Figure 2: Correlation between air gap and deck impact load


Screening for green water loads on the breakwater of a containership Within CRS Green Water Dynamics, the screening methodology was applied to green water loads on the breakwater of a containership at forward speed in waves [4]


number of realisations of two sea states were measured to capture the natural variability of the most extreme crest heights. This dataset was used to find suitable indicators for wave impacts on the deck. It turns out that the crest height of the undisturbed wave at the location of the platform centre is a good indicator. Figure 2 shows the correlation between the remaining air gap (deck height minus crest elevation) and the wave impact load. The platform encountered 38,500 waves in this test campaign, resulting in 100 impacts (the events in the upper right quadrant in Figure 2). Using air gap as an indicator, these 100 impacts could have been generated with just 350 deterministic waves (the events to the right of the vertical broken line). The basin time can then be used to gather much more statistical data, resulting in more reliable impact load distributions and safer designs.


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CFD is well able to capture these loads in predefined wave events selected from an experimental, three-hour irregular wave condition (Figure 1). However, how would one define which event to analyse if no model tests are available? Intuitively, relative wave elevation at the bow seems a good candidate to serve as an indicator for green water loads on the breakwater. This was validated by the experiments: indeed there is a strong relationship between this indicator and the loads. A good screening tool would be able to predict such an indicator value accurately and quickly, even if it cannot predict the loads. Examples of such tools are potential flow seakeeping codes, possibly combined with (non-linear) wave models. Thus, critical events for green water can be selected to analyse in detail in CFD or in a deterministic experiment.


Several lower-order models were validated against the measured loads from the experiments for the green water problem. This showed that the linear potential seakeeping tools could be used to screen for green water events, but their performance is expected to improve when non-linearity in the incoming wave is included in the relative wave elevations.


Screening for impact loads on a deck box Wave-in-deck model tests were carried out in our Offshore Basin with a jacket model to identify air gap exceedance (clearance between water line and deck) and measure related impact loads. A large


Future applications So far, most screening studies have focused on impact phenomena. However, the approach is applicable to other extreme events in waves as well. In theory, studies of bow-flare or stern slamming, course-keeping, propeller ventilation or even loss of stability or parametric roll could benefit from a similar approach. Many of them have a direct impact on the safety and efficiency of operations.


As explained for the containership tests, non-linearity in the incoming waves plays a large role in extreme wave events such as slamming and green water. It is therefore probable that the performance of simple screening tools increases if this non-linearity is included in the screening method. This is the topic of the new CRS project SCREAM that will start this year.


Figure 3: Example of wave impact on a fixed platform deck on jacket structure in MARIN’s Offshore Basin


References [1] Bandringa, H. and Helder, J.A. (2018). On the validity and sensitivity of CFD simulations for a deterministic breaking wave impact on a semi-submersible. 37th OMAE2018, Madrid, Spain.


[2] Bunnik, T., Stansberg, C.T., Pakozdi, C., Fouques, S. and Somers, L. (2018). Useful indicators for screening of sea states for wave impacts on fixed and floating platforms. 37th OMAE, Madrid, Spain.


[3] Bunnik, T., Scharnke, J. and De Ridder, E. (2019). Efficient indicators for screening of random waves for wave impacts on a jacket platform and a fixed offshore wind turbine. 38th OMAE Conference, Glasgow, Scotland, UK.


[4] Van Essen, S.M. (2019). Influence of wave variability on ship response during deterministically repeated seakeeping tests at forward speed. 14th PRADS Conference, Yokohama, Japan.


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