GB2527386A

GB2527386A - Subsea wellhead assembly

Info

Publication number: GB2527386A
Application number: GB1500951.7A
Authority: GB
Inventors: Per Osen
Original assignee: Statoil Petroleum ASA
Current assignee: Equinor Energy AS
Priority date: 2015-01-20
Filing date: 2015-01-20
Publication date: 2015-12-23
Also published as: GB201500951D0

Abstract

Subsea wellhead assembly 1 comprising a template 6, associated with a submarine wellhead 2, and connected to subsea riser system equipment 4. The template frame is attached to the subsea riser system equipment by tension lines L, which are tensioned by a tensioner 12. The template provides lateral support for the subsea equipment and protection (e.g. from trawlers). The subsea riser system equipment may be any equipment for connection to a riser (e.g. Christmas tree, BOP). The attachment wires may feature strain gauges for determining the load on each line. The template may be adapted to be operated by an ROV. A method of installing the template by increasing tension on each line sequentially is included.

Description

SUBSEA WELLHEAD ASSEMBLY

The invention relates to a subsea wehhead assembly and a method of installing a subsea wellhead assembly.

A typical subsea assembly comprises a subsea wellhead to which riser system equipment, such as a blowout preventer and/or a Christmas tree (which may also be referred to as a subsea well) may be connected. The riser system equipment connected to the welihead is typically connected to a riser that extends between this riser system equipment and a surface facility. The riser typically provides a conduit for the drill string and drilling fluids between the subsea well and the surface facility (drilling riser) or for produced fluids (production riser).

It is important that the wellhead assembly integrity is maintained so that structural failure and uncontrolled release of well fluids does not occur. As a result, it is desirable that forces that act on the assembly have as low risk as possible of damaging the assembly.

In a first aspect the present invention provides a subsea welihead assembly, the assembly comprising: a subsea wellhead; a template associated with the wellhead; and subsea riser system equipment connected to the wellhead; wherein the subsea riser system equipment is also connected to the template so that lateral support is provided to the subsea riser system equipment.

In a second aspect, the present invention provides a method of installing a subsea wellhead assembly, the method comprising: providing a subsea wellhead, a template associated with the wellhead, and subsea riser system equipment connected to the wellhead; and connecting the subsea riser system equipment to the template so that lateral support is provided to the subsea riser system equipment.

With the present invention, because the subsea riser system equipment, e.g. blowout preventer (BOP), is connected to the template, it is possible for the template to provide lateral support to the riser system equipment, e.g. BOP, connected to the wellhead. This support may be provided during drilling, completion, and/or workover modes of operation of the wellhead assembly.

The present invention may provide a method of controlling the loads imposed for example by a drilling facility, etc., on a subsea wellhead.

The assembly may be for reducing riser system-induced load effects on the subsea wellhead. Thus the present invention may be considered to provide an

I

assembly or a method for reducing riser system-induced load effects in subsea wellheads.

By lateral support it may be meant that the riser system equipment is supported in a direction that is substantially parallel (or at least partially parallel) to the sea bed or substantially perpendicular to the axis of the well head. When the riser system equipment is connected to the wellhead, the lateral direction may be substantially horizontal.

If the wellhead (to which the riser system equipment is connected) is connected to the template, the connection between the subsea riser system equipment and the template may be in addition to the indirect connection via the wellhead, between the subsea riser system equipment and the template.

With this arrangement, because the subsea riser system equipment (e.g. the BOF) is laterally supported, it is possible for the loads transferred to the wellhead from the riser system (which includes the riser and the riser system equipment) to be reduced, for example loads due to riser system equipment or riser movements. These loads may be cyclic fatigue loads and/or accidental single-loads. The assembly may reduce the loads transferred to the wellhead from the riser system equipment by 50% or more, (e.g. at least 50%, 50% to 60% or at least 75%) compared to a situation without such lateral support.

The riser system equipment may extend vertically up from the wellhead away from the sea bed. The riser system equipment may be connected at its other end to a riser, the upper end of which may be connected to a surface facility such as a floating vessel.

The riser system equipment may be equipment which is attached to the wellhead that facilitates or improves the safety of operations such as drilling and completion in the well. The riser system equipment may for example be a blowout preventer and/or a Christmas/subsea tree. For example, during drilling a blowout preventer may be provided directly on the welihead and during completion a blowout preventer may be provided with a Christmas tree on the wellhead.

The template may also be referred to as a protection frame or a protection envelope. The template for example may be a freestanding frame positionable over a wellhead and its associated components such as a tree. In this case the template may be anchored and mounted on its own dedicated anchoring points and foundations. The template may not be in contact with the wellhead. Alternatively the template may be connected or attached to the wellhead itself. The template may have a wellbay (e.g. a hole) for the well conductor, and thereby may support the wellhead.

The template may be overtrawlable. This means that the template may protect the wellhead and its associated components from damage that could be caused by trawlers operating near the wellhead.

By the template being associated with the wellhead, it may be meant that the template is fixed relative to the welihead. For example, the template may be fixed to the seabed, for example via suction plates, so as to be fixed in a location relative to the location of the wellhead. The template may be located about the wellhead. The template may act as a protection device, such as a cage, to protect the wellhead from damage. The template may be connected to the wellhead and the template may support the wellhead. The template may be associated with a plurality of wellheads, for example, the template may be associated with four wellheads. The template may be a rigid structure/frame that is located about, i.e. around, the wellhead on the sea floor.

The riser system equipment may be connected to the template by means of one or more connection members. For example, the assembly may comprise four (or more) connection members, seven (or more) connection members or the assembly may consist of (i.e. only have) seven connection members. The assembly may comprise between 2 and 12, 5 and 10 or 6 and B connection members.

The connection member may for example be a steel frame that is supported by the template and which supports the riser system equipment.

Each connection member may extend between the riser system equipment and the template. The connection member(s) may be an elongate member that extends between the riser system equipment and the template.

The connection member(s) may extend at an angle between 0 and 90 degrees, 10 and 80 degrees, 40 and 50 degrees, about 45 degrees upwards from the plane of the top of the template towards the riser system equipment.

The connection member(s) may be inclined (relative to the sea floor or the plane of the top of the template) but it may not be vertical.

The connection member(s) may laterally support the riser system equipment and/or may reduce the loads or forces transferred to the wellhead from the riser 1: system equipment compared to an assembly without any connection members.

The connection member(s) may be arranged so as to transmit forces between the riser system equipment and the template. The connection member(s) may be in tension or compression. H The connection member(s) may be a rod or bar which is in compression.

The connection member(s) may each be a steel beam such as a solid steel beam. The connection member(s) may be provided by a rigid frame which is between the template and the riser equipment.

The connection member(s) may be a line which is in tension. The line, for example, could be a wire, rope, cable, tether or chain etc. The line may be formed from a plurality of steel wire parts which are connected together to form a line.

The connection member(s) may rigidly connect the riser system equipment and the template.

The connection members may each be connected to the template and/or the subsea riser system equipment. For example, one end of a connection member may be connected (directly or indirectly) to the template and the other, opposite end of the connection member may be connected (directly or indirectly) to the subsea riser system equipment. The connection member(s) may be directly connected to the subsea riser system equipment and/or the template or the connection member(s) may be indirectly connected to the subsea riser system equipment and/or the template such as via one or more connection parts such as a bracket or clamp which is attached directly to the riser system equipment or the template.

One end of a connection member may be connected (directly or indirectly) to the top frame of the template. The connection to the top frame may be at or near the corners of the top frame (if the top frame is square or rectangular). The other, opposite end of the connection member may be connected (directly or indirectly) to the outer frame of the subsea riser system equipment. This may be at the longitudinally extending corners of the subsea riser system equipment.

The template and riser system equipment may have a nominal aft side (first side) that is opposed to a forward (fwd) side (second side) and a starboard (stb) H side (third side) that is opposed to a port side (fourth side), wherein the port and starboard sides are substantially perpendicular to the aft and forward sides.

The assembly may comprise: 1) a connection member that extends from a position on the template that is forward and port, to a position on the riser system equipment that is aft and port, 2) a connection member that extends from a position on the template that is forward and port, to a position on the riser system equipment that is forward and part, 3) a connection member that extends from a position on the template that is forward and starboard, to a position on the riser system equipment that is forward and port, 4) a connection member that extends from a position on the template that is forward and starboard, to a position on the riser system equipment that is aft and starboard, 5) a connection member that extends from a position on the template that is H aft and starboard, to a position on the riser system equipment that is aft and starboard, 6) a connection member that extends from a position on the template that is aft and starboard, to a position on the riser system equipment that is aft and port, and/or 7) a connection member that extends from a position on the template that is aft and port, to a position on the riser system equipment that is aft and port.

The riser system equipment may have one corner portion to which no connection members are attached. If the riser system equipment is off centre, i.e. towards one edge or corner, of the template, the corner portion of the riser system equipment that is closest the edge or a corner of the template may have no connection members attached thereto. Optionally, all of the other corner portions may have connection members attached thereto. If the riser system equipment is off centre, i.e. towards one edge or corner, of the template, the corner portion of the riser system equipment that is further from the edge or a corner of the template may have the most connection members attached thereto, e.g. three connection members.

If the riser system equipment is off centre, i.e. towards one edge or corner, of the template the corner portion of the template that is furthest from the edge or a 3D corner of the riser system equipment may have the fewest connection members attached thereto, e.g. one or no connection members, The attachments between the connection member(s) (and the connection parts if present) and the riser system equipment and/or template may be designed and located so that the resulting loads exerted on to the riser system equipment or template are within acceptable limits. For example, in relation to the connections between the connection members and the riser system equipment (such as a SOP) these should be carefully designed so as to not cause any damage to the riser system equipment during use. The riser system equipment may not have originally been designed to be used in the subsea wellhead assembly of the present H invention (in which it is connected to the template) and as a result a detailed analysis is required to determine suitable attachment points and attachment means so as to not risk damaging the riser system equipment.

The connection part that is for attaching the connection member(s) to the template may be a bracket. The bracket may weigh less than 1000kg. This is so that the bracket is unlikely to cause damage to the wellhead and its associated components in the event that it is dropped or some other accidental event occurs during installation of the bracket.

The bracket may be arranged to be connected (directly or indirectly) to one or more connection members. For example, the bracket may be designed to be connected (directly or indirectly) to two connection members.

When the template has corners, for example when it comprises a top frame that forms a substantially square or rectangular shape (although the top-frame may not be continuous, i.e. it may not form the whole perimeter of the square or rectangular shape), a bracket may be located on one or more of the corners. For example, a bracket may be located on three corners of the top frame of the template. A bracket may be provided on some of the corners but not all of the corners. This is will depend on the number of connection members to be attached to the template at that location and whether the connection member can be directly connected to the template, for example in a pre-existing hole.

The bracket may be located so that it does not interfere with the operation of the template. For example, if the template comprises a cover, which may for example cover a wellhead when it is not in use, the bracket may be located so as to not prevent the opening and closing of the cover. H The template may comprise one or more support arms. If present, the support arms may extend from one or more of the corners, e.g. of the top-frame, of the template. This support arm may extend at an angle between 0 and 90 degrees, and 80 degrees 40 and 50 degrees or about 45 degrees downwards from the plane of the top4rame towards the sea bed. The support arm may help support the bracket that is installed at the corner of the top-frame.

The connection member(s) may each be provided with a tensioner, i.e. a device that can act to cause a tension on the connection member to which it is attached. The tensioner may be used to put the connection member into tension so as to be able to transmit forces between the riser system equipment and the template. The tensioner may be used to provide a pretension on the connection member(s). This is so that the connection member(s) can be used to reduce (compared to an assembly without connection member(s)) the load which is transmitted to the welihead from the riser system equipment.

The tensioner may be ROy-operable such as a chain jack, a chain hoist or a screw jack tensioner. The tensioner may be a mechanical rope tensioner.

The tensioner may comprise a reversal preventing mechanism, such as a ratchet mechanism, that permits movement in one direction only.

The connection member(s) may be attached to the reversal preventing mechanism. For example, the end of the connection member may comprise an engagement device, e.g. a pull-in head, for engagement with the reversal preventing mechanism of the tensioner.

The tensioner may be located between the template and the respective connection member. The tensioner may therefore be used to provide pretension and to act as a support and connection means for its respective connection member. For example, the tensioner may be attached to the template and the connection member may be attached to a part (such as the reversal preventing device) of the tensioner. H A portion of the tensioner, i.e. a connection portion, may be directly attached to or received directly in the template, such as in a hole in the frame of the template. The hole may be a pre-existing hole in the frame that was used for another purpose such as for holding the frame during installation. The hole may be at, near or towards the corners of the template. The hole may be a transponder bucket.

A portion of the tensioner, i.e. a connection portion, may be directly attached to or received in a connection part, such as the above discussed bracket that may be mounted onto the template. The bracket may be arranged to permit the attachment of two or more tensioners.

In an assembly that comprises a plurality of connection members and a plurality of tensioners, some tensioners may be attached (e.g. received) directly in the template (i.e. in the frame of the template) and some tensioners may be attached to (e.g. received in) a connection part, such as a tensioner support such as the above described bracket, that is mounted on the template or a lifting pad eye connected to the template. H The tensioners may be locked to the template or connection part by a locking device. This is so that the tensioners can be prevented from being lifted off the template or connection part or moved during use.

The tensioner may be arranged so that it can be set up and operated using a remotely operated vehicle (ROy), e.g. a ROV manipulator. This means that the assembly can be installed and set up subsea and at any water depth without difficulty, For example, during installation a deployment wire from the vessel may take the weight of the tensioner and lower it to near the installation site and then an ROV may be used to guide the tensioner into its precise installation position and set it up.

Each connection member may have a rated (permissible) tension of 200-GOOkN, 400 to 500kN or 300 to 400kN, such as about 350kN. The desired rated tension of the connection member will depend on a number of factors, such as the size and weight of the parts of the assembly, the environment is being used in and the likely forces that will act on the assembly.

A force sensor (e.g. tension sensor when the connection members are in tension), such as a load cell, may be provided on each connection member. The force sensor may be a pneumatic line tension sensor.

The force sensor may be arranged so that it can provide force readings during operation. For example, it may display the force so that it can be read by an ROV camera subsea. Alternatively, the force sensor may be arranged to provide H an indication of the force at a location topside, e.g. using a signal cable.

When installing the wellhead assembly, connection parts, e.g. clamps, may be mounted on the riser system equipment before it is deployed subsea, i.e. when the riser system equipment is topside. The connection parts, e.g. clamps, may be attached, such as bolted, onto the wellhead equipment. These connection parts may permit the connection member(s) to be connected to the riser system equipment, i.e. the connection member may be connected directly to a connection part that is mounted on the riser system equipment. For example, the connection part may have an engagement portion to which a connection member can be attached. The connection part may have a plurality of engagement portions so that a plurality of connection members can be attached to a single connection part. F If the riser system equipment, such as a blowout preventer, has a substantially square or rectangular cross sectional shape, the connection parts may be mounted onto the longitudinally extending corners (i.e. corners that are substantially vertical in use) of the riser system equipment. A connection part may be provided on each of these corners of the riser system equipment.

After the connection parts have been mounted on the riser system equipment, the riser system equipment may be deployed subsea and connected to the wellhead in a known manner.

After the riser system equipment is connected to the wellhead, the riser system equipment may be connected to the template, such as by the above described connection member(s). These connection members may have one or more of the optional features discussed above, for example, they may be a line, they may be provided with a force sensor and/or they may be connected to the template via a tensioner that is arranged to be able to pretension the connection member.

The installation method may comprise installing one or more connection parts, such as the above described brackets, onto the template and locking the connection parts in position on the template. The connection parts may be installed onto the template when it is subsea. This may be either before or after the riser system equipment has been connected to the wellhead.

After the riser system equipment has been connected to the wellhead, tensioners may be installed, A tensioner may be installed for each connection member in the assembly.

if there is a plurality of tensioners some tensioners may be connected directly to the template and some tensioners may be connected to a connection part, such as a bracket, installed on the template.

To install the tensioner it may be deployed subsea and then the connection portion of the tensioner may be attached to the template or a connection part, e.g. it may be received in a hole in the template or a hole in a connection part.

Two tensioners may be attached to one connection part.

Once installed, the tensioner may extend in a direction towards the riser system equipment.

The connection member(s) may then be installed. If the connection member is to be provided with a load cell, this may be connected to the connection member before it is connected between the template and the riser system equipment. This H may be before the connection member is deployed subsea.

To connect the connection member between the riser system equipment and the template, one end of the connection member may be connected to the riser system equipment. This may be indirectly via a connection part, such as clamp, that is installed on the welihead assembly. For example, the end of the connection member may have an engagement portion, such as a loop, that can engage with an engagement portion of the clamp. The other end of the connection member may be connected to the template. This connection may be via a tensioner.

A tension, for example 10-4OkN, may be applied to the connection member so as to cause the connection member to engage with the tensioner. When the tensioner comprises a reversal preventing device the force applied to the connection member may cause the end of the connection member to engage with the reversal preventing device, for example this may be a saw tooth interface of a ratchet mechanism.

The installation of the connection members may comprise two steps a) pull in of the connection member into the tensioner, e.g. to around lOkN, which may make the assembly reasonably straight, and b) tensioning the connection member, to increase the tension from, for example, 10kW to about 200kN. The force may vary depending on a number of factors such as the size of the assembly or the forces that are expected during operation.

If there is a plurality of connection members, the connection procedure may be repeated for each connection member.

Once the connection members are installed they may be pretensioned using the tensioner. The tensioner may be arranged so that it can be operated by an ROy.

After pretension has been applied to the assembly, it can provide support to the subsea riser system equipment and relieve the subsea wellhead from part of the bending moment caused for example by a drilling operation. H If there is a plurality of connection members, each connection member may have a different pretension.

If present, the load cell may be used to monitor the tension applied to its respective connection member.

The pretension may be applied to the connection members gradually, e.g. all the connection members may be partially pre-tensioned (relative to the final -11-H intended pretension) such as to 50% of the final pretension and then 75% of the final pretension, before increasing the pretension in all of the lines to 100% of the final pretension. This is so that the forces from the connection members to the riser system equipment can be applied gradually from the connection members to avoid having too large a net tension force on the riser system equipment.

After all of the connection members have been pretensioned, inspection and verification of the pretension may be performed regularly, e.g. about every three hours, until it appears that the system has stabilised.

The components may be deployed subsea using a heave compensated lifting line and/or an ROy. For example, the heave compensated lifting line may be used to lower the components to near the subsea assembly and then an ROV may be used to guide the components into their final position.

Certain components, such as the bracket1 tensioners and other equipment of the assembly may be attached to buoyancy elements during installation to reduce their submerged weight. This is to help reduce the likelihood damage in the event that the component is dropped during installation.

Preferably each connection member is designed, for example with regard to strength and stiffness, to keep the tension within its rated value even when subjected to a worst case accidental load.

Preferably the assembly is designed so that it has a subsea design life of a minimum of 6 months continuous operation. The life can be increased by means of a maintenance program.

In another aspect the present invention provides a subsea weilhead assembly, the assembly comprising: a subsea welihead; a template associated with the wellhead; subsea riser system equipment connected to the wellhead; and a connection member connected between the subsea riser system equipment and the template.

The connection member may provide lateral support to the subsea riser system equipment.

In a preferred embodiment the subsea wellhead assembly, comprises: a subsea wellhead; a template iocated about, and optionally connected to, the welihead; a blowout preventer connected to the wellhead; and a plurality of lines, or other connection members, extending between the subsea riser system equipment and the template so that lateral support is provided to the subsea riser system equipment via the lines or connection members.

The present invention may provide a method of installing a subsea wellhead assembly of any of the above described aspects.

The method of installing a subsea assembly may have any of the features, including the optional or preferable features, of any of the above described aspects.

One or more of the features, including the optional or preferable features, of any of the above described aspects are applicable to any of the other above described aspects of the invention.

Certain preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows a plan view of a subsea wellhead assembly; and Figure 2 shows a perspective view of another subsea wellhead assembly.

A subsea wellhead assembly I is shown in Figure 1 and another subsea wellhead assembly 1 is shown in Figure 2. The subsea well head assembly 1 comprises a well head 2. As illustrated by Figure 1 the assembly may comprise a plurality of wellheads 2, in this case four.

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.

The BOP 4 is connected to the template 6 by tension lines L. In the wellhead assembly 1 of Figure 2 there are four tension lines L and in the wellhead assembly 1 of Figure 1 there are seven tension lines L that are labelled Li to L7.

The tension lines L may be formed from links of steel wire. The tension lines L are each connected at one end to the BOP 4 via a clamp.

The clamps are connected to a part of the frame of the BOP 4.

The tension lines L are each connected at the other end to a tensioner 12.

The tensioners 12 are each connected to the templateS. 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 that is mounted on the template 6.

As shown in Figure 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.

The brackets each have a hole to permit a tensioner 12 to be connected to the bracket. As shown for example in Figure 1, a bracket may be able to be connected to two tensioners 12.

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

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

The tensioners 12 may each be a mechanical rope tensioner.

The tensioner may have a ratchet mechanism. The tension line L may have an engagement portion at one end that can engage with the ratchet of the tensioner 12 to thereby connect the tension line Lto the tensioner 12.

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

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.

For the embodiment shown in Figure 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, and whether the tensioner 12 is connected directly to the template (via a transponder bucket) or the tensioner support.

Line no Template BOP Tensioner connection connection installation _________ location location Li Fwd Port Aft Port Transponder ___________ ___________ ___________ bucket L2 Fwd Port Fwd Port Tensioner __________ ___________ ___________ support H L3 Fwd Stb Fwd Port Tensioner _________ __________ __________ ppport L4 Fwd Stb Aft Stb Tensioner _________ support [5 Aft Stb Aft Stb Tensioner _________ ____________________ support H LB Aft Stb Aft Port Tensioner __________ _______________________ support 7 L7 Aft Port Aft Port Transponder _________ ________________________ bucket The installation of the subsea welihead assembly 1 will now be discussed.

The BOP clamps are installed while the SOP 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 instaHing the SOP 4, but the hook-up of the tension lines [etc. will be performed after the SOP 4 has been installed on the wellhead 2.

The installation of the subsea wellhead assembly 1 may have the following main steps: * Preparing equipment for installation * Performing pre-installation survey * Installing SOP Clamps topside * Installing tensioner supports * Installing and locking tensioners 12 * Preparing tensioners 12 for connection to tension lines L * Hooking-up of tension lines L to the tensioners 12 * Pretensioning the lines L with the tensioners 12 * Performing a post-installation survey Firstly the equipment is prepared for installation. The tension line L may each be connected to a load cell topside.

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.

The tensioner supports may then be installed. This can be achieved by lifting the tensioner support from a cellar deck using a heave compensated lifting line and then lowering the tensioner support to a location, for example 15m, above the template 6. The tensioner support can then be guided by an ROy, 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 to the template 6. The lift wire may then be retrieved so the above steps can be repeated for each tensioner support to be installed.

Next the tensioners 12 are installed. The tensioners 12 may be deployed from the cellar deck using a heave compensated lifting line. The tensioner is lowered to a location, for example to ibm, 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.

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

The ROV may then be used to lock the tensioner 12 in position. This may then be repeated for each tensioner 12. A tensioner 12 is provided for each tension lineL Next the tension lines L are deployed from the cellar deck by using a heave compensated lifting line.

The tension line L is lowered to a location, for example 15 m, above the templateS. Using an ROV one end of the tension line L is hooked onto one of the BOP clamps. The ROV may then be used to guide the other end of the tension line L to the tensioner 12. The ROV may then be used to apply a tension of 10-40 kN to the tension line so as to connect it to the tensioner 12.

This process can then be repeated for each tension line L. The lines L may then be pretensioned using the tensioners 12.

In a preferred embodiment the lines shall be given a pretension as follows: Line L1 120 kN/12 ton Line L2= 100 kN/loton Line L3= 200 kN/ 20 ton Line L4= 210 kN/21 ton Line L5= 100 kNIlO ton Line L6 200 kN/20 ton Line L7= 120 kN/12 ton 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 of the line L that is being tensioned.

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

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.

During this procedure the output of the load cell on each line can be observed after each gradual increase in the pretension using the observation ROy.

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

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.

Depending on the readings taken by the observation ROy, 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 2 tons below the desired tension. It should be noted that if the tension is more than 5 tons from the desired tension a corrective action may be required to rectify the incorrect tension.

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 and thus may not require adjusting of the tensioners to correct this.

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.

* Pre survey of the attachments of the tension lines L to the BOP 4 and tensioner 12.

* Hard line cutter mounted on ROV if contingency cutting is required.

* Cellar deck ready to assist with lifting line.

* Position the ROV at the first tension line L to be unhooked. Relieve the pretension on the tension lines. This should be repeated for each of the tension lines L. Once it is observed that the tension line L is slack, the ROV may be used to H unhook the tension line L from the tensioner 12. Once disconnected from the tension line L the tensioner may be laid down on the roof of the template 6.

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

This process may then be repeated for each of the tension lines L. Following this the tensioners can each be lifted to the surface.

The method may then comprise 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.

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.

To retrieve the tensioner supports, the surface lift line may be attached to the tensioner support, the ROV can be used to release the tensioner support. The ROV may be used to grab the lift wire and guide the tensioner support away from the template 6.

The lift wire may then be used to lift the tensioner support to the surface.

This can then be repeated for each of the tensioner supports.

If desired, the BOP 4 can then be retrieved.

In the case that the tension lines L cannot be slackened the following contingency procedure may be followed.

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 of the BOP 4. The cut tension line L may then be unhooked from its respective tensioner 12.

Claims

CLAIMS: 1. A subsea wellhead assembly, the assembiy comprising: a subsea wellhead; a template associated with the wellhead; and subsea riser system equipment connected to the welihead; wherein the subsea riser system equipment is also connected to the template so that lateral support is provided to the subsea riser system equipment.
2. A subsea wellhead assembly as claimed in claim 1, wherein the riser system equipment is connected to the template by one or more connection members.
3. A subsea welihead assembly as claimed in claim 2, 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 2 or 3, wherein the one or more connection members is a line that is in tension.
5. A subsea wellhead assembly as claimed in claim 2, 3 or 4, wherein the one or more connection members is each provided with a tensioner.
6. A subsea wellhead assembly as claimed in claim 5, wherein the tensioner comprises a reversal preventing device that permits movement in one direction only.
7. A subsea welihead assembly as claimed in claim 5 or 6, wherein the one or more connection members are connected to the template via the tensioner.
8. A subsea wellhead assembly as claimed in any of claims 2 to 7, wherein the one or more connection members are each provided with a force sensor.
9. A subsea welihead assembly as claimed in any of claims 2 to 8, wherein the one or more connection members are each connected to the subsea riser system equipment via a clamp.
10. A subsea wellhead assembly as claimed in any of claims 2 to 9, wherein the one or more connection members are each connected to the template via a bracket.
11. A subsea wellhead assembly as claimed in any preceding claim, wherein the subsea riser system equipment is a blowout preventer. H
12. 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 so that lateral support is provided to the subsea riser system equipment.
13. A method as claimed in claim 13, wherein the subsea riser system equipment is connected to the template whilst they are subsea.
14. The method as claimed in claim 13 or 14, wherein the subseawellhead assembly is the subsea wellhead assembly as claimed in claims ito 12.