Experimental Investigation on a Point Absorber Moored by Taut Mooring System and Mooring Fatigue Analysis

2020 ◽  
Author(s):  
Sheng Xu ◽  
C. Guedes Soares
Keyword(s):  
Fatigue Damage ◽  
Spectral Methods ◽  
Irregular Waves ◽  
Mooring System ◽  
Wire Ropes ◽  
Irregular Wave ◽  
Point Absorber ◽  
Motion Responses ◽  
Linear Spring ◽  
Rainflow Counting

Abstract In this paper, a taut wire mooring system has been designed consisting of three wire ropes symmetrically arranged. The power take-off system is modelled by a linear spring and a heave plate to simulate the linear damping of the power take-off. A series of regular wave tests in head seas are performed to study the dynamics of the system. Irregular wave tests were then conducted to simulate the system performance in the operational sea states, where the irregular waves are modelled by the Jonswap spectrum. The WEC motion responses and mooring tensions are studied. The short term mooring fatigue damage estimated by different spectral methods are compared to the rainflow counting method. The spectral method for estimating fatigue damage include the Dirlik formula, Jiao-Moan method and Tovo-Benasciutti approach. The accuracy of spectral methods for predicting mooring fatigue damage are discussed.

scholarly journals Geometry Optimization of Heaving Axisymmetric Point Absorbers under Parametrical Constraints in Irregular Waves

2021 ◽  
Vol 9 (10) ◽  
pp. 1136
Author(s):  
Jinming Wu
Keyword(s):  
Geometry Optimization ◽  
Irregular Waves ◽  
Small Radius ◽  
Mooring System ◽  
Point Absorber ◽  
Nested Loops ◽  
Absorbed Power ◽  
Point Absorbers ◽  
The Cost ◽  
Motion Constraint

The objective of this work is to identify the maximum absorbed power and optimal buoy geometry of a heaving axisymmetric point absorber for a given cost in different sea states. The cost of the wave energy converter is estimated as proportional to the displaced volume of the buoy, and the buoy geometry is described by the radius-to-draft ratio. A conservative wave-height-dependent motion constraint is introduced to prevent the buoy from jumping out of the free surface of waves. The constrained optimization problem is solved by a two-nested-loops method, within which a core fundamental optimization process employs the MATLAB function fmincon. Results show that the pretension of the mooring system should be as low as possible. Except for very small energy periods, the stiffness of both the power take-off and mooring system should also be as low as possible. A buoy with a small radius-to-draft ratio can absorb more power, but at the price of working in more energetic seas and oscillating at larger amplitudes. In addition, the method to choose the optimal buoy geometry at different sea states is provided.

Fatigue Life Re-Assessment of FSO Spread Mooring System

2017 ◽  
Vol 862 ◽  
pp. 271-277
Author(s):  
Muhammad Irfan
Keyword(s):  
Fatigue Life ◽  
Frequency Domain ◽  
Fatigue Damage ◽  
Summation Method ◽  
Mooring System ◽  
Time Frequency ◽  
Irregular Wave ◽  
Operational Life ◽  
Frequency Domain Approach ◽  
Simple Summation

Re-assessment fatigue life on spread moored system of Floating Storage and Offloading (FSO) vessel moored in irregular wave has been carried out based on hybrid time-frequency domain approach. Previous assessment work result obtained the fatigue life very conservative about thousands of year, whilst existing mooring system was designed for the operational life of 10 years. Different method from previous work was used in fatigue damage calculation approach for undertaking a fatigue mooring analysis in present work. Present works aim to show that the simple summation method used for previous assessment that cause the large variation in fatigue life obtained by comparing different methods of combining fatigue damage. Comparison between the result of fatigue life both previous assessment and present work (re-assessment) of fatigue life are made and discussed.

scholarly journals Study on the Coupled Dynamic Responses of a Submerged Floating Wind Turbine under Different Mooring Conditions

Energies ◽  
2019 ◽  
Vol 12 (3) ◽  
pp. 418 ◽  
Author(s):  
Conghuan Le ◽  
Yane Li ◽  
Hongyan Ding
Keyword(s):  
Wind Turbine ◽  
Natural Frequencies ◽  
Dynamic Responses ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Intermediate Water ◽  
Mooring System ◽  
Irregular Wave ◽  
Coupled Dynamic Analysis ◽  
Motion Responses

A submerged floating offshore wind turbine (SFOWT) is proposed for intermediate water depth (50–200 m). An aero-hydro-servo-elastic-mooring coupled dynamic analysis was carried out to investigate the coupled dynamic response of the SFOWT under different mooring conditions subjected to combined turbulent wind and irregular wave environments. The effects of different parameters, namely, the tether length, pretension and the tether failure, on the performance of SFOWT were investigated. It is found that the tether length has significant effects on the motion responses of the surge, heave, pitch and yaw but has little effects on the tower fore-aft displacements and the tether tensions. The increased pretension can result in the increase of the natural frequencies of surge, heave and yaw significantly. The influence of tether failure on the SFOWT performance was investigated by comparing the responses with those of the intact mooring system. The results show that the SFOWT with a broken tether still has a good performance in the operational condition.

Probabilistic Fatigue Assessment of a Mooring Line Based on Station-Keeping Trials in Ice

2021 ◽  
Author(s):  
Chana Sinsabvarodom ◽  
Bernt J. Leira ◽  
Wei Chai ◽  
Arvid Naess
Keyword(s):  
Fatigue Damage ◽  
Probabilistic Models ◽  
Full Scale ◽  
Mooring Line ◽  
Stress Range ◽  
Station Keeping ◽  
Probability Models ◽  
Line System ◽  
Fatigue Assessment ◽  
Rainflow Counting

Abstract The intention of this work is to perform a probabilistic fatigue assessment of a mooring line due to loads associated with the station-keeping of a ship in ice. In March 2017, the company Equinor (Statoil) conducted full-scale tests by means of station-keeping trials (SKT) in drifting ice in the Bay of Bothnia. The vessel Magne Viking was employed in order to represent a supply vessel equipped with a mooring line system, and the realtime loading during the full-scale measurement was recorded. The second vessel Tor Viking was serving as an ice breaker in order to maintain the physical ice management activities with different ice-breaking schemes, i.e. square updrift pattern, round circle pattern, circular updrift pattern and linear updrift pattern. The fatigue degradation corresponding to these different patterns were investigated. The peaks and valleys of the mooring tension are determined using the min peak prominence method. For the purpose of probabilistic fatigue assessment, the Rainflow-counting algorithm is applied to estimate the mooring stress range. Fatigue assessment based both on Rainflow counting and fitted probabilistic models were performed. For the latter, the stress range distributions from the observed data of mooring loads are fitted to various probability models in order to estimate the fatigue damage. It is found that the stress ranges represented by application of the Weibull distribution for the probabilistic fatigue approach provides results of the fatigue damage most similar to the Rainflow counting approach. Among the different scenarios of Ice management schemes, the circular updrift pattern provides the lowest magnitude of the fatigue degradation.

Irregular Wave and Current Loads on a Fish Farm System

2018 ◽  
Author(s):  
Arnt G. Fredriksen ◽  
Basile Bonnemaire ◽  
Øyvind Nilsen ◽  
Leiv Aspelund ◽  
Andreas Ommundsen
Keyword(s):  
Particle Velocity ◽  
Fish Farm ◽  
Fluid Particle ◽  
Irregular Waves ◽  
Engineering Practice ◽  
Irregular Wave ◽  
Environmental Force ◽  
The Mean ◽  
Particle Velocities ◽  
Farm System

Accurate calculation of the design mooring loads on an aquaculture fish farm mooring system is often a difficult task. The fish farm system has a large horizontal extension with variable environmental conditions across the entire structure. In addition, the drag loads on the fish nets are thought to be the governing environmental force. This means that the mean position of the fish farm is a function of the mean of the fluid particle velocity squared, where the fluid particle velocity must be taken as the sum of current and wave induced fluid particle velocities. Additional offsets will be slowly varying, where the response time will depend on the total mooring stiffness. The magnitudes depend on the height and length on wave groups in the irregular sea state. The paper presents simulations of the response of such a system to a set of combined irregular waves and current conditions. The response evolution in time is discussed as well as parameters affecting the maximum responses in the systems (displacements and loads). Finally, the resulting loads on the fish farm in irregular waves are compared to loads obtained in equivalent regular waves, as this is an often used engineering practice when analyzing the response and mooring loads of a fish farm.

Numerical Study on Hydrodynamic Responses of a Two-Body System in Side-by-Side Operation

2016 ◽  
Author(s):  
Shaowu Ou ◽  
Shixiao Fu ◽  
Wei Wei ◽  
Tao Peng ◽  
Xuefeng Wang
Keyword(s):  
Numerical Study ◽  
Low Frequency ◽  
Optimal Operation ◽  
Hydrodynamic Interactions ◽  
Irregular Waves ◽  
Mooring System ◽  
Time Varying ◽  
Body System ◽  
The Time Domain ◽  
Two Bodies

Typically, in some side-by-side offshore operations, the speed of vessels is very low or even 0 and the headings are manually maneuvered. In this paper, the hydrodynamic responses of a two-body system in such operations under irregular seas are investigated. The numerical model includes two identical PSVs (Platform Supply Vessel) as well as the fenders and connection lines between them. A horizontal mooring system constraining the low frequency motions is set on one of the ships to simulate maneuver system. Accounting for the hydrodynamic interactions between two bodies, 3D potential theory is applied for the analysis of their hydrodynamic coefficients. With wind and current effects included, these coefficients are further applied in the time domain simulations in irregular waves. The relevant coefficients are estimated by experiential formulas. Time-varying loads on fenders and connection lines are analyzed. Meanwhile, the relative motions as well as the effects of the hydrodynamic interactions between ships are further discussed, and finally an optimal operation scheme in which operation can be safely performed is summarized.

Alternative Methodologies for Subsea Flexible Strength Analysis

2018 ◽  
Author(s):  
Anskey A. Miranda ◽  
Fred P. Turner ◽  
Nigel Barltrop
Keyword(s):  
Real World ◽  
Wave Train ◽  
Irregular Waves ◽  
Wave Crest ◽  
Strength Analysis ◽  
New Wave ◽  
Regular Wave ◽  
Wave Analysis ◽  
Sea State ◽  
Irregular Wave

This paper presents a study of the analysis methodologies used to predict the most likely response of flexibles in a subsea environment, with the aim of determining an efficient and reliable prediction methodology. The most accurate method involves simulating multiple wave realisations of a real world sea state, i.e. irregular waves, and post-processing the results to determine the most probable maximum (MPM). Due to the computationally intensive nature of this approach, however, regular wave analysis is typically used to determine flexible response. This approach considers the maximum wave within a design storm at a desired period; the choice of periods may leave room for uncertainty in the conservatism of the approach. With proper screening, regular wave analysis can be a valid yet overly conservative approach resulting in over design and additional cost. However, if screened incorrectly, there is a possibility that the choice of periods could give results that are under conservative. In addition to regular wave analysis, the paper presents two alternative methodologies to determine the most likely response, with the focus on reducing the computational resources required. The first alternative is an ‘Irregular Wave Screen’ approach in which the wave train is screened at areas of interest for waves within a user defined threshold of the maximum wave height, in addition to other user defined parameters. Only waves within these parameters are simulated to determine responses. The second alternative is the ‘New Wave’ approach, which models the most probable wave elevation around the maximum wave crest. The calculated new wave is then placed at the desired location to determine responses. The responses of the Regular, Irregular Wave Screen and New Wave methodologies are compared with the Irregular MPM approach to determine their feasibility to predict the response of flexibles in a real world irregular sea state with lower computational requirements.

scholarly journals STATISTICAL PROPERTIES OF THE MAXIMUM RUN OF IRREGULAR SEA WAVES

1988 ◽  
Vol 1 (21) ◽  
pp. 48 ◽  
Author(s):  
Akira Kimura
Keyword(s):  
Probability Distribution ◽  
Wave Height ◽  
Probability Distributions ◽  
Confidence Regions ◽  
Statistical Properties ◽  
Irregular Waves ◽  
Sea Waves ◽  
Sea State ◽  
Irregular Wave ◽  
Distribution Of The Maximum

The probability distribution of the maximum run of irregular wave height is introduced theoretically. Probability distributions for the 2nd maximum, 3rd maximum and further maximum runs are also introduced. Their statistical properties, including the means and their confidence regions, are applied to the verification of experiments with irregular waves in the realization of a "severe sea state" in the test.

Random Loads Fatigue: The Use of Spectral Methods Within Multibody Simulation

2005 ◽  
Author(s):  
Claudio Braccesi ◽  
Filippo Cianetti ◽  
Luca Landi
Keyword(s):  
Finite Element ◽  
Stress State ◽  
Dynamic Simulation ◽  
Fatigue Damage ◽  
Spectral Methods ◽  
Mechanical Systems ◽  
Frequency Domain Method ◽  
Stress And Strain ◽  
Damage Evaluation ◽  
Multibody Dynamic

The evaluation of the fatigue damage performed by using the Power Spectral Density function (PSD) of stress and strain state is proving to be extremely accurate for a family of random processes characterized by the property of being stationary. The present work’s original contribution is the definition of a methodology which extracts stress and strain PSD matrices from components modelled using a modal approach (starting from a finite element modelling and analysis) within mechanical systems modelled using multibody dynamic simulation and subject to a generic random load (i.e. multiple-input, with partially correlated inputs). This capability extends the actual stress evaluation scenario (principally characterised by the use of finite element analysis approach) to the multibody dynamic simulation environment, more powerful and useful to simulate complex mechanical systems (i.e. railway, automotive, aircraft and aerospace systems). As regards the fatigue damage evaluation, a synthesis approach to evaluate an equivalent stress state expressed in terms of the PSD function of Preumont’s “equivalent von Mises stress (EVMS)”, starting from the complete stress state representation expressed in terms of PSD stress matrix and easily usable in the consolidated spectral methods, is proposed. This approach allows and has allowed the use of the above methods such as the Dirlik formula as a damage evaluation method. An additional result is the conception and implementation of a frequency domain method for the component’s most probable state of stress, allowing quickly identification of the most stressed and damageble locations. The described methodologies were developed and embedded into commercial simulation codes and verified by using as a test case a simple reference multibody model with a simple flexible component.

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