Abstract

To calculate the mechanical stress proficiently, for both non-helix and helically bundled elements, is essential to reduce risk and maintain the product's integrity throughout production, transportation, installation and operation phases. Though, the conversion of such loads into local stress can be a time-consuming process; either due to the magnitude of produced data from global analysis, or by use of an ineffective process.

This paper presents a novel and time efficient approach for modelling both non-helix stress and up to nth order stress hysteresis of helix laid circular elements, using stress coefficients from a local FEA tool. Thus, it forms a link between the global and the local analysis, returning stress results in time domain. These results can for instance be processed further into local fatigue damage, giving an enhanced fatigue life for higher order elements compared to lower order limited techniques.

INTRODUCTION

The local analysis that provides mechanical properties and limitations of the bundled product, is often carried out using in-house or commercial available FEA tools. Transforming time domain results from the global analysis into mechanical stress using these tools is typically impractical, however, and a faster conversion approach that can accurately reproduce the mechanical stress is called for.

Doynov, Nilsen-Aas, Haakonsen, Kan and Bjærum (2007) superficially describes a method for calculating stress in helically laid tensile armor wires in flexible risers, and subsequently evaluate the element's fatigue damage. The mechanical stresses were found using an interpolation routine, combining global analysis with stress transfer functions from a local FEA. A somewhat similar methodology was used to estimate the fatigue damage in fiber optic riser elements presented in Asyikin and Karlsen (2016). They used a tool named Uflexfilter that converts global result into 1st order stresses, based on stress coefficients from the local FEA tool Uflex 2D. Sødahl, Skeie, Steinkjer and Kalleklev (2010) however, approach this issue using the software Helica. This tool has shown to be efficient for both local and fatigue analyses. The efficiency of the software is stated to arise from an analytical 2D calculation, which has been verified by full-scale testing of 1st order bundled steel tubes in an umbilical, Dhaigude, Ekeberg and Sødahl (2016).

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