NUMERICAL AND EXPERIMENTAL STUDIES ON THE SLOW DRIFT MOTIONS AND THE MOORING LINE RESPONSES OF TRUSS SPAR PLATFORMS

An efficient methodology has been developed for the dynamic analysis of offshore floating structures. In this methodology, special attention was given to the second order difference frequency forces and responses. According to this numerical scheme, a MATLAB program named TRSPAR was developed to predict the dynamic responses of truss spar platform in time domain. In this program, the truss spar platform was modeled as a rigid body with three degrees of freedom. Hydrodynamics of the structure, which include the linear and second order wave forces, mean drift forces, added mass, radiation damping, wave drift damping and system stiffness were included in the program. Current and wind forces were also considered showing their effects on the slow drift responses. The wave forces, including inertia and drag forces, were calculated using Morison equation assuming the wave field as undisturbed. An efficient time domain integration scheme was adopted based on Newmark Beta method. Comprehensive experimental studies were conducted and the numerical predictions were systematically compared with model test results. These comparisons consisted of structure’s dynamic responses in different environmental conditions and two structural situations. The first situation was the structure with intact mooring lines and the other one was the structure under mooring line failure. The responses of the platform with mooring line system damage were investigated with the emphasis on finding the critical effects of line failure on the resonant responses. The effects of the second order difference frequency wave forces on the truss spar motion characteristics were examined numerically. Published numerical results were used to verify the developed numerical model in predicting the truss spar dynamic responses when subjected to combined wave, current and wind forces. The effects of strengthening mooring line system on the motion characteristics of the structure were examined numerically. For the assessment of the fluid to mooring nonlinear interactions, a deterministic approach based on lumped mass method with equations of dynamic equilibrium and continuity was adopted. Finally, parametric studies on deepwater mooring line analysis have been conducted for investigating the contributions of the various design parameters on mooring line tension. The experimental results verified the validity of the developed numerical scheme for prediction of the wave frequency and low frequency motions of the truss spar platform with its intact mooring and in the case of mooring line damage condition. RMSD values for the numerical and the experimental results show that the simulated wave frequency responses (WFR) trend was relatively agreed well with the experiments compared to the low frequency responses (LFR). For the intact mooring line condition, RMSD values for the WFR ranged from 109.9 to 182.4 while for LFR were ranged from 499.6 to 550.2. The same has been noticed in the mooring line damage condition in which RMSD values ranged from 107.4 to 323.6 and 209.1 to 1074 for WFR and LFR respectively. With regard to the peak responses, good accuracy has been achieved between the predictions and the measurements. The percentage errors for the peak responses in the intact mooring and the mooring line damage conditions were ranged from 9.5% to 17.3%

Supplementary information on early-stage floating offshore wind platform designs

This document serves as supplementary information to the authors' review paper on early-stage floating offshore wind turbine (FOWT) platform designs. The review paper is the second part in a study on FOWT platform designs, following a review of FOWT platforms which currently have or have previously had a prototype, demonstration, or farm scale project at-sea. The present review covers 86 past and current early-stage platform designs, ranging from early conceptual designs to platforms which have undergone lab tests simulating extreme conditions. In this supplementary information document, more details are provided about all 86 platforms reviewed. For each device, the following is included (if available): (i) a description of the platform and its unique features, (ii) a rough timeline of development, (iii) design goals and constraints, (iv) evolution of the design, (v) lab testing information, and (vi) published dimensions. Two sections are included: one section contains the platforms that are no longer in development (i.e., there has been no new development since 2018), and the other section contains the platforms still in development today. Within each sub-section, platforms designed to hold a single turbine are presented first, then platforms designed to hold multiple turbines, and finally hybrid platforms

DYNAMIC RESPONSE OF SPAR PLATFORM SECURED WITH MOORING SYSTEM USING ANSYS-AQWA

Oil and gas exploration has moved to deeper water in recent years and the type of offshore structures used are also changing. A spar has a deep draft cylindrical hull and is suitable for use in deep water regions. Since spar is a floating structure, it uses mooring lines to hold its position even though the wave and current loading will cause a motion response on the spar. To understand the motion as well as the tension loadings in the mooring lines, a dynamic analysis has been conducted in the time domain. The analysis uses a regular wave profile with 6m height and a period of 10s. The spar is secured with four mooring lines and is subjected to this wave loading and using the Linear Airy Wave Theory for computation. The forces acting on the spar and mooring lines is governed by the Morrison’s Equation. The responses of surge, heave, pitch and tension is then compared to a benchmark case obtained from previous study for validation purpose. Upon validation parametric study on the crosssectional area of mooring lines and the number of mooring lines was performed. Simulation was performed using ANSYS AQWA finite element software and the results obtained are consistent with previous studies

MODEL TEST AND COUPLED DYNAMIC ANALYSIS OF A DEEPWATER FPSO WITH INTERNAL TURRET MOORING SYSTEM

Model test is an effective way to verify numerical dynamic analysis of floating system. The diffraction and radiation analysis is carried out in frequency domain based on potential theory to predict motion response of rigid platform. The quasi static and dynamic methods are usually adopted to simulate mooring system, which determines if the whole system is coupled within the analysis. Here model tests are performed to indicate the accuracy of potential theory and quasi static and dynamic methods for the whole system. A FPSO is tested under regular waves to find its RAO. The FPSO with internal turret mooring system under irregular wave, wind and current are also studied in the deepwater basin of Harbin Engineering University. The results are compared between the model test and numerical models, which show the model test results agree well with the coupled numerical model, while the maximum mooring tensions are under estimated in quasi static analysis

MODEL TEST AND COUPLED DYNAMIC ANALYSIS OF A DEEPWATER FPSO WITH INTERNAL TURRET MOORING SYSTEM

Model test is an effective way to verify numerical dynamic analysis of floating system. The diffraction and radiation analysis is carried out in frequency domain based on potential theory to predict motion response of rigid platform. The quasi static and dynamic methods are usually adopted to simulate mooring system, which determines if the whole system is coupled within the analysis. Here model tests are performed to indicate the accuracy of potential theory and quasi static and dynamic methods for the whole system. A FPSO is tested under regular waves to find its RAO. The FPSO with internal turret mooring system under irregular wave, wind and current are also studied in the deepwater basin of Harbin Engineering University. The results are compared between the model test and numerical models, which show the model test results agree well with the coupled numerical model, while the maximum mooring tensions are under estimated in quasi static analysis

State-of-the-Art Review of Vortex-Induced Motions of Floating Offshore Wind Turbine Structures

The motivation for this study is the fast development of floating offshore wind energy and the immature methodology and engineering practice related to predictions of vortex-induced motions (VIM). Benefiting from the oil and gas industry, in the past several decades, extensive knowledge and experience on vortex-induced vibrations (VIV) on slender marine structures has been gained. As the learnings from these efforts should be transferred and adapted to the renewable energy industry, a state-of-the-art review on influential VIM research has been carried out in this paper, focusing on: (1) engineering practice, (2) model tests, (3) numerical calculation, and (4) field measurement. Engineering gaps and potential research topics are identified as future work.publishedVersio

DYNAMIC RESPONSES OF FLOATING OFFSHORE PLATFORMS WITH LARGE HULLS

Spar and semi-submersible are the most common types of floating offshore platforms used for deepwater operations. The spar consists of a hollow cylindrical deep-draft floating hull that provides buoyancy, with strake surrounding the hull to reduce vortex induce vibration and to held in place by mooring lines. To remain stable, it is important to maintain the centre of gravity always below the centre of buoyancy. The semi-submersible comprises of two horizontal water tight pontoons and number of column units that stand on the pontoons to provide support to the deck structure. It is held in place by mooring lines and dynamic positioning system. Both these types of platforms are made up of large-sized hull for providing buoyancy. As the ratio of the diameter of these structures to the wave length is above 0.2, the wave diffraction theory is the correct theory to be applied for the calculation of wave forces and wave damping, according to the literature. However, the application of diffraction theory, even linear one, is very much complicated and requires very costly commercial software. Hence, many research papers have reported results of dynamic analysis, using Morison equation for such cases, reasoning that for a considerable part of the frequency range, the ratio of diameter to wave length is still below 0.2. This is because of the ease of using Morison equation in programming and the possibility of incorporating the various non-linearity in the analysis. Yet, it has been established that the consultants are using only diffraction analysis for the analysis and design of such platforms. The aim of this study was to determine and compare the responses by both Morison equation and diffraction theory to the model test responses, and to suggest nonlinear multiple regression curves to estimate the structure responses. Model tests were conducted for spar and semi-submersible platform models in the wave tank at the Offshore Engineering Laboratory of Universiti Teknologi PETRONAS and the responses were measured. The respective prototypes were analyzed using a numerical Newmark Beta time domain integration method that was developed by using Matlab program. The platforms were designed as rigid bodies and three degree of freedom; surge, heave and pitch were considered. Linear wave theory and Morison equation were used for wave force determination in time domain analysis. A commercial software was employed to determine responses of the structures by Linear Wave Diffraction module. These results proved that the diffraction theory results were much closer to the actual model test results, thereby proving that using Morison equation for such platforms is not justified. Using the results of the diffraction analysis for a large number of platforms and conducting a non-linear multiple regression analysis, this thesis also suggests formulae to obtain suitable regression curves for predicting the diffraction responses of the spar and semi-submersible for any dimension and draft within the range suggested

COMPARATIVE STUDY OF THE DYNAMIC RESPONSE OF FLOATING PLATFORMS SUBJECTED TO LONG-CRESTED AND SHORT-CRESTED WAVES

Spar and semi-submersible platforms are the common types of large floating offshore platforms operating in deepwater regions. Due to lack of sufficient analytical, experimental and statistical data regarding the performances of such platforms subjected to the real sea condition based on short-crested waves. These types of platforms are normally designed for long-crested waves. In the real sea condition, generally short-crested waves exist

Numerical simulation of the truss spar 'Horn Mountain' using COUPLE

A truss spar, named as Horn Mountain, was deployed in the Gulf of Mexico in 1,650 m of water, approximately 150 km southeast of New Orleans in June 2002. Horn Mountain is operated by British Petroleum (B.P.). Extensive field measurements were made using an integrated marine monitoring system attached to the truss spar. In this study, dynamic analysis of the truss spar interacting with its mooring and riser system was performed using a time-domain numerical code, known as ÂCOUPLEÂ. The simulated results were then compared with the corresponding field measurements made during Hurricane Isidore. During the numerical study, various hydrodynamic parameters which were crucial to the accuracy of predicting the global motions of the truss spar and tensions in mooring lines and risers were scrutinized, such as the drag and added-mass coefficients of heave plates, hard tank and truss beams. Satisfactory agreement between the simulation and corresponding measurements was reached, indicating that the numerical code, COUPLE, can be used to conduct the time-domain analysis of a truss spar interacting with its mooring and riser system under severe storm impact. A comparative study was also conducted to analyze the significance of interaction of risers with the hull structure. Three different cases of coupled analysis are simulated, namely (i) coupled analysis of truss spar interacting with mooring lines, (ii) coupled analysis of truss spar interacting with the mooring lines and the steel catenary risers, (iii) coupled analysis of truss spar interacting with the mooring lines, the steel catenary risers and top tension risers. Major statistical parameters of the global motions of the truss spar and the mooring line tensions for the three cases are compared with the field measurements

Dynamic responses of a spar platform due to waves and current

This report describes a theoretical analysis of the dynamic motion responses of a spar due to wave and current, in correlation to model deviations recorded from laboratory testing