Drilling Tender was experiencing very low operability in calm environment due to uncharacteristically high roll motions which was making the installation of Drilling Equipment extremely difficult. A FEED study was conducted to identify the root cause of the problem and to propose a feasible solution.
Based on the FEED studies, the following were identified.
- Vessel is resonating with swells of around 10s to 12 s.
- The Roll Period of the Vessel is more than that predicted by the Motion Analysis (7 s)
- The underestimation of the Roll period was due to inaccurate estimation of lightship
- The conventional operational solution of draft and GM variation to alleviate the motions was not favorable
- The vessel should have additional means to control the resonant roll motions
In order to have the simplest solution with minimum bearing on the layout and least implication on vessel’s stability the following was proposed.
- Four pairs of controlled flooding tanks, which can be opened/closed to the sea by means of pneumatically, controlled sea chest/air chest valves
- The time period of the tanks and thereby the roll damping can be tuned with the fundamental period of the operating sea state by controlling the valves
- The number and location of the flooding tanks were chosen such that the same has least effect on vessel’s stability, trim and capacities