Rafael A. Watai

Influence of Wave Induced Second-Order Forces in Semisubmersible FOWT Mooring Design

The main focus of this paper is on hydrodynamic modelling of a semisubmersible platform (which can support a 1.5MW wind turbine and is composed by three buoyant columns connected by bracings) with especial emphasis on the estimation of the wave drift components and their effects on the design of the mooring system. Indeed, with natural periods of drift around 60 seconds, accurate computation of the low-frequency second-order components is not a straightforward task. As methods usually adopted when dealing with the slow-drifts of deep-water moored systems, such as Newman?s approximation, have their errors increased by the relatively low resonant periods, and as the effects of depth cannot be ignored, the wave diffraction analysis must be based on full Quadratic Transfer Functions (QTF) computations. A discussion on the numerical aspects of performing such computations is presented, making use of the second-order module available with the seakeeping software WAMIT®. Finally, the paper also provides a preliminary verification of the accuracy of the numerical predictions based on the results obtained in a series of model tests with the structure fixed in bichromatic waves.

Influence of Wave Induced Second-Order Forces in Semi-Submersible FOWT Mooring Design

AZIMUT project (Spanish CENIT R&D program) is designed to establish the technological groundwork for the subsequent development, of a large-scale offshore wind turbine. The project (2010–2013) has analysed different floating offshore wind turbines (FOWT): SPAR, TLP and Semi-Submersible platforms were studied. Acciona, as part of the consortium, was responsible of scale-testing a Semi-submersible platform to support a 1.5MW wind turbine. The floating platform geometry considered in this paper has been provided by the Hiprwind FP7 project and is composed by three buoyant columns connected by bracings. The main focus of this paper is on hydrodynamic modelling of the floater, with especial emphasis on the estimation of the wave drift components and their effects on the design of the mooring system. Indeed, with natural periods of drift around 60 seconds, accurate computation of the low-frequency second-order components is not a straightforward task. As methods usually adopted when dealing with the slow-drifts of deep-water moored systems, such as Newman’s approximation, have their errors increased by the relatively low resonant periods, and as the effects of depth cannot be ignored, the wave diffraction analysis must be based on full Quadratic Transfer Functions (QTF) computations. A discussion on the numerical aspects of performing such computations is presented, making use of the second-order module available with the seakeeping software WAMIT®. Finally, the paper also provides a preliminary verification of the accuracy of the numerical predictions based on the results obtained in a series of model tests with the structure fixed in bichromatic waves.Copyright

A Higher Order Time Domain Rankine Panel Method for Linear and Weakly Non-Linear Computation

This paper presents a higher order time domain boundary elements method based on the Rankine sources for the computation of both linear and weakly non-linear effects for both fixed and free floating bodies. The geometry is described based on surfaces in a standard iges file, considering a NURBS (Non Uniform Rational Basis-Spline) description. The potential function, velocity, free-surface elevation and other quantities are defined using b-splines of arbitrary degree and the floating body interaction is solved using the potential acceleration approach on a Runge-Kutta scheme for time evolution. The integral equation is obtained and solved considering several possibilities for the collocation points, leading to an over-determined system. The integration over the panels is performed using a mixed desingularized-numerical method over Gaussian points. The results comparison are performed with WAMIT solution for a floating sphere concerning wave runup, body motions, velocity field, mean drift components in time domain.Copyright

Analysis of Hydrodynamic Resonant Effects in Side-by-Side Configuration

Seakeeping behavior of a multibody system in side-by-side configuration in head sea condition is discussed in this paper. The system, which can be assimilated to a FLNG and LNG carrier during an offloading operation is composed of a barge and a prismatic geosim with two gap values. Seakeeping tests in regular waves have been performed in the model basin of CEHINAV-Technical University of Madrid (UPM). The movements for the geosim were restricted to the surge, heave and pitch motions (on the vertical plane), whereas the barge was kept fixed. In this way the gap remained constant during the tests. Numerical modeling has been undertaken using WAMIT and an in-house time-domain Rankine Panel Method (TDRPM). Response amplitude operators in terms of movements and wave amplitude in the gap obtained from seakeeping test and numerical models are documented in the paper, illustrating the limitation of the numerical codes regarding the modeling of this hydrodynamic problem. Numerical results indicate a resonant behavior of the waves in the gap for a range of frequencies, with amplitudes much higher than those observed during the tests. Due to the small distances considered in the experiments, these resonant waves are related to longitudinal wave modes in the gap. In order to overcome this problem, a procedure for introducing an external damping factor that attenuates the wave amplitude along the gap in the time-domain RPM is evaluated based on the experimental data.Copyright

A Time-Domain Boundary Elements Method Applied to Multi-Body Simulations With Large Relative Displacements

This paper presents a time domain boundary elements method that accounts for relative displacements between two bodies subjected to incoming waves. The numerical method solves the boundary value problem together with a re-meshing scheme that defines new free surface panel meshes as the bodies displace from their original positions and a higher order interpolation algorithm used to determine the wave elevation and the velocity potential distribution on new free surface collocation points. Numerical solutions of exciting forces and wave elevations are compared to data obtained in a fundamental experimental text carried out with two identical circular section cylinders, in which one was attached to a load cell and the other was forced to move horizontally with a large amplitude oscillatory motion under different velocities. The comparison of numerical and experimental result presents a good agreement.Copyright

Analysis of the Draft Influence on the Numerical Resonant Effects in Side-by-Side Offloading Operation

In the last years hydrodynamic interaction between two vessels in side-by-side configuration is one of the hot issues in offshore floating body dynamics. The paper investigates the hydrodynamical aspects of a floating two body system. The topic is geared towards analysing the influence of the vessel’s draft in side-by-side configuration and in head sea condition. The need to solve this problem arises when one wants to study the hydrodynamic variation for the various stages of a offloading process with a defined operational gap. The system is composed of a barge and a prismatic geosim with a fixed gap value and with two barge’s draft values. Regular wave tests have been performed in the model basin of CEHINAV-Technical University of Madrid (UPM). The motion for the geosim was restricted to the surge, heave and pitch motions (just motions on the vertical plane), whereas the barge was kept fixed. The costant gap value is guaranteed during the tests. A numerical model has been created with WAMIT and with an in-house time-domain Rankine Panel Method (TDRPM). In each case the numerical and experimental response amplitude operators (RAOs) are obtained and compared, researching the limitation of the numerical codes for the gap flow modeling. In the past the gap effects on the numerical results have been studied varying the gap value finding resonant behavior in terms of motion and wave amplitude RAOs. Now the draft value contribution on the hydrodynamic effects is investigated. Also in this case the numerical results indicate a resonant behavior in determined frequencies in motion as well as in wave in the gap, that is not found in the tests. In order to overcome this problem, a procedure for introducing an external damping factor that attenuates the wave amplitude along the gap in the time-domain RPM is evaluated based on the experimental data.Copyright

MPSO Design: Part 1 — Wave Excitation Forces and Moments

The MPSO is characterized by the use of hydrodynamics appendages, such as moonpool, beach and skirts, which improve the hydrodynamic behavior of the unit in waves. This type of platform may be designed for different offshore scenarios as, for example: the possibility of oil and gas storage, dry tree completion system and the use of steel catenary risers (SCR). An optimization procedure to choose the geometric dimensions of the MPSO becomes important in order to achieve the optimum hydrodynamic behavior to operate in harsh environmental conditions for each scenario. The optimization procedure might be useful in the preliminary design phases to reduce the verification time of the solution evaluated with model tests; for that reason it is necessary to create a database with experimental results to make the optimization procedure possible. The main idea of the study is to carry out an extensive experimental model test aimed at obtaining the parameters not well predicted using numerical codes. With this intent, the work is subdivided into three parts: Part 1 – Wave Excitation Forces and Moments; Part 2 – Damping and Added Mass Forces and Part 3 – Optimization Process. Results will be presented in different papers. The first one presents the experimental results for captive tests, the second one the experimental results for forced oscillation tests and the last one the methodology to use the experimental results as input in an optimization tool. The first paper presents the methodology in which nondimensional variables based on MPSO geometric characteristics were defined. These variables were related to a fixed moonpool diameter and they were determined in terms of four geometric dimensions: external diameter; height and diameter of the beach and platform draft. As a consequence, 21 different MPSO model geometries could be defined and experimentally tested in order to obtain the wave excitation forces and moments in 6 DOF. The experiments included transient waves so as to better understand the hydrodynamic behavior of the hull, such as, the response amplitude operator (RAO), cancelation points, the beach/bottom/moonpool effects for the different dimensions. The wave forces and moments obtained experimentally were compared to the results of a numerical code based on potential wave theory.Copyright

A Design Procedure for Evaluating the Air Gap on a Large-Volume Semi-Submersible Platform

This paper presents a design procedure for the evaluation of the air-gap response on semi-submersible platforms subjected to irregular sea conditions. The suggested procedure takes into account both first and second order (low frequency) effects on hull motions when evaluating the air gap. As a first step of the procedure, a large range of sea conditions with different returning periods and directions of incidence are simulated using a frequency domain model. This first step is intended to determine the critical sea conditions regarding the air gap response of that particular floating unit. For those conditions, it is suggested to be performed a more complete analysis of the problem, including time-domain CFD simulations, in order to improve the results, especially for areas that may be susceptible to intense wave run-up effects. Experimental results for some typical sea states of Campos Basin have been employed to validate the procedure using as an example a large displacement four column semi-submersible platform operating at Campos Basin, Brazil. Results have confirmed that the sea state with the highest significant wave height, or peak period, may not lead to the worst air-gap situation. It’s also shown that, although for the critical sea conditions the first order effects were dominant in the air gap response, at many non-critical sea states the second order effects presented magnitudes comparable to those of first order, indicating that the resonant response of the unit should not be disregarded a priori when dimensioning the air-gap of similar deep-draft semi-submersibles.Copyright

Wave Run-up Simulations with a Moving Large Volume Semi-Submersible Platform

Since July 2008, the Numerical Offshore Tank (TPN) of the University of Sao Paulo and Petrobras have been working on a research project intended to improve knowledge and modeling of advanced hydrodynamics topics, such as the wave run-up phenomenon. Among other activities, wave basin tests were performed with small-scale model of a large volume semi-submersible designed to operate in Campos Basin. These tests evidenced significant run-up effects on its squared-section columns for the steepest waves in several design conditions. In order to evaluate the difficulties involved in modeling the wave run-up phenomenon, simplified tests were also carried out with the model fixed and moored in regular waves with varying steepness. Previous studies using a 2nd order BEM model and a VOF CFD code to predict free-surface elevations below the deck under regular waves were presented in Matsumoto et al. (2010). The studies illustrated considerable differences between the wave elevation results in fixed and moored model setup; however, by that time, the analysis of the moored model by a VOF CFD code was not yet complete. This paper, therefore, presents wave run-up estimations with a moving large volume semi-submersible platform performed with the CFD code ComFLOW, which solves the Navier-Stokes equations employing a local height function to the free surface displacement. The phenomenon is investigated by simulating the flow around the semi-submersible model under the influence of high steepness regular waves on a non-uniform grid. Platform motions, derived from a first order BEM code, are imposed and synchronized with the incoming wave. Aiming at avoiding numerical wave reflections, a damping zone is also applied and positioned downstream the platform model. Predicted results are compared to experimental data, measured by seven vertical wave probes located in different positions below the model deck. Although considerably time-consuming, it will be shown that simulations present very good agreement with the experimental results.Copyright