Computation of Slowly Varying Second-Order Hydrodynamic Forces on Floating Structures in Irregular Waves

1989 ◽  
Vol 111 (3) ◽  
pp. 223-232 ◽  
Author(s):  
T. Matsui
Keyword(s):  
Velocity Potential ◽  
Near Field ◽  
Fluid Pressure ◽  
Hydrodynamic Forces ◽  
Second Order ◽  
Irregular Waves ◽  
Floating Structures ◽  
Slow Drift ◽  
Submerged Body ◽  
Element Technique

An exact second-order formulation is presented for computing the slowly varying second-order hydrodynamic forces on floating structures in irregular waves. The near-field approach based on direct integration of the fluid pressure on the submerged body surface is employed in conjunction with numerical first-order solutions by means of the hybrid finite element technique. Green’s second identity is exploited to evaluate the second-order forces due to the second-order velocity potential. Numerical results are presented for the slow drift excitation forces on an articulated column and a semi-submersible platform. It is shown that the contribution from the second-order velocity potential is more significant to the roll moment than to the sway and heave forces on the semi-submersible.

Yaw Drift Moment of a Floating Structure in Waves

2005 ◽  
Author(s):  
Dimitris Spanos ◽  
Apostolos Papanikolaou
Keyword(s):  
Hydrodynamic Forces ◽  
Computational Method ◽  
Irregular Waves ◽  
Linear Potential ◽  
Floating Bodies ◽  
Floating Structure ◽  
Natural Wave ◽  
Positioning Systems ◽  
Floating Structures ◽  
Wave Induced

The wave induced yaw drift moment on floating structures is of particular interest when the lateral yaw motion of the structure should be controlled by moorings and/or active dynamic positioning systems. In the present paper, the estimation of the yaw drift moment in the modeled natural wave environment is conducted by application of a nonlinear time domain numerical method accounting for the motion of arbitrarily shaped floating bodies in waves. The computational method is based on linear potential theory and includes the non-linear hydrostatic terms in an exact way, whereas the higher-order wave-induced effects are partly approximated. Despite the approximate modeling of the second order hydrodynamic forces, the method proved to satisfactorily approach the dominant part of the exerted hydrodynamic forces enabling the calculation of drift forces and of other drift effects in irregular waves. Hence, the subject yaw drift moment in the modeled natural wave environment is derived, resulting to a basic reference for the design of similar type floating structures.

Second-Order Difference-Frequency Loads on FPSOs by Full QTF and Relevant Approximations

2020 ◽  
Author(s):  
Espen Engebretsen ◽  
Zhiyuan Pan ◽  
Nuno Fonseca
Keyword(s):  
Near Field ◽  
Time Domain Analysis ◽  
Second Order ◽  
Difference Frequency ◽  
Irregular Waves ◽  
Mooring Line ◽  
Diffraction Radiation ◽  
Surface Integral ◽  
Order Difference ◽  
Order Diffraction

Abstract This paper investigates three different approximations of the full Quadratic Transfer Function (QTF) for calculating horizontal plane second-order difference-frequency loads on FPSOs, namely Newman’s approximation, full QTF without free surface integral and the white-noise approximation. Second-order excitation loads obtained from approximated QTFs are compared in frequency-domain with those obtained by the full QTFs computed from second-order diffraction/radiation analysis in WADAM. The comparison is performed for a new-build FPSO in a range of water depths and environmental combinations. The full QTFs from second-order diffraction/radiation analysis are further compared to empirical QTFs as identified from cross bi-spectral analysis of model test results in irregular waves. A mesh convergence study is presented for calculating full QTFs by the near-field approach in a second-order diffraction/radiation analysis. The importance of including viscous damping in heave, roll and pitch is illustrated for the mean wave-drift force in surge and sway. FPSO motions and mooring line tensions from fully-coupled time-domain analysis in OrcaFlex is compared when using approximated QTFs and full QTFs from second-order diffraction/radiation analysis.

Assessing the Importance of the Slow Drift Motion of Floating Wind Turbine Platforms

2014 ◽  
Author(s):  
Richard C. Lupton ◽  
Robin S. Langley
Keyword(s):  
Wind Turbines ◽  
Diffraction Theory ◽  
Second Order ◽  
Offshore Wind ◽  
Offshore Platforms ◽  
Floating Structures ◽  
Drift Motion ◽  
Offshore Wind Turbines ◽  
Slow Drift ◽  
Slow Drift Motion

As offshore wind turbines are installed in deeper water, interest is growing in floating wind turbines because, among other reasons, they may become cheaper than fixed-bottom turbines at greater depths. When analysing floating wind turbines, linear diffraction theory is commonly used to model the hydrodynamic loads on the platform. While it well known that slow drift motion due to second-order loads can be important for other floating offshore platforms, it has not yet been established how important such effects are for floating wind turbines. In this paper we aim to give a general result by developing approximate closed-form expressions to estimate the second-order slow drift motion of platforms of different sizes. The values are bench-marked against a typical calculation of the slow-drift response of a platform. The results show that floating wind turbines, which tend to have smaller dimensions than other floating structures, may be expected to show smaller slow-drift motions.

scholarly journals Hydrodynamic Simulation of the Semi-Submersible Wind Float by Investigating Mooring Systems in Irregular Waves

2020 ◽  
Vol 10 (12) ◽  
pp. 4267
Author(s):  
Yu-Hsien Lin ◽  
Cheng-Hao Yang
Keyword(s):  
Time Domain ◽  
Potential Flow ◽  
Near Field ◽  
Three Dimensional ◽  
Irregular Waves ◽  
Viscous Drag ◽  
Floating Platform ◽  
Model Based ◽  
Damping Matrix ◽  
Slow Drift

The present study aims to implement the software ANSYS AQWA to discuss the hydrodynamic analysis of the DeepCwind semi-submersible floating platform in waves based on the potential flow theory by considering the second-order wave exciting force. In this study, the linearized potential-flow hydrodynamic radiation and diffraction problems in the frequency domain were firstly solved by adopting the three-dimensional panel method. Subsequently, the hydrodynamic coefficients and wave loading data were transformed to time domain forms by the Cummins time domain equation as a system loading input. Furthermore, the quadratic transfer function (QTF) matrices with different frequencies and directions deduced based on the near field integration over the mean wetted hull surface were adopted for the calculation of slow-drift forces. In order to represent the damping in a real system for modeling potential flow without Morison’s elements, an additional quadratic damping matrix was added to capture the viscous drag. Eventually, both of the dynamic mooring model based on the lump-mass (LM) approach and the quasi-static mooring model based on the multi-segmented, quasi-static (MSQS) approach are introduced to discuss the mooring effect on the platform hydrodynamics. The effect of wave heading angles on the platform motion is considered as an influential parameter as well.

Slow-Drift Motions of a Moored Two-Dimensional Body in Irregular Waves

1989 ◽  
Vol 33 (02) ◽  
pp. 93-106
Author(s):  
Odd M. Faltinsen ◽  
Rong Zhao
Keyword(s):  
Drift Velocity ◽  
Numerical Procedure ◽  
Second Order ◽  
Irregular Waves ◽  
Order Problem ◽  
Drift Motion ◽  
First Order ◽  
Slow Drift ◽  
Weak Points ◽  
Slow Drift Motion

Weak points in the traditional way of analyzing slow-drift motion are discussed. A theory consistent to second order in wave amplitude and first order in slow-drift velocity for the slow-drift motion of a structure is presented. The interaction between the waves and the local quasi-steady flow due to the slow-drift velocity is incorporated. A new numerical procedure to solve the first-and second-order problem is presented. Generalized Haskind relations for the first-order excitation force and the force due to the second-order potential are derived.

Effect of the Second-Order Potential in the Slow-Drift Oscillation of a Floating Structure in Irregular Waves

1986 ◽  
Vol 30 (02) ◽  
pp. 103-122
Author(s):  
J. A. P. Aranha ◽  
C. P. Pesce
Keyword(s):  
Water Depth ◽  
Narrow Band ◽  
Wave Spectrum ◽  
Low Frequency ◽  
Second Order ◽  
Irregular Waves ◽  
Leading Order ◽  
Floating Structure ◽  
Slow Drift ◽  
The Waves

The slow-drift phenomenon is important when the waves are irregular and the sea spectrum has a relatively narrow band. In this paper an expression is derived for the low-frequency force due to the second-order potential. This expression is the leading-order contribution in the wave spectrum bandwidth and can be exactly determined without computing the second-order potential. It is shown that this effect is of importance when the water depth is relatively shallow or the typical wave period relatively long.

scholarly journals Comparison of Second-Order Loads on a Semisubmersible Floating Wind Turbine

2014 ◽  
Author(s):  
Sébastien Gueydon ◽  
Tiago Duarte ◽  
Jason Jonkman
Keyword(s):  
International Energy Agency ◽  
Oil And Gas Industry ◽  
Second Order ◽  
Offshore Wind ◽  
Irregular Waves ◽  
Massachusetts Institute Of Technology ◽  
Commercial Development ◽  
Energy Agency ◽  
Slow Drift ◽  
Floating Platforms

As offshore wind projects move to deeper waters, floating platforms become the most feasible solution for supporting the turbines. The oil and gas industry has gained experience with floating platforms that can be applied to offshore wind projects. This paper focuses on the analysis of second-order wave loading on semisubmersible platforms. Semisubmersibles, which are being chosen for different floating offshore wind concepts, are particularly prone to slow-drift motions. The slack catenary moorings usually result in large natural periods for surge and sway motions (more than 100 s), which are in the range of the second-order difference-frequency excitation force. Modeling these complex structures requires coupled design codes. Codes have been developed that include turbine aerodynamics, hydrodynamic forces on the platform, restoring forces from the mooring lines, flexibility of the turbine, and the influence of the turbine control system. In this paper two different codes are employed: FAST, which was developed by the National Renewable Energy Laboratory, and aNySIM, which was developed by the Maritime Research Institute Netherlands. The hydrodynamic loads are based on potential-flow theory, up to the second order. Hydrodynamic coefficients for wave excitation, radiation, and hydrostatic forces are obtained with two different panel codes, WAMIT (developed by the Massachusetts Institute of Technology) and DIFFRAC (developed by MARIN). The semisubmersible platform, developed for the International Energy Agency Wind Task 30 Offshore Code Comparison Collaboration Continuation project is used as a reference platform. Irregular waves are used to compare the behavior of this platform under slow-drift excitation loads. The results from this paper highlight the effects of these loads on semisubmersible-type platforms, which represent a promising solution for the commercial development of the offshore deepwater wind resource.

Fluid Pressures on Unanchored Rigid Flat-Bottom Cylindrical Tanks Under Action of Uplifting Acceleration

2009 ◽  
Vol 132 (1) ◽  
Author(s):  
Tomoyo Taniguchi ◽  
Yoshinori Ando
Keyword(s):  
Velocity Potential ◽  
Fluid Velocity ◽  
Fluid Pressure ◽  
Angular Acceleration ◽  
Bottom Plate ◽  
Center Line ◽  
Cylindrical Tank ◽  
Cylindrical Coordinates ◽  
Flat Bottom ◽  
Cylindrical Tanks

To protect flat-bottom cylindrical tanks against severe damage from uplift motion, accurate evaluation of accompanying fluid pressures is indispensable. This paper presents a mathematical solution for evaluating the fluid pressure on a rigid flat-bottom cylindrical tank in the same manner as the procedure outlined and discussed previously by the authors (Taniguchi, T., and Ando, Y., 2010, “Fluid Pressures on Unanchored Rigid Rectangular Tanks Under Action of Uplifting Acceleration,” ASME J. Pressure Vessel Technol., 132(1), p. 011801). With perfect fluid and velocity potential assumed, the Laplace equation in cylindrical coordinates gives a continuity equation, while fluid velocity imparted by the displacement (and its time derivatives) of the shell and bottom plate of the tank defines boundary conditions. The velocity potential is solved with the Fourier–Bessel expansion, and its derivative, with respect to time, gives the fluid pressure at an arbitrary point inside the tank. In practice, designers have to calculate the fluid pressure on the tank whose perimeter of the bottom plate lifts off the ground like a crescent in plan view. However, the asymmetric boundary condition given by the fluid velocity imparted by the deformation of the crescent-like uplift region at the bottom cannot be expressed properly in cylindrical coordinates. This paper examines applicability of a slice model, which is a rigid rectangular tank with a unit depth vertically sliced out of a rigid flat-bottom cylindrical tank with a certain deviation from (in parallel to) the center line of the tank. A mathematical solution for evaluating the fluid pressure on a rigid flat-bottom cylindrical tank accompanying the angular acceleration acting on the pivoting bottom edge of the tank is given by an explicit function of a dimensional variable of the tank, but with Fourier series. It well converges with a few first terms of the Fourier series and accurately calculates the values of the fluid pressure on the tank. In addition, the slice model approximates well the values of the fluid pressure on the shell of a rigid flat-bottom cylindrical tank for any points deviated from the center line. For the designers’ convenience, diagrams that depict the fluid pressures normalized by the maximum tangential acceleration given by the product of the angular acceleration and diagonals of the tank are also presented. The proposed mathematical and graphical methods are cost effective and aid in the design of the flat-bottom cylindrical tanks that allow the uplifting of the bottom plate.

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