DNV Design Considerations

DESIGN CONSIDERATIONS

General - Stiffener/web connectionSize of welding gun compared to typical stiffener heightContinuous welding difficultStart and stop defects

General - Stiffener/web connectionDifficult to weldSmall shear capacityBracket or flat bar addedNot too difficult to weld

General - Stiffener/web connectionCover plate will improve shear capacity and improve force distributionIf possible avoid scallops, snipe corner of collar plate to allow space for longitudinal weld and then weld continuously around collar plate

General - BracketsEdge Stiffening Requirements

General - Tipping Brackets

General - Soft Toe Brackets10 - 15mm

General - Termination of Sniped StiffenersSniping allowed when:buckling stiffenersdynamic loads/vibrations of minor importance30 degrees

General - Pillar Connections to DeckShould not be applied if tension through pillar may occur

General - Pillar Connections/Cross Ties connectionApplicable to pillars/cross ties that may be under compression and tension

General - Alignment in Way of Section JointsEasier to get acceptable alignment when a continuous member is penetrating the bulkhead or deck structure

General - Longitudinals through Web Frames

Longitudinals at Watertight BulkheadLongitudinal continuous through bulkheadLongitudinal terminated at bulkhead with soft-nose brackets

Edge Support of Cut-OutsEdge support when length of free edge exceed 50t or D50tCut-outs to be kept well clear of end of brackets and locations where shear stresses are high

Hard Chine / KnuckleProblem: Cracking due to unfavourable detail design.Possible Cause: The knuckle represents an elastic element in the plate, the effective flange will be reduced and the stresses will increase locally in way of the web / bulkhead, in addition large dynamic pressures act on the planing strip / spray rail.

Cracks in web and shell plating

Hard Chine / KnuckleCHINE SUPPORTKNUCKLE SUPPORTSolution:

Pillar Connections/Cross Ties connectionProblem: Cracking and buckling of frame webs and ties.Possible Cause: Unfavourable detail design.

Pillar Connections/Cross Ties connectionSolution:Additional tripping brackets should be fitted to support cross tie or pillar.Soft toe brackets to be fitted

Transverse Frames - BucklingProblem:Buckling and tripping of frame in way of sharp radius. Position

Transverse Frames BucklingSolution: as illustrated below, or various combinations to increase buckling capability

SKEGS FOR DIRECTIONAL STABILITYProblem: Cracks in various locations starting from weld to bottom plating, edges and slot weldsPossible Cause:Unfavourable detail design.Insufficient stiffeningUnfavourable weld method

Solution:Introduce additional stiffening.Improve weld connections by using double sided fillet welds or full penetration with backing bar where access is difficult.Avoid scallop welds, replace with continuous welds against permanent backingSKEGS FOR DIRECTIONAL STABILITY

SCALLOPS AND CHAIN WELDSProblem:Cracks originating from end of chain weld propagating into stiffener. Cracks originating from scallops propagating into web or shell plating, along HAZ of weldPossible Cause:Vibrations from machinery or high dynamic sealoads. Unfavourable detail design for area. Unfavourable weld execution at starts and stops of chain welds and at ends/terminations of scallops

Solution:Longitudinal stiffeners in area should be welded with double continuous welds. Scallops should be avoided, necessary drain holes may be cut in web above weld to bottom platingSCALLOPS AND CHAIN WELDS

Thrust Bearing, Shaft Bearing and Engine FoundationsProblem:Cracks in various parts of foundations originating from weld toes at ends of brackets, stiffeners and scallops, ends of sniped flanges, or at ends of buttwelds in flanges in way of thickness transitions.Possible Cause:Unfavourable detail design, weld execution at ends, in way of scallops and at buttwelds of thickness transitions. Insufficient thickness of critical parts.

Thrust Bearing, Shaft Bearing and Engine FoundationsSolution:Improve detail design, by avoiding brackets and stiffening with unfavourable details as indicated previously. Increase thickness in certain locations. Improve position and profile of transitions between thick and thinner parts. Improve structural continuity by adding brackets or new flange pieces replacing sniped flanges. Careful grinding of critical welds and rounding of edges of foundations, flanges and transitions will improve fatigue life

Thrust Bearing, Shaft Bearing and Engine FoundationsFoundations supporting oscillatory equipment will be exposed to varying degrees of vibrations at high frequency. Fatigue failure must therefore be taken into consideration. Typical details that may give rise to high stress concentrations and for which it is difficult to obtain a good surface finish are scallops, cutouts, sniped flanges etc.It is therefore very important to maintain structural continuity, avoid scallops and sniped flanges whenever possible and improve details by grinding smooth surfaces at critical locations.

FOIL AND APPENDAGE FOUNDATIONProblem:Cracks in various parts of foundations originating from weld toes at ends of brackets, stiffeners and scallops, or at ends of buttwelds in flanges in way of thickness transitions.Possible Causes:Unfavourable detail design. Poor execution of welds. Insufficient dimensions of critical parts. Lack of structural continuityFoil internal supporting structureCracks in various of the flange junctions and bracket end connections

FOIL AND APPENDAGE FOUNDATIONSolutionImproving detail design by ensuring soft transitions at cruciform joints, positioning of buttwelds outside areas of stress concentrations, avoiding scallops or ensuring proper weld finishing at the scallop toes, fitting of soft toe brackets at ends of girders.

BULKHEAD STIFFENING ARRANGEMENTProblem: Cracks in brackets or stiffener at ends or cracks in bulkhead plating in way of unsupported stiffener endsPossible Cause: Unfavourable detail design. Fatigue cracking from sloshing of liquid in tanks or varying sea pressure

BULKHEAD STIFFENING ARRANGEMENTSolutionSoft toe bracketsProvide full shear connection of websProvide end support to unsupported stiffeners.

BRACKET CONNECTIONSProblemCracks in brackets and longitudinal stiffener, cracks originating from weld toes in various locations propagating into plating or along HAZ of weld.Possible CausesVibrations from machinery or high dynamic sealoads. Unfavourable detail design for the area. Unfavourable workmanship. Insufficient scantlings.

CracksIntermittent welding

BRACKET CONNECTIONSSolutionConnect webs of longitudinals to bulkheads or frames with continuous welds, avoid scallops and weld start / stops in critical positions. Fit soft toe brackets. Toe height should be limited - refer soft toe bracket detail

TRANSVERSE FRAMESProblemCracking in plate buttweld in knuckle. Cracking in webs and bulkheads in way of knuckle.Possible CausesUnfavourable detail design. The knuckle represents an elastic element in the plate, the effective flange will be reduced and the stresses will increase locally in way of the web / bulkhead.

TRANSVERSE FRAMESSolutionKnuckles should be supported with brackets which are properly connected to stiffeners on bulkheads or flanges of web frames. For knuckles with small change of direction, a plate insert made from a bent plate may be introduced to move butt weld out of the knuckle area.

STRUCTURAL DESIGN - POINTS TO REMEMBER1. Bottom slamming will be dimensioning to all structures in this area. Shear area requirement to longitudinal stiffener may be severe.2. Chines and knuckles have to be supported3. Side plating (L>50m) may need increased thickness for shear buckling.4. Vertical pillars to be positioned correctly first time.5. Transverse deck girders will often have long spans which need appropriate section properties.6. Deck plates of catamarans need to be checked for buckling due to transverse bending moment.