Viscous Drift Forces and Responses on a Semisubmersible Platform in High Waves

2009 ◽  
Author(s):  
Petter Andreas Berthelsen ◽  
Rolf Baarholm ◽  
Csaba Pa´kozdi ◽  
Carl Trygve Stansberg ◽  
Amany Hassan ◽  
...  
Keyword(s):  
Low Frequency ◽  
Irregular Waves ◽  
Wave Zone ◽  
Free Surface Elevation ◽  
Viscous Effects ◽  
Sea State ◽  
Viscous Forces ◽  
Wave Basin ◽  
Drift Forces ◽  
Drag Term

In this paper the significance of viscous effects on a moored semisubmersible platform in various sea state conditions is explored. Experimental data from a 1:50 model scale tests in a 50m × 80m wave basin are compared with numerical time domain simulations. Both regular and irregular waves are included, and some tests are also run with current. This paper presents results from a horizontally moored semisubmersible, where we focus on the hull hydrodynamics alone. The emphasis here is low frequency surge responses. Use of conventional potential theory shows large discrepancies when compared with experimental results in high sea states. For surge motion, they are believed to be due to viscous forces in the wave zone. Viscous forces and damping in the numerical model are included by the drag term of Morison’s equation using a total relative velocity approach, which is integrated up to the instantaneous free surface elevation. A common challenge is to choose a suitable Cd coefficient which provides for sufficient excitation force without introducing excessive motion damping. It is found that using a larger Cd coefficient in the wave zone gives larger excitation without influencing the total damping significantly. This way, applying the drag term of Morison’s equation can give results that compare well with measurements. Also, the coefficient is found to be lower in waves with current than in waves only.

Current Effects on a Moored Floating Platform in a Sea State

2008 ◽  
Author(s):  
Carl Trygve Stansberg
Keyword(s):  
Irregular Waves ◽  
Mooring Line ◽  
Viscous Effects ◽  
Floating Platform ◽  
Sea State ◽  
Scaled Model ◽  
Wave Drift ◽  
Wave Basin ◽  
Wave Drift Damping ◽  
Non Gaussian

The significance of current-induced forces and effects on a moored semisubmersible production platform in various sea state conditions is explored, with emphasis on surge motions. Experimental data from 1:55 scaled model tests in a 50m × 80m wave basin are investigated. A description of the current generation is given first. The current in the actual basin is modelled by use of a return current under a false bottom. The importance of modelling a “real” physical current for the proper reproduction of platform responses is pointed out. The semisubmersible tests are carried out with the platform in current only, in irregular waves only, and in combined waves and current conditions. The effects from the current on platform motions and mooring line tensions are investigated. Vortex-Induced motions (VIM) are observed in pure current, depending on the actual combination of current velocity and natural sway period. In combined waves and current the VIM seems to be more or less disappearing. A large effect is seen on the wave drift responses. Both drift forces, non-Gaussian properties and resulting extreme motions and line tensions are significantly increased, especially in high sea states. This is explained through a combination of wave drift damping and viscous effects. At the same time the damping is also increased, but this only partly compensates for the increased forces.

scholarly journals Low Frequency Excitation and Damping of Four MODUs in Severe Seastates With Current

2018 ◽  
Author(s):  
Nuno Fonseca ◽  
Carl Trygve Stansberg ◽  
Kjell Larsen ◽  
Rune Bjørkli ◽  
Tjerand Vigesdal ◽  
...  
Keyword(s):  
Transfer Functions ◽  
Low Frequency ◽  
Irregular Waves ◽  
Experimental Program ◽  
Peak Period ◽  
Numerical Predictions ◽  
Analysis Technique ◽  
Wave Drift Forces ◽  
Drift Forces

Model tests have been performed with four mobile offshore drilling units (MODUs) with the aim of identifying wave drift forces and low frequency damping. The MODUs configuration is different, namely on the number and diameter of columns, therefore the sample is representative of many of the existing concepts. The model scale is the same as well as the wave and current conditions. The experimental program includes irregular waves with systematic variations of the significant wave height, wave peak period, current velocity and vessel heading. The test data is post-processed to identify the surge and sway quadratic transfer functions (QTFs) of the slowly varying excitation, together with the linearized low frequency damping. The post-processing applies a nonlinear data analysis technique known as “cross-bi-spectral analysis” to estimate characteristics of second-order (quadratic) responses from the measured motions and undisturbed incident wave elevation. The empirical QTFs are then compared with numerical predictions to conclude on the role of viscous drift and the applicability of Newman’s approximation for calculation of drift forces in irregular waves. Finally, the empirical drift forces, empirical low frequency damping coefficients and low frequency motions statistics are compared for the three MODUs to conclude on the relation between the Semi configuration and the low frequency responses.

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.

Mean- and Low-Frequency Wave Forces on Semisubmersibles

1982 ◽  
Vol 22 (04) ◽  
pp. 563-572
Author(s):  
J.A. Pinkster
Keyword(s):  
Low Frequency ◽  
Second Order ◽  
Irregular Waves ◽  
Wave Forces ◽  
Frequency Wave ◽  
First Order ◽  
Wave Drift ◽  
Frequency Components ◽  
Wave Drift Forces ◽  
Drift Forces

Abstract Mean- and low-frequency wave drift forces on moored structures are important with respect to low-frequency motions and peak mooring loads. This paper addresses prediction of these forces on semisubmersible-type structures by use of computations based on three-dimensional (3D) potential theory. The discussion includes a computational method based on direct integration of pressure on the wetted part of the hull of arbitrarily shaped structures. Results of computations of horizontal drift forces on a six-column semisubmersible are compared with model tests in regular and irregular waves. The mean vertical drift forces on a submerged horizontal cylinder obtained from model tests also are compared with results of computations. On the basis of these comparisons, we conclude that wave drift forces on semisubmersible-type structures in conditions of waves without current can be predicted with reasonable accuracy by means of computations based on potential theory. Introduction Stationary vessels floating or submerged in irregular waves are subjected to large first-order wave forces and moments that are linearly proportional to the wave height and that contain the same frequencies as the waves. They also are subjected to small second-order mean- and low- frequency wave forces and moments that are proportional to the square of the wave height. Frequencies of second-order low-frequency components are associated with the frequencies of wave groups occurring in irregular waves.First-order wave forces and moments cause the well-known first-order motions with wave frequencies. First-order wave forces and motions have been investigated for several decades. As a result of these investigations, methods have been developed to predict these forces and moments with reasonable accuracy for many different vessel shapes.For semisubmersibles, which consist of a number of relatively slender elements such as columns, floaters, and bracings, computation methods have been developed to determine the hydrodynamic loads on those elements without accounting for interaction effects between the elements. For the first-order wave loads and motion problem, these computations give accurate results.This paper deals with the mean- and low-frequency second-order wave forces acting on stationary vessels in regular and irregular waves in general and presents a method to predict these forces on the basis of computations.The importance of mean- and low-frequency wave drift forces, from the point of view of motion behavior and mooring loads on vessels moored at point of view of motion behavior and mooring loads on vessels moored at sea, has been recognized only within the last few years. Verhagen and Van Sluijs, Hsu and Blenkarn, and Remery and Hermans showed that the low-frequency components of wave drift forces in irregular waves-even though relatively small in magnitude-can excite large-amplitude low- frequency horizontal motions in moored structures. It was shown for irregular waves that the drift forces contain components with frequencies coinciding with the natural frequencies of the horizontal motions of moored vessels. Combined with minimal damping of low-frequency horizontal motions of moored structures, this leads to large-amplitude resonant behavior of the motions (Fig. 1). Remery and Hermans established that low-frequency components in drift forces are associated with the frequencies of wave groups present in an irregular wave train.The vertical components of the second-order forces sometimes are called suction forces. SPEJ p. 563

How to Deal With Basin Modes When Generating Irregular Waves on Shallow Water

2016 ◽  
Author(s):  
Sanne van Essen ◽  
Willemijn Pauw ◽  
Joris van den Berg
Keyword(s):  
Shallow Water ◽  
Water Depth ◽  
Water Waves ◽  
Low Frequency ◽  
Effective Length ◽  
Irregular Waves ◽  
Natural Modes ◽  
Wave Basin ◽  
Basin Geometry ◽  
Basin Mode

Modeling shallow-water waves in a basin with a finite length and width introduces challenges related to low-frequency (LF) waves, especially for testing of moored vessels with long natural periods. Waves in this frequency range are also present in reality, as for instance bound set-down waves and unbound free waves formed by the geometry bathymetry. In model basins, additional unwanted LF wave components will be formed as a side-product of the wave generation and due to the basin geometry though. Standing waves over the basin length and width (basin modes) can generally be identified, which are difficult to dampen using beaches. This is the case for every wave basin, as they all have finite dimensions. Moored structures generally have natural frequencies in the LF range, which may be excited by basin modes with similar frequencies. It is therefore important to understand the natural modes of a basin before tests with moored structures in shallow water are done. The energy of these basin modes increases and their natural frequency decreases with decreasing water depth (waves travel slower in shallow water). Generally, it can be said that the issues with basin modes are present on very shallow water (typically ∼15–30 m water depth full-scale for structures with a length around 200 m at a scale around 1 to 40). The smaller the basin for the same water depth, the higher the basin mode frequencies and the higher the likelihood of resonance problems. The energy and frequencies of the basin modes and their relevance for specific tests depend on the effective length and width of the basin, the water depth, wave conditions and the (mooring stiffness of) the structure under consideration. The influence of these variables is evaluated in the current study. Tests were done in MARIN’s Offshore Basin (OB), but most of the results are also expected to be applicable to other basins. The observed basin mode frequencies during these tests were compared to the theoretical values, and an overview of the unwanted LF wave content as a function of water depth, wave height and period is presented. The energy and shape of individual basin modes is also discussed. Considering these results, a practical approach for future basin projects on shallow water is described.

Damping Identification of the TetraSpar Floater in Two Configurations With Operational Modal Analysis

2019 ◽  
Author(s):  
Antonio Pegalajar-Jurado ◽  
Freddy J. Madsen ◽  
Henrik Bredmose
Keyword(s):  
Wind Turbine ◽  
Modal Analysis ◽  
Numerical Models ◽  
Operating Conditions ◽  
Operational Modal Analysis ◽  
Viscous Effects ◽  
Sea State ◽  
Hydrodynamic Damping ◽  
Wave Basin ◽  
Motion Amplitude

Abstract Second-order hydrodynamic loads can induce motions at the natural frequencies of a floating wind turbine. These resonant responses are highly dependent on the hydrodynamic damping, which is mostly introduced by viscous effects. Numerically, these viscous effects are often represented by a Morison drag term with relative velocity, which introduces forcing, sea state-dependent linear damping and amplitude-dependent quadratic damping. Recent literature shows that calibration of the Morison drag coefficients to decay tests is not sufficient to achieve an accurate response in the numerical models. In addition, calibration of the drag coefficient alone changes both forcing and damping. Hence, following common practice, additional damping terms are needed, which require calibration against operating conditions. In this study, we apply Operational Modal Analysis (OMA) to wave basin results for the TetraSpar floater of Stiesdal Offshore Technologies. The floater was tested at scale 1:60 with the DTU 10MW reference wind turbine, both in the semi and spar configurations. We identify the linearized damping ratio in surge and pitch for different environmental conditions and investigate its dependency on the sea state and the motion amplitude. Our preliminary results show that the damping of the pitch mode follows increasing trends with significant wave height and motion amplitude, whereas the damping in surge presents a less clear tendency. This is linked to the larger damping level, smaller natural frequency and larger OMA uncertainty for surge. The paper concludes with a discussion of the dependency of OMA estimates on the amount of data and its processing.

Effects of Wave Drift Forces on Maneuvering of Ship

2008 ◽  
Author(s):  
Takeshi Kinoshita ◽  
Weiguang Bao ◽  
Motoki Yoshida ◽  
Yasunori Nihei ◽  
Yongze Xu ◽  
...  
Keyword(s):  
Dynamic Positioning ◽  
Positioning System ◽  
Ocean Engineering ◽  
Sea State ◽  
Dynamic Positioning System ◽  
Wave Drift ◽  
Wave Basin ◽  
Wave Drift Damping ◽  
Wave Drift Forces ◽  
Drift Forces

The dynamic positioning system of floating ocean structures requires hydrodynamic force derivatives to construct an accurate maneuvering model. In a severe sea state, the effects of ambient wave field on the maneuvering properties are not negligible. To investigate wave drift forces affecting on maneuvering of a ship relating to dynamic positioning system, an innovative model test, i.e. the Planer Motion Mechanism (PMM) test in waves is discussed in the present paper. Meanwhile, a theory to evaluate wave drift force including wave drift damping and wave drift added mass is summarized. Some examples of experiments done in Ocean Engineering Wave Basin of Institute of Industrial Science, University of Tokyo are presented and compared with calculated results based on the above theory.

The Estimation of Laminar Skin Friction, including the Effects of Distributed Suction

1960 ◽  
Vol 11 (1) ◽  
pp. 1-21 ◽  
Author(s):  
N. Curle
Keyword(s):  
Boundary Layer ◽  
Skin Friction ◽  
Laminar Boundary Layer ◽  
Simple Formula ◽  
Outer Part ◽  
Order Correction ◽  
Viscous Effects ◽  
Viscous Forces ◽  
Exact Theory ◽  
Total Head

SummaryStratford's analysis of the laminar boundary layer near separation uses two physical ideas. In the outer part of the boundary layer, where viscous effects are small, the development is given by the condition that the total head is constant along streamlines, apart from a second-order correction for viscosity. Near the wall, however, viscous forces must balance the pressure forces, and the profile adjusts itself accordingly. Quantitatively these ideas yield a simple formula for predicting separation, which has been found to be particularly accurate.In this paper it is indicated how the same approach may be used to yield the full distribution of skin friction along the wall. Further, the effects of suction may be incorporated into the method. Physically, suction affects the outer part of the boundary layer in that the streamlines are drawn towards the wall when suction is applied. At the wall, the balance between viscous and pressure forces is influenced by the momentum of the fluid which is sucked away. When these effects are accounted for quantitatively, the resulting formula for the skin friction is still very simple.Several examples are considered, and comparison is made with exact theory and with approximate results by other methods. It is indicated that the method has a useful range of validity.

Fully Nonlinear Potential/RANSE Simulation of Wave Interaction With Ships and Marine Structures

2008 ◽  
Author(s):  
Pierre Ferrant ◽  
Lionel Gentaz ◽  
Bertrand Alessandrini ◽  
Romain Luquet ◽  
Charles Monroy ◽  
...  
Keyword(s):  
Degrees Of Freedom ◽  
Stokes Equations ◽  
Wave Interaction ◽  
Irregular Waves ◽  
Navier Stokes ◽  
Viscous Effects ◽  
Navier Stokes Equations ◽  
Fully Nonlinear ◽  
Nonlinear Potential ◽  
6 Degrees Of Freedom

This paper documents recent advances of the SWENSE (Spectral Wave Explicit Navier-Stokes Equations) approach, a method for simulating fully nonlinear wave-body interactions including viscous effects. The methods efficiently combines a fully nonlinear potential flow description of undisturbed wave systems with a modified set of RANS with free surface equations accounting for the interaction with a ship or marine structure. Arbitrary incident wave systems may be described, including regular, irregular waves, multidirectional waves, focused wave events, etc. The model may be fixed or moving with arbitrary speed and 6 degrees of freedom motion. The extension of the SWENSE method to 6 DOF simulations in irregular waves as well as to manoeuvring simulations in waves are discussed in this paper. Different illlustative simulations are presented and discussed. Results of the present approach compare favorably with available reference results.

Numerical Study on Hydrodynamic Responses of a Two-Body System in Side-by-Side Operation

2016 ◽  
Author(s):  
Shaowu Ou ◽  
Shixiao Fu ◽  
Wei Wei ◽  
Tao Peng ◽  
Xuefeng Wang
Keyword(s):  
Numerical Study ◽  
Low Frequency ◽  
Optimal Operation ◽  
Hydrodynamic Interactions ◽  
Irregular Waves ◽  
Mooring System ◽  
Time Varying ◽  
Body System ◽  
The Time Domain ◽  
Two Bodies

Typically, in some side-by-side offshore operations, the speed of vessels is very low or even 0 and the headings are manually maneuvered. In this paper, the hydrodynamic responses of a two-body system in such operations under irregular seas are investigated. The numerical model includes two identical PSVs (Platform Supply Vessel) as well as the fenders and connection lines between them. A horizontal mooring system constraining the low frequency motions is set on one of the ships to simulate maneuver system. Accounting for the hydrodynamic interactions between two bodies, 3D potential theory is applied for the analysis of their hydrodynamic coefficients. With wind and current effects included, these coefficients are further applied in the time domain simulations in irregular waves. The relevant coefficients are estimated by experiential formulas. Time-varying loads on fenders and connection lines are analyzed. Meanwhile, the relative motions as well as the effects of the hydrodynamic interactions between ships are further discussed, and finally an optimal operation scheme in which operation can be safely performed is summarized.

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