Demands for offshore structures such as FPSOs and semi-submersibles have been increased in developing offshore oil and gas fields. Offshore structures constructed in the shipyards should be transported safely to the oil fields. The structures are usually towed by tug boats and towing lines. Since towed offshore structures follow the desired course of the tug boats, the directional stability is one of the indices for safe transportation. If the directional stability is not secured enough, it will be difficult to maintain the course, and eventually, the overall transportation may fail in the worst case. There have been numerous researches to predict the course stability. The prediction on the directional stability of towed vessels in the design stage heavily depends on the model tests in marine industries. The model test is widely used for its reliability, but there are many limitations such as time, cost, measurements and test facilities. Also, it is almost impossible to re-design the hull form for the reason of a poor directional stability after the model test, since the basic design is nearly finished at the model test stage. More importantly, it is difficult to understand the ultimate instability with limited number of data from the model test. Besides the model test, numerical approaches have also been studied. Towing simulations in various conditions are possible through numerical approaches. However, hydrodynamic characteristics of vessels must be known in advance. Empirical or regression formulas can be applied to estimate the hydrodynamic coefficients, but it is applicable only within the database. Also, small changes on the hull form cannot be considered. Especially, the database of the offshore structures is very limited, and thus it is difficult to apply the empirical or regression formulas. Recently, CFD approaches are found to be a useful way to overcome the limitations of the model tests and the previous numerical approaches, e.g. enabling the calculation of various physical quantities. Since, it is more convenient to investigate the effect of hull form, draught and skeg, researches using CFD have been growing at a fast pace.
In the present study, practical methods of predicting and enhancing the directional stability of towed offshore structures using CFD at the initial design phase are discussed. A comparative study for the stability criteria, equilibrium position analysis and CFD towing simulation methods are performed. Towing model tests for typical barge type and ship type floating bodies are also carried out to provide the validation data. Single skeg and twin skeg are considered in the case of the barge type, and two trim conditions are taken into account in the case of the ship type. The hydrodynamic derivatives that are required for the assessment of the stability criteria and the equilibrium position analysis are obtained by performing the virtual drift and PMM simulation using CFD. The motion of the towed bodies is calculated by the direct towing simulation using CFD. The predicted results from the stability criteria, equilibrium position analysis and CFD towing simulation are compared with the towing test results, and the mechanism of instability phenomena is analyzed. It is shown that the directional stability of towing systems could be predicted quantitatively, as well as qualitatively, through the equilibrium position analysis and CFD towing simulation from a practical point of view. Studies to enhance the directional stability of towed FPSOs and semi-submersible are additionally performed by using the CFD towing simulations.
본 연구에서는 설계 초기 단계에서 침로 안정성을 예측하는 방법을 제안하고 안정성을 향상시킬 수 있는 방안을 모색하고자 한다. 예인 안정성을 예측하기 위해서 예인 시스템의 안정성 판별식을 이용하는 방법과 평형점을 구하는 방법, 그리고 시간영역에서 예인 시뮬레이션을 수행하는 방법, 이렇게 세 가지 방법에 대하여 연구를 수행하였다. 예인 시스템의 안정성 판별식과 평형점을 구하는 방법은 정적 해석법으로써, 여기에 필요한 부유체의 유체력 계수는 CFD를 이용한 사항시험과 PMM 시험을 통해서 예측하였다. 그리고 예인 시뮬레이션은 CFD를 이용하여 매 시간 간격마다 부유체에 작용하는 유체력을 계산하여 운동 방정식을 푸는 과정을 거친다. 예측 결과의 검증을 위해서 선박형 부유체와 바아지형 부유체에 대해서 예인 안정성 모형시험을 수행하였다. 각각의 부유체 형상에 대해서 안정 및 불안정한 경우를 선택하여 예인 안정성 예측 결과와 비교를 수행하였다. 그리고 예인 안정성 평가 방법에 대한 비교를 수행하였다.
추가적으로 시간영역에서의 CFD를 이용한 예인 시뮬레이션 기법을 이용하여 다른 형상을 갖는 부유체의 침로 안정성을 예측하였고, 안정성 향상을 위해 스케그 크기와 위치, 선미부 형상, 선수부 형상, 흘수 조건을 달리하여 결과를 검토하였다. 그 중에서 침로 안정성 향상 효과가 있는 안에 대해서 예인 안정성 모형시험을 수행하여 CFD를 통한 예측 결과와 비교를 수행하였다.
