Torgeir Moan

Effect of Flap Type Wave Energy Converters on the Response of a Semi-Submersible Wind Turbine in Operational Conditions

In the present paper the effect of flap type wave energy converters on the response of a floating semi-submersible wind turbine is investigated and reported. Two different layouts with regard to the number of rotating flaps that are utilized are considered and compared with the case of a pure floating semi-submersible wind turbine. Comparisons of response in terms of stability, motions and internal loads are made for selected environmental conditions. The combined operation of the rotating flaps results in an increase of the produced power without affecting significantly selected critical response quantities of the semi-submersible platform.Copyright

Modelling and Analysis of a Semi-Submersible Wind Turbine With a Central Tower With Emphasis on the Brace System

This paper deals with analysis of the OC4 DeepCWind semi-submersible wind turbine, which is provided by NREL through the OC4 project. This concept is a three-column semi-submersible supporting a 5 MW wind turbine on an additional central column.The fact that the semi-submersible floater needs a large water line restoring moment to achieve sufficient stability and the control of the cost based on the steel weight make the design of braces and pontoons very challenging. Effective methods are needed to check the strength of the brace system based on the response forces and moments in the braces under different design environmental conditions, while the floating wind turbine is needed to be considered as an aero-hydro-servo-elastic system.A novel modeling methodology based on the code Simo/Riflex is introduced in this paper. Simo/Riflex is a state-of-the-art code that can account for the coupling effect between rigid body motions and slender structures (e.g. mooring lines, braces and blades) in the time-domain. Simo/Riflex can be combined with Aerodyn, which is a state-of-the-art aerodynamic code, to model the floating wind turbine as an aero-hydro-servo-elastic system, as well as be combined with simplified aerodynamic codes (e.g.TDHMILL) to improve the efficiency of the numerical simulation.The novel modeling method can give the forces and moments in the brace system of the floater under hydrodynamic and aerodynamic loads in the time-domain. In order to get the structural response of the braces, the side columns and the central supporting column are modeled as independent rigid bodies in Simo while the braces are modeled by beam elements in Riflex. Master and slave relationship is applied at the joints in between of the columns and braces.As an application example, the novel modeling method based on the code Simo/Riflex+TDHMILL, which is capable of modeling the floating wind turbine as an aero-hydro-elastic system, has been used to carry out Ultimate Limit State (ULS) design check for the brace system of the OC4 DeepCWind semi-submersible wind turbine based on relevant standards, i.e. NORSOK N00-3, NORSOK N-004, IEC61400-1, IEC61400-3.The modeling method can also be used by other codes which have similar features as Simo/Riflex.Copyright

Design and Analysis of a Braceless Steel 5-MW Semi-Submersible Wind Turbine

This paper introduces the design data and numerical analysis of a braceless steel semi-submersible wind turbine. The hull of the semi-submersible wind turbine is designed to support a reference 5-MW horizontal axis wind turbine at a site in the northern North Sea. The hull is composed of a central column, three side columns and three pontoons. The side columns and pontoons are arranged radially outward from the central column which is used to support the wind turbine. The side columns form the corners of a triangle on the horizontal plane and are connected by the pontoons to the central column at the bottom to form an integrated structure. Numerical analysis has been carried out to analyze the intact stability, natural periods and modes and global dynamic responses in winds and waves. Results of the numerical analysis show that the design has very good intact stability, well designed natural periods and modes, moderate rigid-body motions in extreme environmental conditions and a reasonable structural design. This paper emphasizes the structural responses of the hull considering both the global and local load effects. The global forces and moments in the hull are calculated by carrying out time-domain global analysis and used as inputs for simplified ultimate limit state design checks for structural strength of the hull. The design can be used as a reference semi-submersible wind turbine.

Modeling and Analysis of a 5 MW Semi-Submersible Wind Turbine Combined With Three Flap-Type Wave Energy Converters

Semi-submersible floating structures might be an attractive system to support wind turbines and wave energy converters (WECs) in areas with abundant wind and wave energy resources. The combination of wind turbines and WECs may increase the total power production and reduce the cost of the power.A concept of a semi-submersible with a 5 MW horizontal axis wind turbine combined with three flap-type WECs is presented in this paper. The concept is named as Semi-submersible Flap Combination (SFC). The WECs of the SFC are inspired by an optimized bottom-fixed rotating flap-type wave energy absorber. Each WEC of SFC includes an elliptical cylinder, two supporting arms, a rotational axis and a power take off (PTO) system.A time domain numerical modeling method for the SFC is presented. The numerical model is using the state-of-the-art code Simo/Riflex/Aerodyn. Linear rotational damping is introduced to model the effects of the PTO system. The choice of a PTO damping coefficient and of the mass of the elliptical cylinders has a significant effect on the power generated by the WECs. Such effects have been addressed and discussed in the paper through a sensitivity study.Copyright

Methodology for assessment of the allowable sea states during installation of an offshore wind turbine transition piece structure onto a monopile foundation

In this paper, a methodology suitable for assessing the allowable sea states for installation a transition piece (TP) onto a monopile (MP) foundation with focus on the docking operation is proposed. The TP installation procedure together with numerical analyses are used to identify critical and restricting events and their corresponding limiting parameters. For critical installation phases, existing numerical solutions based on frequency and time domain analyses of stationary processes are combined to quickly assess characteristic values of dynamic responses of limiting parameters for any given sea state. These results are compared against (nonlinear and non-stationary) time domain simulations of the actual docking operations. It is found that a critical event is the structural damage of the TPs bracket supports due to the potential large impact forces or velocities, and a restricting installation event (not critical) is the unsuccessful mating operation due to large horizontal motions of the TP bottom. By comparing characteristic values of dynamic responses with their allowable limits, the allowable sea states are established. Contact-impact problems are addressed in terms of assumed allowable impact velocities of the colliding objects. A possible automatic motion compensation system and human actions are not modeled. This methodology can also be used in connection with other mating operations such as float-over and topside installation.

On the Joint Distribution of Excursion Duration and Amplitude of a Narrow-Band Gaussian Process

The probability density of crest amplitude and duration that exceeds a given level is used in many theoretical and practical problems in engineering that are subjected to fluctuating loads such as wind and wave loads. The presently available joint distributions of amplitude and period are limited to excursion through a mean-level or to describe the asymptotic behavior of high level excursions. This paper extends the knowledge by presenting a theoretical derivation of probability of wave exceedance amplitude and duration for a stationary narrow-band Gaussian process. A density function is suggested that has the salient feature to depend only on the three lowest spectral moments <inline-formula> <tex-math notation=LaTeX>

Dynamic Analysis of Floating Wind Turbines During Pitch Actuator Fault, Grid Loss, and Shutdown☆

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Stochastic dynamic load effect and fatigue damage analysis of drivetrains in land-based and TLP, spar and semi-submersible floating wind turbines

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A fully coupled method for numerical modeling and dynamic analysis of floating vertical axis wind turbines

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Wind-Wave Misalignment Effects on Floating Wind Turbines: Motions and Tower Load Effects

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