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.

scholarly journals Calibration of a Time-Domain Numerical Hydrodynamic Model for Mooring Analysis of a Semi-Submersible

2018 ◽  
Author(s):  
Nuno Fonseca ◽  
Carl Trygve Stansberg
Keyword(s):  
Test Data ◽  
Time Domain ◽  
Transfer Functions ◽  
Numerical Models ◽  
Low Frequency ◽  
Added Mass ◽  
Frequency Wave ◽  
Wave Drift ◽  
Wave Drift Forces ◽  
Drift Forces

The paper presents calibration of a time domain numerical model for the motions of the Exwave Semi in high seastates with current. The time domain equations of motion combine linear radiation, linear diffraction and second order wave drift forces, based on MULDIF diffraction code, with nonlinear forces from quadratic damping and from the mooring system. 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, damping and added mass coefficients. 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”. Initial “uncalibrated” numerical models are based on input from the mooring, vessel mass, MULDIF hydrodynamic analysis, decay tests and current coefficients. They need adjustments for surge and sway. Empirical drift coefficients, natural periods and damping coefficients are then adjusted by matching low frequency surge and sway spectra. Wave-frequency coefficients need no adjustment. Low frequency wave drift forces, damping and added mass need increase in high sea states, in particular with current. Final motion simulations show 30%–40% underestimation in initial simulations, while final calibrated simulations are close to the measured records.

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.

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.

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

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.

scholarly journals Hydrodynamic Responses of a 6 MW Spar-Type Floating Offshore Wind Turbine in Regular Waves and Uniform Current

Fluids ◽  
2020 ◽  
Vol 5 (4) ◽  
pp. 187
Author(s):  
Zhiping Zheng ◽  
Jikang Chen ◽  
Hui Liang ◽  
Yongsheng Zhao ◽  
Yanlin Shao
Keyword(s):  
Perturbation Method ◽  
Time Domain ◽  
Strong Influence ◽  
Offshore Wind ◽  
Regular Waves ◽  
Wave Drift ◽  
Uniform Current ◽  
The Time Domain ◽  
Wave Drift Forces ◽  
Drift Forces

In order to improve the understanding of hydrodynamic performances of spar-type Floating Offshore Wind Turbines (FOWTs), in particular the effect of wave-current-structure interaction, a moored 6MW spar-type FOWT in regular waves and uniform current is considered. The wind loads are not considered at this stage. We apply the potential-flow theory and perturbation method to solve the weakly-nonlinear problem up to the second order. Unlike the conventional formulations in the inertial frame of reference, which involve higher derivatives on the body surface, the present method based on the perturbation method in the non-inertial body-fixed coordinate system can potentially avoid theoretical inconsistency at sharp edges and associated numerical difficulties. A cubic Boundary Element Method (BEM) is employed to solve the resulting boundary-value problems (BVPs) in the time domain. The convective terms in the free-surface conditions are dealt with using a newly developed conditionally stable explicit scheme, which is an approximation of the implicit Crank–Nicolson scheme. The numerical model is firstly verified against three reference cases, where benchmark results are available, showing excellent agreement. Numerical results are also compared with a recent model test, with a fairly good agreement though differences are witnessed. Drag loads based on Morison’s equation and relative velocities are also applied to quantify the influence of the viscous loads. To account for nonlinear restoring forces from the mooring system, a catenary line model is implemented and coupled with the time-domain hydrodynamic solver. For the considered spar-type FOWT in regular-wave and current conditions, the current has non-negligible effects on the motions at low frequencies, and a strong influence on the mean wave-drift forces. The second-order sum-frequency responses are found to be negligibly small compared with their corresponding linear components. The viscous drag loads do not show a strong influence on the motions responses, while their contribution to the wave-drift forces being notable, which increases with increasing wave steepness.

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.

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