Comparative Study of Motions and Drift Forces in Waves and Current

2017 ◽  
Author(s):  
Florian Sprenger ◽  
Elin Marita Hermundstad ◽  
Jan Roger Hoff ◽  
Dariusz Fathi ◽  
Ørjan Selvik
Keyword(s):  
Degrees Of Freedom ◽  
Ocean Basin ◽  
Model Tests ◽  
Measuring System ◽  
Diffraction Radiation ◽  
Station Keeping ◽  
Wave Drift ◽  
Wave Drift Forces ◽  
General Cargo ◽  
Drift Forces

Station keeping analysis is an important activity in the early stages of any vessel/DP project that eventually determines the machinery and thruster configuration and thruster size selection. In order to obtain reliable results, it is crucial to apply engineering tools that realistically represent the flow physics and resulting hydrodynamic forces. Present computer tools are based on the assumption that wave drift- and current forces can be superimposed. However, there are also mutual interaction effects between waves, current and hulls that should be accounted for in the evaluation of the wave drift forces. In MULDIF, a 3D diffraction/radiation panel code developed by SINTEF Ocean within the framework of a JIP, this wave-current-body interaction is taken into consideration by a new potential flow numerical model. A case study with offshore vessels and general cargo ships of different main dimensions has been performed to assess the capabilities of MULDIF for station keeping purposes in wave and current environments. The first-order vessel motions as well as mean second-order drift forces for 0 kn forward speed without current have been calculated. Through an interface to SINTEF Ocean’s vessel response code VERES, MULDIF offers the possibility to include viscous roll damping due to hull friction, flow separation at bilge keels, lift effects as well as normal forces acting on bilge keels and hull pressure created by the presence of bilge keels. This reduces roll motions to a realistic extent as shown by the comparison of RAOs from MULDIF calculations and model tests. Roll reduction tank effects can currently only be considered through the external damping matrix. Model tests for the selected vessels have been performed in SINTEF’s Ocean Basin in a soft-mooring arrangement in different irregular sea states and headings in deep water. The models were equipped with two two-component force transducers, measuring the x- and y- components of the forces. The yaw moments have been calculated from the y-force measurements. In order to measure the vessel motions in six degrees of freedom, an optoelectronic position measuring system has been used. Selected cases illustrate the significant influence of wave-current interaction on motions and drift forces.

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.

Improved Dynamic Positioning Using Wave Feed Forward

2011 ◽  
Author(s):  
Frans Quadvlieg ◽  
Rink Hallmann ◽  
Greg Hughes ◽  
Rick Harris
Keyword(s):  
Wave Field ◽  
Wave Height ◽  
Pressure Sensors ◽  
Model Tests ◽  
Wave Forces ◽  
Feed Forward ◽  
Wave Drift ◽  
Local Wave ◽  
Wave Drift Forces ◽  
Drift Forces

Jo Pinkster made the first attempt to estimate 2nd order wave drift forces. In his PhD thesis from 1980, the first practical application of wave feed forward in DP was demonstrated both theoretically and in model tests. Knowledge of the local wave field was used to estimate the 2nd order wave drift forces. The local wave field was converted in wave forces and fed back in the DP system. The use of this knowledge in a DP system should lead to a better position keeping. Since Pinksters’ thesis 30 years ago, this technique has been tried several times with varying success. However, in 2009, the ‘nut was cracked’ and a good success was undoubtedly demonstrated. In 2008–2009, this method has been developed and applied in DP model tests on a ship equipped with azimuthing thrusters. The use of Wave Feed Forward resulted in a reduction of the watch circle by a factor of two. Important for the success of wave feed forward was the filtering of the measured wave signals to predict the wave forces with a limited delay. The performance is demonstrated during model tests in MARIN’s Seakeeping and Manoeuvring Basin at two speeds of 0 knots and 4 knots, uni-directional and multidirectional seas. Besides the application of wave feed forward for a single ship, wave feed forward is used in a side-by-side condition at zero speed and ahead speed. For both speeds, wave feed forward did not provide a significant improvement in DP accuracy. The objective was to make wave feed forward applicable to: zero and forward speed; on a ship alone and on ships sailing side-by-side; in unidirectional and multi-directional waves, with a realistic amount of sensors and as target wave heights, sea state 3 and 4 were envisaged. To measure the local wave height, wave height measurement sensors as well as pressure sensors were used. The pressure sensors can be mounted below the waterline and deliver an accurate estimation for the wave drift forces as well.

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

2021 ◽  
Author(s):  
Min Zhang ◽  
Junrong Wang ◽  
Junfeng Du ◽  
Nuno Miguel Magalhaes Duque Da Fonseca ◽  
Galin Tahchiev ◽  
...  
Keyword(s):  
Time Domain ◽  
Hydrodynamic Model ◽  
Wave Drift ◽  
Wave Drift Forces ◽  
Drift Forces

Analysis of Semi-Submersible Under Combined High Waves and Current Conditions Compared With Model Tests

2018 ◽  
Author(s):  
Limin Yang ◽  
Arne Nestegård ◽  
Erik Falkenberg
Keyword(s):  
Simulation Model ◽  
Low Frequency ◽  
Model Tests ◽  
Wave Forces ◽  
Viscous Effects ◽  
Numerical Predictions ◽  
Wave Drift ◽  
Zero Current ◽  
Frequency Excitation ◽  
Wave Drift Forces

Viscous effects on the low-frequency excitation force on column based platforms are significant in extreme waves. The wave drift force as calculated by a zero-current potential flow radiation/diffraction code becomes negligible for such waves. In the present study, the effect of current and viscous contributions on the slowly varying wave forces are adjusted by a formula developed in the Exwave JIP, see e.g. [1], which is validated against model test results. This paper presents numerical predictions of low frequency horizontal motions of a semi-submersible in combined high waves and current condition. In the simulation model, frequency dependent wave drift forces from radiation/diffraction code are modified by the formula. Static current forces and viscous damping are modelled by the drag term in Morison load formula using relative velocity between current and floater and with force coefficients as recommended by DNVGL-RP-C205 [2]. Low frequency surge responses calculated by the simulation model are compared with model tests for waves only and for combined collinear and noncollinear wave and current conditions.

Mooring Dynamics Phenomena Due to Slowly-Varying Wave Drift

2002 ◽  
Author(s):  
Michael M. Bernitsas ◽  
Joa˜o Paulo J. Matsuura ◽  
Torgrim Andersen
Keyword(s):  
Dynamic Responses ◽  
Resonance Phenomenon ◽  
University Of Michigan ◽  
Wave Drift ◽  
Mooring Dynamics ◽  
Wave Drift Forces ◽  
The University ◽  
New Phenomena ◽  
External Time ◽  
Drift Forces

The effects of slowly-varying wave drift forces on the nonlinear dynamics of mooring systems have been studied extensively in the past 30 years. It has been concluded that slowly-varying wave drift may resonate with mooring system natural frequencies. In recent work, we have shown that this resonance phenomenon is only one of several possible nonlinear dynamic responses of mooring systems to slowly-varying wave drift excitation. We were able to reveal new phenomena based on the design methodology developed at the University of Michigan for autonomous mooring systems and treating slowly-varying drift as an external time-varying force. In this paper, the U of M methodology is used systematically to reveal seven phenomena induced by mean and slowly-varying drift forces; one of those is resonance. Conceptually, numerous qualitatively different behaviors may be induced. The next step towards comprehensive identification of such phenomena is taken by introducing the method of harmonic balance to study nonautonomous mooring systems.

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