New methods for estimation of extreme wave crest heights have resulted in an increase of the estimated 10,000 year crest height. At the Norwegian Continental Shelf this increase is typically 2 to 4 m, resulting in a crest height of 22 m to 24 m in the Central & Northern North Sea and the Haltenbanken area. As a result several fixed platforms designed prior to 2000 may experience negative air gap if being hit by the 10,000 year wave crest height. Numerical methods have been used for assessing wave-in-deck impact loads. The model tests discussed in this paper were conducted to be used as verification of the numerical codes. For the model tests two sea states along the 10,000 year contour line were considered. Several 3-hour (full scale time) realizations were calibrated in order to capture the natural variability of the most extreme crest heights. For wave deck impact problems, one is merely interested in the few very large wave crests out of a 3-hour simulation. A more efficient test scope would, therefore, be to generate only the largest wave groups of the realizations. For this reason the most extreme crest(s) per sea state were identified and most wave-in-deck tests were conducted by generating only the part of the time series containing the large crest(s). The wave calibration results were discussed in a previous paper, see [1]. For the wave-in-deck model tests, an existing North Sea jacket was built at scale 1:60 and instrumented in order to measure the global loads on the platform deck independently from the loads on the jacket itself. In this paper the model test setup as well as the measured wave-in-deck impact loads are discussed and compared to a simplified load prediction model. The presented results show that the simplified loading model, with wave properties based on Stokes 5th order wave theory, underestimates the measured horizontal deck loads.

Breaking waves have been studied for many decades and are still of interest as these waves contribute significantly to the dynamics and loading of offshore structures. In current MARIN research this awareness has led to the setup of an experiment to determine the kinematics of breaking waves using Particle Image Velocimetry (PIV). The purpose of the measurement campaign is to determine the evolution of the kinematics of breaking focussed waves. In addition to the PIV measurements in waves, small scale wave-in-deck impact load measurements on a fixed deck box were carried out in the same wave conditions. To investigate the link between wave kinematics and wave-in-deck impact loads, simplified loading models for estimating horizontal deck impact loads were applied and compared to the measured impact loads. In this paper, the comparison of the model test data to estimated loads is presented.

Long-crested waves are typically used in the design of offshore structures. However, the corresponding statistics, kinematics and loading are significantly different in short-crested waves and up to date, there is no state-of-the-art methodology to apply short-crested models instead. The objective of the “ShortCresT” Joint Industry Project was to take into account short-crestedness in the design of offshore structures against extreme waves based on a good description of their spectral characteristics, statistics, kinematics, breaking and loading and to deliver (empirical) design recommendations and methods. This paper gives an overview of the findings of ShorTCresT regarding wave crest and height distributions, a comparison of basin and field data, the role of wave breaking, the most realistic directional model, hindcast models as well as the related platform loading.

Experimental or numerical analysis of the response of ships and other floating structures starts with correct environmental modelling. The capabilities of numerical tools are rapidly expanding, but presently the evaluation of extreme events in waves (such as slamming, green water, air-gap exceedance) still requires a combination of experiments and different levels of numerical tools. The present paper describes recent efforts within the Maritime Research Institute Netherlands (MARIN) to improve experimental and numerical wave modelling and especially their combination. The ultimate objective is to be able to reproduce any wave condition from a basin or from sea in numerical tools and vice versa, including a sound treatment of basin effects, numerical effects and statistical variability. The aspects that are of importance in both types of wave modelling are first introduced, after which a number of examples of recent projects is discussed. It can be concluded that important steps were made towards linking experimental and numerical wave modelling, but there are some challenges common to all wave reproductions. Some future planned studies focussing on how to deal with them are discussed as well.

The TLP model tests in CresT [1] showed that there is a significant difference in the maximum load events due to long-crested and short-crested waves of same peak period and significant wave height. This decrease in load amplitudes for increasing spreading was not dominated by the reduction in crest heights, but related to a change in wave excitation. In ShorT-CresT wave-in-deck model tests were carried out with the focus on the physics of impact loading. The primary objective of the platform tests was to link crest height and wave impact with local and global loading on the deck. The model test results showed that the global vertical loads in short-crested waves can be similar to long-crested events, if the wetted deck area is comparable. In other words, the platform deck loading corresponds to the relative short-crestedness of the sea state: if the crest length is at least as large as the characteristic deck dimension, the loads are significantly larger than for lower crest lengths (step change). In this paper the results of the wave-in-deck model tests are presented and discussed. The analysis of the model tests is focused on a comparison between short-crested and long-crested impacts and a comparison of the measurements to a simplified loading model.