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DNV-OS-C201 Structural Design of Offshore Units (WSD method) |
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| Sec.14: Special Considerations for Deep Draught Floaters (DDF) |
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| L: Fatigue |
Guidance note: ---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Early phase evaluation or analysis of fatigue may incorporate
modelling the hull as a beam with associated mass distribution and
simulation of wave actions according to Morison formulation, or
preferably, performing a radiation or diffraction analysis.
Final documentation related to first order wave induced fatigue
damage should incorporate a stochastic approach. This implies establishing
stress transfer functions, which are combined with relevant wave
spectra (scatter diagram) in order to obtain long-term distribution
of stresses. The stress transfer functions should be obtained from
FEM analyses with appropriate simulation of wave loads (radiation/diffraction
analysis). The P-delta effect due to platform roll and pitch must
be taken into account.
| — | hull and deck connections |
| — | collision ring area |
| — | hull and deck and stiffener connections at location of peak wave induced global bending moments |
| — | fairlead area |
| — | hard tank area |
| — | column and brace connections |
| — | strake and hull connections and strake terminations |
| — | riser frame and hull connections |
| — | hard tank and truss spar connections |
| — | tubular joints |
| — | riser porches/hang-off |
| — | tensioner support module/hull |
| — | highly stressed manway areas opened up for construction and closed by welded caps. |
Both global bending effects and local wave induced loads shall
be taken into account in fatigue design of strakes. Local effect
on the hull due to strake induced fatigue loads should be considered
in hull fatigue design.
Sec.14 L
206 VIM load effects from mooring system (global hull in-line
and cross-flow motions) into the fairlead/hull areas shall
be outlined and taken into account. The same applies to VIV load
effects from riser systems into the riser frame and hull areas.
Sec.14 L
207 Allowance for wear and tear shall be taken into account in
areas exposed to e.g. friction and abrasion. For a DDF unit this
will typically be interfaces between hull and risers (keel level,
intermediate riser-frames, deck level). These relative motions are
caused by movements of the unit and risers and subsequent pull-out
and push-up of the risers in the moonpool.
If a DDF may have a draught variation, this should be taken into account in evaluating the splash zone.
Guidance note: ---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
If significant adjustment in draught is possible in order
to provide for satisfactory accessibility in respect to inspection, maintenance
and repair, a sufficient margin in respect to the minimum inspection
draught should be considered when deciding upon the appropriate
design fatigue factors. As a minimum this margin shall be at least
1 m, however it is recommended that a larger value is considered
especially in the early design stages where sufficient reserve should be
allowed for to account for design changes (mass and centre of mass
of the unit). Consideration should further be given to operational
requirements that may limit the possibility for ballasting and deballasting
operations.
When considering utilisation of remotely operated vehicle
(ROV) inspection, consideration should be given to the limitations
imposed on such inspection by the action of water particle motion
(e.g. waves). The practicality of such a consideration may be that
effective underwater inspection by ROV, in normal sea conditions,
may not be achievable unless the inspection depth is at least 10
m below the sea surface.
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