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 during harsh environmental conditions such as hurricanes. During Hurricane Ivan (2004) and Katrina (2005), a number of offshore structures were moved and several tie-down systems were damaged. In the present study, the reaction force and connection capacity of tie-down systems for a TLP and SPAR are investigated. The environmental conditions are taken from the API Bulletin 2INT-MET which has been updated after several major storms during 2004-2005. The hydrodynamic coefficients of the TLP and SPAR are obtained using a 3D diffraction/radiation panel method. The motions of the TLP and SPAR are then simulated in the time domain by using the hull-mooring-riser coupled dynamic analysis tool CHARM3D. Based on the simulated motion and acceleration time series, the inertial and gravity loads on derrick and skid base footing are calculated. In addition to the inertial-gravity loads, wind forces exerted on the derrick are also calculated. All the external forces and resultant hull motions are simulated for 100-year, 200-year and 1000-year storms to observe the derrick structural integrity with increasing environmental intensity. Various environmental headings are also considered to find the maximum reaction forces. In the present method, the phase differences between gravity-inertia forces and wind forces are taken into consideration to obtain more realistic loads on derrick and skid base footings. This research shows that the maximum and minimum load values are appreciably higher for the SPAR. In addition, the direction of external forces is also important to determine maximum reaction forces on footings. The capacities of the clamps in slip, bolt tension, and bolt shear can be also analyzed using the resultant data to provide guidance on appropriate design values.