Fatigue Analysis of Offshore Structures in Indian Western Offshore

2017 ◽  
pp. 143-151
Author(s):  
Nitin Nehra ◽  
Praveen Bhat ◽  
N. Panneerselvam
Keyword(s):  
Offshore Structures ◽  
Fatigue Analysis

Non-Linear Fatigue Analysis for Jacket Offshore Structures

2006 ◽  
Author(s):  
O. Gaidai ◽  
A. Naess
Keyword(s):  
Dynamic Analysis ◽  
Direct Method ◽  
Nonlinear Effects ◽  
Offshore Structures ◽  
Fatigue Analysis ◽  
Offshore Structure ◽  
Structural Fatigue ◽  
Drag Forces ◽  
Random Seas ◽  
Fatigue Estimation

This paper presents different approaches for accounting for nonlinear effects in fatigue analysis. One approach is an application of the quadratic approximation method described in [3, 4] to the stochastic fatigue estimation of jacket type offshore structures. An alternative method proposed is based on a spectral approximation, and this approximation turns out to be quite accurate and computationally simple. The stress cycles causing structural fatigue are considered to be directly related to the horizontal excursions of the fixed offshore structure in random seas. Besides inertia forces, it is important to study the effect of the nonlinear Morison type drag forces. Since no direct method for dynamic analysis with Morison type forces is available, it is a goal to find an accurate approximation, allowing efficient dynamic analysis. This has implications for long term fatigue analysis, which is an important issue for design of offshore structures.

FATIGUE ANALYSIS OF STEEL OFFSHORE STRUCTURES.

1976 ◽  
Vol 60 (4) ◽  
pp. 635-654 ◽  
Author(s):  
AK WILLIAMS ◽  
JE RINNE ◽  
MA MINER ◽  
Keyword(s):  
Offshore Structures ◽  
Fatigue Analysis

Wellhead Fatigue Analysis Method: Benefits of a Structural Reliability Analysis Approach

2012 ◽  
Author(s):  
Torfinn Hørte ◽  
Lorents Reinås ◽  
Jan Mathisen
Keyword(s):  
Reliability Analysis ◽  
Structural Reliability ◽  
Offshore Structures ◽  
Probability Of Failure ◽  
Fatigue Analysis ◽  
Life Extension ◽  
Inspection Planning ◽  
Structural Reliability Analysis ◽  
Extreme Load ◽  
Input Parameters

Structural Reliability Analysis (SRA) methods have been applied to marine and offshore structures for decades. SRA has proven useful in life extension exercises and inspection planning of existing offshore structures. It is also a useful tool in code development, where the reliability level provided by the code is calibrated to a target failure probability obtained by SRA. This applies both to extreme load situations and also to a structural system under the influence of a time dependent degradation process such as fatigue. The current analysis methods suggested for service life estimation of subsea wells are deterministic, and these analyses are associated with high sensitivity to variations in input parameters. Thus sensitivity screening is often recommended for certain input parameters, and the worst case is then typically used as a basis for the analysis. The associated level of conservatism embedded in results from a deterministic analysis is not quantified, and it is therefore difficult to know and to justify if unnecessary conservatism can be removed from the calculations. By applying SRA to a wellhead fatigue analysis, the input parameters are accounted for with their associated uncertainty given by probability distributions. Analysis results can be generated by use of Monte-Carlo simulations or FORM/SORM (first/second order reliability methods), accounting for the full scatter of system relations and input variations. The level of conservatism can then be quantified and evaluated versus an acceptable probability of failure. This article presents results from a SRA of a fictitious but still realistic well model, including the main assumptions that were made, and discusses how SRA can be applied to a wellhead fatigue analysis. Global load analyses and local stress calculations were carried out prior to the SRA, and a response surface technique was used to interpolate on these results. This analysis has been limited to two hotspots located in each of the two main load bearing members of the wellhead system. The SRA provides a probability of failure estimate that may be used to give better decision support in the event of life extension of existing subsea wells. In addition, a relative uncertainty ranking of input variables provides insight into the problem and knowledge about where risk reducing efforts should be made to reduce the uncertainty. It should be noted that most attention has been given to the method development, and that more comprehensive analysis work and assessment of specific input is needed in a real case.

scholarly journals Neural Networks Applied to the Wave-Induced Fatigue Analysis of Steel Risers

2018 ◽  
Vol 2018 ◽  
pp. 1-16 ◽  
Author(s):  
João P. R. Cortina ◽  
Fernando J. M. de Sousa ◽  
Luis V. S. Sagrilo
Keyword(s):  
Neural Networks ◽  
Time Series ◽  
Cross Sections ◽  
Computational Cost ◽  
Offshore Structures ◽  
Fatigue Analysis ◽  
Element Analysis ◽  
Long Time ◽  
Wave Induced ◽  
Hybrid Methodology

Time domain stochastic wave dynamic analyses of offshore structures are computationally expensive. Considering the wave-induced fatigue assessment for such structures, the combination of many environmental loading cases and the need of long time-series responses make the computational cost even more critical. In order to reduce the computational burden related to the wave-induced fatigue analysis of Steel Catenary Risers (SCRs), this work presents the application of a recently developed hybrid methodology that combines dynamic Finite Element Analysis (FEA) and Artificial Neural Networks (ANN). The methodology is named hybrid once it requires short time series of structure responses (obtained by FEA) and imposed motions (evaluated analytically) to train an ANN. Subsequently, the ANN is employed to predict the remaining response time series using the prescribed motions imposed at the top of the structure by the floater unit. In this particular work, the methodology is applied aiming to predict the tension and bending moments’ time series at structural elements located at the top region and at the touchdown zone (TDZ) of a metallic riser. With the predicted responses (tensions and moments), the stress time series are determined for eight points along the pipe cross sections, and stress cycles are identified using a Rainflow algorithm. Fatigue damage is then evaluated using SN curves and the Miner-Palmgren damage accumulation rule. The methodology is applied to a SCR connected to a semisubmersible platform in a water depth of 910 m. The obtained results are compared to those from a full FEA in order to evaluate the accuracy and computer efficiency of the hybrid methodology.

Background for Revision of DNV-RP-C203 Fatigue Analysis of Offshore Steel Structure

2005 ◽  
Author(s):  
Inge Lotsberg
Keyword(s):  
Steel Structure ◽  
Steel Structures ◽  
Offshore Structures ◽  
Fatigue Analysis ◽  
Time Interval ◽  
Fatigue Design ◽  
The World ◽  
New Research ◽  
Made In

The last revision of the DNV recommended practice “Fatigue Analysis of Offshore Steel Structures” is from October 2001. During use of this standard some feed back from designers around the world have been received. Also some new research in the area has been performed in the time interval from it was first developed. It is also realised that the document is being used for fatigue design of some other types of details and structures than was thought of when the document was originally developed. Therefore it was now found convenient to revise the document to incorporate the experience gained and new research and developments made in the area of fatigue of offshore structures the last 7 years since the main content for this recommended practice was developed.

Closed-Form Spectral Fatigue Analysis for Compliant Offshore Structures

1988 ◽  
Vol 32 (04) ◽  
pp. 297-304
Author(s):  
Y. N. Chen ◽  
S. A. Mavrakis
Keyword(s):  
Closed Form ◽  
Fatigue Damage ◽  
Computational Cost ◽  
Power Spectra ◽  
Offshore Structures ◽  
Fatigue Analysis ◽  
Cumulative Fatigue Damage ◽  
High Computational Cost

Spectral fatigue analysis frequently has been applied to welded joints in steel offshore structures. Although, on the theoretical basis, the spectral formulation holds certain advantages over other formulations such as the discrete, design wave type of analysis, numerical methods developed on that basis generally suffer from the shortcomings of lack of precision and high computational cost. This paper synthesizes the uncertainties resulting from modeling errors that are regarded heretofore as unavoidable in an analysis. Such errors are traced to the approximations introduced in handling of wave data, in numerical integration of the response power spectra, and in the integration that leads to the determination of cumulative fatigue damage. To each of these sources of modeling error, a transparent, closed-form method is proposed which not only eliminates the potential errors but, surprisingly, improves the computational efficiency many times. The sensitivity of fatigue damage upon the variability of the shape parameter due to variability of wave environment for the so-called simplified analysis utilizing an idealized mathematical long-term probability density function (for example, the Weibull distribution) is also discussed.

A Practical Spectral Fatigue Analysis Procedure for Topside of Floating Offshore Structures in West Africa

2015 ◽  
Author(s):  
Sungwook Chung ◽  
Minsung Chun ◽  
Kibok Jang ◽  
Youngsuk Suh
Keyword(s):  
West Africa ◽  
Wave Spectrum ◽  
Normal Wave ◽  
Offshore Structures ◽  
Fatigue Analysis ◽  
Analysis Procedure ◽  
Sea State ◽  
Commercial Program ◽  
Wind Sea ◽  
Wrapped Normal

In most offshore projects recently ordered, spectral fatigue analysis is required for design integrity. However, the spectral fatigue analysis is very complicated to implement since it has many variations for parameters and forms of input data, and the classification and commercial software packages are exposing limit to support all those variations. A topside fatigue analysis for a FPSO design in West Africa is one of such a challenging project due to the fact that the specification of the project requires spectral fatigue analysis considering 3-peak Ochi-Hubble wave spectrum, Wrapped normal wave spreading and sea state data with 3 wave components, main swell, secondary swell and wind sea. In this study, a practical spectral fatigue analysis procedure is introduced in order to implement the fatigue analysis using a commercial program SACS. Applying adaptive cosine spreading wave distribution which can approximate Wrapped normal wave spreading is devised for each sea state and the comparison between two wave spreading is carried out. Finally, the proposed methodology is justified by analyzing the characteristics of the sea state in West Africa.

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