FIELD The disclosure relates to semi submersible vessels for manoeuvring offshore infrastructures or parts thereof; in particular, the disclosure may relate to column- stabilised semi submersible vessels with large lift capacities for use during decommissioning of offshore infrastructures. BACKGROUND

The field of offshore infrastructure removal is relatively new and is likely to become more of a focus for activity in the coming years as more offshore infrastructures are decommissioned.

Once an offshore infrastructure has been decommissioned, the infrastructure itself needs to be removed from its offshore location, brought on shore and dismantled. Given the complexity of offshore infrastructures, it is undesirable to dismantle such infrastructures in situ (i.e. offshore). As such, vessels have been developed in order to allow large parts of the infrastructure (i.e. the topsides, jacket etc.) to be removed from the site and transported on shore before being dismantled further.

Some systems have been developed which allow entire topsides to be disconnected from the jacket and transported onshore in a single piece. Such systems may comprise two large vessels such as very large crude containers (VLCC) - a.k.a. oil tankers - which are connected and arranged to receive, separate and support topsides between the two vessels. As such systems may essentially comprise two parts (the oil tankers) designed to move through the water, rather than be stationary, it is often difficult to find a sufficient window of time during which the vessel is steady enough to receive and separate the topsides from the jacket.

Other current systems utilise large ship-based, barge-based or column-stabilized cranes. The offshore infrastructure may be cut up into parts and each part may be hoisted by the crane onto the deck of the ship or transportation barge. These systems are typically not suitable for removing large topsides from infrastructures, and so are often employed in combination with systems such as those described above. Additionally, items hoisted by such systems are often lifted from the seabed/sea level at an angle, and rotated onto their side as they are hauled onto the ship. This can result in damage and pose an undesirable risk of mishap.

It would be desirable to develop a vessel suitable for the use in decommissioning offshore infrastructures which is more robust and efficient than current solutions and provides a large operation window offshore. SUMMARY OF INVENTION

According to an aspect of the present disclosure is a semi submersible vessel for manipulating offshore infrastructures, the vessel comprising:

a first hull portion;

a second hull portion, laterally spaced from the first hull portion;

a central hull portion connecting the first and second hull portions; wherein

the first hull portion, second hull portion and central hull portion collectively define an open receiving section for receiving an offshore infrastructure or a part thereof into between the first hull portion and second hull portion; and

each of the first and second hull portions comprises a plurality of laterally movable support beams arranged to extend from the respective hull portion into the receiving section for supporting an offshore infrastructure or a part thereof.

The vessel may be for use in the decommissioning of offshore infrastructures. Offshore infrastructures may comprise a platform and subsea components comprising, for example, topsides, jacket, conductor string, subsea template and associated piping. The vessel may therefore be for, or configured - i.e. sized and shaped - to separate, cut, lift, lower, support and transport topsides, jackets, piping strings, subsea templates and manifolds or parts of any of the above.

In order to achieve this, the vessel may be suitable for, or configured to, lift up to 10000, 20000, 30000, or over 30000 metric tonnes. The vessel is a semi submersible vessel and so is able to transform from a deep to a shallow draft by deballasting portions of the hull.

The vessel may be configured to move to the location of an offshore infrastructure. When lifting topsides, the vessel may first increase its draft such that it is sitting lower in the water; this may allow the vessel to position its support beams under the topsides. The vessel may be configured to then move into position, adjacent the offshore infrastructure, such that the infrastructure is located between the first hull portion and the second hull portion, in the receiving section, and the support beams are located under either side of the topsides.

Feet on the support beams may engage the topsides and the topsides may be cut from the supporting framework, e.g. the jacket. The hull may then be deballasted such that the vessel rises, lifting the topsides from the jacket.

When lifting (for example) a jacket or a part thereof, the vessel may not need to increase its draft to the same extent as when lifting topsides, as the support beams may not need to be inserted under the payload. Instead, the vessel may be moved into position with the jacket located in the receiving section, between the first and second hull portions. Winches, attached to the support beams, may be connected to the jacket and may be used to lift the jacket from the seabed once the jacket is separated by - for example - remotely operable cutting units.

The vessel may be configured to lift its payload to a height sufficient to allow part of a barge to enter the receiving section, under the payload, in order to receive the payload, which may be lowered onto the barge either by using the winches or by slowly increasing the draft of the vessel. The payload may then be secured to the barge and released by the vessel. It is assumed that, before the vessel is operated, the wells have been made safe and suitable for the removal/installation of an offshore structure or parts thereof. As such, all wells may have been plugged and abandoned in accordance with industry norms, surveys may have been completed and suitable attachment/separation points may have been determined and marked. The semi submersible vessel may be a column-stabilised semi submersible vessel. Use of a column-stabilised semi submersible vessel may improve the motion characteristics of the vessel and thus increase an operational window for dealing with the offshore infrastructure compared to that provided by non-column-stabilised vessels, in particular during poor weather.

The hull portions may be hull parts or sections. Each of the first hull portion and second hull portion may comprise a deck portion and a pontoon portion connected to the deck portion with columns. The central hull portion may comprise a deck portion and a pontoon portion connected to the deck portion with columns.

A, or each, deck portion may be substantially prismatic in shape - that is, having a constant cross portion shape and size along its entire length. A, or each, deck portion may be substantially cuboidal. A, or each, pontoon portion may be prismatic in shape. A, or each, pontoon portion may be cuboidal.

The deck portions of the first hull portion and second hull portion (and the central hull portion, if present) may be separate and discrete, or unitary and integral. The pontoon portions of the first hull portion and second hull portion (and the central hull portion, if present) may be separate and discrete, or unitary and integral. In such an example the vessel may essentially comprise a deck portion and pontoon portion connected to the deck portion by columns; wherein the deck and pontoon portions define the first, second and central section. Use of the above arrangement of deck portions and pontoon portions, or variations of the above arrangement, may provide more stability when the vessel is stationary compared to hull portions which are arranged more hydrodynamically (e.g. ships).

The deck portion may comprise a deck. The deck may comprise a helicopter landing pad, a storage area and any other feature commonly found on crane vessels and offshore decommissioning vessels.

The first and second hull portions may be parallel and laterally spaced to define two sides of a quadrilateral. The central hull portion may extend between the first and second hull portions, to define a third side of the quadrilateral. The first hull portion, second hull portion and central hull portion may define three sides of a rectangle. The fourth side of the rectangle may be open, as may a central portion of the rectangle. The open side and central portion of the rectangle may constitute the receiving section.

Sides of the first hull portion, second hull portion and central hull portion may define the outer profile of the vessel. The outer profile of the vessel, when viewed from above (i.e. along a direction perpendicular to the plane of the sea during use) and considering the receiving section as a part of the vessel, may be a quadrilateral.

The quadrilateral may be a rectangle. It should be noted that the term "rectangle" as used herein is construed to include square, which is a regular-sided rectangle. The quadrilateral-shaped outer profile of the vessel may comprise a first length and a second, substantially perpendicular, length. The ratio of the two lengths may be about 1.0 (e.g. the outer profile is a square). The ratio of the two lengths may be substantially in any of the following ranges: 1 .0 - 1 .1 , 1 .0 - 1.2, 1 .0 - 1.35, 1 .0 - 1 .5, 1 .0 - 2.0; this list is non-exhaustive.

A vessel with a first to second length ratio of close to 1 is more stable when stationary than a vessel with a ratio far greater than 1 . A more stable vessel, with better motion characteristics, can be used to receive, separate and support parts of an offshore infrastructure in rougher seas than a vessel with a ratio far greater than 1 .

The first hull portion, second hull portion and central hull portion may be arranged to form a "U" when viewed from above. The opening of the "U" may form part of the receiving section. The hull portions may be connected end to end, with each connection constituting a 90 degree connection. As such, the three hull portions may define three sides of a rectangle. The hull portions may be of roughly equal length, in which case the three hull portions may define three sides of a square.

The first hull portion and the second hull portion (and optionally the central hull portion) may define a space thereinbetween. The space may be the receiving section. The receiving section may comprise an opening to allow an offshore infrastructure to be moved into between the first and second hull portions. The receiving section may comprise an open expanse between the first and second hull portions. The receiving section may be bounded on two opposite sides by the first and second hull portions. The receiving section may be bounded on a third side by the central hull portion. The receiving section may be located within the space bordered by the first hull portion, second hull portion and the central hull portion. The receiving section may be substantially rectangular.

The receiving section may be a moon pool. The receiving section may be a moon pool with an opening to allow infrastructure to enter the receiving section from the open sea without having to be submerged.

In order to manipulate a part of an offshore infrastructure, the vessel will need to be moved into position. The vessel may comprise a propulsion system for propelling the vessel in any direction in a plane. The propulsion system may comprise any combination of known components suitable for moving and controlling a semi submersible vessel of this scale.

Where features are referred to collectively herein (e.g. "the support beams" or "the feet"), it is to be understood that the term and associated description may apply to a single one of the feature (e.g. "one support beam" or "one foot"), to some of the features, or to each of the features (e.g. "each support beam" and "each foot").

The support beams are laterally movable to increase or decrease the amount by which they extend from the first/second hull portions. The support beams are arranged to move laterally with respect to the vessel, which may be along the longitudinal axis of the support beams themselves. The lateral movement may be achieved by use of an automatic or manually operated motor, or by alternative means. The support beams may be single, load supporting cantilevers. The support beams may be arranged to be inserted under topsides and lift topsides from below. The support beams may be for, or configured for, lifting weight from below. The support beams may be configured (i.e. sized, shaped and attached) such that they can collectively support the weight of the offshore infrastructure on their upper surface(s). The support beams may be built into the corresponding hull portion. This may mean that the support beams may be at least partially located within, or at least part of the support beam must pass through, the corresponding hull portion (e.g. the outer boundaries thereof), as opposed to being attached to an outer surface of the hull portion. This may mean that the support beams are recessed into the corresponding hull portion. The support beams may be arranged to extend out from the (respective one of the) first hull portion or second hull portion. The support beams may be arranged to protrude from a side surface of the respective hull portion. The support beams may be arranged to pass through the corresponding hull portion.

The support beams may be arranged below the top surface (e.g. the top surface of the deck portion) of the first hull portion and second hull portion. The support beams may be arranged such that a top surface of the support beam is level with (e.g. co-planar with) a top surface of the first or second hull portion.

When the support beams extend out from, i.e. through, the first and second hull portions, rather than being mounted onto a surface (e.g. top surface) of the hull portions, it is easier to distribute any loads carried by the support beams. The loads are more easily distributed to the hull and thus local stress concentrations in fixings are reduced or avoided. This, ultimately, may allow greater weights to be lifted by the vessel, or may reduce fatigue and hence prolong the service lifetime of the vessel.

When the top surface of the support beams are substantially level with a deck surface, the deck above the support beams can be used as a lay down area for decommissioning equipment.

The support beams may comprise a foot for contacting and supporting part of an offshore infrastructure (e.g. topsides). The feet may be movable relative to their respective support beam. The feet may be configured to move relative to the support beam in order to compensate for motion of the vessel caused by waves (e.g. heave). That is, the feet may be configured to move relative to the support beams to counteract the wave movement and keep the feet still relative to an offshore infrastructure while the vessel moves relative to the offshore infrastructure due to movement of the waves. The feet may comprise a contact surface for contacting part of an offshore infrastructure (e.g. topsides). A foot or each foot may comprise a sensor configured to detect contact between the foot and part of an offshore infrastructure.

A foot or each foot may comprise a contact lock for fixing the foot relative to a part of an offshore infrastructure. The contact lock may be configured to prevent any slippage between the foot and the part of offshore infrastructure.

One, at least two, or each of the support beams may comprise a winch system for hoisting part of an offshore infrastructure located in the receiving section. At least one of the support beams of the first hull portion and one of the support beams of the second hull portion may comprise a winch system for hoisting part of an offshore infrastructure located in the receiving section. The winch system may be attached to the support beam. The winch system may be integral with the support beam. The winch system may be arranged (e.g. located and of a suitable size) to vertically hoist a part of an offshore infrastructure located in the receiving section.

One of the support beams of the first hull portion and one of the support beams of the second hull portion (e.g. a laterally-adjacent and corresponding support beam) may each comprise a winch system. The winch systems may be arranged to collectively hoist part of an offshore infrastructure located in the receiving section.

The winch systems may be integral with the support beams. The winch systems may be located to extend from an underside of the support beams. The winch systems may be located towards, or at, the end of the support beam which is arranged to extend over the receiving section.

The winch system may comprise a linear winch. The linear winch may have a capacity of up to 1000T.

The winch system may comprise a drum winch. The drum winch may have a capacity of up to 50T, 100T, 150T, 200T, or an alternative value. The winch system may comprise both a linear winch and a drum winch, as described above. The linear winch and drum winch may be configured to work independently and/or in tandem. The drum winch may be used as a winch-cable storage drum for the linear winch. The drum winch may also be configured to operate independently of the linear winch for higher speed pick-ups.

The winch system may comprise winch cables of chain, wire or HDPE ropes. The winch system may comprise one, a couple of or a plurality of, lifting collars for attaching the winches, winches or winch cables thereof to the part of the offshore infrastructure (e.g. a jacket or part thereof). The winch system may comprise one, or a plurality of hydraulically activated lifting attachments. The hydraulically activated lifting attachments may be for use in attaching a jacket or part of a jacket to the winches. The hydraulically activated lifting attachments may be configured to attach to the jacket by attaching the inside of a pole of a jacket. The lifting attachments may be configured to be located inside a leg, or cross-member of the jacket and to hydraulically activate to grip the leg or cross-member. The winch system may be arranged to provide up to 25m, 35m, 45m, 55m, 65m or more than 65m of vertical lift.

Collectively, the winch systems may be configured to lift up to 5000, or up to 6000, 7000 or 8000 tonnes. The winch systems may be configured to collectively lift more than 8000 tonnes.

The vessel may comprise a brace member arranged to extend between the first hull portion and the second hull portion in order to increase the rigidity and strength of the vessel when carrying a load.

The brace member may be arranged to connect to the end of the first and second hull portion distal from the central hull portion. The brace member may be arranged to extend across an entrance of the receiving section. When the first, second and central hull portions define three sides of a quadrilateral when viewed from above, the brace member may be arranged to define the fourth side of the quadrilateral. The brace member may comprise two parts configured to move between a disconnected and a connected arrangement. The two parts may be connected to the first hull portion and second hull portion, respectively. In the disconnected arrangement, the brace member may not extend between the first and second hull portions. In the connected arrangement, the brace member may extend between the first and second hull portions.

When the brace member extends across an entrance of the receiving section i.e. when the brace member is in the connected arrangement, the brace member may prevent part of an infrastructure entering the receiving section. The brace member may therefore need to move to a disconnected arrangement to allow part of an offshore infrastructure to enter the receiving section. Each part of the brace member may be hingedly connected to one of the first and second hull portions respectively. Each part of the brace member may be arranged to rotate upwards from a horizontal orientation towards a vertical or near-vertical orientation to disconnect the brace member and rotate back to a substantially horizontal orientation to connect with the other part of the brace member and connect the brace member. In other examples, the brace member parts may rotate about a vertical axis, or may be arranged to separate and connect by moving laterally with respect to the vessel (which may be parallel to a longitudinal axis of the brace member). The vessel may comprise a crane. The crane may be located on the first, second or central hull portion. The crane may be located on a deck of the first, second or central hull portion.

The vessel may comprise a pipe rack for receiving and supporting pipes hoisted by the winch system or crane.

The vessel may be used to hoist subsea pipe section, using the winch system(s) or crane. The pipe rack may be connected to one of the first or section hull sections and may be arranged to receive pipes or pipe sections once they have been lifted by the vessel. The pipe rack may be arranged to cradle pipe sections parallel to a side of the first, second or central hull section.

The winch systems of multiple support beams may be configured to collectively move the pipes and deposit them in the pipe rack. This operation may be controlled by a control module, which may be configured to individually control the winch systems.

The vessel may comprise pile drivers and may be configured to manoeuvre piles into position and install piles in the seabed.

The vessel may comprise a control module. The control module may comprise any of the following modules. Any control module may comprise a processor and data storage device. The data storage device may comprise computer-readable instructions. The processor may be configured to provide the capabilities of any of the following modules when executing the computer-readable instructions.

The vessel may comprise a dynamic positioning (DP) control module. The DP control module may be configured to control the movement and positioning of the vessel. The DP control module may be configured to control the movement and positioning of the vessel when it approaches (and moves to engage/surround) an offshore infrastructure. The DP control module may comprise a known class 3 DP system.

The vessel (for example the DP control module) may comprise a position reference system. The position reference system may comprise a laser based time of flight system or positional measurement equipment, for example Cy-Scan™ or Fan Beam™. The position reference system may comprise a radar based time of flight system or positional measurement system, for example RadScan™ or Radius™. Targets for the position reference system may need to be located on an offshore infrastructure before the position reference system can be used.

The DP control module may be configured to receive data from a position reference system and use this data to control the propulsion system of the vessel.

The DP control module may be configured to control the movement of the vessel in order to compensate for motion of the vessel caused by waves. The DP control module may be configured to move the vessel to compensate for yaw, sway or surge movements. The DP control module may be configured to move the vessel to keep the vessel stationary with respect to an offshore infrastructure, despite sea movement. The vessel may comprise a foot stabilisation module. The foot stabilisation module may be configured to control the movement of the feet, relative to the support beams, in order to compensate for motion of the vessel caused by waves. The foot stabilisation module may be configured to move the feet to compensate for heave, sway or surge movements. The foot stabilisation module may be configured to control the feet such that they remain substantially stationary with respect to an offshore infrastructure as the vessel moves due to the motion of the waves. The foot stabilisation module may be configured to control the motion of the feet as the vessel is deballasted to engage and lift the (for example) topsides. The vessel may comprise a tether measurement system . The tether measurement system may be connected to the DP control module and/or the foot stabilisation module. The tether measurement system may be configured to output data relating to the position of an offshore infrastructure (e.g. to the DP control module). The data may be received as an input by the DP control module and/or the foot stabilisation module. This information may then be used to control the propulsion mechanisms to manoeuvre the vessel.

The tether measurement system may be configured to measure a position of an offshore infrastructure located in the receiving structure. The position may be measured by attaching a plurality of tethers from the vessel to the infrastructure and using the tethers to measure the location of the infrastructure.

The tether measurement system may comprise a plurality of tethers (connected to the vessel) configured to be connected to an offshore infrastructure located in the receiving section. The tether management system may comprise 4 tethers. The tethers may be flexible. The tethers may be, for example, cord, connecting line, rope or chain. The tethers may be pretensioned.

The tether measurement system may comprise a plurality of attachment points on the vessel, to which corresponding tethers are attached. Attachment points may then be attached to the offshore infrastructure to which the other ends of the tethers are connected.

The tethers may comprise an in-line strain gauge. The in-line strain gauge may be used to determine a force component along the axis of the tether. Each in-line strain gauge may determine a force component. The tether measurement system may sum these force components to determine a total force component. In order to try to keep an offshore structure in the correct location within the receiving section, the tether measurement system (or DP control module) may be configured to manoeuvre the vessel to null this force vector, for example by using the propulsion system.

That is, the force component of the offset vector may, together with force components obtained from the other strain gauges in other tethers, provide a total offset force component of the force vector which the control module (e.g. propulsion control system controlling the propulsion system of the vessel) will attempt to null.

The tether measurement system may comprise a sensor configured to measure an angle of the tethers. The measured angle may be the fleet angle (e.g. an angle between the tether and the horizontal). The sensor may comprise a potentiometer connected to a sheave of a vessel-mounted attachment point or stanchion.

The tether measurement system may be configured to use the in-line strain gauge and sensor to determine a position of an offshore infrastructure within the receiving section. The strain and angle information can be used to determine the force vector of each attachment point on the offshore structure from the corresponding attachment point on the vessel. This information therefore provides a force vector at each possible location points for each tether. Superimposing the force vectors of each tether will therefore allow the tether measurement system to derive the total offset force vector to be nulled by the propulsion control system.

The tethers may comprise a weak point or section, arranged to break at a threshold force such that excessive loads are not transferred to the offshore structure. The tether measurement system may comprise a connecting line. This may be in addition to the tethers. The connecting line may be connected to a constant tension device and may be configured to be attachable to the offshore infrastructure. The constant tension device may be configured to increase or decrease the length of the line as required, in order to maintain a constant tension in the line. The tether measurement system may comprise a device configured to monitor and output data regarding the length of the connecting line; this may be undertaken by the constant tension device. The constant tension device may be a constant tension winch. The tether measurement system may comprise a sensor configured to measure the fleet angle of the connecting line. The sensor may comprise a turntable with integral potentiometer. The length of the line and the fleet angle may be used to determine the distance of the offshore infrastructure from the point at which the connecting line is attached to the vessel. The tether measurement system may comprise a sensor configured to measure an angle in the plane of the sea surface in order to provide a secondary measurement which can be combined with the distance of the offshore infrastructure from the point at which the connecting line is attached to the vessel to determine the location of the offshore infrastructure. Alternatively, the connecting line may be mounted on a trackway on the vessel to monitor transverse displacement of the offshore infrastructure relative to the vessel. The connecting line may be mounted on a trackway on the vessel to monitor and output data regarding the displacement of the offshore infrastructure perpendicular to the connecting line. The tether measurement system may be configured to use the length of the connecting line, the fleet angle of the connecting line; and the displacement of the offshore infrastructure perpendicular to the connecting line, to determine a position of an offshore infrastructure within the receiving section. Data from the tether measurement system may be input into the DP control module or the foot stabilisation module.

The tether measurement system may be configured to be disconnected from the offshore infrastructure once the part to be manoeuvred is fully supported by the vessel. This may be done by disconnecting the tethers from the attachment points and subsequently removing the attachment points from the offshore infrastructure.

The vessel may also comprise remotely operated cutting units configured to cut through sections of the jacket and/or other aspects of the offshore structure. This may allow a part of the offshore structure to be separated and lifted from the rest of the fixed offshore structure.

The vessel may also comprise a ROV control module configured to control ROVs for cutting parts of the offshore infrastructure. The ROV control module may be configured to implement simultaneous cutting of parts of the offshore infrastructure.

Further according to an aspect of the disclosure is a method of lifting topsides of an offshore infrastructure, the method comprising:

manoeuvring a vessel as described anywhere herein to an offshore infrastructure;

positioning the vessel such that the offshore infrastructure is located in the receiving section, between the first hull portion and the second hull portion, and the supporting beams are located below the topsides;

positioning the feet such that they support the topsides;

separating the topsides from the supporting framework by cutting framework; and

deballasting the vessel such that the topsides are lifted from the supporting framework.

Further according to an aspect of the disclosure is a method of lifting a jacket or part thereof of an offshore infrastructure, the method comprising:

manoeuvring a vessel as described anywhere herein to a jacket or a part thereof;

positioning the vessel such that the jacket or a part thereof is located in the receiving section, between the first hull portion and the second hull portion; connecting winch cables of the support beams to the jacket or a part thereof; and

lifting the jacket of a part thereof using the winches and/or deballasting the vessel to lift the jacket or a part thereof vertically upwards. It is to be understood that a vessel according to the disclosure may also be configured to install offshore infrastructure - for example topsides or jackets. Offshore infrastructure may be installed using the same features of the vessel used to remove offshore infrastructure. Generally speaking, the same operations are used to install offshore infrastructure as to uninstall it; the operations are simply undertaken in reverse.

For example, the vessel may be configured to open its brace member, increase its draft, and locate a barge carrying an offshore structure within the receiving section. The supporting beams may then extend and engage the topsides, or the winches may be attached to the jacket or part thereof. The vessel may lift the offshore infrastructure from the barge by deballasting while simultaneously lifting the offshore infrastructure using the winches or movable feet.

Once lifted, the vessel may position itself such that the offshore structure installation location is located in the receiving section. The winches may then be used to lower the jacket into position, or the pontoons may be ballasted to increase the vessel's draft and lower the topsides towards an already-installed jacket. Once the topsides are almost in position, the feet may be used to lower the topsides onto the already-installed jacket. Once the topsides or jacket are in position, normal means may then be employed to connect the topsides to the jacket or the jacket part to existing infrastructure.

When installing parts of an offshore infrastructure, the vessel may be configured to lift a part of an offshore infrastructure off of a barge in a horizontal orientation, and rotate/upend the part into a vertical position for lowering onto the seabed/existing infrastructure. Rotating a supported part of an offshore infrastructure may be achieved by using a plurality of winch systems (and/or cranes located on the deck) and selectively controlling the winches (and/or cranes) to hoist one end or side of the part while lowering the other end or side of the part.

Thus, further according to an aspect of the disclosure is a method of installing topsides on an offshore infrastructure, the method comprising: positioning a vessel as described anywhere herein such that the topsides are located in the receiving section, between the first hull portion and the second hull portion, and the supporting beams are located below the topsides;

positioning the feet such that they support the topsides;

deballasting the vessel such that the topsides are lifted;

manoeuvring the vessel to an offshore infrastructure;

positioning the vessel such that the offshore infrastructure is located in the receiving section, between the first hull portion and the second hull portion; lowering the topsides onto the offshore infrastructure by ballasting the vessel and lowering the feet;

connecting the topsides to the offshore infrastructure.

Further according to an aspect of the disclosure is a method of installing a jacket or a part thereof, the method comprising:

positioning a vessel as described anywhere herein such that the jacket or a part thereof is located in the receiving section, between the first hull portion and the second hull portion;

connecting the winch cables of the support beams to the subsea jacket or a part thereof; and

lifting the jacket or a part thereof using the winches and/or by deballasting the vessel to lift the subsea jacket or a part thereof vertically upwards;

manoeuvring the vessel to an offshore infrastructure or a site for an offshore infrastructure;

positioning the vessel such that the offshore infrastructure or a site for an offshore infrastructure is located in the receiving section, between the first hull portion and the second hull portion;

lowering the jacket or a part thereof using the winches and/or by ballasting the vessel;

connecting the jacket or a part thereof to the offshore infrastructure or a site for an offshore infrastructure. SPECI FIC DESCRIPTION

Examples of the present disclosure will now be described, purely by way of example, in the below figures, in which:

Figure 1 is a perspective view of an example of the present disclosure; Figure 2 is further perspective view of the vessel of figure 1 ; Figure 3 is a top view of the vessel of figure 1 ;

Figure 4 is a detail view of the vessel of figure 1 during use; Figure 5 is a further detail view of the vessel of figure 1 during use;

Figure 6 is a perspective view of a further example of the present disclosure, with the topsides being carried by the vessel;

Figure 7 is a further perspective view of the vessel of figure 6, with a barge located within the receiving section;

Figure 8 is a further perspective view of the vessel of figure 6, with the topsides being lowered onto the barge;

Figure 9 is a further detail view of the vessel of figure 1 during use;

Figure 10 is a further detail view of the vessel of figure 1 during use;

Figure 1 1 is a further detail view of the vessel of figure 1 during use;

Figure 12 is a perspective view of topsides mounted on a barge;

Figure 13 is a perspective view of parts of a jacket mounted on a barge; and

Figure 14 is a close up view of a pipe rack for use with the vessel of figure 1 . DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 shows a semi submersible vessel 10 for manipulating (e.g. separating, lifting, lowering, installing, transporting...) offshore infrastructures or parts thereof. In figure 1 , an offshore structure 12 comprising topsides and a jacket is located within the vessel 10.

The vessel 10 has a first hull portion 14, second hull portion 16 and central hull portion 18 which are each connected substantially at right angles to form a "U"-shaped vessel when viewed from above or below. Each hull portion comprises a deck portion 14a, 16a, 18a and a pontoon portion 14b, 16b, 18b. The deck portions and pontoon portions are connected by columns 14c, 16c, 18c. The pontoon portions 14b, 16b, 18b comprise pumps and supporting equipment for ballasting and deballasting during use. All of the deck portions, pontoon portions and column portions are substantially cuboids.

Various equipment such as cranes, offices, control rooms, maintenance rooms and a helipad are present on, or attached to, the deck of the vessel.

The first hull portion 14, second hull portion 16 and central hull portion 18 collectively define a central area which is arranged to receive an offshore infrastructure - a receiving section. The receiving section is a substantially rectangular area in which an offshore infrastructure may be located. The receiving section is bounded on three sides by the first 14, second 16 and third hull portions 18. The fourth boundary of the receiving section is open, or is bounded by an openable and closable brace member 22.

The brace member 22 extends across the receiving section, between the first hull portion 14 and second hull portion 16. The brace member 22 provides increased structural rigidity to the vessel 10.

The brace member 22 comprises two parts - one connected to each of the first hull portion 14 and the second hull portion 16. The parts are hinged about their respective connections to the first 14 and second 16 hull portions such that they can be independently raised and lowered. When in a lowered position the ends of the two brace member parts engage, forming the brace member 22 which extends between the ends of the first hull portion 14 and section hull portion 16 furthest away from the central hull portion 18.

The brace member 22 extends across an entrance to the receiving section. As such, an offshore infrastructure 12 is prevented from entering or leaving the receiving section when the brace member 22 is in place. To allow an offshore infrastructure 12 to enter or leave the receiving section, the two parts of the brace member 22 need to rotate upward (see for example figure 2), to allow access to the receiving section. The brace member 22 may be configured to be engaged during any lifting or weight-taking procedure in order to provide increased rigidity. The brace member 22 may be configured to be engaged when the vessel is moving with a load.

Each of the first hull portion 14 and the second hull portion 16 comprises a plurality of support beams 20. The support beams 20 are configured to move laterally with respect to the vessel 10 such that they can move inwards and outwards with respect to the receiving section, along their longitudinal axes. The support beams 20 of the vessel of figure 1 are able to move such they almost entirely withdraw from the receiving section.

The support beams 20 are built into an upper area of the corresponding hull portion 14, 16. They are arranged such that they are at least partially located within the hull portion, rather than being attached to the deck the hull. In the present example a top surface of the support beams 20 is substantially co-planar with the top surface of the hull. Having the beams built into the corresponding hull portion 14, 16 can improve load distribution and so reduce stresses and increase the service-life of the vessel 10.

The longitudinal axis of the support beams extends laterally across the vessel i.e. across part of the receiving section such that they extend into the receiving section from either side. The support beams are rectangular, hydraulically-operated structures, which, collectively, can support up to at least 10000, 20000, or 30000 metric tons. The support beams are single-member cantilever structures for collectively supporting topsides on their upper surface.

In figures 1 and 2, the support beams 20 are in a partially-withdrawn arrangement. Accordingly, they are located laterally outwards in order to project into the receiving section as little as possible. This arrangement of the support beams 20 allows an offshore infrastructure 12 to enter the receiving section without being obstructed by a support beam 20.

When removing topsides, the vessel 10 is ballasted so that it lowers in the water and increases its draft prior to positioning the vessel 10 such that the offshore infrastructure 12 is located in the receiving section. The draft is increased to a level whereby the top surfaces of the support beams 20 are lower than the underside of the topsides. The brace member 22 is moved to a disengaged arrangement (i.e. open position). Dynamic Positioning (DP) methods are then used to manoeuvre the vessel 10 such that the offshore infrastructure is located in the receiving section. Once the offshore infrastructure 12 is located in the receiving section, the brace member 22 is engaged, as shown in figure 1 .

Once the offshore infrastructure is located within the receiving section, a tether measurement system is used as an input to the DP method such that the vessel moves relative to the offshore infrastructure as little as possible.

To connect the tether measurement system multiple (normally 4) attachment points are located on the offshore infrastructure. A tether is then connected to each attachment point. The attachment points and tethers can be connected by ROVs.

The tether measurement system (not shown) has 4 tethers connecting the vessel to the offshore structure. The tethers are pretensioned (e.g. to about 0.5 ton). Each tether has an integral strain gauge to provide an indication of a force vector along the tether, and an angle sensor to measure a fleet angle of the tether. Using the strain gauge and fleet angle, a vector can be obtained for each tether and, combining the vectors, an overall force vector for the offshore structure can be obtained. The control module (e.g. DP module) may then use the propulsion system to null this force vector and hence maintain the offshore structure in the desired location. Turning now to figure 2, the same vessel can be seen except with the brace member in an open configuration.

Figure 3 shows the support beams 20 adjacent the offshore infrastructure 12 extended out from their respective hull sections 14, 16 such that their ends are located under the topsides.

Once some, or all, of the support beams 20 have been moved into position, a foot 30 on the end of the respective support beams 20 is moved vertically to abut the underside of the topsides 24. The foot 30 comprises an abutting surface for contacting the underside of the topsides 24 and a movement mechanism connecting the foot 30 to the support beam 20 and controlling the relevant movement of the foot 30 to the support beam 20.

Figure 4 shows a foot engaged with the underside of the topsides.

Movement of the foot 30 may be achieved by a combination of deballasting the vessel 10 such that the whole vessel 10 moves upwards and using the movement mechanism to control the movement of the foot 30 relative to the support beam 20. As the foot approaches the underside of the topsides 24, it is important that the foot 30 does not impact the topsides 24. A foot stabilisation module (not shown) is used to control the movement of the foot 30 and compensates for movement of the vessel due to the sea such that the movement of the foot 30 relative to the topsides 24 is not affected by the heave, sway or surge of the vessel due to the waves. The tether measurement system described above may be used as an input to the foot stabilisation module.

The feet 30 are moved until they make contact with the topsides 24. This is monitored by a load cell integrated into each foot 30. Each foot is secured to the topsides to avoid slippage. A diamond belt cutting unit 28 is attached to each leg of the jacket 26 which connect the topsides 24 to the jacket 26. The cutting units 28 simultaneously cut through each leg 26 after the feet 30 have made contact with the topsides 24. Once all of the legs 26 are cut, the topsides are free from the jacket 26. The vessel 10 is then rapidly deballasted in concert with maximum extension of the foot stabilisation module (a heave compensation mechanism) such that the vessel 10, support beams 20, feet 30 and topsides 24 are quickly moved away from the jacket and the rest of the offshore structure, which remains in the water. Figure 5 shows the topsides 24 after they has been separated and lifted from the jacket 26. It can be seen that three support beams 20 and corresponding feet 30 are supporting the topsides 24 on each side. One of many propulsion units 34 can also be seen in figure 5, attached to the underside of the pontoon portion of the hull. Once the topsides 24 have been separated and lifted from the jacket 26, the vessel 10 can move away from the jacket 26, which remains in the water. If the brace member 22 was closed/engaged after the offshore structure entered the receiving section, it will need to be disengaged before the jacket 26 can leave the receiving section. Once the topsides 24 are supported by the vessel 10, the tether measurement system is disconnected. This is done by disconnecting the tethers from the attachment points on the topsides 24 and removing the attachments points from the topsides 24.

Figure 6 shows an example where the topsides 24 have been separated and lifted from the jacket 26 and is being supported on the support beams 20.

Once the topsides 24 have been removed, it may be necessary to transfer the topsides 24 to a barge 32 in order to transport it back to shore. This can be achieved using a similar procedure as that to remove the topsides 24, except in reverse.

A barge 32 is positioned close to the vessel 10. The brace member 22 is disengaged to allow the barge 32 or a part thereof to enter the receiving section. DP methods are used to position a load supporting section of the barge 32 in the receiving section (i.e. between the first and second hull portions, under the supported topsides 24). This is shown in figure 7. Once the barge 32 is in position, the vessel 10 is ballasted to increase its draft and lower its height in the water. This lowers the topsides 24 towards the barge 32. Once the topsides 24 are close to the load-bearing surface of the barge 32, the feet 30 are moved to gently lower the topsides 24 onto the barge 32. The tether measurement system, DP control module and foot stabilisation module will again be used to ensure the topsides 24 are lowered onto the barge 32 in a controlled manner. Figure 8 shows the topsides 24 being lowered onto the barge 32. Once the topsides 24 are being supported by the barge 32, the supporting beams 20 and feet 30 disengage the topsides 24 and the barge 32 can move out of the receiving section of the vessel 10. The vessel 10 can then deballasted.

Figures 9 and 10 show a jacket 26 located in the receiving section of the vessel 10. In figure 9, four support beams 20a, 20b, 20c, 20d have been extended into the receiving section. The support beams 20a, 20b, 20c, 20d have winch systems. The winches in this example are located on the underside of support beams 20a, 20b, 20c, 20d. Winch cables 36 can be seen descended from the support beams 20a, 20b. The winch cables are attached to the jacket 26 by lifting collars, which are configured to wrap around poles of the jacket 26. Alternatively, the lifting attachment may be of hydraulically actuated design and run internal to the jacket legs. In the shown arrangement, two support beams 20a, 20c, 20b, 20d extend from either side of the receiving section (i.e. two from each of the left and right hull portions). It is to be understood, however, that any number of support beams may be used to an offshore structure part, depending on the size and weight of the part.

The vessel 10 is moved into position, with the jacket 26 located in the receiving section in an analogous manner to that described above with respect to the topsides. However, the vessel 10 does not need to increase its draft at all, or to the same degree, as when lifting the topsides, as the winches allow the jacket 26 to be lifted from above.

Once the winch cables are attached to the jacket 26 (or part thereof) and the winches are operated to support the weight of the jacket part, ROVs are used to separate a part of the jacket 26 from the rest of the jacket 26, by cutting through the legs and cross members (if required) of the jacket. When the jacket part connected to the winches is separated from the rest of the jacket, the freed jacket part is lifted away from the remaining jacket 26, either using the winches, deballasting the vessel to reduce the draft, or a combination of both. The winches on the support beams 20 cooperate to collectively lift the jacket part. The jacket part can be lifted vertically, and does not need to be rotated or rested on a surface - both of which pose risks in terms of damage and losing control of the jacket part. Once the jacket part has been lifted clear of the remaining jacket and moved away from the jacket, a barge can be positioned in the receiving section and the winches can be used to lower the jacket part onto the barge.

Figure 1 1 illustrates the retrieval of a further jacket part - this time from further below the surface of the sea. The method is the same as above, but the winch cables need to extend further below the surface of the sea to connect to the jacket. The winch cables and lifting collars may be connected using remotely-operated vehicles or divers. It is to be understood that different numbers of support beams and different arrangements of winch cables and attachment collars can be used.

Figure 12 illustrates topsides supported on a barge. The topsides supporting frame can be seen on the deck of the barge. Depending on the barge and the situation, different supports for stabilising and holding the topsides on a barge may be utilised. Similarly, figure 13 shows the support for supporting jacket parts on a barge.

Figure 14 shows a pipe rack 38 attacked to one of the first hull portion and second hull portion. The vessel 10 is also suitable for hoisting subsea pipes or pipe sections, using the support beam winches. The vessel 10 therefore may comprise a pipe rack 38, attached to one of the hull portions in the vicinity of the support beams 20. The pipe rack 38 may be substantially between 50 and 150m long or arranged to support lengths of pipe which are substantially between 50m and 150m long. ROVs cut the subsea pipes into approximately 100m lengths and these are then hoisted by the winch system. The winches and support beams 20 are operable to hoist a pipe section from the sea and then position it in the pipe rack 38, for storing the pipes during transit. The present invention has been described above purely by way of example. Modifications in detail may be made to the present invention within the scope of the claims as appended hereto.