Response-based extreme value analysis of moored offshore structures due to wave, wind, and current

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.     

Response analysis of a truss spar in the frequency domain

The nonzero added-mass coefficients of a platform are found by using the transformation law for a second-order tensor, and the repeated application of the parallel-axes theorem. The excitation forces acting on the truss section of the platform are derived by an approach that differs from the conventional one commonly seen in the literature. The force decomposition of the Morison equation is used to add viscous effects to linear equations of motion. The nonlinear equation of motion for the heave of the truss spar is solved without any iteration in the frequency domain. The results obtained from this analysis are compared with results obtained from the conventional numerical approach and with experimental data.     

Large-amplitude motion responses of a Ro–Ro ship to regular oblique waves in intact and damaged conditions

This article presents a nonlinear time-domain simulation method for the prediction of large-amplitude motions of a Ro–Ro ship in regular oblique waves in an intact and a damaged condition. Numerical computations and model tests have been carried out to investigate the dynamic motion responses of Ro–Ro ship Dextra to various wave amplitudes at three different wave headings. The results of numerical and experimental investigations for stern quartering waves are reviewed. Comparisons between predictions and measurements show good agreement except in the roll-resonant region. Nonlinear effects are significant in horizontal modes of motion, and resonant roll motion, and there is strong coupling between all modes of motion in the roll-resonant region for large-amplitude responses. On the other hand, the time-domain simulation technique suffers from numerical drift in horizontal modes of motion as wave amplitude increases. This is due to nonlinear equations of motion and the lack of a restoring force and moment in horizontal motion. Received: April 30, 2002 / Accepted: August 9, 2002 Acknowledgments. II Programme of the European Community Commission under contract No. BRPR-CT97-0513. Address correspondence to: H.S. Chan (hoi-sang.chan@ncl.ac.uk)     

A numerical investigation of the squat and resistance of ships advancing through a canal using CFD

As a ship approaches shallow water, a number of changes arise owing to the hydrodynamic interaction between the bottom of the ship’s hull and the seafloor. The flow velocity between the bottom of the hull and the seafloor increases, which leads to an increase in sinkage, trim and resistance. As the ship travels forward, squat of the ship may occur, stemming from this increase in sinkage and trim. Knowledge of a ship’s squat is necessary when navigating vessels through shallow water regions, such as rivers, channels and harbours. Accurate prediction of a ship’s squat is therefore essential, to minimize the risk of grounding for ships. Similarly, predicting a ship’s resistance in shallow water is equally important, to be able to calculate its power requirements. The key objective of this study was to perform fully nonlinear unsteady RANS simulations to predict the squat and resistance of a model-scale Duisburg Test Case container ship advancing in a canal. The analyses were carried out in different ship drafts at various speeds, utilizing a commercial CFD software package. The squat results obtained by CFD were then compared with available experimental data.