Vincenzo Nava

Progress on the experimental set-up for the testing of a floating offshore wind turbine scaled model in a field site:

This document describes design and realization of a small-scale field experiment on a 1:30 model of spar floating support structure for offshore wind turbines. The aim of the experiment is to investigate the dynamic behaviour of the floating wind turbine under extreme wave and parked rotor conditions. The experiment has been going on in the Natural Ocean Engineering Laboratory of Reggio Calabria (Italy). In this article, all the stages of the experimental activity are presented, and some results are shown in terms of motions and response amplitude operators. Finally, a comparison with corresponding results obtained using ANSYS AQWA software package is shown, and conclusions are drawn. The presented experimental set-up seems promising to test offshore floating structures for marine renewable energy at a relatively large scale in the Natural Ocean Engineering Laboratory field site.

Coupled Surge-Heave-Pitch Dynamic Modeling of Spar-Moonpool-Riser Interaction

Due to the rather intense ongoing development of deep water gas and oil fields, the technical community has devoted considerable attention to the dynamic behavior of Spar floating systems. Spar dynamics exhibits a highly nonlinear behavior due to the presence of various components such as mooring lines, moonpool, and risers. Certain studies have focused on the reduction of the heave response of single-degree-of-freedom Spar models due to the oscillations of water entrapped in the moonpool through the partially closed bottom plates. In this paper, a novel coupled six-degree-of-freedom analytical model of a Spar system comprising top tensioned risers is proposed. The model accounts for the interactions among the Spar hull kinematics (heave, surge, and pitch), the riser kinematics (heave and surge), and the moonpool. This model involves six coupled differential equations comprising nonlinearities associated not only with stiffness and damping but also with inertia terms. A dynamic analysis is performed by subjecting the model to JONSWAP ocean wave spectrum compatible extreme forces (corresponding to the 100 year wave) and to moments applied to the center of gravity computed by means of a standard motion simulation program. Both numerical and analytical techniques (statistical linearization including inertia terms) are used for the determination of the response of the proposed dynamic model, both in the time and the frequency domains. Related parameter study results are reported, including ones pertaining to the dependence of the Spar system motion on the degree of opening of the bottom plates.

Fatigue Analysis of Offshore Wind Turbines on Fixed Support Structures

A stochastic time domain fatigue analysis is pursued on steel tripod and jacket support structures for offshore wind turbines, following the basic concepts of recently validated simplified linear analyses. A set of environmental lumped load cases with corresponding probabilities of occurrence is considered. Results are compared in terms of a lifetime damage equivalent load.

Response of a Spar Platform Under the Occurrence of Extreme Waves

In this note, the response of a Spar Platform is studied when subject to very high waves. The novelty of this paper consists in adopting the Quasi Determinism (QD) theory in its first formulation for the determination of the response of the structure. In fact, one of the peculiarities of QD theory is that it can be adopted under any boundary condition. The Spar Platform has been modeled as a two-dimensional 3-degree-of-freedom (3dof) rigid cylindrical floating body, moored to the seabed, floating in deep water. Further, it is supposed to be a slender body under the action of extreme waves neglecting any diffraction effect. Moreover, such a system is considered to be nonlinear, because of the nonlinear damping derived from the drag force and because of the stiffness of the moorings, which in the model have been studied as nonlinear springs. The non linear coupled set of differential equations of motion have been integrated in order to obtain the response of the structure by means of numerical methods commonly adopted in Literature. Thus, the response under the occurrence of very high wave crests is studied by using Quasi Determinism theory and then the results compared and validated with those obtained through Monte-Carlo simulations.Copyright

Determination of Riser Tensioner Properties From Full-Scale Data

Tensioners are used for supporting various kinds of production and drilling risers on offshore platforms. All permanent production facilities (such as TLP’s and Spar’s) have employed tensioners to support their dry tree production risers. The key design parameters for the tensioners design are stiffness, stroke and friction properties. Full scale onshore testing of production riser tensioners to determine tensioner properties is difficult due to the magnitude of the loads and pressures required for typical deepwater risers, thus designers frequently rely on theoretical values or small scale tests. This paper presents a determination of riser tensioner properties from full scale offshore observations made on the Holstein Spar production facility during 2005 hurricance season. The paper presents a model for the tensioner system that relates the properties needed by platform and riser designers to the physical tensioner systems. The methodology and results from extraction of parameters from the full scale data is presented. The results presented in this paper will provide a better insight to the designers of such systems.Copyright

Effects of the Mooring Line Configuration on the Dynamics of a Point Absorber

The aim of this paper is to study the dynamics of a floating body with characteristics comparable to a point absorber wave energy converter with different mooring systems, in geometrical configuration or in the materials. To this purpose, the dynamics of a moored buoy is investigated. The point absorber is modeled as a spherical buoy in plane two-dimensional motion, and it is studied under the action of irregular unidirectional wind-generated waves, moored to the seabed by means of one, two or three mooring lines. Two different sets of moorings are considered, and typical wires and chains used in offshore technology are considered, leading to a total of 6 case studies.A quasi-static approach is used for modeling the restoring forces needed to keep buoy into station, using an innovative iterative procedure able to predict for each time instant and for each cable the lay down length of the cable, being each mooring line allowed to be taut or slack. Approaches in the time and frequency domains are used to obtain the system responses in intermediate waters, where these facilities are usually installed. Results for all case studies are compared both in terms of statistics of response and tensions on the top of the cable.Copyright

Effects of Second-Order Extreme Waves on the Dynamics of a Non-Linear Floating Body

Nowadays technical and scientific communities are increasingly interested in the development of technologies for floating devices which can serve different purposes both in coastal and offshore environment. Thus, strong effort is required in the development of correct and efficient algorithms for studying the behavior of such structures under the action of sea wave loadings. At this purpose, in the past few years several approaches were investigated, both in frequency and in time domains, using linear and non-linear structural models and linear and non-linear wave theories.In this note, the effects of nonlinearities in the wave model on the dynamics of a non-linear floating rigid body model are calculated using the second-order Quasi-Determinism (QD) theory (see [1], [2]; [3]) under the action of extremely high waves. Structural nonlinearities consist essentially in non linear damping and nonlinear stiffness due to mooring lines, following the model shown in Nava & Arena, [4]. Numerical nonlinear simulations were performed by means of an algorithm based on the approach showed in Zheng et al. [5], and the results compared to those provided by the nonlinear QD theory.The purpose of this note is to show not only the effects of nonlinearities in the behavior of a floating body and to compare them with those obtained from a linear approach, but also to estimate them through non linear QD theory under the occurrence of a large wave in order to evaluate the reliability of the proposed approach.Copyright

Non-Linear Random Wave Groups With a Superimposed Current

In this paper a new solution for non-linear random wave groups in the presence of a uniform current is obtained, by extending to the second-order the Boccotti’s ‘Quasi-Determinism’ (QD) theory. The second formulation of the QD theory gives the mechanics of linear random wave groups when a large crest-to-trough wave height occurs. Here the linear QD theory is firstly applied to the wave-current interaction. Therefore the nonlinear expressions both of free surface displacement and velocity potential are obtained, to the second-order in a Stokes’ expansion. Finally some numerical applications are presented in order to analyze both the wave profile and the wave kinematics.Copyright