| dc.description.abstract | The push for optimizing current technology in semisubmersible system enables reduction in semisubmersible overall size. One field that see potential optimization is the riser operation philosophy, which in certain cases become the restricting factor of utilizing smaller semisubmersible. In adverse sea-state riser stroke becomes large such that it results in large topside deck, hence large hull. The objective of this thesis is to employ the magneto-rheological damper (MR Damper) in riser system to ameliorate the riser stroke in storm event in order to reduce the deck spacing. This leads to reduction in deck size. In addition, this thesis looks into suitable semisubmersible that generates favorable motion to be used hand-in-hand with the MR damper. Finally, this thesis investigates the operational philosophy of the riser tensioner system and MR Damper that result in the most optimum riser stroke. The analysis develops a new shallow draft low-heave semisubmersible with the resulting favorable riser stroke of 7.55m (24.76ft) without MR Damper. Using this as the host vessel, the analysis finds that the most optimum use of MR Damper on riser tensioner system is by having the MR Damper constantly engage to the riser tensioner ring so that it can dissipate energy when the stroke is at its highest speed, that is when the stroke is at nominal (at zero stroke). Then applying linear damping coefficient of 9000 kN/ms^-1 onto the MR Damper reduces riser stroke from 7.55m (24.76ft) to 4.52m (14.81ft), well within the target stroke of 4.57m (15ft). The reduction of the stroke is attributed to the energy dissipation in semisubmersible heave motion and riser top motion, leading to lower heave motion and riser top motion. Implementing an up-scaled MR Damper numerical model results in total stroke close to linear damping MR Damper model, that is 4.91m (16.11ft). In conclusion, the application of MR damper in riser tensioner system results in a lower riser stroke, the utilization of a low-heave semisubmersible also results in a lower riser stroke, and the constantly-engaged MR Damper during storm event allows an effective use of MR Damper. | en |
