Response-based extreme value analysis of moored offshore structures due to wave, wind, and current
ATILLA INCECIK1, JOHN BOWERS2, GILL MOULD2, and OGUZ YILMAZ3
1 Department of Marine Technology, Armstrong Building, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK
2 University of Stirling, Stirling, UK
3 University of Glasgow, Glasgow, UK
Abstract： Floating moored offshore structures have a significant future in offshore operations as an attractive economic alternative to fixed structures in deep waters and/or in areas where there is no existing infrastructure. This paper describes
an analysis procedure based on the structure variable approach to estimate load and response values of a moored offshore platform at a given return period by taking into account the joint occurrence of wave, wind, and current. The results show that the most severe mooring loads may not occur when wind, wave, and current are collinear and are at their maximum design values, i.e., the 50- or 100-year case. It is recommended that the extreme mooring design loads for moored offshore systems should be determined through a range of physical or numerical simulations where wave, wind,
and current are noncollinear and act with less severe magnitudes than the 50- or 100-year case. This recommendation has also been adopted in the ITTC/Ocean Engineering Committee recommendations to the ITTC Conference held in September 1996.
Key words： floating offshore platforms, response-based design, multivariate extreme value analysis
Address correspondence to： A. Incecik
Received for publication on March 4, 1997; accepted on Aug. 12, 1998
Introduction
Floating and compliant offshore structures have a significant future in offshore operations as they are an attractive economic alternative to fixed structures in deep waters and/or in areas where there is no existing infrastructure. However, recent failings of mooring lines have prompted further reviews of the design of offshore systems. Whilst previous studies have examined some engineering and environmental aspects, there is a need to develop a new design criteria for the design of mooring lines. Existing design criteria for mooring analysis1,2 are based on the 100-year oceanographic design criteria.
During the course of this research, an analysis procedure based on the structure variable approach, illustrated in Fig.1, has been developed to estimate load and response values at a given return period by taking into account the joint occurrence of the magnitudes and directions of the wave, wind, and current. This procedure
involves：
― the development of a motion response simulation of the specified system;
― parametric experiments with the simulation to explore the influences of wind, wave, and current;
― summarizing the parametric results in the form of a simple function which provides an approximation to the system's response;
― the transformation of the environmental time series into a time series of forces using the response function;
― an extreme value analysis of the forces' time series to provide estimates of the N-year return forces.
The tools developed during the study have been successfully applied to estimate the extreme mooring forces of a semisubmersible, as illustrated in Figs.2 and 3. The nonlinear time-domain tools developed in the study can model the behavior of semisubmersibles by taking into account steady and dynamic wind, and first- and second-order wave and current forces. These tools have been validated with experimental measurements.3,4
Motion response simulation of a semisubmersible under wave, wind, and current loading
Motion response prediction of offshore structures may be carried out using time domain or frequency domain
