Strain lntensification Due to Material Discontinuity at Field Joints Adjacent to Thick Wall Insulation Coating

High pressure/high temperature (HP/HT) subsea pipelines are increasingly being designed to buckle in a controlled manner in order to relieve axial loading. HP/HT pipelines often require insulation coating to prevent hydrate formation and to help manage internal corrosion. Thick wall polymer coatings are a common choice of insulation material in such cases. A change in local bending stiffness, for example due to the change in external coating at field joints, can result in strain localisation leading to a higher local curvature than would have occurred in an otherwise homogenous system. Industry knowledge of the influence of thick wall polypropylene is limited. The proportion of total bending stiffness attributed to the coating is much larger than typically observed in non-insulated coating systems. The polymer coating is likely to behave in an elastic manner even at elevated strain levels, i.e. those typically observed in lateral buckles. It is shown herein that significant strain intensification can occur at field joints even when the field joints appear to be of a similar configuration to that of the nominal linepipe coating. This paper presents the results of finite element (FE) analyses (using solid body elements) that have been employed to demonstrate, in highly loaded conditions such as laterally buckled pipelines, the influence of thick wall insulation coatings on strain intensification at field joints. Furthermore analytical equations have been derived as an alternative methodology for calculation of strain intensification at field joints. It is demonstrated that, even in highly loaded conditions, the analytical relationship correlates very well to results determined via the FE technique.