DNV GL along with Ultra Deep launched a Joint Industry Project (JIP) to validate the loxodromic assumption, i.e. slip is assumed to occur in axial helix direction only, in bending stress calculation used in local analysis of umbilicals and flexible pipes. The validation is performed by comparison of the numerical model described by Skeie, Steinkjer and Sødahl (2010), hereafter called HELICA, with high quality bending tests, contributed to the JIP. Loxodromic assumption is increasingly used in the industry in local stress analysis of umbilicals and flexible risers. The results showed a remarkably good agreement between the stress calculations and the stress measurements, thereby confirming the validity of the loxodromic assumption. The accuracy in the prediction of umbilical steel tube stresses achieved also suggests that the largest uncertainty in fatigue prediction has moved from the stress assessment and back towards the global loads prediction.
This paper presents the work carried out in the JIP on a high-tension umbilical specimen to compare stress predictions achieved using HELICA with stress measurements performed on an umbilical sample, along with details on how the work was carried out, findings and conclusions. The loxodromic assumption validation methodology is described in detail by Dhaigude, Ekeberg and Sødahl (ISOPE-2016-TPC-1060).
A test piece of an umbilical was instrumented with 48 strain gauges and fatigue-tested in a full-scale flex fatigue rig at TMT's laboratories in California. This represents a heavily instrumented umbilical that generated a lot of valuable data. The umbilical test piece was instrumented while carefully documenting the umbilical's position and orientation inside the bend stiffener; the relative longitudinal position of all sensors; the orientation of all tubes at the sensor locations; as well as all sensors' orientation on each individual tube (Dhaigude, Ekeberg 2015). Upon completion of the test, umbilical test piece was dissected.