CFD-Based Numerical Wave Basin for Global Performance Analysis

2016 ◽  
Author(s):  
Guangyu Wu ◽  
Jang Whan Kim ◽  
Hyunchul Jang ◽  
Aldric Baquet
Keyword(s):  
Performance Analysis ◽  
Surface Flow ◽  
Design Tool ◽  
Offshore Platform ◽  
Mooring Line ◽  
Mooring Lines ◽  
Wave Modeling ◽  
Wave Basin ◽  
Global Performance ◽  
Numerical Wave

Several recent benchmark studies have demonstrated that Computational Fluid Dynamics (CFD) is capable of capturing both nonlinear and viscous effects in offshore marine hydrodynamics and predicting well certain wave- and current-induced offshore platform motion. In order to apply CFD for practical global performance analysis of a complete hull-mooring-riser coupled floating system, we develop an advanced numerical wave basin that combines CFD, nonlinear irregular wave modeling, and finite-element mooring modeling. Specifically, CFD is used to simulate the violent free-surface flow with hull motions; nonlinear wave modeling is applied to generate a realistic wavefield and provide initial and far-field conditions to CFD for efficient long-duration simulation; and mooring modeling is two-way coupled with CFD to account for dynamic mooring response and its effects on hull motion. In this study, to demonstrate the capability of such tool, the global performance of a semi-submersible with 4 mooring lines in a 3-hour extreme sea state is simulated for both head and quartering sea. The simulation results are compared to model test data of hull motion, mooring line tension, and relative wave elevation around the hull for validation. It is shown with spectrum and statistics that the simulations predict well the platform’s global performance in all frequency ranges, including low frequency where the mooring lines have the greatest influence on the motion response. Compared to the predictions from a conventional global performance design tool that is based on diffraction analysis and empirical coefficients, the CFD results show significant improvements. The encouraging results from this study indicate that a CFD-based numerical wave basin, although still computationally expensive, is technically ready to be a complementary tool to physical wave basin for offshore platform global performance design.

A New Nonlinear Coupled Analysis Tool for Floating Structures

2012 ◽  
Author(s):  
Zhiling Li ◽  
Carlos Llorente ◽  
Cheng-Yo Chen ◽  
Chang Ho Kang ◽  
Edmund Muehlner ◽  
...  
Keyword(s):  
Performance Analysis ◽  
Coupled Analysis ◽  
Mooring Line ◽  
Analysis Tool ◽  
Wave Load ◽  
Mooring Lines ◽  
Dynamic Interactions ◽  
Floating Structures ◽  
Coupled Method ◽  
Global Performance

For the global performance analysis of a floater, the traditional semi-coupled method models mooring lines/risers as nonlinear massless springs and ignores 1) the inertial effects from mooring lines/risers, 2) the current and wave load effects on mooring lines/risers, and 3) the dynamic interaction between mooring lines/risers and the floater. However, these effects are deemed critical for deepwater and ultra deepwater floating structures as they may have a significant impact on the floaters’ motions and mooring line/riser tensions. This paper presents the development and verification of a time-domain nonlinear coupled analysis tool, MLTSIM-ROD, which is an integration of a recently developed 3D rod dynamic program, ROD3D, with the well-calibrated floater global performance analysis program, MULTISIM (Ref [9]). The ROD3D was developed based on a nonlinear finite element method and merged with MULTISIM by matching the forces and displacements of mooring lines/risers with the floater at their connections. MLTSIM-ROD can thus predict the floater’s large displacement/rotation motions and mooring line/riser tensions including all the coupled effects between the floater and mooring lines/risers. In this paper, global performance predictions for a SPAR in the Gulf of Mexico in deepwater were carried out using MLTSIM-ROD. The results were then verified with those from other coupled analysis programs. The paper also presents the results of motions and mooring line/riser tensions of the SPAR using both the coupled and semi-coupled methods. The results from the coupled and semi-coupled analyses indicate that the floater’s motions and mooring line/riser tensions could be significantly influenced by the dynamic interactions between the floater and mooring lines/risers. Hence, the coupled method needs to be considered for deepwater floating structures.

Damage Detection of Offshore Platform Mooring Line Using Artificial Neural Network

2018 ◽  
Author(s):  
Djoni E. Sidarta ◽  
Jim O’Sullivan ◽  
Ho-Joon Lim
Keyword(s):  
Neural Network ◽  
Artificial Neural Network ◽  
Damage Detection ◽  
Total Mass ◽  
Offshore Platform ◽  
Mooring Line ◽  
Ann Model ◽  
Station Keeping ◽  
Mooring Lines ◽  
Artificial Neural

Station-keeping using mooring lines is an important part of the design of floating offshore platforms, and has been used on most types of floating platforms, such as Spar, Semi-submersible, and FPSO. It is of great interest to monitor the integrity of the mooring lines to detect any damaged and/or failures. This paper presents a method to train an Artificial Neural Network (ANN) model for damage detection of mooring lines based on a patented methodology that uses detection of subtle shifts in the long drift period of a moored floating vessel as an indicator of mooring line failure, using only GPS monitoring. In case of an FPSO, the total mass or weight of the vessel is also used as a variable. The training of the ANN model employs a back-propagation learning algorithm and an automatic method for determination of ANN architecture. The input variables of the ANN model can be derived from the monitored motion of the platform by GPS (plus vessel’s total mass in case of an FPSO), and the output of the model is the identification of a specific damaged mooring line. The training and testing of the ANN model use the results of numerical analyses for a semi-submersible offshore platform with twenty mooring lines for a range of metocean conditions. The training data cover the cases of intact mooring lines and a damaged line for two selected adjacent lines. As an illustration, the evolution of the model at various training stages is presented in terms of its accuracy to detect and identify a damaged mooring line. After successful training, the trained model can detect with great fidelity and speed the damaged mooring line. In addition, it can detect accurately the damaged mooring line for sea states that are not included in the training. This demonstrates that the model can recognize and classify patterns associated with a damaged mooring line and separate them from patterns of intact mooring lines for sea states that are and are not included in the training. This study demonstrates a great potential for the use of a more general and comprehensive ANN model to help monitor the station keeping integrity of a floating offshore platform and the dynamic behavior of floating systems in order to forecast problems before they occur by detecting deviations in historical patterns.

Interaction Between Mooring Line Damping and Response Frequency as a Result of Stiffness Alteration in Surge

2006 ◽  
Author(s):  
Lars Johanning ◽  
George H. Smith ◽  
Julian Wolfram
Keyword(s):  
Limit State ◽  
Extraction Process ◽  
Tidal Range ◽  
Mooring Line ◽  
System Response ◽  
Mooring Lines ◽  
Wave Basin ◽  
A Chain ◽  
Conversion Efficiencies ◽  
Line Design

The design and operation of a chain mooring for a wave energy converter (WEC) is considered. Experimental measurements of a mooring line were conducted in the Heriot-Watt University wave basin at a scale of 1:10. The laboratory procedures were designed to resemble tests undertaken earlier in the year at ‘full’ scale in 24 m water depth. This paper describes and compares these measurements and relates the results to earlier work on mooring lines by Webster [1]. Measurements of both the damping and response frequencies of the mooring are described. Although the present results support partly the conclusions of the earlier work, care must be taken in how these are applied when one is considering mooring line design for WECs. It is concluded that there are significant differences for a WEC for both operational and limit state design in comparison with a more conventional offshore system such as an FPSO or CALM. Although the primary requirement is still one of station-keeping two further considerations may be of great importance. Firstly if a ‘farm’ of devices is to be considered then limitations in sea space may necessitate that the devices be relatively densely packed. This will mean that the ‘footprint’ of the mooring should be constrained, to ensure that the moorings from each device do not interfere with one another and this will have great significance for the loading experienced by the line. This can be exacerbated by variations in tidal range which will have a larger effect in comparison with a conventional deepwater mooring. A second factor may arise if the mooring system response is critical to the WEC energy extraction process. If the mooring becomes part of the ‘tuned’ system then changes in the mooring properties of damping and natural frequency could seriously affect energy conversion efficiencies.

Experimental and Numerical Study on Large Truncation of Deepwater Mooring Line

2009 ◽  
Author(s):  
Yihua Su ◽  
Jianmin Yang ◽  
Longfei Xiao ◽  
Gang Chen
Keyword(s):  
Dynamic Response ◽  
Water Depth ◽  
Numerical Study ◽  
Unit Length ◽  
Low Frequency ◽  
Model Tests ◽  
Mooring Line ◽  
Mooring System ◽  
Mooring Lines ◽  
Wave Basin

Modeling the deepwater mooring system in present available basin using standard Froude scaling at an acceptable scale presents new challenges. A prospective method is to truncate the full-depth mooring lines and find an equivalent truncated mooring system that can reproduce both static and dynamic response of the full-depth mooring system, but large truncation arise if the water depth where the deepwater platform located is very deep or the available water depth of the basin is shallow. A Cell-Truss Spar operated in 1500m water depth is calibrated in a wave basin with 4m water depth. Large truncation arises even though a small model scale 1:100 is chosen. A series of truncated mooring lines are designed and investigated through numerical simulations, single line model tests and coupled wave basin model tests. It is found that dynamic response of the truncated mooring line can be enlarged by using larger diameter and mass per unit length in air. Although the truncated mooring line with clump presents a “taut” shape, its dynamic characteristics is dominated by the geometry stiffness and it underestimates dynamic response of the full-depth mooring line, even induces high-frequency dynamic response. There are still two obstacles in realizing dynamic similarity for the largely truncated mooring system: lower mean value of the top tension of upstream mooring lines, and smaller low-frequency mooring-induced damping.

Detection of Mooring Line Failure of a Spread-Moored FPSO: Part 1 — Development of an Artificial Neural Network Based Model

2019 ◽  
Author(s):  
Djoni E. Sidarta ◽  
Ho-Joon Lim ◽  
Johyun Kyoung ◽  
Nicolas Tcherniguin ◽  
Timothee Lefebvre ◽  
...  
Keyword(s):  
Neural Network ◽  
Artificial Neural Network ◽  
Numerical Simulations ◽  
Learning Algorithm ◽  
Offshore Platform ◽  
Training Data ◽  
Mooring Line ◽  
Ann Model ◽  
Mooring Lines ◽  
Artificial Neural

Abstract Artificial Intelligence (AI) has gained popularity in recent years for offshore engineering applications, and one such challenging application is detection of mooring line failure of a floating offshore platform. For most types of floating offshore platforms, accurately detecting any mooring line damage and/or failures is of great interest to their operators. This paper demonstrates the use of an Artificial Neural Network (ANN) model for detecting mooring line failure for a spread-moored FPSO. The ANN model representation, in terms of its input variables, is based on assessing when changes in a platform’s motion characteristics are in-fact indicators of a mooring line failure. The output of the ANN model indicates the status condition for the mooring lines (intact or failed). This ANN model only requires GPS / DGPS monitoring data and does not require data on the environmental conditions at the platform. Since the mass of an FPSO changes with the stored volume of oil, the vessel’s mass is also an input variable. The ANN training uses the results from numerical simulations of a spread-moored FPSO with fourteen mooring lines. The numerical simulations create the FPSO’s response to a range of metocean conditions for 360-degree directions, and they cover several levels of vessel draft (mass). Furthermore, the simulations cover both the intact mooring configuration and the full permutation where each of the fourteen mooring lines is modeled as broken at the top. The global performance analysis of the FPSO is presented in a different paper (Part 2 of these paper series). The training of the ANN model employs a back-propagation learning algorithm and an automatic method for determining the size of ANN hidden layers. The trained ANN model can detect mooring line failure, even for vessel draft (mass), sea states and environmental directions that are not included in the training data. This demonstrates that the ANN model can recognize and classify patterns associated with mooring line failure and separate such patterns from those associated with intact mooring lines under conditions not included in the original training data. This study reveals a great potential for using an ANN model to monitor the station keeping integrity of a floating offshore platform with changing storage, or mass status, and to detect mooring line failure using only the vessel’s mass and deviations in the platform’s motions derived from GPS / DGPS data.

Detection of Mooring Line Failure of a Spread-Moored FPSO: Part 2 — Global Performance Analysis Using MLTSIM

2019 ◽  
Author(s):  
Johyun Kyoung ◽  
Ho-Joon Lim ◽  
Djoni E. Sidarta ◽  
Nicolas Tcherniguin ◽  
Timothee Lefebvre
Keyword(s):  
Performance Analysis ◽  
Numerical Simulations ◽  
Motion Analysis ◽  
Time Domain ◽  
Model Tests ◽  
Mooring Line ◽  
Fast Time ◽  
Ann Model ◽  
Wide Range ◽  
Global Performance

Abstract This paper presents Part 2 in the development of an Artificial Neural Network (ANN) model for detection of mooring line failure of a spread-moored FPSO, global performance analysis used to generate the training and test data for the study. The development of an ANN model for detection of mooring line failure requires a comprehensive training data that is most practically available from the results of numerical simulations. Time domain analysis is necessary to capture the nonlinear behavior of a moored FPSO system and to represent the behavior of the physical system as accurate as possible. Given the wide range of sea-state conditions, of direction of the sea-states and of draft conditions of the FPSO, the number of time domain simulations is easily larger than 100,000. Therefore, an accurate and numerically efficient tool is necessary for carrying this task. The FPSO hull motion analysis is performed using MLTSIM, a TechnipFMC in-house, nonlinear time domain floating body motion analysis program. MLTSIM captures various non-linear load and response effects such as mooring stiffness, riser loads, drag and drift forces, as well as various user defined loads. MLTSIM is a numerically efficient and fast time domain solver which can run on both high-performance computing (HPC) system and a single laptop. Numerical model of a FPSO system has been validated using the results of model tests. In addition, the results of numerical simulations, in terms of hull motions and mooring line tensions, are compared with the results of model tests and a commercial software OrcaFlex. This well-calibrated model is then used for generating the numerical data required for the development of the ANN model.

scholarly journals Parametric Study of a Taut Compliant Mooring System for a FOWT Compared to a Catenary Mooring

2020 ◽  
Vol 8 (6) ◽  
pp. 431 ◽  
Author(s):  
Magnus Thorsen Bach-Gansmo ◽  
Stian Kielland Garvik ◽  
Jonas Bjerg Thomsen ◽  
Morten Thøtt Andersen
Keyword(s):  
Parametric Study ◽  
Environmental Benefits ◽  
Mooring Line ◽  
Mooring System ◽  
Mooring Lines ◽  
Wave Energy Converters ◽  
Cost Of Energy ◽  
Wave Basin ◽  
Line Angle ◽  
Energy Converters

The catenary mooring system is a well recognized station keeping method. However, there could be economical and environmental benefits of reducing the footprint. In the last decades, more focus has been given to synthetic mooring lines and different mooring layouts to optimize the levelized cost of energy (LCOE) for offshore renevable energy converters such as wave energy converters. Therefore, this work presents a parametric study of two important parameters, namely the mooring line angle and line pretension, for a taut mooring configuration focusing on the dynamic response when applied to the TetraSpar floating foundation compared to a catenary mooring system. The work is based on experimental results conducted in the wave basin at Aalborg University (AAU) and compared to analytical stiffness calculations. In addition, a numerical model was tuned based on the main dynamics to achieve the tension response. The results showed satisfying dynamic behavior where the angle and pretension mainly influenced the surge and yaw natural periods. The motion response showed similar behavior between the chosen parameters, and larger pitch amplitudes were found compared to the catenary system.

Prediction of Offshore Platform Mooring Line Tensions Using Artificial Neural Network

2017 ◽  
Author(s):  
Djoni E. Sidarta ◽  
Johyun Kyoung ◽  
Jim O’Sullivan ◽  
Kostas F. Lambrakos
Keyword(s):  
Neural Network ◽  
Artificial Neural Network ◽  
Numerical Simulations ◽  
Offshore Platform ◽  
Mooring Line ◽  
Offshore Platforms ◽  
Ann Model ◽  
Station Keeping ◽  
Mooring Lines ◽  
Hidden Neurons

Station-keeping is one of the important factors in the design of offshore platforms. Some offshore platforms, such as Spar, Semi-submersible and FPSO, use mooring lines as a mean for station-keeping. Tensions in the mooring lines are one of the key factors in station-keeping. The design of an offshore platform and its mooring lines is based on computed motions of the platform and associated mooring line tensions from numerical simulations using a software code on the basis of metocean criteria. This paper presents an Artificial Neural Network (ANN) model for the prediction of mooring line tensions based on the motions of the platform. This ANN model is trained with time histories of vessel motions and corresponding mooring line tensions for a range of sea states from the results of numerical simulations. After the model is trained, it can reproduce with great fidelity and very fast the mooring line tensions. In addition, it can generate accurate mooring line tensions for sea states that were not included in the training, and this demonstrates that the model has captured the knowledge for the underlying physics between vessel motions and mooring line tensions. The paper presents an example of the training and the validation of the model for a semi-submersible offshore platform for a range of sea states. The training of the ANN model employed a back-propagation learning algorithm. In this algorithm the computed output error is back-propagated through the neural network to modify the connection weights between neurons. The training started with a small number of hidden neurons, and the model grew adaptively by adding hidden neurons until either the target output convergence is achieved or a maximum number of additional hidden neurons is reached. The ANN model discovers nonlinear relationships between the input and output variables during training. The paper presents comparison of time series of mooring line tensions for sea states that were and were not included in the training between those from the numerical simulations and those computed by the trained ANN model. Fatigue assessment is also used to quantitatively measure the accuracy of the ANN model prediction of the time series of mooring line tensions. The paper presents the results of fatigue assessment using various stages of the ANN models with different number of hidden neurons. This shows that the additional hidden neurons improve the prediction of the ANN model of the mooring line tensions for sea states that were and were not included in the training. This approach of prediction of mooring line tensions based on vessel motions using ANN model paves the way to the development of an ANN-based monitoring system. Also, this ANN study demonstrates a great potential for the use of a more general and comprehensive ANN model to help monitor the dynamic behavior of floating systems and forecast problems before they occur by detecting deviations in historic patterns.

Methodology for Application of Model Test Data in TLP Global Design

2006 ◽  
Author(s):  
Amal C. Phadke ◽  
Alaa M. Mansour ◽  
Edward W. Huang
Keyword(s):  
Performance Analysis ◽  
Test Data ◽  
Model Test ◽  
Time Domain ◽  
Higher Order ◽  
Model Tests ◽  
Wave Basin ◽  
Global Performance ◽  
Numerical Tools ◽  
Tendon Tension

Sophisticated frequency and time domain software tools are available for the global performance analysis of Tension Leg Platforms (TLP). Time-domain tools allow realistic simulation of the response of TLP systems compared to frequency-domain tools, but they are generally computationally intensive. Rapid advances in computer technology have made it possible to employ sophisticated time-domain techniques as primary tools for the global performance analysis of TLP systems. However, response characteristics such as higher-order tendon response, wave-runup, airgap etc. cannot still be accurately predicted using the available numerical tools. Wave basin model tests, therefore, are indispensable to designers for estimating responses that cannot be reliably predicted. At the same time, using model tests alone as an analysis tool is not practical due to large number of design cases typically defined in global performance analysis. It is necessary to verify and calibrate numerical tools using model test data prior to their application in global performance analysis. This paper describes a methodology for calibrating and correlating predicted response from time-domain software tools against wave basin model tests. The application of correlation data in conjunction with predicted response to obtain various design quantities of interest has been investigated. Discussion for determination of model test correlated design maximum/minimum tendon tension, higher-order tendon tension response, and incorporation of vortex induced motion is presented.

scholarly journals Transient Responses Evaluation of FPSO with Different Failure Scenarios of Mooring Lines

2021 ◽  
Vol 9 (2) ◽  
pp. 103
Author(s):  
Dongsheng Qiao ◽  
Binbin Li ◽  
Jun Yan ◽  
Yu Qin ◽  
Haizhi Liang ◽  
...  
Keyword(s):  
Service Condition ◽  
Mooring Line ◽  
Mooring System ◽  
Transient Effects ◽  
Floating Platform ◽  
Transient Responses ◽  
Mooring Lines ◽  
Performance Deterioration ◽  
Steady State Responses ◽  
Influence Process

During the long-term service condition, the mooring line of the deep-water floating platform may fail due to various reasons, such as overloading caused by an accidental condition or performance deterioration. Therefore, the safety performance under the transient responses process should be evaluated in advance, during the design phase. A series of time-domain numerical simulations for evaluating the performance changes of a Floating Production Storage and Offloading (FPSO) with different broken modes of mooring lines was carried out. The broken conditions include the single mooring line or two mooring lines failure under ipsilateral, opposite, and adjacent sides. The resulting transient and following steady-state responses of the vessel and the mooring line tensions were analyzed, and the corresponding influence mechanism was investigated. The accidental failure of a single or two mooring lines changes the watch circle of the vessel and the tension redistribution of the remaining mooring lines. The results indicated that the failure of mooring lines mainly influences the responses of sway, surge, and yaw, and the change rule is closely related to the stiffness and symmetry of the mooring system. The simulation results could give a profound understanding of the transient-effects influence process of mooring line failure, and the suggestions are given to account for the transient effects in the design of the mooring system.

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