Loads on Tie-down Systems for Floating Drilling Rigs During Hurricane Conditions

Tie-down systems are used to fasten drilling rigs to the deck of offshore structures to prevent them from moving in harsh environmental conditions such as hurricanes. During hurricanes Ivan (2004) and Katrina (2005), failures in tie-down systems allowed several rigs to move and be damaged. In this study, the reaction force and connection capacity of tie-down systems for a tension leg platform (TLP) and a SPAR are investigated by analyzing the dynamics of the drilling rig and its substructure. For the analysis and simulations in time domain, the hull-mooring-riser coupled dynamic analysis program developed by the authors was used. The 100-year and 1000-year hurricane conditions are taken from the API Bulletin 2INT-MET, which reflects updated conditions following major storms during 2004-05. Based on the simulated motion and acceleration time series, the inertial and gravity loads on derrick and skid base are calculated, in addition to dynamic wind forces for various wind-wave-current (W-W-C) collinear headings. Then the loads that could cause tensile/shear/slip failure at derrick and skid-base footings are computed and analyzed. A new concept called instant/progressive slip failure is introduced and demonstrated. Also assessed are the contributions of the centrifugal forces and rotary inertia moments that are often neglected in design practice. Copyright by The International Society of Offshore and Polar Engineers.

Loads on Tie-down Systems for Floating Drilling Rigs During Hurricane Conditions Conference Paper