Investigating the effects of pipe live pressure on the design of composite overwrap repairs

Pipelines are widely used in oil and gas industry both in offshore and onshore applications. After several years of operation, steel pipelines typically suffer from internal or external metal loss due to erosion and/or corrosion. More than 60% of the oil and gas transmission pipelines around the world are more than 40 years old and in urgent need of rehabilitation in order to re-establish their maximum operating capacity. Different repair methods are available for pipeline rehabilitation; the most recent method exploits the special characteristics of fibre-reinforced polymer (FRP) matrix composite material for rehabilitation. The corroded part of a pipeline is reinforced by wrapping composite material around the pipe. In 2006, two international codes ISO-24817 (International Organization for Standardization 2006) and ASME PCC-2 (The American Society of Mechanical Engineering 2011) were published in order to assist engineers in designing reliable composite overwrap repairs. For the case when the corroded pipe contributes to the load carrying capacity, the codes calculate the repair thickness based on the pipe diameter, remaining wall thickness, pipe and composite material properties, composite allowable strain, design pressure and the live pressure, which is the internal pressure in the pipe at the time of repair application. In this study, a range of design scenarios are modelled using analytical equations and Finite Element method to assess the validity of including live pressure in the design. Results indicate that the repair thickness is independent of the live pressure and hence an appropriate modification is proposed to the existing design equation.