SUBSEA WELLHEAD ASSEMBLY

Abstract

A subsea wellhead assembly 1 comprises: a subsea wellhead 2; a template 6 associated with the wellhead; subsea riser system equipment 4 connected to the wellhead and one or more connection members. The subsea riser system equipment 4 is also connected to the template 6 by the one or more connection members so that lateral support is provided to the subsea riser system equipment 4 from the template. A method of installing the subsea wellhead assembly 1 is also provided.

Claims

1. A subsea wellhead assembly, the assembly comprising: a subsea wellhead; a template associated with the wellhead; subsea riser system equipment connected to the wellhead; and one or more connection members, wherein the subsea riser system equipment is connected to the template by the one or more support members so that lateral support is provided to the subsea riser system equipment from the template.

2. A subsea wellhead assembly as claimed in claim 1, wherein the subsea wellhead assembly is for reducing riser system induced load effects in the subsea wellhead.

3. A subsea wellhead assembly as claimed in claim 1, wherein the one or more connection members each extends between the riser system equipment and the template.

4. A subsea wellhead assembly as claimed in claim 1, wherein the one or more connection members is a line that is in tension.

5. A subsea wellhead assembly as claimed claim 1, wherein the one or more connection members are each provided with a tensioner.

6. A subsea wellhead assembly as claimed in claim 4, wherein the tensioner comprises a reversal preventing device that permits movement in one direction only.

7. A subsea wellhead assembly as claimed in claim 5, wherein the tensioner has an extended position and a retracted position and is movable between the two positions so as to permit a tension to be exerted on the connection member.

8. A subsea wellhead assembly as claimed in claim 5, wherein the one or more connection members are connected to the template via the tensioner.

9. A subsea wellhead assembly as claimed in claim 1, wherein the one or more connection members are each provided with a force sensor.

10. A subsea wellhead assembly as claimed in claim 1, wherein the one or more connection members are each connected to the subsea riser system equipment via a clamp.

11. A subsea wellhead assembly as claimed claim 1, wherein the one or more connection members are each connected to the template via a bracket.

12. A subsea wellhead assembly as claimed claim 1, wherein the subsea riser system equipment is a blowout preventer.

13. A method of installing a subsea wellhead assembly, the method comprising: providing a subsea wellhead, a template associated with the wellhead, and a subsea riser system equipment connected to the wellhead; and connecting the subsea riser system equipment to the template using one or more connection members so that lateral support is provided to the subsea riser system equipment from the template.

14. A method as claimed in claim 13, wherein the subsea riser system equipment is connected to the template whilst they are subsea.

15. The method as claimed in claim 13, wherein the subsea wellhead assembly comprises: a subsea well head; a template associated with the wellhead; subsea riser system equipment connected to the wellhead; and one or more connection members, wherein the subsea riser system equipment is connected to the template by the one or more support members so that lateral support is provided to the subsea riser system equipment from the template.

Description

[0149] Certain preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

[0150] FIG. 1 shows a plan view of a subsea wellhead assembly;

[0151] FIG. 2 shows a perspective view of another subsea wellhead assembly;

[0152] FIG. 3 shows a tensioner support;

[0153] FIG. 4 shows the tensioner support being installed on a template;

[0154] FIG. 5 shows a tensioner;

[0155] FIG. 6 shows a side view of the tensioner in a fully retracted position;

[0156] FIG. 7 shows a side view of the tensioner in a fully extended position;

[0157] FIG. 8 shows a part of a subsea wellhead assembly including a clamp on the riser system equipment;

[0158] FIGS. 9, 10 and 11 show example tension lines;

[0159] FIG. 12 shows a clamp on the wellhead assembly;

[0160] FIG. 13 shows a tension line being pulled onto a tensioner using an installation guide line; and

[0161] FIG. 14 shows an installation guide line.

[0162] A subsea wellhead assembly 1 is shown in FIG. 1 and another subsea wellhead assembly 1 is shown in FIG. 2. The subsea wellhead assembly 1 comprises a wellhead 2. As illustrated by FIG. 1 the assembly may comprise a plurality of wellheads 2, in this case four.

[0163] Subsea riser system equipment, in this case a blowout preventer (BOP) 4, is attached to the wellhead 2. The attachment between the BOP 4 and the wellhead 2 may be via a Christmas/subsea tree 3. A subsea template 6 is associated with the wellhead 2 to which the BOP 4 is attached. The template 6 will be fixed to the sea bed by means of suction plates 8. This means that the template 6 will be fixed relative to the wellhead 2. The template 6 may be connected to, and support the wellhead 2.

[0164] The BOP 4 is connected to the template 6 by tension lines L. In the wellhead assembly 1 of FIG. 2 there are four tension lines L and in the wellhead assembly 1 of FIG. 1 there are seven tension lines L that are labelled L1 to L7. The tension lines L are formed from links of steel wire. The tension lines L are each connected at one end to the BOP 4 via a clamp 10 (as shown in FIG. 8 and FIG. 12 for example).

[0165] The clamps 10 are bolted onto a part of the frame of the BOP 4. The clamps 10 each have a number of protrusions to which an end connection portion of the tension line L can connect.

[0166] The tension lines L are each connected at the other end to a tensioner 12. The tensioners 12 are each connected to the template 6. Some of the tensioners 12 are received directly in a hole (that may be referred to as a transponder bucket) near a corner of the template 6 and other tensioners 12 are received in a tensioner support/bracket 14 that is mounted on the template 6.

[0167] As shown in FIGS. 3 and 4 for example, the tensioner supports (also referred to as a bracket) 14 are shaped to fit onto a corner portion of the template 6. As shown in FIG. 1, the template 6 may comprise support arms 16 at each corner of the template 6. These support arms 16 each extend at about 45 degrees downwards from the plane of the top of the template 6 towards the seabed. These support arms 16 together with the top frame of the template 6 can be used to support the bracket 14.

[0168] The bracket 14 may be locked in position on the template 6 by means of a locking device 18. The locking device 18 may extend through a gap located between a support arm 16 of the template 6 and a leg that extends between the top frame and a suction plate 8 on the sea bed. The locking device 18 may act to lock the bracket 16 to the template 6.

[0169] The brackets 14 each have a hole to permit a tensioner 12 to be connected to the bracket 14. As shown for example in FIGS. 1 and 8, a bracket may be able to be connected to two tensioners 12.

[0170] The wellhead assembly 1 may not comprise any brackets 14 as shown in FIG. 2 and the tensioners 12 may be connected directly to the template 6.

[0171] Each tension line L may have a load cell 20 thereon. This permits the tension in each line L to be measured during installation and operation of the subsea wellhead assembly 1.

[0172] The tensioners 12 may each be a mechanical rope tensioner as shown in FIGS. 5, 6 and 7.

[0173] The tensioners 12 in a wellhead assembly 1 may be of different lengths. For example, some tensioners 12 may be longer tensioners whilst some tensioners 12 may be shorter tensioners (with reference to the other tensioners 12 in the assembly 1).

[0174] The tensioner 12 comprises a connection portion in the form of a guide bolt 22 (not shown in FIGS. 6 and 7) and a main body portion 24. The main body 24 may rotate by 360 degrees about the axis of the guide bolt 22 and may pivot relative to the guide bolt to permit the main body 24 to extend at a desired angle to the template 6 once it is installed.

[0175] The guide bolt 22 may be received in the template 6 or in a bracket 14 as discussed above. The tensioner 12 may then be locked in position by a locking pin (not shown) that passes through a hole 23 in the bottom of the guide pin 22.

[0176] The tensioner 12 has a ratchet mechanism 26. The tension line L may have an engagement portion 27 at one end that can engage with the ratchet 26 of the tensioner 12 to thereby connect the tension line L to the tensioner 12.

[0177] The ratchet 26 can act to accommodate slack that may occur in the tension line L during operation of the subsea wellhead assembly 1.

[0178] The tensioner has a guide funnel 28 through which the end portion of the tension line L that engages with the ratchet 26 can be received and guided.

[0179] The tensioner 12 is movable between a retracted position as shown in FIG. 6 and an extended position as shown in FIG. 7. This may be achieved using an ROV when then tensioner 12 is subsea.

[0180] The tension line L may be attached to the tensioner 12 when it is in the extended position or a partly extended position (as shown in FIG. 13). The tensioner may then be moved to a more retracted position so as to put a pretension on the tension line L.

[0181] The template and riser system equipment may have a nominal aft side that is opposed to a forward (fwd) side and a starboard (stb) side that is opposed to a port side, wherein the port and starboard sides are substantially perpendicular to the aft and forward sides.

[0182] For the embodiment shown in FIG. 1 the below table lists for each of the seven tension lines L, where it is connected to the template, where it is connected to the BOP 4, whether the tensioner 12 is connected directly to the template (via a transponder bucket) or the tensioner support 14, what the tension line L is formed from and whether the tensioner is a longer or a shorter (relative to the other tensioners) tensioner 12.

TABLE-US-00001 Template BOP Length Line connection connection Tensioner of no location location installation Description tensioner L1 Fwd Port Aft Port Transponder 2 parts steel Long bucket wire L2 Fwd Port Fwd Port Tensioner 2 parts steel Long support wire L3 Fwd Stb Fwd Port Tensioner Grommet Short support L4 Fwd Stb Aft Stb Tensioner 1 part steel Short support wire L5 Aft Stb Aft Stb Tensioner 1 part steel Long support I wire L6 Aft Stb Aft Port Tensioner 1 part steel Long support wire L7 Aft Port Aft Port Transponder 2 parts steel Long bucket wire

[0183] FIG. 9 shows a tension line L formed from 2 parts steel wire, FIG. 10 shows a tension line L formed from 1 part steel wire and FIG. 11 shows a tension line L formed from a grommet.

[0184] The installation of the subsea wellhead assembly 1 will now be discussed. The BOP clamps 10 are installed while the BOP 4 is on a deck, prior to subsea activities. The remaining equipment, which is part of the assembly 1, shall be installed subsea. The tensioners 12 may be installed on the template 6 prior to installing the BOP 4, but the hook-up of the tension lines L etc. will be performed after the BOP 4 has been installed on the wellhead 2.

[0185] The installation of the subsea wellhead assembly 1 may have the following main steps: [0186] Preparing equipment for installation [0187] Performing pre-installation survey [0188] Installing BOP Clamps 10 topside [0189] Installing tensioner supports 14 [0190] Installing and locking tensioners 12 [0191] Preparing tensioners 12 for connection to tension lines L [0192] Hooking-up of tension lines L with pull-in head [0193] Pretensioning the lines L with the tensioners 12 [0194] Performing a post-installation survey

[0195] Firstly the equipment is prepared for installation. The tensioners 12 may each be pre-installed with an installation guide line 30 (shown in FIGS. 13 and 14) or fore-runner that is used to aid the operation of connecting the tension line L to the tensioner 12. The installation guide line 30 is a line with a link or a hook 32 at one end for connection to a tension line L and a link or hook 34 at the other for connection to an ROV. The installation guide line 30 may be fed through the tensioner 12 topside and then used subsea to pull the tension line L into connection with the ratchet 26 of the tensioner 12.

[0196] The tension line L may each be connected to a load cell 20 topside.

[0197] Next the subsea steps are explained. An ROV is used to verify that the transponder buckets in the template 6 are clean and free from debris. The transponder buckets may then be cleaned if required.

[0198] The tensioner supports 14 may then be installed. This can be achieved by lifting the tensioner support 14 from a cellar deck using a heave compensated lifting line and then lowering the tensioner support 14 to a location, for example 15m, above the template 6. The tensioner support can then be guided by an ROV, which grabs the lifting line, to the intended installation position on the template 6. The ROV may then be used to lock the tensioner support 14 to the template 6. This may be achieved by pushing the locking mechanism 18 into the tensioner support and through a portion of the template 6.

[0199] The lift wire may then be retrieved so the above steps can be repeated for each tensioner support 14 to be installed.

[0200] Next the tensioners 12 are installed. The tensioners 12 may be lifted off the basket and deployed from the cellar deck using a heave compensated lifting line. The tensioner is lowered to a location, for example to 15 m, above the template 6. The tensioner 12 may be installed in the transponder bucket in the template 6 or in a hole on one of the installed tensioner supports 14.

[0201] An ROV may be used to grab the tensioner, pull and guide it to the transponder bucket or a hole in the tensioner support 14.

[0202] The ROV may be used to align the hole in the guide bolt 22 to a hole in the bottom of the transponder bucket or tensioner support 14.

[0203] The ROV may then be used to install a locking pin through a hole in the transponder bucket or tensioner support 14 and the hole 23 in the guide pin 22 so as to lock the tensioner 12 in position. This may then be repeated for each tensioner 12. A tensioner 12 is provided for each tension line L.

[0204] Each tensioner 12 may then be set into its extended position by the ROV.

[0205] Next the tension lines L, which each have a pull-in head 27, are deployed from the cellar deck by using a heave compensated lifting line.

[0206] The tension line L is lowered to a location, for example 15 m, above the template 6. Using an ROV one end of the tension line L is hooked onto one of the BOP clamps 10. The ROV may then be used to guide the other end of the tension line L with pull-in head 27 to the tensioner 12. The pull-in head is connected to one end of the pre-installed installation guide line 30 in the tensioner 12 (as shown in FIG. 13).

[0207] The ROV may then be used to apply a tension of 10-40 kN to the installation guide line 30 so as to pull the pull-in head 27 into the saw tooth interface of the ratchet mechanism 26 on the tensioner 12.

[0208] This process can then be repeated for each tension line L.

[0209] The lines L may then be pretensioned by moving each of the tensioners 12 towards its retracted position until the desired tension is achieved.

[0210] In a preferred embodiment the lines shall be given a pretension as follows:

[0211] Line L1=120 kN (12 ton)

[0212] Line L2=100 kN (10 ton)

[0213] Line L3=200 kN (20 ton)

[0214] Line L4=210 kN (21 ton)

[0215] Line L5=100 kN (10 ton)

[0216] Line L6=200 kN (20 ton)

[0217] Line L7=120 kN (12 ton)

[0218] As used herein the term “ton” refers to a metric tonne, i.e. 1000 kg. When used as a force measure, it may mean the force equivalent to the weight of 1000 kg mass, i.e. the force=1 ton×9.81 m/s.sup.2=9810 N.

[0219] The process of tensioning the lines L may be as follows. The method may include locating an observation ROV in place to observe the load cell 20 of the line L that is being tensioned.

[0220] The method may then include tightening all of the tension lines L with a low torque equaling less than 10 kN. Following this all the tension lines L may in turn be tightened to 50% of the final desired pretension.

[0221] The tension lines L may then again in turn be tightened to 75% of the final pretension. Finally, the tension lines L may then again in turn be tightened to 100% of the final pretension.

[0222] During this procedure the output of the load cell 20 on each line can be observed after each gradual increase in the pretension using the observation ROV.

[0223] Inspection and verification of the presentation in the lines L may be performed every 3 hours after the installation is complete.

[0224] Once it is observed that the system 1 has stabilised, the inspection intervals can be extended to longer periods, such as 6 hours and then 12 hours until the system appears to be entirely stable.

[0225] Depending on the readings taken by the observation ROV, e.g. an ROV camera, the tension in the tension lines L may be adjusted using the tensioners 12 to obtain the desired pretension. For example, a tensioner 12 may be adjusted if the average tension is more than 20 kN (2 tons) below the desired tension. It should be noted that if the tension is more than 50 kN (5 tons) from the desired tension a corrective action may be required to rectify the incorrect tension.

[0226] If some lines L have too low tension and some too high tension (e.g. variations due to lower riser inclination), then it may not be necessary to adjust the tension in the tension lines L. This for example may occur due to load variations on the riser e.g. natural loads from ocean current variations, and thus may not require adjusting of the tensioners to correct this.

[0227] If it is desired to uninstall the assembly, e.g. when the BOP 4 is to be detached from the wellhead 2, the following procedure may be followed. [0228] Pre survey of the attachments of the tension lines L to the BOP 4 and tensioner 12. [0229] The observation ROV may be used if needed. [0230] Torque tool (TT) mounted on ROV and calibrated. [0231] Hard line cutter mounted on ROV if contingency cutting is required. [0232] Cellar deck ready to assist with lifting line. [0233] Position the ROV at the first tension line L to be unhooked. Relieve the pretension on the tension lines L by moving the tensioner 12 towards its extended position. This should be repeated for each of the tension lines L.

[0234] Once it is observed that the tension line L is slack, the ROV may be used to unhook the tension line L from the tensioner 12. This may be achieved by connecting an ROV hook to the pull-in head 27 and then lifting a thimble of the pull-in head 27 clear of the ratchet mechanism 26 on the tensioner 12 for the tension line L.

[0235] Once disconnected from the tension line L the tensioner may be laid down on the roof of the template 6.

[0236] The other end of the tension line L may then be unhooked from the clamp 10 mounted on the BOP 4. The disconnected tension line L may then be lifted to the surface.

[0237] This process may then be repeated for each of the tension lines L.

[0238] The tensioners 12 may then each be retracted using an ROV. Following this the tensioners can each be lifted to the surface.

[0239] The method may then comprise retrieving the locking pin that locks down the tensioner 12 to the template 6 from the drilled hole in the bottom of the transponder bucket or the tensioner support 14. This may be followed by attaching the surface lift line to the tensioner to permit the tensioner 12 to be lifted vertically and then lifting the tensioner 12 out of the transponder bucket or tensioner support. The ROV may be used to assist the lift operation and guide the tensioner 12 out of the transponder bucket or tensioner support 14.

[0240] The tensioner 12 can then be lifted to the surface, the retrieved tensioner may be placed in the basket for transport to shore. This process may be repeated for each of the tensioners 12.

[0241] To retrieve the tensioner supports 14, the surface lift line may be attached to the tensioner support 14, the ROV can be used to release the tensioner support 14 by pulling out the locking mechanism 18. The ROV may be used to lock the locking mechanism 18 in an open position with a locking wedge. The ROV may be used to grab the lift wire and guide the tensioner support away from the template 6.

[0242] The lift wire may then be used to lift the tensioner support 14 to the surface. This can then be repeated for each of the tensioner supports 14.

[0243] If desired, the BOP 4 can then be retrieved.

[0244] In the case that the tension lines L cannot be slackened by extending the tensioner 12 the following contingency procedure may be followed.

[0245] A hard line cutter may be used to cut the tension line L, this may be achieved by cutting the connection portion used to connect the tension line to the clamp 10 of the BOP 4. The cut tension line L may then be unhooked from its respective tensioner 12.