WO2014098605A1

WO2014098605A1 - Offshore installation method, e.g. by floatover, and system.

Info

Publication number: WO2014098605A1
Application number: PCT/NL2013/050945
Authority: WO
Inventors: Mark Riemers
Original assignee: Suction Pile Technology Bv
Priority date: 2012-12-21
Filing date: 2013-12-21
Publication date: 2014-06-26
Also published as: NL2012008B1; EP2943617A1; NL2012008A

Abstract

Method for offshore installation of an offshore structure being part of an assembly comprising an offshore structure loaded onto a transport vessel floating in a body of water, this assembly being designed for installation of the offshore structure onto the sea floor or an already installed foundation structure, e.g. jacket, by a floatover operation. Preferably the barge draft starts at maximum, complete topside load, step 1, then reaches close to minimum, start of step 5, then reaches close to maximum, end of step 5, then reaches close to minimum, end of step and then quickly reaches minimum, step 7.

Description

Offshore installation method, e.g. by floatover, and system.

This invention relates to offshore installation of an offshore structure wherein the offshore structure is supported by a transport vessel and at the location of installation is transferred from the transport vessel to a foundation, for which transfer a floatover operation is applied. The foundation can be the seabed or an in advance installed substructure, e.g. j acket .

EP2216447A1 (Sip2New) , EP1101872A2 ( Sip2 ) ,

WO2011071385A1 (Sip23) and WO02088475A1 ( Sip3 ) disclose floatover installation. EP2397400A2, US2010316449A1 and US6540441B1 also disclose floatover installation. EP2397400A2 discloses in Fig. 1-5 and the corresponding specification the principle of a floatover operation.

A barge with the offshore structure, e.g. topside (also termed topsides), on board moves with six degrees of freedom in response to the sea state at the installation site. Thus the upright legs of the topside move relative to the mating upright legs of the jacket. If this relative movement is too much, the floatover must wait for lower sea states. Devices are known to be applied to restrain relative motion in the horizontal plane (e.g. bumper means) between the mating legs or to reduce vertical impact loads between the mating legs (e.g. LMU (leg mating unit) ) .

There is a need for an improved system and method of loading an offshore structure, e.g. topsides, onto an offshore foundation. The object of the invention is versatile. According to an aspect the object is allowing the structure to enjoy the benefits of remote, e.g. nearshore or onshore, assembly.

According to an aspect the object is to avoid the use of a heavy lift crane vessel. According to an aspect the object is to avoid the use of impact damping elements, e.g. a LMU. According to an aspect the object is to minimize or avoid ballasting or deballasting of the barge during floatover which helps to reduce the time required for a floatover. The object could also be a combination of one or more of the above or elsewhere in the application disclosed objects.

According to the invention the topside is supported by a transport vessel, e.g. a floating barge, pontoon or floater and this barge is floated out to the installation site where the topsides or deck or different offshore structure is loaded onto the jacket by a floatover operation such that after installation the topsides is supported by the jacket. The jacket or different substructure can be floating type or supported or founded by the seabed. The offshore structure supported by the barge could in the alternative be immediately loaded onto the seabed, e.g. if the offshore structure has an integral foundation, e.g. comprising one or more suction piles for embedment into the seafloor.

The topside has one or more elements to mate with one or more elements at the foundation. Typically, the topside and the jacket each have one or more upright legs designed to be in register and mated when the topside is finally installed onto the jacket. During installation the legs of the topside are made to mate with, e.g. stab onto, the legs of the substructure .

Typically, as known in general, for the floatover method the topsides is temporarily coupled with, e.g. loaded onto one or more barges near land and the barges are floated to and above the jacket.

Typically the topside is loaded onto a single barge hull. During floatover the single barge hull floats within the slot (the slot being the region between two parallel rows of upright legs of the jacket) . Such slot must be designed to be devoid of any framing members and the slot must be deeper than the maximum draft of the barge. When within the slot the barge floats above much of the jacket.

As an alternative two or more parallel barges could be used, e.g. if a slot at the jacket is absent. During floatover the jacket fits in the space between the two barges and the topside spanning between the two barges (catamaran floatover) .

In the floatover position the upright legs of the topside are registered with the upright legs of the jacket, such that the topside is ready to mate with the jacket and be transferred from the barge. In the floatover position, one or more of the following steps are carried out so that the topsides can be transferred from the barge to the jacket: the barge is ballasted downward and/or deballasted upward; means, e.g. TJU (topside jack unit) or DSU (deck support unit) supporting the topsides above the barge deck (for example sand jacks) are retracted and/or extended (or expanded and/or contracted) or removed to change the vertical distance between the topside and barge (in particular their contacting faces); the upright legs of the topside are moved upward and/or downward relative to the topside .

Preferably one or more of the following features are applied to the arrangement of barge and topside supported by the barge: advance means, preferably linear actuators, e.g. jack means, to advance, preferably in a positive manner, upward and/or downward in leg longitudinal direction of the upright legs of the topside relative to the topside (or vice versa), which advance means could be embodied by strand jacks or motoric driven winches lengthening and shortening hoisting cables, which means preferably are designed for selective switching between retaining mode and free running mode; the advance means have elongate suspension elements designed to bear both tension and compression loads, e.g. since they are flexural rigid and/or buckle rigid, e.g. taut strands, co-extending with the legs of the topside and mounted to the topside legs at the opposite leg ends; the advance means are double acting and/or

hydraulically driven; removable or retractable supports, e.g. linear actuators, e.g. hydraulic jacks (also termed TJU or topside jack units) located between the barge and the topside to change the vertical distance between the barge bearing surface and topside; shock absorbers at the legs of the topside to dampen the vertical impact force at the time of stabbing the topside legs onto the jacket legs are absent; quick couplings or parts thereof at the mating ends of the topside legs to couple with the mating ends of the jacket legs; the elongate suspension elements and/or the topside legs preferably extend above and below the topside at the time the topside is supported by the barge; the TJU's provide a stroke of at least 30 centimetre (e.g. typically approximately 100 centimetre) which stroke provides equal change of the vertical distance between the barge and the topside.

Preferably in the retaining mode the jack means hold the topside legs and command their longitudinal advancement up and/or down and the drive force provided by the jack means is transferred to the topside legs for the advancement, preferably positive advancement, of the topside legs relative to the topside, while in the free running mode the topside legs are released from the jack means such that the topside legs are free to longitudinally advance up and down relative to the topside .

The strand jacks could be replaced by alternative means, preferably removably mounted to the topside to be removed after completion of the installation, which allow a kind of climbing or different movement of the topside up or down along an elongate suspension element. In case of the strand jack, the elongate suspension element is provided by the strands along which the hydraulic jack or ram (expansion body) climbs by alternatingly or intermittently or successively clamping to the strands with its first or second longitudinally spaced clamps and move these clamps longitudinally to and from each other. The topside is preferably rigidly mounted to the expansion body to move with the expansion body along the suspension element while the expansion body successively expands and contracts. In case of double acting the advance means provide both upward and downward movement, preferably any or both in a positive manner.

The term positive preferably means that the advance means applies a driving force in the direction of movement. For example if the topside legs are lowered relative to the topside to mate with the jacket legs the advance means could apply to the legs a downward directed driving force in addition to the downward directed gravity force. Such is impossible with a hoisting cable but possible with a linear actuator, e.g. modified strand jack. A prior art strand jack is limited to lifting a load. One of the advantages of the elongate suspension elements being flexural rigid or buckle rigid is that the expansion body can climb in both longitudinal directions along the suspension elements, such that a positive displacement both up and down of the topside legs relative to the topside is obtained. In case of a flexible pulling element, e.g. a cable or rope or strand, the flexural or buckle rigidity can be obtained by mounting the cable to be pulled taut such that the cable is pretensioned and thus can withstand both upward and downward movement of a body mounted to the cable and loaded by an upward or downward, respectively, force. Alternatively e.g. a beam or a rail, possibly mounted to a topside leg, is (by nature) a flexural or buckle rigid elongate suspension element, however not easy to remove from the topside.

Preferably, during floatover one or more of the following steps are carried out in the order given, or in any different order: while the topside bears onto the barge the topside legs and jacket legs are brought in mutual elongation and

subsequently the topside legs are lowered while moving up and down with the barge floating on the sea waves and then united with the topside legs (or the topside legs are engaged with the sea floor) ; while the topside legs are united with the jacket legs (or sea floor) the topside is allowed to move free up and down along the legs (free running mode) while resting onto the barge floating on the sea waves; while the topside legs are united with the jacket legs (or sea floor) the topside is held immovably relative to the legs while supported by the barge (retaining mode); while the topside legs are united with the jacket legs (or sea floor) the topside is moved up along the legs while still supported by the barge such that the legs increasingly support the topside and thus the barge is increasingly unloaded and thus looses draft, preferably during this step the upward movement of the topside supported by the barge is substantially free from influence from the sea waves (retaining mode) ; supporting means (e.g. TJU) between the barge and topside are actuated, e.g. expanded to force the barge and topside vertically away from each other to increasingly load the barge and thus increase its draft; after activation of the supporting means the topside is further moved up along the legs to increasingly unload the barge; supporting means (e.g. TJU) between the barge and topside are actuated, e.g. contracted, to quickly increase the vertical distance between the topside and bearing surface of the barge to avoid the unloaded barge impacting the topside from below while moving up and down due to the sea waves, preferably wherein such actuation takes place at the time the barge is still supporting part of the topside load such that the barge is quickly completely unloaded; during actuation of the supporting means to increase the distance the topside is moved up along the legs; moving up the topside along the legs is always in retaining mode.

In an embodiment the topside legs are permanently fixed to the jacket legs (e.g. by welding) before the procedure of lifting the topside from the barge is started. Preferably, however, these legs are temporary coupled or gravity coupled during lifting and only after the barge is removed the permanent fixture is applied.

In an embodiment the TJU or equivalent expandable support element in its one position, e.g. retracted or contracted, does not support the topside, in other words a different supporting part associated with the barge supports the topside, while in its second position, e.g. expanded or extended, indeed supports the topside. Such case is illustrated by steps 4 - 6 of the steps 1 - 10 of the attached drawing. In step 4 the TJU (referenced as jacks 6) are completely retracted and the topside bears onto fixed supports such that the TJU's do not carry any load from the topside. In step 5 the barge is pushed down by extension of the TJU's such that the TJU's are loaded by the topside and the fixed supports do not carry any load from the topside. This situation is maintained in step 6. The advantage of using the TJU's only as temporary supports during floatover, preferably only after lifting of the topside is started, is that the TJU's do not need to bear the complete weight of the topside. Since in step 5 part of the topside weight is carried by the topside legs. As an alternative the assembly could be that in the one position of the TJU's they bear only part of the topside weight while the other part of the topside weight is carried by a different supporting part associated with the barge. Thus the design is such that the TJU's will carry only part of the topside weight.

Preferably the draft of the barge fluctuates during the floatover due to fluctuating load from the topside and only by the time of retracting the TJU's reaches minimum draft. Preferably the draft starts at maximum (complete topside load, step 1), then reaches close to minimum (start of step 5), then reaches close to maximum (end of step 5), then reaches close to minimum (end of step 6) and then quickly reaches minimum ( step 7 ) .

Presently preferred embodiments are illustrated in the drawing.

Fig. 1 - 4 show a first embodiment in side view, end view and top view, before and after installation;

Fig. 5 - 10 show in perspective view a second embodiment at different steps during a floatover operation;

Fig. 11 shows in side view a detail of fig. 7;

Fig. 12 shows a perspective view of the completed structure according to fig. 5 - 10;

Fig. 13 shows in sectional side view an upright leg during the floatover operation at two different stages;

Fig. 14 shows a schematic side view of the releasable clamps of a strand jack;

Fig. 15 - 18 show schematic side views of the operating principle of the double acting strand jacks during floatover operation according to the invention; and

Fig. 19 and 20 show schematically in side view an embodiment of the double acting hydraulic jack assembly of the strand jacks according to the invention.

Fig. 21-22 show steps of a possible floatover procedure.

According to fig. 1 - 4, an offshore structure having an integral foundation of suction piles is installed onto the seafloor by a floatover operation. Fig. 1 and 2 show in side and end view the stage prior to floatover, fig. 3 shows in a view similar to fig. 1 the floatover completed and the barge 7 sailed away. Fig. 4 shows a topview of the topside 2, the four legs 1 and the suction piles 11. Also the sea surface 8, the sea floor 12, and the TJU's 6 are shown.

According to fig. 5 - 10, first a jacket is installed onto the seafloor and subsequently the topsides is installed onto the jacket by a floatover operation. Top strand anchor blocks 3 are shown. The topside legs 1 can be coupled to the jacket legs 9 one at a time (individually) or simultaneously. In fig. 6 installation of the jacket is completed. The suction piles 11 are penetrated into the seafloor 12. The legs 9 project above the water level 8. The floatover procedure for the topside 2 now commences.

In fig. 7 the legs 1, 9 are registered. In fig. 8 the legs 1 are lowered onto the legs 9 and mated and coupled with them. In fig. 9 the topside 2 is separated from the barge 7 by upward movement of the topside 2 along the legs 1. In fig. 10 the barge 7 is sailed away.

Fig. 11 illustrates of the Fig. 5-10 embodiment

schematically one of the topside legs 1, the topside 2 (partly) , the strand anchor blocks 3 at top and bottom end of the leg 1 (above and below the topside 2), the hydraulic jack 4 and the strands 5 of the strand jack associated with a single leg 1. Also illustrated are one of the hydraulic jacks 6 (TJU), the barge 7 (partly), the water line 8 and one of the jacket legs 9. The strands 5 are pulled taut between the blocks 3 such that the strands 5 can withstand a pulling load in both directions .

The legs 1 are designed to support the topside onto a foundation. The blocks 13 support the topside 2 while loaded on the barge 7.

By operation of the jack 4 (expansion and contraction of the distance a) the topside 2 is made to climb up and down along the strands 5 (direction of arrows A and B) . By operation of the jack 6 (expansion and contraction of the distance b) the barge moved to and from the topside (direction of arrows C and D) . The floatover operates as follows: as soon as the legs

1 and 9 are registered, the leg 1 is positively lowered by the appropriate action of the jack 4 while the barge 7 and topside

2 maintain their mutual position. Typically, the barge 7 (and thus the topside 2) will rise up and down due to sea waves.

The leg 1 nears the leg 9 and finally mates and couples with leg 9. Male and female parts of quick couplings (Quick couplings are typically a temporary fixture) at the facing ends of legs 1 and 9 are engaged and activated during mating, thus legs 1 and 9 become fixedly connected by the quick couplings (e.g. similar to Lynx connector, supplied by Oil States Industries, Inc., Arlington, Texas, USA) . At that time the jack 4 releases the strands 5 to enter the free running mode. In the free running mode the topside 2 and jack 4 are free to move up and down (caused by sea waves) along the fixed legs 1 and strands 5.

An alternative coupling is provided by gravity force which keeps the legs coupled and engaged by merely the weight of the topside leg.

During coupling some offset of the mating legs 1, 9 is allowed, as fig. 13 shows (to the left at the start, to the right as coupling is finished) .

While in the free running mode the mating legs 1 and 9 are permanently connected by welding (or permanent connecting is done at a later time) . Also the TJU 6 is contracted to its minimum of 1 meter stroke, thus topside and barge have minimum distance while still mutually connected.

After completion of welding the legs and contraction of the TJU, the jack 4 is again connected to the strands 5 in preparation of positive upward movement of the topside 2 along the legs 1. As soon as jack 4 holds the strands 5, the position of the topside relative to the legs 1 is fixed. Due to contraction of the jack 4 while holding the strands 5 at its top and releasing the strands 5 at its bottom, the topside 2 climbs upward along the legs 1, causing the barge 7 becoming progressively unloaded from the topside, such that the barge draft decreases. The additional draft of the barge due to full topside load is three meters. At the time this additional draft is decreased to 1 meter since two thirds of the topside load is supported by the legs 1 while the topside climbs upward, thus before the topside is completely unloaded from the barge, the TJU's 6 are fully extended and provide a one meter stroke. Since the strand jacks are in the retaining mode, expansion of the TJU's 6 provides that the barge is pushed one meter deeper into the water .

Upward climbing of the topside caused by continued action of the strand jacks continues while the maximum expansion of the TJU's 6 is maintained. At the time the topside is completely unloaded from the barge, the TJU's 6 are contracted. Thus the barge 7 and topside 2 are separated due to simultaneous upward climbing of the topside 2 and contraction of the supports 6, such that the topside 2 is quickly released from the barge 7. The barge 7 (moving with the sea waves) impacting the topside

2 from below is thus restricted to a minimum.

The jacket of fig. 12 has a suction piles foundation. The suction piles 11 could be replaced by a different foundation means, e.g. for piles which are rammed into the sea floor.

Fig. 14 illustrates the clamping principle of a strand jack. Illustrated are a strand 2, clamping wedges 3 and an inclined slide face 7. The clamping wedges 3 are fixed to the hydraulic jack. Actually, the wedges are always contacting the strand 2, preferably urged towards the strand by a spring means (not shown) . If the strand 2 moves in the direction of the arrow, the wedges 3 initially are dragged along due to friction between strand 2 and wedges 3 and thus are forced towards strand 2 since the wedges 3 slide along face 7. This results in the wedges

3 clamping and thus fixing the strand 2 between them (clamping position) . Forcing the strand 2 opposite the arrow provides release of the wedges 3 (standby position) . This is the retaining mode since each time the strand 2 moves according to the arrow, the wedges 3 will automatically clamp the strand 2. To avoid this, a means should be added to avoid that the wedges are dragged along with the strand 2 (freerunning position) . Such means is not illustrated, however such could be embodied by retaining members that can selectively be switched between a retaining position in which they retain the wedges 3 from being dragged along by the strand 2 (freerunning mode) , and a releasing position in which they are released from the wedges 3 such that the wedges are dragged along by the strand

2 or are in the standby position (retaining mode) . Such retaining members e.g. deactivate the spring means urging the wedges 3 against the strand 2.

For clamping the strand 2 moving in the opposite direction, the wedges 3 and face 7 should be turned upside down.

In stead the strand 2 moves relative to the wedges 3 and face 7, the assembly of wedges 3 and face 7 could move relative to the strand 2, yielding the same clamping, standby and freerunning positions and retaining and freerunning modes.

Fig. 15 - 18 illustrate a topside 1, strand 2, wedges 3 - 6 of the strand jack. Wedges 3 and 4 are for clamping the strand 2 moving in the one direction, wedges 5 and 6 are for clamping the strand 2 moving in the opposite direction (or vice versa the wedges move and the strand is immovable) .

Fig. 15 shows two stages (left first and right second) during climbing (arrow b) of the topside 1 relative to the strand 2, and thus topside leg, with positive displacement. The operation starts at the left hand strand 2 (first stage) . Wedges 3 clamp the strand 2, Wedges 4, 5 and 6 are in the standby position. The distance between wedges 3 and 4 is decreased by contraction of the associated hydraulic jack (arrow a) . The wedges 5 and 6 follow without changing distance between wedges 4, 5 and 6 and also the topside 1 follows. When this moving of wedge 4 towards wedge 3 is completed, the stage (second stage) as shown for the right hand strand 2 is obtained in which wedge

3 is now in the standby position (arrow e) , wedge 4 is now in the clamping position (arrow d) . The hydraulic jack associated with the wedges 3 and 4 starts now to expand (arrow c) to move wedge 3 away from wedge 4. Wedges 4-6 and also topside 1 do not move relative to the strand 1 during this stage. The dotted lines illustrate the location of the wedges 4-6 at the beginning of the first stage. After completion of the second stage, a further first stage can commence, etc. During climbing of the topside 1, the level of point P at strand 2 is constant.

Fig. 16 shows two stages (left first and right second) during descending (arrow b) of the strand 2 and thus topside leg (not shown) relative to the topside 1, with positive displacement. The operation starts at the left hand strand 2 (first stage) . Wedges 3 and 5 clamp the strand 2. Wedges 4 and 6 are in the standby position all the time. Wedges 5 bear the weight of the associated topside leg. Wedges 3 are moved towards wedges 4 (arrow a) by contracting the associated hydraulic jack. As a result point P at strand 2 descends and also wedges 5 descend (arrow a) for the same amount. Wedges 5 descend due to contracting of the associated hydraulic jack. Wedges 4 and 6 and topside 1 keep their level all the time. When this descending movement of wedges 3 and 5 is completed, the stage (second stage) as shown for the right hand strand 2 is obtained in which wedges 3 and 5 are still clamping strand 2. Wedges 3 and 5 are now brought (arrow c) to their initial level (according to first stage) illustrated with dotted lines by first releasing wedges 3 and reclamping the strand at their initial level and subsequently releasing wedges 5 and reclamping the strand 2 at their initial level. Thus, during second stage wedges 3 temporarily carry the topside leg weight while wedges 5 are in the standby position and move upward to their initial level. As soon as wedges 3 and 5 have taken their initial levels, the first stage is obtained again and a next lowering step can commence, ect . With this technique lowering the topside leg is not depending from gravity force since the strand jack can positively drive the leg downward.

In stead of using wedge 5, wedge 6 could be used for bearing the leg weight. Then wedge 6 would move in phase with wedge 3. During the first (descend) stage wedge 6 preferably moves away from wedge 5, e.g. by expansion of the associated jack.

Fig. 17 shows an alternative to Fig. 16. Fig. 17 shows two stages (left first and right second) during descending (arrow b) of the strand 2 and thus topside leg (not shown) relative to the topside 1, without positive displacement. The operation starts at the left hand strand 2 (first stage) . Wedges

5 clamp the strand 2, wedges 6 are in the standby position. Wedges 3 and 4 are in the standby position all the time. Wedges 5 bear the weight of the associated topside leg. Wedges 5 are moved towards wedges 6 (arrow a) by contracting the associated hydraulic jack. As a result point P at strand 2 descends and also wedges 5 descend (arrow a) for the same amount. Wedges 3, 4 and 6 and topside 1 keep their level all the time. When this descending movement of wedges 5 is completed, the stage (second stage) as shown for the right hand strand 2 is obtained in which wedges 5 are brought in their standby position (arrow e) and wedges 6 now clamp the strand 2. Wedges 5 are now brought (arrow c) to their initial level (according to first stage) illustrated with dotted lines. Thus, during second stage wedges

6 temporarily carry the topside leg weight while wedges 5 are in the standby position and move upward to their initial level. As soon as wedges 5 have taken their initial levels, the first stage is obtained again and a next lowering step can commence, etc.

Also different ways for climbing and descending by using the alternating or intermittent movement between wedges 3 and 4 and wedges 5 and 6 and their alternating or intermittent clamp and standby modes are feasible.

Fig. 18 illustrates the free running mode. Fig. 18 shows two stages (left first and right second) during unrestricted up and down (arrow b) movement of the strand 2 and thus topside leg (not shown) relative to the topside 1 (or vice versa) . The operation starts at the left hand strand 2 (first stage) . All wedges 3-6 are in their freerunning position all the time. Point P at strand 2 is free to move up (second stage) and subsequently can freely return to its initial level (first stage) . The level of wedges 3-6 and topside remains unchanged all the time. In the alternative case, the level of point P remains unchanged all the time and the level of wedges 3-6 and topside 2 changes up and down in an equal amount .

Fig. 19 shows an assembly of two hydraulic jacks, both double acting. Their barrels 21, 22 are fixedly mounted to the topside 1. Their shafts 23 are completely extended and are retractable (arrow a) . Wedges 3 are associated with the one shaft. Wedges 6 are associated with the other shaft 23. Wedges 4 are associated with barrel 21. Wedges 5 are associated with barrel 22. In this embodiment the barrels 21, 22 face towards each other.

Fig. 20 shows an alternative assembly in which the two jacks are head to tail. In a further alternative the barrels 21, 22 face away from each other (in other words the shafts 23 are sandwiched between the barrels 21, 22.

The barrels 21, 22 and shafts 23 could also be functionally integrated into a single jack in a manner to allow the relative movement of the wedges 3-6.

By way of fig. 11 and fig. 21 and 22, steps 1-10 of a floatover procedure are illustrated. During step 1 (illustrated in fig. 11), the barge 7 is located between the pre-installed jacket. In step 2 the legs 1 are lowered onto the jacket 9. In step 3 the barge 7 and the topside 2 are in the free running mode. In step 4 lifting of the topside 2 by the jacks 4 is started, thus the barge 7 is increasingly unloaded. In step 5

(illustrated in fig. 21) at first the barge 7 still bears part of the topside 2 load through the blocks 13 while the topside 2 is kept at a fixed level and subsequently the jacks 6 are expanded such that the barge 7 is pushed deeper into the water 8 and the topside 2 is freed from the blocks 13. In step 6, while the jacks 6 are kept expanded, the topside 2 is lifted by the jacks 4 thus the barge 7 is increasingly unloaded, preferably until 80% of the load from the topside 2 is unloaded from the barge 7. During the whole duration of step 6, the jacks 6 transfer the load from the topside 2 to the barge 7 while the blocks 13 are unloaded. During step 7 (illustrated in fig. 22) the jacks 6 are contracted while the topside 2 is kept at constant level or is moved upward, thus the barge 7 is quickly completely unloaded. In step 8 the topside is further lifted by the jacks 4. In step 9 the barge 7 is sailed away. In step 10 the installation is completed.

Claims

1. Method for offshore installation of an offshore structure being part of an assembly comprising an offshore structure loaded onto a transport vessel floating in a body of water, this assembly being designed for installation of the offshore structure onto the sea floor or an already installed foundation structure, e.g. jacket, by a floatover operation.

2. Method according to claim 1, wherein during floatover a topside, supported by a barge, is installed onto an already installed jacket and the topside and the jacket each have upright legs designed to be in register and mated when the topside is finally installed onto the jacket and during floatover the legs of the topside are registered and made to mate with the legs of the substructure while the barge floats above the jacket between its legs, and expandable jacks (6) are active between the topside (2) and the barge (7) to change their mutual vertical distance, wherein the draft of the barge fluctuates during the floatover due to fluctuating load from the topside and only by the time of retracting the jacks (6) reaches minimum draft.

3. Method according to claim 2, the barge (7) draft starts at maximum (complete topside load, step 1), then reaches close to minimum (start of step 5) , then reaches close to maximum (end of step 5), then reaches close to minimum (end of step 6) and then quickly reaches minimum (step 7) .

4. Method according to any of claims 1-3, after coupling the topside legs with the jacket legs, the topside (2) moves upward along the legs (1), causing the barge (7) becoming progressively unloaded from the topside, such that the barge draft decreases, subsequently, before the topside is completely unloaded from the barge, the jacks (6) are extended such that they push the barge deeper into the water and subsequently while the jacks (6) are kept extended, the topside moves further upward along the legs (1) causing the barge (7) becoming progressively unloaded from the topside, such that the barge draft decreases, subsequently, before the topside is completely unloaded from the barge, the jacks (6) are contracted such that the barge (7) and topside (2) are separated.

5. Method according to any of claims 1-4, advance means, e.g. strand jacks, let the topside climb up and down along the legs in a positive manner in both directions.

6. Method according to any of claims 1-5, the means for moving the topside up and down along the legs are selectively switched between retaining mode and free running mode.

7. Method according to claim 6, in the retaining mode the means control the relative position between the topside and the topside legs, while in the free running mode the topside legs are free to longitudinally advance up and down relative to the topside.

8. Method according to claim 6 or 7, prior to coupling the topside with the jacket the retaining mode applies and subsequently one switches from retaining to free running mode and after the topside is coupled with the jacket one switches from free running to retaining mode and after that the floatover operation is completed.

9. Assembly comprising an offshore structure loaded onto a transport vessel floating in a body of water, this assembly being designed for installation of the offshore structure onto the sea floor or an already installed foundation structure, e.g. jacket, by a floatover operation.

10. Assembly according to claim 9, provided with expandable means (6) active between the topside (2) and the barge (7) to change their mutual vertical distance; and/or advance means to let the topside climb up and down along the legs in a positive manner in both directions, preferably designed to be selectively switched between retaining mode and free running mode.