Reducing uncertainty in time domain fatigue analysis of offshore structures using control variates

2021 ◽  
Vol 149 ◽  
pp. 107192
Author(s):  
Ruifeng Chen ◽  
Ying Min Low
Keyword(s):  
Time Domain ◽  
Offshore Structures ◽  
Fatigue Analysis ◽  
Control Variates

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

System Identification of Jack-Up Platform by Spectral Analysis

2010 ◽  
Author(s):  
X. M. Wang ◽  
C. G. Koh ◽  
T. N. Thanh ◽  
J. Zhang
Keyword(s):  
Spectral Analysis ◽  
System Identification ◽  
Time Domain ◽  
Time History ◽  
Offshore Structures ◽  
Wave Load ◽  
History Of ◽  
Structural Responses ◽  
Jack Up ◽  
Numerical Strategy

For the purpose of structural health monitoring (SHM), it is beneficial to develop a robust and accurate numerical strategy so as to identify key parameters of offshore structures. In this regard, it is difficult to use time-domain methods as the time history of wave load is not available unless output-only methods can be developed. Alternatively, spectral analysis widely used in offshore engineering to predict structural responses due to random wave conditions can be used. Thus the power spectral density (PSD) of structural response may be more appropriate than time history of structural responses in defining the objective (fitness) function for system identification of offshore structures. By minimizing PSD differences between measurements and simulations, the proposed numerical strategy is completely carried out in frequency domain, which can avoid inherent problems rising from random phase angles and unknown initial conditions in time domain. A jack-up platform is studied in the numerical study. A search space reduction method (SSRM) incorporating the use of genetic algorithms (GA) as well as a substructure approach are adopted to improve the accuracy and efficiency of identification. As a result, the stiffness parameters of jack-up legs can be well identified even under fairly noisy conditions.

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.

Computing Large Amplitude Motions of a Floating Offshore Structure in the Time Domain

2008 ◽  
Author(s):  
Wei Qiu ◽  
Hongxuan Peng
Keyword(s):  
Time Domain ◽  
Large Amplitude ◽  
Offshore Structures ◽  
The Body ◽  
Surface Boundary ◽  
Offshore Structure ◽  
Floating Structure ◽  
Time Step ◽  
Large Amplitude Motions ◽  
The Time Domain

Based on the panel-free method, large-amplitude motions of floating offshore structures have been computed by solving the body-exact problem in the time domain using the exact geometry. The body boundary condition is imposed on the instantaneous wetted surface exactly at each time step. The free surface boundary is assumed linear so that the time-domain Green function can be applied. The instantaneous wetted surface is obtained by trimming the entire NURBS surfaces of a floating structure. At each time step, Gaussian points are automatically distributed on the instantaneous wetted surface. The velocity potentials and velocities are computed accurately on the body surface by solving the desingularized integral equations. Nonlinear Froude-Krylov forces are computed on the instantaneous wetted surface under the incident wave profile. Validation studies have been carried out for a Floating Production Storage and Offloading (FPSO) vessel. Computed results were compared with experimental results and solutions by the panel method.

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