Shallow Water Effects On Low-Frequency Wave Excitation of Moored Ships

2004 ◽  
Author(s):  
Robert Grant ◽  
Monica Holboke
Keyword(s):  
Shallow Water ◽  
Low Frequency ◽  
Wave Excitation ◽  
Frequency Wave ◽  
Water Effects

Low Frequency Wave Forces and Wave Induced Motions of a FPSO in Shallow Water

2007 ◽  
Author(s):  
Longfei Xiao ◽  
Jianmin Yang ◽  
Zhiqiang Hu
Keyword(s):  
Shallow Water ◽  
Wave Spectrum ◽  
Low Frequency ◽  
Wave Force ◽  
Wave Forces ◽  
Frequency Wave ◽  
Wave Basin ◽  
The Difference ◽  
The One ◽  
Wave Induced

The low frequency (LF) response of a soft yoke moored 160kDWT FPSO in shallow water is investigated by conducting frequency domain computations and wave basin model tests. An incident wave with Hs = 4.1m and Tp = 8.9s is applied. An obvious LF part appears in the measured wave spectrum at water depth of 16.7m. As a result, the 1st order LF wave force exists and is much larger than the 2nd one. The difference of the spectrums is about one hundred times. The LF wave drift force increases enormously. Consequently, much larger resonant surge response is induced. The LF surge amplitude at h = 16.7m is about 7 times the one at h = 29.0m and 9 times the one in deep water, although the 2nd order response changes a little. Therefore, in very shallow water, LF part of incident waves should be taken into account carefully and LF wave forces and wave induced motions will be very serious.

Laboratory Wave Modelling for Floating Structures in Shallow Water

2006 ◽  
Author(s):  
Carl Trygve Stansberg
Keyword(s):  
Shallow Water ◽  
Low Frequency ◽  
Second Order ◽  
Wave Groups ◽  
Large Wave ◽  
Frequency Wave ◽  
Wave Modelling ◽  
Floating Structures ◽  
Wave Basin ◽  
Wave Drift Forces

The analysis of moored floating vessels in shallow water requires special attention, when compared to similar problems in deep water. In particular, low-frequency wave drift forces need to be studied. Model testing is essential in validation of numerical prediction tools for these problems. Wave-group induced low-frequency wave components is an important part of the problem. Their reproduction in laboratories needs special attention. In general, two types of low-frequency waves are present: “bound” waves following the wave groups, and “free” waves propagating with their own speed. The former is included in second-order numerical codes for floater is included in second-order numerical codes for floaters, while the latter is normally not. Therefore, identification and possible reduction of the free components is of interest. A practical way to do this in a large wave basin is described in this paper. Results from generation of bi-chromatic waves without and with correction are presented. Corrected results show a clear reduction of the free wave component.

Viscous Drift Force and Motion Analysis of Semi-Submersible in Storm Sea States Compared With Model Tests

2017 ◽  
Author(s):  
Limin Yang ◽  
Erik Falkenberg ◽  
Arne Nestegård ◽  
Jørn Birknes-Berg
Keyword(s):  
Frequency Response ◽  
Potential Flow ◽  
Low Frequency ◽  
Model Tests ◽  
Viscous Drag ◽  
Wave Forces ◽  
Linear Wave ◽  
Wave Excitation ◽  
Frequency Wave ◽  
Drift Force

Standard analysis models applied for motions of moored floaters are based on potential flow perturbation methods with wave frequency response governed by first order wave forces and low-frequency response governed by second-order difference frequency wave forces. These models have been shown to have limitations in extreme sea states where nonlinear wave excitation and viscous drag forces above still water level may dominate. This effect is particularly visible for the low frequency excitation since the potential flow contribution goes to zero for long waves. In the present study non-linear wave excitation and viscous drag contributions on a semi-submersible is modelled by Morison’s load formula since the columns and pontoons are slender elements. A numerical simulation model is developed using SIMO [6], in which viscous forces and damping are included by the drag term of Morison equation and with drag coefficients recommended from DNV-RP-C205 [1]. Low frequency surge responses calculated by the combined potential flow drift forces and viscous drag from Morison load model are compared with model tests for waves only and for combined wave and current conditions. A simplified formula for current and viscous effects on wave drift force, generalized to non-collinear conditions is presented and compared with model test results.

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.

scholarly journals Triggered Convection, Gravity Waves, and the MJO: A Shallow-Water Model

2013 ◽  
Vol 70 (8) ◽  
pp. 2476-2486 ◽  
Author(s):  
Da Yang ◽  
Andrew P. Ingersoll
Keyword(s):  
Shallow Water ◽  
Large Scale ◽  
Low Frequency ◽  
Propagation Speed ◽  
Water Model ◽  
Shallow Water Model ◽  
Horizontal Scale ◽  
Frequency Wave ◽  
Large Scale Structures ◽  
Scale Structures

Abstract The Madden–Julian oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics. Despite its primary importance, a generally accepted theory that accounts for fundamental features of the MJO, including its propagation speed, planetary horizontal scale, multiscale features, and quadrupole structures, remains elusive. In this study, the authors use a shallow-water model to simulate the MJO. In this model, convection is parameterized as a short-duration localized mass source and is triggered when the layer thickness falls below a critical value. Radiation is parameterized as a steady uniform mass sink. The following MJO-like signals are observed in the simulations: 1) slow eastward-propagating large-scale disturbances, which show up as low-frequency, low-wavenumber features with eastward propagation in the spectral domain, 2) multiscale structures in the time–longitude (Hovmöller) domain, and 3) quadrupole vortex structures in the longitude–latitude (map view) domain. The authors propose that the simulated MJO signal is an interference pattern of westward and eastward inertia–gravity (WIG and EIG) waves. Its propagation speed is half of the speed difference between the WIG and EIG waves. The horizontal scale of its large-scale envelope is determined by the bandwidth of the excited waves, and the bandwidth is controlled by the number density of convection events. In this model, convection events trigger other convection events, thereby aggregating into large-scale structures, but there is no feedback of the large-scale structures onto the convection events. The results suggest that the MJO is not so much a low-frequency wave, in which convection acts as a quasi-equilibrium adjustment, but is more a pattern of high-frequency waves that interact directly with the convection.

scholarly journals Uncertainty Assessment of CFD Investigation of the Nonlinear Difference-Frequency Wave Loads on a Semisubmersible FOWT Platform

2020 ◽  
Vol 13 (1) ◽  
pp. 64
Author(s):  
Lu Wang ◽  
Amy Robertson ◽  
Jason Jonkman ◽  
Yi-Hsiang Yu
Keyword(s):  
Low Frequency ◽  
Uncertainty Assessment ◽  
Difference Frequency ◽  
Offshore Wind ◽  
Wave Loads ◽  
Wave Excitation ◽  
Wave Load ◽  
Frequency Wave ◽  
Floating Offshore Wind Turbines ◽  
Frequency Excitation

Current mid-fidelity modeling approaches for floating offshore wind turbines (FOWTs) have been found to underpredict the nonlinear, low-frequency wave excitation and the response of semisubmersible FOWTs. To examine the cause of this underprediction, the OC6 project is using computational fluid dynamics (CFD) tools to investigate the wave loads on the OC5-DeepCwind semisubmersible, with a focus on the nonlinear difference-frequency excitation. This paper focuses on assessing the uncertainty of the CFD predictions from simulations of the semisubmersible in a fixed condition under bichromatic wave loading and on establishing confidence in the results for use in improving mid-fidelity models. The uncertainty for the nonlinear wave excitation is found to be acceptable but larger than that for the wave-frequency excitation, with the spatial discretization error being the dominant contributor. Further, unwanted free waves at the difference frequency have been identified in the CFD solution. A wave-splitting and wave load-correction procedure are presented to remove the contamination from the free waves in the results. A preliminary comparison to second-order potential-flow theory shows that the CFD model predicted significantly higher difference-frequency wave excitations, especially in surge, suggesting that the CFD results can be used to better calibrate the mid-fidelity tools.

The Dynamic Response of a Moored Vessel in Shallow Water Using Boussinesq Equations

2009 ◽  
Author(s):  
Byeong W. Park ◽  
Rae H. Yuck ◽  
Seok K. Cho ◽  
Hang S. Choi
Keyword(s):  
Shallow Water ◽  
Bottom Topography ◽  
Boussinesq Equations ◽  
Linear Wave ◽  
Linear Potential ◽  
Wave Excitation ◽  
Wave Interactions ◽  
Water Effects ◽  
Excitation Force ◽  
Wave Excitation Force

In this study, firstly nonlinear waves in shallow water were simulated by using the Boussinesq equations. The simulated waves represented well the wave deformations such as shoaling and refraction as well as non-linear wave interactions among wave components as they approach coastal region from far field. The velocity components of the simulated waves at an arbitrary location in the fluid domain can be computed most effectively by introducing the so-called utility velocity. By taking the deformed wave field into account, the motion response of a moored floating barge was analyzed. The wave excitation and radiation force were estimated by the Constant Panel Method (CPM) based on linear potential theory. In order to estimate the wave excitation force including shallow water effects, the wave height and the wave velocity components obtained from the Boussinesq simulation were used. This approach used to estimate the wave excitation force including shallow water effects is herein referred to as Hybrid Boussinesq-CPM. An example calculation was made for the Pinkster barge, which is supposed to be located in a specific bottom topography and moored by the Tower Yoke Mooring System. The results were compared with those obtained for the equivalent constant water depth condition. The comparison showed that the motion responses obtained by the Hybrid model were larger than those for the even bottom case. In particular, the horizontal surge motion was significantly enlarged because of two facts: the wave deformation due to the bottom topography and the low frequency waves caused by nonlinear wave-wave interactions. The enlarged horizontal surge motion is important for mooring design in shallow water.

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