WO2009093911A1

WO2009093911A1 - A device in a combined wellhead/pipe string

Info

Publication number: WO2009093911A1
Application number: PCT/NO2009/000024
Authority: WO
Inventors: Espen Pettersen
Original assignee: VASSHELLA AS
Current assignee: VASSHELLA AS
Priority date: 2008-01-25
Filing date: 2009-01-21
Publication date: 2009-07-30
Anticipated expiration: 2010-07-25
Also published as: EP2245262B1; EP2245262A1; EP2245262A4; NO20080477L; NO328221B1

Abstract

Combined wellhead (21) and pipe string (31) apparatus arranged to be lowered into the outer casing (8') which is guided into the well on the seabed, where the casing (8') usually is moulded to the seabed formation, where the wellhead (2') normally is welded to the end of the pipe string ( 3'), as showed. A joint is arranged between the pipe string (31) and the wellhead (21), thus replacing the welding, the joint will be adapted to transfer axially applied forces and simultaneously cancel the bending moments.

Description

A device in a combined wellhead/pipe string

The present invention relates to a combined wellhead and pipe string assembly designed for installation within an outer casing string, which casing is installed in a well bore in the sea bed and the casing is generally cemented to the sea bed formation, where such wellhead normally is welded to the end of said pipe string

Such welded wellheads have been installed at the sea bed in a number of locations around the world. Recently one has been aware that the combined wellhead and pipe string, at the point where the pipe string is set into the sea bead and the wellhead is welded to the top of the pipe string and projects from the sea bed, over time can result in fatigue in the weld joint between the pipe string and the wellhead.

This results from the fact that the wellhead is subjected to alternating flexural or bending moments during drilling and other pipe string activity from a floating vessel and through a blow out preventer stack (BOP) installed at the top of the wellhead. This can be added to the motions of the vessel resulting from sea waves, oceanic currents, weather and wind. These alternating flexural moments are transferred from the wellhead, through the welded connection and further to the pipe string itself. The pipe ^v string, however, is near completely restrained to the sea bed and has little chance to move. Since a weld connection is present, this inherently will mean that the weld between the pipe string and the wellhead constitutes the weak spot and forms a place for potential fatigue.

In the Norwegian sector of the North Sea (NCS) this fatigue is calculated by using DnV RP C203 which provides the S/N curve Cl for through passing butt weld which is grinded on both sides.

According to the present invention an assembly of the introductory kind is provided which is distinguished in that a joint is introduced between the pipe string and the wellhead, and thus replaces said weld, which joint is able to transfer axially acting forces at the same time as the joint suspends flexural moments. In a preferred embodiment the joint comprises an elastomeric matrix located at the interface between the wellhead and the pipe string.

In one embodiment the elastomeric matrix can include at least one embedded reinforcing lamella. Thus it will be possible to regulate load capacity and rigidity.

The elastomeric matrix can be in the form of a rubber material, natural or synthetic, possibly resilient polymer material.

Further the elastomeric matrix can be vulcanized, glued or similar to a sheet of metal which in turn is fixed to the wellhead.

In turn the elastomeric matrix can be vulcanized, glued or similar to a sheet of metal which in turn is fixed to the pipe string.

In one embodiment the wellhead is provided with a radially inwardly directed flange having upwardly facing surface for placement of the elastomeric matrix, alternatively the sheet of metal.

In turn the pipe string may have a radially outwardly directed flange having downwardly facing surface for placement of the elastomeric matrix, alternatively the sheet of metal.

The respective sheets of metal can for example be secured to the wellhead, respectively the pipe string, by means of bolt connections.

In addition at least one sealing ring can be included in the joint structure in order to provide for pressure integrity between the pipes.

Preferably a clearance is present between the peripheral circumferential surface of the pipe string flange and the internal surface of the wellhead. This provides a limited freedom of motion. Further a clearance may be present between the external surface of the pipe string and the internal surface of the wellhead flange, which provides a limited freedom of motion.

Other and further objects, features and advantages will appear from the following description of a preferred embodiment of the invention, which is given for the purpose of description, and given in context with the appended drawings where:

Fig. 1 shows a longitudinal section through a conventional welded connection between a wellhead and a pipe string according to the prior art,

Fig. 2 shows a longitudinal section through an exemplary embodiment of an articulated joint between the wellhead and the pipe string according to the invention,

Fig. 3 shows in a top view an end flange of a pipe string,

Fig. 4 shows a cross sectional view through the joint connection of fig. 2 in enlarged scale, and

Fig. 5 shows a further enlarged detailed view of the joint connection shown in figure 4.

With reference to figure 1 , the traditional wellhead 2 according to the prior art is firstly shown. A first, large dimension outer casing 8 is initially lowered into a well bore B in the sea bed 7. Typical diameter is 30". This outer casing 8 is normally 60-100 meters long and is cemented into the well bore B. The well bore B is predrilled down through the formation of the sea bed in a length corresponding to the length above. The drill bit used is normative for the dimension of the well bore B.

Then the dimension of the drill bit is decreased and the drill bit is lowered through the first, outer casing 8 down to the bottom of the first well bore B. The drilling is resumed and a well of several hundred meters is drilled with this reduced dimension. Then a second, next outer casing 3 is lowered through the outer casing 8 and further down into the well bore with reduced dimension. The casings 3, 8 are assembled of shorter single casing pipes, alternatively stands consisting of three preassembled single pipes, which are joined together by means of threaded connections as they are lowered down into the well bore. The second outer casing 3 terminates in a wellhead 2 at top, which has inner and outer coupling means which are commonly known in the art. The outer coupling means are intended to engage with a termination part 1 at the end of the outer casing 8, which termination part 1 has corresponding, internal coupling means. The wellhead 2 has an annular abutment 5 resting against an inside shoulder 5a formed internally of the termination part 1.

The wellhead 2 is traditionally welded to the top of the casing 3 at a place indicated by the reference number 4 in the drawings. The weld is typically a butt weld grinded on both sides. Between the outer casing 8 and the second outer casing 3 an annulus 6 is formed. The annulus 6 is filled with cement C approximately up to the weld 4. The termination part 1 and the wellhead 2 projects from the sea bed as indicated by the reference number 7.

One or more additional casings (not shown), having decreasingly smaller cross sectional dimension, can be installed within the casing 3, all in accordance with the needs regarding the drilling depth and the nature of the formation of the sea bed. However, the number of pipes has less significance for the invention.

The wellhead 2 forms the connecting point for a Christmas three (not shown) which is normally landed from the surface. The Christmas three has required safety mechanisms in the form of valves and rams in order to be able to handle different situations during well drilling and possibly subsequent production of oil and/or gas.

Figure 2 shows in longitudinal section a combined wellhead/pipe string assembly 10 according to the present invention in a contemplated installed situation, i.e. substantially vertically oriented. This is the situation it is referred to in this specification when it comes to the terms "upper", "lower", "top", "bottom", "upwardly", "downwardly" etc.

Those parts also found in figure 1 have the same reference numbers, but with the addition of a mark'.

The combined wellhead/pipe string assembly 10 is installed into the sea bed 7 in the very same way as illustrated in figure 1. This means that the heaviest casing 8' is firstly cemented to the well bore before the casing 3' is cemented to the casing 8', fully corresponding to what is shown and described with reference to figure 1.

Briefly explained, an articulated "joint" is now introduced between the casing 3' and the wellhead 2'. This implies that the wellhead 2' is no longer able to transfer flexural moments to the casing 3', in the following generally termed pipe string 3'. However, the "joint" is able to transfer axially acting forces between the wellhead 2' and the pipe string 3'.

The wellhead 2' is now a separate part which is in connection with the pipe string 3' via an elastomeric matrix 11 located in the interface between the wellhead 2' and the pipe string 3'. The elastomeric matrix 11 can be in the form of a rubber material, natural or synthetic, alternatively a polymer material.

In the illustrated embodiment the wellhead 2' has in its lower end 2a an inwardly directed flange 2b. The radially inwardly directed flange 2b has an upwardly facing surface 2c which forms a supporting surface for placement of the elastomeric matrix 11, as shown in further detail in figure 4 and 5.

In turn, the pipe string 3' has in its upper end 3a a radially outwardly directed flange 3b. The radially outwardly directed flange 3 b has a downwardly facing surface 3 c which forms abutment surface for location of the elastomeric matrix 11, as shown in closer detail in figures 4 and 5.

Figure 3 shows the pipe string 3' viewed from above, in particular the outwardly directed flange 3b.

As illustrated in detail in figures 4 and 5, the elastomeric matrix 11 can in one embodiment include one or more embedded reinforcing lamellas 12, such as annularly formed plates of metal. These will be able to assist in regulating the load capacity and the rigidity according to specific requirements and demands. The elastomeric matrix 11 constitutes part of a supporting element 13, which in turn acts and performs like a "joint" having limited freedom of motion, consisting of a lower annular reinforcing plate 14, an upper annular reinforcing plate 15 and the elastomeric matrix 11 located between the reinforcing plates 14, 15. The reinforcing plates 14 and 15 are vulcanized or glued to the matrix 11. One or more annular reinforcing plates, or lamellas 12, can possibly be vulcanized into the elastomeric matrix 11. The number and the size of the reinforcing plates 12 will be factors that dictate the load capacity and rigidity of the support elements 13.

Moreover, the support element 13 is secured to respective flange surfaces 2c and 3c by means of a number of bolts (not shown) that is passed through apertures 2e, 3e (figures 3 and 4) in the flanges 2b and 3b of the wellhead 2' and the pipe string 3' respectively, and further down into corresponding threaded holes (14e, 15e) in the respective reinforcing plates 14 and 15 of the support element 13.

In order to obtain complete sealing integrity, one or more sealing rings 9 can be provided between the respective flanges 2b, 3b and reinforcing plates 14, 15.

In addition, as made clear in detailed figure 5, a clearance d] is intentionally provided between the peripheral circumferential surface 3d of the pipe string flange 3b and the internal surface 2f of the wellhead 2'. A similar clearance d₂ is present between the external surface 3f of the pipe string 3' and the internal surface 2d of the wellhead flange 2b. This is provided to enable smaller motions between the parts without introducing bending or flexural moments of significance. Thus the limited freedom of motion is ensured.

As indicated in figure 2, the wellhead 2' according to the invention is assembled of two parts, an upper part and a lower part 2a which are welded together as indicated by the weld seam W₁. Correspondingly, the upper part 3a of the pipe string 3' is welded to the remainder of the pipe string by means of the weld seam w₂ as indicated in figure 2. This is carried out in this way to be able to assemble the respective parts to each other, i.e. the pipe string 3', the support element 13 and the wellhead 2'. Thus it is to be understood that the lower part 2a of the wellhead 2' needs to be installed within the upper part 3a of the pipe string 3', with the support element 13 between the flanges 2b and 3b, such as shown in figure 4, before the respective welds W₁ and w₂ can be performed.

This, however, may be prepared and pre welded at the workshop on shore, ready for later installation in the sea bed.

Claims

P a t e n t c l a i m s

1.

Combined wellhead (2') and pipe string (3') assembly (10) designed for installation within an outer casing (8'), which casing is installed in a well bore (B) in the sea bed (7) and the casing (8') is generally cemented to the sea bed formation, said wellhead (2') normally being welded to the end of said pipe string (3'), characterized in that a joint (13) is introduced between the pipe string (3') and the wellhead (2'), and thus replaces said weld, which joint (13) is able to transfer axially acting forces at the same time as the joint suspends flexural moments.

2.

The assembly as defined in claim 1, characterized in that said joint forms a supporting element which includes an elastomeric matrix (11) located at the interface between said wellhead (2') and the pipe string (3').

3.

The assembly as defined in claim 1 or 2, characterized in that said elastomeric matrix

(11) includes at least one embedded reinforcing lamella (12), such as a plate of steel.

4.

The assembly as defined in claim 1, 2 or 3, characterized in that said elastomeric matrix (11) is in the form of a rubber material, natural or synthetic, alternatively a polymer material.

5.

The assembly as defined in any of the claims 1-4, characterized in that said elastomeric matrix (11) is vulcanized, glued or similar to a sheet of metal (14) which in turn is fixed to the wellhead (2').

6.

The assembly as defined in any of the claims 1-5, characterized in that said elastomeric matrix (11) is vulcanized, glued or similar to a sheet of metal (15) which in turn is fixed to the pipe string (3').

7.

The assembly as defined in any of the claims 1-6, characterized in that said wellhead

(2') has an radially inwardly directed flange (2b) having upwardly facing surface (2c) for placement of the elastomeric matrix (11), alternatively the sheet of metal (14).

8.

The assembly as defined in any of the claims 1-7, characterized in that said pipe string

(3') has an radially outwardly directed flange (3 b) having downwardly facing surface

(3c) for abutment of the elastomeric matrix (11), alternatively the sheet of metal (15).

9.

The assembly as defined in any of the claims 1-8, characterized in that said respective sheets of metal (14, 15) is secured to the wellhead (2'), respectively the pipe string (3') by means of bolt connections.

10.

The assembly as defined in any of the claims 1-9, characterized in that at least one sealing ring (9) in addition is included in the joint structure (13).

11.

The assembly as defined in any of the claims 1-10, characterized in that a clearance (di) is present between the peripheral circumferential surface (3d) of the pipe string flange (3b) and the internal surface (2f) of the wellhead (2').

12.

The assembly as defined in any of the claims 1-11, characterized in that a clearance (d₂) is present between the external surface (3f) of the pipe string (3') and the internal surface (2d) of the wellhead flange (2b).