Finite Element Models for Integrity Assessment of Flexible Riser with Damaged Tensile Wires

Abstract

Many of the flexible risers in service do not meet their documented service life. A common failure mode is the rupture of the abrasive external layer which, in itself, is not critical, but seawater ingress may corrode tensile wires over time and, in worst case, cause breakage. Damaged tensile armour could significantly reduce the load capacity of the riser. Integrity assessment of flexible riser with damaged tensile armour based on stress concentration factors (SCF) could predict the remaining fatigue life of the structure. SCF could be based on experimental tests, but they are expensive. Finite element analyses can be used instead at a much lower cost. In this thesis, three finite element models are created in Abaqus to replicate a 2.5 inch flexible riser. In the first model, the tensile wires are represented by beam elements and the other layers by shell elements. In the second and third model, the tensile wires are represented by solid elements and the other layers are represented by shell and solid elements, respectively. However, only the two last models are used in the analyses. To reduce simulation time, the carcass and pressure armour are modelled in a simplified manner with equivalent material properties. A convergence study is performed to find the minimum element size needed to yield good results. The equivalent material properties used in the simplified layers are evaluated by comparing the hoop stress to analytical values which are in good agreement. Two load cases are considered in the stress analyses, where one is tension and the other is bending. The models are verified by comparing the axial stiffness to the physical one and axial wire stress due to bending to analytical values found in the literature. Both are in good agreement. A simulation is performed for the second model with two ruptured wires in the outer tensile layer. A tress concentration factor is established based on the current stress results for the mean axial wire stress. The stress concentration factor is compared to results in the literature and show good agreement.