NL2021201B1

NL2021201B1 - Method for installation of an offshore structure using one or more cranes

Info

Publication number: NL2021201B1
Application number: NL2021201A
Authority: NL
Inventors: Overdulve Leon; Van Mierlo Stefan; Nagtegaal Thijs; Diekerhof Roel
Original assignee: Holding Oos Ii B V
Priority date: 2018-06-28
Filing date: 2018-06-28
Publication date: 2020-01-07
Also published as: WO2020005066A1

Abstract

Method for lifting a constructional structure of an offshore platform, wherein the constructional structure has a weight of at least 100000 kg and a base having an upper side an a lower side with at least three spaced apart lower supports adapted for axially supporting the structure on one or more upper supports of another constructional structure ofthe platform, the method comprising: connecting hoisting wires to or nearthe lower supports and below the center of mass of the constructional structure, in such a manner that when an axially directed lifting force is applied on the wires this axially directed force is substantially transferred via the lower supports to the rest of the base and in such a mannerthat, while the axially directed lifting force is applied on the wires, the hoisting wires remain spaced apart from distal lower ends ofthe lower supports; and while the constructional structure is in an substantially upright orientation, lifting the constructional structure by using one or more cranes to apply an axially directed lifting force on each ofthe wires in such a mannerthat the wires extend substantially parallel to each other.

Description

Field of the invention

The present invention relates to a method for lifting a constructional structure of an offshore platform using one or more cranes at an offshore location. A constructional structure is herein defined as a structure that is part of, or is to form part of, the offshore platform, has a weight of at least 100000 kg and comprises a base having a lower side with at least three spaced apart lower supports that are adapted for axially supporting the structure on one or more upper supports of another constructional structure of the platform. Such supports are typically are adapted for, together, substantially completely supporting the weight of the constructional feature on the another constructional feature that has already been installed as part of the offshore platform. The constructional structure may optionally comprise a number of modules that are supported on the base.

Background art

When lifting heavy constructional structures for an offshore platform, it is common practice to provide the constructional structure that is to be lifted with some form of reinforcement to which wires can be attached for lifting the constructional structure. For instance, it is known to provide a topside or a topside module, e.g. a living quarters module, for an offshore platform with reinforcement structures at or near its upper side, so that hoisting wires can be connected to the reinforcement structures.

South-Korean patent application KR 20170048737 describes a method for installation of a top-side module on the top side of an offshore structure, using a marine crane that is provided on an overhead barge. The known top-side module installation method comprises: a step of installing each column having a hollow interior to allow internal lifting tool (ILT) equipment to be inserted into opposing points along the periphery of a topside module to achieve convenient installation of the top side module using the ILT equipment; a step of inserting suspended ILT equipment into each column and then fixing the ILT equipment with the column; a step of lifting the topside module using the marine crane after fixing the ILT equipment with the column to be transferred to the top side of a marine structure and then, setting the correct position of the top side module; and a step of fixing the top side module in such a manner as to be placed on the top side after setting the right position of the top side module on the top side of the offshore structure. The columns are typically fixed to the module while on-shore, and provide reinforced points of attachment that may be engaged by the ILT equipment without causing substantial damage to the module.

Though the known method may shorten installation time for various topside modules, a drawback is that the installation of the columns along opposing points along the periphery of the topside module requires significant amounts of reinforcements to be added to the topside modules.

It is an object of the present invention to provide a method which at least partially overcomes this drawback.

Summary of the invention

To this end, the present invention provides a method for lifting a constructional structure of an offshore platform, wherein the constructional structure has a weight of at least 100000 kg, preferably at least 250000 kg, and comprises a base having a lower side with at least three spaced apart lower supports that are adapted for axially supporting the structure on one or more upper supports of another constructional structure of the platform, the base further comprising an upper side which faces away from the lower side, wherein the method comprises the steps of: connecting hoisting wires to or near the lower supports and below the center of mass of the constructional structure, in such a manner that when an axially directed lifting force is applied on the hoisting wires this axially directed force is substantially transferred via the lower supports to the rest of the base, and in such a manner that, while the axially directed lifting force is applied on the wires, the hoisting wires remain spaced apart from distal lower ends of the lower supports; and while the constructional structure is in an substantially upright orientation, lifting the constructional structure by applying an axial lifting force on each of the wires in such a manner that the wires extend substantially parallel to each other. For such heavy structures it is typically not feasible to loop hoisting wires around the entire lower side of the structure as the force exerted on the loop may cause damage to the wires and/or the structure. Moreover, a lower portion of the loop may become clamped between a lower side of the structure and an upper side of another structure onto which the first structure is to be placed. Instead in the prior art, when lifting very heavy structures which are to be lifted, it has been common practise to attach hoisting wires with their distal ends at or near the top of the structure, i.e. above the constructional structure’s center of mass, so that the structure may easily be kept in an upright orientation during lifting and subsequent placement of the structure on another structure. This typically requires attaching significant reinforcements to the structure at positions above a horizontal plane through the structure’s center of mass. In contrast, according to the present invention, the hoisting wires are connected to or near the lower supports, and preferably at the lower side of the base. This allows the lower supports, which have as their main function to axially support the constructional structure on top of the another constructional structure, to be used to bear most of the weight of the structure as it is lifted using the cables. The number of further reinforcement structures for attachment of hoisting wires at points well above the constructional structure center of mass can thus be reduced and may in some cases be zero. This also results in a decrease in amount of material that is used for reinforcement structures which have as their main function to allowing the structure to be lifted. Moreover, no or less time is needed to install reinforcements on the structural construction whose main purpose is to allow lifting thereof, the lifting method may provide more time efficient manner for attaching hoisting cables to a constructional structure. The method of the invention is particularly suitable for lifting a constructional structure which has already been placed with its lower supports on upper supports of another constructional structure, as it requires no or only a few reinforcement structures to be installed on the upper side of the base prior to lifting to ensure structural integrity of the constructional structure during lifting. The method may thus be used for lifting the constructional structure of the offshore platform vertically upwards from said platform, e.g. when dismantling the platform.

The lower supports are preferably formed as columns which extend on the lower side of the base. At the points where the columns are to be supported on the upper supports, the columns are preferably oriented with their principal axes in the vertical direction.

The hoisting wires remain substantially parallel to each other during lifting to reduce the risk of sharp angled contact between those portions of the wires which extend above the supports and the structure itself, which would cause the wires the wear and/or damage to the constructional structure. Additionally, the as the wires remain substantially parallel during lifting, the force exerted thereby on the lowers supports will be substantially vertically directed, i.e. parallel to the direction in which the lower supports are typically strongest.

As, during lifting, the hoisting wires remain spaced apart from the distal lower ends of the lower supports, so that structure can be placed with its lower supports onto the another constructional structure, without being placed on top of the hoisting wires. The distal lower ends of the lower supports are preferably spaced vertically apart from the lower side of the base by a distance of at least 0,5 meter.

The constructional structure may for example be a topside with a number of support columns at its lower side which form the lower supports, and the another constructional feature may then be a jacket having an equal number of upper supports on its upper side for axially supporting the lower supports thereon. In such a case the hoisting wires will be attached at or near the support columns at the lower side of the base of the topside; as these columns are sufficiently strong to support the weight of the topside they provide excellent points of attachment for the hoisting wires.

The hoisting cables are preferably connected to the lower supports in such a manner that, when they are disconnected from the lower supports the hoisting wires can moved away from the constructional feature while the hoisting wires remain substantially parallel to each other.

In an embodiment the wires are connected to or near the lower supports at a location below a horizontal plane through the lower side of the base. During lifting, the parallel wires thus extend through the horizontal plane, substantially vertically along the lower supports. One or more wires may be connected to a lower support for instance via an internal lifting tool that clamps against an inner surface of the lower support at a position below the horizontal plane, an external lifting tool that is clamped around the lower support at a position below the horizontal plane, a padeye or trunnion that is welded to the lower support at a position below the horizontal plane, and the like.

In an embodiment during said lifting, the sum of axial lifting forces exerted on the hoisting wires is at least 70%, preferably at least 90% of the axial force required for vertically lifting the constructional structure. During lifting, the greater portion of the weight of the constructional structure is thus borne by the lower supports.

In an embodiment the upper supports of the another constructional structure extend above the water surface, wherein the another constructional structure has a lower portion that extends below the water surface.

In an embodiment the constructional structure has a center of mass, and, when the structure is lifted while in the substantially upright orientation, the center of mass is arranged at a level above each of the lower supports. Typically, when mounted and seen in projection onto a horizontal plane, a constructional structure of an offshore platform is adapted such that its lower supports span an area within which the center-of-mass is arranged, and such that the lower supports are sufficiently spaced apart from each other that, when the constructional structure is supported on its lower supports, it stably maintains an upright orientation. By lifting such a constructional structure from its lower supports, it can easily be maintained in a substantially upright orientation during lifting.

In an embodiment, each of lower supports is spaced apart from the other lower supports by a distance of at least 10 meters.

In an embodiment at least two of said hoisting wires are connected to a substantially rigid lifting beam, wherein said at least two said hoisting wires are connected to the lifting beam at points substantially corresponding to the points where said hoisting wires are connected to or near the lower supports. For instance, if the center point of a first lower support is spaced apart by a predetermined distance form the center point of a second lower support, then the distance between wires connecting the rigid lifting beam and the respective first and second lower support will be substantially the same, allowing the wires to run substantially vertically from the points at or near lower supports to the lifting beam during lifting. When internal lifting tools are used, the points of connection of the hoisting wires with the lifting beam will generally correspond to the center axes of the lower supports in which the lifting tools are inserted. If the hoisting wire is attached to an external surface of a lower support, e.g. via a padeye or trunnion that is welded to the external surface, or via an external clamping tool that is clamped around the external surface, then the points of connection of the hoisting wires with the lifting beam will be offset with respect to the center axes. During lifting the hoisting wires are preferably substantially fixedly connected at said points of the lifting beam. More preferably, each hoisting wire has a length substantially equal to the vertical distance between the point where the wire is connected to the lifting beam and the point at or near which the wire is connected to its corresponding lower support.

In an embodiment at least three of said hoisting wires are connected to a substantially rigid lifting frame, wherein said at least three hoisting wires are connected to the lifting frame at points substantially corresponding to two or more of the lower supports. During lifting the hoisting wires are preferably substantially fixedly connected at said points of the lifting frame. In this manner it is ensured that at least the wires connected tot the lifting frame are substantially parallel to each other during lifting of the structure. Preferably, all hoisting wires are connected to a same lifting frame, with the lifting frame, when seen in projection onto a horizontal plane during lifting, spanning all of the at least three lower supports. More preferably, each hoisting wire has a length substantially equal to the vertical distance between the point where the wire is connected to the lifting frame and the point at or near which the wire is connected to its corresponding lower support.

In an embodiment the one or more cranes are arranged on one or more vessels, such as one or more barges, which float on the water. Preferably, all of the hoisting wires are attached, via a single lifting frame, to a single crane. However, when two or more cranes on separate vessels are used, with at least one crane on each vessel, the hoisting wires from separate cranes can be connected to the constructional structure, and each crane can be controlled such that the orientation of the constructional structure remains substantially upright during lifting. This allows very heavy constructional features to be lifted, even if the cranes on the separate vessels move somewhat differently from each other due to waves.

In an embodiment, one or more of the one or more cranes is arranged on a jack-up vessel, wherein during lifting the jack-up vessel is substantially rigidly supported on the sea floor; and/or one or more of the one or more cranes is arranged on a platform that is fixed to and substantially rigidly supported by the sea floor.

In an embodiment the method comprises subsequent steps of: positioning the lifted constructional structure to a position in which the lower supports are substantially axially aligned with the one or more upper supports of the another constructional structure; and lowering the constructional structure until the lower supports are substantially completely supported on the one or more upper supports of the another constructional structure. Thus, the constructional structure is first lifted and subsequently lowered with its lower supports onto the upper supports of the another structure. In this manner, the constructional module can be efficiently mounted to form part of the offshore platform under construction.

In an embodiment when the constructional structure is being lifted in the upright orientation, and when viewed in projection onto a horizontal plane, the parallel portions of the wires are arranged within 5 meters from the peripheral edge of the base, and preferably completely within said peripheral edge. Wear and/or shifting of the wires due to contact with the peripheral edge of the base is thus avoided. Preferably, when viewed in said projection, the parallel portions of the lie within the peripheral edge of the base at a distance of at least 5 meters from the peripheral edge.

In an embodiment when the constructional structure is being lifted in the upright orientation, and when viewed in projection onto a horizontal plane, the area spanned by the lower supports is smaller than or equal to the area spanned by the base in said projection. This is typically the case when the constructional structure is a topside. Preferably, the area spanned by the lower supports is three fourth or less of the area spanned by the base.

In an embodiment when the constructional structure is being lifted in the upright orientation, and when viewed in projection onto a horizontal plane, each of the lower supports is spaced apart from the peripheral edge of the base by a distance greater than or equal to 4 times a length of said lower support along its principal axis in said projection. For instance, if a lower support has a cylindrical shape with an outer radius of 3,5 meters, then the closest distance of said lower support to the peripheral edge is at least 14 meters.

In an embodiment one or more of the wires are directly connected to the lower support surfaces. For instance, a hoisting wires can be tied in a knot around the outer surface of its corresponding lower support, or the ends of the wire may be provided with a sling that is arranged around the lower support.

In an embodiment the direct connection is achieved by tying said one or more of the wires around the lower support surface on a lower side of the base.

In an embodiment the one or more hoisting are connected to the lowers supports such that, during lifting, the wires extend from the lower side of the base, through openings in the base towards the upper side of the base. For instance, when the constructional module that is to be lifted using the method is a topside or module that is to be arranged on the topside, the base of the constructional structure provided with openings which extend from the upper side of the base and debauch on the lower side of the base within the lower supports and/or substantially adjacent to an outer surface of the lower supports. E.g., when an internal lifting tool is attached at the end of a wire, then during lifting, the wire may extend through an opening that debauches on the lower side of the base within a corresponding lower support so that the wire may partially extend within the lower support. Alternatively, then the opening debauches on the lower side of the base substantially adjacent, e.g. within 5 meters, preferably within 3 meters, of the connection between the lower support to the lower side of the base, then the wire can be attached to an outer surface of the lower support, e.g. via an external lifting tool, by attaching the wire to a padeye or trunnion that is provided on the outer surface of the lower support, or by tying the wire around the outer surface of the lower support.

In an embodiment, one or more of said lower supports comprises a leg having a substantially cylindrical or conical leg inner surface, wherein one or more of said wires is provided at an end thereof with an internal lifting tool adapted for engaging the leg’s inner surface.

In an embodiment one or more of said lower supports comprises a leg having a substantially cylindrical leg outer surface around which an external lifting tool is attached, wherein one or more of said wires is attached to said external lifting tool.

In an embodiment wherein one or more of said lower supports is provided with a padeye or or trunnion to which one or more of the wires is attached.

In an embodiment the method further comprises, prior to lifting of the constructional structure, welding a reinforcement structure to the lower support, wherein the reinforcement structure is provided with a padeye or trunnion.

In an embodiment the constructional structure comprises or is a topside, wherein the one or more lower supports are formed by legs of the topside, and wherein the another constructional structure is a jacket. I

In an embodiment the constructional structure comprises or is a topside module, wherein the one or more lower supports are formed by formed by legs of the topside module, and wherein the another constructional structure is a topside. Examples of topside modules are living quarters for personnel, a helideck module, and a processing module for processing hydrocarbons.

In an embodiment the constructional structure comprises or is a jacket, wherein the one or more lower supports are formed by lower ends of legs of the jacket, and wherein the another constructional structure is a jacket foundation. The jacket typically has a base that extends substantially horizontally between the jacket legs above the lower ends of the legs.

Short description of drawings

The present invention will be discussed in more detail below, with reference to the attached drawings, in which same reference numerals refer to the same structures or elements, and in which:

Figs. 1 A, 2A and 3A respectively illustrate how a constructional structure in the form of a top side is lifted according to the method of the invention;

Figs. 1 Β, 2B and 3B respectively show bottom views from horizontal planes indicated by lines l-B, ll-B and lll-B of Figs. 1A, 2A and 3A;

Fig. 4 illustrates the method of the invention in which a constructional structure in the form of a top side is lifted using cranes on two separate vessels; and

Figs. 5A, 5B and 5C respectively illustrate different manners in which the hoisting wires can be connected to a constructional structure in accordance with the method of the invention.

Description of embodiments

Figs. 1A and 1B respectively show a schematical side view and a view along direction l-B of Fig. 1A, which illustrate an embodiment of the method according to the invention. Shown are a barge 100, floating on the water and having a portion 101 below the water surface W and a portion 102 above the waterline. A crane 110 is provided on the barge 100, having main wire 111 at the end of which a hook 112 is provided. A further wire 113, which runs substantially vertically, is connected to a number of wires 114a,114b which in turn are each attached to a corner of a lifting frame 120. Hoisting wires 130a,130b,130c,130d are attached to the lifting frame 120 and extend substantially parallel to each other. At their lower ends, each of the hoisting wires is attached to an internal lifting tool, which clamps against an inner surface of lower supports 164a,164b,164c,164d of topside 160.

Constructional structures comprising the topside 160, jackets 170 and 180 form part of an offshore platform 105 together with foundation 190 that is fixed on the sea floor 106. Each of the constructional structures 160,170,180 comprises a substantially horizontal base 161,171,181 below which lower supports extend in such a manner that distal lower portions of the lower supports are vertically spaced apart from the horizontal base 161,171,181. The foundation 190 comprises four upper supports, two of which, 195a, 195b, are shown in Fig. 1A, and on top of which four lower supports of jacket 180 are axially supported. It will be understood that though the schematical side view of Fig. 1A shows only two lower supports 164a,164b, 174a,174b, 184a,184b of each of the constructional structures 160,170,180, each of these constructional structure is provided with four such lower supports. The lower supports of each constructional structure are adapted for together substantially completely supporting the weight of the constructional structure. The topside 160 comprises a base in the form of a lower deck 161, and the lower supports 164 of the topside extend on a lower side of the base 161. The partially cut away detail of lower support 164a shows an internal lifting tool 131 a which clamps against an inner surface of the lower support and is connected to hoisting wire 130a. The hoisting wire 130a thus extends from an end thereof that is attached to the lifting frame 120, through the base 161 of the topside 160 to an inner space of the lower support 164a, where the wire is connected to the lower support by means of the internal lifting tool 131 a at a position well below the center of mass C of the topside 160. When an substantially vertically directed axial force Fa is exerted thereon, the hoisting wires 130a,130b, 130c,130d extend in parallel from points below the center of mass C at or near the lower supports, to corresponding points of attachment with the lifting frame. Thus, during lifting, axial force exerted on the hoisting cables is transferred via the lower supports to the base 161 of the topside 160. The combined lifting force that is exerted by the cables, via the lower supports to the base and rest of the topside, is sufficient to lift the topside. That is, the combined axially directed lifting force that is exerted, through the four lower supports, on the topside 160 during lifting will be substantially equal to the mass of the topside times the gravitational acceleration of about 9.80665 m/s².

Fig. 1B schematically shows a bottom view of the topside 160 and lifting frame 120 from the level of the plane indicated by line IB in Fig. 1A in which hollow lower supports 164a,164b,164c,164d at a lower side 162 of the base 161 can be seen. Though the lifting frame 120 would typically not be visible in the bottom view, it has been indicated using dotted lines to illustrate the points of attachment between the lifting frame 120 and the hoisting wires 130a,130b,130c,130d. The hoisting wires extend through the lower supports along vertical axes indicated by reference numerals 133a,133b,133c,133d, and are connected to a corresponding internal lifting tool 131a,131b,131c,131d that clamps against an inner surface of its corresponding the hollow lower support.

In the view of Fig. 1B, the center of mass C is arranged within an area spanned by the lower supports, in order to provide stability when the topside is lifted in an upright orientation. Though in the bottom view of Fig. 1B, the area spanned by the lower supports 164a-164d is smaller than the area spanned by the bottom side 161 of the base 161, stability during lifting is improved by spacing each of the lower supports a distance of at least 10 meters apart from the other lower supports. The lower supports are thus also spaced apart from outer peripheral edge E of the base 160.

Figs. 2A and 2B illustrates another method for lifting such a topside 160, in which instead of an internal lifting tool an external lifting tool is used which clamps around an outer surface of the lower support 130a, as show in the detail of lower support 130a in Fig. 2A. Fig. 2B schematically shows a bottom view of the topside along the plane indicated by line Il-B in Fig. 2A, again with the lifiting frame 120 shown in dotted lines. The hook 112 here is attached directly to wires 114a, 114b which are connected to the lifting frame 120. The hoisting wires 130a - 130d run, from lifting frame 120, parallel to each other at the exterior sides of the lower supports 130a,130b to the corresponding external lifting tools. In order to allow the hoisting wires to pass through the base 160, the base is provided with openings 167a-167d which extend through the base 161, from an upper side 163 thereof to a lower side 162 of the base. In the view shown in Fig. 2B, each openings is arranged substantially adjacent to a portion of the outer surface of its corresponding lower support, in the present example the center of the opening is within 3 meters of the outer surface of the corresponding lower support. During lifting, the four hoisiting wires can thus extend substantially in parallel to each other from their points of attachment to the corresponding external lifting tools, and to their points of attachment to the lifting frame 120.

Figs. 3A and 3B illustrates another method for lifting a topside 160, in which instead of an internal or external lifting tool, padeyes are used for connecting the lower ends of the hoisting wires to the lower supports. Fig. 3A schematically shows the topside 160 as it is being lifted as well as a detail of hoisting cable 130a that is connected by such at padeye to lower support 164a of the topside 160. Fig. 3B schematically shows a bottom view of the topside 160, with the lifting frame 120 again indicated in dotted lines. As in the embodiment of Fig. 2A and 2B, the hoisting wires 130a-130d run through openings 167a-167d in the base, wherein the openings are substantially adjacent to their corresponding lower supports. Though not shown, instead of padeyes, trunnions or the like may be attached to the outer surfaces of the lower supports, for connecting the hoisting wires to lower supports.

Fig. 4 schematically shows the topside 160 as it is lifted using hoisting wires that are connected to two different cranes 110,210 that are provided on two different vessels 100,200. Each crane is connected, via a main wire cable 111,211, hook 112,212 and lifting beam 120’, 220, to hoisting wires. Thus, crane 210 is connected, via lifting beam 220 to two separate hoisting wires indicated schematically by reference numerals 230a,230b, and crane 110 is connected, via lifting beam 120’to two separate hoisting wires indicated schematically by reference numerals 130a, 130b. The hoisting wires are connected to corresponding lower supports of the top side at a position well below the topside’s center of mass C. As the lower supports are already provided for axially supporting the weight of the topside thereon, the lower supports are suffiently strong to withstand the forces exerted thereon during lifting.

Figs. 5A,5B and 5C schematically illustrate different positions where the hoisting wires can be attached to a constructional structure 360 according to the method of the invention. The constructional structure is provided with a base 361, having a lower side 362 and an upper side 363. The base is arranged below the constructional structure’s center of mass C, and is provided with lower supports 364a-364d which extend at and from the lower side of the base 361. In Fig. 5A four hoisting wires 330a-330d run parallel to each other from two separate lifting beams 120’ and to below the lower side 362 of the base where they are connected to the lower supports. In Fig. 5A the connections are made via internal lifting tools that are arranged within the lower supports.

In Fig. 5B, the hoisting wires 330a-330d are attached to the constructional structure 360 by means of padeyes that are welded onto upper side of the base at positions directly above the lower supports 364a-364d lower where the constructional feature is axially supported by the lower supports. At corresponding positions on the lower side of the base the lower supports are fixed to the base, e.g. by welding. The hoisting wires 330a-330d are connected to the padeyes at positions well below the center of mass C. When a vertically directed lifting force is exerted on the parallel hoisting wires 330a-330d, a substantial portion of this force is transmitted via the padeys to the lower supports and subsequently to the rest of the base.

In Fig. 5C, support extensions 374a-374d have been welded onto the lower supports 364a364d, wherein internal lifting tools (indicated schematically) have been arranged in the support extensions at a position well below the center of mass C but above the lower side 362 of the base. In this manner support extensions can be used that have a length that smaller than, e.g. half or less, the height of the constructional structure 360. When a vertically lifting force is exerted on the parallel hoisting wires, a substantial portion of this force is transmitted via the extensions 374a-374d to the lower supports and subsequently to the base 360.

In each of Figs. 5A,5B and 5C, the hoisting wires run parallel to each other and are connected to or near the lower supports at positions well below the center of mass. The lower end point of each hoisting wire thus lies well below the constructional structure’s center of mass C and is attached to a lower support which provided a readily available strong point by which the structure may be lifted. When a lifting force is exerted on the cables, a substantial portion of this force is transmitted via the padeyes to the lower supports and subsequently to the base.

The present invention has been described above with reference to a number of exemplary embodiments as shown in the drawings. Modifications and alternative implementations of some parts or elements are possible, and are included in the scope of protection as defined in the appended claims. In particular, though the examples illustrated in the figures show lifting of a constructional structure in the form of a topside, the skilled person will understand that the method can be used to lift constructional structures such as a jacket module as well.

Claims

Conclusions

A method for hoisting a structural structure (160; 170; 180) from an offshore platform (1), wherein the structural structure has a weight of at least 100,000 kg and comprises a base (161; 171; 181) has a bottom side (162) and a top side (163), the structural structure further comprising at least three spaced-apart lower supports (164, 164b, 164c, 164d; 174a, 174b; 185a, 185b) that are on the bottom side of the bases are arranged and arranged for axially supporting the structure on one or more upper supports (175a, 175b, 175c, 175; 195a, 195b) of another structural structure (170) of the platform, characterized in that the method comprises the steps of:

connecting hoisting wires (130a-130d) to or near the lower supports and below the center of gravity (C) of the structural structure, in such a way that when an axially directed hoisting force (Fa) is applied to the hoisting wires in this axially directed force in substantially via the lower supports (164, 164b, 164c, 164d) is transferred to the rest of the base and in such a way that, while the axially directed hoisting force is applied to the wires, the hoisting wires remain spaced from distal lower ends of the lower supports; and while the structural structure (160; 170; 180) is in a substantially upright orientation, hoisting the structural structure by using one or more cranes (110,210) to apply an axially directed lifting force (Fa) to each of the wires in such a way that the wires extend substantially parallel to each other.

The method of claim 1, wherein the wires are connected to or near the lower supports at a location below a horizontal plane through the bottom of the base.

A method according to claim 1 or 2, wherein during lifting, the sum of axial lifting forces exerted on the lifting ropes is at least 70%, preferably at least 90% of the axial force required to vertically raise the structural structure hoisting.

4. Method as claimed in any of the foregoing claims, wherein at least two of the hoisting wires are connected to a substantially rigid hoisting beam, wherein the at least two hoisting wires are connected to the hoisting beam at points which substantially correspond to two or more of the lower hoisting beams. to support.

The method of any one of claims 1-3, wherein at least three of the hoisting wires are connected to a substantially rigid hoisting frame (120), wherein the at least three hoisting wires are connected to the hoisting frame at points substantially corresponding to three or more of the lower supports.

Method according to any of the preceding claims, wherein the one or more cranes are arranged on one or more vessels (100) floating on the water.

A method according to any one of the preceding claims, wherein the hoisting cranes are arranged on a platform during hoisting that has a bottom that is in contact with and supported by the sea floor.

A method according to any one of the preceding claims, comprising subsequent steps of: positioning (120) the raised structural structure to a position in which the lower supports are substantially axially aligned with the one or more upper supports of the other structural structure ( 40); and lowering (130) the structural structure until the lower supports are substantially fully supported on the one or more upper supports of the other structural structure.

The method of any one of the preceding claims, wherein, when the structural structure is hoisted in the upright orientation, and when viewed in projection on a horizontal plane, the parallel parts of the wires are arranged within the peripheral edge of the base.

The method of claim 9, wherein when the structural structure is hoisted in the upright orientation, and when viewed in projection on a horizontal plane, the area tensioned by the lower supports is smaller than or equal to the area projected in the projection by the base is tensioned.

A method according to claim 8 or 9, wherein when the structural structure is hoisted in the upright orientation, and when viewed in projection on a horizontal plane, each of the lower supports is spaced from the peripheral edge of the base with a spacing greater than or equal to 4 times the length of the lower support along its major axis in the projection.

A method according to any one of the preceding claims, wherein the step of connecting the hoisting wires comprises, for each of the hoisting wires, connecting one end of the wire to one of the lower support surfaces.

A method according to any one of the preceding claims, wherein one or more of the wires are provided with a loop that is directly connected to the lower support surfaces.

The method of claim 13, wherein the direct connection is achieved by the one or more of the wires around the lower support surface at a bottom of the base.

A method according to any one of the preceding claims, wherein the one or more hoisting wires are connected to the lower supports such that, during hoisting, the wires extend from the bottom of the base through openings in the base to the top of the base. base.

A method according to any one of the preceding claims, wherein one or more of the lower supports comprises a leg comprising a substantially cylindrical or conical leg inner surface, one or more of the wires being provided with an internal hoisting tool at one end thereof that is adapted to engage the inner surface of the leg.

A method according to any one of the preceding claims, wherein one or more of the lower supports comprises a leg having a substantially cylindrical outer surface around which an external hoisting tool is attached, one or more of the wires being attached to the external hoisting tool.

A method according to any one of the preceding claims, wherein one or more of the lower supports is provided with a lifting eye or stud to which one or more of the wires is attached.

The method of claim 18, wherein the method further comprises, prior to hoisting the structural structure, welding a reinforcement structure on the lower support, wherein the reinforcement structure is provided with a lifting eye or stud.

The method of any one of the preceding claims, wherein the structural structure comprises one or more of:

a topside, wherein the one or more lower supports are formed by legs of the topside, and wherein the other structural structure is a jacket;

a topside module, wherein the one or more lower supports are formed by legs of the topside module, and wherein the other structural structure is a topside;

a jacket, wherein the one or more lower supports are formed by lower ends of the jacket, and wherein the other structural structure is a foundation of a jacket foundation.

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