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Event screening will identify waves that do result in an impact (ticked in green) or not (crossed in red)


Joining forces to capture extreme loads


The possible occurrence of negative air gaps on offshore structures in storm conditions has recently attracted significant attention. A new JIP called Screenin’ aims to reduce the statistical uncertainties of wave-in-deck load estimates. Tim Bunnik & Joop Helder, t.bunnik@marin.nl


T


he consequence of having insufficient clearance between the water level and the underside of a platform


deck during an extreme wave event can be severe - the resulting wave-in-deck loads can compromise structural integrity or even lead to casualties. Increasing the deck level is however, associated with high costs. In order to achieve a sensible balance between risk and cost, accurate predictions of air gap and the associated wave-in-deck loading are required.


Obtaining sufficient information on the probability of occurrence of wave-in-deck events and the variability in the associated impact loading is challenging because extreme waves are rare. Prediction of air gap and wave loading is therefore associated with significant statistical uncertainty.


Statistical uncertainty MARIN has investigated the statistical distribution of extreme wave crests in two previous JIPs: Crest and ShortCrest. Recently, the line of research was advanced further by quantifying scale effects associated with wave basin testing of wave-in-deck loads in the BreaKin JIP (see page 16). Now, MARIN is starting up a new JIP to link these obtained insights to the probability of the occurrence of wave-in-deck loads - the Screenin’ JIP.


10 report


The traditional approach to address and reduce wave-in-deck uncertainty is to model test (or simulate) many 3-hour realisations of critical sea states. Although effective, this approach is time consuming and costly.


Faster than traditional methods Within the Screenin’ JIP, a novel approach will be investigated to obtain more statistical data within a limited amount of time. Simplified screening models will be used, in combination with the traditional ‘complete’ model (i.e. model test or CFD). The simplified screening models are used to identify which individual wave crests will result in impact and which will not. The screening models should incorporate sufficiently accurate physics for impact identification, but be much faster than the ‘complete’ model when it comes to screening a large number of sea states.


When the screening is completed, individual events can be run in the ‘complete’ model instead of running full 3-hour realisations. As such, much more information can be obtained on the probability distributions of the impact loads. The focus of this JIP will be on wave-in-deck loads, but it is expected that similar methodologies can also be applied to slamming or green water loading on FPSOs and offshore wind turbines.


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