The Effect on Dynamic Steel Tube Umbilical Fatigue Performance Associated With Designing for Elevated Temperature
Authors: Fletcher-Woods Jamie , Noble Jake , Balfour Lewis 
Published: October 2017

The expansion of umbilical functionality to include power cables and high temperature fluid transportation (for applications such as gas lift) has led to the requirement for a high temperature steel tube riser umbilical, as shown in Figure 1. The combination of the elevated temperature and the dynamic service conditions create a unique design brief for steel tube umbilicals.

The following paper presents a case study evaluating a steel tube riser umbilical capable of transporting hot gas at 70°C from an FPSO to a well head at approximately 800m water depth. The material selection for the steel tube is super duplex stainless steel (SDSS) and due to the high temperature, the corrosion resistant coating selected to ensure corrosion resistance in seawater at 70°C is fusion bonded epoxy protected with a bonded polypropylene outer layer (3LPP).

The fatigue performance of the dynamic steel tube umbilicals is highly dependent on the frictional loads between the components developed due to tension and bending. This loading is most critical in the bend stiffener location at the riser umbilical’s interface with the FPSO structure. The fatigue critical component is usually determined to be one of the super duplex steel tubes within the umbilical.

The frictional loads are a function of the coefficient of friction between the interacting components and the contact load developed between the layers of helically wound components. This contact load increases with tension.

The paper considers the effects of the polypropylene material selection and the elevated operating temperature on the friction interface between the steel tubes. The work assesses the corresponding change in fatigue damage through the service life in comparison to more common temperatures and materials used in dynamic steel tube umbilicals.

Changes in contact load between elements of a friction interface are known to affect the friction coefficient. The contact load across all interfaces has been varied to help understand how the coefficient of friction may be affected by different tensile loads or umbilical designs. Including this variable in the test program also ensures that the friction is quantified at a contact load relative to the design case considered.

To assess the differences in friction a scope of component level friction testing is presented and the results are processed through umbilical local fatigue analysis software to establish the implications on fatigue performance.

The umbilical structure is designed to free flood in between the components in service, and the bend stiffener region is submerged for the design case in question. Full scale flex-fatigue testing of the dynamic umbilical and the bend stiffener are however conducted in a dry environment. The impact of this on the severity of the test and in comparison to the in-service condition are assessed using component level wet and dry testing to provide changes in friction for sensitivity analysis.

In addition to the loading on the tubes, the fatigue performance of the material being loaded is also considered. the fatigue performance of welded super duplex tubing has previously been tested and documented at ambient room temperature conditions and therefore the effect on the welded steel tube fatigue performance due to the increase in temperature for this application has been quantified to ensure the proposed design curve is suitable. cyclic bending stress experienced by the steel tubes during dynamic service (2).