Abstract
Mechanically lined pipe (MLP) is a lower cost alternative to clad pipe, and is used to carry corrosive fluids in oil and gas applications. The pipe includes a liner which is made from corrosion resistant alloy (CRA). The liner is fixed at both ends using seal welds and a length of weld overlay. This results in a junction between the backing steel, weld overlay and liner, which is called the transition point or the triple point. When lined pipe is subjected to fatigue loading, the triple point has been shown to be location where fatigue cracks initiate. Flaws present at the triple point will grow in service. When a flaw breaches the liner, the backing steel will be exposed to the corrosive fluid, limiting the remaining life of the pipeline. It is therefore of interest to perform fracture mechanics calculations to understand fatigue crack growth from the triple point, so that the operational service life of MLP can be calculated if a flaw is present at the triple point. In order to do this, a stress intensity factor, K, is needed which describes the stress intensity at a crack tip at the triple point. However, owing to the unique geometry of this location, no stress intensity factor solution currently exists for the triple point location. To overcome this challenge, parametric finite element models were developed to calculate stress intensity factors for triple point crack-like flaws. This paper presents results from a work package from a joint industry project whose aim was to understand the fatigue performance of mechanically lined pipe. Finite element analysis was used to develop K solutions for the triple point in mechanically lined pipe by developing geometry-specific Y-factors for the MLP triple point geometry. An engineering critical assessment was then performed. The calculated fatigue life agreed well with results from full scale resonance fatigue tests and demonstrates the value of using FEA to derive geometry-specific K solutions for this geometry.
