the foundation.

The connection between the supporting structure and the drilling pipe and/or produc tion riser may be formed as a clearance-free connection . To achieve a clearance-free connection, the supporting structure may comprise a coupling formed as a sleeve that surrounds a portion of the drilling pipe and/or production riser in a press fit, or as a sleeve that has been shrunk around a portion of the drilling pipe and/or production riser.

The foundation may be a suction foundation which is provided with a housing with an open bottom and a top that projects up above the unconsolidated mass and is closed with a top cover.

The suction foundation may enclose the wellhead, and the top cover may be connect ed to the lower portion of the drilling pipe and/or production riser and form said sup porting structure. An upper portion of a low-pressure barrier and an upper portion of a high-pressure barrier in the well may be supported in a supporting pipe in the suction foundation.

The ratio of a bending moment absorbed by the supporting structure to the calculated bending moment around the wellhead coupling may be at least 1 :2, alternatively at least 3:4, alternatively at least 9: 10. Thereby, the extent to which the wellhead cou- pling is to be relieved to give the wellhead coupling an optimum dimensioning can be controlled.

In what follows, an example of a preferred embodiment is described, which is visual ized in the accompanying drawings, in which:

Figure 1 shows, in a side view, a principle drawing of a tension-leg platform con- nected to three wells via marine drilling pipes or production risers;

Figure 2 shows a side view, on a larger scale, of a portion of a well provided with a first embodiment of a system for stabilizing a riser;

Figure 2a shows, in section, a portion of a riser support provided with a yielding portion; Figure 3 shows a side view of a portion of a well provided with a second embodi ment of a system for stabilizing a riser;

Figure 4 shows a side view of a portion of a well provided with a third embodi ment of a system for stabilizing a riser;

Figure 5 shows a side view, on a smaller scale, of a portion of a well provided with a fourth embodiment of a system for stabilizing a riser;

Figure 6 shows a side view of a portion of a well provided with a fifth embodi ment of a system for stabilizing a riser; and

Figure 7 shows, highly simplified, the elements that absorb strain when a cou pling portion between a wellhead and a drilling pipe or production riser is subjected to a bending moment from a horizontal load component. Reference is first made to figure 1, in which several subsea wells 1 extend downwards from a seabed 51 and are connected to an installation 2, known per se, which is float ing on a water surface 41, shown here as a tension-leg platform which is vertically anchored to the seabed 51 by means of several tension legs 21 extending through a water mass 4 to respective tension-leg anchors 211 that are fixed in an unconsolidat ed mass 5 under the water mass 4. The wells 1 are connected to the tension-leg plat form 2 by means of respective risers 3 forming low-pressure barriers to the surround ings and being used for passing a drill string (not shown) and other equipment when the wells 1 are being established and maintained and for passing conveying pipes for well fluids (not shown) by production from the wells 1. To each of the risers 3 there are connected necessary well valve assemblies 22 on the tension-leg platform 2. The well valve assemblies 22 comprise all types of valves and safety devices that are nec essary for controlling the production from and the maintenance of the wells 1, includ ing blowout preventers.

Currents in the water mass 4 are visualized by the arrows 42. The currents 42, wind and waves (not shown) can cause horizontal drift by the riser platform 2 from its nor mal position above the wells 1.

Reference is now made to figure 2, in which a first exemplary embodiment of the sys tem for stabilizing the riser 3 is shown, an upper portion of a well 1 being shown in greater detail. A low-pressure barrier 11 in the form of a conductor and/or casing ex tends in a manner known per se from a foundation 14 and down through the uncon solidated mass 5 into a borehole (not shown) in an underlying underground structure (not shown). A high-pressure barrier 12 in the well 1 in the form of a well pipe (tub ing) is arranged internally in the low-pressure barrier 11. The low- and high-pressure barriers 11, 12 project up above the foundation 14 and form a wellhead 13.

By means of a wellhead coupling 311, the wellhead 13 forms a fluid-tight connection of the high-pressure barrier 12 of the well 1 and a lower portion 31 of the riser 3.

At a distance from the wellhead coupling 311, the lower riser portion 31 is supported against the foundation 14 by means of a supporting structure 32 which comprises abutments 321, shown here as several supports 321 distributed around the lower riser portion 31. The supports 321 abut supportingly against the foundation 14. A support ing-structure coupling 322 forms the connection between the supporting structure 32 and the riser 3. In a preferred embodiment, three to eight supports 321 are used.

Reference is now made to figure 3 in which a second exemplary embodiment of the system for stabilizing the riser 3 is shown. Here, the foundation 14 is formed as a suc tion foundation 14 with a housing 141 with an open bottom and a top that is closed with a top cover 142. The suction foundation 14 is provided with a supporting pipe 143 extending through it, fitting tightly against the top cover 142. The low-pressure barrier 11 of the well 1 in the form of a conductor and/or casing extends in a manner known per se through the supporting pipe 143 down through the unconsolidated mass 5 and down into the borehole (not shown) in the underlying underground structure (not shown). The high-pressure barrier 12 of the well 1 in the form of a well pipe (tub ing) is arranged internally in the low-pressure barrier 11. The low-pressure and high- pressure barriers 11, 12 project up above the top cover 142 of the suction foundation 14 and form the wellhead 13.

By means of the wellhead coupling 311, the wellhead 13 forms a fluid-tight connection of the high-pressure barrier 12 of the well 1 and the lower portion 31 of the riser 3.

At a distance from the wellhead coupling 311, the lower riser portion 31 is supported against the suction foundation 14 by means of the supporting structure 32 which comprises abutments 321, shown here as several supports 321 distributed around the riser portion 31, which abut supportingly against an edge portion of the top of the suc tion foundation 14. A supporting-structure coupling 322 forms the connection between the supporting structure 32 and the riser 3. Reference is now made to figure 4, in which a third exemplary embodiment of the sys tem for stabilizing the riser 3 is shown. A suction foundation 14 with a housing 141 with an open bottom and a top that is closed with a top cover 142 accommodates the wellhead 13. The suction foundation 14 projects up above the seabed 51, and a por tion of the housing 141 extends downwards in the unconsolidated mass 5. A portion 31 of the riser 3 is supported in the top cover 142 of the suction foundation 14. In this embodiment, the suction foundation 14 forms both the supporting structure and the foundation of the system.

Reference is now made to figure 5, in which a fourth exemplary embodiment of the system for stabilizing a riser 3 is shown. A template 14a covering several wells 1 forms a foundation for the risers 3 of all the wells 1, the supporting structures 32 of the risers 3 being supported on the template 14a. In this embodiment, the template 14a is fixed to several foundations 14 placed on the seabed 51. It may be an ad vantage if the foundations 14 are fixed in the unconsolidated mass 5 under the seabed 51 by means of several poles 14b that have been hammered down through cut-outs in the foundations 14 and into the unconsolidated mass 5, as is shown in the right-hand part of figure 5.

Figure 6 shows a variant of the exemplary embodiment according to figure 5, the template 14a in this fifth exemplary embodiment being secured to several suction foundations 14 that have been inserted, in a manner known per se, into the unconsol- idated mass 5 under the seabed 51.

It is an advantage if the supporting structure 32 is secured to the foundation 14 or the template 14a in such a way that both compressive and tensile forces may be transmit ted between the supporting structure 32 and the foundation 14.

The currents 42 in the water mass 4, the horizontal drift of the riser platform 2 from its normal position over the wells 1 and the weight of the riser 3 result in a horizontal load component that without the use of the system according to the invention would have subjected the wellhead coupling 311 to a bending moment M , see figure 7. The riser 3 and the supporting structure 32 are connected to each other in a way that makes it possible for the supporting structure 32 to absorb a bending moment Mf as a reaction to said deflection of the riser 3 by the imposed load component U, as a vertical load component L v is transmitted to the foundation 14. The supporting- structure coupling 322 may be arranged in such a way that the lower portion 31 of the riser 3 may exhibit a certain deflection before it hits the supporting structure 32 and the further load is substantially absorbed by the supporting structure 32. The design of the supporting-structure coupling 322 and the dimensioning of the supporting struc ture 32 can thereby be used to control how great a load the well-head coupling 311 and the wellhead 13 may be subjected to. If the supporting-structure coupling 322 is formed in such a way that it does not permit movement between the riser 3 and the supporting structure 32, the wellhead coupling 311 will practically not be subjected to loads caused by deflection of the riser 3.

Reference is now made to figure 2a. The portion of load transmission may be con trolled by each of the supports 321 comprising a yielding portion 321a, for example in the form of a hydraulic cylinder 321a which, by great strain, may yield by safety valves or accumulators (not shown) in an associated hydraulic system (not shown) allowing hydraulic fluid to escape from the cylinder 321a when the load exceeds a set limit value, so that the support 321 yields to the load, and the hydraulic cylinder 321a returns to its normal position when the load falls below said limit value.

Figure 7 shows the statics of the supporting structure 32 in principle. Solid, sloping connecting lines between horizontal and vertical lines indicate that the connection is rigid. Broken, sloping connecting lines indicate that the connection may allow a limited relative movement, as has been described for the supporting-structure coupling 322 above.

When the supporting structure 32 that is attached to the lower portion 31 of the riser 3 is subjected to a horizontal load component , the supporting structure 32 is sub jected to a vertical load U, which is transmitted to the suction foundation 14 at a dis tance from the centre axis of the well 1 through the abutment of the supporting struc ture 32 against the suction foundation 14, as is shown in the right-hand part of figure 7. In addition, an opposite portion of the supporting structure 32, if appropriately at- tached to the suction foundation 14, may be subjected to a vertical tensile force, as is indicated in the left-hand part of figure 7. Broken-line arrows indicate resulting reac tion forces arising in the suction foundation 14. Mf is the portion of the bending mo ment Mw that is absorbed by the supporting structure 32. Without the use of the in vention described herein, the entire bending moment M would have to be absorbed by the wellhead coupling 311. How large the portion Mf will be, will vary, depending on how great a deviation by the riser 3 the supporting structure 322 will allow before the riser 3 is laid into supporting abutment against the supporting structure 32. Calcula tions show that it is perfectly possible to dimension the supporting structure 32 for absorbing at least 9/10 of the bending moment M that the wellhead coupling 311 had to absorb in a conventional design of the wellhead 13 and its adjacent elements.

In addition to the fact that the system according to the invention relieves the wellhead coupling 311, the use of a foundation in the form of the suction foundation 14 accord ing to the second exemplary embodiment of the system reduces the risk of destabiliz ing established wells 1 by later establishing of further wells within or near the group of established wells 1. This is because the suction foundations 14 both give better sup port of the wells 1 against the unconsolidated mass 5 and reduce the risk of washing out unconsolidated mass 5 when the low-pressure barrier 11 of a new well is inserted into the unconsolidated mass 5 through the supporting pipe 143 of the suction founda tion 14. It should be noted that all the above-mentioned embodiments illustrate the invention, but do not limit it, and persons skilled in the art may construct many alternative em bodiments without departing from the scope of the attached claims. In the claims, reference numbers in brackets are not to be regarded as restrictive.

The use of the verb "to comprise" and its different forms does not exclude the pres- ence of elements or steps that are not mentioned in the claims. The indefinite article "a" or "an" before an element does not exclude the presence of several such elements.