Long-Term Analysis Applied to Mooring Systems Design

2020 ◽  
Author(s):  
Pedro Seabra ◽  
Luis Volnei Sudati Sagrilo ◽  
Paulo Esperança
Keyword(s):  
Time Domain ◽  
Environmental Conditions ◽  
Systems Design ◽  
Environmental Data ◽  
Coupled Analysis ◽  
Offshore Structure ◽  
Short Term ◽  
Extreme Response ◽  
Term Analysis

Abstract Nowadays, the most used methodology to predict line tensions is the short-term coupled analysis, where the mooring system responses are obtained by a time-domain analysis for only some specific design combinations of extreme environmental conditions. This mooring analysis demands certain considerations and it is not the best way to obtain the offshore structure responses. The advances in both quantity and quality of collected environmental data and the increase of the computers processing power has enabled to consider the approach of more accurate long-term methodologies for mooring systems design. This paper proposes a numerical/computational procedure to obtain the extreme loads (ULS) acting on offshore platforms’ mooring lines. The work is based on the methodology of long-term analysis, employing a 10-yr long short-term environmental dataset of 3-h sea-states, where each short-term environmental condition is composed of the simultaneously observed environmental parameters of wave (sea and swell), wind and current. The methodology is applied to the analysis of three different mooring systems: a) spread-moored FPSO, b) Semi-Submersible platform and c) turret-moored FPSO. The Bootstrap approach is employed in order to take into account the statistical uncertainty associated to the estimated long-term most probable extreme response due to the limited number of short-term environmental conditions. The work was carried out using Dynasim software [1] to generate the time domain tension time series, which were later post-processed by using computational codes developed with Python software. Longer short-term numerical simulations lengths than the short-term period (3-h) have been investigated in order to understand the influence of this parameter on the final extreme long-term top tensions.

Long-term extreme response analysis for semi-submersible platform mooring systems

2020 ◽  
pp. 147509022097651
Author(s):  
Yuliang Zhao ◽  
Sheng Dong
Keyword(s):  
Environmental Data ◽  
Distribution Model ◽  
Response Analysis ◽  
Accurate Assessment ◽  
Response Distribution ◽  
Short Term ◽  
Floating Structure ◽  
Extreme Response ◽  
Term Analysis

The accurate assessment of long-term extreme responses of floating-structure mooring system designs is important because of small failure probabilities caused by long-term and complex ocean conditions. The most accurate assessment would involve considering all conceivable sea states in which each sea state is regarded as a stochastic process and performing nonlinear time-domain numerical simulations of mooring systems to estimate the extreme response from a long-term analysis. This procedure would be computationally intensive because of the numerous short-term sea states involved. Here, a more feasible approach to evaluate the long-term extreme response is presented through immediate integration combined with Monte Carlo simulations. A parameter fitting procedure of the short-term extreme response distribution under irregular wave conditions is employed to solve the long-term response integration. Case studies were conducted on a semi-submersible platform using environmental data measurements of the Gulf of Mexico and a joint distribution model of the environmental parameters was considered. This approach was observed to be effective and the results were compared with those of traditional methodologies (univariate extreme value design and environmental contour methods). The differences were reflected using a reliability analysis of mooring lines, which indicated that the design standards must be stricter when using long-term analysis.

Statistical Methods for Prediction of Characteristic Loads for Free Fall Lifeboats Based on CFD Screening Results

2013 ◽  
Author(s):  
Vidar Tregde ◽  
Arne Nestegård
Keyword(s):  
Environmental Conditions ◽  
Time History ◽  
Free Fall ◽  
Response Analysis ◽  
Short Term ◽  
Sea State ◽  
Screening Process ◽  
History Of ◽  
Term Analysis

Computational Fluid Dynamics (CFD) has been used in a screening process to calculate characteristic loads for a Free Fall Lifeboat (FFLB) during impact and submergence. The link between various input, e.g. environmental conditions and host specific data, resulting structural loads and motion of the lifeboat is explored. The screening can be used together with host specific environmental conditions to find structural design loads and motion restrictions. Response based analysis have been developed for both short term and long term predictions. For the short term predictions a sea state given by (Hs, Tp) on the 100-year contour line is identified and a three hour irregular sea state is simulated. This time history of surface elevations is used for a large number of random lifeboat drops. From these random drops a distribution of wave height and corresponding wave steepness is derived which is then input to an interpolation in the database of CFD screening results. The resulting responses are fitted to a Weibull distribution and the 90% quantile in this short term load distribution is determined. The long term response analysis is further developed from the short term analysis. The short term distributions for each (Hs, Tp) are combined with the probability of occurrence of the sea state, and long term distributions are derived for the responses similar to the short term analysis. The screening results are used to identify critical load cases which are further investigated.

Global Performance and Mooring Analysis Using Long Term Analysis Methodology

2016 ◽  
Author(s):  
Mubing Xu ◽  
Anil Sablok ◽  
Oddgeir Dalane
Keyword(s):  
High Speed ◽  
Line Tension ◽  
Mooring Line ◽  
Return Periods ◽  
Short Term ◽  
Analysis Methodology ◽  
Extreme Response ◽  
Global Performance ◽  
Term Analysis

The long term analysis is performed to predict the hull global motion and mooring strength of a Spar platform and the results from long term analysis are compared with the predictions from the short term analysis. The long term motions are also used to investigate the long term riser strength response in Ref. [5]. The results includes the lateral offset, heel angle, heave motion and the mooring line tension at the fairleads. In the short term analysis, the environment events with various return periods and various realizations are considered for each environment load. The Gumbel fitting is used to predict the extreme response. In the long term analysis methodology, the prediction of the global performance and the mooring tension are based on 56-year hindcast wind and wave data with 3-hour intervals. Weibull fitting is used to predict the extreme response for various return periods. The comparison between the long term and the short term predictions indicate that the short term predictions are generally conservative compared to the more accurate, but computationally expensive long term analysis method. The long term methodology is not widely adopted currently due to the computation inefficiency. However, it is expected that this long term methodology could provide a better option in the future with the consideration of its accuracy and the application of high speed computer.

Long-Term Distribution of the Extreme Values of Offshore Structural Response by Finite-Memory Nonlinear System Modelling

2011 ◽  
Author(s):  
N. I. Mohd Zaki ◽  
G. Najafian
Keyword(s):  
Nonlinear System ◽  
Probability Distribution ◽  
Extreme Values ◽  
System Modelling ◽  
Wave Loading ◽  
Offshore Structure ◽  
Short Term ◽  
Extreme Response ◽  
Finite Memory

Offshore structures are exposed to random wave loading in the ocean environment and hence the long-term probability distribution of the extreme values of their response to wave loading is of great value in the design of these structures. Due to nonlinearity of the drag component of Morison wave loading and also due to intermittency of wave loading on members in the splash zone, the response is often non-Gaussian; therefore, simple techniques for derivation of the extreme response probability distributions are not available. However, it has recently been shown that the short-term response of an offshore structure exposed to Morison wave loading can be approximated by the response of an equivalent finite-memory nonlinear system (FMNS). In this paper, the approximate FMNS models are used to determine both the short-term and the long-term probability distribution of the response extreme values with great efficiency.

On the Long-Term Reliability Analysis of a Point Absorber Wave Energy Converter

2017 ◽  
Author(s):  
Jarred Canning ◽  
Phong Nguyen ◽  
Lance Manuel ◽  
Ryan G. Coe
Keyword(s):  
Wave Energy ◽  
Reference Model ◽  
Wave Energy Converter ◽  
Environmental Data ◽  
Energy Converter ◽  
Short Term ◽  
Point Absorber ◽  
Extreme Response ◽  
Term Response

Of interest in this study is the long-term response and performance of a two-body wave point absorber (“Reference Model 3”), which serves as a wave energy converter (WEC). In a previous study, the short-term uncertainty in this device’s response was studied for an extreme sea state. We now focus on the assessment of the long-term response of the device where we consider all possible sea states at a site of interest. We demonstrate how simulation tools may be used to evaluate the long-term response and consider key performance parameters of the WEC device, which are the heave and surge forces on the power take-off system and the power take-off extension. We employ environmental data at a designated deployment site in Northern California. Metocean information is generated using approximately 15 years of data from this site (National Data Buoy Center site no. 46022). For various sea states, a selected significant wave height and peak period are chosen to describe representative conditions. Then, using a public-domain simulation tool (Wave Energy Converter Simulator or WEC-Sim), we generate various short-term time-domain response measure for these sea states. Distribution fits to extreme response statistics are generated, for each bin that represents a cluster of sea states, using the open-source toolbox, WDRT (WEC Design Response Toolbox). Long-term distributions for each response variable of interest are estimated by weighting short-term distributions by the likelihood of the sea states; from these distributions, the 50-year response can be derived. The 50-year response is also estimated using an approximate but more efficient inverse reliability approach. Comparisons are made between the two approaches.

LOCA analysis of BWR-4/Mark-I nuclear power plant with TRACE

Kerntechnik ◽  
2021 ◽  
Vol 86 (2) ◽  
pp. 128-142
Author(s):  
J.-J. Huang ◽  
S.-W. Chen ◽  
J.-R. Wang ◽  
C. Shih ◽  
H.-T. Lin
Keyword(s):  
Nuclear Power ◽  
Coupled Model ◽  
Short Term ◽  
Containment System ◽  
Loss Of Coolant Accident ◽  
Main Steam Line ◽  
Main Steam ◽  
Temperature Responses ◽  
Term Analysis

Abstract This study established an RCS-Containment coupled model that integrates the reactor coolant system (RCS) and the containment system by using the TRACE code. The coupled model was used in both short-term and long-term loss of coolant accident (LOCA) analyses. Besides, the RELAP5/CONTAN model that only contains the containment system was also developed for comparison. For short-term analysis, three kinds of LOCA scenarios were investigated: the recirculation line break (RCLB), the main steam line break (MSLB), and the feedwater line break (FWLB). For long-term analysis, the dry-well and suppression pool temperature responses of the RCLB were studied. The analysis results of RELAP5/CONTAN and TRACE models are benchmarked with those of FSAR and RELAP5/GOTHIC models, and it appears that the results of the above four models are consistent in general trends.

Strength Performance of Steel Catenary Riser Using Short Term and Long Term Analysis Methodologies

2016 ◽  
Author(s):  
Feng Wang ◽  
Roger Burke ◽  
Anil Sablok ◽  
Kristoffer H. Aronsen ◽  
Oddgeir Dalane
Keyword(s):  
Bending Moment ◽  
Current Profile ◽  
Short Term ◽  
Sea State ◽  
Strength Performance ◽  
Steel Catenary Riser ◽  
Analysis Methodology ◽  
Riser Design ◽  
Term Analysis

Strength performance of a steel catenary riser tied back to a Spar is presented based on long term and short term analysis methodologies. The focus of the study is on response in the riser touch down zone, which is found to be the critical region based on short term analysis results. Short term riser response in design storms is computed based on multiple realizations of computed vessel motions with various return periods. Long term riser response is based on vessel motions for a set of 45,000 sea states, each lasting three hours. The metocean criteria for each sea state is computed based on fifty six years of hindcast wind and wave data. A randomly selected current profile is used in the long term riser analysis for each sea state. Weibull fitting is used to compute the extreme riser response from the response of the 45,000 sea states. Long term analysis results in the touch down zone, including maximum bending moment, minimum effective tension, and maximum utilization using DNV-OS-F201, are compared against those from the short term analysis. The comparison indicates that the short term analysis methodology normally followed in riser design is conservative compared to the more accurate, but computationally more expensive, long term analysis methods. The study also investigates the important role that current plays in the strength performance of the riser in the touch down zone.

Long-Term Analysis Applied to Mooring Systems Design

2021 ◽  
Author(s):  
Pedro Seabra ◽  
Luis Sagrilo ◽  
Paulo De Tarso T. Esperan\xe7a
Keyword(s):  
Systems Design ◽  
Term Analysis

Random Waves and Capsize Probability Based on Large Amplitude Motion Analysis

2002 ◽  
Author(s):  
Jan O. de Kat ◽  
Dirk-Jan Pinkster ◽  
Kevin A. McTaggart
Keyword(s):  
Time Domain ◽  
Random Waves ◽  
Extreme Wave ◽  
Small Probability ◽  
Safe Operation ◽  
Short Term ◽  
Physical Behavior ◽  
Wind Climate ◽  
Large Amplitude Motion

The objective of this paper is to apply a methodology aimed at the probabilistic capsize assessment of two naval ships: a frigate and a corvette. Use is made of combined knowledge of the wave and wind climate a ship will be exposed to during its lifetime and of the physical behavior of that ship in the various sea states it is likely to encounter. This includes the behavior in extreme wave conditions that have a small probability of occurrence, but which may be critical to the safe operation of a ship. Time domain simulations provide the basis for deriving short-term and long-term statistics for extreme roll angles. The numerical model is capable of predicting the 6 DOF behavior of a steered vessel in wind and waves, including conditions that may lead to broaching and capsizing.

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