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.

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.

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.

Comparison of Time and Frequency Domain Analysis With Full Scale Data for the Horn Mountain Spar During Hurricane Isidore

2006 ◽  
Author(s):  
Arcandra Tahar ◽  
John Halkyard ◽  
Mehernosh Irani
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Full Scale ◽  
Frequency Domain Method ◽  
Drag Forces ◽  
The Time Domain ◽  
Scale Data

The Horn Mountain Spar is located in 1,654 m of water about 135 km from Venice, Louisiana in the Gulf of Mexico. The facility was instrumented extensively to measure key spar and riser response parameters (Edwards et. al. 2003). Halkyard et. al. (2004) and Tahar et. al. (2005) have compared measured spar responses such as motion and mooring line tensions with numerical predictions. This paper extends the work done on comparison of the full scale data during hurricane Isidore. All previous numerical simulations were based on a time domain analysis procedure. One concern related to this method is that it is computationally intensive and time consuming. In the initial stages of a project, a frequency domain solution may be an effective tool compared with a fully coupled time domain analysis. The present paper compares results of time domain and frequency domain simulations with field measurements. Particular attention has been placed on the importance of the phase relationship between motion and excitation force. In the time domain analysis, nonlinear drag forces are applied at the instantaneous position. Whereas in the frequency domain analysis, nonlinear drag forces are stochastically linearized and solutions are obtained by an iterative procedure. The time domain analysis has better agreement with the field data compared to the frequency domain. Overall, however, the frequency domain method is still promising for a quick and approximate estimation of relevant statistics. With advantages in terms of CPU time, the frequency domain method can be recommended as a tool in pre-front end engineering design or in a phase where an iterative nature of design of an offshore structure takes place.

scholarly journals A Comparison of Numerical Approaches for the Design of Mooring Systems for Wave Energy Converters

2020 ◽  
Vol 8 (7) ◽  
pp. 523 ◽  
Author(s):  
Imanol Touzon ◽  
Vincenzo Nava ◽  
Borja de Miguel ◽  
Victor Petuya
Keyword(s):  
Frequency Domain ◽  
Wave Energy ◽  
Time Domain ◽  
Linear Time ◽  
Line Tension ◽  
Mooring System ◽  
Mooring Lines ◽  
Viscous Forces ◽  
And Performance ◽  
Inertia Forces

This paper analyses the numerical outcome of applying three different well-known mooring design approaches to a floating wave energy converter, moored by means of four catenary lines. The approaches include: a linearized frequency domain based on a quasistatic model of the mooring lines, a time domain approach coupled with an analytic catenary model of the mooring system, and a fully coupled non-linear time domain approach, considering lines’ drag and inertia forces. Simulations have been carried out based on a set of realistic combinations of lines pretension and linear mass, subject to extreme environmental conditions. Obtained results provide realistic cost and performance indicators, presenting a comparison in terms of total mooring mass and required footprint, as well as the design line tension and structure offset. It has been found that lines’ viscous forces influence significantly the performance of the structure with high pretensions, i.e., >1.2, while there is acceptable agreement between the modelling approaches with lower pretensions. Line tensions are significantly influenced by drag and inertia forces because of the occurrence of snap loads due to the heaving of the floater. However, the frequency domain approach provides an insight towards the optimal design of the mooring system for preliminary designs.

Investigation on the Use of Different Approaches to Mooring Analysis and Appropriate Safety Factors

2012 ◽  
Author(s):  
Sojan Vasudevan ◽  
Paul Westlake
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Line Tension ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Mooring Line ◽  
Mooring System ◽  
Safety Factors ◽  
Dynamic Time ◽  
The Time Domain

This paper presents the results of the analyses of a twelve line catenary mooring system using a quasi-static method in the frequency domain, and uncoupled and coupled dynamic methods in the time domain. The latter is found to produce significantly higher tensions. The reasons for these differences are investigated. The minimum line tension safety factors required by design codes do not distinguish between uncoupled and coupled dynamic analyses and some codes use the same factors even for quasi-static analyses. Consequently, the present mooring system passes the acceptance criteria based on quasistatic frequency domain and uncoupled dynamic time domain analyses but does not meet the same criteria when a coupled dynamic time domain analysis is employed. It is understood that because the coupled time domain analysis determines the vessel motions using all forces the accuracy of mooring line tension estimation will be improved over other methods. Hence the application of less conservative safety factors is proposed.

scholarly journals An Improved Algorithm for Indirect Time domain Analysis of Thin Wire Structures

2012 ◽  
Vol 8 (1) ◽  
pp. 12
Author(s):  
Siniša Antonijević
Keyword(s):  
Fourier Transform ◽  
Frequency Domain ◽  
Time Domain ◽  
Adaptive Sampling ◽  
Time Domain Analysis ◽  
Optimization Method ◽  
Computational Time ◽  
Frequency Domain Method ◽  
The Time Domain ◽  
Domain Transformation

The algorithm for an indirect time domain (TD) analysis using arbitrary frequency domain method is presented.In previous work, an optimized version of the indirect timedomain method based on adaptive sampling iterative algorithm used in combination with inverse Fast Fourier Transform (FFT) was presented. In this paper, the algorithm is further improved by circumventing the use of inverse FFT altogether, and using the simple formula based on the analytical inverse Fourier transform. The proposed approach is simpler and more convenient for use with the underlying iterative optimization method, as only a part of the frequency domain (FD) signal has to be transformed to the time domain during each iteration procedure, the frequency domain discretisation does not have to be uniform, and the time domain discretisation does not have to be performed. These advantages can also lead to the reduced computational time cost of the frequency to time domain transformation when compared with the standard inverse FFT.

scholarly journals Time and Frequency Domain Dynamic Analysis of Offshore Mooring

2021 ◽  
Vol 9 (7) ◽  
pp. 781
Author(s):  
Shi He ◽  
Aijun Wang
Keyword(s):  
Dynamic Analysis ◽  
Frequency Domain ◽  
Time Domain ◽  
Linearization Method ◽  
Euler Method ◽  
Single Component ◽  
Hydrodynamic Drag ◽  
Lumped Mass ◽  
Mooring Lines ◽  
The Time Domain

The numerical procedures for dynamic analysis of mooring lines in the time domain and frequency domain were developed in this work. The lumped mass method was used to model the mooring lines. In the time domain dynamic analysis, the modified Euler method was used to solve the motion equation of mooring lines. The dynamic analyses of mooring lines under horizontal, vertical, and combined harmonic excitations were carried out. The cases of single-component and multicomponent mooring lines under these excitations were studied, respectively. The case considering the seabed contact was also included. The program was validated by comparing with the results from commercial software, Orcaflex. For the frequency domain dynamic analysis, an improved frame invariant stochastic linearization method was applied to the nonlinear hydrodynamic drag term. The cases of single-component and multicomponent mooring lines were studied. The comparison of results shows that frequency domain results agree well with nonlinear time domain results.

scholarly journals Analysis of Three-Phase Wye-Delta Connected LLC

Energies ◽  
2021 ◽  
Vol 14 (12) ◽  
pp. 3606
Author(s):  
Jing-Yuan Lin ◽  
Chuan-Ting Chen ◽  
Kuan-Hung Chen ◽  
Yi-Feng Lin
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Operation Time ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Complete Analysis ◽  
Plane Analysis ◽  
Analysis Methods ◽  
Three Phase

Three-phase wye–delta LLC topology is suitable for voltage step down and high output current, and has been used in the industry for some time, e.g., for server power and EV charger. However, no comprehensive circuit analysis has been performed for three-phase wye–delta LLC. This paper provides complete analysis methods for three-phase wye–delta LLC. The analysis methods include circuit operation, time domain analysis, frequency domain analysis, and state–plane analysis. Circuit operation helps determine the circuit composition and operation sequence. Time domain analysis helps understand the detail operation, equivalent circuit model, and circuit equation. Frequency domain analysis helps obtain the curve of the transfer function and assists in circuit design. State–plane analysis is used for optimal trajectory control (OTC). These analyses not only can calculate the voltage/current stress, but can also help design three-phase wye-delta connected LLC and provide the OTC control reference. In addition, this paper uses PSIM simulation to verify the correctness of analysis. At the end, a 5-kW three-phase wye–delta LLC prototype is realized. The specification of the prototype is a DC input voltage of 380 V and output voltage/current of 48 V/105 A. The peak efficiency is 96.57%.

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.

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