ON THE DISTRIBUTION OF WAVE IMPACT LOADS ON OFFSHORE STRUCTURES

For offshore structures in harsh environments, horizontal wave impact loads should be taken into account in design. Shafts on GBS structures, and columns on semisubmersibles and TLPs are exposed to impact loads. Furthermore, if the crest height exceeds the available freeboard, the deck may also be exposed to wave impact loads. Horizontal loads due to waves impacting on the structure are difficult to quantify. The loads are highly intermittent, difficult to reproduce in model tests, have a very short duration and can be very large. It is difficult to calculate these loads accurately and the statistical challenges associated with estimating a value with a prescribed annual probability of occurrence are formidable. Although the accurate calculation of crest elevation in front of the structure is a significant challenge, industry has considerable experience in handling this problem and the analysis results are usually in good agreement with model test results. The present paper presents a statistical model for the distribution of horizontal slamming pressures conditional on the incident crest height upwave of the structure. The impact load distribution is found empirically from a large database of model test results where the wave impact load was measured simultaneously at a large number of panels together with the incident crest elevation. The model test was carried out on a circular surface piercing column using long simulations of longcrested, irregular waves with a variety of seastate parameters. By analyzing the physics of the process and using the measured crest elevation and the seastate parameters, the impact load distribution model is made seastate independent. The impact model separates the wave impact problem in three parts: - Given an incident crest in a specified seastate, calculate the probability of the crest giving a wave impact load above a threshold. - Given a wave impact event above a threshold, calculate the distribution of the resulting peak load. - Given a peak load, calculate the distribution of slamming pressures at one spatial location. The development of the statistical model is described and it is shown that the model is appropriate for fixed and floating structures and for wave impact with both columns and the deck box.