The Performance, Safety and Production Benefits of SPS Structures ...

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InnerShellOuterShellCentrelineBulkheadSideSideLongitudinalsLower StrakeMiddle StrakeUpper StrakeLower StrakeMiddle StrakeUpper StrakeStiffenersLower StrakeUpper StrakeHP 280x1214 mm13 mm12 mm15 mm14 mm13 mmHP 220x1015 mm13 mmSPS 6-40-6(5700mm a/b)SPS 5-35-5SPS 6-30-6(5700mm a/b)Main Deck Plate11 mmSPS 6-25-6Longitudinals HP 220x10Transverse Web Frame Spacing 3000mm 3500mmTable 4.3 Summary of Hull Girder Section PropertiesHull Girder Section Modulus, m 3Location Conventional SteelScantlingsSPS ScantlingsKeel 7.720 6.376Tank Top 12.734 10.352Main Deck 5.255 4.534Table 4.2 lists the basic scantlings of both designs andprovides a comparison of the two different structures andTable 4.3 gives a summary of hull girder sectionproperties.4.3 DESIGN ASSESSMENT SUMMARYFE analysis of the mid-section was carried out usingLR’s ShipRight direct calculation procedures, using FEmodels as shown in Figure 4.4. An example set of thetank design pressures used in the FE analysis is given inTable 4.4.StructuralComponentTable 4.4 Example Tank Design PressuresPressure(kPa)Tank Top 95OuterBottomInner SideShellOuter SideShell949394Main Deck 24LongitudinalBulkhead95CommentPressure at centreline assuming tank at98% capacity and relatively heavy cargo(specific gravity = 1.0)Pressure equal to draft (T) plus 0.5 timespassing wave height (h w)LR Direct Calculation Guidance Notes h w= 4.6x10 -2 L e -0.0044L = 3.42 mNote: Part 4, Chapter 9, Section 4, Table9.4.1 lists a design head for outer bottomh T2 = T + 0.5C w > 1.2T (T = 7.65 m,C w = 7.71x10 -2 L e -0.0044L = 5.74 m, 1.2T =9.18m)Pressure at centreline, assuming tank at98% capacity and relatively heavy cargo(specific gravity = 1.0)Pressure at baseline equal to outer bottomPressure reduces to h w at load waterline(LWL) and to zero at h w above LWL draftLR Part 3, Chapter 3, Section 5, Table3.5.1, h 2 = L/56Pressure at base of bulkhead equal to tanktop pressurePressure reduces to zero at the main deckThe hull section modulus at the main deck, keel and tanktop for the steel and SPS designs are listed in Table 4.3.The SPS deck and keel section moduli are 4.8% and7.5% less than the all-steel design. In comparison to theall-steel design the balance of material (ratio of deck tokeel moduli) about the neutral axis is improved in theSPS ship. The primary bending stresses have not beenevaluated since, at the time of the design study, the stillwater bending moments for the ship were not available.The results of the FE analysis were assessed against thestress criteria listed in Table 4.5 The stress comparisonof the two designs is summarised in Table 4.6Table 4.5 Double Hull Tanker - Allowable StressCriteriaStructuralMemberApplied LoadingStressComponentMaximumAllowable Stress,MPaGradeGeneralAHull girder loadsσ 175 / k L 175Hull Girder (still water & wavecombined)τ 110 / k L 110Local loads and hullgirder loadsσ 216 / k 216Local loads σ e 177 / k 177Local loadsPlating 1(in line with hull σ x 108 / k 108girder stress)Local loads(transverse to hull σ y 147 / k 147girder stress)Longitudinal Local loads σ 100 / k 100Stiffeners Local loads τ 83 / k 831. Primary structure plating including inner and outer hull andlongitudinal bulkheadsNotes: • σ e = von Mises equivalent stress• σ = bending stress• τ = average shear stress over depth of web plate• For LR Grade A steel: k = 1.0, k L = 1.0Table 4.6 Analysis Stress Results – Local Pressure LoadsStructuralMemberStressQuantityAllowableStressCriteriaStress, MPaAll-SteelScantlingsSPSσ x 108 88 106Tank Top Plating σ y 147 128 118σ e 177 113 110Tank Top Stiffs σ x 100 87 -Outer BottomPlatingOuter BottomStiffenersInner Side ShellPlatingOuter Side ShellPlatingσ x 108 89 102σ y 147 124 119σ e 177 115 111σ x 100 103 -σ x 108 136 * 86σ y 147 84 112σ e 177 123 102σ x 108 86 88σ y 147 59 91σ e 177 78 848
Main Deck σ x 108 - 104Plating σ y 147 - 31σ x 108 - 109Longitudinalσ y 147 - 138Bulkhead Platingσ e 177 - 131Note: * local peak stress at the location of plate thickness change,anticipated that an extension of thicker lower plate by 200 mmremoves high stress.4.4 QUANTITY COMPARISONSFollowing the design study, a comparison of the twooptimised designs was made. A 15m length of the midsectionwas used and the comparisons for weight, surfacearea and other important parameters are summarised inTables 4.7 and 4.8.StructureTable 4.7 Weight ComparisonWeight, tonnesConventionalSteelScantlingsSPSScantlingsDifferencetonnes %Double Bottom 133.4 116.8 -16.6 -12.4Side Shell 102.1 109.9 +7.8 +7.6Longitudinal24.0 22.4 -1.6 -6.6BulkheadMain Deck 50.3 52.0 +1.7 +3.4Totals 309.8 301.1 8.7 -2.8Table 4.8 Other Comparisons Between Conventional andSPS DesignsStructureWeight, tonnesConventionalSteelScantlingsSPSScantlingsDifferenceDelta %Painted SurfaceArea, m 2 6280 5025 1255 -20# StiffenerIntersections310 10 310 -97Weld Volume,cm 3 215000 93000 122000 -574.5 CONCLUSIONS AND FURTHER WORKThe design study verifies that SPS technology can beapplied to a double hull tanker and assessed successfullyagainst industry-standard assessment procedures. Thestudy demonstrates that SPS offers a number of benefitsto the design of double hull tankers:• The weight of the SPS design is slightly less than theconventional steel design. The weight comparisonincludes the mass of elastomer core.• The elimination of secondary stiffeners provides a20% reduction in internal surface area and representsa major saving in terms of paint volume, ease andquality of application.• The weld volume reduction of 50% is achievedthrough the elimination of stiffeners and theassociated intersections with transverse primarystructure. This represents significant savings inproduction time and cost.• The SPS design has an uncluttered internalarrangement of structure and can accommodate areduced double bottom height of up to 600mmwithout compromising access for inspection. Thisprovides a potential for an additional 6% of cargocarrying capacity.• Inspection and maintenance are greatly enhanced forthe SPS design, with easier access, faster inspectionand significantly fewer locations with potential forcoating breakdown and structural failure.There is further work to be completed. The designassessment needs to include a more complete review oflongitudinal strength and verification by a classificationsociety. There is yet more scope for optimisation of theSPS structure and further gains in structural efficiencyare expected.5. PRODUCTION METHODOLOGIESThe design study summarised in Section 4 above showsthat SPS structures can be designed with weight savingsand strength equivalence to conventional ship structures.Production of SPS panels involves some key processeswhich are unique to SPS technology.• The internal surfaces of the steel cavities needpreparation to achieve the required surface roughnessof at least 60 microns and a surface cleanliness of SA2 ½. The preparation uses standard steel fabricationpractices and is best achieved using steel grit orcorundum, which have suitably hard particles and anangular profile.• The internal cavities must be clean and dry prior toinjection. They must also be airtight in order toprevent elastomer leakages during injection.• SPS panels are restrained during injection and curing.This ensures that the face plates remain flat. On aproduction line this may be achieved using simpleportable frames which can be clamped at the edges.• The injection process usually takes around 8 to 12minutes. At this stage the temperature of the panelsshould be no less than 20°C.To realise the maximum commercial benefit of SPStechnology, successful integration of SPS into theproduction lines of shipyards is essential. Workcontinues with yards in Europe, Asia and N. America toimplement SPS into shipyard construction processes.Currently there are two main options for production ofSPS structures:• Use of pre-fabricated SPS panels delivered to theproduction line• Integration of SPS panel fabrication within theproduction lineIn the first option SPS panels are pre-fabricated to thedesigner’s specification and dimensions, away from theproduction line. This option allows for SPS panels to be9
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