This invention relates to offshore vessels having lifting mechanisms for substructures to be placed on or removed from the seabed or other offshore installations. International Patent Publication No. WO-A-00/27693 discloses an apparatus and method for installation of a deck onto an offshore structure. The apparatus can be configured either for flotation and transportation or for fixed hydrocarbon drilling operations. The apparatus is self- floating and includes a deck, at least one pontoon, and at least one lifting support connecting each pontoon to the deck. The or each pontoon has sufficient composite buoyancy to provide the apparatus with net positive buoyancy. In the flotation configuration, the deck is supported by the lifting supports which are in turn mounted on the pontoons. The lifting supports are typically in compressed position so that the deck is relatively close to the pontoons and the water and the apparatus is sufficiently buoyant and stable for passages in open water. In operational- configuration, the entire weight of the apparatus is supported by the offshore structure upon which the deck has been installed. Installation of the apparatus on an offshore substructure having an upper end adapted to support the weight of the deck and the pontoons is accomplished by transporting the apparatus in the flotation configuration to a location proximate to the substructure. The upper end of the substructure may also be elevated above the surface of the water. The deck is then elevated an amount sufficient to permit positioning of the deck over the upper end of the substructure by extending the lifting supports. The apparatus is then moved on the surface of the water with the lifting supports extended to position the deck at a selective location over the upper end of the substructure. After positioning, the lifting supports are retracted until the weight of the apparatus is transferred from the water to the substructure. The lifting supports are further retracted to lift the pontoons to a desired elevation above the surface of the water. This invention provides a vessel for installing or removing a structure at or from an offshore -site, the vessel being U-shaped in plan to provide an open-ended well to enable the vessel to be manoeuvred into position around an offshore site, and a plurality of lifting devices on the vessel for supporting/raising/lowering the structure for placement at or removal from the offshore site, wherein each lifting device comprises a tower mounted on the U-shaped vessel, a hoist on each tower for supporting the offshore structure and variable water ballast means providing a counterweight for each hoist for operating the hoists to raise, hold in equilibrium or lower the structure. More specifically each hoist may have one part depending on the side of the tower adjacent the well in the vessel for supporting the offshore structure and another part attached to a water ballast container, the arrangement being such that the water ballast containers counter-balance the weight of the structure and allow the structure to be raised or lowered by varying the amount of water ballast held in the containers. In one arrangement according to the invention means are provided for supplying water to or releasing water from each container to enable the counter-weight provided by the container to be varied for raising, lowering or holding in equilibrium the offshore structure suspended from the lifting devices. In any of the above arrangements the towers of the suspension devices may comprise columns for holding water, means being provided for supplying water to and releasing water from the columns to set a requisite water level in the columns and the water ballast containers of the hoists float in the columns, the arrangement being such that the volume of water in the containers may be varied to support the offshore structure suspended in equilibrium and the water level within the columns may be raised and lowered for raising and lowering the containers to raise and lower the offshore structure. In the latter case the cross-section of the column is determined in accordance with the cross-section of the ballast container to provide a lifting device having a required "spring stiffness". In the case where a load carrier is suspended from the lifting devices to receive and support an offshore structure, the ballast container may be a main ballast container and a supplemental ballast container may be provided partially immersed in the main container and connected to the hoist with means to supply/release water from the supplemental container for balancing the load carrier only and means are provided for coupling the supplemental container to the main container for counterbalancing both the load carrier and offshore structure on the load carrier. The bottom of the supplemental container of each lifting device may be submerged in the water ballast of the main container and the cross-section of this bottom section is determined in relation to the cross-section of the main container to provide the required "spring stiffness" during lifting of the load carrier, this spring stiffness being different from the spring stiffness required during lifting of the offshore structure. Furthermore the supplemental container reduces in diameter towards its lower end to enable the "spring stiffness" provided by the supplemental container to be varied according to the extent it is submerged in the water ballast of the main container to enable a gradual transition to be made between the supplemental container, acting on its own and the main and supplemental ballast containers acting together. Each column may have an additional container outside the column which may be filled with water and a valve control is provided for regulating the flow of water between the column and the supplemental container to enable the stiffness of the spring provided by the water column to be adjusted from a low value with the valve open to a high value with the valve closed. In the latter arrangement the control valve may have means for gradually adjusting the valve opening to vary the "spring stiffness" of the arrangement. In a further arrangement the control valve may have means for setting the valve at a requisite setting to provide a required "spring stiffness". In any of the above arrangements the offshore structure or load carrier for the offshore structure may extend over the deck of the vessel on either side of the well so that the . structure/carrier can be lowered onto the deck and the load on the lifting devices relieved for transit of the structure. For example the base of the U shaped member may be adapted to receive the structure for transit of the offshore structure. Preferably at least two towers are provided on each side of the U-shaped vessel. In any of the above arrangements the lifting devices may be adapted for lowering the offshore structure onto a barge positioned in the well of the vessel for transiting of the offshore structure. Also in any of the above arrangements the vessel may comprise twin side-by-side members extending generally parallel to one another and spaced apart to provide the open ended well and a transverse member connected between the side members at one end thereof. In the latter case the side members or the transverse member or both side members and transverse member provide a flotation for the vessel . Also in the latter case the side members are provided by floating hulls and the transverse member may also be provided by a floating hull. In an alternative construction the side members may be provided by floating hulls and the transverse member is provided by a bridging member which is non-floating. The side members of the vessel are secured to an end or the sides of a floating barge. For example the side members of the vessel may be mounted in cantilever fashion on the barge to extend from the side of the barge and do not contribute to the flotation of the vessel. In a further arrangement the side members may be floatable and extend outwardly from the barge contributing to the overall flotation of the vessel. In the latter case the side members of the vessel are Z shaped in elevation each side member having one limb of the Z overlying the deck of the barge and being secured thereto and the other limb of the Z extending outwardly of the barge to contribute to the flotation of the vessel . In any of the above arrangements a plurality of lifting devices may be mounted on the vessel on each side of the well in the vessel. In addition a lifting device or devices may be provided on the base of the U-shaped vessel. Again in any of the above arrangements the lifting devices may be arranged to provide relatively low resistance to horizontal movement of the suspended structure with respect to the vessel to limit imposition of horizontal forces on the offshore site, the suspended structure or the vessel during placement on or removal of the structure from the- site. The invention also provides a vessel for installing or removing a structure at or from an offshore site, the vessel being U-shaped in plan to provide an open-ended well to enable the vessel to be manoeuvred into position around an offshore site, and a plurality of lifting devices on the vessel for supporting/raising/lόwering the structure for placement at or removal from the offshore site, wherein each lifting device comprises a column, mounted for vertical movement on the vessel for supporting the structure to be placed on or removed from the offshore site and means to raise and lower the columns with respect to the vessel to raise and lower the structure. The columns may have suspension elements which depend from the lifting devices for supporting a structure to be placed on or removed from the offshore site. The suspension elements may comprise cables, bars or other structural elements, and the length of the suspension elements may be determined such that the structure has said relatively low resistance to horizontal movement with respect to the vessel . The length of the suspension elements is preferably such that the effective resistance to horizontal movement of a suspended structure is less than 800 tonnes per metre. A plurality of columns may be mounted on the vessel on each side of the well in the vessel. Also a column or columns may be provided on the base of the U-shaped vessel. A load carrier may be suspended from the columns over the well in the vessel for supporting an offshore structure for placement or removal at an offshore site. The load carrier may be engageable with the vessel for transporting the- offshore structure to or from a site. At least the lower part of each column may comprise a container for receiving a volume of water and the upper ends of the columns having said suspension means ■ from which the offshore structure may be suspended whereby varying the volume of the ballast in the towers can be used to raise, lower or hold the offshore structure in equilibrium. More specifically the vessel has moon pools extending through the vessel in which the columns are slidable, the moon pools having guiding means for guiding the columns. In the latter case the columns may be arranged so that they may rise until the lower ends of the columns are received in the moon pools in the vessel for transit of the vessel. In any of the latter arrangements the lower ends of the columns may have encircling plates projecting outwardly of the columns, the mass and resistance of which to movement through the water increases the natural period of the floating suspension columns thus reducing their vertical motions. Also in any of the latter arrangements the container provided by the lower part of the column may be a closed container. More specifically the container provided by the lower part of the column may extend above water level and may be open topped. The offshore structure or load carrier for the offshore structure may extend over the deck of the vessel on either side of the well so that the structure/carrier can be lowered onto the deck and the load on the lifting devices relieved for transit of the structure. Preferably the base of the U shaped member is adapted to receive the structure for transit of the offshore structure. It is further preferred that the columns are adapted for lowering the offshore structure onto a barge positioned in the well of the vessel for transiting of the offshore structure. In one arrangement according to the invention the vessel may comprise twin side-by-side members extending generally parallel to one another and spaced apart to provide the open ended well and a transverse member connected between the side members at one end thereof. For example the side members or the transverse member or both side members and transverse member may provide flotation for the vessel. In any of the above arrangements the lifting devices may be arranged to provide relatively low resistance to - S -

horizontal movement of the suspended structure with respect to the vessel to limit imposition of horizontal force on the offshore installation, the offshore structure or the vessel itself during placement on or removal of the structure from an offshore installation. More specifically the lifting devices have suspension elements which depend from the lifting devices for supporting a structure to be placed on or removed from the offshore site. For example the suspension elements comprise cables, bars or other structural elements. In accordance with the invention the length of the suspension elements is determined such that the offshore structure has said relatively low resistance to horizontal movement with respect to the vessel. Thus the length of the suspension elements may be such that the effective resistance to horizontal movement of a suspended offshore structure is resisted is less than 800 tonnes per metre.

The following is a description of a number of specific embodiments of the invention, reference being made to the accompanying drawings in which: Figure 1 illustrates four different concepts of offshore vessels having lifting mechanisms for offshore towers or other substructures; Figure 2 shows two general arrangements of an offshore vessel in plan view; Figure 3 is a more detailed view of a vessel and lifting structure showing a deck or other substructure raised above the seabed mounted substructure for receiving or removal of a deck; Figure 4 is a similar view to that of Figure 3 showing the deck lowered to a position to be transported by a pontoon; Figure 5 illustrates a first embodiment of vessel according to the invention holding a deck suspended above an offshore substructure on which it is to be installed or from which it is to be removed; Figure 6 illustrates a second embodiment of vessel according to the invention and lifting structure; Figure 7 illustrates a third embodiment of vessel and lifting structure showing a deck in the installed position on an offshore substructure; Figure 8 illustrates a further embodiment of vessel and lifting structure showing the deck in the installed position on the offshore structure; Figure 9 is a similar view to that of Figure 8 showing the- deck raised above the offshore structure; Figure 10 is a similar view to Figures 8 and 9 showing the deck lowered to a position to be transported by a pontoon; Figure 11 is a similar view to that of Figures 8 to 10 showing a variant of the structure; Figure 12 shows a further embodiment of vessel and lifting structure for a deck with the deck lifted off an offshore substructure on which it has been mounted; Figure 13 is a similar view to that of Figure 12 with the deck shown in a position to be transported by the vessel; Figure 14 shows a variant of the embodiment of Figures 12 and 13; Figures 15 and 16 show a further embodiment of a vessel and lifting structure; and Figures 17 to 26 show still further embodiments of vessels and lifting structures.

The various embodiments of the invention which will now be described in detail all have the following characteristics in common: • a vessel 20, consisting of two or more parallel bodies 21a, 21b connected at one end by a transverse body 22, these bodies 21a,21b, 22 forming a U-shape, of which at least one of the bodies 21a, 21b,22 is in the form of a hull which provides flotation for the vessel and load carried by the vessel. Preferably at least the parallel bodies 21a and 21b are in the form of flotation hulls,- • a topside or substructure to be placed on or removed from the seabed or a seabed installation 10; • two or more suspension columns or towers 30, located on the-vessel bodies of which at least two towers 30 are on opposite sides of the structure to be placed or removed 10; • a lifting support 41 extending across the well of the vessel to carry the structure be placed or removed 10; • two or more suspension means 40 connecting the lifting support structure 41 and the suspension towers 30, of which at least two of the suspension means 40 are at opposite sides of the topside 10; • displacement means 50,60,39 to move the suspension means 40 vertically relative to the vessel; • the suspension means 40 have sufficient length, such that a relatively soft horizontal (pendulum) spring stiffness is generated of less than 800 tonnes per meter. The suspension means 40 can be cables, bars or other structural elements. The lifting support structure 41 can be a separate structure, engaging with the underside of the topside 10, but can also be a structural element - either permanent or temporary for the sake of installation or removal - of topside 10 itself. The concepts differ in the way the suspension means 40 are vertically moved. With these concepts, a topside 10 can be placed on or removed from an offshore facility support structure 11, being a steel jacket, a mono-pile, a concrete gravity structure or a floating support structure like a SPAR, a SEMI or an FPSO or an offshore substructure can be placed on or removed from the seabed. The soft horizontal spring is necessary to limit the horizontal forces at the top of the suspension towers 30 as a result of vessel motions when engaging the lifting support structure 41 with the structure to be placed or removed 10 and tensioning the suspension means 40. The horizontal spring is created by the pendulum system of lifting support structure 41 plus topside 10 suspended from the relatively long suspension means 40. At least two suspension towers 30 are placed on the bodies 21a,21b as can be seen in Figure 2a. Alternatively, two towers may be placed on the bodies 21a,21b and a third tower on the transverse body 22 as shown in Figure 24. However, the preferred option is four suspension towers 30 two by two placed at either side of the U-shaped well 23 between the parallel bodies 21a, 21b as shown in Figure 2b. During installation of a topside 10, the U-shaped vessel 20 is manoeuvred around a pre-installed facility support structure 11 with the topside 10 suspended from the suspension towers 30. During removal of a topside 10, the U-shaped vessel 20 is manoeuvred around an existing offshore installation consisting of a facility support structure 11 with topside 10, whereupon the topside 10 is suspended from the suspension towers 30, lifted free from the facility support structure 11 and removed. One offshore vessel having a lifting arrangement is shown in detail in Figures 2 to 4 and will now be described in detail. The U-shaped vessel comprises a pair of parallel flotation hulls 21a, 21b connected at one end by a transverse flotation hull 22. Suspension towers 30 are rigidly fixed to the hulls 21a,21b (and/or to the transverse .hull 22) of the vessel on the foundation structures 34 built in to the hulls. The suspension tower structure 31 is of the type used in jack-up structures and can be a plated or braced box of a triangular, rectangular, circular or other shape. Each suspension tower 30 is provided with a shuttle 50 which can be moved up and down along the suspension tower, using displacement mechanisms known from the jack-up technology, e.g. rack-and-pinion gear driven jacks, telescopic hydraulic rams or a system of cables, chains and pulleys (linear winches) . The shuttles 50 each have an inwardly extending cantilever support 51 from which vertical suspension means 40 of fixed length run to the lifting support structure 41. For installation of a topside (or substructure) , the topside 10 is suspended from the suspension means 40 at a location remote from the facility support structure 11. Subsequently, the topside 10 is raised to a level which is amply above the facility support structure 11, by using the displacement mechanisms of the shuttles 50. Then the U- shaped vessel 20 is manoeuvred around the facility support structure 11, whereupon the topside 10 is lowered by the shuttles 50 until it is engaged with facility support structure 11. For removal, the lifting support structure is brought underneath the topside 10 and connected to suspension means 40. Then the topside 10 is lifted free from the facility- support structure 11 using the displacement means of shuttles 50. Once the topside is sufficiently free from the facility support structure, the U-shaped vessel 20 is moved away from the facility support structure 11, whereupon the topside 10 is lowered by shuttles 50 before transport. The topside 10 is lowered to be landed either on the two parallel bodies 21a,21b or on a cargo barge-70 brought into the U-shaped opening 23. The topside is lowered by displacing the shuttles 50 downwards using their displacement mechanisms, until the topside has been landed and the load in suspension means 40 is zero. Then the suspension means 40 can be disconnected from the lifting support structure 41. The lifting support structure 41 can be transported to shore together with the topside 10; alternatively it can be retrieved from underneath the topside 10 after this has been landed on the cargo barge 70. Instead of transporting the topside 10 to shore using a cargo barge 70, the topsides can be left resting on the two parallel bodies 21a,21b or be skidded to the transverse body 22 for transport . Reference is now made to Figure 5 of the drawings which shows a first embodiment of the invention. The suspension towers 30 are rigidly fixed to the parallel bodies 21a,21b or the transverse body 22 by means of the foundation structure 34. The suspension tower structure 31 is a closed or braced box of a triangular, rectangular, circular or other shape. The suspension means 40 are a cable or a system of cables connected to a counterweight container 60. Each suspension tower 30 is provided with a system of sheaves 62 supporting the suspension means 40, which runs from the lifting support structure 41, vertically to the system of sheaves 62 and over the system of sheaves downward to the counterweight container 60. The counterweight container 60 is filled with a ballast 61, preferably water, for lifting the topside 10. The counterweight containers 60 may move up and down inside or outside the suspension towers 30. Installation and removal of topside 10 is done in the same way as described for the first embodiment, -except that in the second embodiment the topside is lifted and lowered by successive ballasting and de-ballasting the counterweight containers 60 with ballast 61, preferably water. A variant of the first embodiment is shown in Figure 6. The counterweight container 60 moves up and down inside the suspension tower 30. The counterweight container 60 is partly filled with water for making equilibrium with the weight of the lifting support structure 41 and further filled with water for making equilibrium with the weight of the lifting support structure 41 plus topside 10. The underside of counterweight 60 is submerged in a volume of water 63 contained inside suspension tower 30. For this purpose, the structure of the suspension tower 31 is a closed box of triangular, rectangular, circular or another cross-section. The counterweights 60 are moved upwards or downwards (and thus the topside 10 downwards or upwards) by raising or lowering the level of water volume 63. The difference in cross-section between the outside of counterweight 60 and the inside of suspension tower 30 gives a certain spring stiffness to the lifting system 60,40,41,10. This spring stiffness can be influenced by carefully selecting the cross-section of counterweight 60 in relation to that, of suspension tower 30. A further variant of the second embodiment is shown in Figures 7 to 10. A secondary counterweight container 64 is provided above the primary counterweight container 60. The secondary counterweight container 64 has a reduced bottom section 65 which is, at the underside, submerged in the ballast water 61 inside the primary counterweight container 60. When partly filled with ballast water 66, the secondary counterweight container 64,65 makes equilibrium -with the lifting support structure 41. Its upward and downward motions are controlled by moving the level of the ballast water 61 inside the primary counterweight container 60 up and down, or by moving the entire primary counterweight container 60 plus ballast water 61 up and down by changing the volume of water 63 inside the suspension tower 30. By moving the secondary counterweight container 64 downward, the lifting support structure 41 is moved upward, until it is engaged with the underside of topside 10. The secondary counterweight container 64,65 is further filled with water until the lifting support structure 41 exerts a predetermined pre-tensioning force on topside 10. During this pre-tensioning operation, the reduced bottom section 65 provides certain spring stiffness to the lifting system 64,40,41. The spring stiffness can be influenced by carefully selecting the cross-section of bottom section 65. Once lifting support structure 41 has been engaged with topside 10 and pre-tensioned, the primary counterweight container 60 is raised by raising the level of water volume 63 inside the suspension tower 30, until the primary counterweight container 60 is engaged with the secondary counterweight container 64. After engagement, both counterweight containers are connected together by locking means 67. During this operation, the spring stiffness of the system of the lifting system changes from the stiffness determined by the cross-section of bottom section 65 in water volume 61 to the stiffness determined by the cross- section of primary counterweight 60 in water volume 63. If this change is too abrupt, bottom section 65 can be given a more gentle transition shape 65a from a small cross-section at the bottom, corresponding with a "soft spring", to a wider cross-section at the top, corresponding with a "stiff spring" . The spring stiffness of the secondary lift system 64,40,41 can then be made gradually stiffer by raising the water level 61 with respect to the bottom section 65. The upward force on topside 10 is increased by filling counterweight containers 64 and 60 completely and by lowering the level of water volume 63, until the upward force makes equilibrium with the weight of lifting support structure 41 plus topside 10. Then the topside 10 is lifted free from the facility support structure 11 by further lowering the level of water volume 63. Once the topside 10 is free from the facility support structure 11, the U-shaped vessel 20 is manoeuvred away from the facility support structure 11. Once the U-shaped vessel 20 has been moved away from the facility support structure, the topside is lowered to be landed either on the two parallel bodies 21a,21b or on a cargo barge 70 brought into the U-shaped opening 23. The topside is lowered by raising the level of the water volume 63 inside the suspension towers 30, until the topside has been landed and subsequently discharging the ballast water 66 and 61 from respectively the counterweight containers 64 and 60, until the load in suspension means 40 is zero. Then the suspension means 40 can be disconnected from the lifting support structure 41. The lifting support structure 41 can be transported to shore together with the topside 10; alternatively it can be retrieved from underneath the topside 10 after this has been landed on the cargo barge 70. Instead of transporting the topside 10 to shore using a cargo barge 70, the topsides can be left resting on the two parallel bodies 21a,21b or be skidded to the transverse body 22 for transport. The embodiment described in Figures 7 to 10- can be further varied as shown in Figure 11. During lifting the topside 10 off the facility support structure 11, large spring stiffness of the lifting system is required. This is realized by a small difference in cross-section between the inside of the suspension tower 30 and the counterweight container 60 in comparison with the cross-section of the counterweight container 60 itself. When the counterweight container 60 is displaced over a small distance, the level of water volume 63 is displaced in opposite direction over a much large distance, generating a high amount of counteracting buoyancy and giving the system a high apparent stiffness. The smaller the difference is between cross- sections 63 and 60, the stiffer the spring. The spring stiffness required during lifting may be too high during first contact and during tensioning of the contact between lifting support structure 41 and topside 10. In order to reduce the spring stiffness during first contact and tensioning, an extra water tank 90 is added containing a volume of water 91 with a free water surface and a cross- sectional area which is considerably larger than the difference between the cross-sections 63 and 60. The water volume 91 can be connected with the water volume 63 inside the suspension tower 30 via a large valve 92. During first contact and tensioning, the valve is open and counterweight container 60 is moving up and down in the common water plane of water volumes 63 and 91, generating low spring stiffness. During tensioning, the valve 92 is gradually closed, gradually increasing the spring stiffness of the system, until the counterweight container 60 moves up and down in the water plane of water volume 63 only and the spring stiffness is high. The valve 92 may be partly open when intermediate spring stiffness is desired under specific circumstances. A further arrangement is shown in Figure 12 in which the floating suspension towers 30 can move up and down in moon pools 80 provided in the parallel bodies 21a,21b or the transverse body 22. The floating suspension towers 30 are held upright under load relative to the bodies 21a,21b,22 using guiding means 83, which may be positioned in or above or below the moon pools 80. In these guiding means, sliding means 84 are provided (e.g. lignum vitae, Teflon, bearings, rollers or wheels) to allow for vertical motions of the suspension towers 30 relative to the bodies 21a,21b, 22, such vertical motions evolving from waves or ballasting. An important aspect of the invention in this embodiment is that the stability of the floating suspension towers during lifting of the topside 10 is derived from the guiding means 83, firmly connected to the bodies 21a,21b,22. The structure of the bottom part of the suspension tower 36 is a closed box of triangular, rectangular or circular or other shape and can contain a volume of ballast water 39. The structure of the top part of the suspension tower 37 can be a plated or braced box of a triangular, rectangular, circular or other shape. The length of the closed bottom part 36 is such that the transition to the open top part 37 stays always above water. Each suspension tower 30 is provided with a cantilevering support 38 at its top, from which support vertical suspension means 40 of fixed length run to the lifting support structure 41. The suspension means 40 can be a cable, a bar or another structural element. The lifting support structure 41 and topside 10 can be raised and lowered by respectively decreasing and increasing the volume of ballast water 39. Installation, removal and transport of a topside 10 is done in the same way as described in the first embodiment, except that in this arrangement the topside is lifted and lowered by successive ballasting and de-ballasting of the bottom parts 36 of the suspension towers 30. Referring to Figure 13, after the lifting support structure 41 has been disconnected from the suspension means 40, "the ballast water 39 can be discharged and the suspension towers 30 raised. In this way, the draft of the U-shaped vessel 20 can be reduced for entering a harbour. Optionally, the suspension towers 30 may be fixed in the moon pools 80 during transport. The motion behaviour of the embodiment of Figures 12 and 13 can be improved by adding a so-called added-mass plate 35 to the underside of the suspension tower 30 as shown in Figure 14. The plate increases the natural period of the floating suspension towers, thus reducing its vertical motions. Optionally, the shape of the underside of the moon pool 80 may be adapted in order to engage with the shape of the added-mass plate 35 and to be able to retrieve the suspension tower 30 fully into the moon pool 80. A variant of the arrangement of Figures 12 and 13 is shown in Figures 15 and 16. The closed bottom part 36 of suspension towers 30 is always fully submerged during all engaging with and lifting and lowering of the topside 10. The top part 37 is an open structure, with a cross-sectional area designed to provide the desired spring stiffness during engaging, lifting and lowering operations. For engaging and lifting the topside 10, the suspension towers 30 are raised by discharging ballast water 39 from the closed bottom part 36. For lowering he topside, landing it on the transport medium and releasing the lift force to zero, the volume of ballast water 39 is increased. Installation, removal and transport of a topside 10 is done in the same way as described in the first embodiment, except that in this variant the topside is lifted and lowered by successive ballasting and de-ballasting of the bottom parts 36 of the suspension towers 30. After the lifting support structure 41 has been disconnected from the suspension means 40, the ballast water 39 can be discharged and the suspension towers 30 raised. In this way, the draft of the U-shaped vessel 20 can be reduced for entering a harbour. Optionally, the suspension towers 30 may be fixed in the moon pools 80 during transport. In a variant applicable to any of the above embodiments shown in Figure 17 the two parallel hulls 21a,21b provide a catamaran type vessel carrying four suspension towers 30 and the transverse body 22 at one end of the vessel is in the form of a bridge deck located above water. Alternatively the two parallel hulls 21a,21b provide flotation and carry four suspension towers 30 as shown in Figure 18. The transverse body 22 at the end is also a flotation hull and is wide enough to carry the topside 10 during transport . After topside 10 has been lowered onto the parallel bodies 21a,21b, it can be skidded to the transverse floater 22 via two skid beams 71. A further variation is shown in Figure 19 in which two parallel bodies 21a,21b are flotation hulls, carrying four suspension towers 30 and the transverse body 22 is provided by a cargo barge. The parallel hulls 21a,21b are welded to the stern of the cargo barge 22 via weld 24. Skid beams 71 are provided between the parallel floaters 21a,21b and the cargo barge 22 in order to skid topside 10 to the cargo barge for transport. With this concept several topsides 10 can be collected on the cargo barge 22 for transport to the shore. Figure 20 shows a variant in which the two parallel bodies 21a, 21b are flotation hulls, carrying four suspension towers 30 and the transverse body 22 connects the two parallel floaters 21a, 21b and has a part protruding over the stern of a cargo barge 70. The protruding part has a ballast tank 25 which can be filled with ballast water in order to ballast the U-shaped vessel 20 on to the stern of the cargo barge 70. Skid beams 71 are provided on the cargo barge 70 and skid beams 72 on the parallel floaters 21a,21b at exactly the same spacing and height in order to skid the topside 10 to the cargo barge for tansport. Supports 73 are provided underneath the skid beams 71 on the cargo barge 70 and skid beams 72 on the parallel floaters 21a, 21b in order to have the skid beams 71,72 at equal level over the parallel hulls 21a,21b, the transverse body 22 and the cargo barge 70. The supports 73 have a depth equal to the depth of the transverse body 22. With this concept several topsides 10 can be collected on the cargo barge 70 for transport to the shore. A further variation is shown in Figures 21 and 22 in which the two parallel hulls 21a,21b are Z-shaped hulls. One leg of the Z-shape is protruding over a transverse body 22. The transverse body 22 can be a cargo barge, a semi- submersible heavy-lift vessel or another transport vessel with a flat deck. The other legs of the Z-shaped hulls carry the four suspension towers 30. The protruding part has a ballast tank 26 which can be filled with ballast water in order to ballast the Z-shaped parallel hulls 21a, 21b on to the transverse body 22. Skid beams 71 are provided on the transverse hull 22 and skid beams 72 on the parallel hulls 21a, 21b at exactly the same spacing and height. For transport, the topside 10 is skidded to the deck of transverse body 22, being a cargo barge, a semi-submersible heavy-lift vessel or another transport vessel with a flat deck. After the topside 10 has been skidded to the deck of 22," the Z-shaped hulls 21a, 21b are de-ballasted and sailed away from transverse hull 22. The Z-shaped hulls 21a, 21b can even be re-ballasted on to 22 at another location. With this concept several topsides 10 can be collected on the transverse hull 22 for transport to the shore. In a variant shown in Figure 23 applicable to the embodiments of Figures 12 to 16, the two parallel bodies 21a,21b may be non-floating. The parallel bodies 21a,21b can be purpose-built outriggers or standard cargo barges. One end of the parallel bodies 21a,21b is resting on the transverse body 22. This end is provided with a ballast tank 26 which can be filled with water in order to ballast the parallel bodies 21a, 21b on to the transverse body 22. The transverse body 22 can be a cargo barge, a semi- submersible heavy-lift vessel or another transport vessel with a flat deck. The other end of the parallel bodies 21a,21b cantilevers above the water. This end is provided with moon pools 80, guiding means 83 and sliding means 84 giving guidance and support to the four floating suspension towers 30. Skid beams 71 are provided on the transverse floater 22 and skid beams 72 between the lifting support structure 41 and the topside 10. Skid beams 71 and 72 have exactly the same spacing. For skidding of the topside 10 to the deck of the transverse body 22, the lifting support structure 41 is lowered in between the two parallel bodies 21a,21b until the top of skid beams 72 lines up exactly with the top of skid beams 71. Before topside 10 can be skidded to the deck of transverse body 22, it must be ensured that system of the floating suspension towers 30 plus topside 10 moves in unison with the system of transverse body 22 plus cantilevering parallel bodies 21a, 21b. For this purpose, a brake system 81 is provided in the moon pools 80 to dampen the relative motions between the two independently floating systems 30 + 10 and 22 + 21a,21b. Once the relative motions are zero and the tops of the skid beams 71 and 72 are at the same level, a locking system 82 between the suspension tower 30 and the moon pool 80 secures both systems 30 + 10 and 22 + 21a, 21b and prohibits further relative motions. For transport, topside 10 is skidded to the deck of transverse body 22, being a cargo barge, a semi-submersible heavy-lift vessel or another transport vessel with a flat deck. During skidding topside 10 in the direction of the deck of transverse body 22, the volume of ballast water 39 in the bottom part 36 of the suspension towers 30 is gradually increased in order to make equilibrium with the decreasing load oh skid beams 72 and to prevent high uplift forces on the cantilevering parallel bodies 21a,21b. After the topside 10 has been skidded to the deck of 22, the parallel bodies 21a,21b can be detached by a combination of (partly) submerging the deck of transverse body 22, shifting the ballast water in the parallel bodies 21a,21b and using uplift buoyancy from the suspension towers 30 whilst keeping the locking system 82 engaged. The parallel bodies 21a,21b can even be re-installed at another location on transverse body 22 by following the detachment procedure in opposite sequence. With this concept several topsides 10 can be collected on the transverse body 22 for transport -to the shore. All arrangements of the U-shaped vessel 20 can also be used for placing an offshore structure 11 like a jacket on the seabed 100 or for removing it from the seabed. The lifting support structure 41 is engaged with the offshore structure 11 and lifted from the seabed 100. " For transport after removal, the offshore structure 11 is raised as high as possible and then sailed to a sheltered shallow water location, where the structure is demolished. Figures 25 and 26 show the concept of Figures 2 to 4 applied to the placement or removal of a seabed mounted structure.