Motion response analysis of a Sevan FWT moonpool foundation

Blog written by Master Student in SFI BLUES, Borgar Larsen, 2021-2022, NTNU (collaboration with Sevan SSP).

For several years Sevan SSP have been pioneers within providing an alternative to the traditional ship-shaped, turret moored designs. The Sevan SSP hull is a cost-effective alternative, that with geostationary design avoids the need for a costly turret, while allowing for a larger number of risers. The Sevan SSP hull is a proven and credible alternative to traditional offshore, floating designs. As the offshore wind market is moving into deeper and harsher environments with larger turbines, there is a call for cost-efficient floating foundation technologies. For this reason Sevan SSP are trying to implement this hull into the floating offshore wind sector, as an innovative solution for foundations for floating wind turbines (FWT). As a part of my master’s thesis, experiments in operational conditions have been conducted on a two-dimensional representation of the Sevan SWACH foundation, together with numerical simulations, during the spring of 2022. The reason for this was to investigate if adding horizontal baffles on the internal walls may help contribute to reduced motion.

Figure 1: Concept image of the Sevan SWACH foundation for FWT.

As a means of mitigating motion, internal baffles fitted to the vertical walls of the moonpool does not seem to improve the overall motion response in operational conditions. Results from experiments indicated that while motion in heave experience a slight decrease from adding baffles, motion in pitch increases, as shown in Figure 4. A parametric study conducted numerically on different variations of drafts-to-moonpool diameter ratios reveals that the rigid body motions in pitch are highly sensitive to draft. Heave RAOs in operational conditions experience a significant reduction with increasing drafts, as well as a slight shift in peak periods. Cancellation periods also experience a slight shift towards higher periods. The RAOs in pitch experience the most significant changes, both in response amplitude and cancellation periods. Pitch motion is most sensitive for h/Dmp ≤ 0.67. Here h is the draft and Dmp is the moonpool diameter. This is shown in Figure 5.

Figure 2: Model used for experiments in Ladertanken without internal baffles (left), and model fitted with one internal baffle below the free surface (right)

Four different configurations were tested in experiments conducted in Ladertanken, at the Marine Technology Centre at NTNU in Trondheim. The same base model, shown in Figure 2, was used for all four configurations. The first configuration, C1, was the base model without baffles. Baffles mounted in different locations on the internal walls of the two-dimensional representation of the model made up the other three configurations. Configuration C3 was fitted with two baffles on each side, with a spacing of 60mm from each other, while configuration C2 and C4 was fitted with only one baffles on each side. The model was a floating freely, horizontally moored model, and subjected to incident regular waves with periods ranging from T = 0.6s − 1.3s in model scale. By using Froude scaling this corresponds roughly to T = 5s – 11s in full scale.

Each configuration showed some dependency on the different values of wave steepness that was tested. When the different configurations were compared to each other the dependency on wave steepness was more noticeable. By adding baffles, the motion response in each DOF was dampened for steeper waves. Configuration C2 and C4 showed the smallest increase in maximum motion. C3, with two baffles fitted on each side, showed less dampened response than C2 and C4. C1, with no fitted baffles, showed the most increase in maximum motion, and least dampened response. The surface elevation of the moonpool showed only small changes in response for each configuration, for both steep and less steep waves.

In general, experimental and numerical results were seen as satisfactory with regards to validity from comparison. Some discrepancies were present due to viscous effects in the experiments that were not accounted for in the numerical simulations which assumes linear potential flow theory. This is especially noticeable in pitch RAOs shown in Figure 5, where nonlinear damping from vortex shedding was present due to the bottom skirts of the model, as well as the fitted baffles.

Figure 3: Heave RAO for each configuration compared to numerical results from configuration C1.  Wave steepness

Figure 4: Pitch RAO for each configuration compared to numerical results from configuration C1.  k = wave number.
Figure 5: Numerical RAO in pitch for configuration C1.