Impact of Bathymetry on the Mooring Design of an Offshore Floating Unit

2016 ◽  
Author(s):  
Vivek Jaiswal ◽  
Srinivas Vishnubhotla ◽  
Sean Cole ◽  
Robert B. Gordon ◽  
Partha Sharma
Keyword(s):  
Time Domain ◽  
Previous Method ◽  
Mooring Line ◽  
Mooring System ◽  
Analysis Program ◽  
Design Assessment ◽  
True Representation ◽  
The Time Domain ◽  
Constant Slope ◽  
The Impact

Moored offshore floating units may operate in regions where the bathymetry changes significantly over the mooring spread. Traditional mooring analysis methods make the assumption that the seabed slope is constant along the azimuth direction of each mooring line. This assumption, while reasonable for a seabed with nearly constant slope, can lead to significant errors with respect to the line tensions and vessel offsets in the mooring design assessment when the seabed slope is variable. This paper demonstrates the impact of bathymetry changes on the mooring design with the help of numerical analysis examples. The floating vessel considered is a semi-submersible moored by an eight point all chain catenary mooring system. Two methods of analysis are compared. In the first method, a true representation of the seabed surface that accounts for all variations in the bathymetry is used. In the second method, the anchor depths and the seabed slopes at the anchor locations estimated in the previous method are used, however, with the assumption of constant seabed slopes along the line azimuth directions. The dynamic analysis program Orcaflex is used for performing the numerical analyses in the time domain for both the methods. Differences in the performance of the mooring system are demonstrated by comparing the static and the dynamic line tensions as well as the vessel offsets in different environmental conditions. The paper also discusses how maximum offsets and line tensions are estimated.

Time-Domain Coupled Dynamic Analysis of Mooring Systems in Extreme Sea Condition

2018 ◽  
Author(s):  
Xuliang Han ◽  
ShiSheng Wang ◽  
Bin Xie ◽  
Wenhui Xie ◽  
Weiwei Zhou
Keyword(s):  
Dynamic Analysis ◽  
Body Surface ◽  
Time Domain ◽  
Mooring Line ◽  
Mooring System ◽  
Dynamic Coupling ◽  
Spar Platform ◽  
Coupled Dynamic Analysis ◽  
Motion Responses ◽  
The Time Domain

In order to predict the coupled motion and external wave load for the design of deepwater floating structure system, based on the three-dimensional time-domain potential flow theory, this paper present the indirect time-domain dynamic coupling method and the body nonlinear dynamic coupling method. The perturbation expansion theory is adopted to evaluate hydrodynamic on the fixed mean wetted body surface for the former method. The transient free surface Green function has been extended and applied to calculate the nonlinear hydrodynamic on the instantaneous wetted exact body surface for the latter method. The finite element model is employed to solve dynamic response of mooring line. Then asynchronous coupled method is adopted to achieve the coupled dynamic analysis of platform and mooring lines. The time-domain motion responses and spectrum analysis of Spar platform are verified and compared with the traditional indirect time-domain coupling dynamic method when the mooring system is completed. Also the time-domain motion responses and statistical characteristic of Spar platform are investigated with one mooring line broken in extreme sea condition. Some conclusions are obtained, that is, dynamic coupling effects are significant and transient position hydrodynamic calculation of platform has a great influence on the low frequency motion. The results also show that the influence on the global performance of mooring system is different when the broken line is in different place. A remarkable influence occurs when the broken mooring line is in the head-wave direction.

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.

Optimized Mooring Line Simulation Using a Hybrid Method Time Domain Scheme

2014 ◽  
Author(s):  
Niels Hørbye Christiansen ◽  
Per Erlend Torbergsen Voie ◽  
Jan Høgsberg ◽  
Nils Sødahl
Keyword(s):  
Hybrid Method ◽  
Time Domain ◽  
Computation Time ◽  
Mooring Line ◽  
Mooring Lines ◽  
Fem Model ◽  
Input Variables ◽  
The Time Domain ◽  
The Cost ◽  
Short Time

Dynamic analyses of slender marine structures are computationally expensive. Recently it has been shown how a hybrid method which combines FEM models and artificial neural networks (ANN) can be used to reduce the computation time spend on the time domain simulations associated with fatigue analysis of mooring lines by two orders of magnitude. The present study shows how an ANN trained to perform nonlinear dynamic response simulation can be optimized using a method known as optimal brain damage (OBD) and thereby be used to rank the importance of all analysis input. Both the training and the optimization of the ANN are based on one short time domain simulation sequence generated by a FEM model of the structure. This means that it is possible to evaluate the importance of input parameters based on this single simulation only. The method is tested on a numerical model of mooring lines on a floating off-shore installation. It is shown that it is possible to estimate the cost of ignoring one or more input variables in an analysis.

Selection of random vibration theory procedures for the NGA-East project and ground-motion modeling

2021 ◽  
Vol 37 (1_suppl) ◽  
pp. 1420-1439
Author(s):  
Albert R Kottke ◽  
Norman A Abrahamson ◽  
David M Boore ◽  
Yousef Bozorgnia ◽  
Christine A Goulet ◽  
...  
Keyword(s):  
Ground Motion ◽  
Time Domain ◽  
Random Vibration ◽  
Amplitude Spectrum ◽  
Motion Modeling ◽  
Peak Response ◽  
Vibration Theory ◽  
Random Vibration Theory ◽  
The Time Domain ◽  
The Impact

Traditional ground-motion models (GMMs) are used to compute pseudo-spectral acceleration (PSA) from future earthquakes and are generally developed by regression of PSA using a physics-based functional form. PSA is a relatively simple metric that correlates well with the response of several engineering systems and is a metric commonly used in engineering evaluations; however, characteristics of the PSA calculation make application of scaling factors dependent on the frequency content of the input motion, complicating the development and adaptability of GMMs. By comparison, Fourier amplitude spectrum (FAS) represents ground-motion amplitudes that are completely independent from the amplitudes at other frequencies, making them an attractive alternative for GMM development. Random vibration theory (RVT) predicts the peak response of motion in the time domain based on the FAS and a duration, and thus can be used to relate FAS to PSA. Using RVT to compute the expected peak response in the time domain for given FAS therefore presents a significant advantage that is gaining traction in the GMM field. This article provides recommended RVT procedures relevant to GMM development, which were developed for the Next Generation Attenuation (NGA)-East project. In addition, an orientation-independent FAS metric—called the effective amplitude spectrum (EAS)—is developed for use in conjunction with RVT to preserve the mean power of the corresponding two horizontal components considered in traditional PSA-based modeling (i.e., RotD50). The EAS uses a standardized smoothing approach to provide a practical representation of the FAS for ground-motion modeling, while minimizing the impact on the four RVT properties ( zeroth moment, [Formula: see text]; bandwidth parameter, [Formula: see text]; frequency of zero crossings, [Formula: see text]; and frequency of extrema, [Formula: see text]). Although the recommendations were originally developed for NGA-East, they and the methodology they are based on can be adapted to become portable to other GMM and engineering problems requiring the computation of PSA from FAS.

scholarly journals Calibration of ultrasonic hardware for enhanced total focusing method imaging

2020 ◽  
Vol 62 (7) ◽  
pp. 408-415
Author(s):  
M Ingram ◽  
A Gachagan ◽  
A Nordon ◽  
A J Mulholland ◽  
M Hegarty
Keyword(s):  
Time Domain ◽  
Pressure Profile ◽  
Side Lobe ◽  
Destructive Testing ◽  
Experimental Variation ◽  
Focal Position ◽  
Measured Time ◽  
The Time Domain ◽  
Ray Path ◽  
The Impact

Experimental variation from ultrasonic hardware is one source of uncertainty in measured ultrasonic data. This uncertainty leads to a reduction in the accuracy of images generated from these data. In this paper, a quick, easy-to-use and robust methodology is proposed to reduce this uncertainty in images generated using the total focusing method (TFM). Using a 128-element linear phased array, multiple full matrix capture (FMC) datasets of a planar reflection are used to characterise the experimental variation associated with each element index in the aperture. Following this, a methodology to decouple the time-domain error associated with transmission and reception at each element index is presented. These time-domain errors are then introduced into a simulated array model used to generate the two-way pressure profile from the array. The side-lobe-to-main-lobe energy ratio (SMER) and beam offset are used to quantify the impact of these measured time-domain errors on the pressure profile. This analysis shows that the SMER is raised by more than 6 dB and the beam is offset by more than 1 mm from its programmed focal position. This calibration methodology is then demonstrated using a steel non-destructive testing (NDT) sample with three side-drilled holes (SDHs). The time delay errors from transmission and reception are introduced into the time-of-flight (TOF) calculation for each ray path in the TFM. This results in an enhancement in the accuracy of defect localisation in the TFM image.

Coupled Time-Domain Hydro-Elastic Simulation for Submerged Floating Tunnel Under Wave Excitations

2021 ◽  
Author(s):  
Chungkuk Jin ◽  
Sung-Jae Kim ◽  
MooHyun Kim
Keyword(s):  
Time Domain ◽  
Domain Model ◽  
Mooring Line ◽  
Wave Forces ◽  
Rod Theory ◽  
Discrete Module ◽  
Simulation Based ◽  
Submerged Floating Tunnel ◽  
The Time Domain ◽  
Elastic Simulation

Abstract We develop a fully-coupled time-domain hydro-elasticity model for the Submerged Floating Tunnel (SFT) based on the Discrete-Module-Beam (DMB) method. Frequency-domain simulation based on 3D potential theory results in multibody’s hydrodynamic coefficients and excitation forces for tunnel sections. Subsequently, we build the time-domain model with the multibody Cummins equation and external stiffness matrix from the Euler-Bernoulli and Saint-Venant torsion theories. We establish the mooring line model with rod theory and couple components with translational springs at their respective connection locations. We then compare the dynamic motions, wave forces, and mooring tensions between the present and Morison-equation-based elastic models under regular wave excitations at different submergence depths. The present model is especially important for the shallowly submerged tunnel in which the Morison model shows exaggerated motions, especially at high-frequency range.

Research on the Static and Dynamics Characteristics of Soft Yoke Mooring System Based on Multi-Rigid Body Interaction

2020 ◽  
Author(s):  
Hongwei Wang ◽  
Zizhao Zhang ◽  
Gang Ma ◽  
Rongtai Ma ◽  
Jie Yang
Keyword(s):  
Time Domain ◽  
Rigid Bodies ◽  
Coupled Analysis ◽  
Mooring System ◽  
Accurate Analysis ◽  
Restoring Force ◽  
Stiffness Characteristics ◽  
The Time Domain ◽  
Multi Body ◽  
Good Agreement

Abstract Select the common mooring system-soft yoke mooring system as the research object. The soft yoke mooring system is regarded as a structure composed of multiple rigid bodies, and the theoretical analysis of multi-body dynamics is used to discuss the interaction of multi-rigid bodies. The classical HYSY113 FPSO is selected as an example, for the soft yoke mooring system, the stiffness characteristics and static restoring force curved compared with those of software OrcaFlex, and they are in good agreement, which verify the reliability of the formula derived, and it is a prerequisite for the accurate simulations in further steps. Coupled analysis to the whole system in time domain is also carried out both in OrcaFlex and AQWA, and the representative response of the FPSO under different environmental conditions is compared, the results are consistent well with each other. It is a good reference for the future study in this field. Good static characteristics are a prerequisite for accurate analysis of time-domain motion. By comparing the results in the time domain, it is found that under the same working conditions, the analysis results calculated by different commercial software (AQWA and OrcaFlex) may be different. We need to perform design analysis based on the characteristics of the software.

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.

Analysis of Vehicle Longitudinal Dynamics for Longitudinal Ride Comfort

2006 ◽  
Author(s):  
Shiang-Lung Koo ◽  
Han-Shue Tan ◽  
Masayoshi Tomizuka
Keyword(s):  
Time Domain ◽  
Ride Comfort ◽  
Vehicle Control ◽  
Switching Behavior ◽  
Mode Switching ◽  
Longitudinal Motion ◽  
Analytical Prediction ◽  
Longitudinal Model ◽  
The Time Domain ◽  
The Impact

Longitudinal ride comfort is one of the most crucial features to most advanced vehicle control systems. Literature review shows that the ride comfort analysis in vehicle longitudinal motion can be divided into two categories: time domain and frequency domain. Most vehicle longitudinal control designs incorporate jerk and acceleration constraints from the time-domain comfort criterion. However, the vehicle longitudinal characteristics in the frequency range important to passenger ride comfort are rarely discussed in the vehicle control literature. This paper proposes an improved vehicle longitudinal model that captures tire and suspension modes accurately and investigates the impact of these often-ignored vehicle resonant modes to ride comfort. This study shows that the "tire-mode switching behavior" affects longitudinal ride comfort of a stopping vehicle rather than the suspension. A passenger car was tested as an example, and the collected data verified the analytical prediction from the improved vehicle longitudinal model.

scholarly journals Spillover effects from news to travel and leisure stocks during the COVID-19 pandemic: Evidence from the time and frequency domains

2021 ◽  
pp. 135481662110584
Author(s):  
Ying Wang ◽  
Hongwei Zhang ◽  
Wang Gao ◽  
Cai Yang
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Spillover Effects ◽  
Short Term ◽  
Time Frequency ◽  
Frequency Domains ◽  
Media Hype ◽  
The Time Domain ◽  
The Impact

The impact of the COVID-19 pandemic on tourism has received general attention in the literature, while the role of news during the pandemic has been ignored. Using a time-frequency connectedness approach, this paper focuses on the spillover effects of COVID-19-related news on the return and volatility of four regional travel and leisure (T&L) stocks. The results in the time domain reveal significant spillovers from news to T&L stocks. Specifically, in the return system, T&L stocks are mainly affected by media hype, while in the volatility system, they are mainly affected by panic sentiment. This paper also finds two risk contagion paths. The contagion index and Global T&L stock are the sources of these paths. The results in the frequency domain indicate that the shocks in the T&L industry are mainly driven by short-term fluctuations. The spillovers from news to T&L stocks and among these T&L stocks are stronger within 1 month.

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