scholarly journals Efficient Fatigue Analysis of Helix Elements in Umbilicals and Flexible Risers: Theory and Applications

2012 ◽  
Vol 2012 ◽  
pp. 1-22 ◽  
Author(s):  
Geir Skeie ◽  
Nils Sødahl ◽  
Oddrun Steinkjer
Keyword(s):  
Cross Section ◽  
Fatigue Analysis ◽  
Special Importance ◽  
Stress Cycle ◽  
Cross Sectional ◽  
Stick Slip ◽  
Cross Sectional Analysis ◽  
Analysis Scheme ◽  
Sectional Analysis

Fatigue analysis of structural components such as helix tensile armors and steel tubes is a critical design issue for dynamic umbilicals and flexible pipes. The basis for assessment of fatigue damage of such elements is the long-term stress cycle distribution at critical locations on the helix elements caused by long-term environmental loading on the system. The long-term stress cycle distribution will hence require global dynamic time domain analysis followed by a detailed cross-sectional analysis in a large number of irregular sea states. An overall computational consistent and efficient fatigue analysis scheme is outlined with due regard of the cross-sectional analysis technique required for fatigue stress calculation with particular attention to the helix elements. The global cross-section is exposed to pure bending, tensile, torsion, and pressure loading. The state of the different cross-section elements is based on the global response. Special emphasis is placed on assessment of friction stresses caused by the stick-slip behavior of helix elements in bending that are of special importance for fatigue life assessments. The described cross-sectional analysis techniques are based on an extensive literature survey and are hence considered to represent industry consensus. The performance of the described calculation scheme is illustrated by case studies.

Efficient Fatigue Analysis of Helix Elements in Umbilicals and Flexible Risers

2010 ◽  
Author(s):  
Nils So̸dahl ◽  
Geir Skeie ◽  
Oddrun Steinkjer ◽  
Arve Johan Kalleklev
Keyword(s):  
Fatigue Analysis ◽  
Stress Cycle ◽  
Cross Sectional ◽  
Stick Slip ◽  
Analysis Techniques ◽  
Cross Sectional Analysis ◽  
Flexible Pipes ◽  
Analysis Scheme ◽  
Sectional Analysis

Fatigue analysis of helix elements such as tensile armors and steel tubes are critical design issues for dynamic umbilicals and flexible pipes. The basis for assessment of fatigue damage of such elements is the long-term stress cycle distribution at critical locations on the helix elements caused by long-term environmental loading on the system. Establishment of the long-term stress cycle distribution will hence require global dynamic time domain analysis followed by cross-sectional analysis in a large number of irregular sea states. An overall computational consistent and efficient fatigue analysis scheme is outlined in this paper. A detailed description is given of cross-sectional analysis techniques required for fatigue stress calculation in helix elements exposed to bending, tensile and pressure loading. Special emphasis is placed on assessment of friction stresses caused by the stick/slip behaviour of helix elements in bending. Such effects are known to be of special importance for fatigue life assessments of deepwater applications of flexible pipes and umbilicals due to increased contact pressure between the individual elements in the cross-section. The described cross-sectional analysis techniques are based on an extensive literature survey and hence considered to represent industry consensus. The performance of the described calculation scheme is illustrated by case studies.

Consistent Free Span VIV Fatigue Analysis of Flexibles

2017 ◽  
Author(s):  
Mário Caruso ◽  
Xu Han ◽  
Nils Sødahl
Keyword(s):  
Fatigue Life ◽  
Cross Section ◽  
Fatigue Analysis ◽  
Life Assessment ◽  
Response Analysis ◽  
Stick Slip ◽  
Analysis Scheme ◽  
Industry Practice ◽  
Current Loading ◽  
Calculation Procedures

The fatigue life assessment of free spanning flexible products, such as subsea cables and umbilicals, due VIV requires special attention to the structural properties due to the stick/slip behaviour of helix elements in bending. Essential parameters for assessment of stick/slip effects in free span VIV response are the structural damping in the stick regime (i.e. umbilical behaves as a solid cross section due to friction between the helix elements) as well as the additional damping introduced by the hysteretic damping due to the stick/slip behaviour in bending. Furthermore, consistent fatigue stress recovery considering the stick/slip behaviour in bending is essential for fatigue life predictions. The consistent evaluation of stick/slip behaviour requires more sophisticated calculation procedures due to the non-linearity it introduces. Hence, industry practice has been to consider simplified, linear calculation procedures. However, future flexibles utilization may be much benefitted by a consistent stick/slip treatment in free spanning VIV fatigue assessments, as it may allow for longer allowable free span lengths or longer fatigue life. The overall objective of the paper is to establish a consistent free span VIV analysis methodology for flexibles in compliance with requirements given in ISO 13628-5 ‘Subsea Umbilicals’ and the overall philosophy of DNV-RP-F105 ‘Free Spanning Pipelines’. A consistent fatigue analysis scheme for VIV in free spans is outlined using commercially available state-of-the-art computer programs for free span VIV response analysis (FatFree) and cross section stress analysis (Helica). The performance of the calculation scheme is demonstrated by case studies in a complex long-term current loading environment. It is shown that consistent treatment of the mechanical properties of flexibles is essential for VIV fatigue life assessments of free spans.

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