Tie-down systems are used to fasten drilling rigs to the deck of offshorestructures during harsh environmental conditions such as hurricanes. During HurricaneIvan (2004) and Katrina (2005), a number of offshore structures were moved and severaltie-down systems were damaged. In the present study, the reaction force and connectioncapacity of tie-down systems for a TLP and SPAR are investigated. The environmentalconditions are taken from the API Bulletin 2INT-MET which has been updated afterseveral major storms during 2004-2005. The hydrodynamic coefficients of the TLP andSPAR are obtained using a 3D diffraction/radiation panel method. The motions of theTLP and SPAR are then simulated in the time domain by using the hull-mooring-risercoupled dynamic analysis tool CHARM3D. Based on the simulated motion andacceleration time series, the inertial and gravity loads on derrick and skid base footingare calculated. In addition to the inertial-gravity loads, wind forces exerted on the derrickare also calculated. All the external forces and resultant hull motions are simulated for100-year, 200-year and 1000-year storms to observe the derrick structural integrity withincreasing environmental intensity. Various environmental headings are also considered to find the maximum reaction forces. In the present method, the phase differencesbetween gravity-inertia forces and wind forces are taken into consideration to obtainmore realistic loads on derrick and skid base footings. This research shows that themaximum and minimum load values are appreciably higher for the SPAR. In addition,the direction of external forces is also important to determine maximum reaction forceson footings. The capacities of the clamps in slip, bolt tension, and bolt shear can be alsoanalyzed using the resultant data to provide guidance on appropriate design values.
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