水深对动力定位能力的影响

Effect of water depth on dynamic positioning capability

在作业过程中,海工船需应对不同水深的海域,而水深对流力和波浪力有影响。针对这一问题,对比5个水深下的环境载荷,并应用二次规划算法求解推进器最大功率利用率,进而分析水深对动力定位能力的影响。结果表明,随着水深的减小,流力和平均波浪漂移力均呈上升趋势。不同水深下波浪力的差异集中在0.5~1.5 rad/s的波频范围,谐摇会显著增加其对应的艏摇力矩。横浪下,动态载荷的影响已不可忽略,且合力矩的方向由风力主导。环境载荷的变化直接影响了推进器的使用功率。船舶的动力定位能力受首部伸缩推功率的限制,对应100°浪向。当水深为15 m时,该推进器处于满负荷状态;随着水深增至50 m,功率利用百分数降至约60%。因此,对于现有吃水条件,当水深小于20 m,环境载荷和推进器功率骤增,对动力定位的安全性提出了极大挑战;超过40 m后,水深的影响已不再明显,可近似视为无限水深。

During operation,water depth is variable for offshore vessel,which poses effects on current and mean wave drift forces.For this reason,this work reveals the effect of water depth on dynamic positioning capability by comparing environmental loads at five different water depth.The corresponding thruster maximum power percentages are solved by quadratic programming method.The results demonstrate that with the decreasing of water depth,both current and mean wave drift forces show rising trends.The differences in wave drift force are concentrated in the range from 0.5 to 1.5 rad/s,harmonic roll can enhance yaw moments to a large extent.The effect of dynamic loads can not be neglected at beams,total moments are dominant by wind.The changes in environmental loads directly affects thruster power.Dynamic positioning capacity is restricted by retractable thruster at fore,corresponding to 100 degree wave direction.When water depth is 15 m,the thruster is at full-loaded state.With water depth increasing to 50 m,the maximum power percentage is correspondingly decreased to about 60%.In sum,for the given draft in this work,when water depth is less than 20 m,it has witnessed rapid increases in environmental loads and thruster maximum power percentage,posing great challenges to safe dynamic positioning.On the other hand,water depth exceeding 40 m can be approximately regarded as infinite depth.