Analysis Of Mooring System Research Articles

Design of Experiments Technique Applied to Artificial Neural Network Models for FPSO Mooring System Analysis

Online monitoring of mooring system response for the FPSO platform in any operational condition is so far challenging for machine learning (ML). This paper presents a new dynamic NARX ANN model for time series of mooring tension and a static MLP model for the offset chart prediction of a taut-leg moored FPSO with different working scenarios. A novel method for supervised feature selection of the dataset was applied to determine the most influential design features. Additionally, a design of experiments (DOE) technique was implemented for test matrix creation, simulation, database generation, and supervised selection characteristics in ML. The DOE analysis revealed that the mooring configuration, platform loading condition, and environmental loads alter the platform’s six-degree-of-freedom motion response patterns. These input data were used to predict the mooring tension and the offset chart of the floater. The results include the fair values of statistical error for mooring tension (R2 = 0.8–0.98, E ≈ 1.3–5.7%, RMSE ≈ 6–66 kN) and platform offset (E ≈ 0.1–1 m) prediction when testing the trained models with unseen data representing new operational conditions. Faster convergence can be achieved by adding non-numeric (string) input values to dataset numeric features. Supervised feature selection of the dataset is a step forward in ML to improve prediction accuracy.

System reliability analysis of mooring system for floating offshore wind turbine based on environmental contour approach

The system reliability of the mooring system for floating offshore wind turbines (FOWT) is analyzed in this paper. Various failure modes such as mooring line breaking, fatigue and floater offset are included and solved as a series system. This work employed a new mooring breaking assessment algorithm, wherein the effect of fatigue damage on breaking strength is simulated by introducing functional dependences into the coupled dynamic analysis. In this research, the extreme design loads are estimated using the environmental contour (EC) approach and a logarithmic function is proposed to describe the relationship between the extreme response and the target return period to simplify the full long-term analysis. The time-domain coupling analysis of the FOWT is performed considering all one-year sea states, the long-term fatigue damage was approximated based on linear criterion. The fatigue damage can contribute significantly to the breaking strength of mooring lines. The results indicate that ignoring the dependence between the residual strength and cumulative fatigue damage would cause under-estimated results. The calculated failure probability of the floating system is larger than that of any single failure mode, while it is mainly dominated by the main single failure mode with the largest failure probability.

Offshore Fish Farms: A Review of Standards and Guidelines for Design and Analysis

While moving fish farms to offshore sites can be a more sustainable way to expand farmed fish production, the fish pens have to contend with a harsher environment. Thus, it is necessary to draw on offshore engineering competences for designing and analysing the offshore fish farming infrastructure. This paper reviews existing design and analysis guidance from maritime classification and national/international authorities that can be applicable for offshore fish farms. Based on the existing design guidelines, a review of design criteria for offshore fish farms under the following subtopics is provided: design life, design environmental loads, combining environmental loads, and miscellaneous load conditions. This review on the global performance analysis procedures and methods is presented based on practices used for neighbouring industries, such as offshore oil and gas and wind energy production, under the following subtopics: hydrostatic analysis, hydrodynamic analysis, and mooring system analysis with introducing theoretical background and modelling techniques. This paper also highlights limitations and cautions when using these design and analysis methods. Providing this comprehensive information, as well as commentary on their applications, will help engineers and designers to develop offshore fish farming infrastructure with confidence.

DESIGN OF A BUOYANT SYSTEM WITH DISCRETE FORCE DISTRIBUTION

The stability and dynamic positioning of any floating structure is essential during offshore or on shore activities. It is therefore necessary to carry out offshore mooring system analysis in order for it to withstand extreme environmental forces of wind, wave, and current that will act on the structures. This project is focused on designing a fit-for-purpose buoyant mooring system to achieve the stability and dynamic positioning of a floating system. This system must provide a discreet non-linear force distribution within a given range. The sea-water depth range at the shore was also provided. For a certain stretch in the mooring line, due to rise in the level of water, the decrement in the required buoyant force was to decrease non-linearly. In order to meet that requirement a mechanism with only one piston-cylinder connected to mooring line was devised. This was the simplest solution that could be achieved for the desired outcome. Any other mechanism, according to observations, increases the complexity of the system, which was to be avoided (as demanded in the problem).Once the string is stretched the piston is pulled downwards. This increases the total volume occupied by the mechanism thus increasing the buoyant force (Archimedes Principle). But there is a linear relationship between the stretch of string and increase of buoyant force. In order to make this relationship non-linear a multi-groove step pulley was used. The mooring line passing over this pulley produced a non-linear pull in the piston (since the radius of the each step changes non-linearly). As a result buoyant force also changes non-linearly for the same stretch of the mooring line. CATIA was used for 3D modeling of the system, and better understanding of the mechanism. The above mentioned mechanism was the simplest which could be achieved, because no complex tools were required for its construction and it also provided the exact values as demanded.

Platform Motions and Mooring System Coupled Solver for a Moored Floating Platform in a Wave

In advance of building moored floating offshore platforms, in recent years, there has been a greater demand for two-way coupled simulations between a motion solver based on the viscous flow theory and a mooring line model, including cable dynamics. This paper introduces open-source libraries such as MoorDyn (the lumped-mass mooring line model) and OpenFOAM (the computational fluid dynamics libraries). It describes the methods by which they can be coupled bi-directionally. In each time step, the platform motions calculated by OpenFOAM are transferred to MoorDyn as the boundary conditions for the mooring system analysis. In contrast, MoorDyn calculates the restoring force and moment due to the mooring system and transfers them to OpenFOAM. The restoring force and moment act on the platform as the external force and moment for the platform motions in the next time step. The static tension and profile of the mooring system, dynamic tension of the mooring system, and free decay motions of the floating buoy in the still water were simulated to check the accuracy of OpenFOAM and MoorDyn. The coupled solver was used to produce simulations of the moored decay motions of the floating buoy in the still water and the moored motions with the Stokes 5th order wave. All simulation results were compared and showed good agreement with the numerical solution and experiment results. In addition, the characteristics of each solver were investigated.

Comparative Analysis and Differential Study on Dynamic Response of Tension Type Mooring and Catenary Mooring System

Mooring system is an important factor affecting the safety and motion law of floating body. It is very important to study the hydrodynamic characteristics of different mooring methods for offshore safety engineering and stability analysis of mooring system. Taking oc4 as a floating body hydrodynamic model and considering the combined action of wind, wave and current, the hydrodynamic characteristics and system stability of taut and catenary mooring mode are compared and analyzed respectively. In addition, the differences and variation rules of dynamic response of floating body under different mooring conditions are explored and summarized. Combined with the time-domain analysis theory and simulation results, the coupled motion equations of taut and catenary mooring system are improved, and the corresponding relationship between slow drift motion and different mooring modes is summarized. It provides an important theoretical basis and support for the research of time domain variation law of mooring system and marine safety control.

Mooring Analysis of SPAR Type Floating Offshore Wind Turbine in Operation Condition due to Heave, Roll, and Pitch Motions

In Indonesia, one of the regions that have good potential for Floating Offshore Wind Turbine (FOWT) installations is the Natuna Sea which has a fairly good average wind speed in the range of 10.0 - 15.0 m/s. In addition, the Natuna Sea has a water depth of around 50-250 m with relatively mild wave height. This condition is a suitable environment for the installation of the FOWT. Therefore, it is necessary to investigate the appropriate mooring system configuration due to the motion of the structure. This paper examined the analysis of mooring system of the SPAR type FOWT structure due to its global motion through numerical modelling by using open source software FAST. The analysis was carried out for operation condition, by varying three configurations of the mooring system for the direction of environmental loads (waves and winds), namely 0 degrees (perpendicular in front of the turbine), 45 degrees, and 90 degrees (the turbine side direction) and wind speed. With the scope of such analysis, it could be properly examined the mooring system due to the global motion of heave, roll and pitch of the Spar.

A Study on the Dynamic Analysis of Mooring System During Hook-up Installation

A Study on the Dynamic Analysis of Mooring System During Hook-up Installation

Validation of a dynamic mooring model coupled with a RANS solver

Standard design procedures and simulation tools for marine structures are aimed primarily for use by the offshore oil and gas. Mooring system restoring forces acting on floating offshore structures are obtained from a quasi-static mooring model alone or from a coupled analysis based on potential flow solvers that do not always consider nonlinear mooring-induced restoring forces, fluid structure interactions, and associated hydrodynamic damping effects. This paper presents the validation of a dynamic mooring system analysis technique that couples the dynamic mooring model with a Reynolds-averaged Navier-Stokes (RANS) equations solver. We coupled a dynamic mooring model with a RANS equations solver, and analyzed a moored floating buoy in calm water, regular and irregular waves and validated our motion and mooring force predictions against experimental measurements. The mooring system consisted of three catenary chains. The analyzed response comprised decaying oscillating buoy motions, linear and quadratic damping characteristics, and tensile forces in mooring lines. The generally favorable comparison of predicted buoy motions and mooring forces to experimental data confirmed the reliability of our implemented coupling technique to predict system response. Additional comparative results from a potential flow solver demonstrated the benefits of the coupled dynamic mooring model with RANS equations. The successful validated tool of coupling the dynamic mooring model with the RANS solver is available as open source, and it shows the potential of the coupled methodology to be used for analyzing the moored offshore structures.

Multi-mode reliability analysis of mooring system of deep-water floating structures

This paper proposes a comprehensive method for reliability analysis of a deep-water mooring system. The novelty of the newly developed method is that it considers the failure modes of floater offset excess (the function of the intact mooring system), mooring line breakage (safety), and fatigue (persistence) as part of the mooring system's reliability assessment. In addition, the quantitative relation between mooring line breaking strength and cumulative fatigue damage is considered. In this research, the annual maximum floater offset and mooring line tension are calculated based on the wave condition with a 1-year return period. Long-term mooring line tension ranges are obtained from coupled analysis in the time domain for all the sea states in the wave scatter diagram and the fatigue damage is assessed using the Palmgren–Miner rule and T-N curve. Then, the mooring system's reliability is investigated by running Monte Carlo simulations. The results demonstrate that the failure probability considering the three failure modes is much greater than that of any single mode and neglecting the correlation between different failure modes leads to significant underestimation of the mooring system failure probability. Moreover, the sensitivity analysis indicates that the underestimation proportion is obviously affected by the fatigue exponent (m).

Proposal of Wear Estimation Method for Mooring Chain of Floating Structures

Up to now, wear estimation method for mooring chain of floating facilities has not been established, and it currently relies on empirical methods. In this research, numerical analysis combining 1) wear analysis between links by finite element method and 2) response analysis of mooring system was performed to establish a method for estimating wear amount of mooring chain during operation. The validity of the proposed method was confirmed by comparing with the actual measurement value in the actual sea area. As a result, it was found that the wear amount estimated by our proposed method is generally close to the measured value. However, it was also confirmed that the estimated wear amount at the touchdown point between the seabed and mooring chain was considerably overestimated. That result could be attributed to insufficient consideration of the effects caused between the links ( e.g. snap load, elastic deformation and friction), the behavior of the links ( e.g. sticking, rolling and non-contact) and the external forces ( e.g. wind and current ).

Analysis of Mooring System of Shuttle Tanker on Jack-up Platform

Analysis of Mooring System of Shuttle Tanker on Jack-up Platform

Fatigue reliability analysis of mooring system for fish cage

Fish cages in the open sea are exposed to cycle loads due to irregular wave climate during their service life, and thus the fatigue reliability assessment of mooring system should be conducted to ensure the safe operation. The aim of this study is to evaluate the fatigue failure probability of mooring system for fish cage. Numerical simulation of net cage in random waves is performed and the time dependent approach is applied to conduct the fatigue reliability analysis of shackle chains based on S-N curve method. The sensitivity analysis of fatigue reliability of mooring line to the uncertainty of random variables in the fatigue limit state is conducted. In addition, the system reliability for mooring system is analyzed and the effect of the initial pretension and safety factor on system reliability is investigated. The results indicate that a case without the initial pretension on anchor lines is helpful to decrease the failure probability of mooring system and the safety factor of mooring lines in the current regulation is conservative for the system reliability against fatigue damage.

섬유로프 계류시스템의 크리프 효과가 부유체의 운동응답 및 계류선의 장력 변화에 미치는 영향에 관한 연구

Growing demand and rapid development of the synthetic fiber rope in mooring system have taken place since it has been used in deep water platform lately. Unlike a chain mooring, synthetic fiber rope composed of lightweight materials such as Polyester(polyethylene terephthalate), HMPE(high modulus polyethylene) and Aramid(aromatic polyamide). Non-linear stiffness and another failure mode are distinct characteristics of synthetic fiber rope when compared to mooring chain. When these ropes are exposed to environmental load for a long time, the length of rope will be increased permanently. This is called 'the creep phenomenon'. Due to the phenomenon, The initial characteristics of mooring systems would be changed because the length and stiffness of the rope have been changed as time goes on. The changed characteristics of fiber rope cause different mooring tension and vessel offset compared to the initial design condition. Commercial mooring analysis software that widely used in industries is unable to take into account this phenomenon automatically. Even though the American Petroleum Institute (API) or other classification rules present some standard or criteria with respect to length and stiffness of a mooring line, simulation guide considers the mechanical properties that is not mentioned in such rules. In this paper, the effect of creep phenomenon in the fiber rope mooring system under specific environment condition is investigated. Desiged mooring system for a Mobile Offshore Drilling Unit(MODU) with HMPE rope which has the highest creep is analyzed in a time domain in order to investigate the effects creep phenomenon to vessel offset and mooring tension. We have developed a new procedure to an analysis of mooring system reflecting the creep phenomenon and it is validated through a time domain simulation using non-linear mooring analysis software, OrcaFlex. The result shows that the creep phenomenon should be considered in analysis procedure because it affects the length and stiffness of synthetic fiber rope in case of high water temperature and permanent mooring system.

Hydrodynamic response simulation of Catenary mooring in the spar truss floating platform under Caspian Sea conditions

Nowadays, oil platform's moorings failure one of the most important challenges of the drilling industry in deep seas for the establishment and stability of the platform during operation. The behavior of these moorings plays an important role in the preservation of the platform with interaction by waves and environmental forces. In this regard a sample of non-classical Spar Truss platform was analyzed using Moses software in the Caspian Sea. Also hydrodynamic analysis of mooring system based on three-dimensional diffraction method was used; this analysis is based on time dependent and six degrees of freedom motion. Therefore response amplitude operator (RAO) of platform moorings was obtained by incident wave. The effect of removing a platform mooring was studied. The results indicate a high gradient change in Surge Movement. Also the results of this study can be used as a benchmark for spar platforms design in Caspian sea.

基于静力学分析的系泊系统状态浅析

系泊系统分析是通过获取系泊系统的自身参数以及周围环境参数，对系泊系统的钢桶和各节钢管的倾斜角度、浮标的吃水深度、和游动区域进行求解，从而判断系泊系统的工作状态。首先，通过对系泊系统整体和各部件的力学分析建立模型，并引入悬链线方程求解锚链方程，运用递推方程求解未知参数。其次，将数学建模题目中已知参数代入，利用穷举法求解临界风速，判断系统运动状态，在不同情况下分别运用MATLAB编程求解。最后，利用穷举法求出重物球重量变化范围。从而实现系泊系统状态的判断与分析。 Through the mooring system’s own parameters and the surrounding environment parameters, mooring system analysis is to solve steel drums and sections of the steel pipe tilt angle, buoy of the draft depth, and swimming area of the mooring system, which determine the working state of the mooring system. Firstly, the model is established by static analysis of the mooring system and the components, and the catenary line equation is introduced to solve the equation of anchor chain, recurring equations to solve the unknown parameters. Secondly, we substitute the known parameters of subject into the equation, and use the exhaustive method to solve the critical wind speed and to determine the state of motion of the system. In different circumstances, we respectively use MATLAB to solve the equation. Finally, the range of heavy weight is determined by exhaustive method so as to realize the state judgment and analysis of mooring system.

A novel underwater measurement method for mooring system using self-contained technique

This article introduces a prototype measurement method for investigating deep-water mooring systems. Measurement information and specifications are determined according to mooring system analysis. Mooring line information is difficult to measure underwater, particularly for aging platforms. To solve this problem, a novel mooring line tension measurement strategy based on lumped mass method is proposed. A type of self-contained sensor is designed and manufactured to realize dynamic response measurement of mooring systems underwater. The sensor is described in detail, including its design requirement, hardware design, and installation mode. A comprehensive field test system was constructed on a semi-submersible platform in South China Sea. Prototype data, including the data measured during typhoons, are introduced and discussed to improve the accuracy, feasibility, and stability of the proposed measurement method.

Dynamic Response Analysis on Submerged Floating Tunnel due to Hidrodynamic Loads

This paper presents mooring system analysis on submerged floating tunnel (SFT) with a number of mooring configurations and the influence of mooring angle variations to its dynamic response. Working loads considered on the tunnel are current and wave loads. The tunnel structure is assumed of steel tubewith 150 m length and 5 m of diameter. Mooring configurations used are varied with 12, 16, 20 and 28 lines, with mooring angle respective to thevertical line are 30, 45 and 60 degrees. The structure was numerically modeled and the results showed that the biggest tension of 241.53 tons occurred for the case of wave heading 45 degree, 30 degree mooring inclination and with 28 mooring lines. This case generatedlarge lift force so that a very high lines tension occurred.

Mooring Pattern Optimization Using A Genetic Algorithm

In this paper, a constraint Genetic Algorithm is used for the purpose of mooring pattern optimization. The Genetic Algorithm is applied through a mathematical formulation which is introduced to define a typical mooring system optimization problem. The mathematical formulation is used in a case study on a spread moored crane barge, operating in the vicinity of a jacket type platform, in order to minimize its surge motions towards the platform. For this purpose, a set of criteria regarding clearances between anchors and seabed preinstalled facilities (pipelines), and also between the crane barge and the jacket platform are presented and considered. An automatic process of repetitive analyses implementing a MATLAB code as an interface between the Genetic Algorithm and a mooring system analysis program is used, and an optimum solution is resulted by performing 4000 quasi-dynamic analyses in time domain. The effectiveness of the Genetic Algorithm in leading to an optimum mooring system pattern is studied and it is shown that using a proper formulation of the problem, the Genetic Algorithm can be a very useful tool for finding an optimum pattern for mooring systems in fields with constraints on anchor locations and vessel motions.

Fatigue analysis of mooring system for net cage under random loads

Mooring system for net cage plays a pivotal role in the aquaculture industry. Much room exists for further development in relation to the mooring line analysis. A statistical analysis of stress range of mooring line is presented. The fatigue design analysis is based on the time domain analysis of net cage and mooring system in irregular waves. The statistical properties of stress range are analyzed using the Weibull plots, histogram, damage plot and evaluation of its applicability in relation to simplified methods. In addition, the fatigue damage of mooring line is also analyzed using spectrum analysis method and compared with the result from rainflow counting method. A visual inspection of Weibull plots indicates a good fit, and the shape parameter is typically between 0.8 and 1.2. The fatigue damage is underestimated based on the Weibull parameters, while it is predicted accurately based on the Rayleigh distribution. The results also show that the fatigue damage of mooring line can be underestimated 10–30% using the spectrum analysis method.