Analysis of Extreme TLP Tether Tension Amplitudes With Comparison to Model Tests

2005 ◽  
Author(s):  
Thomas B. Johannessen ◽  
Arild K. Sandvik
Keyword(s):  
Model Test ◽  
Transfer Functions ◽  
Model Tests ◽  
Second Order ◽  
Surface Zone ◽  
Test Results ◽  
Analysis Model ◽  
Load Model ◽  
Analysis Methods ◽  
The Time Domain

The Snorre TLP has been operating in the Northern North Sea since 1992. During the last year, an extensive reanalysis of tether tension, TLP motion and airgap has been carried out in order to ensure that the TLP can withstand updated environmental conditions and design requirements. It is the object of the present paper to describe the analysis methods which have been employed to determine extreme tether loading and to compare analysis results with model test results. The model tests were carried out in the facilities at MARIN in the Netherlands and considered more than 200 three hour realizations of irregular seastates with and without wind and current. Typical fatigue seastates as well as extreme 100 year and 10 0000 year seastates were investigated for both head and quartering seas. The extensive database of model test results gives an opportunity to study the accuracy of the analysis methods. The analysis of the TLP response was carried out in the time domain using the SIMO software developed by Marintek. In the work described here, the results from a purely linear radiation-diffraction analysis is used as the main load model in SIMO together with slender element additions based on the incident wave kinematics. The slender elements give loads on tethers and risers, viscous loads on the TLP hull and also provide a simple third order load model for the excitation of high frequent response in the surface zone. SIMO has also been implemented with second order quadratic force transfer functions, but it is concluded that the second order loading does not give significant contributions to extreme tether tension. It is concluded that, at least for the Snorre TLP, the relatively simple analysis model gives good agreement with measured extreme tether tension provided that the TLP is modeled carefully. By filtering the tether tension time series, the contributions due to wave frequent and high frequent (resonant) tether tension may be separated. It is shown that the main discrepancy between model test and analysis results of tether tension lies not in the magnitude but in the simultaneity of wave frequent and high frequent tether response. If this is due to the tendency for large waves to propagate towards the front of the nonlinear wave envelope, this may be extremely difficult to capture in analysis models.

Improved Strake Design for Vortex Induced Motions of Spar Platforms

2005 ◽  
Author(s):  
Mehernosh Irani ◽  
Lyle Finn
Keyword(s):  
Model Test ◽  
Model Tests ◽  
Test Results ◽  
Test Program ◽  
Test Set ◽  
Test Methodology ◽  
Spar Platforms ◽  
Truss Spar ◽  
Set Up

An extensive model test program was conducted to explore the effectiveness of alternate strake designs to reduce Truss Spar VIV response. Different strake configurations were tested to minimize VIV response. The paper presents results of the model tests. The model test set-up is described, important parameters that are modeled (including hull and truss geometry, strake configuration, mass and mooring properties) and considerations of instrumentation and test methodology are discussed. The paper also describes the analysis of the test results and shows the effectiveness of new strake design. The present results are compared with VIV response of existing Truss Spars with conventional strake design.

Model Testing for Vortex Induced Motions of Spar Platforms

2004 ◽  
Author(s):  
Mehernosh Irani ◽  
Lyle Finn
Keyword(s):  
Gulf Of Mexico ◽  
Model Test ◽  
State Of The Art ◽  
Model Testing ◽  
Model Tests ◽  
The State ◽  
Test Results ◽  
Test Set ◽  
Vortex Induced Vibrations ◽  
Set Up

The state-of-the art in model testing for Vortex Induced Vibrations (VIV) of Spars is presented. Important issues related to Spar VIV model testing are highlighted. The parameters that need to be modeled including hull geometry, strake configuration, mass and mooring properties and, considerations of test set-up and instrumentation are discussed. Results are presented from model tests of an as-built Spar deployed in the Gulf of Mexico. It is shown that the model test results compare well with the VIV responses measured in the field.

scholarly journals Model-Scale/Full-Scale Correlation of NRC-OCRE’s Model Resistance, Propulsion and Maneuvering Test Results

2015 ◽  
Author(s):  
Michael Lau
Keyword(s):  
Test Data ◽  
Model Test ◽  
Model Tests ◽  
Full Scale ◽  
Test Results ◽  
Sea Trial ◽  
Model Scale ◽  
Performance Predictions ◽  
Ice Strength ◽  
Scale Data

There are a variety of model ices and test techniques adopted by model test facilities. Most often, the clients would ask: “How well can you predict the full scale performance from your model test results?” Model-scale/full-scale correlation becomes an important litmus test to validate a model test technique and its results. This paper summarizes the model-scale/full-scale correlation performed on model test data generated at the National Research Council - Ocean, Coastal, and River Engineering’s (NRC-OCRE) test facility in St. John’s. This correlation includes ship performance predictions, i.e., resistance, propulsion and maneuvering. Selected works from NRC-OCRE on the USCGC icebreaker Healy, the CCGS icebreaker Terry-Fox, the CCGS R-Class icebreakers Pierre Radisson and Sir John Franklin and the CCGS icebreaker Louis S. St. Laurent were reviewed and summarized. The model tests were conducted at NRC-OCRE’s ice tank with the correct density (CD) EGADS model ice. This correlation is based on the concept that a “correlation friction coefficient” (CFC) can be used to predict full-scale ship icebreaking resistance from model test data. The CFCs have been compared for correlation studies using good-quality full-scale information for the five icebreaker models in the NRC-OCRE’s model test database. The review has shown a good agreement between NRCOCRE’s model test predictions and full-scale measurements. The resistance and power correlation were performed for five sets of full-scale data. Although there is substantial uncertainty on ice thickness and ice strength within the full scale data sets that contributes to data scattering, the data suggest a conservative estimate can be obtained to address reasonably this uncertainty by increasing the model prediction by 15% that envelopes most data points. Limited correlation for maneuvering in ice was performed for the USCGC icebreaker Healy. Selected test conditions from the sea trials were duplicated for the maneuvering tests and turning diameters were measured from the arcs of partial circles made in the ice tank. Performance predictions were then compared to the full-scale data previously collected. Despite some discrepancy in ice strength and power level between the model tests and sea trial, the model data agree well with the sea trial data except for three outliers. Otherwise, the maneuvering data show a good correlation between the model test and sea trial results.

Comparative Study of Statistical Analysis Methods for Nonlinear Hydrodynamic Responses of Offshore Structures

2020 ◽  
Vol 142 (4) ◽  
Author(s):  
Dong-Hyun Lim ◽  
Yonghwan Kim
Keyword(s):  
Statistical Analysis ◽  
Probability Distribution ◽  
Transfer Functions ◽  
Volterra Series ◽  
Wave Spectrum ◽  
Offshore Structures ◽  
Second Order ◽  
Analysis Methods ◽  
Extreme Response ◽  
Statistical Analysis Methods

Abstract The present study compares the statistical analysis methods for nonlinear hydrodynamic responses in offshore engineering. In particular, the Kac–Siegert and Hermite-moment methods were compared for estimating the probability distribution of the second-order responses represented via the two-term Volterra series. The Kac–Siegert method analytically formulates the probability density function (PDF) of the second-order Volterra series using an eigenvalue problem constructed with the frequency-domain transfer functions and the wave spectrum, whereas the Hermite-moment method utilizes the statistical moments to determine the coefficients of the fitting function. In addition, the probability distribution of the peak values in the second-order Volterra series with high spectral bandwidth was derived explicitly. The fatigue damage rate and the extreme response were estimated analytically. The accuracy and applicability of each method were investigated by comparing the methods with the results of the direct sampling obtained from the time series.

Model Tests With an FPSO in Design Environmental Conditions

2004 ◽  
Author(s):  
Jesper Skourup ◽  
Martin J. Sterndorff ◽  
Susan F. Smith ◽  
Xiaoming Cheng ◽  
R. V. Ahilan ◽  
...  
Keyword(s):  
Model Test ◽  
Environmental Conditions ◽  
Transfer Functions ◽  
Model Tests ◽  
Irregular Waves ◽  
Wind Condition ◽  
Test Programme ◽  
Wave Basin ◽  
Wide Range ◽  
Wave Conditions

An extensive model test programme has been carried out with a turret moored FPSO model in design environmental conditions. The model tests were carried out in the 3D offshore wave basin at DHI Water & Environment at a scale of 1:80. The objectives of the model tests were two-fold: 1. To determine quadratic transfer functions for the slow-drift forces. 2. To determine the turret moored FPSO response in design environmental conditions with wave spreading. The model tests were made with a wide range of monochromatic and bi-chromatic wave conditions and also with long- and short-crested irregular wave conditions. For the tests in design conditions the irregular waves were combined with the corresponding wind condition. The model (which was segmented into two parts) was equipped with instruments to measure forces in mooring lines and turret, 6 dof motions of the FPSO, bending moments on the FPSO hull and wave run-up on the FPSO model. The present paper describes the details of the experimental work and the measurements made in the tests. Comparisons between model test results themselves to demonstrate the effects of wave spreading on the responses and comparisons to numerical results are given. The model test programme is part of the REBASDO project, funded by the European Union, and involving companies and institutions from several European Countries. The overall objective with the REBASDO project is to develop met-ocean and hydrodynamic models, which will capture the significant features of directional wave effects on FPSO design so that relevant enhancements in the design process can be incorporated in the future.

Simulation of Second-Order Roll Motions of a FPSO

2008 ◽  
Author(s):  
Fla´via Rezende ◽  
Xiao-Bo Chen ◽  
Marcos D. Ferreira
Keyword(s):  
Transfer Functions ◽  
Model Tests ◽  
Second Order ◽  
Vertical Position ◽  
Wave Loads ◽  
Natural Period ◽  
New Developments ◽  
Linear Waves ◽  
Instantaneous Position ◽  
Key Parameter

The roll motions are a key parameter on the design of FPSOs that operate in moderate and severe environmental conditions. To reduce the magnitude of roll motions, some techniques based on changing the vertical position of gravity center are used to put the roll natural period outside of the frequency range of the linear waves. However, recent model tests and also numerical calculations have shown that the vessel may still experience large roll motions which are considered to be induced by second-order wave loads. Further to the work in Rezende et al. (2007) to compute the roll response in frequency domain, new developments to perform simulations in time domain are presented here. In this new method, variations of second-order roll moments dependent on the roll and heave motions are taken into account consistently. It is shown that, unlike the horizontal loads, the quadratic transfer functions of the vertical loads depend on the instantaneous position of the vessel. The variation of the roll moment with the heave position of the vessel has been considered more important than the variation obtained only with the inclination of the vessel. Furthermore, numerical results of roll simulations are compared with model tests results and presented in the paper.

Model Test Data Correlations With Fully Coupled Hull/Mooring Analysis for a Floating Wind Turbine on a Semi-Submersible Platform

2014 ◽  
Author(s):  
Bonjun Koo ◽  
Andrew J. Goupee ◽  
Kostas Lambrakos ◽  
Ho-Joon Lim
Keyword(s):  
Wind Turbine ◽  
Model Test ◽  
Model Tests ◽  
Test Results ◽  
Scaled Model ◽  
Floating Wind Turbine ◽  
Mooring Dynamics ◽  
Set Up ◽  
Fully Coupled ◽  
Turbine Model

The DeepCwind floating wind turbine model tests were performed at MARIN (Maritime Research Institute Netherlands) with a model set-up corresponding to a 1:50 Froude scaling. In the model tests, the wind turbine was a scaled model of the National Renewable Energy Lab (NREL) 5MW, horizontal axis reference wind turbine supported by three different generic floating platforms: a spar, a semi-submersible and a tension-leg platform (TLP) (Ref. [1] and [2]). This paper presents validation of the MLTSIM-FAST [3] code with DeepCwind semi-submersible wind turbine model test results. In this integrated program, the turbine tower and rotor dynamics are simulated by the subroutines of FAST [4], and the hydrodynamic loads and mooring system dynamics are simulated by the subroutines of MLTSIM. In this study, fully coupled hull/mooring dynamics and second-order difference-frequency response are included in MLTSIM-FAST. The analysis results are systematically compared with model test results and show good agreement.

Bearing capacity of foundations on a weak sand layer overlying a strong deposit

1982 ◽  
Vol 19 (3) ◽  
pp. 392-396 ◽  
Author(s):  
A. M. Hanna
Keyword(s):  
Bearing Capacity ◽  
Model Test ◽  
Classical Equation ◽  
Model Tests ◽  
Sand Layer ◽  
Test Results ◽  
Dense Sand ◽  
Design Charts ◽  
Sand Deposit ◽  
Compact Sand

The ultimate bearing capacity of footings resting on subsoils consisting of a weak sand layer overlying a strong deposit has been investigated. Based on model tests of strip and circular footings in a loose or compact sand layer overlying a dense sand deposit, the classical equation of bearing capacity of footings on homogeneous sand was extended to cover cases of these footings in layered sands where the upper layer is the weaker. The theory compared well with the available model test results. Design charts are presented.

Correlation Allowances in Model Tests Results: A Delicate Balance Between Performance, Accuracy and Commercial Interests?

2016 ◽  
Author(s):  
Gerco Hagesteijn ◽  
Patrick Hooijmans ◽  
Karola van der Meij
Keyword(s):  
Model Test ◽  
Performance Prediction ◽  
Model Tests ◽  
Full Scale ◽  
Test Results ◽  
Power Performance ◽  
Low Light ◽  
Sea Trial ◽  
Performance Accuracy ◽  
Full Scale Tests

Model tests at ballast and design draught are used to convert the sea trial results from the ballast trial draught to the contractual design draught. Correlation allowances in model test results and their effect on the trial performance prediction are of major importance. Nowadays it is not only typical to verify the contract speed but also the EEDI certification requires a verification of the speed power performance of the vessel. The use of a to favorable CA-value may lead to attractive performance figures, but also leads to higher fuel consumption figures than expected. Furthermore the design point of the propeller is affected, which leads to a too low light running margin and in some cases to erosive cavitation. During a study, large spreading in the values of the correlation allowances for design draughts have been found for merchant vessels tested at different model test institutes, but at ballast trial draught the spreading is much less. Can it happen that some institutes select favorable correlations allowances on the basis of inaccurate trial data of shipyards? Or should we accept a large spreading in correlation allowances and have these indeed been confirmed by sea trials at design draught? This paper will present a discussion using the experience of a large full scale trial database as well as the accuracy of model and full scale tests.

Numerical Simulations and Truncation Design of a Deepwater Internal Turret FPSO Before Model Test

2010 ◽  
Author(s):  
Hongwei Wang ◽  
Yong Luo ◽  
Xiaoming Cheng
Keyword(s):  
Numerical Simulations ◽  
Model Test ◽  
Time Domain ◽  
Model Tests ◽  
Test Model ◽  
Mooring System ◽  
Static Characteristics ◽  
Mooring Lines ◽  
Horizontal Dimension ◽  
The Time Domain

Model tests of an internal turret moored deepwater FPSO will be conducted in the deepwater offshore basin recently completed in Harbin Engineering University. Considering the limitations of the basin facilities and to reduce the complexity of fixing risers on the basin bottom, the 13 risers are simplified and combined to 4 risers. In addition, the horizontal dimension of the basin is not able to accommodate mooring system at the test model scale. Therefore, certain mooring lines have to be truncated. Two truncation methods are proposed according to the similitude of static characteristics of the mooring system. The first method only truncates the portion of the line that lies on the seabed and the second method truncates the suspended line portion. Numerical simulations have been carried out in the time domain with mooring lines coupled with FPSO. The calculated results are found to agree well with those of the prototype mooring system. Both truncation methods are found to be robust and reliable which provide a valuable reference for the forthcoming model tests.

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