Dynamic Analysis of Splash-zone Crossing Operation for a Subsea Template

Subsea templates are steel structures used to support subsea well compo­nents.Normally,offshore crane vessels are employed to install them to the target location on the seabed.Crossing the splash-zone during the lowering of a subsea template is considered the most critical phase during the instal­lation due to slamming loads and needs to be studied to provide the oper­ational weather criterion during the planning phase.In this study,dynamic response analysis has been carried out to evaluate the allowable sea states for the plash-zone crossing phase of the subsea templates.The numerical model of the lifting system,including the crane vessel and the subsea tem­plate,is firstly built in the state-of-the-art numerical program SIMA-SIMO.Then,dynamic analysis with time-domain simulations is carried out for the lifting system under various sea states.The disturbed wave field due to the shielding effects from the installation vessel is considered when calculat­ing the hydrodynamic forces on the template.Statistical modelling of the dynamic responses from different wave realizations is used to estimate the extreme responses of various sea states.The application of the generalized extreme value distribution and Gumbel distribution in fitting the extreme responses is discussed.Moreover,the influence of the shielding effects from the vessel,as well as the influence of the changing size of the suction anchor on the hydrodynamic responses and the allowable sea states are studied.

Numerical analysis of an over-boarding operation for a subsea template

Subsea templates are normally transported to the installation site on the deck of a crane vessel.After being lifted off from the deck,the template is 1)over-boarded from the initial location to the target position by the side of the vessel;2)lowered through the splash zone;3)further lowered down to the seabed and 4)finally positioned and landed.All the mentioned phases should be evaluated.Usually,the splash zone crossing phase is taken as the critical phase and analyzed to define the installation weather criterion.The over-boarding phase has not been the focus of analysis due to a large involvement of human actions and little involvement of hydrodynamic effects.During offshore operations,the offshore manager may decide to decrease the defined installation weather criterion if the risk of the personnel safety on deck during over-boarding phase is considered high.Thus,it is of great need to quantify the operational criterion for such operation.The objective of this paper is to perform numerical analyses and define the allowable sea states for a safe over-boarding operation.The numerical analyses using time-domain simulations have been performed in various sea states.Tugger lines have been modelled to control the motions of the template during the operation.The pendulum motions of the subsea template are considered as the critical responses for the assessment of the allowable sea states.