WAVE-INDUCED LOADS ON FIXED OFFSHORE STRUCTURES AN ASSESSMENT OF 'WAVE-BY-WAVE' LOAD VARIABILITY AND BIAS
Philip Jonathan and Paul.H. Taylor, Shell Research Rijswijk, The Netherlands
Abstract
A theory is presented to estimate the characteristics of wave-loads on fixed offshore structures given the occurrence of a maximum of surface elevation time-series at some sensor location. The theory assumes a random Gaussian ocean environment, together with mixed inertia and drag loading defined according to the Morison equation. The analysis is based on the statistics of a Gaussian process near an extreme excursion of a different but related Gaussian process. The resulting software is used to explore the influence of wave field directionality (spreading) and sensor location on expected wave loads and their variability. Comparisons of theory with full-scale offshore measurements at the Tern structure in the northern North Sea indicate good agreement.
It is demonstrated further that the standard interpretation of spreading as solely a velocity reduction factor in a unidirectional wave force calculation may not account correctly for the reduction in peak drag force on a fixed structure in spread seas. The extent of the inaccuracy is a function of structural dimensions; in general broadside loads are over-predicted and end-on loads slightly under-predicted.
A set of calibration curves is included with which to estimate adjusted spreading values for use with standard fluid loading packages, which enables the effects of directional spreading to be estimated realistically.
DYNAMICS OF OFFSHORE STRUCTURES UNDER SEA WAVES AND EARTHQUAKE FORCES
Arvind K. Jain ,Department of Civil Engineering, Indian institute of Technology, Delhi, Hauz Khas, New Delhi, INDIA
ABSTRACT
The analysis and design of Offshore Structures is a complicated process and requires several assumptions and approximations. The structures have to resist a hostile environment and the loads acting on them in a typical ocean environment are many such as wind, waves, tides, currents, ice, earthquakes, temperature loads, operational loads and so on. It is necessary to design an offshore structure such that it can respond to moderate severe environmental loads without damage and be capable of resisting severe environmental loads without seriously endangering the occupants.
In this paper, a study of the dynamic analysis of offshore structures in random seas to inputs of earthquake ground motions is presented. P-M spectrum is used for sea wave representation and the Morison equation defines the wave forcing function. Kanai-Tajimi's PSDF is used for the ground acceleration due to earthquakes. Response analysis is carried out using the time domain random vibration approach. It has been observed that the hydrodynamic damping is higher in random seas than in still water and sea waves induce a reducing effect on the seismic response.
