Experimental Study on the Effects of Mooring System on Air Gap Response of Semi-Submersible Platform

The air gap response is crucial for the safe design and operation of large-volume floating platforms such as semi-submersible and tension leg platforms. It is a complex task to perform numerical simulation on the air gap response considering the wave free surface elevation and the motions of the floating vessel. Therefore, the prediction of air gap response still relies heavily on model tests. This paper attempts to investigate the effects of the mooring system, especially the effects of the length of mooring lines, on the air gap response of semi-submersible platform based on model tests results. The scaled model of the semi-submersible platform is supported by a symmetric mooring system composed of 8 mooring lines. A set of model tests with different length of mooring lines was performed in the State Key Laboratory of Ocean Engineering basin at Shanghai Jiao Tong University, and the air gap responses of 15 locations were measured using wave probes. The results indicate that the mooring system plays an important role in the air gap response of semi-submersible platform.

Nonlinear Amplifications in the Air Gap Response of a Deep Draft Semi-Submersible

This paper addresses the nonlinear amplifications in the upwelling crests of air gap responses from the wave basin model test of a deep draft semi-submersible in extreme wave condition. Contributions from nonlinear incident waves, vessel motions and wave-body interactions are analyzed separately. Results from the analyses suggest that, nonlinear amplification factors are larger than predictions from second order corrected model for incident and diffracted waves. Low frequency roll and pitch motions will induce larger nonlinear negative vertical motions for points in down-wave area, thus worsen air gap performance. Upwellings are highly related to diffracted wave elevations. Local run-ups due to highly nonlinear wave-body interactions around column walls could result in the nonlinear amplification factors in this area to be up to 230%.

Theoretical and Experimental Analysis of Air Gap Response and Wave-on-Deck Impact of Floating Offshore Structures

A comparative study between the theoretical and experimental analysis of air gap response and potential wave-on-deck impact forces of floating offshore structures is the main topic of this study. Both motion of the platform and the local wave elevation are important in air gap responses and wave impact forces. So, accurate and efficient computational analysis of wave induced loads and resulting platform’s responses and wave elevation is important in the prediction of air gap and evaluation of possible wave impact force. Numerical modelling for air gap and wave impact prediction is particularly complicated in the case of floating offshore structures because of their large volume, and the resulting effects of wave diffraction and radiation. Therefore, for new floating platforms, the model tests are often performed as part of their design process. The overall aim of this study is to introduce a simplified numerical method with sufficient accuracy suitable for preliminary design stages of a floating offshore platform to predict the air gap response using hybrid method and to evaluate the vertical wave impact force using Wagner-based method. The results obtained from the proposed method have been compared with those obtained from the experiments carried out in the wave tank of the Newcastle University.