OA10876A - Method for assembling a floating offshore structure - Google Patents

Abstract

An assembly method involves mating the topsides (26) on a floating offshore structure such as a spar hull (10), joining components of the spar hull (10), and/or a combination or variation of these procedures. Generally, the installation is carried out as follows. Once at the mating site, the spar hull component (12) is upended, outfitted with controls and hardware (20) necessary to effectuate controlled submergence of the spar hull component, and submerged to a predetermined depth below the water surface to have a zero water plane area. The second component (26) to be mated with the top of the spar hull component (12) is positioned to float above the submerged spar hull component (12). The draft of the submerged spar hull component (12) is decreased until the two components (12, 26) are fully engaged.

Description

-1-

METHOD FOR ASSEMBLING A FLOATING OFFSHORE STRUCTURE

BACKGROUND OF THE INVENTION 5 1. Field of the Invention

The invention is generally related to the installation of decks on offshore structures and more particularly to theassembly of floating offshore structures. 2. General Background 10 In the offshore drilling industry, unlike ships which can be fully assembled at an inshore facility, many types cf oildrilling or production facilities require part of the assemblyto take plac-e either at the field location itself or afloat priorto tow to the installation site. For example, it is, by the L5 nature of the design of jacket type production platforms, that the production deck (topsides) be installed after the jacket hasbeen installed and piled to the sea floor. The topsides aretypically installed in one or more pièces using heavy lift marinecrânes. This can be a costly and weather sensitive operation. 10 Also, additional cost is incurred due to the additicnal logistical support for the final hook-up of the topsides to theplatform which must then take place offshore. The offshore hcok-up problem is further aggravated if the topsides requires severallifts and/or the production platform is in a remote location. 5 Concrète GBS production platforms and floating production platforms such as Spar Platforms can provide the option, due totheir buoyancy capacity, of avoiding the cost associated withoffshore heavy lift operations by allowing a "float over" deckinstallation operation. Using this prior art method, a fully 010876 -2- ! completed deck is loaded on barges in a catamaran configuration,the platform is ballasted down to a reduced freeboard, and thetcpsides floated over the platform. The platform is deballasted,thereby picking up the topsides and lifting it to the proper 5 élévation above the water line. However, the transport of the topsides and the mating operation itself must take place infairly benign conditions.

Due to the large draft of Spar type platforms, thetraditional construction sequence, for Steel hull Spars, involves 10 the joining of structural sections in the horizontal position, followed by upending of the entire Spar hull to the verticalposition. The structural sections may consist of either platéehull tank sections only, or combination of plated tank and trusstype sections. Such Spar type platforms are described in U.S. 15 Patents No. 4,702,321 and 5,558,467. As a conséquence of a horizontal assembly and upending sequence, the topsides can onlybe installed after the upending operation and thus must takeplace in a location with substantial water depth. This canresuit, depending on geographical location, in either: 20 the topsides having to be installed offshore in a non- sheltered area, which means the deck transport andinstallation become weather sensitive operations;or possibly require a long tow of the fully assembled Sparto the production site, if the risk of an offshore deck :-5 installation is too high and the topsides must be installed in a sheltered location.

It can be seen that the présent State of the art in the -3- »._*·<« si /u (Ί1^ ι«"·ΐΜ>ί^·*^ j J d 7 6 installation of topsides on a floating offshore structure suchas a Spar type hull includes shortcomings which hâve not beenadequately addressed.

SUMMARY OF THE INVENTION 5 The invention addresses the above shortcomings. What is ♦ provided is a method of mating the topsides on a floatir.goffshore structure such as a spar hull, joining components of aspar hull, and a combination or variation of these procedures.Generally, the installation is carried out as follows. The spar 10 hull component is towed to the mating site and upended, outfitted with Controls and hardware necessary to effectuate controlledsubmergence of the spar hull component, and submerged to apredetermined depth, thereby positioning the submerged spar hullcomponent either floating above the seabed or, alternatively, 15 resting on it. The second component to be mated with the top of the spar hull component is positioned above the submerged sparhull component. The draft of the submerged spar hull componentis decreased until the two components are fully engaged.

BRIEF DESCRIPTION OF THE DRAWINGS 20 For a further understanding of the nature and objects of the présent invention reference should be made to the followingdescription, taken in conjunction with the accompanying drawingin which like parts are given like reference numerals, andwherein: 25 Fig. l illustrâtes a spar hull which has been upended and submerged in position floating above the seabed, with the topsides ready to be moved into position over the hull. 08 7 6 -4-

Fig. 2 illustrâtes the topsides in position over the hullas the hull is moved upward into engagement with the topsides.

Fig. 3 is a plan view of the topsides.

Fig. 4 illustrâtes a variation of the procedure of Fig. 15 and 2 where the topside and hard tank components hâve been previously engaged and are then moved into position over the truss component of the spar.

Fig. 5 illustrâtes a variation of the procedure of Fig. 4where the hard tank component, without the topsides, is engaged 10 with the truss component.

Fig. 6 illustrâtes a variation of the procedure of Fig. 1where a spar hull which has been upended is submerged in positionresting on the seabed, with a selected water depth, and bottomportion of the hull penetrating the seabed to a predetermined 15 depth.

Fig. 7 illustrâtes the topsides in position over the hullof Fig. 6 as the hull is moved upward, thereby reducing thepénétration of the bottom hull portion in the seabed and engagingwith the topsides. 20 Fig. 8 illustrâtes an alternate embodiment of the invention where shock absorbing materials are provided.

Fig. 9 illustrâtes an alternate embodiment of the inventionwhere matching grooves and protrusions are provided on thecomponents. 25 Fig. 10 is a plan view of the topsides that illustrâtes the protrusions on the structural framing.

Fig. 11 is a plan view of the spar hull that illustrâtes the 11 0 n < grooves on the structural framing.

Fig. 12 is a view taken along lines 12-12 in Fig. 11.

Fig. 13 is a plan view of an alternate embodiment to Fig. 10-12. 5 Fig. 14 is a view taken along lines 14 -14 in Fig. 12. Fig. invention. 15 illustrâtes an alternate embodiment cf the Fig. invention. 16 illustrâtes an alternate embodiment of the 10 Fig. 17 illustrâtes an alternate embodiment of the invention.

DETAILED DESCRIPTION OF THE PPEFERRED EMBODIMENT

Fig. 1 illustrâtes the situation where a floating offshorestructure such as a spar hull 10 has been towed to a mating site 15 having sufficient water depth to carry out the mating operation between the topsides 26 and hull 10. The mating site may aisebe the final operational site. If towed horizontally, the sparhull 10 is upended into its normal vertical operating positionand then submerged a predetermined depth below the water surface 20 to provide a zéro water plane area (no surface piercing éléments) across the spar hull 10. The spar hull 10 illustrated isgenerally comprised.of a truss section 12, a hard tank section14, and a soft tank section 16. Such a spar hull is describedin U. S. Patent No. 5,553,467. For the purpose of the invention, 25 the hard tank section 14 is provided with a plurality of stabbing posts 18 or cther arrangement for effecting the alignment and engagement of the components. It shoulc. be understcod that the 6- c10876 type of spar hull shown, a truss spar, is only an example of thetype of floating structure that may be used in the method of theinvention. Other floating offshore structures, such as a sparhull described in U.S. Patent Ho. 4,702,321 may also be used in 5 the method of the invention.

Once the spar hull 10 is upended, weighted draft contrcl

Unes 20 such as heavy chains, which are partly resting on thesea floor 21, are attached to the lower end of the spar hull 10,and a ballast control line 22, or "life line", is attached 10 between the spar hull 10 and a surface vessel 24 with air compressors on board. The weighted draft control Unes may befree hanging from the spar or attached to anchors 11, therebyalso serving for station keeping purposes. To increase theweight of the draft control Unes, clump weights 13 or bundles 15 of chain can be attached to the Unes near the seabed. The spar hull 10 is then submerged by ballasting tanks in the spar hulland, with the aid of the draft control lines 20 which change thesubmerged weight of the spar hull as chain is lifted on or offthe bottom, equilibrium at the desired depth is achieved. The 20 ballast control line 22 is used to control and adjust buoyancy of the spar hull 10 as necessary during ballasting forsubmergence and, later, deballasting upward into engagement withthe topsides 26.

Alternatively, the submerged depth of the spar hull can be 25 controlled by the ballast control lines 22 in conjunction with a length of buoyancy éléments 23 either rigidly or flexibly attached to the top of the hull and floating at the surface. The , ΰ 1 U ? ? 6 . -7“ ► submerged weight of the structure is changed by the number cr length of buoyancy éléments pulled below the surface as the hullsubmergence draft is progressively increased to the desireddepth. 5 Another useful alternative shown in Fig. 6 is to provide the bottom extremity of the spar hull component 10 with spud piles15, cans, or a mud mat. This allows the spar hull component tobe submerged at a site with pre-determined water depth and soi!conditions and ballasted to allow the structure to rest on the 10 sea bed and the spud piles 15, if so provided, to penetrate the sea floor 21. In this manner, spud piles 15 provide stationkeeping capability by means of shear résistance of the piles inthe soil for surviving design level storms while providinçsufficient latéral compliancy to avoid large environmentally 15 induced bending loads in the submerged structure. Overturning is prevented by the location of the center of buoyancy beingabove the center of gravity for the submerged structure.

Once the spar hull 10 is submerged to the proper depth,either floating above the sea floor or sitting on the sea bed 20 with the spud piles penetrating the sea floor, one or more maneuvering vessels 28 are used to move the self-floatingtopsides 26 into floating position above the spar hull 10. Itshould be noted that, depending upon the distance from thefabrication site to the mating site, the topsides may be towed 25 out in a self-floating mode or on a heavy lift vessel and then put afloat during the appropriate weather window. As seen in

Fig. 2 and 3, a plurality of mating Unes 30 may be used, each ΰ I ü 8 7 * ' -8- * engaged with a winch 32 mounted on the topsides 26. The topsides 26 is provided with a plurality of sleeves 34, vertical bores,sized to receive the stabbing posts 18. The mating lines 3 0 areeach run down through their respective sleeves 34 and attached 5 to the stabbing posts 18 using divers or underwater robots. The draft of the spar hull 10 is decreased by deballasting, crwinching, or a combination of both to engage the stabbing posts18 into the sleeves 34. It is désirable for the tension on themating lines 30 to be at a level sufficient to insure that the 10 stabbing posts 18 remain properly aligned with their respective sleeves 3 4 and to cause the two components to move togethervertically, thereby eliminating contact forces during the matinçoperation.

Once the connections between the topsides 26 and spar hull 15 10 are temporarily secured, the spar hull 10 is deballasted te raise the topsides 26 to a safe freeboard. The connectionsbetween the topsides 26 and spar hull 10 are then completed, theballast control line 22, and draft control lines 20 are removed.If required, solid ballast is then installed. If not already at 20 the final drilling or production site, the completed spar is towed to the final site.

Alternatively, in Fig. 7, both the topsides and the top ofthe spar hull component are provided with a plurality of sleeves34 sized to receive alignment posts 18. The alignment posts IS 25 are lowered from the topsides 2 6 through the matching sets of hull and topsides sleeves 34, thereby effectuating alignment of the two components without restraining them vertically. The spar » ίΛϊ i» ¢- hull is then deballasted to make contact between the hull andtopsides and further deballasted to raise the topsides to a safeélévation above the water.

Fig. 4 illustrâtes an alternative wherein the truss section12 and the hard tank section 14 are moved to the mating site asseparate components. This allows the topsides 26 to be mated tothe hard tank section 14 prior to tow out to the mating site. Thestabbing posts 18 or similar arrangement are provided at theupper end of the truss section 12 and the sleeves 34 forreceiving the stabbing posts are provided at the lower end of thehard tank section 14. As illustrated, the first component ofthe offshore structure, the truss section 12 is submerged asdescribed above. The second component of the offshore structure,the hard tank section 14 and topsides 26, are then floated intoposition above the truss section 12. The draft of the trusssection 12 is decreased until the two components are fullyengaged. The mating Unes 3 0 and winches 32 are used asdescribed above.

Fig. 5 illustrâtes another alternative wherein thecomponents of the offshore structure may be separated into threecomponents (truss section, hard tank section, and topsides) , withonly the truss section 12 and hard tank section 14 being shown.The mating procedure is carried out as described above. The maindifférence is that the upper portion of the hard tank section isalso provided with stabbing posts 19 which are received insleeves 34 in the topsides as seen in Fig. 2. The truss section12 (first component) is submerged, the hard tank section 14 610ô76 -10- (second component) is floated into position over the trusssection 12, and the draft of the truss section 12 is decreaseduntil both of these components are fully engaged. The sameprocedure may then be used for mating the topsides 26 (third 5 component) to the hard tank section as described above.

With the forces generated during initial contact of the tvo components usually being the limiting factors for these types cfmating operations, the principles of this invention can befurther enhanced to carry out the operations in more severa 10 environmental conditions by incorporating certain provisions, either singly or in combination, into the designs of the sparhull components to be joined together. One means to reduce loadsproduced during the initial contact stages of the matingoperation is to provide the stabbing posts 18 or sleeves 34 with 15 shock absorbing material, indicated by numéral 25 in Fig. 8.

Another alternative, seen in Fig. 9, is to use the stabbing posts 18 for alignment purposes while allowing the stabbing posts18 to remain unrestricted vertically within the réceptacles 34throughout the operation. In this case, the floating and 20 submerged components are provided with matching grooves 27 and protrusions 29 to carry out the load transfer and finalalignment. The matching grooves and protrusions are providedwith shock absorbing material 25, thereby reducing the loadscreated as the two components are gradually brought into contact. 25 As the matching grooves and protrusions are engaged, the latéral alignment of the two components is secured and full load transfer can take place by deballasting the submerged hull component to 11- 01 0 8 7 6 the reguired freeboard.

It is also considered to be structurally bénéficiai to hâvethe matching grooves and protrusions 27, 29 aligned with or asextensions of the main structural framing of the hull componentsto be joined. In the example illustrated in Fig. 10-12, the mainstructural framing on which the grooves and protrusions 27, 29are fixed comprises the main structural bulkheads 31A, Brespectively in the floating topside component 26 and thesubmerged spar hull 10.

Fig. 13, 14 illustrate an alternate embodiment to that ofFig. 10-12 where the matching grooves 29 and protrusions 27 arerespectively fixed to a circular ring bulkhead 33 on the topsides26 and the perimeter of the submerged spar hull 10.

Fig. 15 illustrâtes an alternate embodiment to that of Fig.13-14 where the upper end of the spar hull 10 is provided witha circular shoulder bulkhead 37 around the perimeter of the hullwhich engages with the circular ring bulkhead 33 in the topsides26. Fendering 39 may also be provided on the circular ringbulkhead 33. This embodiment éliminâtes the need for the stabbing posts described above.

Fig. 16 illustrâtes another embodiment to those of Fig. 15and 2 where the circular shoulder 37 around the perimeter of thehull is replaced with an alignment bevel 42 around the bottomperimeter of the circular ring bulkhead 33 on the floatingtopside 2 6 to guide the submerged hull 10 as it engages thetopsides opening. Also, hydraulic chain jacks 38 or similarhoisting devices are shown to be fixed to heavy plates 43. Heavy ,« "ί ’W'** 010876 — 12 — plates 43 fit inside deck recesses 45 and are seated on shcckabsorbing raaterial 25. In combination, the heavy plates 43,shock absorbing raaterial 25, and the deck recesses 45 act as acompilant or shock absorbing plunger assembly 44. The joininç 5 with the spar hull 10 is carried out in a similar nanner as previously described where the topsides 26 is positioned abovethe spar hull 10, mating Unes 3 0 are attachée! to the top of thespar hull 10, and chain stoppera 40 are engaged, thereby leckingthe mating Unes 30 to heavy plates 43. The tension in matir.g 10 Unes 30 is increased bv reducing the air pressure in ballast tanks 35 in a controlled manner by means of ballast centroi line22, thereby allcwing more water to enter the tank through ballasttank inlet 36. Alternatively, water can be pumped into ballasttank 35 by pumping water through ballast control line 22. As the 15 tension is increased to a pre-determined level due te the increasing submerged weight of hull 10, the two competents areincreasingly caused to move vertically together. Durinç theperiod between attaching mating lines 30 and to the peint thatthe weight of the submerged spar hull 10 is sufficient to cause 20 the two hull components to move together, anv shock loads which could occur in a seaway are absorbed by the shock absorbingplunger assembly 44 on which the chain stoppers are mounted.After attaining the pre-determined tension in the mating lines30, the chain stoppers 40 are released and the joining operation 25 can continue by operating the chain jacks. Proper latéral alignment of the spar hull 10 and topsides 26 during the engagement operation is aided by alignment bevel 42. This -13 invention allows the two components to be safely joineà inseaways without the undesirable impact loads which couleotherwise occur using prior art methods. Another benefit of thésinvention over the prior art is the ability to put operations enhold while waiting on favorable weather or fully reverse theoperation at any point of engagement with minimal risk to the twocomponents or personnel. Due to the reversibility of theoperation, the invention can also be used to disassemble two sparcomponents which hâve been joined using the invention.

Alternatively, in Fig. 17, both the topsides 26 and the topof the spar hull 10 are provided with a plurality of sleeves 34sized to receive jacking posts 46. The jacking posts 46 arelowered from the topsides 26 through the matching sets of hulland topsides sleeves 34 and are connected to the hull 10 by meansof locking connectors 48, thereby effectuating alignment of thetwo components initially without restraining then vertically.Jack-up devices 47 mounted on the topsides 26 are then activatedagainst jacking posts 46, thereby decreasing the submerged draftof the spar hull component 10 until contact is made in acontrolled manner while restraining the undesirable verticalrelative motions between the two components caused byenvironmental forces. In this embodiment of the invention, thestep of ballasting or deballasting the submerged component canbe eliminated. Also, the use of shock absorbing material can beeliminated by the use of this embodiment. The principles andadvantages of this embodiment can be realized using a number ofother types of jacking, winching, or hoisting devices generally •1 4. .LA. 0i0876 «14- known in the art.

Because many varying and differing embodinents may be madewithin the scope of the inventive concept herein taugr.t andbecause many modifications may be made in the embodinent herem 5 detailed in accordance with the descriptive requirement cf the law, if is to be understood that the details herein are to beinterpreted as illustrative and not in a limiting sense.

Claims (33)

1. 01ϋ«76 -15- What is clairaed as invention is:

   1. A method of mating a topsides to the hull of a floatingoffshore structure, comprising: a. submerging the hull of the floating offshore structureto a predetermined depth below the water surface to providea zéro water plane area across the hull of the floatingoffshore structure; b. floating the topsides above the hull of the floatingoffshore structure; and c. decreasing the draft of the hull of the floatingoffshore structure to cause the topsides and hull to fully engage.
2. 2. The method of claim 1, further comprising attaching draftcontrol Unes to the hull of the floating offshore structurebefore submerging the hull.
3. 3. The method of claim 2, wherein the draft control Unes areweighted with one end resting on the sea floor.
4. 4. The method of claim 1, further comprising controlling thebuoyancy of the hull through a control line having one end incommunication with the hull and the opposite end in communicationwith buoyancy Controls on a surface vessel.
5. 5. The method of claim 3, wherein the control line includesbuoyancy éléments.
6. 6. A method of mating first and second components of a floatingoffshore structure, comprising: a. submerging the first component of the floating offshore structure to a predetermined depth below the water surface < t ό . 6 — 16 — to provide a zéro water plane area aorcss the firsrcomponent; b. floating the second component of the floating offshorestructure above the first component; and c. decreasing the draft of the first componenr cf thefloating offshore structure to cause the first and secondcomponents to fully engage.
7. 7. The method of claim 6, further comprising attachinç weighteddraft control Unes to the first component cf the floatingoffshore structure before it is submerged.
8. 8. The method of claim 6, further comprising controlling thebuoyancy of the first component by attaching tucyant conmroiUnes between the first component and a surface vessel.
9. 9. The method of claim 6, further comprising the use of winchesand Unes to effect engagement of the first and secondcomponents.
10. 10. The method of claim 6, wherein the first and secondcomponents are respectively provided with stabbing pcsts andstabbing réceptacles.
11. 11. The method of claim 6, further comprising: a. submerging the first component until said firstcomponent rests vertically on the seafloor; and b. ballasting said first component on the seafloor toachieve a pre-determined on bottom weight.
12. 12. The method of claim 11, further comprising: a. providing spud piles on the lower portion of the firso component; and . 2.» 010876 17- b. ballasting the first component until the spud pilespenetrate the seafloor to a pre-determined depth.
13. 13. The method of claim 6, wherein the first and secondcomponents are respectively provided with matching grooves and protrusions. *
14. 14. T ht method of clain 13, wherein the matching grooves andprotrusions are provided with shock absorbing material.
15. 15. The method of claim 13, wherein the matching grooves andprotrusions are fixed to the main structural bulkheads of thefirst and second components.
16. 16. The method of claim 15, wherein the main structural bulkheadof the second component is circular and is aligned with the cuterperimeter of the first component.
17. 17. The method of claim 16, wherein the first component isprovided with a circular recessed shoulder sized to receive theinner perimeter of the circular bulkhead of the second component.
18. 18. The method of claim 9, wherein the weight of the firstcomponent is increased to a predetermined level prior toeffecting the engagement of the first and second components.
19. 19. The method of claim 9, wherein the winches are mounted onshock absorbing material.
20. 20. The method of claim 9, wherein said shock absorbing materialis contained within a plunger assembly.
21. 21. The method of claim 6, wherein the second component isprovided with an alignment bevel.
22. 22. The method of claim 21, wherein one or more main structuralbulkheads of the second component are aligned with corresponding 18- 010876 main structural bulkheads of the first component.
23. 23. The method of claim 22, wherein said corresponding mainstructural bulkheads in the first and second components are circular.
24. 24. The method of claim 22, wherein shock absorbing material isfixed to said corresponding main structural bulkheads.
25. 25. A method of mating first and second components of a floatingoffshore structure, comprising: a. submerging the first component of the floating offshorestructure to a predetermined depth below the water surface; b. floating the second component of the floating offshorestructure above the first component; c. attaching mating Unes between the first and secondcomponents ; ,d. ballasting the first component to a predeterminedweight so as to minimize the relative motions between thefirst and second components; e. fully engaging the first and second components bv meansof winches acting on the mating Unes; and f. raising the second component to a safe élévation abovethe water surface by deballasting the first component.
26. 26. The method of claim 25, wherein the mating Unes are rigidtubular members.
27. 27. The method of claim 25, wherein submergence of the firstcomponent provides a zéro water plane across the first component.
28. 28. The method of claim 25, wherein the winches are mounted onshock absorbing material. 010876 -19-
29. 29. The method of claim 25, wherein the shock absorbing materialis contained within a plunger assembly.
30. 30. The method of claim 25, wherein the second coaponer.t isprovided with an alignaient bevel.
31. 31. The method of claim 30, wherein one or more main structural bulkheads of the second component are aligned with correspondonsmain structural bulkheads of the first component.
32. 32. The method of claim 31, wherein the corresponding mainstructural bulkheads in the first and second ccmponents are 10 circular.
33. 33. The method of claim 32, wherein shock absorbing material isfixed to the corresponding main structural bulkheads.