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Description:
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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 . |