The paper describes the basic principles of an integrated computer program system for three dimensional static and dynamic analysis of deep water flexible tubular structures.
The deflected configuration and stresses of the structure, statically loaded, are evaluated by means of a step-by-step procedure. Then, the dynamic response about the static configuration is determined. Either the frequency and the transient solution are performed. In the transient solution the nonlinearities due to wave forces, to the time variable constraints, to the relative motion between structure and water and between structure and sea-bottom, are taken into account. The procedure allows to perform a complete simulation of the behaviour of the structure with rather sophisticated static and dynamic load conditions. Moreover, a statistical evaluation of the short term structural response is presented.
The improved capabilities in deep water pipelaying and the need to move ahead offshore ocean activities, require a great care in executing design of flexible structures. Considering the large money losses consequent to a pipe or a riser failure in deep water, it is essential to perform a more is hesitated analysis of such structures in relation to the actual environmental conditions.
The need of extending static methods to include dynamic effects has been indicated by many authors /1/,/2/,/3/and confirmed by both model and field tests /4/, /5/.
The proposed computer system performs the static and dynamic analysis of offshore structures as marine risers and sealines (laying I abandoning, recovering, on bottom free spans) allowing a correct and comprehensive analysis of such structures I essential to engineer deep water operations.
This system is part of a wider computerized procedure, illustrated in fig. 1. Starting from the environmental conditions and from the geometry of the marine system the dynamic analysis of the vessel and the static analysis of the flexible structure are performed. The outputs of these analyses are the input, together with the environmental loads, for the dynamic model of the flexible structure. The dynamic analysis may be performed either in the frequency or in the time domain. In the second case it is possible to take into account the most important nonlinearities.
Within this complete procedure, the paper deals with the static and dynamic analyses of the flexible structure.
The proposed computer system takes into account all the most important static and dynamic nonlinear effects necessary to be considered in simulating the behaviour of flexible marine tubular structures. The static nonlinearities are:
This type is usually referred as "geometric nonlinearity" and occurs when deflections are large enough to cause sensible changes in the geometry of the structure. In the problems considered the equilibrium configuration is determinedstarting from an unreformed structure very different in geometry from the final structure (i.e. : deflections are of the same order of the structural dimensions). In this case the equilibrium equations must be formulated for the deformed configuration and not for the initial configuration as in the usual theory.