Time Domain Structural Fatigue Analysis of Floating Offshore Platforms: A Response Based Technique

2021 ◽  
Author(s):  
Johyun Kyoung ◽  
Sagar Samaria ◽  
Jang Whan Kim
Keyword(s):  
Time Domain ◽  
Fatigue Analysis ◽  
Offshore Platforms ◽  
Structural Fatigue

Time Domain Structural Fatigue Analysis of Floating Offshore Platforms: A Response Based Technique

2020 ◽  
Author(s):  
Johyun Kyoung ◽  
Sagar Samaria ◽  
Jang Whan Kim
Keyword(s):  
Spectral Method ◽  
Fatigue Damage ◽  
Time Domain ◽  
Structural Response ◽  
Fatigue Analysis ◽  
Offshore Platform ◽  
Offshore Platforms ◽  
Time Step ◽  
Structural Fatigue ◽  
Non Linear

Abstract This paper presents a response-based, time-domain structural fatigue analysis of a floating offshore platform. The conventional technique for structural fatigue assessments of offshore platforms uses a linear, frequency-domain analysis based on the spectral method. Although this conventional method is computationally efficient, there is a room for improving accuracy and reducing uncertainties because it cannot accurately address non-linear loadings on the offshore platform. Such non-linear loads arise from the wave, wind, and current as well as from the riser and mooring systems; these non-linearities necessitate large factors of safety that lead to conservative design and frequent inspection. As an extension of previous work (Kyoung et al.[12]), this study presents the development of a time-domain, structural fatigue analysis that explicitly addresses non-linear loading on the platform. The external load time-histories are directly mapped onto the structure at every time interval to create a stress-based response with the varying environment. In each time step, the load mapping accurately captures the phase relationship between the external loading and hull inertial response. Therefore, present method reduces uncertainties in the fatigue damage computation and overcomes the assumptions of spectral method. Present load component-based approach is applied onto a finite element structural model, which provides unit structural response at locations of interest. Time history of structural response is obtained by synthesizing the obtained unit stress-based structural response with environmental loading and platform motion response. Fatigue damage can be computed from the obtained time series of structural response using rain-flow counting. As an application, a conventional semisubmersible platform is used to evaluate structural fatigue damage for a given wave scatter diagram. A comparison between results from this response-based time-domain approach and the conventional spectral method is presented.

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.

Time Domain Fatigue Life Analysis of Offshore Jacket Structure

2019 ◽  
Author(s):  
Yan Wei Wu
Keyword(s):  
Fatigue Life ◽  
Fatigue Damage ◽  
Time Domain ◽  
Offshore Wind ◽  
Support Structure ◽  
Structural Fatigue ◽  
Long Time ◽  
Fatigue Life Analysis ◽  
Stress Concentration Factors ◽  
The Time Domain

Abstract Offshore wind system encountered wind, wave, current, soil, and other environmental loads. The support structure is randomly loaded for a long time, which is more likely to cause fatigue damage. In this paper, the NREL 5MW wind turbine and OC4 jacket support structure is selected to perform the time domain fatigue analysis. Commercial software Bladed and SACS are used to perform the required structural responses and fatigue strength calculations. The Stress Concentration Factors (SCF) and S-N curves for the stress calculations of tubular joints are adopted based on the recommendation of DNV GL guidelines. The magnitude of the stress variation range and the corresponding number of counts are obtained by using the rain-flow counting algorithm. Finally, the Palmgren-Miner’s rule is adopted to calculate the cumulative damage ratio and the fatigue life can then be estimated. Fatigue damage ratio and structural fatigue life of each joint during 20 years of operation period are evaluated.

The Consideration of Welding Profiles in Fatigue Evaluation of Structure Design

2006 ◽  
Author(s):  
D. Zhang
Keyword(s):  
Fatigue Life ◽  
Engineering Design ◽  
Structural Design ◽  
Structure Design ◽  
Offshore Platforms ◽  
Efficient Design ◽  
Structural Fatigue ◽  
Different Shapes ◽  
Fatigue Evaluation ◽  
The Impact

Structural fatigue plays a very important role in plating structural design. There are a lot of efforts in finding effective ways to improve the fatigue life of the plating structure. One of the means is to use welding profile. Welding profile is widely used in fatigue sensitive areas to improve the fatigue life of offshore platforms. How to evaluate the impact of welding profile to fatigue life calculation has always been the topic of engineering design. This paper will study the impact of different shapes of welding profiles to the fatigue life of structure through a real project example, and discuss its application in platform design. Different welding profiles have large impact on fatigue life; and an achievable good welding profile can improve the fatigue life dramatically. With the help of welding profiles, offshore engineers can achieve a more efficient design of the structure.

Long-Term Fatigue Analysis of Deepwater Risers in the Time Domain

2014 ◽  
Author(s):  
Yidan Gao ◽  
Ying Min Low
Keyword(s):  
Time Domain ◽  
Time Domain Analysis ◽  
Fatigue Analysis ◽  
Sea State ◽  
Need To Evaluate ◽  
Nature Of Time ◽  
The Time Domain ◽  
Computational Difficulty ◽  
Deepwater Risers

A floating production system is exposed to many different environmental conditions over its service life. Consequently, the long-term fatigue analysis of deepwater risers is computationally demanding due to the need to evaluate the fatigue damage from a multitude of sea states. Because of the nonlinearities in the system, the dynamic analysis is often performed in the time domain. This further compounds the computational difficulty owing to the time consuming nature of time domain analysis, as well as the need to simulate a sufficient duration for each sea state to minimize sampling variability. This paper presents a new and efficient simulation technique for long-term fatigue analysis. The results based on this new technique are compared against those obtained from the direct simulation of numerous sea states.

Direct Calculation of Fatigue Damage of Ship Structure Details

2011 ◽  
Author(s):  
Zhiyuan Li ◽  
Jonas W. Ringsberg
Keyword(s):  
Fatigue Damage ◽  
Time Domain ◽  
Direct Calculation ◽  
Fatigue Analysis ◽  
Fe Model ◽  
Stress Concentration Factors ◽  
Rainflow Counting ◽  
Global And Local

Fatigue assessment of ships using the direct calculation approach has been investigated by numerous researchers. Normally, this approach is carried out as either a global model analysis, or as a local model structural analysis. The current investigation presents a case study of a container vessel where the global and local analyses procedures are combined. A nonlinear time-domain hydrodynamic analysis followed by a global FE analysis is employed to screen for the most severe locations of the global ship’s hull with regard to fatigue damage. Once these locations have been identified, a sub-modelling technique is employed to transfer global loads from the global FE model to local FE models that have high resolution of elements for local structure details. Results from a selection of local FE model simulations are presented. Stress concentration factors at four critical locations are calculated and compared with values recommended by classification guidelines. Results are presented from a short-term fatigue analysis which has been carried out using the rainflow counting method. Finally, a long-term fatigue analysis is performed in time-domain using a designed wave scatter diagram of representative sea states.

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