Application of Peak Distribution Method for Response Based Analysis of Mooring Lines Under Tropical Storms

2020 ◽  
Author(s):  
A. Ghasemi ◽  
Y. Drobyshevski ◽  
M. Kimiaei ◽  
M. Efthymiou
Keyword(s):  
Time Domain ◽  
Nonlinear Behavior ◽  
Extreme Value Distribution ◽  
Time Domain Analysis ◽  
System Response ◽  
Response Analysis ◽  
Tropical Storms ◽  
Mooring Lines ◽  
Distribution Method ◽  
Large Variability

Abstract Response based analysis (RBA) is a comprehensive approach for the prediction of extreme responses and design metocean conditions of offshore facilities. For RBA, the structural system needs to be modelled, and its behavior analyzed when subjected to large metocean datasets, usually comprising thousands of different sea states. Due to the dynamic and nonlinear behavior of mooring systems in floating structures, application of conventional time domain analysis for RBA of these systems is a computationally demanding process. Hence, investigation of faster solvers and more efficient methods for the RBA is inevitable. Peak distribution method (PDM), which has recently been introduced and used for response analysis of mooring systems under extreme design conditions, is a possible solution to reduce the computational efforts in RBA by reducing the number of simulations. This study explores the utilization of the PDM for RBA of the mooring system of a turret-moored large FPSO subjected to tropical storms. Large variability of metocean parameters within such storms limits the applicability of intuitive judgement for the selection of governing sea states. The results are compared through both time-domain and frequency-domain simulations and a computationally efficient methodology is proposed. It provides a general robust framework of computing the extreme value distribution of the system response. The proposed methodology can be used for RBA of mooring lines tension under storm conditions comprising large number of sea states.

A Preliminary Study of a Rigid Semi-Submersible Fish Farm for Open Seas

2017 ◽  
Author(s):  
Lin Li ◽  
Muk Chen Ong
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Time Domain Analysis ◽  
Fish Farm ◽  
Response Analysis ◽  
Frequency Domain Method ◽  
Mooring System ◽  
Viscous Effects ◽  
Support Structure ◽  
Mooring Lines

The development of reliable fish farm structures for open seas becomes more and more important. One of the challenges is to design a robust structure to withstand the harsh offshore environmental loads. This paper investigates a semi-submersible type offshore fish farm system for open seas. This system consists of a semi-submersible support structure with pontoons and braces, a catenary mooring system and net cages. The support structure is designed to be rigid to resist severe offshore conditions. A preliminary hydrodynamic and response analysis is carried out for this concept. The linear hydrodynamic properties using different composite models with panel and Morison elements are computed. Based on the hydrodynamic analysis, linearised frequency-domain and coupled time-domain analysis are performed to predict the extreme motions of the support structure and the extreme tensions in the mooring lines. The results indicate that the frequency-domain method underestimates the extreme responses, and the couplings between the structure and the mooring system need to be considered in the time-domain. Responses using various hydrodynamic models are also compared to evaluate the influences of the viscous effects from the pontoons and the nets of this fish farm concept.

Dynamic Analysis for the Design of CALM System in Shallow and Deep Waters

1997 ◽  
Vol 119 (3) ◽  
pp. 151-157 ◽  
Author(s):  
Y.-L. Hwang
Keyword(s):  
Mathematical Model ◽  
Dynamic Analysis ◽  
Model Test ◽  
Time Domain ◽  
Time Domain Analysis ◽  
Mooring Lines ◽  
Deep Waters ◽  
Wave Drift ◽  
The Mathematical Model ◽  
Survival Condition

This paper presents a time domain analysis approach to evaluate the dynamic behavior of the catenary anchor leg mooring (CALM) system under the maximum operational condition when a tanker is moored to the terminal, and in the survival condition when the terminal is not occupied by a tanker. An analytical model, integrating tanker, hawser, buoy, and mooring lines, is developed to dynamically predict the extreme mooring loads and buoy orbital motions, when responding to the effect of wind, current, wave frequency, and wave drift response. Numerical results describing the dynamic behaviors of the CALM system in both shallow and deepwater situations are presented and discussed. The importance of the line dynamics and hawser coupled buoy-tanker dynamics is demonstrated by comparing the present dynamic analysis with catenary calculation approach. Results of the analysis are compared with model test data to validate the mathematical model presented.

A Review of Mooring Analysis Methodologies for Permanent Offshore Installations

2020 ◽  
Author(s):  
Spiro J. Pahos ◽  
Georgina Maldonado ◽  
Paul C. Westlake
Keyword(s):  
Time Domain ◽  
Line Strength ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Mooring Line ◽  
System Response ◽  
Mooring System ◽  
Deterministic Approach ◽  
Safety Factors ◽  
Analytical Approaches

Abstract Traditionally mooring line strength assessment is based on a deterministic approach, where the mooring system is evaluated for a design environment defined by a return period. The mooring system response is then checked against the mooring strength to ensure a required factor of safety. Some codes adopt a deterministic approach [1], [2], [3]. Other codes like [4] adopt a partial safety factor format where uncertainties are addressed through load factors for load components and material factors for line strength. Industry practices give guidance on mooring analysis methodology together with analysis options like coupled, de-coupled, time domain, frequency domain and the associated line tension safety factors. Prior work has demonstrated that discrepancies in mooring line tensions are observed when different analytical approaches are used [5]. Namely, the mooring line tensions of a semi-submersible unit in a coupled time domain analysis, were found to be non-compliant, whereas those calculated using a decoupled time domain analysis returned compliant tensions. This work focuses on a coupled dynamic analysis where all inertial, hydrodynamic and mechanical forces are assessed to determine the subsequent motions. Despite being considered the most accurate to capture the true dynamic response, a coupled analysis is also the least efficient in terms of the required computer resources and engineering effort [1]. This paper presents further discussion on the above observation in mooring tensions and also considers differences in the installation’s excursion. All responses are evaluated in the time domain where the nonlinear dynamic behavior of the mooring lines, slowly varying wave drift forces and coupling effects are captured. Agreement is found in the present computations, carried out with two renowned hydrodynamic codes, which validate former results and reiterate the need to distinguish between time domain methods and recommended appropriate safety factors accordingly.

Response Analysis on Asymmetric Twin Tower Building with Large Bases Subjected to Seismic Action

2011 ◽  
Vol 117-119 ◽  
pp. 174-179
Author(s):  
Qiong Fen Wang ◽  
Ji Yao ◽  
Liang Cao ◽  
Jian Bin Xie ◽  
Chun Li Guo
Keyword(s):  
Time Domain ◽  
Structural Design ◽  
Time Domain Analysis ◽  
Response Analysis ◽  
Seismic Action ◽  
Analysis Method ◽  
Structural Dynamic ◽  
Fem Model ◽  
Structural Dynamic Characteristics ◽  
High Building

In this paper, asymmetric twin tower high buildings with large bases were studied and the FEM model of asymmetric twin tower high building with large bases was established. Then the structural dynamic characteristics was analyzed, the x-direction and y-direction level seismic excitation were input separately with the help of time-domain analysis method, and the acceleration time-domain curve of the top of the structure was obtained, the reference of structural design was provided.

Seismic Analysis of Intake Tunnel for Nuclear Power Plant

2011 ◽  
Vol 243-249 ◽  
pp. 3513-3517
Author(s):  
Jie Zhao ◽  
Yang Zheng ◽  
Gui Xuan Wang
Keyword(s):  
Power Plant ◽  
Nuclear Power Plant ◽  
Time Domain ◽  
Nuclear Power ◽  
Seismic Analysis ◽  
Time Domain Analysis ◽  
Internal Force ◽  
Response Analysis ◽  
Plant Structure ◽  
Seismic Response Analysis

Aiming at the nuclear power plant structure and adopting time domain analysis approach, the seismic analysis of an intake tunnel for nuclear power plant is performed with FLAC3D in this paper. Contraposing the characters of the field rock of the nuclear plant, the internal force distribution of the tunnel under different wall rocks is researched and analysed, and the envelope diagram of inner force in the lining of the tunnel is drawn. The obtained law can provide the basis for the seismic response analysis of tunnels.

Stochastic Response Analysis of Deepwater Structures in Short-Crested Random Seas

1999 ◽  
Vol 121 (3) ◽  
pp. 181-186 ◽  
Author(s):  
P. Teigen ◽  
A. Naess
Keyword(s):  
Time Domain ◽  
Large Volume ◽  
Time Domain Analysis ◽  
Order Theory ◽  
Offshore Structures ◽  
Response Analysis ◽  
Random Waves ◽  
Stochastic Response ◽  
Cpu Time ◽  
Random Seas

The paper discusses the problem of estimating the response statistics of moored large-volume offshore structures subjected to short-crested random waves. A general second-order theory is described that makes it possible to carry out the entire analysis in the frequency domain, which is computationally more efficient than time domain analysis, which generally requires considerably more CPU time to reach the same level of accuracy.

Route to Chaos of an Articulated Offshore Loading Platform

2004 ◽  
Author(s):  
A. P. Shashikala
Keyword(s):  
Wind Waves ◽  
Nonlinear Behavior ◽  
Mooring Line ◽  
Response Analysis ◽  
Mooring Lines ◽  
Restoring Force ◽  
Highly Nonlinear ◽  
Waves And Currents ◽  
Time Domain Response ◽  
The Time Domain

Articulated Loading Platforms are compliant structures which undergo excessive displacements due to large hydrodynamic loads produced by wind, waves and currents. Prediction of nonlinear behavior of these complex structures in the nonlinear environment is extremely difficult. The discontinuity in the mooring line stiffness at the equilibrium position due to slackening of the mooring line between tower and tanker introduces nonlinearity in the equation of motion. An attempt to study the effect of the highly nonlinear restoring force and hydrodynamic forces on the system was performed. The time domain response analysis was done on the basis of approximate analytical investigations. The solution of the nonlinear simultaneous equations was performed and the results were interpreted by means of phase plots and poincare mapping. The effect of forcing amplitude on the behavior of the system was studied by varying the frequency ratio. Possible occurrence of sub harmonic and chaotic responses and hence different routes to chaos were also identified. These results can be made use of in the design of mooring lines to avoid structural instabilities in the evolving offshore environment.

Hydrodynamic Response Analysis of Combined Spar Wind Turbine and Fish Cage for Offshore Fish Farms

2020 ◽  
Vol 20 (09) ◽  
pp. 2050104
Author(s):  
Y. I. Chu ◽  
C. M. Wang
Keyword(s):  
Wind Turbine ◽  
Frequency Domain ◽  
Time Domain ◽  
Fish Farming ◽  
Fish Farm ◽  
Constant Current ◽  
Response Analysis ◽  
Comparison Study ◽  
Mooring Lines ◽  
Hydrodynamic Response

This paper is concerned with the hydrodynamic response of a novel offshore fish farm that combines a floating spar wind turbine and a fish cage (named as COSPAR for brevity). The open net steel cage is octagonal in shape with a partially porous wave fence at its top end to attenuate wave energy for a calm fish farming environment as well as to keep predators out. The deep draught spar is made from concrete for its bottom half and from steel for its top half. The spar carries a control unit and a 1[Formula: see text]MW wind turbine that provides the required power to operate the offshore salmon fish farm. The COSPAR fish cage has four catenary chains as mooring lines attached to mid length of the spar (outside the fish cage) so as to mitigate tension force in the mooring lines and to reduce the benthic footprint. ANSYS Design Modeler and Aqwa are used to perform the hydrodynamic response analysis of free-floating condition of COSPAR in the frequency domain and coupled analysis involving COSPAR and the mooring lines in the frequency domain and time domain. Environmental conditions, representing 5-year, 20-year and 50-year wave return periods with a constant current flow at an exposed fish farming site in Storm Bay of Tasmania, Australia, are adopted for the analyses. A comparison study is made against having a floating fish cage only (i.e. without the bottom half concrete of the spar) with four catenary chains attached to side vertical columns of the cage so that the fish cage behaves like a semi-submersible cage. Based on the comparison study, the COSPAR fish cage shows enhanced hydrodynamic responses in the following respects: (1) more stable motion responses in heave and pitch against wave and current forces, (2) less susceptible to the viscous damping when it is assumed by a linearized drag force of Morison elements in the frequency domain and (3) reduction of tension forces in the mooring lines. Interestingly, the pitch motion response of COSPAR fish cage in the frequency domain is in close agreement with the time domain result due to its greater pitching stiffness that reduces nonlinear effects from viscous drag and mooring interaction.

Implementation and Application of Modal Analysis During Time Domain Dynamic Simulation of Floating Offshore Systems

2006 ◽  
Author(s):  
Fabri´cio Nogueira Correˆa ◽  
Breno Pinheiro Jacob
Keyword(s):  
Modal Analysis ◽  
Dynamic Simulation ◽  
Time Domain ◽  
Time Domain Analysis ◽  
Jacobi Method ◽  
Hydrodynamic Behavior ◽  
Mooring Lines ◽  
Dynamic Simulations ◽  
Floating System ◽  
Nonlinear Time Domain

This paper presents the implementation and application of modal analysis during nonlinear time-domain dynamic simulations of floating offshore systems. The simulations are performed by a fully coupled nonlinear time-domain analysis methodology, which considers the interaction between the hydrodynamic behavior of the hull and the structural/hydrodynamic behavior of the mooring lines and risers. Considering the nonlinear variation of the stiffness and added mass of the floating system with time, the objective is to assess the variation of the natural periods of vibration for the 6-DOF of the floating system (surge, sway, heave, roll, pitch and yaw). To accomplish this goal, the generalized eigenvalue problem associated to the system is assembled, and the Generalized Jacobi Method is employed to solve this problem and determine natural periods of the system, at selected time intervals during the dynamic simulation. Case studies are selected to assess the variation with time of the natural periods, considering two different types of floating systems: the ITTC semi-submersible platform, for which experimental results are available; and a CALM monobuoy system. The results obtained stresses the importance of the calculation of natural periods in different positions of the system: due to the marked nonlinear behavior of the mooring lines and risers, the natural periods can show considerable variations, for instance, from the neutral design position to an equilibrium position under action of current.

Prediction of Mooring Line Tensions for Hurricane Conditions in the Gulf of Mexico

2014 ◽  
Author(s):  
Jan Mathisen ◽  
Torfinn Hørte
Keyword(s):  
Short Interval ◽  
Extreme Value Distribution ◽  
Line Tension ◽  
Time Domain Analysis ◽  
Extreme Value ◽  
Value Distribution ◽  
Mooring Line ◽  
Response Analysis ◽  
Short Term ◽  
Tension Distribution

A probabilistic metocean model for hurricane conditions is briefly described. The model is based on site-specific, hindcast data and defines the time variation of the metocean conditions during the realisation of a hurricane at the site. The annual extreme value distribution of mooring line tension for a large, semi-submersible, mobile drilling unit is computed. Time domain analysis is applied to obtain the short-term, extreme value distribution of line tension, conditional on stationary metocean conditions. A large number of different conditions are considered. A response surface is used to interpolate on the short-term distribution parameters in order to describe the tension response during the varying conditions associated with the passage of a hurricane. The hurricane duration is split into a sequence of 15-minute intervals such that the conditions can be assumed stationary during each such short interval. The tension distribution, conditional on the realisation of a hurricane, is accumulated across the sequence of short intervals. The distribution of hurricanes is taken into account to obtain the tension distribution in a random hurricane. Finally, the frequency of hurricanes is taken into account to give the annual extreme distribution of line tension. The characteristic tension computed using 10-year return conditions and the ISO 19901-7 design standard is found to correspond to a return period of 29 years in the test case. The effects of various assumptions in the design analysis are investigated. Sensitivities to simplifications of the metocean model are considered. The effects of uncertainties in the response calculation and in the distribution of peak significant wave height during hurricanes are quantified and included in the response analysis.

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