Statistical Properties of mechanically generated surface gravity waves: a laboratory experiment in a three-dimensional wave basin

A wave basin experiment has been performed in the MARINTEK laboratories, in one of the largest existing three-dimensional wave tanks in the world. The aim of the experiment is to investigate the effects of directional energy distribution on the statistical properties of surface gravity waves. Different degrees of directionality have been considered, starting from long-crested waves up to directional distributions with a spread of ±30° at the spectral peak. Particular attention is given to the tails of the distribution function of the surface elevation, wave heights and wave crests. Comparison with a simplified model based on second-order theory is reported. The results show that for long-crested, steep and narrow-banded waves, the second-order theory underestimates the probability of occurrence of large waves. As directional effects are included, the departure from second-order theory becomes less accentuated and the surface elevation is characterized by weak deviations from Gaussian statistics

Stochastic asymmetry properties of 3D gauss-lagrange ocean waves with directional spreading

In the stochastic Lagrange model for ocean waves the vertical and horizontal location of surface water particles are modeled as correlated Gaussian processes. In this article we investigate the statistical properties of wave characteristics related to wave asymmetry in the 3D Lagrange model. We present a modification of the original Lagrange model that can produce front-back asymmetry both of the space waves, i.e. observation of the sea surface at a fixed time, and of the time waves, observed at a fixed measuring station. The results, which are based on a multivariate form of Rice’s formula for the expected number of level crossings, are given in the form of the cumulative distribution functions for the slopes observed either by asynchronous sampling in space, or at synchronous sampling at upcrossings and down-crossings, respectively, of a specified fixed level. The theory is illustrated in a numerical section, showing how the degree of wave asymmetry depends on the directional spectral spreading and on the mean wave direction. It is seen that the asymmetry is more accentuated for high waves, a fact that may be of importance in safety analysis of capsizing risk

Reliable and Efficient Injury Assessment for Free-Fall Lifeboat Occupants During Water Entry: Correlation Study Between Lifeboat Acceleration Indicators and Simulated Human Injury Responses

The evacuation of personnel from an offshore installation in severe weather conditions is generally ensured by free-fall lifeboats. During the water entry phase of the launch, the lifeboat may be subject to large acceleration loads that may cause harmful acceleration-induced loads on the occupants. The present/common methodology for assessing the occupant safety of free-fall lifeboats uses one single characteristic launch to perform injury risk analysis for a given free-fall lifeboat launch condition that includes e.g. weather conditions, lifeboat and host installation loading conditions. This paper describes an alternative methodology to fully assess the risk of injury for lifeboat occupants during water entry by introducing a correlation model between acceleration load indicators and injury responses. The results are presented in terms of seating matrices showing critical seat rows, in which the probability of being injured exceeds a pre-defined threshold