Dnv Design Considerations

DESIGN CONSIDERATIONS

General - Stiffener/web connection Size of welding gun compared to typical stiffener height Continuous welding difficult Start and stop defects 50mm

Approval Centre Sydney

General - Stiffener/web connection Difficult to weld Small shear capacity

Bracket or flat bar added Not too difficult to weld

Approval Centre Sydney

General - Stiffener/web connection Cover plate will improve shear capacity and improve force distribution

If possible avoid scallops, snipe corner of collar plate to allow space for longitudinal weld and then weld continuously around collar plate Approval Centre Sydney

General - Brackets

Edge Stiffening Requirements Approval Centre Sydney

General - Tipping Brackets

Approval Centre Sydney

General - “Soft Toe” Brackets

10 - 15mm

Approval Centre Sydney

General - Termination of Sniped Stiffeners

30 degrees

Sniping allowed when: •buckling stiffeners

D<25mm

•dynamic loads/vibrations of minor importance Approval Centre Sydney

General - Pillar Connections to Deck

Should not be applied if tension through pillar may occur

Approval Centre Sydney

General - Pillar Connections/Cross Ties connection

Applicable to pillars/cross ties that may be under compression and tension Approval Centre Sydney

General - Alignment in Way of Section Joints Bulkhead or deck

Easier to get acceptable alignment when a continuous member is penetrating the bulkhead or deck structure

Section Joint

Approval Centre Sydney

General - Longitudinals through Web Frames

Approval Centre Sydney

Longitudinals at Watertight Bulkhead

Longitudinal continuous through bulkhead Longitudinal terminated at bulkhead with softnose brackets Approval Centre Sydney

Edge Support of Cut-Outs

Edge support when length of free edge exceed 50t or D≥ 50t Cut-outs to be kept well clear of end of brackets and locations where shear stresses are high Approval Centre Sydney

Hard Chine / Knuckle Problem: 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.

Integral spray rail

Cracks External spray rail Cracks in web and shell plating

Approval Centre Sydney

Hard Chine / Knuckle Solution:

CHINE SUPPORT

KNUCKLE SUPPORT

Approval Centre Sydney

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

Buckling and tripping of frame

Position Cracks

Approval Centre Sydney

Pillar Connections/Cross Ties connection

Solution: Additional tripping brackets should be fitted to support cross tie or pillar. Soft toe brackets to be fitted

Soft toe brackets

Add tripping brackets

Approval Centre Sydney

Transverse Frames - Buckling Problem:Buckling and tripping of frame in way of sharp radius. Position Buckled web and tripped flange

Approval Centre Sydney

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

Additional buckling stiffener

Tripping brackets

Additional bracket Tripping brackets

Approval Centre Sydney

SKEGS FOR DIRECTIONAL STABILITY Problem: Cracks in various locations starting from weld to bottom plating, edges and slot welds Possible Cause: Unfavourable detail design. Insufficient stiffening Unfavourable weld method

Bottom Pl.

Cracks

Approval Centre Sydney

SKEGS FOR DIRECTIONAL STABILITY 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 backing

Additional Stiffening / Continuous Welding

Approval Centre Sydney

SCALLOPS AND CHAIN WELDS Problem: Cracks originating from end of chain weld propagating into stiffener. Cracks originating from scallops propagating into web or shell plating, along HAZ of weld Possible 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 Approval Centre Sydney

Crack

Cracks in web along HAZ of weld, crack in bottom plating

SCALLOPS AND CHAIN WELDS 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 plating

Re-weld with continuous welding

Scallops avoided, necessary drain holes cut above weld.

Approval Centre Sydney

Thrust Bearing, Shaft Bearing and Engine Foundations Problem: 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.

Cracks in bracket supports starting from bracket ends and scallops

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Cracks in foundation flanges in way of thickness transitions

Approval Centre Sydney

Thrust Bearing, Shaft Bearing and Engine Foundations Solution: 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

New thicker transverse webs cut from one piece welded without scallops and with weld toes ground. Reposition girder webs inside line of gear bed bolting.

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Reposition weld in relation to supports and increase taper

Approval Centre Sydney

Thrust Bearing, Shaft Bearing and Engine Foundations Foundations 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.

Approval Centre Sydney

FOIL AND APPENDAGE FOUNDATION Problem: 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 continuity

Foil internal supporting structure

Cracks in various of the flange junctions and bracket end connections

Approval Centre Sydney

FOIL AND APPENDAGE FOUNDATION Solution Improving 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.

Approval Centre Sydney

Use proper soft transitions between girder flanges and soft toe brackets at girder ends

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

Cracks Cracks

Typical cracks at unsupported ends of vertical stiffeners

Approval Centre Sydney

BULKHEAD STIFFENING ARRANGEMENT Solution Soft toe brackets Provide full shear connection of webs Provide end support to unsupported stiffeners.

End connection of bulkhead stiffeners Approval Centre Sydney

Soft toe brackets, stiffener webs welded continuously to bulkhead.

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

Cracks

Intermittent welding

Approval Centre Sydney

BRACKET CONNECTIONS Solution Connect 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

Soft bracket possibly of increased size.

Webs welded continuously.

Approval Centre Sydney

TRANSVERSE FRAMES Problem Cracking in plate buttweld in knuckle. Cracking in webs and bulkheads in way of knuckle. Possible Causes Unfavourable 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.

Approval Centre Sydney

Cracks in webs and along shell plating butt weld

Crack in longitudinal butt weld in shell

Cracks in webs from scallop

TRANSVERSE FRAMES Solution Knuckles 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.

Insert bent plate

Add brackets to support knuckle

Alt. 1, on both sides of frame

Approval Centre Sydney

Alt. 2, on one side of frame

STRUCTURAL DESIGN - POINTS TO REMEMBER 1. 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 supported 3. 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. Approval Centre Sydney

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