Viscous Drift Forces on a Semi-Submersible Type Mega-Float Comprised of Circular Cylinders

2004 ◽  
Author(s):  
Shunji Sunahara
Keyword(s):  
Potential Flow ◽  
Flow Theory ◽  
Circular Cylinders ◽  
Mooring System ◽  
Potential Flow Theory ◽  
Wave Drift ◽  
Long Wave ◽  
Drift Force ◽  
Wave Drift Forces ◽  
Drift Forces

An estimation of the wave drift forces acting on a semi-submersible type Mega-float supported by very many columns is very important in order to design its mooring system. It is known that the wave drift forces acting on a train of multiple vertical circular cylinders may be determined using the potential flow theory. However, it has recently been reported that the large wave drift forces acting on a large scale model of a semi-submersible type Mega-float, comprised of many simple circular cylinders, for long wave periods, cannot be explained by the potential flow theory. In addition the forces seem to have a significant influence on the design of its mooring system. At first, it seemed that the measured forces were viscous drift forces. The viscous drift forces are in proportion to the square of the wave particle velocity or the cube of the wave height. Of course, the existence of viscous drift forces has already been established, but it was considered that the forces acting are conditional in that the flow is apt to shed, for example on complex under-water shapes, on radiation problem, in larger height or longer period waves. Also it was thought that the forces acting on simple circular cylinders were negligibly small from the viewpoint of engineering applications. Finally, it was not accurately verified that the forces were viscous drift forces. In this study, model tests were carried out. The wave loads acting on a 16-column platform model and the hydrodynamic forces acting on each column of the model were simultaneously measured. The contribution of the viscous drift force component on the wave drift force acting on a train of vertical circular cylinders was also investigated in detail. It was confirmed that significant viscous drift forces act on circular cylinders for long wave periods. Furthermore an applicable region of viscous and potential components of the wave drift forces acting on vertical circular cylinders was obtained.

On Approximations of the Wave Drift Forces Acting on Semi-Submersible Platforms With Heave Plates

2016 ◽  
Author(s):  
Bernard Molin ◽  
Jean-Baptiste Lacaze
Keyword(s):  
Wave Field ◽  
Scattered Wave ◽  
Diffraction Radiation ◽  
Morison Equation ◽  
Wave Drift ◽  
Horizontal Wave ◽  
Heave Plate ◽  
Drift Force ◽  
Wave Drift Forces ◽  
Drift Forces

The horizontal wave drift force acting on a vertical floating column, without then with a heave plate, is considered. Computations are performed with a diffraction-radiation code and through the Morison and Rainey equations. Focus is on wave frequencies around the heave resonance where the drift force may be significant, even though the scattered wave-field being weak. It is found that the Morison equation overpredicts the drift force while Rainey equations perform rather well.

Mooring of Semi Submersibles in Extreme Sea States: Simplified Models for Wave Drift Forces and Low Frequency Damping

2018 ◽  
Author(s):  
Kjell Larsen ◽  
Tjerand Vigesdal ◽  
Rune Bjørkli ◽  
Oddgeir Dalane
Keyword(s):  
Low Frequency ◽  
Scale Model ◽  
Small Scale ◽  
Mooring System ◽  
Viscous Effects ◽  
Empirical Correction ◽  
Wave Drift ◽  
Total Wave ◽  
Wave Drift Forces ◽  
Drift Forces

This paper presents results from extensive small-scale model testing of three semi submersibles together with an overview of damping contributions of low frequency motions. The objectives of the model tests were to verify empirical correction formulas for viscous wave drift forces and to recommend and validate theoretical low frequency damping models. The main parameters of the semis such as displacement, number of columns and diameter of columns were intentionally varied in order to assess the effects on total wave drift forces and corresponding damping. The results show that viscous effects significantly increase the total wave drift forces in extreme sea states. The presence of current increases the effect. As expected, the viscous contribution to wave drift is especially important for semis with slender columns. A revised empirical correction formula for wave drift forces is proposed based on model test results. An overview of the different low frequency damping effects is given. Damping from viscous forces on the hull and damping from the mooring system are the most important sources of damping for the moored semis. A simplified model to calculate mooring system damping is proposed. For accurate prediction of low frequency motions of moored semi submersibles in extreme sea states, a damping level in the range 40–70% of critical damping should be applied for surge and sway when the empirical correction formulas for wave drift forces are applied.

Numerical Evaluation of Ship Maneuvering Performance in Waves

2018 ◽  
Author(s):  
Min-Guk Seo ◽  
Bo Woo Nam ◽  
Yeon-gyu Kim
Keyword(s):  
Irregular Waves ◽  
Time Signal ◽  
Regular Waves ◽  
Ship Maneuvering ◽  
Wave Drift ◽  
Mmg Model ◽  
Free Running ◽  
Drift Force ◽  
Wave Drift Forces ◽  
Drift Forces

This paper considers a numerical computation of ship maneuvering performance in waves. For this purpose, modular-type maneuvering model (MMG model) is adopted and wave drift forces and moments are included in maneuvering equation of motion. Wave drift forces ware calculated using a seakeeping program based on higher-order Rankine panel method. When calculating the wave drift force acting on a ship, the forward speed, wave heading, wave period and drift angle of the ship are considered as key parameters. It means that ship’s lateral speed is also included to calculate wave drift force. Numerical simulations are carried out in regular waves using S175 containership and computation results are validated by comparing them with results of free-running model test. Using the developed program, numerical simulation in irregular waves are, also, conducted and discussion is made on the sensitivities of time signal of wave elevation on turning performance.

Model Tests of Pipe-Laying Vessel: Results and Analysis

2011 ◽  
Author(s):  
Alevtina N. Kulikova ◽  
Victor G. Platonov ◽  
Sandro Foce
Keyword(s):  
Dynamic Characteristics ◽  
Physical Modeling ◽  
Center Of Gravity ◽  
Irregular Waves ◽  
Flexural Stiffness ◽  
Mooring System ◽  
Wave Drift ◽  
Component Systems ◽  
Wave Drift Forces ◽  
Drift Forces

The paper deals with the physical modeling of kinematic/dynamic characteristics of multi-component systems in a seakeeping basin. The paper presents the results of model seakeeping tests conducted to study the behavior of a combination comprising the barge + stinger + roller + pipe + mooring system in irregular waves under combined wind and current effects in the process of pipe-laying operations. The tests were conducted with variation of wave headings and flexural stiffness of pipeline. The data contained in the paper can be useful for modeling multi-component systems operating at sea. The following abbreviations are used: mooring system – MS, WDF – wave drift forces, CL plane – centerline plane of barge, CoG – center of gravity.

Wave Drift Forces and Low Frequency Damping on the Exwave Semi-Submersible

2017 ◽  
Author(s):  
Nuno Fonseca ◽  
Carl Trygve Stansberg
Keyword(s):  
Potential Flow ◽  
Low Frequency ◽  
Second Order ◽  
Difference Frequency ◽  
Frequency Wave ◽  
Wave Drift ◽  
The Difference ◽  
Wave Drift Forces ◽  
The Relationship ◽  
Drift Forces

The paper presents realistic horizontal wave drift force coefficients and low frequency damping coefficients for the Exwave semi-submersible under severe seastates. The analysis includes conditions with collinear waves and current. Model test data is used to identify the difference frequency wave exciting force coefficients based on a second order signal analysis technique. First, the slowly varying excitation is estimated from the relationship between the incoming wave and the low frequency motion using a linear oscillator. Then, the full quadratic transfer function (QTF) of the difference frequency wave exciting forces is defined from the relationship between the incoming waves and the second order force response. The process identifies also the linear low frequency damping. The paper presents results from cases selected from the EXWAVE JIP test matrix. The empirical wave drift coefficients are compared to potential flow predictions and to coefficients from a semi-empirical formula. The results show that the potential flow predictions largely underestimate the wave drift forces, especially at the low frequency range where severe seastates have most of the energy.

Simulation of Low Frequency Motions in Severe Seastates Accounting for Wave-Current Interaction Effects

2017 ◽  
Author(s):  
Babak Ommani ◽  
Nuno Fonseca ◽  
Carl Trygve Stansberg
Keyword(s):  
Test Data ◽  
Time Domain ◽  
Low Frequency ◽  
Interaction Effects ◽  
Force Coefficients ◽  
Wave Drift ◽  
Current Interaction ◽  
Drift Force ◽  
Wave Drift Forces ◽  
Drift Forces

Today’s industry practice assumes wave drift forces on floating structures can be computed from zero current wave drift force coefficients for the stationary floater, while simplified correction models introduce current effects and slow drift velocity effects. The paper presents an alternative approach which overcomes some of the limitations of today’s procedures. The method, to be applied together with a time domain solution of the low frequency motions, is based on pre-calculation of mean wave drift force coefficients for a range of current velocities. During the low frequency motions simulation, the wave drift forces induced by the irregular waves are computed from the mean drift coefficients corresponding to instantaneous relative velocity resulting from the current and the low frequency velocities. A simple interpolation model, based on a quasi-steady assumption, is applied to obtain the drift forces in time-domain. Since calculation of the wave drift forces on Semi-submersibles in severe sea states with fully consistent methods is out of reach, a semi-empirical model is applied to correct the potential flow wave drift force coefficients. This model takes into account viscous effects, that are important in high seastates, and wave-current interaction effects. The paper compares the wave drift forces and the related low frequency motions computed by the proposed method, with results applying “standard” methods and with model test data. The test data was obtained in the scope of the EXWAVE JIP, with model tests designed to investigate wave drift forces in severe seastates and assess the wave-current interaction effects.

Time-Domain Hydrodynamic Model for Mooring Analysis of a Spread Moored FPSO With Calibration of Wave Drift Forces

2020 ◽  
Author(s):  
Min Zhang ◽  
Junrong Wang ◽  
Junfeng Du ◽  
Nuno Fonseca ◽  
Galin Tahchiev ◽  
...  
Keyword(s):  
Numerical Model ◽  
Test Data ◽  
Time Domain ◽  
Transfer Functions ◽  
Low Frequency ◽  
Linear Wave ◽  
Mooring System ◽  
Wave Drift ◽  
Wave Drift Forces ◽  
Drift Forces

Abstract The paper presents calibration and validation of a time domain numerical model for mooring analysis of a spread moored FPSO in moderate seastates with and without current. The equations of motion are solved in the time domain with a fully coupled method, accounting for linear wave frequency (WF) radiation and diffraction, second order wave drift forces and nonlinear low frequency (LF) damping. The mooring system dynamics is solved by a FEM. Uncalibrated numerical models are based on input from the mooring system, vessel mass, radiation/diffraction analysis, decay tests and current coefficients. WF responses are very well predicted by standard radiation/diffraction linear analysis, therefore the focus is on the LF responses. LF motions are underpredicted by the uncalibrated numerical model. Calibration is performed by comparing simulations with model test data and adjusting hydrodynamic coefficients known to be affected by uncertainty. These include wave drift force coefficients and LF damping. Correction of the drift coefficients is based on empirical quadratic transfer functions (QTFs) identified from the test data by a nonlinear data analysis technique known as “cross-bi-spectral analysis”. The LF damping coefficients are then adjusted by matching low frequency surge and sway spectra from the model tests and from the simulations.

Maximum Likelihood Method as a Means to Estimate the Directional Wave Spectrum and the Mean Wave Drift Force on a Dynamically Positioned Vessel

2002 ◽  
Author(s):  
Olaf J. Waals ◽  
A. B. Aalbers ◽  
J. A. Pinkster
Keyword(s):  
Maximum Likelihood Method ◽  
Wave Spectrum ◽  
Likelihood Method ◽  
Feed Forward ◽  
Wave Drift ◽  
Directional Wave ◽  
The Mean ◽  
Drift Force ◽  
Wave Drift Forces ◽  
Drift Forces

Feed forward in control theory is a method in which real time information about system disturbance is fed into the controller to improve its performance. As such, feed forward of the wave drift forces would improve DP behavior of a ship in terms of fuel consumption as well as position keeping. In the present study the wave drift forces have been divided in a constant part and a low frequent oscillating part. The constant part directly depends on the directional wave energy spectrum. In this paper the directional spectrum and mean drift force will be estimated from six relative wave height measurements on a dynamically positioned vessel. The Extended Maximum Likelihood Method (EMLM) is known to make a reliable estimate of the directional wave spectrum from wave measurements at fixed locations in the wave field. For a wave feed forward application the EMLM had to be implemented on a moving ship. Six relative wave height probes have been installed on board of a shuttle tanker. The EMLM has been applied to these relative motions and the low frequent yawing motion has been taken into account to calculate an earth bound spectral estimate. The estimate for the spectrum is based on a 30min average and is updated every minute in a moving average algorithm. Finally, the mean wave drift force is calculated for the actual heading of the ship.

Yaw motion analysis of a FPSO turret mooring system under wave drift forces

2018 ◽  
Vol 74 ◽  
pp. 170-187 ◽  
Author(s):  
J. Sanchez-Mondragon ◽  
A.O. Vázquez-Hernández ◽  
S.K. Cho ◽  
H.G. Sung
Keyword(s):  
Motion Analysis ◽  
Mooring System ◽  
Wave Drift ◽  
Wave Drift Forces ◽  
Drift Forces

Analysis of Wave Drift Forces on a Floating Wave Energy Converter

2008 ◽  
Author(s):  
Nuno Fonseca ◽  
Ricardo Pascoal ◽  
Joa˜o Marinho ◽  
Tiago Morais
Keyword(s):  
Transfer Function ◽  
Wave Energy ◽  
Time History ◽  
Wave Energy Converter ◽  
Mooring System ◽  
Energy Converter ◽  
Wave Drift ◽  
The Mean ◽  
Wave Drift Forces ◽  
Drift Forces

Wave drift forces acting on floating wave energy converters (WEC) are often the most important loading component for the design of the mooring system. On one hand these forces may be, at least, one order of magnitude larger than wind and current forces, and on the other hand the floating structure and mooring system may respond dynamically to the slowly varying wave drift forces. The paper presents an analysis of the wave drift forces on an articulated floating wave energy converter. Particular attention is given to the effects of the wave energy extraction on the time history of the horizontal drift forces. The hydrodynamic calculations are carried out by a frequency domain Green function panel method, resulting on the transfer function of the WEC motions as well as the transfer function of the mean drift forces. The power takeoff system is represented by a simple linear model where the extracted power is related to the relative velocity in the articulation and the damper of the PTO. With the transfer function of the mean drift forces, the variance spectrum of these same forces is calculated for stationary irregular seastates, and finally time histories of the drift forces are produced for typical operational conditions.

Sign in / Sign up

Export Citation Format

Share Document