A Subsea Wellhead Clamp Assembly

Abstract

A subsea wellhead clamp assembly 10 comprising a collar 11 having at least one arm 11a,b pivotally connected to a body 11c and relatively movable between an open receiving configuration (fig.6) and a

Description

According to the present invention, there is provided a subsea wellhead clamp assembly for clamping onto a profile on a subsea wellhead for retrieval purposes, the assembly comprising: a collar having at least one arm pivotally connected to a body at a pivot point, and movable relative to the body around the pivot point between an open configuration in which the arm has at least one free end and wherein an opening in the collar is open to receive a wellhead between the arm and the body, and a closed configuration, in which the body and the arm are closed around the wellhead; a locking mechanism adapted to lock the arm in the closed configuration; a driving mechanism adapted to drive the movement of the arm relative to the body in a first direction toward the open configuration; and wherein the clamp assembly is adapted to store potential energy during the relative movement of the arm(s) and the body in the first direction, and wherein the clamp assembly is adapted to use the stored potential energy to urge the relative movement of the arm(s) and the body in a second direction toward the closed configuration. The invention also provides a method of retrieving a subsea wellhead from a subsea well, the method comprising clamping a clamp assembly to the subsea wellhead, the assembly comprising: a collar having at least one arm pivotally connected to a body at a pivot point, and movable relative to the body around the pivot point between an open configuration in which the arm has at least one free end and wherein an opening in the collar is open to receive a wellhead between the arm and the body, and a closed configuration, in which the body and the arm are closed around the wellhead; a locking mechanism adapted to lock the arm in the closed configuration; and a driving mechanism; wherein the method comprises driving the movement of the arm relative to the body in a first direction toward the open configuration and thereby storing potential energy in the clamp assembly; passing the subsea wellhead through the collar opening and receiving the subsea wellhead within the open collar; releasing the stored potential energy and thereby urging the relative movement of the arm and body in a second direction toward the closed configuration around the subsea wellhead; locking the arm in the closed configuration; and lifting the clamp assembly and connected wellhead to the surface. Optionally the clamp assembly is applied to a profile on the subsea wellhead, and has a matching profile on the inner surface of the arm and / or body to engage with the profile and to resist relative axial movement of the collar to the wellhead during retrieval. Optionally the profile can comprise a flange, and / or a recess. Optionally the driving mechanism comprises a winch, adapted to pull a cable to move the arm relative to the body in the first direction under power, and adapted to pay out the cable (optionally under tension) during relative movement of the arm and the body in the second direction. Optionally the cable is in tension in both directions. Optionally the drive mechanism has an open setting, in which it drives movement from the closed to the open configuration under power, and a closed setting, in which it controls relative movement of the arm(s) and the body in the second direction. Optionally during movement in the second direction, the drive mechanism retards the speed of relative movement of the arm(s) and the body to within safe parameters, so that the arms are lowered back to the closed configuration in a controlled manner. The potential energy stored during movement in the first direction is released during the relative movement of the arm(s) and the body in the second direction, and is used to urge the movement in the second direction. Optionally the collar has at least two arms. Optionally the arms and the body comprise annular sections. Optionally at least some of the annular sections describe equal angular dimensions. For example, where there are two arms, each arm can describe the same dimension of arc, e.g. each arm can have an arc of 90°, or 100°, or 120° etc. Optionally each arm has the same dimensions. Optionally the arm can have the same arc as the body, e.g. 180°, if there is one arm, or 120°, if there are two arms and one body, but optionally the body can have a greater angular dimension than the arms in some cases. Optionally each arm is pivotally connected to a respective pivot point at opposite ends of the body. Optionally the collar is suspended by rigging. Optionally each of the body and each arm has a respective rigging line, optionally at least two lines. Optionally each rigging line is connected to a common fixing such as a shackle or ring, which can be raised a lowered from the surface by a crane on a ship. Optionally the rigging lines are of equal length. Optionally the arm(s) and the body are restricted to move in the same plane, e.g. by the pivot connection. Optionally in the open configuration, the rigging lines between the common suspension point and each segment are subjected to unequal tension, with tension in first rigging lines extending between the common suspension point and first portions of the collar adjacent the opening being higher than tension in second rigging lines between the common suspension point and second portions of the collar spaced from the opening, and wherein in the closed configuration, the differential in the tension of the first and second rigging lines is reduced, and is optionally approaching (or is) zero. Optionally in the closed configuration, the arm(s), body and opening are in a common first horizontal plane, and wherein in the open configuration, the opening of the collar is raised above the first horizontal plane. Optionally, suspension of the collar from the rigging biases the collar toward the closed configuration, for example, the weight of the arm(s) passively urges the relative movement of the arm(s) and body in the second direction. Optionally the locking device comprises a locking pin, optionally having a latch comprising a resilient mechanism to retain the locking pin within a locking aperture. Optionally the resilient mechanism is energised when inserting the pin into the aperture, and resists pull-out of the pin from the aperture. Optionally, the pin can be removed from the aperture by a two stage process, optionally by moving the pin through the aperture in a single direction, e.g. pushing the pin through the aperture, where pulling the pin in the opposite direction of insertion is typically resisted, e.g. by the resilient mechanism. Optionally the pin has a cap acting as a stop, which limits movement of the pin through the aperture, and optionally the cap can be detached following insertion to permit onward movement of the pin through the aperture, for example, in the same direction as insertion, to release it from the aperture, optionally by unscrewing the cap from the pin. Optionally the pin and aperture can have antirotation formations such as cooperating recesses and splines to resist rotation of the pin when engaged in the aperture, thereby permitting unscrewing of the cap relative to the pin when the pin is in place within the aperture. Optionally the drive mechanism can be powered from an ROV, optionally having a drive socket that can be stabbed by an ROV drive tool, such as a rotating head. Optionally the locking pin can be inserted and the cap disconnected also by means of an ROV drive tool. Hence, the clamp assembly does not require on-board power supplies. Optionally the or each drive mechanism has a brake mechanism, which is adapted to arrest relative movement of the arm and body when activated, and which can be selectively activated and deactivated. Optionally the brake mechanism is biased by a resilient member into a configuration in which the brake is activated. Optionally the brake mechanism is activated when the ROV drive tool is disconnected from the drive mechanism. Optionally the brake mechanism comprises an arrangement of cogs associated with the winch (e.g. associated with the winch drum) which engage with a spline resisting movement of the cogs and thereby the winch drum when the cogs are engaged with the spline. Optionally the cogs and spine are biased into engagement when the ROV drive tool is disengaged from the winch, thereby locking the winch drum against movement, and are disengaged, free permitting movement of the winch drum, when the ROV drive tool is engaged with the winch. Optionally the collar is segmented, having at least two segments, optionally three, and at least one pivot connection between adjacent segments. Optionally the segments have similar circumferential dimensions. Optionally the segments are annular, and optionally the assembled and closed collar is circular. Each segment is optionally connected via at least one rigging line (optionally two or more rigging lines) to a common suspension point, for example a ring or shackle, adapted to connect to a hoist line of a surface vessel. Optionally each rigging line between the common suspension point and each segment has a uniform length. Optionally opening the collar tensions some of the rigging lines more than others, for example, the rigging lines between the common suspension point and a first portion e.g. at the arm(s) are subjected to higher tension when the collar is open than the corresponding lines between the common suspension point and second portions e.g. on the body, and spaced further from the opening than the first portions. Optionally the difference in tension in the rigging lines reduces toward zero as the collar approaches the closed configuration, and typically in the closed configuration, the tension in the rigging lines between the common suspension point and each of the segments is or is close to zero. Hence the closed configuration has a greater stability than the open configuration. Optionally the open configuration stores more potential energy than the closed configuration. The present invention also provides a subsea wellhead clamp assembly for clamping onto a profile on a subsea wellhead for retrieval purposes, the assembly comprising: a segmented collar having at least one arm segment pivotally connected to a body segment at a pivot point, and movable relative to the body segment around the pivot point between an open configuration in which the arm segment has at least one free end and wherein an opening in the collar is open to receive a wellhead between the arm segment and the body segment, and a closed configuration, in which the body segment and the arm segment are closed around the wellhead; a locking mechanism adapted to lock the arm segment in the closed configuration; a driving mechanism adapted to drive the movement of the arm segment relative to the body segment in a first direction toward the open configuration; wherein each collar segment is adapted to connect via a rigging line to a common suspension point, and wherein each rigging line connecting the common suspension point with each segment has a uniform length; and wherein the clamp assembly is adapted to store potential energy during the relative movement of the arm(s) and the body in the first direction, and wherein the clamp assembly is adapted to use the stored potential energy to urge the relative movement of the arm(s) and the body in a second direction toward the closed configuration. Optionally in the closed configuration, the collar segments (e.g. the body and the at least one arm) are disposed in a generally horizontal plane, suspended below the common suspension point via the rigging lines extending between each segment and the common suspension point. Optionally as the collar moves from the closed to the open configuration, the segments tilt from the generally horizontal plane into a non-horizontal plane, in which first portions forming free ends of the segments at the collar opening are raised above at least a second portion of the collar diametrically opposite (e.g. interposed midway between) said first portions of the arm. The second portion can be a portion of the body segment, e.g. a pivot point of the body segment if there is only one arm, and optionally a central portion of the body segment if there is more than one arm. Optionally the first portions rise above the second portions during opening, and fall back toward horizontal during closing. The various aspects of the present invention can be practiced alone or in combination with one or more of the other aspects, as will be appreciated by those skilled in the relevant arts. The various aspects of the invention can optionally be provided in combination with one or more of the optional features of the other aspects of the invention. Also, optional features described in relation to one aspect can typically be combined alone or together with other features in different aspects of the invention. Any subject matter described in this specification can be combined with any other subject matter in the specification to form a novel combination. Various aspects of the invention will now be described in detail with reference to the accompanying figures. Still other aspects, features, and advantages of the present invention are readily apparent from the entire description thereof, including the figures, which illustrates a number of exemplary aspects and implementations. The invention is also capable of other and different examples and aspects, and its several details can be modified in various respects, all without departing from the spirit and scope of the present invention. Accordingly, each example herein should be understood to have broad application, and is meant to illustrate one possible way of carrying out the invention, without intending to suggest that the scope of this disclosure, including the claims, is limited to that example. Furthermore, the terminology and phraseology used herein is solely used for descriptive purposes and should not be construed as limiting in scope. In particular, unless otherwise stated, dimensions and numerical values included herein are presented as examples illustrating one possible aspect of the claimed subject matter, without limiting the disclosure to the particular dimensions or values recited. All numerical values in this disclosure are understood as being modified by "about". All singular forms of elements, or any other components described herein are understood to include plural forms thereof and vice versa. Language such as "including", "comprising", "having", "containing", or "involving" and variations thereof, is intended to be broad and encompass the subject matter listed thereafter, equivalents, and additional subject matter not recited, and is not intended to exclude other additives, components, integers or steps. Likewise, the term "comprising" is considered synonymous with the terms "including" or "containing" for applicable legal purposes. Thus, throughout the specification and claims unless the context requires otherwise, the word “comprise” or variations thereof such as “comprises” or “comprising” will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Any discussion of documents, acts, materials, devices, articles and the like is included in the specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention. In this disclosure, whenever a composition, an element or a group of elements is preceded with the transitional phrase "comprising", it is understood that we also contemplate the same composition, element or group of elements with transitional phrases "consisting essentially of”, "consisting", "selected from the group of consisting of, “including”, or "is" preceding the recitation of the composition, element or group of elements and vice versa. In this disclosure, the words “typically” or “optionally” are to be understood as being intended to indicate optional or non-essential features of the invention which are present in certain examples but which can be omitted in others without departing from the scope of the invention. References to directional and positional descriptions such as upper and lower and directions e.g. “up”, “down” etc. are to be interpreted by a skilled reader in the context of the examples described to refer to the orientation of features shown in the drawings, and are not to be interpreted as limiting the invention to the literal interpretation of the term, but instead should be as understood by the skilled addressee. In particular, positional references in relation to the well such as “up” and similar terms will be interpreted to refer to a direction toward the point of entry of the borehole into the ground or the seabed, and “down” and similar terms will be interpreted to refer to a direction away from the point of entry, whether the well being referred to is a conventional vertical well or a deviated well. Brief Description of the Drawings In the accompanying drawings: Figure 1 shows a plan view of a collar of a subsea wellhead clamp assembly; Figure 2 shows a side view of the Fig 1 collar; Figure 3 shows a drive mechanism of the Fig 1 collar; Figures 4 and 5 show a locking pin of the Fig 1 collar; Figures 5-13 show sequential views of the clamp assembly being applied to a wellhead; and Figure 14 shows a section view of the clamp assembly connected to a wellhead. Referring now to the drawings, a subsea wellhead clamp assembly 10 comprises a collar 11 divided into three segments comprising a body 11c and first and second arms 11a, 11b. The arms 11a,b are pivotally connected to the body 11c by pivot pins 15, at opposite ends of the body 11c, as best shown in figure 1. The three segments are generally arcuate, and define approximately equal angular dimensions of around 120°. The collar 11 is adapted to close (in a closed configuration) around a wellhead Wto be recovered from an oil or gas well; in this closed configuration, the collar 11 is generally circular as best shown in Fig 1, and is dimensioned to match the outer diameter of the wellhead W. One end of each of the arms 11a, 11 b is pivotally connected to the body by the pivot pins 15 in a cantilever manner, and the opposite free ends of each arm 11a, 11b together form an opening 16, and are adapted to interengage to close the collar 11 around the wellhead W. Figure 1 shows the clamp assembly 10 in the closed configuration, with the free ends of the arms 11a, 11b forming the opening 16 having cooperating lobes that are interconnected by locking pin 13, which extends through apertures 12a, 12b, in the lobes once they are aligned during overlap of the lobes, as best seen in figure 8, to lock the arms 11a, b in the closed configuration. The inner surface of the arms 11a, b and the body 11c are typically profiled to engage a recess on the wellhead W, as best shown in Fig 14, when the collar 11 is closed around the wellhead W. The arms 11a,b in this example are constrained by the pivot pins 15 to move pivotally in the same plane as the body 11c, and to resist movement outwith that plane, so that when the arms 11a,b close, the lobes on the free ends of the arms 11a,b are aligned in a suitable manner to interconnect. As shown in figures 1 and 2, the body 11c has a drive member for each arm 11a, 11b in the form of a winch 14a,b acting on each arm in this example, as best seen in figure 3. Each winch 14a,b is disposed on the body 11c, has a drive socket adapted for cooperation with a rotary head on an ROV drive tool, and is adapted to be rotated in order to wind in a cable 14c, best visible in figure 2, which is connected between one of the winches 14a,b on the body 11c, and an anchor point on an outer surface of the corresponding arm 11a,b. The opening 16 on the collar 11 is opened by rotation of the rotary head on the ROV engaged in the socket on one winch (e.g. 14a) which powers the winch 14a to reel in the cable 14c, pulling the corresponding arm 11a closer to the winch 14a on the body 11c, thereby driving movement of the arm 11a from a position close to or at the closed position shown in figure 1 toward an open position, best seen in figure 6. The winch 14a is normally driven to pull in the arm 11a, and then the winch 14b on the other side is then similarly and sequentially operated to move the other arm 11b in the same manner, but both arms can optionally be moved at the same time if required, and if sufficient ROV tooling is available. Once the winch 14a has pulled in the cable 14c to the desired extent (for example, when the arm 11a is fully opened and the opening 16 is wide enough to receive the wellhead W) the ROV drive tool detaches from the winch 14a. Optionally this triggers the winch 14a to apply a braking mechanism to arrest movement of the cable 14c, typically in each direction. This can optionally be done by different braking mechanisms in the winch 14a, for example, the drive socket on the winch 14a which is engaged by the drive head on the ROV drive tool can be spring-loaded to enable free movement of the socket when the ROV drive tool is engaged in the socket and is pressing axially into the socket (for example rotation of the socket to drive rotation of the winch drum attached). The socket can be arranged to be urged by a spring against a spline or sprocket which resists and / or arrests movement of the socket on the winch 14a when the drive head on the ROV drive tool disengages from the socket on the winch 14a. Other braking mechanisms can be adopted to brake movement of the winch (and thereby lock the cable against uncontrolled pay-out) once the ROV drive tool disengages. The same winch braking mechanism can be provided on the other winch 14b. This permits each winch 14a,b to be operated sequentially by a single ROV to move from a position at or near to the closed configuration toward the open configuration, and then to retain that position thereby storing the potential energy in the clamp assembly while the ROV then moves the other arm(s) in a similar manner. The locking pin 13 in this example is shown in Figs 4 and 5. The pin 13 has a body 13b, with a number of legs (four in this case) formed of resilient material such as spring steel, and having latch formations in the form of a lip extending radially outwardly from the end of each leg. During insertion into the apertures 12 in the lobes of the arms 11, the resilient legs on the pin 13 are deformed radially inwards, until the lip on each leg passes completely through the aperture, and springs radially outwards beneath the outermost lobe, thereby latching the pin in place within the aperture. The aperture 12 in this example has internal splines (visible in Fig 8) which permit the insertion of the pin 13 into the aperture 12 only in certain orientations, and which resist relative rotation of the pin 13 in the aperture 12. The pin 13 has a cap 13c which screws onto the top of the body 13b, and which has a drive socket on its upper face, adapted to cooperate with a drive head on an ROV drive tool (optionally the same drive head that operates the winches 14). For insertion, the cap 13c and the body 13b are made up, with the screw tight between them, permitting the ROV to insert the pin 13 into the aperture in a straight line, once the splines are aligned with the corresponding formations on the pin 13. After the lips on the ends of the legs are hooked under the lowest lobe of the interconnected arms 11a,b, the pin 13 is held firmly in place against axial withdrawal from the aperture 12 (by means of the lips) and by rotation within the aperture 12 (by means of the splines). Hence the locking pin 13 prevents movement of the arms 11a,b from the closed position once locked in place. The collar 11 is typically lowered from the surface crane on the operating vessel and is supported by rigging lines 17 between each of the arms 11a,b and body 11c (typically 2 rigging lines / segment, although more or fewer lines can be provided in other case). The rigging lines extend between each segment and a common ring 18 from which the hoist line extends vertically to the crane of the ship. In this example, the rigging lines are the same length, which has the effect that the resting position of the collar 11 is the closed position, where the rigging lines adopt the same angle that is close to the axis of the hoist line between the ship and the ring 18 and where the collar 11 is disposed in a generally horizontal configuration, directly below the ring. When the collar is open, in the configuration shown in Fig 6, the free ends of the arms 11a,b forming the opening 16 are raised above the body segment 11c of the collar (disposed diametrically opposite to the opening 16) so that the collar is no longer horizontal. The weight of the arms 11 a,b naturally urges them from the extended open configuration toward the closed configuration, such that the winches 14a,b must actively hold the cables 14c in tension to maintain the open configuration while the collar 11 is being applied to the wellhead W. When the collar 11 is to be applied to the wellhead W, the arms 11a,b are opened by driving the winches 14a,b in succession. This sequentially raises the free ends of the arms 14a,b slightly above the body segment 11c, applies additional tension to first rigging lines 17a between first portions of the collar corresponding to the arm segments 11a,b and the ring, as compared with second rigging lines 17b between the ring 18 and second portions of the collar corresponding to the body segment 11c. This stores potential energy in the clamp assembly 11. When the first winch 14a has pulled the arm 11a to the desired open configuration, the ROV drive tool disengages from the winch 14a, which activates the winch brake to resist rotation of the winch drum and thereby hold the tension in the cable, so keeping the arm 11a in the open configuration. The same ROV drive tool can then deploy on the other winch 14b to move the other arm 11 b in the same manner. Thus the arms are typically sequentially moved toward the open configuration and locked there by the winch brake, holding the positions of the arms in the open configuration by tension in the cables 14c on each side. In the closed configuration, the collar segments are all disposed in a generally horizontal plane, suspended below the ring 18 via the rigging lines 17a,b extending between each of the collar 11 and the ring 18. However, in the open configuration, the segments are in a non-horizontal plane, because the free ends of the arms 11a,b at the collar opening 16 are raised above the body diametrically opposite (e.g. interposed midway between) the free ends of the arms 11a,b. Driving the winches 14a,b to the open configuration uses energy from the ROV power pack, and the clamp assembly is adapted to store the potential energy as it moves toward the open configuration. In the open configuration the arms 11a,b naturally try to close under their own weight and by the action of the rigging lines 17, and are only held from doing so by the tension in the cables 14c. Thus, when the collar 11 is open, it is applied to the wellhead W and the ROV drive tool then reengages with each winch 14a,b to lower each arm 11a,b in a controlled manner with the cable 14c still typically under tension from the open configuration toward (and eventually to) the closed configuration, with the arms wrapped around the wellhead, and engaging in the wellhead profile or under the wellhead flange. Once the collar 11 is closed around the wellhead W in an axial position on the wellhead where the inner profile on the collar 11 engages with the corresponding profile on the outer surface of the wellhead W (as shown best in Fig 14, where the collar 11 is engaged in the recess below an upper flange) and the apertures 12a, b in the lobes are mutually aligned, the pin 13 is inserted through the apertures 12 in the lobes to lock the arms 11a, b in the closed position. The collar 11 can be applied to the wellhead typically before the wellhead is cut below the seabed surface (usually 3-5m below the seabed). Normally the wellhead is cut from the inner bore of the wellhead W, using a separate cutting tool. Once the collar 11 is locked in position on the wellhead profile, the hoist rope is then winched in on the surface crane, and the wellhead is recovered to the surface. The collar 11 can be removed from the wellhead W by removing the pin 13 from the aperture. In this example, the resilient lip on the pin 11 typically permits movement of the pin 13 only in one direction in the aperture as it resists pull-out of the pin 13 when the lips are engaged. Further the cap 13c on the pin 13 acts as a stop member to limit the extent of axial movement of the pin 13 into the aperture, so once in place in the aperture 12, the pin 13 cannot be removed without deactivating the stop mechanism on the cap. This is done by unscrewing the cap from the pin body, again by the drive socket on the ROV drive tool, which dislocates the cap from the pin body, and allows the pin to drop out of the aperture, or be pushed out by the ROV drive tool. In this example, the pin and aperture have anti-rotation formations such as cooperating recesses and splines to resist rotation of the pin when engaged in the aperture, thereby permitting unscrewing of the cap relative to the pin when the pin is in place within the aperture. This can be performed when the wellhead has been retrieved to the surface, or alternatively, in the case of emergencies, when the collar 11 needs to be removed from the wellhead before retrieval.

Claims

1 A subsea wellhead clamp assembly for clamping onto a profile on a subsea wellhead for retrieval purposes, the assembly comprising:a collar having at least one arm pivotally connected to a body at a pivot point, and movable relative to the body around the pivot point between an open configuration in which the arm has at least one free end and wherein an opening in the collar is open to receive a wellhead between the arm and the body, and a closed configuration, in which the body and the arm are closed around the wellhead;a locking mechanism adapted to lock the arm in the closed configuration;a driving mechanism adapted to drive the movement of the arm relative to the body in a first direction toward the open configuration; andwherein the clamp assembly is adapted to store potential energy during the relative movement of the arm(s) and the body in the first direction, and wherein the clamp assembly is adapted to use the stored potential energy to urge the relative movement of the arm(s) and the body in a second direction toward the closed configuration.2 A subsea wellhead clamp assembly as claimed in claim 1, wherein the driving mechanism comprises a winch, adapted to pull a cable to move the arm relative to the body in the first direction under power, and adapted to pay out the cable during relative movement of the arm and the body in the second direction.3 A subsea wellhead clamp assembly as claimed in claim 2, wherein the cable is in tension in both first and second directions of movement.4 A subsea wellhead clamp assembly as claimed in any preceding claim, having at least two arms.5 A subsea wellhead clamp assembly as claimed in any preceding claim, wherein the collar is segmented, having at least two segments, and at least one pivot connection between adjacent segments.6 A subsea wellhead clamp assembly as claimed in claim 5, wherein the collar segments comprise annular segments, wherein at least two of the segments have equal angular dimensions.7 A subsea wellhead clamp assembly as claimed in any preceding claim, wherein the arm(s) and the body are restricted to move in the same plane by the pivot connection.8 A subsea wellhead clamp assembly as claimed in any preceding claim, wherein the collar is suspended by rigging lines from a common suspension point, wherein each of the body and the arrn(s) has at least one respective rigging line connected to the common suspension point.9 A subsea wellhead clamp assembly as claimed in claim 8, wherein the rigging lines are of equal length between the collar and the common suspension point.10 A subsea wellhead clamp assembly as claimed in claim 8 or claim 9, wherein in the open configuration, the rigging lines between the common suspension point and each segment are subjected to unequal tension, with tension in first rigging lines extending between the common suspension point and first portions of the collar adjacent the opening being higher than tension in second rigging lines between the common suspension point and second portions of the collar spaced from the opening, and wherein in the closed configuration, the differential in the tension of the first and second rigging lines reduces toward zero.11 A subsea wellhead clamp assembly as claimed in any preceding claim, wherein in the closed configuration, the arm(s), body and opening are in a common first horizontal plane, and wherein in the open configuration, the opening of the collar is raised above the first horizontal plane.12 A subsea wellhead clamp assembly as claimed in any preceding claim, wherein the locking device comprises a locking pin adapted to engage in an aperture formed by segments on opposing sides of the opening, wherein the locking pin has a latch comprising a resilient mechanism to retain the locking pin within the aperture.13 A subsea wellhead clamp assembly as claimed in claim 12, wherein the pin has a head comprising a stop member, which limits movement of the pin through the aperture.14 A subsea wellhead clamp assembly as claimed in claim 13, wherein the stop member can be selectively deactivated following insertion of the pin into the aperture to permit disengagement of the pin with the aperture.15 A subsea wellhead clamp assembly as claimed in claim 13 or 14, wherein the pin and aperture have anti-rotation formations to resist rotation of the pin when engaged in the aperture.16 A subsea wellhead clamp assembly as claimed in any preceding claim, wherein the or each drive mechanism has a brake mechanism, adapted to arrest relative movement of the arm and body when the brake mechanism is activated and to permit relative movement of the arm and the body when the brake mechanism is deactivated.17 A subsea wellhead clamp assembly as claimed in claim 16, wherein the brake mechanism is adapted to be activated by engagement of an ROV drive tool with a drive mechanism, and deactivated by disengagement of the ROV drive tool from the drive mechanism.18 A method of retrieving a subsea wellhead from a subsea well, the method comprising clamping a clamp assembly to the subsea wellhead, the assembly comprising:a collar having at least one arm pivotally connected to a body at a pivot point, and movable relative to the body around the pivot point between an open configuration in which the arm has at least one free end and wherein an opening in the collar is open to receive a wellhead between the arm and the body, and a closed configuration, in which the body and the arm are closed around the wellhead;a locking mechanism adapted to lock the arm in the closed configuration; and a driving mechanism;wherein the method comprises driving the movement of the arm relative to the body in a first direction toward the open configuration and thereby storing potential energy in the clamp assembly;passing the subsea wellhead through the collar opening and receiving the subsea wellhead within the open collar;releasing the stored potential energy and thereby urging the relative movement of the arm and body in a second direction toward the closed configuration around the subsea wellhead;locking the arm in the closed configuration; and lifting the clamp assembly and connected wellhead to the surface.19 A method as claimed in claim 18, wherein the driving mechanism comprises a winch, and wherein the method includes tensioning a winch cable to pull the arm into an open configuration.20 A method as claimed in any one of claims 18-19, wherein the drive mechanism has a brake mechanism, and wherein the method includes activating the brake mechanism to hold the arm and body in the open configuration.21 A method as claimed in any one of claims 18-20, wherein the collar is segmented, and wherein in the closed configuration, the collar segments are disposed in a generally horizontal plane, suspended below a common suspension point via rigging lines extending between each segment and the common suspension point, and wherein as the collar moves from the closed configuration to the open configuration, first portions forming free ends of the segments at the collar opening are raised above at least a second portion of the collar diametrically opposite (e.g. interposed midway between) said first portions.22 A subsea wellhead clamp assembly for clamping onto a profile on a subsea wellhead for retrieval purposes, the assembly comprising:a segmented collar having at least one arm segment pivotally connected to a body segment at a pivot point, and movable relative to the body segment around the pivot point between an open configuration in which the arm segment has at least one free end and wherein an opening in the collar is open to receive a wellhead between thearm segment and the body segment, and a closed configuration, in which the body segment and the arm segment are closed around the wellhead;a locking mechanism adapted to lock the arm segment in the closed configuration; a driving mechanism adapted to drive the movement of the arm segment relative to the body segment in a first direction toward the open configuration;wherein each collar segment is adapted to connect via a rigging line to a common suspension point, and wherein each rigging line connecting the common suspension point with each segment has a uniform length; andwherein the clamp assembly is adapted to store potential energy during the relative movement of the arm(s) and the body in the first direction, and wherein the clamp assembly is adapted to use the stored potential energy to urge the relative movement of the arm(s) and the body in a second direction toward the closed configuration.23 A subsea wellhead clamp assembly as claimed in claim 22, wherein in the closed configuration, the collar segments are disposed in a generally horizontal plane, suspended below the common suspension point via the rigging lines extending between each segment and the common suspension point, and wherein in the open configuration, the segments are in a non-horizontal plane, in which first portions forming free ends of the segments at the collar opening are raised above at least a second portion of the clamp diametrically opposite (e.g. interposed midway between) said first portions.24 A subsea wellhead clamp assembly as claimed in claim 22 or 23, wherein in the open configuration, the rigging lines between the common suspension point and each segment are subjected to unequal tension, with tension in first rigging lines extending between the common suspension point and first portions of the collar adjacent the opening being higher than tension in second rigging lines between the common suspension point and second portions of the collar spaced from the opening and wherein in the closed configuration, the differential in the tension of the first and second rigging lines is reduced.25 A subsea wellhead clamp assembly as claimed in any one of claims 22-24, wherein the or each drive mechanism has a brake mechanism, adapted to arrest relative movement of the arm and body when the brake mechanism is activated andcmto permit relative movement of the arm and the body when the brake mechanism is deactivated.

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