A highly efficient umbilical analysis scheme for consistent fatigue life calculation of helix elements in umbilicals and flexible risers is briefly discussed in this paper. This scheme is developed by DNV GL and used in the commercially available umbilical analysis tool ‘HELICA’. The calculation efficiency is due to an analytical calculation of helix bending stresses assuming so called loxodromic helix geometry (i.e. slip is assumed to take place in the axial helix direction only). DNV GL along with Ultra Deep launched a Joint Industry Project (JIP) to validate the loxodromic assumption, which is used in local analysis stress calculations of umbilicals and flexible pipes, by comparing calculations with high quality bending test measurements. Validation against high/low tension bending tests (Ekeberg, Dhaigude (2016); Dhaigude, Ekeberg (2016)) is considered to cover umbilicals installed at normal to deep water depths. Details of the validation methodology and sample results are presented in this paper.
In recent years, there have been significant advancements made in umbilical technology. Subsea umbilicals are designed with multipurpose functionality including subsea control and monitoring, chemical injection, gas lift, and electric power transmission etc. Deepwater dynamic umbilicals pose additional challenges due to high end termination loads and potential fatigue concerns. As a result, umbilical designs have become increasingly complex and require advanced tools and methods for design, analysis and testing.
In order to achieve flexibility during bending, elements of umbilical or flexible pipe are arranged in a helical geometry. When the umbilical or pipe cross section is bent, initially the helical elements stick with the pipe. Once the frictional resistance is overcome, these elements slip and therefore release stored frictional stresses, as shown in Figure 1. Thus the helical geometry and relative slip between components makes the umbilical compliant. However, assessment of fatigue stresses in helical elements becomes complex due to such stick/slip behavior.
