Abstract
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.
Highlights
The required bending flexibility of risers and umbilicals is achieved by arranging strength and functional elements in a helix geometry
The helix geometry allows the elements to slip in order to release axial stresses built up by cross-sectional bending
The following can be concluded: i a pronounced peak in the cycle histogram is observed at 15 MPa. This corresponds to the maximum friction stress range; ii the effect from frictional stresses is decisive for fatigue life prediction
Summary
The required bending flexibility of risers and umbilicals is achieved by arranging strength and functional elements in a helix geometry. The helix geometry allows the elements to slip in order to release axial stresses built up by cross-sectional bending. This mechanism is essential for arranging flexible risers and umbilicals in compliant configurations that are capable of absorbing loads due to floater motions in harsh environmental conditions. Fatigue analysis of helix elements such as tensile armors and steel tubes is a critical design issue for umbilicals and flexible pipes. Different types of helix elements may be applied in the same layer to model, for example, umbilicals. All elements are assumed to have linear elastic material properties
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