Design of Mechanically Lined Pipe MLP for Lateral Buckle Zones – MLP Qualification and Calibrated Wrinkling Criteria Using FEA

Abstract

Mechanically Lined Pipe (MLP) is a pipe with a layer of corrosion resistant alloy (CRA) mechanically bonded to carbon steel pipe. MLP is mostly used in offshore pipelines for HPHT (high pressure and high temperature) conditions with corrosive contents. These type of pipe joints are largely used for HPHT offshore pipelines conveying corrosive fluids, but mostly limited to ‘non-buckle’ zones due to the concerns of liner wrinkling, fatigue damage and triple point failure resulting from high strain and stress ranges in the lateral buckle zones. Recently, MLP's have been used for the entire length, including in the buckle zones of a dual HPHT multi-phase infield flowlines. Planned buckles have been designed at specific intervals to ensure controlled lateral buckling. Extensive qualification procedures are required to be undertaken to validate the use of these types of pipe at the buckle zones with high strains and stress ranges during repeated shut down cycles. The qualification procedure for MLP at the buckle zone are discussed in this paper. The paper also presents an optimized formula, calibrated based on the qualification procedure and Finite Element (FE) analysis for determining the optimized onset strain for liner wrinkling. Initially, a thorough review of the concerns in the industry for employing MLP at the lateral buckle zone was undertaken and include the requirements for qualification/testing currently specified in the industry, such as bend test/liner wrinkling, and and full-scale fatigue tests. Liner wrinkling is caused by excessive compressive strain that occurs during de-pressurisation of the flowline. To assess the risks of wrinkling, the compressive strain for the onset of liner wrinkling is evaluated using analytical calculations. This is followed by a comprehensive discussion on the qualification steps adopted for addressing the fatigue, triple point failure and wrinkle that enabled to use MLP at buckling and fatigue sensitive zones. The qualification procedure undertaken has demonstrated better fatigue performance and wrinkling onset strains considerably higher than those evaluated analytically. The findings from the qualification and material tests are used to calibrate the ABAQUS FE analysis simulations to obtain optimised wrinkling onset criteria. An optimized wrinkling onset criteria for a range of D/t ratios defined based on Finite Element (FE) analysis results. In summary, this paper provides reliable guidance for the qualification of MLP at the lateral buckle zones. The paper also presents the methodology and results for determining an optimized strain for the onset of liner wrinkling based on ABAQUS FE analysis.