Model Tests and Numerical Prediction of the Low Frequency Motions of a Moored Ship in Shallow Water With a Wave Splitting Method

2020 ◽  
Author(s):  
Nuno Fonseca ◽  
Galin Tahchiev ◽  
Sébastien Fouques ◽  
Carl Trygve Stansberg ◽  
José Miguel Rodrigues
Keyword(s):  
Shallow Water ◽  
Test Data ◽  
Numerical Models ◽  
Splitting Method ◽  
Low Frequency ◽  
Ocean Basin ◽  
Model Tests ◽  
Harmonic Waves ◽  
Frequency Wave ◽  
Numerical Predictions

Abstract Prediction of shallow water low frequency (LF) motions of vessels in the context of mooring analysis is challenging. Model tests are often performed to calibrate and validate numerical models and, in this way, reduce the uncertainty. Model tests are part of the positioning system qualifying process. However, model tests also present challenges and uncertainties related to parasitic low frequency wave systems which are unavoidable in shallow water ocean basin conditions. The paper presents model tests with a ship moored in shallow water (20 m), the analysis and discussion of the test data and comparisons with numerical predictions. The focus is on the low frequency motions and related wave drift forces. The tests have been performed in harmonic waves, bi-harmonic waves and irregular seastates, including conditions with and without current. The first part of the study consists of analysing the wave field measured by a long array of wave sensors distributed along the ocean basin. The analysis provides split wave systems, namely the low frequency components including the bound wave, the incoming free parasitic wave, the reflected component and additional very long waves. The second part proposes a method to calibrate and validate mooring analysis numerical models, based on comparisons with model test data which includes the unavoidable effects from parasitic waves. Simulations of LF motions with the calibrated model show a good agreement with the measurements.

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.

Analysis of Semi-Submersible Under Combined High Waves and Current Conditions Compared With Model Tests

2018 ◽  
Author(s):  
Limin Yang ◽  
Arne Nestegård ◽  
Erik Falkenberg
Keyword(s):  
Simulation Model ◽  
Low Frequency ◽  
Model Tests ◽  
Wave Forces ◽  
Viscous Effects ◽  
Numerical Predictions ◽  
Wave Drift ◽  
Zero Current ◽  
Frequency Excitation ◽  
Wave Drift Forces

Viscous effects on the low-frequency excitation force on column based platforms are significant in extreme waves. The wave drift force as calculated by a zero-current potential flow radiation/diffraction code becomes negligible for such waves. In the present study, the effect of current and viscous contributions on the slowly varying wave forces are adjusted by a formula developed in the Exwave JIP, see e.g. [1], which is validated against model test results. This paper presents numerical predictions of low frequency horizontal motions of a semi-submersible in combined high waves and current condition. In the simulation model, frequency dependent wave drift forces from radiation/diffraction code are modified by the formula. Static current forces and viscous damping are modelled by the drag term in Morison load formula using relative velocity between current and floater and with force coefficients as recommended by DNVGL-RP-C205 [2]. Low frequency surge responses calculated by the simulation model are compared with model tests for waves only and for combined collinear and noncollinear wave and current conditions.

Experimental Validation of a New Shallow Water CALM Buoy Design

2013 ◽  
Author(s):  
A. Neil Williams ◽  
Williams G. McDougal
Keyword(s):  
Shallow Water ◽  
Experimental Validation ◽  
Numerical Models ◽  
Water Environment ◽  
Model Testing ◽  
Model Tests ◽  
Hydrodynamic Efficiency ◽  
Shallow Water Environment

This paper presents an overview of the model testing of a new turret-type CALM buoy concept developed by WISON for shallow water (20m–80m) applications. In the WISON design the outer body of the buoy is hexagonal, a geometry that allows for ease of fabrication while retaining hydrodynamic efficiency. The overall objective of the model tests was to demonstrate the performance of this new design for a typical shallow water environment under both operating and survival conditions. Additionally, the model tests were intended to provide data to calibrate the numerical models for buoy motions and line tensions used in the design, and to give guidance regarding the suitability of the buoy freeboard and deckhouse arrangement.

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.

Resonant Vibrations of Riser Guide Tubes Due to Wave Impact

2004 ◽  
Author(s):  
Arne Nestega˚rd ◽  
Arve Johan Kalleklev ◽  
Kjell Hagatun ◽  
Yu Lin Wu ◽  
Sverre Haver ◽  
...  
Keyword(s):  
High Frequency ◽  
Rapid Change ◽  
Ocean Basin ◽  
Model Tests ◽  
Slender Body ◽  
Irregular Waves ◽  
Resonant Vibrations ◽  
Wave Impact ◽  
Numerical Predictions ◽  
Steep Waves

The Kristin platform is a catenary moored semi-submersible production vessel (SSPV) intended for production of gas at the Kristin field at Haltenbanken. Kristin has 24 riser guide tubes for tie in of flexible risers, umbilicals and electric cables to the riser balcony. The riser guide tubes (RGT) provide the necessary guiding, support and protection for risers and cables. The guide tubes run vertically from the deck and through the extended east pontoon. The guide tubes are welded to the pontoon and horizontally supported at the underside of the balcony deck. During model tests of the Kristin platform performed in the Ocean Basin laboratory at Marintek, high frequency in-line vibrations of the RGTs were observed during passage of steep waves. The resonance period for the individual RGTs is 0.3 sec. To mitigate the vibration problem, a vibration suppression arrangement of stiff rods was introduced between the guide tubes. Model tests were performed with respect to extreme- and fatigue loads in regular and irregular waves, with and without the suppression arrangement. The model included the floating framework representing the hull and the 24 RGTs with correct diameter and resonance period. The model was suspended in a horizontal mooring system, giving resonance periods in surge and sway close to the prototype platform. A load-response model for the interaction between large steep waves and vertical flexible cylinders has been developed. A slender body load model derived from Morison’s equation is shown to be able to excite the resonant vibrations. The dominant part of the loading comes from the rapid change of added mass momentum, giving rise to an additional slamming term in the load formulation. The structural response is calculated from a recognized non-linear slender body response program. Numerical simulations have been carried out and compared with model tests for both regular and irregular waves. The numerical predictions confirm the effect observed in the model tests; i.e. connecting the tubes generally leads to a reduction of the high frequency response amplitudes.

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.

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