Global analysis of floating offshore wind turbines with shared mooring system

Floating wind turbines (FWTs) with shared mooring systems can be one of the most cost- effective solutions in reducing mooring costs. First, the static configuration of a shared line is estimated using the elastic catenary equation. The present study investigates the global responses of two FWT with a shared mooring system. Two shared mooring configurations with different horizontal distances between the FWTs are considered. In the first configuration, the FWTs are placed 750m apart; and in the second configuration, they are placed 1000m apart. Two different environmental conditions (ECs) are used to simulate the global responses of the system in time domain. The shared mooring line results in higher extreme motions in surge and sway (degree of freedoms) DOFs due to the reduction of mooring restoring stiffness. The lower mooring restoring stiffness can be attributed to the reduction of one seabed anchoring point for each FWT as compared to a single FWT with three anchors installed. In the rotational DOFs, the shared mooring line configurations result in slight mean offset in each direction and significant increase in the motion standard deviations. This is caused by the reduced mooring stiffness associated with the change in platform orientation.

Propulsion Performance Analysis of Wave-powered Boats

With the development of oceanographic research and marine environment protection, mobile marine platforms are applied for ocean observation for a long journey. Wave-powered boats are capable of applying wave motion to propel itself and make a long-duration survey. This paper presents the dynamics of the wave-powered boat under the excitation of the heave motion and pitch motion. Taking the wave-powered boat with double fins as an example, the heave and pitch motions of the boat are obtained by ANSYS-AQWA firstly. Then the relationship between propulsion performance and three factors, including wave height, wave period, and restoring stiffness of torsion spring, was analyzed through multibody dynamics software ADAMS. With the increase of sea state from level 1 to level 4 the average propulsion speed increased from 0.4m/s to 1.4m/s. Under the same wave height and period, with the increase of restoring stiffness of torsion spring from 0.0125N·m/deg to 0.3N·m /deg, the propulsion speed of the wave-powered boat increases first and then decreases, and there exists an optimum stiffness. Through the calculation it is found that when the restoring stiffness of torsional spring is increased from 0.025N·m /deg to 0.2N·m /deg with the sea state level 1 to 4, the wave powered boat has better propulsion performance.

Non-Linearly Restoring Performance and its Hysteresis Behavior of Dynamic Catenary

Catenary is increasingly used as mooring-line and riser system as the water depth gets larger due to its lower cost and easier installment. Its dynamic response and restoring performance become more complicated, as the length of the mooring-line become larger, and the structural and fluid dynamics the mooring-line become consequently more obvious. Compared to the quasi-static method where the static restoring force is mainly involved, the dynamic behaviors and its hysteresis of the catenary mooring-line are considered here so as to comprehensively examine the non-linearly restoring performance of mooring-lines. Based on the 3d dynamic vector equations along with the modified FEM simulations, the hysteresis character of the restoring stiffness and the influences of the catenary dynamics on its restoring performance are presented and discussed. It is found that, principally owing to the damping and inertial effect coming from the fluid and structural dynamics, the restoring force of the mooring-line depends on both the structural displacement and velocity. Moreover, the dynamic stiffness behaves as a hysteresis loop, instead of a curve. Our numerical results show that the energy consumption during one period rises nonlinearly with the increase of the body frequency ωd and amplitude A0. And, the influence of nonlinear restoring stiffness on the structural response along with the slack-taut phenomenon caused by structural /hydrodynamic inertia and damping is discussed.

Dynamic Analysis of a Tension Leg Platform Under Combined Wind and Wave Loads Within the Typhoon Area

The dynamic analysis of a Tension Leg Platform (TLP) is investigated by combining wind and wave loads within the typhoon area. By considering different typhoon parameters, such as the tangential velocity, the radius of maximum velocity, the translational direction and velocity, the model of typhoon is established. The wave height and period are obtained by the empirical formula related to the parameters of typhoon. The nonlinear restoring stiffness of TLP is derived with the set-down motion of platform and the coupled motion of the tension leg and platform. The results in this paper indicates that typhoon has a major impact on the safety of the platform in production operation, and it is also a threat to the strength of tension legs and risers.

DESIGN OF ISOLATED BRIDGES USING POLYNOMIAL FRICTION PENDULUM ISOLATOR

This paper is aimed to develop a design procedure of Polynomial Friction Pendulum Isolator (PFPI), a various-frequency sliding isolator, for decreasing the seismic responses of isolated bridges. Although sliding isolators have been widely used to mitigate seismic hazard, it may be not effective in decreasing the seismic responses of isolated structures subjected to near-field ground motions. The sliding surface of the PFPI is defined by a sixth-order polynomial function to avoid resonance under near-field ground motions. The restoring stiffness of the PFPI possesses softening section as well as hardening section. The structural acceleration response can be decreased by decreasing the restoring stiffness in softening section while the structural displacement response can be decreased by increasing the restoring stiffness in hardening section. However, it is difficult to determine the design parameters of PFPI in practical implementations. This study proposes a design procedure for the PFPI based on the bridge seismic design code in Taiwan. Designers can follow this procedure to easily design the bridge with PFPIs which satisfies the requirements of the code. The bridge with PFPIs designed by using this procedure is analyzed to realize the dynamic nonlinear responses of the bridge under artificial strong earthquake. The results show that the PFPIs effectively decrease the seismic responses of isolated bridges as compared with non-isolated bridges.