Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

A fundamental study of active method for experiment of mooring line included model
Mitsushi Watanabe
Ocean Engineering Department, National Maritime Research Institute, Japan
Mitaka, Tokyo, Japan,

by numerical simulation. Although this method has been studied by anumber of researchers (Dahl, 2010, Cao and Tahchiev, 2013), theymainly focused on numerical conceptual studies and were not studied inan actual basin. A previous study (Cao and Tahchiev, 2013) evaluatedonly one cut ratio, which is the ratio between partial line length and fullline 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 waslimited. To acquire knowledge for the actual model test in basins,
various combinations should be evaluated.

ABSTRACTThis paper describes a fundamental study about the active method of
applying entire mooring forces or behaviors of any point of a mooringline using a number of actuators. The application limitation wasinvestigated by changing the replacement ratio and oscillatedenvironment. Two kinds of one-degree-of-freedom (1DOF) forced
oscillation model tests were conducted to determine the limitation: oneusing a partial mooring line model and the other using a full-length
mooring line model to obtain the correct value of the top tension andthe motion of the actuator mounted position. In addition, a number ofnumerical 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 bytruncation cause fatal differences on the top tension amplitude whenoscillated by the wave period range. However, the deterioration has abad effect than the delay in a long period range.

On the basis of the aforementioned information, this study’s aim is
acquiring knowledge of the active method for practical application
through experimental and numerical study.

EXPERIMENTAL STUDY
A fundamental study that investigates the application limitation bychanging the cut ratio and oscillated environment was executed using a
vertical shaped line. The ratio consists of the length of a physicallymodeled mooring line and a replacement mooring line. Two kinds ofmodel tests were conducted to determine the limitation.

KEY WORDS: Mooring; Model Test; Truncated Model; ActiveMethod; Real-time Hybrid Simulation; Forced Oscillation

INTRODUCTION

1DOF forced oscillation test using partial mooring linemodel

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 thecorrect dynamic effect. A disadvantage of this method is that it takes alot of time. In addition, the correct behavior of a mooring line andfloater 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 areused with numerical simulation software under real-time conditions tocalculate actuators signals. By using these methods, all mooring
systems (Vilsen, Sauder, and Sorensen 2017) or some partial linelengths (Cao and Tahchiev, 2013) have been replaced by numericalsimulations.
This paper focuses on an active method that replaces partial line lengths

One of the model tests was a one-degree-of-freedom (1DOF) forced
oscillation test using a partial mooring line model. The test wasconducted in a temporary basin mounted on the floor of the offshore
engineering basin at the National Maritime Research Institute, Japan(NMRI). The water depth was set to 0.8 m. Figure 1 shows the top viewof the basin. The oscillator was mounted on the fixed sub carriage, andthe top of the mooring line was oscillated. The opposite end of the line
was connected to an actuator mounted on the bottom of the basin.
When starting the oscillation, the signal for the actuator was calculated
in real time and the actuator moved accordingly. The oscillation
frequency, amplitude, and cut ratio were changed, and the top tensionof the mooring line and motion of the cut point were measured. Table 1
shows the specification of the mooring line, Fig. 2 shows the cut ratio,
Fig. 3 shows the signal flow of the experimental system, and Fig. 41941