DNV Design Considerations

DESIGN CONSIDERATIONS

Approval Centre Sydney

General - Stiffener/web connection

50mm

Size of welding gun compared to typical stiffener height

Continuous welding difficult

Start and stop defects


Difficult to weld

Small shear capacity

Bracket or flat bar added

Not too difficult to weld



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



General - BracketsGeneral - Brackets

Edge Stiffening Requirements


General - Tipping BracketsGeneral - Tipping Brackets


General - “Soft Toe” BracketsGeneral - “Soft Toe” Brackets

10 - 15mm


General - Termination of Sniped StiffenersGeneral - Termination of Sniped Stiffeners

D<25mm

Sniping allowed when:

•buckling stiffeners

•dynamic loads/vibrations of minor importance

30 degrees


General - Pillar Connections to DeckGeneral - Pillar Connections to Deck

Should not be applied if tension through pillar may occur


General - Pillar Connections/Cross Ties connectionGeneral - Pillar Connections/Cross Ties connection

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


General - Alignment in Way of Section JointsGeneral - Alignment in Way of Section Joints

Bulkhead or deck

Section JointEasier to get acceptable

alignment when a continuous member is penetrating the bulkhead or deck structure


General - Longitudinals through Web FramesGeneral - Longitudinals through Web Frames


Longitudinals at Watertight BulkheadLongitudinals at Watertight Bulkhead

Longitudinal continuous through bulkhead

Longitudinal terminated at bulkhead with soft-nose brackets


Edge Support of Cut-OutsEdge Support of Cut-Outs

Edge support when length of free edge exceed 50t or D50t

Cut-outs to be kept well clear of end of brackets and locations where shear stresses are high


Hard Chine / KnuckleHard 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.

Cracks in web and shell plating

Integral spray rail

Cracks

External spray rail


Hard Chine / KnuckleHard Chine / Knuckle

CHINE SUPPORT KNUCKLE SUPPORT

Solution:


Pillar Connections/Cross Ties connectionPillar Connections/Cross Ties connection

Problem: Cracking and buckling of frame webs and ties.

Possible Cause: Unfavourable detail design.

Buckling and tripping of frame

Cracks

Position


Pillar Connections/Cross Ties connectionPillar Connections/Cross Ties connection

Add tripping brackets

Soft toe bracketsSolution:

Additional tripping brackets should be fitted to support cross tie or pillar.

Soft toe brackets to be fitted


Transverse Frames - Buckling

Problem:Buckling and tripping of frame in way of sharp radius.

Buckled web and tripped flange

Position


Transverse Frames Buckling

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

Tripping brackets

Additionalbuckling stiffener

Additional bracket

Tripping brackets


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 methodCracks

Bottom Pl.


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

SKEGS FOR DIRECTIONAL STABILITY


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

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

Crack


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

SCALLOPS AND CHAIN WELDS

Scallops avoided, necessary drain holes cut above weld.

Re-weld with continuous welding


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.

a : a

Cracks in bracket supports startingfrom bracket ends and scallops

Cracks in foundation flanges in way of thickness transitions

aa



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

Reposition weld in relation tosupports and increase taper

a : a

New thicker transverse webs cut fromone piece welded without scallops andwith weld toes ground. Reposition girderwebs inside line of gear bed bolting.

aa


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.



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


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.

FOIL AND APPENDAGE FOUNDATION

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

Typical cracks at unsupported ends of vertical stiffeners

Cracks

Cracks


BULKHEAD STIFFENING ARRANGEMENT

Solution

Soft toe brackets

Provide full shear connection of webs

Provide end support to unsupported stiffeners.

End connection of bulkhead stiffeners

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


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

BRACKET CONNECTIONS

Soft bracket possibly of increased size.

Webs welded continuously.


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.

Cracks in webs from scallop

Crack in longitudinal butt weld in shell

Cracks in webs and alongshell plating butt weld


SolutionKnuckles 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.

TRANSVERSE FRAMES

Alt. 2, on one side of frame

Alt. 1, on both sides of frame

Add brackets to support knuckle

Insert bent plate


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.

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DNV Design Considerations

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