Wave Sector Dependent Contour Lines

2007 ◽  
Author(s):  
Gro Sagli Baarholm ◽  
Sverre Haver ◽  
Carl M. Larsen
Keyword(s):  
Linear Response ◽  
Contour Line ◽  
Exceedance Probability ◽  
Wave Direction ◽  
Short Term ◽  
Sea State ◽  
Extreme Loads ◽  
Contour Lines ◽  
Term Analysis

This paper is concerned with estimating the response value corresponding to given annual exceedance probability. In principle, this requires that a full long term analysis is executed. For a linear response this can easily be done. For a non-linear response quantity however, where time domain simulations are required in order to obtain the short term stochastic structure a full long term analysis will be time consuming. An approximate method to determine the long-term extremes by considering only a few short term sea states is outlined. All sea states corresponding to a certain probability of occurrence and are given by a contour line of Hs, Tp for each wave direction. The advantage of the method is that a proper estimate of the long term extreme can be obtained by considering the most unfavourable sea state along the contour line. This will make possible practical estimation of the extreme loads the structure is exposed to. The purpose of the present paper is to illustrate how to apply directional contour lines in order to obtain a characteristic design value according to requirements regarding the marginal exceedance probability.

Application of Contour Line Method to Estimate Extreme Ship Hull Loads Considering Operational Restrictions

2001 ◽  
Vol 45 (03) ◽  
pp. 228-240
Author(s):  
Gro Sagli Baarholm ◽  
Torgeir Moan
Keyword(s):  
Linear Response ◽  
Nonlinear Response ◽  
Contour Line ◽  
Time Domain Simulation ◽  
Short Term ◽  
Sea State ◽  
Peak Period ◽  
Extreme Loads ◽  
Long Time

This paper is concerned with estimating the response value corresponding to a return period, say 100 years. In principle, this response needs to be obtained by combining the response in all the sea states. The response for a given sea state, specified by the significant wave height, Hs, and the peak period, Tp, can be obtained in the frequency domain for the linear response. Time domain simulation is required to obtain the nonlinear response, and long time series are required to limit the statistical uncertainty in the simulations. Especially in the latter case, it is crucial to introduce ways to improve the efficiency in the calculation. A method to determine the long-term extremes by considering only a few short-term sea states is outlined. The sea states have a certain probability of occurrence and are identified by a contour line in the (Hs TJ-plane. This will make possible practical estimation of the extreme loads the vessels are exposed to.

Long Term Analysis for Estimation of Wave Slamming Pressures for Spar Design

2015 ◽  
Author(s):  
Ho-Joon Lim ◽  
Gunnar Lian ◽  
Sverre Haver ◽  
Oddgeir Dalane ◽  
Bonjun Koo ◽  
...  
Keyword(s):  
Model Tests ◽  
Design Parameters ◽  
Exceedance Probability ◽  
Extreme Wave ◽  
Short Term ◽  
Sea State ◽  
Norwegian Sea ◽  
Wave Slamming ◽  
Term Analysis

A long term analysis was performed to determine extreme wave slamming loads on the Aasta Hansteen Spar, the first production and storage Spar to be installed in the Norwegian Sea. The Spar will experience high slamming pressures on the hull due to harsh environments in the field. Extensive model tests were performed to measure the wave slamming pressure which is one of challenging design parameters. The slamming loads were measured with a 3×3 array of force transducer panels attached to the Spar hull. The extreme slamming loads were estimated from 3-hour simulations of the 100-yr and 10000-yr wave environments at the Aasta Hansteen field in the Norwegian Sea. The wave simulations included fourteen sea states, and each sea state was represented by as many as 20 realizations. Based on model test data, short term analysis of 3-hour extreme pressure at each tested sea state was performed using the Gumbel distribution. Due to high variability of 3-hour maximum pressures, a long term analysis was required to investigate the proper percentile level to be used in the design. The paper presents a long term statistical methodology for extreme wave slamming loads that is used to calculate long term slamming pressures corresponding to a specified annual exceedance probability of q (e.g., q = 10−2 and q = 10−4). The paper also derives the appropriate non-exceedance probability for a short term wave environment that reproduces the long term pressures of a specified annual exceedance probability, q. Various sensitivity analyses (e.g., on the two Gumbel parameters, number of realizations, etc.) were performed to validate the short term target percentiles and associated extreme pressures derived from this approach. Details of the model tests and methodology to define the design pressure profile above mean water level (MWL) are presented in a companion paper of this Conference.

Environmental Contour Lines: A Method for Estimating Long Term Extremes by a Short Term Analysis

2008 ◽  
Author(s):  
S. Haver ◽  
S. R. Winterstein
Keyword(s):  
Major Part ◽  
Offshore Structures ◽  
Environmental Load ◽  
Short Term ◽  
Sea State ◽  
Contour Lines ◽  
Linear Problems ◽  
Term Analysis ◽  
Do So

Design of offshore structures involves the calculation of reliable estimates for loads and responses corresponding to annual exceedance probabilities of 10-2 and 10-4. In order to do so, all sources of inherent randomness must be accounted for, i.e. the short term variability of say the 3-hour extreme value in a given sea state should be combined with the long term variability of the sea state characteristics. This calls for some sort of a long term analysis. For linear or nearly linear problems this can easily be done, while such an analysis becomes more complicated and time consuming for strongly non-linear response problems. The difficulties are greater if a major part of the environmental load is of an on-off nature. This paper illustrates an approximate approach, the environmental contour line method, for obtaining proper estimates of long term extremes utilizing a short term analysis. Examples are also included.

Application of Environmental Contour Lines on a Flexible Riser

2010 ◽  
Author(s):  
Gro Sagli Baarholm ◽  
Sverre Haver
Keyword(s):  
Time Domain ◽  
Linear Response ◽  
Contour Line ◽  
Exceedance Probability ◽  
Response Analysis ◽  
Wave Direction ◽  
Flexible Riser ◽  
Peak Period ◽  
Non Linear

This paper is concerned with estimating the response value corresponding to an annual exceedance probability. In principle, this response needs to be obtained by combining the response statistics in all sea states and wave headings. The probability distribution for a given sea state and wave direction, specified by the significant wave height, Hs, the peak period, Tp, and wave direction, β, can be obtained by frequency domain analysis for linear response. Time domain simulations are, in general, required to obtain the stochastic structure of a non-linear response quantity. To limit the statistical uncertainty, the time domain simulations must be sufficiently long. Therefore, a simplified method is necessary to improve the efficiency of the direct calculation of the long-term response value in the non-linear case. A method to determine the long-term extremes by considering only a few short term sea states is outlined. The sea states have a certain probability of occurrence and are identified by a contour line in the (Hs,Tp) plane. This will make possible practical estimation of the extreme loads the structure is exposed to. The contour line approach is merely suggested as a method for predicting load- and response maxima corresponding to a given annual exceedance probability without having to carry out a full long term analysis. The advantage with this concept is that the environmental and response analysis is decoupled. This is very convenient if the problem under consideration is of a very non-linear nature — in particular if characteristic values for design are to be established directly from model tests. The method is an approximate method, but seems to give results of reasonable accuracy for most problems. The purpose of the present paper is to present the contour line method applied to estimate responses on a flexible riser, lazy wave configuration, located in the North Sea.

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 of Extreme Global Restoring Loads on a FPSO Turret Structure Using a Coupled Model

2018 ◽  
Author(s):  
Isabel Jimenez Puente ◽  
Kjell Larsen
Keyword(s):  
Coupled Model ◽  
Element Model ◽  
Contour Line ◽  
Structural Element ◽  
Mooring Lines ◽  
Probability Of Exceedance ◽  
Extreme Loads ◽  
Annual Probability ◽  
Term Analysis

A turret structure can be a major design driver for FPSO systems. Therefore, careful attention needs to be given to the estimation of extreme loads on this structural element. This paper presents an all seas long-term analysis of the extreme global restoring forces acting on a FPSO turret structure, and a comparison with the results obtained through the contour line approach. The analysis is performed in the time domain using a coupled model, where the floater is modelled in the software SIMO, and the risers and mooring lines are represented by a Finite Element Model in RIFLEX. The characteristic responses of the turret structure with q-annual probability of exceedance are estimated from a full long-term analysis where both the short and long-term variability are considered. These results are compared to those obtained through the long-term estimate from the contour line approach when assuming 90th percentile for the worst sea state with q-annual probability of exceedance. The results from the full long-term analysis will allow us to verify the adequate percentile level to be used with a contour line approach when estimating extreme turret structure loads.

Prediction of Extreme Roll Motion on an FPSO Using Long Term Statistics

2002 ◽  
Author(s):  
Tone M. Vestbo̸stad ◽  
Sverre Haver ◽  
Odd Jan Andersen ◽  
Arne Albert
Keyword(s):  
Probability Distribution ◽  
Wind Waves ◽  
Weather Conditions ◽  
Full Scale ◽  
Roll Motion ◽  
Short Term ◽  
Sea State ◽  
Term Analysis

This paper presents a method for predicting extreme roll motion on an FPSO using long-term statistics. The method consists of a long-term simulation where a database of consecutive short-term sea states with combined weather conditions, including direction and magnitude of wind, wind waves and swell waves, is used. The vessel heading in given weather conditions is simulated. For each combined sea state, the short-term roll motion maxima are calculated to form a long-term probability distribution, and the extreme roll motion, e.g. the 100-year value, can be estimated from the distribution. For an example FPSO, the results from the long-term analysis have been compared with full-scale measurements, giving a validation of the method. This paper is a shortened version of [1].

Comparison of Design Wave Approach and Short Term Approach With Increased Wave Height in the Evaluation of Whipping Induced Bending Moment

2012 ◽  
Author(s):  
Quentin Derbanne ◽  
Fabien Bigot ◽  
Guillaume de Hauteclocque
Keyword(s):  
Wave Height ◽  
Return Period ◽  
Significant Wave Height ◽  
Bending Moment ◽  
Short Term ◽  
Sea State ◽  
Significant Wave ◽  
Non Linear ◽  
Term Analysis

The evaluation of extreme bending moment corresponding to a 25 years return period requires very long simulations on a large number of sea states. This long term analysis is easy to do with a linear model of the ship response, but is impractical when using a time consuming model including non linear and slamming loads. In that case some simplified methods need to be applied. These methods are often based on Equivalent Design Waves (EDW) which are calibrated on the extreme linear value. The general practice is to define the EDW as a regular wave. A very simple method is to compute the non linear bending moment applying the pressure correction on the hull without recomputing the ship motions. A better method is to recompute in time domain the non linear ship response on this Design Wave. It is even possible to define a more realistic Design Wave, taking into account the frequency and directional content of the sea states used in the long term analysis: those waves are called Response Conditioned Wave and Directional Response Conditioned Waves. The different methods are applied to an Ultra Large Container Ship (ULCS). Hydro-structure calculations are carried out on a severe design sea state, taking into account Froude-Krylov pressure correction, slamming forces and whipping response. Results of a very long computation are compared to the results of the Design Wave approaches. Another method is proposed to compute very rare events. It is based on an artificial increase of the significant wave height of the sea state, and the assumption of the independence of the non linear effects to the significant wave height. Using this method it is possible, with a simulation of only a few hours, to predict a very rare short term event, corresponding to a very long return period. The results are compared to the Design Wave results and appear to be much more precise.

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.

Environmental Contour Lines for Design Purposes: Why and When?

2004 ◽  
Author(s):  
Sverre Haver ◽  
Gudmund Kleiven
Keyword(s):  
Time Domain ◽  
Exceedance Probability ◽  
Response Analysis ◽  
Short Term ◽  
Structural Problems ◽  
Contour Lines ◽  
Convenient Approach ◽  
Probabilistic Structure ◽  
Term Response

Methods of prediction of structural loads corresponding to a required target annual exceedance probability are reviewed. Particular attention is given to utilization of environmental contour lines for such a purpose. This approach is based on using short term methods for predicting adequate estimates of the q-probability response. The environmental contour line approach is a very convenient approach if complicated structural problems are considered. For such problems one will often have to involve numerical time domain analyses or model tests to reveal the short term probabilistic structure of the response maxima, making a full long term response analysis impossible for most practical problems.

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