This chapter examines the sensitivity of structure loads and responses, particularly those of compliant and dynamic systems, to changes in the environment model. The main parameters investigated are those associated with the wave: the use of regular and irregular wave models, long- and short-crested waves, sensitivity to wave spectral truncation frequency and resolution, and to the choice of the amplitude and phase spectra. The extraction of extreme loads from irregular wave data poses special problems. Low-frequency resonant systems, such as moored vessels or tension leg platforms, are often sensitivity to wave grouping and second-order processes, and to the way in which these are modelled.
The choice of environmental modelling parameters is linked to that of design philosophy. Conventional jacket structure design does not try to represent the environment in a realistic way; indeed, the attempt to improve ‘realism’ may be dangerous, perhaps reducing an importing source of conservatism in the process. This approach aims to produce a safe and economic structure without unnecessary design effort, replying heavily on post experience. The traditional design package therefore has to seen as a single entity, some aspects of which are conservative, and others not so. It is this unity that makes it difficult to bring in new information, or to reconcile the design package with research data.
The traditional approach is gradually being challenged by reliability-based methods, where parameter variations are considered as part of the design, and the whole approach may be reconciled more easily with research data and with the real sea.
Some of the sources of conservatism or otherwise in the traditional design process will now be discussed, and the results indicate a few of the difficulties that may arise in introducing a more realistic model.
Loads on a conventional jacket structure are invariably calculates using Morison's equation. The force per unit length on a structural member is expressed as the sum of drag and inertial components:
Where u andû are components of fluid velocity and acceleration normal to the member, p is the water density, D and A are the member's effective diameter and cross-sectional area, and Cd', Cm are drag and inertia force coefficients.
Over the years, there has been much controversy among researchers about the values of Cdand Cm. Research has shown that these values depend on a complex range of parameters such as the Keulegan-Carpenter and Reynolds number, type and effective height of marine fouling- with Cd reaching as high as 1.9 with only a small amount of marine roughness.
Design practice has tended to ignore much of this variability. Very low values of the drag coefficient, in the range Cd = 0.6–.08, continue to be used because there are other compensating factors in the design process which provide sufficient conservatism. One such factor lies in the research data itself: high values of Cd tend to be associated with low value of Cm representing a change in phase of the force on an individual member rather than a change in its magnitude.