Steep fronted waves can induce extremely large impact pressures on the hull of moored ship-type offshore structures. Several incidents confirmed that these loads can result in significant damage to the bow. As part of the SAFE-FLOW Joint Industry Project these loads were investigated with a dedicated series of model tests. Detailed analysis of the results gave important insight in the wave impact hydrodynamics. It was observed that the local steepness of the waves has a large influence on the magnitude of the occurring bow slams and that the slamming could be characterised as a wave crest phenomenon. Therefore high pressures can be experienced in the upper bow and forecastle. The paper first presents the test philosophy, set-up and dedicated measurements. Then the developed prediction method and its results will be discussed in detail.
Several incidents in recent years have revealed that wave impact loading is an important issue for moored floating production systems. Wave impact damage has been experienced by both the Foinaven and Schiehallion FPSOs. Impact loading differs from other types of wave loads by their very short rise time. The rise times, defined as the time from the application of the slam till the peak in the loading, are in the order of milliseconds. The pressures resulting from the large impact loads are often described with a slam coefficient times the fluid density and the velocity squared. Most of these slam coefficients are based on drop tests in which a structure is dropped in the water with a known velocity. Campbell and Weynberg (1980) derived these coefficients for a cylinder. The magnitudes of the slam coefficients provided in literature differ significant and for application for the wave loading problem the impact velocities need to be known.