An Experimental Investigation of Wave Impacts on the Deck of a Gravity Based Structure

The occurrence of wave impacts is a critical feature in the design and re-assessment of many offshore structures. With evidence of increasing storm severity, and with subsidence an important characteristic of some mature fields, the quantification of impact loads arising on both the columns and the underside of these structures remains a difficult but important issue. In contributing to this debate, this paper presents the results of a physical model study of a gravity based structure. This comprises an arrangement of storage caissons located on the sea bed, with four large diameter surface-piercing columns supporting the topside structure at a significant height above mean sea level. The purpose of the study was to provide new physical insights into the nature of the wave-structure and the local wave-wave interactions. This has been achieved by employing a large number of traditional wave gauges within the vicinity of the model structure and by complementing this data with two new measuring techniques in order to quantify the behaviour of the water surface in close proximity to the columns. These observations provide a clear understanding of how a large volume structure modifies the incident waves and why wave-structure and subsequent wave-wave interactions lead to a higher probability of wave impacts on the overlying deck. In particular, considerable attention was paid to the nature of the wave run-up on the front face of the columns. The results show that far from being a highly localised effect, involving a thin sheet of water, the run-up associated with a steep wave can involve significant volumes of water, traveling at very high velocities, leading to the occurrence of large impact pressures acting over substantial areas. Estimates of the run-up velocity are coupled with slamming coefficients to provide load predictions which are comparable to the measured wave-in-deck loads. Perhaps surprisingly, it is also shown that the highest and steepest waves do not always cause the largest impacts.