WO2012049289A1

WO2012049289A1 - Reducing wear on well head

Info

Publication number: WO2012049289A1
Application number: PCT/EP2011/067981
Authority: WO
Inventors: Arve Tvitekkja; Hallgeir MELBØ
Original assignee: Aker Subsea AS
Current assignee: Aker Solutions AS
Priority date: 2010-10-15
Filing date: 2011-10-14
Publication date: 2012-04-19
Anticipated expiration: 2013-04-15
Also published as: NO20101432A1; NO333482B1

Abstract

A well head assembly for sub sea wells, comprising a well head housing (1) secured to a well casing by welding along the top portion of the latter in a weld zone (8). The lower part of the well head housing (1) has an area (3) with a reduced thickness at a distance from the weld zone (8), so as to be flexible and to absorb bending force induced on the well head housing (1) during riser operations.

Description

Reducing wear on well head FIELD OF THE INVENTION The present invention in general relates to an improved well head assembly and in particular to an improved well head housing which is relatively more flexible and is capable of absorbing bending stress, caused by riser connection during drilling, completion and work-over operations. Particularly, the present invention relates to a well head assembly according to the preamble of claim 1 and to a well head housing according to the preamble of claim 8.

TECHNICAL BACKGROUND OF THE INVENTION

A well head is used primarily as the suspension point and pressure seals for the casing strings extended from the bottom of the hole sections to the surface pressure control system or blow out preventer (BOP). After the well is drilled the surface pressure control is provided by a Christmas tree which is installed on the top of the well head.

A riser is connected on the top of the blow out preventer (BOP) and the housing in turn is connected to the well head casing by welding. When a riser is connected to the top of the BOP it creates a very high bending momentum on the connecting surface of the lower part of the well head housing and the upper part of the casing, i.e. at the welded joint area. In a sub-sea drilling operation the design criterion is that the weld should endure at least 500 days with a riser connected. It is noted that when the well is in production a riser is connected only during the work-over operations. The safety factor is very stringently set by the regulation at 10. Therefore, the connection of the well head housing and well head casing has to endure stress for 5000 days of operation with a riser connected.

Any break in the weld between the well head housing and the casing will lead to blow out and there will be little possibility of sealing the well, at least not without having to pump large amounts of concrete down the well and thereby permanently closing the well. Therefore the welded area between the casing and the housing is probably the most critical and vulnerable area in the whole assembly.

In a conventional well head the casings are also tied back to the well head to reduce the risk of break in the connection and to maintain the pressure integrity.

In order to avoid such welded connection, it is thought of making the casing and the housing as one piece structure. This will avoid the problem but such a structure will require special running and installation equipment and process and is thereby not practicable in the existing systems.

US patent 6,196,323 discloses a solution to maintain the pressure integrity by providing a well head system comprising a well head housing, a surface casing and one or more successive liners without the need for using a casing string, thereby reducing the load of the system.

However, the issue of withstanding heavy bending moment on the welded area of the housing-casing joint during riser operation and fatigue of the welded joint area remain a problem.

The present invention aims to solve this problem by providing a unique designing of the well head housing.

OBJECTS OF THE INVENTION

The primary object of the present invention is to provide a well head assembly which is capable of reducing the bending moment/stress experienced on its welded joint area.

It is yet another object of the invention to use lesser quantity of forged material used for manufacturing the well head housing without affecting its strength and stability.

It is a further object of the invention to reduce the risk of blow out of the well head during riser operation. It is yet further object of the invention is to provide a well head assembly which conforms to the regulatory criterion and safety standard for offshore wells. SUMMARY OF THE INVENTION

The present invention meets the above objects by a well head assembly for sub sea wells, comprising a well head housing secured to a well casing, the housing being secured to the casing by welding along the top portion of the latter in a weld zone, wherein the lower part of the well head housing has an area with a reduced thickness at a distance from the weld zone, so as to be flexible and to absorb bending force induced on the well head housing during riser operations.

Thereby reduced fatigue on the weld zone is achieved and hence a longer expected life time of the well head assembly.

By achieving the thickness reduction by reducing the outer diameter of the well head housing in the zone a continuous annulus bore is maintained.

If the well head housing is made of forged material having a higher fatigue strength than the weld zone, it can be ensured that the bending can be absorbed without undue fatigue.

If the assembly is adapted to endure at least 5000 days of riser connection during drilling, completion or work-over operations, the regulatory requirements will be met.

The invention also meets the above objectives by a well head housing for a sub sea well head assembly, adapted for securing to a well casing, the housing (1 ) being secured to a well casing by welding along the top portion of the latter in a weld zone, wherein the lower part of the well head housing has an area with a reduced thickness at a distance from the weld zone, so as to be flexible and to absorb bending force induced on the well head housing during riser operations. BRIEF DESCRIPTION OF THE DRAWINGS A more detailed and non-limiting description of some exemplary embodiments are given in the following with reference to the drawings, in which,

Figure 1 is a sectional view of the well head assembly in a state of completion showing the different parts as is existing in the state of the art.

Figure 1 A is an illustrative sectional view of the well head assembly showing the different parts without completion as is existing in the state of the art.

Figure 2 is a sectional view of the improved well head assembly according to the the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention has been described with reference to a preferred embodiment and drawings for the sake of understanding.

The accompanying Figure 1 shows a sectional view of a well head assembly with its internal components in completed state as is existing in the state of the art. A well head housing 1 is received within a conductor casing 2. The conductor casing 2 has on its upper part a receiving portion 3 designed to receive and lock the lower part of the well head housing 1 . The joint, between the well head housing 1 and the well casing 2 is secured by a clamping system 4. A well casing 7 has been inserted into the conductor casing 2 and is cemented in place in the well. A production tubing 6 is inserted within the well head housing and extends within the well casing 7. A riser (not shown) may be attached to the top of the well head housing 1 to provide for well access from the sea surface. During production a Christmas tree is installed on top of the well head having the flow controlling equipment such as isolation valves and choke equipment to control the flow of well fluids during production. The other associated parts of a well head assembly are excluded from the figure for the sake of simplicity as they are well known in the art and not by way of limitation.

The lower part of the well head housing 1 is welded to the top of the well casing 7 by a weld 8. When a riser is connected to the top of the well head housing 1 , it creates a very high bending moment on the lower part of housing 1 and the upper part of the casing 7 during work-over operations.

Figure 1A is another sectional view of the well head assembly showing the different parts without well completion as is existing in the state of art. Similar reference numerals refer to similar parts of the embodiment as discussed with respect to the above mentioned figure for the sake of simplicity in understanding. During the work over operation, the welded area 8 of the prior art experiences, as discussed, severe bending stress and momentum when a riser is connected at the top of the well head due its different movements as a result of the wave induced movements of the work-over vessel at the surface, to which the upper part of the riser is connected. Therefore there is a potential risk for such a welded joint to break due to fatigue resulting in a blow out.

The inventors of the present invention have come up with a unique solution to this problem which greatly reduces such risk. The invention aims at reducing the resulting bending momentum experienced by the welded joint 8 of the housing 1 and the well casing 7.

In order to achieve such a result, the invention is aiming to move the absorption zone of the bending moments away from the welded joint 8 between the housing 1 and the casing 7.

The well head housing 1 is made of forged material which is relatively soft and ductile and its flexibility and fatigue characters therefore are relatively better than the casing 7. This provides a unique opportunity to provide more flexibility in the lower end of the housing 1 . As illustrated in figure 2, which illustrates a sectional view of the inventive well head assembly as claimed in the present disclosure, this is done by reducing the diameter of the well head housing at the lower end thereof in a zone 3' some distance away from the welded area between the housing and the casing, thereby making a zone 3' with a reduced stiffness as compared with the welded area 8. In this concept the zone 3' becomes more flexible. As this zone 3' is within the fairly ductile well head housing 1 it can endure more bending cycles without excessive fatigue. The solution is therefore to reduce the wall thickness in the lower part of the housing 1 in a zone 3' some distance away from the welded area 8. This design imparts the well head housing 1 improved flexibility to endure increased bending moment. The reduced thickness of the housing helps in transferring the bending moment to the receiving portion 3 of the conductor 2, so that more momentum is absorbed by the conductor 2 and less is transferred to the welded zone 8. Preferably the wall thickness in the zone 3' is reduced by reducing the outer diameter of the housing only, so to maintain an annulus bore within the casing and housing as even as possible.

The thickness of the inner diameter of the wellhead housing 1 may be reduced by a proportion of the normal diameter of the wellhead housing member in order to facilitate such flexibility.

It is found that a proportionate reduction of the diameter of the housing makes the assembly capable of exhibiting better flexibility, without affecting the strength and stability of the forged material in comparison to the existing assemblies.

The invention has been described with the help of a preferred embodiment and several alterations and modifications may be made without departing from the sprit and scope of the invention as described above and as defined in the appended claims.

Claims

Patent claims 1.

A well head assembly for sub sea wells, comprising a well head housing (1 ) secured to a well casing (7) by welding along the top portion of the latter in a weld zone (8), ch aracte rised i n that the lower part of the well head housing (1) has an area (3') with a reduced wall thickness compared to the wall thickness of the well casing (7), at a distance from the weld zone (8), so as to be flexible and to absorb bending force induced on the well head housing (1) during riser operations.

2.

The well head assembly according to claim ^cha racteri sed i n that the thickness reduction is achieved by reducing the outer diameter of the well head housing (1) in the zone (3').

3.

The well head assembly according to claims 1 to 2, ch aracteri sed i n that well head housing is made of forged material having a higher fatigue strength than the weld zone (8).

4.

The well head assembly according to claim 1 to 3, cha racte ri sed i n that the assembly is adapted to endure at least 5000 days of riser connection during drilling, completion or work-over operations.

5.

A well head housing for a sub sea well head assembly, adapted for securing to a well casing (7) by welding along the top portion of the latter in a weld zone (8), cha racte ri sed i n that the lower part of the well head housing (1 ) has an area (3') with a reduced wall thickness compared to the wall thickness of the well casing (7), at a distance from the weld zone (8), so as to be flexible and to absorb bending force induced on the well head housing (1) during riser operations.

6.

The well head housing according to claim 5, cha racte ri sed i n that the thickness reduction is achieved by reducing the outer diameter of the well head housing (1 ) in the zone (3').

7.

The well head housing according to any of the claims 5 to 6,

cha racte ri sed i n that the well head housing is made of forged material having a higher fatigue strength than the weld zone (8).

8.

The well head housing according to any of the claims 5 to 7,

cha racte ri sed i n that the housing is adapted to endure at least 5000 days of riser connection during drilling, completion or work-over operations.

1.