Experimental Flow-Induced Motions of a FOWT Semi-Submersible Type (OC4 Phase II Floater)

Abstract

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.

CFD code comparison, verification and validation for a FOWT semi-submersible floater (OC4 Phase II)

“…The first sub-section focuses on the validation case-study of a free floating 3D cylinder experimentally studied in Palm et al (2016) and used for validation purposes by Rivera-Arreba et al ( 2019). The second sub-section focuses on the decay study of the OC4 platform designed by Robertson et al (2014) and experimentally studied by Gonçalves et al (2020). Each sub-sections is divided into two parts with the former describing the numerical set-up and the latter presenting and analysing the results.…”

“…The study on the OC4 semi-submersible platform was performed with a scaled down (1:72.73) model of the original design. The scaled down model was studied experimentally by Gonçalves et al (2020) and the numerical results were compared with these experimental measurements. Surge and sway decay tests were performed with both solvers with restriction body motion (1DoF).…”

“…In OpenFOAM, an adjustable time-step is chosen with an initial value of 0.0025s and a maximum Courant number limitation of Co < 2 and an interface based Courant number limitation of Co α < 0.25. The resultant motion is matched with the experimental results of Gonçalves et al (2020) through comparison of the natural periods and damping coefficients. At first, only 1 DoF calculations are performed for each decay test focusing on the primary motion that is to be captured.…”

“…In this work, we will focus on two CFD solvers, namely the open-source generalist code OpenFOAM, and the community-based maritime-focused code ReFRESCO. The case-study is the decay analysis of the OC4 semi-submersible platform designed by Robertson et al (2014) and experimentally studied by Gonçalves et al (2020). The numerical results will be compared with the experimental data.…”

“…The original design for the semisubmersible was developed for the DeepCwind project and further optimised in the OC4 project work. This platform, in a scaled-down version (1:72.73), was experimentally studied byGonçalves et al (2020) in a towing-tank. In this work, the scaled-down model is numerically studied with OpenFOAM and ReFRESCO and compared with the experimental results ofGonçalves et al (2020).…”