This paper describes a fundamental study about the active method of applying entire mooring forces or behaviors of any point of a mooring line using a number of actuators. The application limitation was investigated by changing the replacement ratio and oscillated environment. Two kinds of one-degree-of-freedom (1DOF) forced oscillation model tests were conducted to determine the limitation: one using a partial mooring line model and the other using a full-length mooring line model to obtain the correct value of the top tension and the motion of the actuator mounted position. In addition, a number of numerical studies were conducted to investigate calculation accuracy by truncation and the effect of signal transfer delay. These studies showed that the delay and deterioration of calculation accuracy by truncation cause fatal differences on the top tension amplitude when oscillated by the wave period range. However, the deterioration has a bad effect than the delay in a long period range.
In a moored floater model experiment, a number of truncated methods have been used to overcome the limit of depth and width of a model test basin. In many cases, the horizontal static stiffness of a mooring system is modeled using a number of springs and wires (Ma et al., 2019). A final evaluation is conducted in a simulation model with the correct dynamic effect. A disadvantage of this method is that it takes a lot of time. In addition, the correct behavior of a mooring line and floater model, including viscous damping, line stiffness, etc., cannot be observed directly. To address this problem, a number of active methods have been studied that apply entire mooring forces or behavior of any point of a mooring line using a number of actuators. These methods are used with numerical simulation software under real-time conditions to calculate actuators signals. By using these methods, all mooring systems (Vilsen, Sauder, and Sorensen 2017) or some partial line lengths (Cao and Tahchiev, 2013) have been replaced by numerical simulations.
This paper focuses on an active method that replaces partial line lengths by numerical simulation. Although this method has been studied by a number of researchers (Dahl, 2010, Cao and Tahchiev, 2013), they mainly focused on numerical conceptual studies and were not studied in an actual basin. A previous study (Cao and Tahchiev, 2013) evaluated only one cut ratio, which is the ratio between partial line length and full line length. The value was 0.625 and it was composed of 1,000 m's partial line length and 1,600 m's full line length. For actual model tests in basins, it is necessary to evaluate various ratios. The research conducted a forced oscillation test using the top position of the line. However, the combination of the amplitude and oscillation period was limited. To acquire knowledge for the actual model test in basins, various combinations should be evaluated.