Fracture Mechanics Based Mooring Fatigue Analysis for a Semi-Submersible Subjected to Triple Narrow-Band Loading Processes

2019 ◽  
Author(s):  
Xutian Xue ◽  
Nian-Zhong Chen ◽  
Yongchang Pu
Keyword(s):  
Fatigue Life ◽  
Fracture Mechanics ◽  
Fatigue Analysis ◽  
Second Order ◽  
Combination Method ◽  
Mooring System ◽  
Initial Surface ◽  
Loop Current ◽  
Mooring Lines ◽  
First Order

Abstract In the general offshore operating locations, mooring systems are normally considered to be primarily affected by environmental loadings induced by waves, wind and current. WF motion induced by first-order waves, LF motion induced by second-order waves and wind would make the most contribution to the fatigue damage of mooring lines. However, it was reported recently that in the Gulf of Mexico (GoM), the fatigue life of mooring lines can be significantly affected by the vortex induced motion (VIM) induced by loop current. The aim of this presented paper is to address the influence of VIM on fatigue life of the mooring system operating at the central of the GoM through performing a fracture mechanics (FM) based fatigue analysis for an offshore mooring system. A frequency-domain mooring analysis for the semi-submersible is conducted where WF motion induced by first-order waves, LF motion induced by second-order waves and wind, and VIM induced by loop current are taken into account. WF motion, LF motion and VIM are treated as three independent loading processes. A wide-band loading combination method is then used for predicting the loading processes acting on the mooring system combining WF motion, LF motion and VIM. A fracture mechanics based analysis is performed to examine the fatigue life of mooring system, in which initial surface cracks in previous existence are assumed to grow from the surfaces of mooring chain links connecting to the fairleads. The stress intensity factor ranges to estimate the crack growth in the FM based analysis are obtained from a finite element (FE) analysis.

T-N Curves and Fracture Mechanics Based Mooring Fatigue Analysis for a Semi-Submersible

2017 ◽  
Author(s):  
Xutian Xue ◽  
Nian-Zhong Chen
Keyword(s):  
Fracture Mechanics ◽  
Fatigue Failure ◽  
Line Tension ◽  
Fatigue Analysis ◽  
Mooring Line ◽  
Mooring System ◽  
Bottom Line ◽  
Mooring Lines ◽  
Element Analysis ◽  
Chain Link

This paper is to perform T-N curves and fracture mechanics based fatigue analysis for mooring lines of a semi-submersible installed in Gulf of Mexico (GoM). The wave frequency (WF) and the low frequency (LF) load processes are regarded as two random processes and the load combination of the two processes is considered. Frequency-domain analysis is then conducted for calculating the tension ranges along hybrid-material mooring lines induced by motions of WF, LF and the combined WF and LF. The narrow-banded spectral method is used for calculating the mooring line tension and the fatigue damage of mooring lines is estimated by T-N curves and fracture mechanics based approaches. The fracture mechanics based analysis is combined with a finite element analysis to predict crack propagation at different locations of a studless chain link. It was found that the crown section of a mooring chain is the most critical location subjected to fatigue failure in a studless chain link. The most critical points prone to fatigue failure of the catenary and taut mooring systems designed for the semi-submersible are at the bottom end of top line and the top end of bottom line, respectively. Also, fatigue lives of mooring lines in the catenary mooring system are generally longer than those of the taut mooring system. In addition, a comparison between fatigue lives of mooring lines predicted by T-N curves and fracture mechanics based approaches shows that the results estimated by both approaches are generally comparable.

Coupled Dynamic Analyses of Deep-Water Semi-Submersible With New Spread Mooring System

2020 ◽  
Author(s):  
S. Chandrasekaran ◽  
Arvind Kumar Jain ◽  
Syed Azeem Uddin
Keyword(s):  
Fatigue Life ◽  
Deep Water ◽  
Production Systems ◽  
Irregular Waves ◽  
Mooring System ◽  
Mooring Lines ◽  
Dynamic Positioning System ◽  
Exploration And Production ◽  
Ocean Environment ◽  
Offshore Oil

Abstract Offshore complaint structures dominate the deepwater oil exploration and production due to their adaptive geometric form and well-established construction practices. Semi-submersible is one of the widely preferred, floating production systems due to its form-dominant ability, better stability characteristics, and best constructional features. It is usually position-restrained using a dynamic-positioning system (active-restraining) or mooring system (passive-restraining); being less-sensitive to freak ocean environment is an added advantage. The Semi-submersible, chosen for the present study is based on a similar configuration of a 6th generation deep-water Hai Yang Shi You (HYSY) – 981 platforms, commissioned by the China National Offshore Oil Corporation (CNOOC) in 2012. A sixteen-point, spread catenary-mooring without submerged buoy (case-1) in the form of chain-wire-chain type configuration is used for position-restraining. Response behavior of the semi-submersible with a conventional spread catenary-mooring system with a submerged buoy (case-2) is compared. API spectrum is used for computing wind loads, while the JONSWAP spectrum is used to represent irregular waves for various directions of wave heading. The effect of non-linearly varying current is considered up to 10% of water depth. Numerical analyses of the semi-submersible are carried out under 10-years, and 100-years return period events using Ansys Aqwa. Under wind, wave, and current loads, motion responses of the Semi-submersible at 1500 m and 2000 m water depths are investigated for both the cases in time-domain. Dynamic mooring tension variations arise from the environmental loads are further investigated for a fatigue failure using the S-N curve approach. It is found that the fatigue life of the mooring lines after the inclusion of the buoy is enhanced. It was also observed that, during failure of mooring lines there is an increase in tension of the mooring lines which are adjacent to the failed mooring lines and this is due to the transfer of mooring load and hence reducing their fatigue life.

Determination of the Effect of Second Order Motions of Moored MODU on Wellhead Fatigue

2014 ◽  
Author(s):  
Dara Williams ◽  
Patrick Ashton
Keyword(s):  
Fatigue Damage ◽  
Water Depth ◽  
Low Frequency ◽  
Fatigue Analysis ◽  
Second Order ◽  
First Order ◽  
Analysis Techniques ◽  
Wave Drift ◽  
Wave Drift Forces ◽  
Drift Forces

As has been noted in industry publications and conferences in the recent past the use of more modern deepwater capable 5th and 6th generation semisubmersible drilling rigs in relatively shallow water applications (when compared to design water depth) is likely to become more commonplace. Water depths of 500m or less will necessitate the use of mooring systems in order to maintain position close to the well centre whilst drilling. For fatigue assessments of moored MODUs, the current industry practice to estimate fatigue damage in the drilling riser and the wellhead, using global riser analysis techniques, is to consider both wave and VIV fatigue effects. Standard wave fatigue analysis considers two key response parameters, firstly the impact of the hydrodynamic loading on the riser joints due to drag forces, inertia and added mass effects, and secondly the effects of vessel motions on the riser system and wellhead loading. Standard practice for wave fatigue analysis is to consider only first order motion effects as described by the vessel RAO (response amplitude operator). However, for a moored MODU low frequency (100s-200s period) vessel response can have a significant impact on the overall vessel motions. The actual response and magnitude of MODU motion will be influenced by the size and displacement of the vessel in addition to the configuration of the mooring system. First order lateral motions for a semisubmersible tend to increase as wave period is increased and therefore at lower periods first order motions can be quite low. However, the opposite can be said of wave drift forces that contribute to second order response. Although the wave drift forces are largest for lower wave periods, these low period drift forces have a significant influence on the resulting long period second order response of a moored MODU. This has important implications for drilling riser and wellhead fatigue analysis as in many cases the critical seastates for fatigue damage are low period seastates with a large number of occurrences. Thus the current global analysis techniques for fatigue calculations may lead to an underestimation of fatigue damage contribution from low period seastates. The purpose of this paper is to present the key conclusions and findings of a study carried out in order to determine the effects of low frequency moored MODU motions on wellhead fatigue. These results are derived from a case study of a moored 6th generation semi-submersible drilling vessel in 500m water depth.

Fracture Mechanics Approach to Fatigue Analysis in Design

1978 ◽  
Vol 100 (2) ◽  
pp. 113-120 ◽  
Author(s):  
R. P. Wei
Keyword(s):  
Fatigue Life ◽  
Fracture Mechanics ◽  
Crack Growth ◽  
Environmental Variables ◽  
Life Prediction ◽  
Fatigue Analysis ◽  
Growth Data ◽  
Engineering Structures ◽  
The Relationship

Many engineering structures are subjected to cyclically varying (fatigue) loads during service. Fatigue analysis, or the estimation of fatigue lives on such structures, is therefore an essential part of engineering design. In this paper, the fracture mechanics approach to fatigue analysis is described. This approach has evolved over the last decade, and is based on the assumptions (i) that there are preexisting flaws or cracks in a structural component, or that cracks are initiated early in the life of the component, and (ii) that the fatigue life of the component is determined principally by the rate of growth of these cracks under cyclic loading. Characterization of the rate of fatigue crack growth in terms of fracture mechanics parameters is discussed. The relationship between these parameters and those commonly used in fatigue analysis is identified. A procedure for estimating fatigue lives from crack growth data is outlined. The importance of various loading and environmental variables on fatigue life prediction is considered. A number of example problems are given to illustrate the procedure and the various effects.

Low-Frequency Phenomena Associated With Vessels Moored at Sea

1975 ◽  
Vol 15 (06) ◽  
pp. 487-494 ◽  
Author(s):  
J.A. Pinkster
Keyword(s):  
Wave Height ◽  
Low Frequency ◽  
Second Order ◽  
Irregular Waves ◽  
Ship Motions ◽  
Wave Forces ◽  
Mooring System ◽  
Frequency Wave ◽  
First Order ◽  
The Mean

Abstract The influence of the low-frequency-wave-drifting force on the motions of moored vessels and the loads in the mooring system is demonstrated from results of model tests in irregular waves. The origin of the wave drifting force is discussed and methods for calculating the mean drifting force are reviewed. To facilitate calculation of the low-frequency-wave drifting force on an object in irregular waves, an existing method using the mean drifting force in regular waves is generalized. The results of calculations using the method introduced in this paper are compared with previously published test results. Finally, some remarks are added concerning effects that have not been accounted for in existing calculation methods. Introduction A vessel moored at sea in stationary conditions with regard to waves, wind, and current is subjected to forces that tend to shift it from the desired position. For a given vessel and position in the position. For a given vessel and position in the horizontal plane, the motions depend on both the mooring system and the external forces acting on the vessel. In steady conditions, the forces caused by a constant wind and current are constant quantities for a given heading angle of the vessel. The forces caused by a stationary irregular sea are of an irregular nature and may be split into two parts: first-order oscillatory forces with wave parts: first-order oscillatory forces with wave frequency, and second-order, slowly varying forces with frequencies much lower than the wave frequency.The first-order oscillatory wave forces on a vessel cause the well known ship motions whose frequencies equal the frequencies present in the spectrum of the irregular waves. These are the linear motions of surge, sway, and heave and the three angular motions of roll, pitch, and yaw. In general, the first-order wave forces are proportional to the wave height, as are the ensuing motions. The magnitude of the linear oscillatory motions is in the order of the height of the waves.The second-order wave forces, perhaps better known as the wave drifting forces, have been shown to be proportional to the square of the wave height. These forces, though small in magnitude, are the cause of the low-frequency, large-amplitude, horizontal motions sometimes observed in large vessels moored in irregular waves. Tests run in irregular waves in wave tanks of the Netherlands Ship Model Basin revealed a number of properties and effects of the low-frequency-wave properties and effects of the low-frequency-wave drifting force that are discussed here using the results of two test programs.The first of these programs concerns tests run with the model of a 125,000-cu m LNG carrier moored in head seas with an ideal linear mooring system. The second program deals with a 300,000-DWT VLCC moored with a realistic nonlinear bow hawser to a single-buoy mooring in waves, wind, and current coming from different directions.The results of the tests with the LNG carrier are shown in Figs. 1 through 3, while the results of the tests with the 300,000-DWT VLCC are shown in Fig. 4. All results are given in full-scale values. Fig. 1 shows the wave trace and the surge motion of the LNG carrier to a base of time. SPEJ P. 487

Experimental Results on the Combined 1st and 2nd Order Dynamics of a Small-Scale Taut-Leg Mooring Line

2005 ◽  
Author(s):  
Alexandre N. Simos ◽  
Andre´ L. C. Fujarra ◽  
Fa´bio G. Palazzo
Keyword(s):  
Second Order ◽  
Experimental Results ◽  
Mooring Line ◽  
Small Scale ◽  
Experimental Paradigm ◽  
Current Profile ◽  
Mooring Lines ◽  
Towing Tank ◽  
Calibration And Validation ◽  
First Order

A set of experimental results on the dynamics of taut-leg mooring-lines was generated by means of towing-tank tests. These results will be employed by Petrobras as an experimental paradigm for the calibration and validation of numerical codes based on finite-elements method (FEM). The setup allowed combining first and second order motions on the top of the line with different amplitudes and frequencies. The first order motions were emulated by means of circular harmonic motions while alternate horizontal translations represented the drift motions. It was also possible to emulate a uniform in-plane current profile along the suspended length of the model. The model was composed by three different segments. An intermediate rubber hawser connected the top and anchor chain segments and allowed considerable elongation of the line during the tests. Tension at the top of the model was measured by means of a load cell and the second-order motion was registered optically. This paper presents the procedure adopted for the tests and also some preliminary comparisons between experimental results and numerical simulations.

The Impact of the Second Order Vessel Motion on the Fatigue Life of Steel Catenary Risers

2004 ◽  
Author(s):  
Ghiath Mansour
Keyword(s):  
Fatigue Life ◽  
Early Stage ◽  
Second Order ◽  
Wind Loading ◽  
Coupled Analysis ◽  
Steel Catenary Riser ◽  
Simplified Approach ◽  
First Order ◽  
Vessel Motion ◽  
The Impact

Fatigue is a primary factor in assessing the feasibility of a steel catenary riser solution for production and export of offshore hydrocarbons from floating production systems. The first and second order vessel motion due to wave and wind loading is a major source of fatigue. While the first order motion fatigue can be readily estimated through the vessel response amplitude operators, assessing the impact of the second order motion is time and effort consuming especially in generating the vessel motion time traces, more so for coupled analysis. However, inclusion of the second order motion at early stages of the riser feasibility assessment, when the mooring system design is far from final, is essential in predicting the riser fatigue life. In this article, a simplified approach for estimating the impact of the second order motion at an early stage in the design is presented based on the first order motion analysis and the vessel mean static offsets as obtained from frequency domain analysis.

scholarly journals A Fracture Mechanics Approach to Life Prediction of Turbine Blades

1993 ◽  
Author(s):  
N. S. Vyas ◽  
Sidharth ◽  
J. S. Rao
Keyword(s):  
Fatigue Life ◽  
Fracture Mechanics ◽  
Crack Initiation ◽  
Life Prediction ◽  
Residual Life ◽  
Low Cycle Fatigue ◽  
Turbine Blades ◽  
Combination Method ◽  
Prediction Algorithm ◽  
Hysteresis Curve

Emerging blade technologies are finding it increasingly essential to correlate blade vibrations to blade fatigue in order to asses the residual life of existing blading and for development of newer designs. In this paper an analytical code for Dynamic Stress Analysis and Fatigue Life Prediction of blades is presented. The life prediction algorithm is based on a combination method, which combines the local strain approach to predict the initiation life and fracture mechanics approach to predict the propagation life, to estimate the total fatigue life. The conventional stress based approach involving von Mises theory along with S-N-Mean stress diagram suffer from the drawback that they do not make allowance for the possibility of development of plastic strain zones, especially in cases of low cycle fatigue. In the present paper, strain life concepts are employed to analyse the crack initiation phenomenon. Dynamic and static stresses incurred by the blade form inputs to the life estimation algorithm. The modeling is done for a general tapered, twisted and asymmetric cross section blade mounted on a rotating disc at a stagger angle. Blade damping is nonlinear in nature and a numerical technique is employed for estimation of blade stresses under typical nozzle excitation. Critical cases of resonant conditions of blade operation are considered. Neuber’s rule is applied to the dynamic stresses to obtain the elasto-plastic strains and then the material hysteresis curve is used to iteratively solve for the plastic stress. Static stress effects are accounted for and crack initiation life is estimated by solving the strain life equation. Crack growth formulations are then applied to the initiated crack to analyse the propagation of crack leading to failure. The engineering approximations involved are stated and the algorithm is numerically demonstrated for typical conditions of blade operations.

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