Wave Front Steepness and Influence on Horizontal Deck Impact Loads

Abstract

In design storm sea states, wave-in-deck forces need to be analysed for fixed and floating offshore platforms. Due to the complex physics of wave impact phenomena, numerical analyses should be complemented by model test data. With a large statistical variability, such experiments usually involve running many 3-h storm realisations. Efforts are being done to establish efficient procedures and still obtain improved statistical accuracy, by means of an initial simplified screening based on parameters derived from the incident wave record only. Here, we investigate the vertical rise velocity of the incident wave elevation at a fixed point in space, which indirectly measures both the local slope and the near-surface orbital velocity. A derived simple deck slamming model is also suggested, to support the check of the physical basis of the approach. Correlation against data from a GBS wave-in-deck model test is used for checking this model. The results show that, although there is a significant random scatter in the measured impact forces, especially in the local slamming forces but also in the global forces, there is a correlation to the rise velocity. Comparisons to the simple load model also show promising results when seen on background of the complex physics and random scatter of the impact problem.

Selected Papers from the Future Paths and Needs in Wave Modelling Workshop

“…Clear and quantified variability effects are demonstrated which should be considered within engineering applications; the effects are strongest for the third-order model. A related random scatter is seen in the wave/load correlation study in [5]; the variability is even stronger there because of the very strong nonlinear wave-structure slamming physics.…”

“…In model testing for design load estimation in given sea states, it is important to represent relevant wave events, critical for such design loads, and numerical models must properly take into account nonlinear effects such as the above mentioned nonlinear wavewave interaction [1,4]. Ideally, for efficient testing one should only include critical events that are important for the design loads [5,6,12]. Extreme crests are clearly non-Gaussian, and a set of point observation data in 17 m depth show extremes well above both second and third-order models [1].…”

“…Not only the extreme crest heights, but also the extreme local steepness or slope of individual energetic wave events matter for the impact on structures. This is addressed in Stansberg [5] where correlations between local front steepness estimates and resulting wave-in-deck impact loads from a model test are demonstrated. The maximum orbital velocity in such waves, closely related with the local slope, is another critical parameter addressed in van Essen et al [6].…”

“…Proper estimation of design load and response from random wave tests (or numerical simulations) quite often requires an extremely large amount of model test (and/or numerical) data for robust extreme value statistics, such as in slamming for example. A need to establish improved procedures for identifying and running tests with critical wave events only is addressed in [5,6,12]. A possible choice for a critical wave parameter for screening is investigated in [5], where correlations against wave-in-deck model test impact data are recognized despite the type of random scatter that is typical for such data.…”

“…As an outcome of the International Workshop "Future needs and challenges in wave modelling", held by SINTEF in Trondheim, Norway, 21-22 October 2019, this Special Issue includes 10 full Journal scientific papers [1][2][3][4][5][6][7][8][9][10] and two Technical notes [11,12], which have been prepared based on a selection of the Workshop presentations. All publications were peer-reviewed according to Journal standards by international experts.…”