Diffraction/Radiation of 135,000M3 Storage Capacity LNG Carrier in Shallow Water: A Benchmark Study

2009 ◽  
Author(s):  
Mamoun Naciri ◽  
Emmanuel Sergent
Keyword(s):  
Shallow Water ◽  
Load Transfer ◽  
Storage Capacity ◽  
Transfer Functions ◽  
Second Order ◽  
Diffraction Radiation ◽  
Hull Form ◽  
Benchmark Study ◽  
Wave Drift ◽  
Key Issues

The HAWAI (sHAllow WAter Initiative) JIP was launched in 2005. The objective was to improve the reliability of Offshore (LNG) Terminals by combining the expertise of offshore hydrodynamics and coastal engineering to better address key issues regarding motion and mooring prediction methods in shallow water. One of the key issues identified was the diffraction/radiation calculation as this is the main foundation of all motion and mooring analyses. Comparisons of second order wave drift load transfer functions predicted by leading diffraction/radiation software for a typical 135,000m3 storage capacity LNG Carrier (LNGC) had shown notable differences (see Ref [1]). A benchmark study was launched for a standard LNGC in 15m water depth. Seven leading commercial diffraction/radiation software were used for this comparison (AQWA, DELFRAC, DIFFRAC, DIODORE, HYDROSTAR, WADAM and WAMIT). Comparison was first done by specifying the hull form in CAD format and then by specifying the mesh. First and second order results are presented and conclusions are drawn regarding the robustness of these codes.

A New Interpolation Method for FPSO Motion and Wave Drift Load Transfer Functions

2011 ◽  
Author(s):  
Ste´phanie Stafrach ◽  
Mamoun Naciri
Keyword(s):  
Spectral Density ◽  
Shallow Water ◽  
Load Transfer ◽  
Storage Capacity ◽  
Transfer Functions ◽  
Interpolation Method ◽  
New Method ◽  
Wave Drift ◽  
Floating System ◽  
Response Amplitude Operators

A new method of direction-wise interpolation is proposed. Its merits are first presented by considering the interpolation of 1st order motion Response Amplitude Operators (RAOs) and 3D (ω1, ω2, θ) wave drift load Quadratic Transfer Functions (QTFs). In a recent publication (see Ref. [1]), the importance of wave spreading on the spectral density of wave drift loads for a standard storage capacity LNG Carrier (135,000m3) in shallow water has been demonstrated. The computation of above-mentioned spectral densities requires the precalculation of a large number of 4D (ω1, ω2, θ1, θ2) wave drift load QTF and interpolations between the calculated directions. Application of the new method is investigated in this more challenging context. Examples are selected in the buoyant LNG floating system area with an LNG Carrier in shallow water and an FLNG in deep water.

Experimental Study of Slow-Drift Ship Motions in Shallow Water Random Waves

2011 ◽  
Author(s):  
Carl Trygve Stansberg ◽  
Trygve Kristiansen
Keyword(s):  
Spectral Analysis ◽  
Shallow Water ◽  
Transfer Functions ◽  
Second Order ◽  
Ship Motions ◽  
Random Wave ◽  
Random Waves ◽  
Low Frequencies ◽  
Wave Basin ◽  
Drift Forces

Slowly varying motions and drift forces of a large moored ship in random waves at 35m water depth are investigated by an experimental wave basin study in scale 1:50. A simple horizontal mooring set-up is used. A second-order wave correction is applied to minimize “parasitic” long waves. The effect on the ship motion from the correction is clearly seen, although less in random wave spectra than in pure bi-chromatic waves. Empirical quadratic transfer functions (QTFs) of the surge drift force are found by use of cross-bi-spectral analysis, in two different spectra have been obtained. The QTF levels increase significantly with lower wave frequencies (except at the diagonal), which is special for finite and shallow water. Furthermore, the QTF levels frequencies at low frequencies increase significantly out from the QTF diagonal. Thus Newman’s approximation should preferrably not be used in these cases. Using the LF waves as a direct excitation in a “linear” ship force analysis gives random records that compare reasonably well with those from the cross-bi-spectral analysis. This confirms the idea that the drift forces in shallow water are closely correlated to the second-order potential, and thereby by the second-order LF waves.

Slowly Varying Wave Drift Forces Analyzed From Model Test Data on a Moored Ship in Shallow Water

2009 ◽  
Author(s):  
Carl Trygve Stansberg ◽  
Csaba Paˆkozdi
Keyword(s):  
Noise Reduction ◽  
Shallow Water ◽  
Model Test ◽  
Transfer Functions ◽  
Numerical Study ◽  
Wave Frequency ◽  
Irregular Waves ◽  
Spectral Peak ◽  
Frequency Range ◽  
Wave Drift

Model test estimation of quadratic transfer functions (QTFs) is investigated for slowly varying wave drift excitation on a large moored ship in shallow water. Cross-bi-spectral analysis in irregular waves is used. A numerical study is run first, with a known, synthetical QTF model characterized by a strong off-diagonal variation, combined with a very lightly damped linear slow-drift dynamical system. The purpose is to check the accuracy of the analysis. For this simple model, a good accuracy is obtained in the estimated QTF. This is because of a refined noise reduction method which works well in this case. The wave frequency range of valid estimates is where the wave spectrum S(f) is higher than 7% of the spectral peak. Without the refinement, the useful range is reduced to where S(f) is higher than 15% of the spectral peak, based on a 3-hour sea state simulation. The method is then applied on experimental surge motion records from 1:50 scaled model tests carried out in an offshore basin, simulating 15m water depth. It is found that the QTF estimation procedure works reasonably well, but the accuracy is lower than that in the numerical study because the refined noise reduction could not be used due to the particular characteristics of the QTF. Therefore a basic version without the refinement had to be used. Still, results appear to be fairly reliable in the reduced wave frequency range with S(f) > 15% of the spectral peak, i.e. from 0.07Hz to 0.10Hz in this case.

Wave Spreading and Wave Drift Loads for a Standard LNG Carrier in Shallow Water

2010 ◽  
Author(s):  
Emmanuel Sergent ◽  
Mamoun Naciri
Keyword(s):  
Natural Gas ◽  
Shallow Water ◽  
Natural Frequency ◽  
Water Depth ◽  
Storage Capacity ◽  
Mooring System ◽  
Spectral Densities ◽  
Floating Structures ◽  
Wave Drift ◽  
The Impact

The need for LNG export and import terminals is anticipated to grow as natural gas progressively accounts for a larger fraction of worldwide consumed energy. These terminals are preferably located nearshore i.e. in relatively shallow water. Design of floating structures is most of the time performed assuming long-crested waves. In shallow water, diffraction of waves by a variable bathymetry can result in wave spreading i.e. in short crested seas. The effect of short crested seas on the wave drift load spectral densities for a 135,000m3 storage capacity LNG Carrier in 15m water depth is investigated. It is shown that the impact of wave spreading on drift loads depends on the natural frequency of the moored vessel and thus on the stiffness of the mooring system under consideration. Although response calculations are not performed herein for reasons to be discussed, it is conceivable that wave spreading could adversely affect loading/offloading terminal availability for stiff moorings.

Ship Motion Predictions: A Comparison Between a CFD Based Method, a Panel Method and Measurements

2010 ◽  
Author(s):  
R. H. M. Huijsmans ◽  
R. van ‘t Veer ◽  
M. Kashiwagi
Keyword(s):  
Load Transfer ◽  
Numerical Solutions ◽  
Transfer Functions ◽  
Panel Method ◽  
Ship Motions ◽  
Internal Load ◽  
Seminal Paper ◽  
Benchmark Study ◽  
Computational Procedures ◽  
Hull Forms

In the past 50 years the research into the behavior of ships in seaway has shown a great deal of progress. From analytical solutions of 2-d hydrodynamics as derived by Ursell in 1949 to complex 3-d CFD numerical solutions that can be used nowadays. The first consistent approach for ship motions in time domain was derived by prof. T.F.Ogilvie [] as presented in his seminal paper in Bergen in 1964. Throughout the years however ship hull forms developed and the need for validation of computational procedures for the calculation of the ship motions has never gone away. In this paper we present a selection of results of a benchmark study performed by some 12 companies and universities using their state of the art computational tools. In this benchmark study the results of model tests of a modern container vessel are used. The results presented in this paper show that the panel method as described in this paper, based on the disturbed steady flow, leads to acceptable transfer functions for ship motions. The CFD approach used in this paper also produces acceptable motion transfer functions. However the results from the CFD computation for the internal load transfer functions do show a larger scatter when comparing with the results from model test.

Second-Order Low-Frequency Wave Forces on a SPM Offloading Tanker in Shallow Water

2009 ◽  
Vol 131 (4) ◽  
Author(s):  
S. Ma ◽  
M. H. Kim ◽  
S. Shi
Keyword(s):  
Shallow Water ◽  
Transfer Functions ◽  
Low Frequency ◽  
Wave Force ◽  
Second Order ◽  
Irregular Waves ◽  
Mooring Line ◽  
Frequency Wave ◽  
Water Region ◽  
Shallow Water Region

This paper studies the influence of three different calculation methods of the second-order low-frequency (LF) wave-force quadratic transfer functions (QTFs) for a single point mooring (SPM) tanker system in relatively shallow water region. The multivessel-mooring hawser coupled dynamic analysis is used to simulate the floater relative motions and mooring and hawser tensions. Because the SPM tanker is deployed in shallow water region and the slowly varying drift motions are to be dominant in typical operational conditions, the accurate calculation of LF wave-force QTFs become important especially for mooring and hawser-tension prediction. The practically popular Newman’s approximation and another approximation excluding complicated free-surface integrals are used to calculate the LF QTFs on the offloading tanker and they are compared with the complete QTF results. Further comparison is carried out by calculating the resulting LF wave-force spectra and motion time histories and analyzing their impacts on hawser and mooring line tensions. Through the example studies, the limitation of the Newman’s approximation in the case of shallow water and longer period irregular waves is underscored.

Second-Order Low-Frequency Wave Forces on a SPM Offloading Tanker in Shallow Water

2008 ◽  
Author(s):  
S. Ma ◽  
S. Shi ◽  
M. H. Kim
Keyword(s):  
Shallow Water ◽  
Transfer Functions ◽  
Low Frequency ◽  
Wave Force ◽  
Second Order ◽  
Dynamic Responses ◽  
Mooring Line ◽  
Wave Forces ◽  
Frequency Wave ◽  
The Impact

This paper studies the influence of three different calculation methods of the second-order low-frequency (LF) wave forces on the tanker responses and hawser/mooring tensions in relatively shallow water region. The vessel-mooring-riser coupled dynamic analysis computer program HARP is used to simulate the coupled dynamic responses of offloading tanker moored to a SPM (Single Point Mooring). Because the SPM is supposed to be deployed in shallow water and the slowly varying drift motions of the tanker are to dominate the motion responses in typical operational conditions, the accurate calculation of LF wave-force quadratic transfer functions (QTFs) becomes important especially for mooring and hawser tensions. Like common practice, the so-called Newman’s approximation and another approximation method without including complicated free-surface integrals are first used to calculate the LF QTFs on the offloading tanker and they are compared with the complete QTF results. Further comparison is performed by calculating the resulting LF wave-force spectra and response time series by using the three different methods. The impact of the three different approaches on vessel surge motions and hawser/mooring line tensions is also addressed.

scholarly journals A Finite-Volume Icosahedral Shallow-Water Model on a Local Coordinate

2009 ◽  
Vol 137 (4) ◽  
pp. 1422-1437 ◽  
Author(s):  
Jin-Luen Lee ◽  
Alexander E. MacDonald
Keyword(s):  
Shallow Water ◽  
Finite Volume ◽  
Spherical Surface ◽  
Second Order ◽  
Water Model ◽  
Numerical Dissipation ◽  
Shallow Water Model ◽  
Finite Volume Model ◽  
Volume Model ◽  
Cartesian Coordinate

Abstract An icosahedral-hexagonal shallow-water model (SWM) on the sphere is formulated on a local Cartesian coordinate based on the general stereographic projection plane. It is discretized with the third-order Adam–Bashforth time-differencing scheme and the second-order finite-volume operators for spatial derivative terms. The finite-volume operators are applied to the model variables defined on the nonstaggered grid with the edge variables interpolated using polynomial interpolation. The projected local coordinate reduces the solution space from the three-dimensional, curved, spherical surface to the two-dimensional plane and thus reduces the number of complete sets of basis functions in the Vandermonde matrix, which is the essential component of the interpolation. The use of a local Cartesian coordinate also greatly simplifies the mathematic formulation of the finite-volume operators and leads to the finite-volume integration along straight lines on the plane, rather than along curved lines on the spherical surface. The SWM is evaluated with the standard test cases of Williamson et al. Numerical results show that the icosahedral SWM is free from Pole problems. The SWM is a second-order finite-volume model as shown by the truncation error convergence test. The lee-wave numerical solutions are compared and found to be very similar to the solutions shown in other SWMs. The SWM is stably integrated for several weeks without numerical dissipation using the wavenumber 4 Rossby–Haurwitz solution as an initial condition. It is also shown that the icosahedral SWM achieves mass conservation within round-off errors as one would expect from a finite-volume model.

Analysis on the Performance of Deckwetness of Sandglass-type and Cylindrical Floating Bodies

2016 ◽  
Vol 60 (03) ◽  
pp. 145-155
Author(s):  
Ya-zhen Du ◽  
Wen-hua Wang ◽  
Lin-lin Wang ◽  
Yu-xin Yao ◽  
Hao Gao ◽  
...  
Keyword(s):  
Transfer Functions ◽  
Linear Response Theory ◽  
Second Order ◽  
Irregular Waves ◽  
Wave Loads ◽  
Floating Bodies ◽  
Drift Motion ◽  
First Order ◽  
Series Of Experiments ◽  
Deck Wetness

In this paper, the influence of the second-order slowly varying loads on the estimation of deck wetness is studied. A series of experiments related to classic cylindrical and new sandglass-type Floating Production, Storage, and Offloading Unit (FPSO) models are conducted. Due to the distinctive configuration design, the sand glass type FPSO model exhibits more excellent deck wetness performance than the cylindrical one in irregular waves. Based on wave potential theory, the first-order wave loads and the full quadratic transfer functions of second-order slowly varying loads are obtained by the frequency-domain numerical boundary element method. On this basis, the traditional spectral analysis only accounting for the first-order wave loads and time-domain numerical simulation considering both the first-order wave loads and nonlinear second-order slowly varying wave loads are employed to predict the numbers of occurrence of deck wetness per hour of the two floating models, respectively. By comparing the results of the two methods with experimental data, the shortcomings of traditional method based on linear response theory emerge and it is of great significance to consider the second-order slowly drift motion response in the analysis of deck wetness of the new sandglass-type FPSO.

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