Environmental Directionality Considerations in MODU Location Assessments

Abstract

ABSTRACT The effects of considering environmental directionality in the assessment of Mobile Offshore Drilling Unit (MODU) locations are examined in this paper. A specific North Sea jack-up location is reviewed. Traditionally, the suitability for operation of a MODU at a given location is determined by simultaneously applying the maximum design wind, wave, and current values simultaneously from the worst possible directions for assessing stresses, overturning moments, etc. These environmental loads are treated as a quasi-static load case. Stresses, overturning moments, and preload requirements resulting from this extreme load case are estimated and compared to design allowables. Many site-specific assessments are now beginning to include second-order effects and are also attempting to evaluate the dynamic responses of MODU's. Peak static loads and peak dynamic loads do not necessarily occur simultaneously. The possibility of overly conservative load and stress predictions exists if an extreme environment is simultaneously applied from the worst directions in these evaluations. An argument for considering rig orientation and environmental directionality is presented. Results obtained from a directionality assessment are reviewed. It is concluded that site assessments based on environmental directionality considerations are a reasonable and prudent indicator of safe MODU locations.