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

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pre-fabricated at a single site for distribution to manydifferent locations. The advantages of this method is thata specialised facility using fast, modern productiontechniques can produce panels with varying designparameters, rapidly and in great numbers. Theeconomies of scale of such plant mean that theproduction costs are very low for each unit area ofstructure. This methodology is well suited to the casewhere small ship yards require SPS components to beincluded in a new vessel, or specialist fabricatorsbuilding accommodation blocks, hatch covers and othership components for delivery as complete blocks tolarger ship yards.In the second option larger areas of SPS structure arefabricated and injected with polymer as an integratedprocess on the production line. A typical ship productionfacility would use a process similar to the following:• Steel plates are cleaned and prepared as required andbrought to the production line where they are joinedusing automated welding to create large panels.• The large panels are moved along the line to a pointwhere secondary stiffeners are introduced, positionedand welded to the plating. Although automatedwelding techniques are employed, this process can bevery lengthy due to the sheer number of secondarystiffeners and the amount of welding to be deposited.• Once the secondary stiffeners are attached, primarystructures (e.g. floors, webs, girders and stringers)which have already been prepared with openings andcut-outs for the secondary stiffeners are introduced tothe production line. These are attached to the largeplate panel using automated welding, but in additionthe intersections of the primary and secondarystructure need to be carefully welded in a sequencewhich minimises the risk of distortions and residualstresses. This is a very time consuming part of theproduction process and one where great attention toquality is required. Not only do the intersectionscreate difficult areas for automated welding to beused, but lugs and collars can add complexity andtime to the process.• Once the primary structure is added, the panelsbecome 3D sub-assemblies which can then be erectedand assembled into larger blocks ready for erectinginto the main ship structure.In integrating SPS into a production line similar to theabove, the process will be modified as follows.• Steel plates are cleaned and prepared as required andbrought to the production line where they are joinedusing automated welding to create large panels thatwill form either the inner or outer SPS face plate.• The inner SPS face plate is moved along the line tothe point where primary structures (e.g. floors, webs,girders and stringers) which have already beenprepared with access openings and cut-outs fordrainage, are introduced to the production line. Nosecondary stiffeners are required, eliminating manymetres of welding and the complexity ofintersections.• The outer SPS face plate is moved along the line tothe point where the perimeter bars are welded to theedges of the plate and SPS spacers are added toprovide the necessary control over the core depth.Once this process is complete, the inner SPS faceplate (complete with primary structure) is placed overthe outer SPS face plate and welded to the perimeterbars to form an air-tight cavity ready for theelastomer injection.• The complete panel is moved along the line to the“injection bay” where the panel is injected withelastomer resin. Care is needed at this point tocontrol the temperature of the panel during the curingstage of the elastomer and also to restrain the panelsas described above.The above are only outline descriptions of the processesand do not enter into detail, but the important point isthat the SPS production process eliminates the timeconsuming step of introducing the secondary stiffenersand the associated intersections with primary structure.Initial estimates have indicated that the amount of timespent in attaching secondary stiffeners may be as high asdouble that required to complete the steps involving SPScavity forming and elastomer injection. This can save upto 8 hours elapsed time in the production of a singlepanel sub-assembly.6. INSPECTION AND MAINTENANCEThe simplified structures made possible with SPStechnology offer benefits for inspection and maintenance.The ballast tanks of conventional double hull tankershave significant internal structure, often with manythousands of intersections.A principal concern for double hull tankers is theapplication, inspection and maintenance of coatings inthe ballast tanks. In the 14k products tanker exampleoutlined in Section 3, the reduced surface area of coatingrepresents approximately 20% of the ballast tank internalarea. The elimination of secondary stiffeners also meansthe elimination of edges and corners where coating breakdown is initiated.Inspection of inner hull spaces is very time consuming,due to the cellular nature of the space and the necessityof inspecting the intersections. This often requiresclimbing over each longitudinal stiffener and having toinspect under flanges using torches. Where coatingbreakdown is found, it is necessary to note the location;and remedial action can be laborious, time consumingand expensive. SPS structures are inherently less costlyto apply original coatings, less susceptible to coatingbreakdown and easier to inspect and re-coat.10
Repair of damaged SPS panels is a four-step processrequiring cutting and removal of the damaged steel plate,routing of elastomer, installation of a new steel plate andinjection of elastomer. The procedure to repair adamaged SPS section is illustrated in Figure 6.1.Top Steel FaceBacking BarsSteps 3 and 4: SPS ReinstatementOnce the repair cavity is dry and cleared of any debris,backing bars are tack welded in place. The replacementplate is welded in place with a square butt weld. Newelastomer is injected into the repair cavity and theinjection port and vent holes are sealed. The newlyinjected elastomer bonds to the existing elastomer.ElastomerTop Steel FaceElastomerTop Steel FaceElastomerSquare Groove ButtBottom SteelFaceBottom SteelFaceTop Steel FaceVenting PortInjection PortBottom SteelFace7. CONCLUSIONSThis paper gives an overview of SPS technology and itspotential application to double hull tankers. A designstudy undertaken by Intelligent Engineering is used todemonstrate the viability of using SPS for the hullstructure of a small products tanker.SPS technology is finding a wide application in both thecivil and marine industries. The number of marineapplications is growing and new construction projectsinclude components such as decks and machinerycasings, as well as complete hulls for inland waterwaysvessels. SPS technology has a number of clear benefitsto offer the marine industry:Enhanced Safety and Environmental Protection• The energy absorption characteristics provideexcellent resistance to impact from collisions andgrounding. This is of particular relevance to doublehulled tankers, since the risk of hull perforation andsubsequent flooding is greatly reduced, aiding fastand safe recovery of ships.• Fire protection to A60 standard and above can beachieved with SPS panels. In many cases noadditional fire insulation is required.• SPS also provides resistance to blast and ballistics,enabling protection of key areas of the ship fromaccidental damage and from deliberate attack fromterrorism or piracy.Figure 6.1 SPS Repair ProcedureStep 1: Cut Steel Face platesThe damaged SPS steel facing plate is cold cut, hydro cutor back gouged to free it from the adjacent undamagedplate. The damaged plate is removed by applyingsufficient heat to soften the elastomer to allow it to befreed. The cutting methods may be cold cutting withsteel or carbide disks, air-carbon-arc gouging, or highpressure jet with a water/abrasive mixture.Step 2: Elastomer Removal from Damaged SectionThe elastomer is routed, ground or hydro-blasted to aminimum of 30mm from the cut edge. The methods forrouting/gouging may be hand-held power tools (routersand angle grinders) or high pressure water jet equipment.Reduced Costs• SPS enables the design of simpler structures and theelimination of secondary stiffeners and associatedconnection details.• The elimination of stiffeners can remove severalkilometres of welding and many thousands of squaremetres of internal surface area. Economies aretherefore available in construction and tank coating.• With efficient pre-fabrication of SPS panels (eitherthrough separate production of panels, or throughintegration of SPS structures to the production line),cost reductions are achievable.• Inspection and maintenance costs are reduced. SPSstructures are faster and simpler to inspect, and havefar fewer high risk locations for coating breakdown,corrosion and fatigue cracking.Improved Space Utilisation and Crew Comfort• SPS enables simpler structures. The lack of stiffenersimproves access and space utilisation.11
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