Page 249 - Dynamic Loading and Design of Structures

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Screening in order to identify joints with high dynamic stresses and stress concentration,
which require more detailed fatigue analyses, may be undertaken by using the first approach.
s0 may be based on the nominal member stress for the extreme event and an appropriate stress
concentration factor, and the shape parameter γcould be obtained by general guidance.
Detailed fatigue analyses should be performed using conservative deterministic methods or
frequency domain techniques and, in particular situations, by TDA. Stochastic approaches
should be applied for dynamic sensitive structures. For linear systems, frequency domain
techniques are efficient.
More complete time domain approaches may especially be necessary in case of strong non-
linearities (e.g. associated with local splash zone behaviour), at least to calibrate simpler
methods.

5.5.4 Non-linear system assessment for ultimate or accidental limit states
Current ultimate strength code checks of marine structures are commonly based on load
effects (member and joint forces) that are obtained by a linear global analysis while
resistances of the members and joints are obtained by experiments or theory which account
for plasticity and large deflection. This methodology then focuses on the first failure of a
structural component and not the overall collapse of the structure, which is of main concern in
view of the failure consequences. The advent of computer technology and the finite element
method have made it possible to develop analysis tools that include second order geometrical
and plasticity effects and to account for possible redistribution of the forces and subsequent
component failures until the system’s collapse.
Ultimate strength analysis aims at providing a more realistic measure of the overall strength
of a platform, by using methods to account for global and inelastic features (e.g. to represent
redistribution of loads to alternative paths).
Initially such methods were developed for seismic analysis and for calculating the residual
strength of systems with damage (e.g. according to the accidental limit state checks). More
recently, such methods have also been applied to reassessment of ageing structures to
determine the ultimate capacity of the intact system as well as the global strength after
fatigue-induced fracture of members in connection with inspection planning.
Models which have been used to idealize structural members include phenomenological
models and various finite element-type models (see e.g. Hellan et al., 1994; Hellan, 1995;
Nichols et al., 1997). Cost-effective solutions are obtained by using large deformation theory
for beam elements and special displacement functions (e.g. Livesley ‘stability’ functions) and
concentrating the material nonlinearities in yield hinges at predefined locations or at locations
where maximum stress occurs. Yield hinge models are developed with different refinements,
from yield hinges with zero extension along the element to models that account for the
extension of the yield hinge; with elastic—perfectly plastic or gradual plastification