Case study

Sector: Oil and Gas

Objective: Derive tolerable embedded circumferential flaws in girth welds for mainline clad pipes subjected to static and fatigue loads during installation and subsequent operation.

Client benefits: TWI's finite element analysis (FEA) based ECA allowed the client to produce a table of tolerable flaw acceptance criteria for girth weld inspection during the installation of the pipeline. This work also helped to reduce incidences of weld repair and to meet the tight pipeline installation schedule. As a result, significant cost savings were made in relation to the pipeline project and its future maintain operations.

Project background: Metallurgically clad pipes were used for the mainline pipeline in a gas production project. The pipes were manufactured from clad steel plate using a hot roll metallurgical bonded process, that included a series of mechanical and thermal heat treatment processes. The girth welds between the pipes were made with a MIG process using a 'duplex' welding wire. In order to assess the condition of the welds, TWI undertook a series of finite element analyses to derive J driving force versus applied axial strain for various sizes of circumferential flaws and carried out fracture toughness tests of the material in which flaws might be located.

Solution: By comparing the fracture toughness required from the FEA derived driving force curve with the experimentally determined fracture toughness, it has been possible to provide an estimate of the maximum tolerable flaw size to avoid failure for a given set of loading conditions.

For more information please contact: nmo@twi.co.uk