A Calibrated Service Life Wear Model for the Pressure Armor Layer in Unbonded Flexible Pipes

Abstract An important design driver of an unbonded flexible pipe is often the fatigue life of the tensile armor layers. However, in pursuit of providing the market with durable, high-spec flexible pipe structures suited for increasingly higher operating bore pressures, it has also been found that the wear of the pressure armor layer should also be understood and quantified to assess the service life of a flexible pipe. This paper presents a novel approach to getting a direct coupling between the full-scale modeling of the flexible pipe and the wear of the pressure armor. This was done by measuring the wear of pressure armor samples extracted from an in-plane bending test portfolio. In this way, it has been possible to correlate the wear level to a large range of loads, pipe diameters, steel grades, and pressure armor geometries. These measurements, combined with other details of full- and small-scale tests, have made it possible to develop a calibrated model that assesses the wear in the pressure armor layer. In this manner, it is now possible to quantify the wear in service conditions and use it to establish the influence on the service life. The originality in this work is twofold: Based on wear measurements on pressure armor wires extracted from full-scale specimens, the wear rate in the pressure armor layer is shown to scale with slip amplitude. This phenomenon is known generally but has not been reported earlier for the unbonded flexible pipe. Further, the wear data from full-scale test specimens has been used to produce a calibrated model. Even though the failure mode in the pressure armor has shown to be rare, it is important to include the relevant phenomenon in the model as pressures increase.