Engineering Critical Assessment of Triple-Point Flaws in Mechanically Lined Pipes

There is increasing demand for subsea transport of well-produced fluids through corrosion resistant pipelines such as stainless steel or bimetallic pipes. The latter are made of carbon steel (CS) pipe and a thin (typically 3.0 mm thick) internal layer of corrosion resistant alloy (CRA) such as 316L, 625, 825 or 904L. The CRA and CS layers are adhered either metallurgically or mechanically by a means of an interference fit. Although less mature than other products, mechanically lined pipes (MLP) are more readily available and economical than both stainless steel or hot-roll bonded (HRB) metallurgically clad pipes. One of the gaps that remains to be filled relates to a reliable assessment method for confirming the MLP integrity during offshore installation and subsea service. The CRA liner in MLP is metallurgically bonded to CS at pipe ends by overlay welding, typically deposited using alloy 625 consumable. At the triple-point interface between the liner, overlay and host pipe, cracks may initiate from fabrication flaws and grow during installation or in service. Therefore, the engineering critical assessment (ECA) should be carried out to evaluate the risk of triple-point cracks breaching the CRA layer at any stage of the pipeline’s life cycle. Currently, no recognised ECA approach exists to allow completing such an assessment. Therefore, a bespoke integrity assessment procedure has been developed and validated both numerically and via laboratory testing. This paper outlines the ECA procedure for triple-point flaws in subsea MLP pipelines, which is undertaken through a combined analytical and numerical calculation and small-scale fracture testing. Fatigue crack growth due to cyclic loading is estimated using a geometry-specific stress intensity factor (SIF) solution derived by finite-element analysis (FEA). Ductile tearing during fracture load events is quantified by small-scale fracture testing on specimens with a representative geometry, designed to match the crack tip constraint of triple-point cracks in MLP.

Nanoscale Interphase Characterization of Agglomerated MWCNT in Composites Connected to Mode I Fracture

The authors investigate the effect of carbon nanotubes (CNT) on the microstructure, nanomechanical properties, and fracture performance of three-phase polymer matrix composites (PMC). Two types of carbon fiber (CF)-Epoxy-CNT composites with different nanofiller distribution were studied at the nanoscale with PeakForce Quantitative Nanomechanical mapping technique (PFQNM) and macroscale with mode I fracture testing to clarify the relationship between nanofiller interphase properties and mode I fracture performance. CNT agglomerates were identified on the polished sample surface in well-dispersed and agglomerated form. AFM data showed the inhomogeneity of nanoscale local mechanical properties in CNT-rich zones. Variation in material properties is attributed to voids, CNT alignment, and changes in density of the matrix and CNT nanoparticles. A higher resolution AFM scanner and Field Emission Scanning Electron Microscopy are necessary to observe nano-scale interphase mechanical properties and CNT orientation, respectively. Mode I interlaminar fracture testing demonstrated the effectiveness of CNT nanoparticles in preventing crack-jump and fiber-bridging effects. GIC for FCNT is 0.345±0.06 N-mm/mm2 at crack initiation, compared to 0.28±0.03 N-mm/mm2 for the plain epoxy reference sample. CNT nanoparticles increase the energy required for interlaminar fracture by promoting crack deflection and strengthening the interphase between CF and epoxy matrix through increased interfacial surface area.