Mechanical behaviour of lined pipelines under welding and impact

Abstract

The research presented in this thesis covers two critical problems regarding lined pipes: dynamic impact and welding. A lined pipe consists of an inner layer (the liner) made of corrosion resistant alloy (CRA), e.g. AISI304 stainless steel, and an outer layer made of low carbon steel, e.g. carbon-manganese steel, C-Mn. To manufacture the lined pipe, a special heat treatment, known as tight fit pipe (TFP), based on cooling the liner to -200°C, heating the backing pipe to +500°C and inserting the liner inside the outer pipe, was used in this work. Both welding and impact with external objects are responsible for accumulating high levels of plastic strains and residual stresses which could lead to failure in the pipe sometime after the impact or the welding. The special welding process used in lined pipes typically consists of the overlay welding (inner welding) of the liner with the C-Mn steel pipe for each segment and the girth welding (outer welding) of the two segments. To simulate this welding process using the ABAQUS code, nonlinear heat-transfer and mechanical finite-element (FE) analyses have been conducted. A distributed power density of the moving welding torch and a non-linear heat transfer coefficient accounting for both radiation and convection have been used in the analysis and implemented in ABAQUS user-subroutines. The modelling procedure has been validated first against previously published experimental results for stainless steel and carbon steel pipe welding separately. The model has been then used to determine the isotherms induced by the one-pass weld overlay and the one-pass girth welding and to clarify their influence on the transient temperature field and residual stress in the lined pipe. Furthermore, the influence of the cooling time between weld overlay and girth welding and of the welding speed have been examined thermally and mechanically as they are key factors that can affect the quality of lined pipe welding. The same FE numerical procedure to analyse line pipe welding is then applied to simulate six cases experimentally tested in the lab within this project. Furthermore, two cases have been analysed first, namely a reference case, in which the effect of the TFP pre-heat treatment is neglected, and a second one where the pre-heat treatment has been taken into consideration. During welding, the FE thermal history and mechanical strain results for both cases correlate well with the experimental ones in the region with the highest residual stresses, because the effect of initial residual stresses is cancelled in the regions subject to very high temperatures. After welding, the numerical and experimental results have proved that the initial residual stresses due to the TFP pre-heat treatment are reasonably important in the liner whereas they are practically negligible in the C-Mn pipe. The same reference case is then compared numerically and experimentally with further five parametric cases to study the effect of welding properties (weld overlay and girth welding materials), geometric parameters (using weld overlay and liner) and welding process parameters (heat input). The numerical temperature fields and residual stresses are in good agreement with their experimental counterparts for all cases. The dynamic impact problem is a crucial one for lined pipes because of the reduction in the thickness of the outer pipe ensured by the internal protection from corrosion given by a thinner liner. In this case, the lined pipe is more affected by potential impact with external objects (so-called ‘third party interference’ in the Oil and Gas industry). In general, a dent produced by a freely dropped weight is responsible to a large extent of catastrophic failure in pipelines. Therefore, in this work, 3D FE models have been developed to simulate the mechanism of vertical free drop of a weight from different heights resulting in damage in the pipe. Models have been executed using a three-dimensional non-linear explicit-dynamics FE code, ABAQUS/EXPLICIT. In order to precisely simulate the response of the pipe to subsequent impacts and spring back, an elastic-plastic constitutive law is adopted using the isotropic Hooke’s law and a Von Mises yield criterion, with work hardening based on an isotropic hardening rule associated with the equivalent plastic strain rate. Strain-rate dependent properties are specified for both materials, C-Mn and AISI304, to take into account the change in velocities during impact. The numerical strain results are reasonably consistent with the experimental ones recorded by four strain gauge rosettes positioned symmetrically around the dent centre. Numerical and experimental results are comprehensively analysed and discussed also in terms of practical implications in the industry.