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Feature 2
Fatigue in focus
Bureau Veritas has developed a new fatigue assessment methodology for
typical details of very large gas carriers. Philippe Cambos, BV head of oil
and gas technical section, gives an overview.
L
PG carriers have experienced a picture of loads, a 3D fine mesh model gave in the sliding mode and the resultant
sudden growth in size, and we now yielding assessment, and finally a 3D very damage. Friction forces were applied in
commonly see ships of 80,000m
3
fine mesh model focused in on the detail. the longitudinal and transverse directions
capacity. They are built for very long service For the fine mesh the element size was taken depending on the load case. The static
lives, yet they pose special problems of as the plate thickness. friction coefficient used was 0.3 whereas
fatigue, due to their large open tanks and Each type of tank support was fully dynamic was 0.15. The results of the
the tank/hull coupling. modelled and then subject to fatigue calculations clearly show fatigue stresses
Accordingly, Bureau Veritas has developed analysis in both longitudinal and spread across the hull supports.
special techniques focusing on the detail transverse directions, and in way of the When it came to the fatigue assessment
of VLGC structure, to design out fatigue hull structure. on the anti-rolling keys a different
and to assess fatigue in service. The vessels The 3D coarse mesh model focused methodology to that used for the vertical
considered in a major study were large on the yielding and buckling of primary supports was deployed, due to the gap in
LPG carriers, typically up to 230m loa, with members in tanks and holds. That in turn the key.
prismatic tank structures. The key fatigue yielded the forces on the keys of the tanks The three stage approach was based on
areas requiring attention were determined for supports assessment and the boundary calculating the fatigue damage without
to be the feet of main frames of the side conditions for fine mesh assessment. anti-upper rolling key to define what
shell, shell longitudinals with transverse The assumptions for fatigue analysis probability level there is of contact in the
webs, knuckles between double bottom and were: sailing factor 0.85; ship sailing 40% anti-rolling keys, then a repeat calculation
hopper tanks, brackets in the cargo tanks, ballast and 60% full load; Bureau Veritas with the anti-rolling key merged, ie in
tank supports and keys, and tank dome. design S-N curve based on British standard; contact, followed by a combination of the
Fatigue calculations are based on a upright and inclined ship condition and damages obtained in steps one and two.
deterministic approach using notch corrosion environment (not applicable in BV’s conclusions, validated on vessels
stresses, calibrated on the results of a full the cargo hold). in service, are that the methods developed
hydrodynamic analysis taking into account Both internal and external dynamics produce clear and correct fatigue damage
both hull and cargo movement. Both the of sea conditions and cargo and ballast assessment. That in turn points to remedial
cargo tanks and the hull structure had to be internally were considered in the dynamic actions at the design and build stage,
modelled, because the reaction forces in the load modelling. including the need for brackets to be
hull structural members and tank supports With the right loads and friction forces, with soft toes, both in the transverse and
depends on the stiffness of both. The model there were three steps to the fatigue longitudinal direction. Local grinding may
and analysis were built up in the following assessment of the supports. These were be requested during construction to get
way. to calculate all the merged loads on the weld detail correct. For anti-rolling keys
A 3D FEM model using VeriSTAR Hull supports with no sliding, then to calculate a gap is necessary, and in almost all cases
was used at three levels of focus. A three the probability of sliding, and then to go local reinforcements are required, based on
hold coarse mesh model was used for the big on to calculate the stresses on the supports fatigue assessment. NA
Fatigue assessment of vertical supports.
Limit between static and dynamic friction,
wherein the total horizontal dynamic force
in the supports is calculated.
The Naval Architect February 2008 57
NA Feb 08 - p57.indd 57 05/02/2008 09:41:40
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