Jin Wang

Conventional catenary mooring systems for floating offshore wind turbines (FOWTs) face significant challenges related to high installed cost and large seabed footprint. To address these challenges, this paper proposes a novel length-varying tension-fixed (LVTF) mooring concept characterized by a compact seabed footprint. Theoretical formulations are developed, and parametric study results are presented on the fundamental station-keeping characteristics of a single-anchor LVTF mooring concept for a 15 MW semi-submersible FOWT. Preliminary design of a multi-anchor passive LVTF mooring system in 100 m water depth with a mooring anchor radius of 70 m is performed for a 16 MW multi-column Spar-type FOWT for potential deployment in the South China Sea. A wave basin model test campaign is conducted to investigate the station-keeping performance of the passive LVTF mooring system for the Spar-type FOWT under extreme typhoon conditions. The model test results, including platform global motions and mooring line tensions, are compared with those of the conventional catenary mooring design. It is found that the small-footprint passive LVTF mooring system can provide adequate station-keeping performance for the 16 MW Spar-type FOWT with peak mooring line tension significantly lower than that of the equivalent catenary system. A preliminary cost analysis indicates that an installed-cost reduction of up to 65% may potentially be achieved in comparison with the conventional catenary mooring design.

Conventional catenary mooring systems for floating offshore wind turbines (FOWTs) face significant challenges related to high installed cost, large seabed footprint, and supply chain constraints. To address these challenges, this paper proposes a novel length-varying tension-fixed (LVTF) mooring concept characterized by a compact seabed footprint. Theoretical formulations are developed, and parametric study results are presented on the fundamental station-keeping characteristics of a single-anchor LVTF mooring concept for a 15 MW semi-submersible FOWT. Preliminary design of a multi-anchor passive LVTF mooring system in 100 m water depth with a mooring anchor radius of 70 m is performed for a 16 MW multi-column Spar-type FOWT for potential deployment in the South China Sea. A wave basin model test campaign is conducted to validate the design of the passive LVTF mooring system for the Spar-type FOWT under extreme typhoon conditions. The model test results, including platform global motions and mooring line tensions, are compared with those of the conventional catenary mooring design. It is found that the small-footprint passive LVTF mooring system can provide adequate station-keeping forces for the 16 MW Spar-type FOWT. A preliminary cost assessment indicates a potential installed-cost reduction of approximately 67% in comparison with the conventional catenary mooring design.

The dynamics of superharmonic resonant sloshing in a vertical cylindrical container are experimentally investigated. The container is first driven by longitudinal driving at a frequency near ω11/3. Three types of superharmonic single-crest wave regimes, i.e., stable planar, stable swirling, and irregular, are observed. Both the wave patterns and the corresponding frequency domains of planar and swirling at superharmonic resonance are similar to those at the primary harmonic resonance. However, the evolution of the irregular wave at the superharmonic resonance is considerably more complex than that at the primary harmonic resonance, because the superharmonic irregular sloshing involves six movement directions: longitudinal, transverse, counterclockwise, clockwise, and two diagonal directions. Then, the container is driven by longitudinal driving at a frequency close to ω21/2. Three types of superharmonic double-crest wave regimes, i.e., stable planar, stable swirling, and irregular, are observed. Interestingly, the configuration of these wave regimes in the (Ω/ω21, ax)-plane bears a striking similarity to that of the superharmonic single-crest wave regimes. Overall, the experimental measurements are qualitatively consistent with the theoretical predictions provided by the weakly nonlinear theory. Furthermore, the bandpass filtering method is employed to clarify the superharmonic double-crest resonance mechanism. The superharmonic double-crest wave patterns are considerably more complex than the superharmonic single-crest wave patterns, because the superharmonic double-crest wave can be regarded as a combination of a harmonic single-crest wave that oscillates at the driving frequency and a superharmonic double-crest wave that oscillates at twice the driving frequency.

Three-dimensional (3D) resonant liquid sloshing is experimentally investigated in a square-base container with and without a vertical baffle subjected to harmonic horizontal forcing. This study aims to understand the influence of baffle height on the sloshing response, including viscous damping, natural frequency, frequency ranges of different wave modes, wave profiles, and impact pressures on the sidewall and roof. A relatively large liquid depth and three different vertical baffles with varying heights are considered. To elucidate the experimental observations, a potential-based analytical solution for natural sloshing modes and an asymptotic modal system for the frequency ranges of wave modes are employed. Theoretically, four types of sloshing wave modes exist, i.e., planar standing, diagonal standing, regular rotating, and irregular chaotic wave modes, for a clean container. However, diagonal standing waves are difficult to experimentally observe because their effective frequency domain coincides then with those for the planar ones. Therefore, the other three wave modes are focused on herein. When the amplitude of the rotating and chaotic waves is large, the corner of the roof experiences extremely high impact pressures. Consequently, the frequency ranges of rotating and chaotic waves evidently decrease with increasing baffle height. Furthermore, the sloshing wave amplitude and maximum impact pressure at the roof decreases due to the suppression effects of the baffle.

The effect of ring baffles on suppressing the three-dimensional (3D) resonant sloshing in an upright cylindrical container is experimentally investigated. The main objectives of this work are to examine the effectiveness of various baffle configurations, to establish the stability boundaries of the stable steady-state waves in the unbaffled and baffled containers, to provide accurate experimental data for the verification of the analytical and numerical models, and to prompt future investigations. For this purpose, hundreds of sloshing experiments are conducted in a cylindrical container with or without a ring baffle. An analytical potential-flow solution and an asymptotic multimodal method are used to elucidate the experimental results. It is found that the vertical location of the ring baffle has small influence on the fundamental natural frequency of the system; however, it has a significant influence on the viscous damping and the damping rate increases gradually with the ascension of the baffle. When the distance between the baffle and the free liquid surface is sufficiently large, the system exhibits three types of resonant wave patterns, namely stable planar, stable swirling, and irregular chaotic. These wave patterns are qualitatively and quantitatively similar to those in the unbaffled container. When the baffle is near the free liquid surface, neither the chaotic waves nor the swirling waves take place, but a new wave pattern with the characteristic of multiple wave crests is observed. Probably, this is the first time that the 3D resonant sloshing in the baffled cylindrical container has been systematically investigated.

In order to systematically investigate the suppression effect of a vertical baffle on three-dimensional (3D) swirling and chaotic sloshing in a square-based tank subjected to horizontal harmonic excitation, hundreds of experiments are conducted in a clean tank and baffled tanks with three different configurations. Specifically, the vertical baffle is mounted on the tank bottom parallel to the longitudinal direction, the transverse direction, or the diagonal direction. This experimental work finds that there are four sloshing wave regimes in a clean tank—planar, square-like, swirling, and chaotic—which can be described by the asymptotic multimodal theory. Furthermore, there are only two wave regimes in a longitudinal-baffle tank, the planar and swirling regimes, and the occurrence of swirling requires that the excitation amplitude is sufficiently large. It is confirmed that the longitudinal baffle has a significant suppression effect on the swirling and chaotic motions of the sloshing waves, even though it is parallel to the direction of tank movement. Furthermore, the suppression effect of the diagonal baffle is similar to but somewhat smaller than that of the longitudinal baffle. However, when the transverse baffle is mounted on the bottom of the tank, it is difficult to excite the rotation of the sloshing wave. Therefore, the suppression effect of a bottom-mounted baffle depends largely on the included angle between the vertical baffle and the tank movement direction.