Wave Crest Forces on Offshore Platforms Using Irregular Wave Theory

Abstract

ABSTRACT Wave force on deck beams are not usually a consideration in offshore platform design. An air gap is provided between the crest of the design wave and the lower deck. Therefore, no standard procedures for predicting wave forces on deck beams have been developed. Large platform subsidence introduces the possibility of waves contacting the deck beams if no other remedial structural work is undertaken. Wave forces on deck beams could be an important consideration when confirmingplatform structural integrity for those platforms which are not designed for subsidence. Standard procedures were developed for predicting wave forces on deck beams using regular waves as described by stokes Fifth Order Wave Theory. Regular wave theory is commonly used for offshore platform design; however, it is recognized by designers as a conservative approach since "real world" sea states consist of an irregular wave spectrum. A theoretically valid procedure based on irregular wave theory was developed which approximates "real world" conditions better than do the regular wave procedures. This report presents a method of predicting wave forces on deck beams using irregular waves derived from JONSWAP spectra. Also presented are procedures for predicting the number of wave contacts on a beam during storms, and the duration of wave loading on the deck in seconds. The total load to the platform deck by irregular wave forces may be lower than conventional predictions for any level of subsidence. The indications are that Subsiding structures would also experience l7sS cumulative metal fatigue damage at any g1ven subs1dence level than that predicted by the stokes Fifth Order Wave Theory. INTRODUCTION Wave pressures of design waves and breaking waves on deck beams of offshore structures have been the Subject of a considerable amount of research as well as model tank testing. P. Broughton (Reference [1]) reports on model testing on the subsiding North Sea Ekofisk platform 2/4-C as well as the nonlinear structural calculations which followed. These calculations proved that this structure still had a considerable reserve strength (through elastoplastic behavior) after wave crests would start to reach the cellar deck beams. Model tank tests by Kjeldsen and stork (Reference [2]) indicated that total in-line wave forces and overturning moments measured in breaking waves exceed those measured in the much higher non-breaking waves by a factor of three. They concluded that wave pressures computed from a regular lO0 -year design wave specified only by its height and period will not always lead to a conservative wave force computation on bluff bodies in the wave crest zone. This conclusion certainly would seem true if wave crests reach thebluff areas of platform decks when forces have a slamming character. Wave slamming is known to cause large wave forces, which in some cases are known to have caused disaster. The most well known event is said to be the tragic capsizing of the semi submersible "Ocean Ranger" offshore Newfoundland. The capsizing was initiated by the impact of a breaking wave, causing a control room window to fail.