Modeling Crossing Random Seas by Fully Non-Linear Numerical Simulations

Bimodal spectrum wave conditions, known as crossing seas, can produce extreme waves which are hostile to humans during oceanic activities. This study reports some new findings of the probability of extreme waves in deep crossing random seas in response to the variation of spectral bandwidth through fully non-linear numerical simulations. Two issues are addressed, namely (i) the impacts of the spectral bandwidth on the changes of extreme wave statistics, i.e., the kurtosis and crest exceedance probability, and (ii) the suitability of theoretical distribution models for accurately describing the wave crest height exceedance probability in crossing seas. The numerical results obtained by simulating a large number of crossing sea conditions on large spatial-temporal scale with a variety of spectral bandwidth indicate that the kurtosis and crest height exceedance probability will be enhanced when the bandwidth of each wave train becomes narrower, suggesting a higher probability of encountering extreme waves in narrowband crossing seas. Meanwhile, a novel empirical formula is suggested to predict the kurtosis in crossing seas provided the bandwidth is known in advance. In addition, the Rayleigh and second-order Tayfun distribution underestimate the crest height exceedance probability, while the third-order Tayfun distribution only yields satisfactory predictions for cases with relatively broader bandwidth regarding the wave conditions considered in this study. For crossing seas with narrower bandwidth, all the theoretical distribution models failed to accurately describe the crest height exceedance probability of extreme waves.

Design waves and statistics of linear gap resonances in random seas

Water wave resonance between two side-by-side vessels is a multimode resonant hydrodynamic phenomenon with low damping. The potential flow damping and viscous damping inside the gap play a significant role, influencing the amplitudes of the gap resonances. The frequencies of the gap modes can be well predicted by linear potential flow theory, while much effort has been made to explore the nature of the viscous damping. A series of experiments is conducted to explore the temporal (Zhao et al., Journal of Fluid Mechanics, vol. 812, 2017, 905–939) and spatial structure (Zhao et al., Journal of Fluid Mechanics, vol. 883, 2020, A22) of the resonant responses along the gap. Ultimately, it is of practical interest to understand the response statistics along the gap in random seas, to facilitate decision making for safe offshore operations. Following our previous studies which focused on new physics, here we identify the design waves that produce the most probable maximum responses under unidirectional random linear wave excitation. This is achieved through an efficient prediction model within linear theory. Combining the experimental data and linear potential flow calculations, we provide the lower and upper bounds of gap responses, bracketing possible responses at field scale. The statistical model is expected to be of practical importance for offshore operations.

A NUMERICAL STUDY OF FREAK WAVE GENERATION IN RANDOM SEAS OVER A SUBMERGED BAR

Freak waves, also called rogue waves and giant waves, are much larger and steeper than the surrounding waves, can cause severe accidents, and can be formed in both coastal and offshore regions. The past researchers on freak waves in coastal regions are mainly focused on the statistical properties, and the generation mechanism of such large waves are not yet discussed intensively. The aim of the present study is to examine the generation process of freak waves in unidirectional propagating random waves over a submerged bar using a fully nonlinear numerical wave model, SWASH. It was found that freak waves are readily formed at the seaward part of the crest of the bar and gradually emerged from an intense wave group. The enhancement of the bound higher harmonics in the shoaling process is the main reason to form such large waves in shallow water. On the crest bar of the bathymetry, the extreme wave gradually vanished, mainly due to the releasing of bound higher-harmonics to free wave components.

Investigation of tendon dynamics effects on tension leg platform response in random seas

This study investigated tendon dynamics effects of tension leg platform models with tendons modelled by finite element spring and beam elements for uncoupled and coupled tension leg platform in random waves and current environment. The purpose of the study is to proffering numerical solution of single mathematical equation of motion for fully integrated-coupled tension leg platform floater with tendons model. Structural modelling of complete tension leg platform is achieved with the help of ABAQUS/Standard finite element tools which is incorporated with ABAQUS/Aqua module for the application of hydrodynamic loadings on the partially submerged tension leg platform hull and fully submerged platform tendons. For the uncoupled tension leg platform model, weight, inertia, hydrodynamic force and damping forces are ignored on the tendons modelled with springs, while the stiffness of the tendons is considered as a static restoring force. The coupled tension leg platform model had all the forces applied on the tendons modelled with beam elements. Conclusively, modelling and analysis of the tension leg platform as uncoupled and coupled models have expanded our understanding to know that surge motion response is fairly predicted by the two models, however, heave and pitch motions, and variations in tendon tension differ significantly; hence, coupled tension leg platform model is recommended. The influence of a removed tendon due to accident or maintenance on the tension leg platform motions is also reported.