Numerical prediction of green water loads on ships

The main objective of the present study has been to investigate problems related to shipping green water on deck of a floating vessel by use of numerical methods. A Navier-Stokes solver with a free surface capturing scheme, similar to the VOF method (Hirt and Nichols, 1981) has been applied. The green water problem occurs when the relative free surface motion exceeds the freeboard of a floating vessel and the deck of the vessel is wetted. In the present context only FPSO type vessels, with zero forward velocity have been investigated. Apart from an introduction to the numerical method and simple tests of the free surface capturing scheme, the study can be devided into two parts: First, a number of simplified test cases have been investigated, which are all related to green water loads. These cases only include elements of the full green water problem and are primarily used as verification of the numerical model. These cases include dam break, cylinder impact, tank sloshing and wave crest impact on horizontal structures. Second, three test cases closely related to green water problems have been investigated. These include wave run-up on a wall, a green water problem with a fixed vessel and a full green water problem with relative ship motion included. Great effoort has been made to validate and verify the numerical method and all computations have been verified by use of several computational grids with increasing resolution and validated by comparison to experimental data. Results from the dam break and cylinder impact cases document that the numerical model accurately can predict complicated free surface problems and both water height and pressures/ loads compares well with experimental data. The last stage of the dam break problem, where the water returns from impact with the wall on the opposite side of the tank is very sensitive to initial conditions, making this part of the problem difficult to reproduce exactly. This is also documented by comparison to results obtained by use of other numerical methods.