Dynamics of a Floating Spar in Regular Waves

Journal of Marine Science and Technology, 2018

The investigation of the interaction of floating structures with very high waves, also known as freak or rogue waves, is of crucial importance for the analysis of their ultimate design conditions. The representation of such waves is usually achieved through computationally intensive numerical simulations. In this paper, a deterministic approach is proposed, to represent extreme wave groups in the space-time domain. The free surface profile for a Gaussian sea is obtained by means of the Quasi-Determinism theory, and the corresponding dynamic response of a spar-type support for floating offshore wind turbines in parked rotor conditions is analyzed. The Quasi-Determinism theory and the nonlinear equation of motion of the structure are coupled through an in-house time-domain numerical code. Wave forces and structure motions in surge, heave and pitch are obtained. A parametric analysis is carried out to investigate the effects of the criteria used for the definition of the extreme wave, its position of occurrence and the initial conditions in terms of body motions. The results obtained give a clear insight into the physics of the wave-structure interaction phenomenon for extremely high waves in Gaussian seas and allow to identify a few load combinations, corresponding to the severest wave conditions for the floating structure. Keywords Extreme waves in Gaussian seas • Quasi-determinism theory • Floating offshore wind turbines • Spar

Ships and Offshore Structures, 2016

A coupled numerical analysis has been carried out for the structural responses and motion behaviour of a classical spar structure subjected to irregular waves represented by JONSWAP (Joint North Sea Wave Project) spectrum. The motion of the floating body is restrained by the four catenary mooring lines with the generation of tension due to change in their nonlinear profile. The surface of the spar is represented by hydrodynamic pressure panels while cables are discretised using a series of Morison elements. A comprehensive sensitivity analysis under two sea depths is carried out by changing (1) the length of mooring lines, (2) the vertical position of fairlead point, (3) incident angle of long crested unidirectional single spectrum, and (4) the number of short crested sub segmented spreading spectra. The effect of second-order hydrodynamic loading on the structure is also taken into account in the analysis procedure by using quadratic transfer function.

2011

Abstract—Truss spars are used for oil exploitation in deep and ultra-deep water if storage crude oil is not needed. The linear hydrodynamic analysis of truss spar in random sea wave load is necessary for determining the behaviour of truss spar. This understanding is not only important for design of the mooring lines, but also for optimising the truss spar design. In this paper linear hydrodynamic analysis of truss spar is carried out in frequency domain. The hydrodynamic forces are calculated using the modified Morison equation and diffraction theory. Added mass and drag coefficients of truss section computed by transmission matrix and normal acceleration and velocity component acting on each element and for hull section computed by strip theory. The stiffness properties of the truss spar can be separated into two components; hydrostatic stiffness and mooring line stiffness. Then, platform response amplitudes obtained by solved the equation of motion. This equation is non-linear due...

International Journal of Ocean System Engineering, 2014

Heave plates have been widely used to enhance viscous damping and thus reduces the heave response of Spar platforms. Single heave plate attached to the keel of the Spar has been reported in literature (Tao and Cai 2004). The effect of double heave plates on hydrodynamic response in random waves has been investigated in this study. The influence of relative spacing L d /D d (D d-the diameter of the heave plate) on the hydrodynamic response in random waves has been simulated in wave basin experiments and numerical model. The experimental investigation has been carried out using 1:100 scale model of Spar with double heave plates in random waves for different relative spacing and varying wave period. The influence of relative spacing between the heave plates on the motion responses of Spar are evaluated and presented. Numerical investigation has been carried out to investigate effect of relative spacing on hydrodynamic characteristics such as heave added mass and hydrodynamic responses. The measured results were compared with those obtained from numerical simulation and found to be in good agreement. Experimental and numerical simulation shows that the damping coefficient and added mass does not increase for relative spacing of 0.4 and the effect greater than relative spacing on significant heave response is insignificant.

Kapal: Jurnal Ilmu Pengetahuan dan Teknologi Kelautan

Pitching mode is more crucial than heaving mode in assessing floating offshore wind turbine (FOWT) motion characteristics, especially the operation of the SPAR substructure. The aim of this paper is to develop an experimental method for improving the SPAR substructure to minimize unnecessary pitch motion. Toward this end, three vertical plate configurations based on the novel SPAR are being developed, known as the 3VP, 4VP, and 5VP models. In consideration of 0⁰, 30⁰, 60⁰, 90⁰-incidence, the pitch response characteristics of the proposed novel SPAR models are comprehensively evaluated in terms of submerged volume ratio, wave-induced motion, non-dimensional damping coefficient, and percentage of motion reduction. The model test results indicate that the 4VP model outperforms the other novel models with respect to dynamic response, particularly the incidence of 0⁰ and 90⁰. This study implies that the novel SPAR development is both feasible and effective in the modification of SPAR-typ...

International Journal of Civil and Structural Engineering, 2010

Offshore Spar platforms are the floating platforms and behave as moored structures in horizontal as well as vertical planes in deep oceans. Under the influence of wind, superstructure of the spar is subjected to aerodynamic drag forces in the windward direction and the underwater portion is exposed to random waves and currents. Spar motions have time periods falling in the wind excitation frequency range due to its compliant nature. Dynamic response analysis of spar platforms to a low frequency wind forces with random waves is presented in the paper. Nonlinear effects due to variable submergence of the structure, hydrodynamic forces and cable tension variation are also considered. The random sea state is characterized by Pierson-Moskowitz sea surface elevation spectrum. The wave forces on the elements of the structure are calculated using Morison's equation with Airy's linear wave theory ignoring diffraction effects. The fluctuating wind has been estimated using Emil Simiu&#...

2005

It is shown that, in the context of a linear theory, all radiation actions of fluid on a floating body can solely be represented by the fluid kinetic and potential energy associated with the wetted surface of the body. In this regard, it is indicated that the linear radiation damping can be expressed by a part of the fluid kinetic energy which has a bilinear form. The linear problem of a floating body motion is then studied in the context of a general linear dynamical system with such form of kinetic energy. From the Lagrange's equations of motion, an equation of motion is derived which generates the linear damping force directly from the bilinear kinetic energy without using any dissipation function. A variant of Hamilton's principle is introduced as the variational generator of this equation of motion. It has been shown that in the context of a linear theory for a floating body with six degrees of freedom each of the 6x6 added mass and damping matrices contains three disti...

Ocean Engineering, 2017

Several advanced floating structures have been proposed and developed with varying cost effectiveness and productiveness in deep water exploration. Among them, Spar platforms have been accepted as an efficient platform for the exploration. Many research works have been conducted on floating structure but a few on Spar platform. Nonlinear dynamic analysis of a 3D model of floating Spar platform structure is a resourceful tool to predict the responses, where the main body of the Spar hull and mooring lines are considered as an integrated coupled system. To define accurately the interaction between the Spar and mooring lines, coupled dynamic analysis was found to be appropriate for studying responses in the deep sea. Numerical simulation and motion analyses were carried out with the ABAQUS/AQUA. The responses of Spar platform were extracted and evaluated in time histories along with Response Amplitude Operator (RAO). The behaviours of coupled Spar platform have been investigated under real sea environments for increasing water depth to ultra-deep together with the load variability employing sea current for surge, heave, pitch and mooring tension responses. Motions show the consistency in the behaviour of Spar platform responses. Surge response indicates the static offset of the platform due to the static current force under wave plus current. The current force compresses oscillations and reduce heave and pitch magnitude. For larger water depth the platform responses reduce significantly due to the increased damping of mooring line.

16th Australasian …, 2011