Page 146 - Marine Structural Design

122 Part I Structural Design Principles

Openings in main hull,
Transverse frames,
Flare tower,
Riserporches,
Major process equipment seats.
Any turret structure will be exposed to high levels of dynamic action. The following actions
should be considered throughout the fatigue design of turret structures:
Dynamic fluctuations of mooring line tension,
Dynamic actions (tension and bending moment) from risers,
Local varying hydrodynamic pressure due to wave action and vessel motion,
Reactions in the bearing structure due to the other effects,
Inertia actions due to accelerations of vessel motions including variations in internal fluid
pressure,
Fluctuating reactions in pipe supports due to thermal and pressure induced pipe
deflections.
Local stress ranges are determined from dynamic pressures acting on panels and accelerations
acting on the equipment and topside. Other environmental actions also affect part of the
structure as local stresses with a variety of ranges.
The transfer function for the dynamic pressure could either be used directly to calculate local
stress transfer functions and combined with the global stress transfer function or a long-term
pressure distribution could be calculated. At least, the following dynamic pressure components
should be considered:
Double hull stresses due to bending of double hull sections between bulkheads,
Panel stresses due to bending of stiffened plate panels,
Plate bending stresses due to local plate bending.
Global and local stresses should be combined to give the total stress range for the detail in
question. The global and the local stress components differ in amplitude, phase, and location.
The method of combining these stresses for the fatigue damage calculation will depend on the
location of the structural detail.
Local, detailed FE-analysis (e.g. unconventional details with insufficient knowledge about
typical stress distribution) should be undertaken in order to identify local stress distributions,
appropriate SCFs, andor extrapolated stresses which will be utilized in the fatigue evaluation.
Dynamic stress variations through the plate thickness shall be documented and considered in
such evaluations.
During the fatigue assessment, tine element mesh models will be developed for critical stress
concentration details that do not comply with the stress concentration factors given in
recognized standards. The size of the model should be such that the assumptions made for the
boundary conditions do not affect the calculated hot spot stresses significantly. Element sizes
for stress concentration analysis should be of the same order of magnitude as the plate
thickness. Normally, shell elements may be used for the analysis.