This article was downloaded by: [195.229.242.55] On: 02 July 2011, At: 05:00 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK The IES Journal Part A: Civil & Structural Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tiea20 Design considerations of a loadout skid frame for a 14,000-ton upper hull structure L. Y. Cheung a & K. G. Foong a a Marine Engineering Services Pte Ltd., 29 International Business Park, No. 07-05, Acer Building, Tower B, Singapore, 609923 Available online: 26 Oct 2007 To cite this article: L. Y. Cheung & K. G. Foong (2008): Design considerations of a loadout skid frame for a 14,000-ton upper hull structure, The IES Journal Part A: Civil & Structural Engineering, 1:1, 83-95 To link to this article: http://dx.doi.org/10.1080/19373260701620287 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan, sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
Transcript
This article was downloaded by: [195.229.242.55]On: 02 July 2011, At: 05:00Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK
The IES Journal Part A: Civil & Structural EngineeringPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tiea20
Design considerations of a loadout skid frame for a14,000-ton upper hull structureL. Y. Cheung a & K. G. Foong aa Marine Engineering Services Pte Ltd., 29 International Business Park, No. 07-05, AcerBuilding, Tower B, Singapore, 609923
Available online: 26 Oct 2007
To cite this article: L. Y. Cheung & K. G. Foong (2008): Design considerations of a loadout skid frame for a 14,000-ton upperhull structure, The IES Journal Part A: Civil & Structural Engineering, 1:1, 83-95
To link to this article: http://dx.doi.org/10.1080/19373260701620287
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching and private study purposes. Any substantial or systematicreproduction, re-distribution, re-selling, loan, sub-licensing, systematic supply or distribution in any form toanyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses shouldbe independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims,proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly inconnection with or arising out of the use of this material.
Design considerations of a loadout skid frame for a 14,000-ton upper hull structure
L.Y. Cheung* and K.G. Foong
Marine Engineering Services Pte Ltd., 29 International Business Park, No. 07-05,Acer Building, Tower B, Singapore, 609923
(Received 17 April 2007; final version received 14 May 2007)
The objective of this study is to document the design considerations of a Loadout Skid Frame structure to loadout a14,000 tons heavy Upper Hull Deck of a semi-submersible platform. It highlights the thinking that goes into thedesign to satisfy all the design constraints starting from load path consideration, dimension limitation to weldingaccess and material availability. In essence, the purpose of the design is to support the new Jurong Shipyardfabrication method for semi-submersible platforms. This useful skid frame has since been used to loadout two upperhull structures, one in June 2003 and the other in May 2004.
In a highly competitive international fabricationmarket, the bidder with the lowest bid stands a goodchance of winning a job and reducing fabricationmanhour plays a crucial role in coming up with a lowbid. Using better and up-to-date construction equip-ment on site or setting up a yard in a cheaper laborcost country can help but using an innovative idea toincrease productivity will probably produce the bestresult with the least capital investment. This studydocuments the design principles of a skidding structureto support an innovative fabrication method proposedby Jurong Shipyard to fabricate semi-submersibleplatforms.
In the fabrication of offshore structures, one way tominimize fabrication manhour is to fabricate as manyitems as possible on the ground, then lift them up inthe air for final assembly (Cheung et al. 1998). Thelifting capacity of the available cranes in the yard willlikely dictate the size of the components that should befabricated. In recent years, many Oilfield Operatorshave gone further to take full advantage of the savingsby fabricating the entire deck structure onshore, thenloaded it out in one piece and set it onto a pre-installedsubstructure by a float-over method. In this dev-elopment strategy, they saved both fabrication andinstallation cost (Gerwick 1986). This float-overmethod is not a new technique as it has been practicedin the North Sea for over 20 years. In the North Sea, asuper heavy topside deck of a concrete gravityplatform is usually fabricated onshore, loaded out
onto two or three barges for mating with a pre-setgravity base concrete structure, and then sailed awayto site for final installation. Recently, Statoil also usedthe float-over method to fabricate the Visund semi-submersible platform with a topside weight of 25,000tons. Mating between the substructure and the deckwas done in sheltered water. The difference betweenthis type of float-over installation and the standardoffshore float-over method is that the former is donenear shore where the weather condition is perhapsbetter whereas the standard float-over is done onsite inopen sea with a pre-installed jacket. However, bothmethods involve one fixed body and one floating body.If mating were done between two floating bodies, thetask would be infinitely more difficult.
Whether it is component fabrication or completedeck fabrication, the underlined message is thatonshore fabrication is the cheapest and the quickestway to do fabrication, since every operation can becarefully controlled and quality can be ensured andhence better productivity and greater cost saving.However, float-over installation is not always thecheapest as it depends on the availability of heavyinstallation equipment and the cost of extra steelneeded for this kind of operation, for example, theweight of the present Loadout Skid Frame is over3,000 tons and some of the plate thicknesses are morethan 80 mm thick. So far, oil companies operating inthis South-East Asia region prefer float-over installa-tion method for heavy decks; say over 5,000 tons, whenbig derrick barges are not readily available. In the realworld, one has to weigh the cost of this kind of
The IES Journal Part A: Civil & Structural Engineering
Vol. 1, No. 1, February 2008, 83 – 95
ISSN 1937-3260 print/ISSN 1937-3279 online
� 2008 The Institution of Engineers, Singapore
DOI: 10.1080/19373260701620287
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fabrication and installation methods against otheralternative solutions to come up with the least cost.
In this study, we are dealing with a skid-over matingmethod inside a dry dock. Jurong Shipyard wants tofabricate an upper hull of a semi-submersible platformonshore and then skid it out to mate with two pre-positioned pontoons inside the dry dock. Theirobjective is to reduce fabrication schedule. In 1999,Hyundai Heavy Industries used a similar concept tofabricate the 25,500 tons semi-submersible drilling rig,the RBS-8M, where the topside deck weight is 11,000tons. They first fabricated the topside deck on theground, and then used strand jacks to lift the deck 38 mabove ground (Cho et al. 2001). The pontoons beingfabricated on-site and located on either side of the deckwill then be pushed into position for final mating. Theircost saving was derived from on-the-ground fabricationand installation since it eliminated the need of the drydock. But, they still had to loadout the assembledstructure onto a submersible cargo barge and put it intothe water by submerging the cargo vessel. Thisoperation can be very expensive. This study documentshow the Loadout Skid Frame was conceived anddesigned. The Jurong Shipyard engineers did all theloadout planning and job execution.
2. Design criteria and constraints
The design of the Loadout Skid Frame has to meet thefollowing fabrication and mating requirements:
(1) The planned loadout sequence is to firstfabricate the Upper Hull Deck near the drydock-head (see Figure 1). The Upper HullDeck is skidded into the dry dock as shown inFigures 2 and 3. The pontoons are de-ballasted to mate with the Upper Hull Deck.The assembly is floated out of the dry dock.
(2) During loadout, BOS (bottom of steel) of theUpper Hull Deck is set at elevation (þ)20,100 mm to provide 500 mm clearancebetween top of the supporting column of the
pontoon and the Upper Hull Deck. The pon-toons would have been pre-positioned insidethe dry dock prior to loadout (see Figure 4).
(3) The center-to-center separation of the twolongitudinal loadout trusses SL and PL (asshown in Figure 5) is set at 36 m to provideenough sideway clearance from the edge of thecolumn legs.
Figure 1. Upper Hull at starting position before loadout.
Figure 2. Upper Hull at halfway position.
Figure 3. Upper Hull at final position.
Figure 4. Cross section view of pontoons inside dry-dock.
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(4) The existing service tunnel next to the dock-head is to be protected during loadout. Thismeans that the skid beams have to bridgeacross the tunnel.
(5) The reinforcement inside the dry dock isavoided such that other yard activities cancontinue during fabrication of the semi-submersibles.
(6) Provide access driveways underneath theskid trusses (SL and PL) during fabrica-tion. This is to enable construction equip-ment to move in and out of the center areaof the Upper Hull structure duringfabrication.
(7) Provide additional supports for the UpperHull Deck on the cantilever sides. This is tosuit the planned block fabrication sequence(see Figure 5). All these supports must beremoved prior to loadout.
(8) Provide a safe working platform for the entireunderside of the Upper Hull Deck. This is forthe block installation sequence.
(9) The Loadout Skid Frame is to be designedfor skid-out as well as pullback operationsbecause it will be re-used to loadout two moresemi-submersibles in the coming years.
(10) Materials will be sourced from Singaporewhenever possible to cut down procurementtime. This means that some old or datedmaterials may have to be re-certified due tounavailability of mill certificates.
Figure 5. Upper Hull blocks loading diagram.
3. Design considerations
3.1. Load path
The Upper Hull of the semi-submersible is a squarebox having a plan dimension of 74.42 m by 74.42 m.In one direction, there are six transverse frames,including the two side shells, TF0, TF2, TF6, TF12,TF16 and TF18. In the other direction, excluding theside shells, there are two longitudinal bulkheads, PL7and SL7 (see Figure 5). The center moonpool isbounded by TF6, TF12, PL7 and SL7. After mating,the Upper Hull will be supported by four columns atthe four corners of the semi-submersible platform. Thecolumn leg size is 15.86 m by 15.86 m.
Since the chosen loadout direction is in line withthe longitudinal bulkheads and the Upper Hull is to befabricated in 12 blocks, it is natural to have sixtransverse trusses (TFs) and two longitudinal trusses inthe proposed skid frame configuration as shown inFigure 6. The basic load path of the structure duringconstruction is to take the loads from the TFs to thetwo longitudinal skid trusses, which are supported by atotal of 12 skid shoes, 6 per truss. The loadout skidshoes are located at the 12 intersection points betweenthe 6 TFs and the 2 longitudinal trusses (PL and SL).The skid shoes are shown in Figures 11 and 12. Thisload path is correct during block assembly. This isbecause during fabrication, each block will be fabri-cated at different times according to the projectschedule and when it is lifted into position on top ofthe skid frame, it is acting like a weight without overall
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structural stiffness of the Upper Hull. It is clear thatthe weight will be transferred from the TFs to the twoouter longitudinal trusses. One should bear in mindthat during block assembly, there are 12 additionalsupporting towers supporting the blocks (Figure 5).However, after all the blocks are welded up, the TFswill become less effective. Since all the transverseframes in the Upper Hull have been designed to spanbetween the two longitudinal frames, PL7 and SL7,most of the deck load will be transferred directly to thetwo longitudinal frames by virtue of the hull stiffness.The true structural behavior of the combined UpperHull and the Loadout Skid Frame system is howeversomewhere in between, depending on how the 300 mmby 300 mm wooden blocks are wedged into position(Figure 7), the offset between the skid trusses and thelongitudinal bulkheads, the block assembly sequence,the heavy topside equipment loads and equipmentinstallation sequence. One can conclude that there aretwo possible load paths to transfer the inner loadsacting on the TFs to the two longitudinal loadout skidtrusses, SL and PL. One is during block assembly andthe other is during loadout. Before loadout, all 12supporting towers will be removed and this change ofthe boundary conditions will create a different loadpath.
For clearance reasons, the longitudinal skid trusseswill have to be offset from the two longitudinalbulkheads (SL7 and PL7) by 3.35 m (see Figure 5),and the effect of the resulting eccentric moments on theUpper Hull Deck have to be checked. It is the offsetthat creates soft intersection points along the skidtrusses and makes the load path for loadout differ fromthe load path for the in-place condition. Since theUpper Hull Deck is fabricated in blocks and thenlifted into position for assembly in air, temporarysupporting towers and temporary out-rigging trusses
from the skid frame need to be provided. These outrigging trusses and towers will be removed beforeloadout and the load path will be altered again. For thefabrication condition, the structure has to be checkedfor various block installation sequences for all threesemi-submersibles, as they may not have identicalweight distribution. However, in the present design, itis assumed that all three semi-submersibles have thesame dimensions and framing spacing, otherwise, theskid frame may have to be modified to suit a newlayout.
3.2. Selection of structural configuration andconnection details
Once the load paths are selected and understood, it isstraightforward to design the skid frame. To providemaximum stability during loadout and bigger momentarm to resist side load, which is caused by the wind inthe transverse direction, the two vertical longitudinaltrusses (SL and PL) are spaced at 36 m apart, themaximum spacing allowed, and the loads will govertically down to the loadout skid shoes. If the twolongitudinal trusses were slanted to reduce the 36 mspan, the horizontal component of the Upper HullDeck load at each column base would require a verylarge member to carry this horizontal force and theresulting connecting details would be very complicatedand expensive to fabricate. Direct simple load pathswill always cost less. The selected structural config-uration is shown on Figures 6–10. It represents asimple direct load-transfer design. Two 3-D views arealso provided in Figures 11 and 12.
For access requirement, the depth of all the TFsshould not be more than 7.5 m. This is to allowworkers to move freely underneath the entire UpperHull Deck without stepping over too many beams
Figure 6. Loadout Skid Frame. Top framing plan @ (þ) 13290.
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during emergency evacuation and to provide freeaccess to construction equipment on the ground(Figure 10). There will be no diagonal bracingsobstructing passageways.
During the loadout, the skid frame structure willexperience different foundation stiffness. For theonshore portion, many skin friction piles were installedto support the skidway; therefore, support pointsettlements will be small. When the Upper Hull Deckis being pulled into the dry dock, the same topside loadwill have to go through another supporting structureinside the dry dock before going down to the dockfloor, which is supported by smaller piles. Therefore,settlements inside the dry dock are expected to be
greater. The self-imposed allowable differential settle-ment is 20 mm, which is the limiting criterion to keepthe Upper Hull and the loadout system within theallowable stresses during the loadout operation. Tosatisfy this allowable differential settlement require-ment, the proposed skid shoe design should accom-modate rotation capabilities and the structure insidethe dry dock and the dock floor supporting piles mustbe stiff enough not to have settlements more than20 mm. This is a difficult design requirement to meetespecially when the dock floor is not to be hacked. Inaddition, the two longitudinal trusses (SL and PL)must be capable of re-distributing the load due todifferential settlement of 20 mm or more. The
Figure 7. Loadout Skid Frame. Top framing plan @ (þ) 15100.
Figure 8. Bottom framing plan of Loadout Skid Frame.
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re-distribution should take place among all the 6 skidshoes within the same skid truss, either SL or PL.
In a standard offshore platform design, direct tube-to-tube connections would be the preferred connectiondetails. However, this type of detail will call foropening up work-points to provide a 50 mm gapfor welding and it usually leads to the requirement forheavy wall can material with special through thicknessproperties such as API Spec 2H SupplementaryRequirement S4 (American Petroleum Institute1999). This is to guard against laminar tearing in thickmaterial. This special heavy wall steel will usually take4 – 5 months for delivery to Singapore or may belonger since steel mills do not produce through
Figure 9. Longitudinal truss row.
Figure 10. Transverse truss row.
Figure 11. Isometric view of Loadout Skid Frame (lookingdown).
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thickness plates on a regular basis and the smallquantity ordered must fit into their regular rollingschedule. This long delivery will affect Jurong Shipyardfabrication schedule. Therefore, it was decided to gofor gusset connection details, which do not requirethrough thickness properties because the load istransferred by shear. However, gusset connectionswill produce heavier structures.
With the view to keep the weight as low aspossible, it is necessary to avoid eccentricities in theskid frame structure. Therefore, gusset plates are usedextensively to maintain single work-point at all thenodes. Whenever possible, overlaps are avoided forall the incoming braces and adequate welding accessprovided. In areas where overlapping is necessary,special welding details are required. If the singlework-point design approach is not adopted, many ofthe gusset plate thicknesses would be more than100 mm thick and welding could be more trouble-some (see Figure 13).
In normal offshore platform fabrication, if weldingwere to follow AWS D1.1 standard (AmericanWelding Society 2000), direct tube-to-tube connectionswould require welders to have more stringent weldingqualifications. By using gusset details, this type ofwelder pre-qualification is not needed and the edgepreparation between the gusset plate and the incomingslotted tubular is very simple. It should be noted thatunless shipyard welders are engaged in jacket fabrica-tion on a continuous basis, they usually do not haveapproved qualification to weld tubular connections.Therefore, welders would have to be re-qualified ifplate gusset connections are not adopted. It is bothtime-consuming and expensive. Tubular structuresrequire highly trained welders, fitters and NonDestructive Testing (NDT) specialists.
NDT and weld-repair procedures are much easierfor simple butt welds than tubular connection details.It is also easier to handle the pre-heat and post-heattreatments as per AWS code (American WeldingSociety 2000), if required, and not many new weldingprocedure qualifications would have to be prepared forthe job.
3.3. Methods of skidding loadout
In a normal loadout operation of offshore structures,the structure is pulled onto a floating barge, which isbutting against the bulkhead by anchor lines. Wincheson the barge or on land or barge mounted strand jackscan provide the pulling force. However, due to actionand reaction, the barge will exert a pushing force to thequay wall of the bulkhead and horizontal frictionalforce under the skid shoes will also be applied to allthe pile heads supporting the skid ways as shown in
Figure 14. This is the reason why the supporting pilesshould be designed for axial and lateral loads (Cheungand Gho 2002).
In the present loadout system, we do not have afloating system inside the dry dock and tidal variationdoes not exist. As long as the differential settlementis kept small, the loadout skid beam can bemade continuous (Figure 15). The skid beam itself isin self-equilibrium, so there is no need to design thesupporting piles under the skid beams to take the fulllateral load. However, the very long skid beam itselfmust be checked for beam-column interaction. One of
Figure 13. Typical joint details in loadout skid frame.
Figure 14. Typical offshore barge loadout.
Figure 12. Isometric view of Loadout Skid Frame (lookingup).
Figure 15. Dry dock loadout system.
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the key indicators for successful loadout is to monitordifferential settlement between the dock head andvertical deflection inside the dry dock. This means thatthe structure inside the dry dock must be carefullydesigned to ensure the settlement limitation is met. Theother design objective is to spread the topside loadequally to all the piles inside the dry dock so that theywill not be overstressed.
3.4. Foundation
During loadout, the soil must be able to support thehighly concentrated traveling loads coming downfrom the skid shoes. Owing to the height limitation,the 500 mm gap requirement for mating and basedon 458 spread, there is no way to spread out theload to a longer distance. This implies that bearingstress on the soil will be so high such that piles willbe needed to avoid this type of bearing failure.Therefore, a piled skidway is required for theloadout operation. The pile design was handled byother civil engineering consultants who were engagedby Jurong Shipyard.
Because of the differential settlement of 20 mm, thereaction forces at the dock head can increase greatlyduring loadout and this has been taken care of byinstalling a few 1.3 m diameter piles at the dock headand dry dock interface. In other offshore platformfabrication yards, a special load-relief platform isinstalled to handle this load fluctuation. Thereare other types of bulkhead stability issues associatedwith loadout of heavy structures (Cheung 1989 and
Cheung and Gho 2002), but they are outside the scopeof this study.
4. Structural design
4.1. Design of skid frame
The structure has been designed and code checkedusing standard offshore software, SACS (EDI 2001).All members and connections were checked againstAPI-RP-2A-WSD (American Petroleum Institute2000) and AISC (American Institute of Steel Con-struction 1989) codes for interaction equations andpunching shear checks. Welding specifications, proce-dures and qualifications are to follow AWS D1.1(American Welding Society 2000). Basically, the skidframe system including design and fabrication of allitems is to follow standard offshore practice. The finiteelement model is shown in Figure 16.
For an offshore design, the number of piles in aplatform is usually not more than 20. But in thepresent problem, there are more than 150 piles. First ofall, the Upper Hull Deck is sitting on many 300 mm by300 mm timber blocks placed on top of the skid frameas shown in Figure 7. To simulate this condition, weused gap spring elements, which can take compressionbut no tension. This is the first nonlinearity ofthe analysis. Once the loads have been transferreddown to the skid beams, nonlinear soil and nonlinearstructural-pile interaction effect will come in and hencethe presence of second nonlinearity. The computerprogram using special built-in iteration procedure canhandle this kind of nonlinear analysis.
Figure 16. Complete computer model for Upper Hull, Loadout Skid Frame and Support structure inside the dry dock.
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Figure 17. Skid shoe and rocking details.
Figure 18. Skid shoe link-bar details.
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All diagonal braces attached to the 12 columns, nomatter whether they are from the longitudinal trussesand/or the TFs, are connected to the top of thecolumns for two reasons. One is to make sure load re-distribution, if required, can be accomplished muchquicker as bracing pattern can greatly affect loadtransfer (Cheung 1990). If there is a need for load re-distribution due to differential settlement, we want toget the load out of the troubled leg and transfer toother skid shoes as soon as possible; before it reachesthe skid shoe below, otherwise the column or the shoemay be over-stressed. The second reason is that theconnection between the skid shoe and the column baseis already very highly stressed and it is better not tomake it more congested than it needs to be. There issimply not enough space to do proper welding andfitting up of all the components. Welding access is amajor consideration in the connection design.
Secondary trusses were designed and installed totransfer all the loads in between the TFs. They are pin-ended. This is to make sure that no end moments couldget into the TFs and the only induced moments arecaused by eccentricities of the pin connections. Therewere also few connections that failed the punchingshear checks and were reinforced by grout injection if
under compression load or by gusset plates if undertension forces.
In certain areas of the TFs (TF6 and TF12) wheretubular member sizes available in the Singaporemarket at the time of fabrication, were not big enoughto carry the load, shear plates were used instead.
Since one of the design constraints is to usewhatever materials can be obtained in the Singaporemarket to cut short the procurement period, some ofthe dated materials have to be down-graded to accountfor corrosion and others were tested locally to ensurethe required minimum yield stress is met. In fact, thewhole design has been modified many times to caterfor what can be purchased in the local market and notbased on what is needed in the design. In certain areas,the stress ratios are close to unity and in other areas thestress ratios are relatively low. This is a commonpractice in the offshore industry.
Lateral restraints are provided to take care ofthe design wind load due to 31.3 m/sec (70 mph)wind. However, these restraints will not restrictthermo expansion during fabrication under the hotsun.
Tubular connection details were either checked bythe computer program using punching shear equations
Figure 19. Loadout jacks mounting arrangement.
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as per API RP-2A (American Petroleum Institute2000) or by elastic finite element analysis with thecombined stress less than yield stress. In offshorepractice, we usually set the limit to 90% of yield stress.In areas that punching shear check has failed andthicker heavy wall can material is not available, groutinjection is used to overcome the problem. Otherconnection details were checked by hand.
4.2. Design of skid shoe
All the topside loads are transferred to the skid shoesvia shear plates located inside the column legs(Figure 17). Each leg has two shear plates, which arebutt welded to the inside of the 1,524 mm (60 in.)diameter column. The shear plates will then transferthe load to the shoe by bearing. To allow formisalignment, the half circular bearing plates on bothside-webs of the shoe have been oversized to cater for12 mm installation tolerance.
In between the two shear plates, an additionalfabricated T-beam was added to the top of the halfpipe to provide additional load path to transfer theload to shoe if the need arises. The rotation capabilityto handle the differential deflection is also provided bytwo tightly fitted half pipes. Traveling speed is assumedto be about 2.54 mm/sec (6 in. per minute). On thebasis of past experience in heavy structure loadoutfrom other offshore platform fabrication yards andfurther confirmed by simple calculation, such speedwill not generate significant inertia forces on the UpperHull Deck as long as the whole loadout process iscarried out very slowly. The Upper Hull is sitting onthe skid frame and is not welded down.
On the basis of past experience and also confirmedby site measurement, the expected static friction shouldbe less than 7% of the loadout weight and the dynamicfriction is much lower. However, the design staticfriction for the present system is 20% of the totalloadout weight. The actual pulling force required tobreak the static friction will depend on the type ofgrease used and the levelness of the skid beam. Being anew skid way, the 20% assumption is reasonable.
The timber underneath the shoe is the Balau timberwith a very high allowable compressive stress parallelto grain. They are vacuum treated with timberpreservative to guard against insect, fungal and termiteattack. This is to ensure they can be re-used for futureloadouts. Since the pulling force is applied only to thefront skid shoes, we have to make sure that this loadwill only be transferred to the rest of the skid shoeswithout going through the deck. If this is not done, theskid frame will be over-stressed at the front columnsand may cause major failure to the upper hullstructure. To overcome this difficulty, a special gusset
connection is provided as shown in Figure 18. Thisinnovative detail will allow the connection to bend orrotate in one direction without transmitting anypulling load to the deck. A usual padeye and shacklepin detail will not work in this case.
Skid shoes are fitted with anchor bracketsto accommodate the cable jacks as shown inFigures 19–21. They are designed for pulling fromeither direction. The pulling force must be balanced oneither side of the skid shoe and the shoe is not designedto twist. In general, if it is a single point pull, the loadshould be applied at the centerline of the skid shoe. If itis in tandem pull, the force must be applied equally on
Figure 20. Skid shoe jacking bracket details, type 1.
Figure 21. Skid shoe jacking bracket details, type 2.
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both sides of the shoe. In the present design, thepulling forces come from two independent sources andone would expect certain amount of jerking motion tohappen.
4.3. Design of skid beam
One of the most critical areas is to bridge across thetunnel near the dock head. The designed span is 7.2 mas shown in Figures 22 and 23.
During pulling, the skid beam is in self-equilibriumand is subject to very large axial load and bending
moment, therefore side restraints must be provided tocut down the slenderness ratio. At the end of the skidbeam where the cable jack anchor block is located, aspecial holding down detail must be provided to keepthe skid beam in place. Jurong Shipyard provided thisdetail using stationary barges. Since pulling is done attwo locations using two sets of cable jacks, racking willoccur during loadout. The skid frame has been checkedfor 5% differential racking forces, but it is stillnecessary to minimize the jerking forces due to unequalpull. This is very much an operational problem thatrequired very close cooperation from the loadout crew.
Figure 22. Skid beam arrangement.
Figure 23. Skid beam cross section.
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5. Concluding remarks
The advantage of this fabrication method usingLoadout Skid Frame is that during mating, everystep can be carefully controlled in a stable environmentinside the dry dock. In other fabrication methods suchas the one used by Hyundai in 1999, they also enjoyedthe same advantage but the total cost for lifting thedeck for mating, skidding the two pontoons intoposition as well as the final loadout and tiedownoperation using a submersible material barge wouldprobably cost more than the Jurong method. It isextremely difficult to find a submersible barge in thisregion capable of carrying 25,000 tons concentratedtopside load and the cost could be prohibitive to bringin a big barge from elsewhere. Therefore, the presentskid/mate fabrication method is not very expensiveconsidering the productivity gained and the extra costcan be spread over three projects.
Jurong Shipyard engineers have since successfullyloaded out and mated Upper Hull Decks with theLower Hull Pontoons on two occasions. One was onJune 18, 2003 and the latest one was on May 19, 2004.The success of this fabrication method by JSL clearlydemonstrated their ability and the usefulness ofinnovative approach in fabrication and also showedthe importance of the concept of Design Economics inEngineering, which emphasizes the need to come upwith designs to suit fabrication and installation togenerate more profits (Cheung 1990). This is muchcheaper than heavy investment in new equipment toimprove productivity. Both investment and innovationare essential, but from the Singapore perspective,innovation is much more important in view of thehigh labor cost in Singapore.
Acknowledgements
The authors wish to express their thanks to the followingpersons for their cooperation: Mr. Seow Tan Hong, Mr. GohKing Kwee and Dr. Zhong Kui (from Jurong ShipyardPrivate Limited), Associate Professor Choo Yoo Sang andProfessor N E Shanmugam (from the National University ofSingapore), Assistant Professor Gho Wie Min (fromNanyang Technological University) and Mr. Yeo Ah Teeand Mr. Ng Seng Chow (from Marine Engineering ServicesPte Ltd).
References
American Institute of Steel Construction, 1989. AISCspecification for structural steel buildings (allowablestress design). 9th ed. Chicago, IL: AmericanInstitute of Steel Construction.
American Petroleum Institute, 1999. API spec 2Hspecification for carbon manganese steel plate foroffshore structures. 8th ed. Washington DC: Amer-ican Petroleum Institute.
American Petroleum Institute, 2000. API RP 2A(WSD), Recommended practice for planning, de-signing, and constructing fixed offshore platforms(working stress design). 21st ed. Washington DC:American Petroleum Institute.
Cheung, L.Y., 1989. SIA hangar roof at Changiairport: Bidding, fabrication and installation.Journal of The Institution of Engineers, Singapore,29, 67–81.
Cheung, L.Y., 1990. Design economics of offshorestructures. Journal of The Institution of Engineers,Singapore, 30, 73–84.
Cheung, L.Y., Gho, W.M., Fung, T.C. and Soh, C.K.,1998. Design economics of offshore structures:Eccentric jacket. Journal of The Institution ofEngineers, Singapore, 38, 42–48.
Cheung, L.Y. and Gho, W.M., 2002. Effect of soil-structure-barge interaction for loadout analysis ofoffshore steel jackets. Journal of The Institution ofEngineers, Singapore, 42, 30–35.
Cho, K.R., Kim, Y.S. and Fern, D.T., 2001. 11000 tDeck superlift for RBS-8M drilling semi-submer-sible. In: Proceedings of the Institution of CivilEngineers, Structures and Buildings 146, May 2001,Issue 2, 203–216.
EDI, 2001. Structural analysis computer system(SACS), Version 5.1. Engineering Dynamics Inc.
Gerwick, B.G., 1986. Charter 10 concrete offshoreplatforms (gravity-base structures) construction ofoffshore structures, Wiley.
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