Recent Developments in the Analysis of Wave Drift Forces, Low-Frequency Damping and Response

1987 ◽  
Author(s):  
R.G. Standing ◽  
W.J. Brendling ◽  
D. Wilson
Keyword(s):  
Low Frequency ◽  
Wave Drift ◽  
Recent Developments ◽  
Wave Drift Forces ◽  
Drift Forces

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.

Determination of the Effect of Second Order Motions of Moored MODU on Wellhead Fatigue

2014 ◽  
Author(s):  
Dara Williams ◽  
Patrick Ashton
Keyword(s):  
Fatigue Damage ◽  
Water Depth ◽  
Low Frequency ◽  
Fatigue Analysis ◽  
Second Order ◽  
First Order ◽  
Analysis Techniques ◽  
Wave Drift ◽  
Wave Drift Forces ◽  
Drift Forces

As has been noted in industry publications and conferences in the recent past the use of more modern deepwater capable 5th and 6th generation semisubmersible drilling rigs in relatively shallow water applications (when compared to design water depth) is likely to become more commonplace. Water depths of 500m or less will necessitate the use of mooring systems in order to maintain position close to the well centre whilst drilling. For fatigue assessments of moored MODUs, the current industry practice to estimate fatigue damage in the drilling riser and the wellhead, using global riser analysis techniques, is to consider both wave and VIV fatigue effects. Standard wave fatigue analysis considers two key response parameters, firstly the impact of the hydrodynamic loading on the riser joints due to drag forces, inertia and added mass effects, and secondly the effects of vessel motions on the riser system and wellhead loading. Standard practice for wave fatigue analysis is to consider only first order motion effects as described by the vessel RAO (response amplitude operator). However, for a moored MODU low frequency (100s-200s period) vessel response can have a significant impact on the overall vessel motions. The actual response and magnitude of MODU motion will be influenced by the size and displacement of the vessel in addition to the configuration of the mooring system. First order lateral motions for a semisubmersible tend to increase as wave period is increased and therefore at lower periods first order motions can be quite low. However, the opposite can be said of wave drift forces that contribute to second order response. Although the wave drift forces are largest for lower wave periods, these low period drift forces have a significant influence on the resulting long period second order response of a moored MODU. This has important implications for drilling riser and wellhead fatigue analysis as in many cases the critical seastates for fatigue damage are low period seastates with a large number of occurrences. Thus the current global analysis techniques for fatigue calculations may lead to an underestimation of fatigue damage contribution from low period seastates. The purpose of this paper is to present the key conclusions and findings of a study carried out in order to determine the effects of low frequency moored MODU motions on wellhead fatigue. These results are derived from a case study of a moored 6th generation semi-submersible drilling vessel in 500m water depth.

Low Frequency Motions of LNG Carriers Moored in Shallow Water

2004 ◽  
Author(s):  
Mamoun Naciri ◽  
Bas Buchner ◽  
Tim Bunnik ◽  
Rene´ Huijsmans ◽  
Jerome Andrews
Keyword(s):  
Shallow Water ◽  
Low Frequency ◽  
Test Program ◽  
Analysis Methodology ◽  
Wave Drift ◽  
Water Effects ◽  
Near Shore ◽  
Numerical Tools ◽  
Wave Drift Forces ◽  
Drift Forces

With the LNG market booming, the need for reliable and safe means of transferring LNG from a producing, floating facility to an LNG carrier and from this carrier to a near-shore terminal is becoming acute. The Soft Yoke Mooring and Offloading (SYMO©) system has recently been model tested in MARIN’s offshore basin. Results of these tests are presented. Insight has been gained, from these model tests and from the calibration of numerical tools performed thereafter, on the following issues: • The inherent weakly damped nature of a moored LNG carrier, • Shallow water effects in wave drift forces, • The effect of current on drift forces, • The structure of low frequency long waves in a shallow water basin. These issues will be discussed and guidance regarding their importance will be provided. Consequences in terms of system design, mooring analysis methodology and model test program will be discussed.

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

Estimating Second Order Wave Drift Forces and Moments for Calculating DP Capability Plots

2019 ◽  
Author(s):  
Saeed Barzegar Valikchali ◽  
Mitchell Anderson ◽  
David Molyneux ◽  
Dean Steinke
Keyword(s):  
Wind Waves ◽  
Low Frequency ◽  
Offshore Structures ◽  
Second Order ◽  
Design Stage ◽  
Wave Direction ◽  
Dynamic Positioning System ◽  
Wave Drift ◽  
Wave Drift Forces ◽  
Drift Forces

Abstract The DP capability plot is a useful tool to show the limitations of a dynamic positioning system for ships or offshore structures under loading from wind, waves and ocean currents. At the preliminary design stage, it is desirable to use fast methods for calculating the forces and moments caused by the environment, preferably without the need for CFD simulations or model experiments. Empirical methods are available for predicting aerodynamic forces and moments, and hydrodynamic forces and moments from currents, but little is published for second order wave drift forces. Wave drift forces and moment calculations have been carried out using WAMIT, for a series of ship hulls from OSVs to VLCCs and the effects of wave direction and frequency on the Surge, Sway, and Yaw forces and moment have been studied. The presentation of the results allows the user to interpolate the resulting drift forces and moments as a function of wave direction for a given ship size. In terms of wave drift loads calculation, it is found that the very large vessels are dominant in the low frequency waves, while smaller size ships are in high frequencies. The wave frequency and direction in which maximum drift load occurs depends on the ship size.

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.

Wave Drift Forces in Directional Seas in Shallow Water

2009 ◽  
Author(s):  
J. A. Pinkster
Keyword(s):  
Shallow Water ◽  
Near Field ◽  
Low Frequency ◽  
Computational Method ◽  
Direct Evaluation ◽  
Beam Seas ◽  
Wave Drift ◽  
Wave Effects ◽  
Wave Drift Forces ◽  
Drift Forces

Wave drift forces in shallow water are dominated by low frequency bound wave effects. The effect of short-crestedness of the waves on the wave drift forces is investigated based on direct evaluation of the near field pressure equations in irregular directional seas. The background to the method is treated and examples of results showing some of the details of the computational method are given. Some trends found for the effect of the short-crestedness on the second order wave drift forces on a typical LNG carrier moored in shallow water are presented. Finally, results are given of low-frequency horizontal motions of the LNG carrier in directionally spread head, bow-quartering and beam seas for a range of directional spreading values.

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.

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