Wave excited roll motion poses danger for moored offshore vessels such as Floating Production Storage and Offloading (FPSO) because they cannot divert to avoid bad weather. Furthermore, slack cargo tanks are almost always present in FPSOs by design. These pose an increased risk of roll instability due to the presence of free surfaces. The most common method of determining roll damping is roll decay tests, yet very few test have been performed with liquid cargo, and most liquid cargo experiments use tanks that span the entire width of the vessel; which is seldom the case for full scale FPSO vessels during normal operations. This paper presents a series of roll decay test carried out on a FPSO model with two two-row-prismatic tanks with different filling levels. To directly investigate the coupling between the liquid sloshing and the vessel motion, without modifying the damping, tests were performed at a constant draft. The equivalent linear roll damping coefficients consisting of linear, quadratic and cubic damping terms are analyzed for each loading condition using four established methods, the Quasi-linear method, Froude Energy method, Averaging method and the Perturbation method. The results show that the cubic damping term is paramount for FPSOs and at low filling levels, were the FPSO is more damped. Recommendations regarding the applicability of the methods, their accuracy and computational effort is given and the effect of the liquid motion on the vessel motion is discussed.

中文翻译：

---

两排水箱中FPSO纯轧辊伴随液体晃荡的非线性轧辊阻尼实验确定

波浪激发的横摇运动对停泊的海上船舶（如浮式生产储油库和卸油库（FPSO））构成危险，因为它们无法转移以避免恶劣的天气。此外，根据设计，FPSO中几乎总是存在松弛的液货舱。由于存在自由表面，这些增加了辊不稳定的风险。确定侧倾阻尼的最常见方法是侧倾衰减测试，但是对液态货物进行的测试很少，大多数液态货物实验都使用横跨整个船宽的液舱。在正常运行期间，全尺寸FPSO船舶很少出现这种情况。本文介绍了在FPSO模型上进行的一系列滚动衰减测试，该模型使用了两个具有不同填充水平的两排棱柱形储罐。为了直接研究液体晃动和容器运动之间的耦合，在不改变阻尼的情况下，以恒定的吃水深度进行测试。使用拟定线性方法，弗洛德能量方法，平均方法和摄动方法四种建立的方法，针对每种载荷条件，分析了由线性，二次方和三次方阻尼项组成的等效线性侧倾阻尼系数。结果表明，立方阻尼项对于FPSO至关重要，在FPSO阻尼较大的情况下，在低填充水平下。给出了有关这些方法的适用性，准确性和计算量的建议，并讨论了液体运动对血管运动的影响。使用拟定线性方法，弗洛德能量方法，平均方法和摄动方法四种建立的方法对每种载荷条件的二次和三次阻尼项进行了分析。结果表明，立方阻尼项对于FPSO至关重要，在FPSO阻尼较大的情况下，在低填充水平下。给出了有关这些方法的适用性，准确性和计算量的建议，并讨论了液体运动对血管运动的影响。使用拟定线性方法，弗洛德能量方法，平均方法和摄动方法四种建立的方法对每种载荷条件的二次和三次阻尼项进行了分析。结果表明，立方阻尼项对于FPSO至关重要，在FPSO阻尼较大的情况下，在低填充水平下。给出了有关这些方法的适用性，准确性和计算量的建议，并讨论了液体运动对血管运动的影响。