FPSOの定点保持に関する研究(続報)

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Drift forces are generally too small to influence the oscillatory motions of the ship but they can nevertheress be important in certain circumstances, particularly in considering the drifting motions of ships in the horizontal plane. At zero or very low speeds large excursions may result from even a very small wave-induced force if it is steady over a long interval of time. In the previous paper, we examined the motions of FPSO system and FPSO-Shuttle tanker system under the external forces. Furthermore, we applied the ILQ optimal servo theory to design the control system of DPS for two systems. Generally, in the design of the control system for DPS loads due to current, wind, and wave drift forces are generally of equal importance. Therefore, in this paper, numerical simulations are used to assess control performances of FPSO under external forces. For numerical simulations, we carried out calculations by changing the values of the wave length, ζ, and the wave height, λ. In addition, in order to investigate optimal hawser length l_h, we evaluated hawser tension and control performance which are affected by changing the hawser length. As the result of the numerical simulations, we obtained the following conclusion. For ζ=1m〜2m and λ=1L_1〜2L_1 (L_1: Ship length of FPSO), control performances are affected not by the wave length and the wave height but the time constant. But for ζ≥3m and λ≤0.5L_1, since the wave length and the wave height are too large to influence the control performances, time constant should be selected as small as possible within the limits of achievable control objective. Finally, from the calculation for the change of hawser length, we take 0.7L_1〜1.0L_1 as the optimal hawser length. The reason for this result lies in the fact that for short hawser length, l_h<0.5L_1, the hawser tension tends to significant and the control performance is good. But for long hawser length, l_h>1.2L_1, the two items showed opposite tendency in comparison to the results of short hawser length.
社団法人日本船舶海洋工学会の論文
2002-03-20

FPSOの定点保持に関する研究(続報)

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