Flow-Induced Motions (FIM) are an issue for floating offshore structures, such as multi-column platforms, as the phenomenon can decrease the fatigue life of the mooring, riser, and cable systems. The new concept of Floating Offshore Wind Turbines (FOWT) have a multi-column design that may be subjected to FIM. In the past, FIM was studied mainly for Oil & Gas platforms installed in deep waters. However, the FIM phenomenon of FOWT has been insufficiently explored. To rectify this, model tests were performed for the Semisubmersible (SS) Floating System design developed for the DeepCwind project (OC4 Phase II). This paper will investigate the presence of FIM and show its importance in the design process of FOWT. Three different incidence angles of the current were tested, namely 0, 90, and 180 deg. For each heading, thirty reduced velocities were tested, across the range 8,000<Re<70,000. The results showed amplitudes in the transverse direction of around 70% of the diameter of the platform column, which is similar to those observed for deep-draft (DD) SS with circular columns. Note that these amplitude values are larger for a floater with a circular column, than for a platform with square columns. The results showed that as FIM occurred for this specific FOWT SS, its effect has to be considered in the mooring system and electric cable design.

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Flow-Induced Motions (FIM) is an issue for multi-column platforms, and the phenomenon can decrease the fatigue life of the mooring, riser, and cable systems. In the past, FIM was studied mainly for platforms installed in deep waters. The new concepts of Floating Offshore Wind Turbines (FOWT) have multi-column design and may therefore observe FIM. However, FOWTs have been installed in shallow water and, in this case, the FIM remains insufficiently investigated. To address this issue, FIM model tests were performed for the – Semisubmersible (SS) Floating System design developed for the DeepCwind project (OC4 Phase II). The goal of this paper is to investigate the presence of FIM for this type of system to show the importance of FIM in the design of FOWT. Three different incidence angles of the current were tested, namely 0, 90 and 180 degrees. For each heading, thirty reduced velocities were tested. The results showed amplitudes in the transverse direction of around 70% of the diameter of the platform column, which is similar to the ones observed for the deep-draft (DD) SS with circular columns and larger than for the platforms with square columns. The results showed that FIM was present for this specific FOWT SS investigated and that it may thus be essential to consider when designing the mooring system, as an increment in the total cost of the platform may make the system economically unfeasible. When extrapolating the results for the full-scale configuration, the FIM synchronization occurred for current velocities from 0.5m/s up to 1.2m/s, and the maximum nondimensional nominal amplitudes for the motions in the transverse direction reached 70% of the external column diameter.

The Flow-Induced Motions (FIM) is an essential topic on multi-column platforms due to the effect on the mooring line fatigue life. Vortex-Induced Motions (VIM) or galloping behavior can be observed for an array of four columns with square sections. The presence of pontoons showed to be important for changing the flow around the array and promoting different amplitude behaviors of the motions in the transverse direction mainly. This article aims to understand the effect of the presence of four pontoons on the FIM of a semi-submersible platform (SS) with four square section columns. Model tests of a floating system supported elastically utilizing four springs were performed in a towing tank. Five different pontoon ratios were tested, namely P/L = 0, 0.25, 0.50, 0.75, and 1.00; where P is the pontoon height (the dimension in the vertical direction), and L is the length of the square column face. The draft condition was kept constant as H/L = 1.5; where H is the draft of the platform. The spacing ratio of the columns was S/L = 4; where S is the distance between column centers. Two incidence angles of the current were carried out, namely 0 and 45 degrees. The amplitudes in the transverse direction (direction perpendicular to the incidence current) decreased by increasing the pontoon ratio for 0 and 45-deg incidences. The pontoons positioned aligned to the flow significantly reduced the amplitudes in the transverse direction since the pontoon presence in this position modified the incident wake in the downstream columns. The pontoon presence needs to be well investigated to choose the best condition to avoid raising the FIM or mitigating the FIM.