Installation Methodologies for a Tension Leg Platform Under Ocean Swell Conditions

Abstract

A common procedure of installing a Tension Leg Platform is to float it over the tendons and lower it over the tendons. A key phase of this procedure is lock-off, i.e. the instant when the tendons become firmly attached to the TLP hull. The lock-off position of the tendons on the hull should be within specified tolerances. Lock-off on the tendons higher than targeted results in a reduced draft which implies reduced displacement and therefore typically reduced payload capacity or reduced payload reserve. Lock-off on the tendons lower than targeted results in increase in draft and brings the deck closer to mean water which adversely affects the airgap requirements. Hence lock-off at the correct draft is a key aspect of a successful installation procedure. The general geometric properties of several TLPs are such that there is a significantly more heave motion under swell conditions than under sea states with much smaller peak periods (for the same significant wave height). These larger motions under swell conditions can make it harder to achieve the desired lock-off elevation. Such swell conditions are known to occur of the West Coast of Africa. Waiting for the swell to subside may not be an attractive alternative for TLP installation as these swells may be present for extended periods of time. Two engineering solutions are provided which reduce the heave motion of a free floating TLP. Both of these solutions are temporary changes to the TLP itself and should be implemented as a part of the TLP design spiral. One of the solutions implies an increase of TLP mass; the second solution reduces the vertical stiffness of the free floating TLP. The heave natural period is significantly increased in both methods, and heave motions are reduced. Both methods increase the heave response amplitude operator (RAO) for periods below 8 to 12 seconds and reduce the heave RAO magnitudes above 12 seconds. Implementing one of these methods in the installation procedure may be most applicable to situations where the installation window may contain prolonged periods of persistent swell.