Multi diameter pigging for Åsgard Commissioning and pigging the 710 km 42 " x 28 " Åsgard Pipeline

Statoil has designed and tested dual diameter pipeline pigs to perform various tasks during operations, precommissioning and commissioning of the 710km 42"x28" Åsgard pipeline. The concepts developed have been put through a major design and testing program to demonstrate that the various functional requirements have been met. The paper describes the development process as well as field experience from the initial pigging runs of the Åsgard Pipeline. Background The Åsgard field, currently the world’s largest subsea development, comprises of a large number of subsea manifolds connected to two processing facilities. the Asgard A FPSO oil centre and the Åsgard B semisubmersible gas centre. The Åsgard Transport Pipeline will transport dense phase hydrocarbon gas approx. 710km from Åsgard B to the onshore processing terminal at Kårstø. The diameter of the pipeline is 42”, but the water depth excluded a conventional 42” steel riser. Production will therefore be achieved using a number of 16" flexible risers connected to a subsea Export Riser Base (ERB). The Åsgard Project and partners initially evaluated a conventional 42” subsea launching facility against a reduced diameter concept. The 42” launcher enabling the use of conventional single diameter pigs, but at a high cost and high-risk development for the subsea structure. The alternative of a 28” sub sea launcher was eventually selected, which required specially designed 28”/42” pigs. The 28" ERB is connected to the 42" pipeline via a 28" expansion spool with mechanical connectors and a 28” to 42” transition piece. Operational pigs will be pre-loaded into cassettes, lowered to the seabed and connected to the 28” manifold prior to launching through the 28” section of line and into the 42” pipeline for travel the 710 km to the onshore plant at Kårstø. During precommissioning and commissioning of the pipeline, pigs have been run the opposite direction, from the landfall site, in order to perform flooding, cleaning, electronic gauging and de-watering of the line. These activities are referred to as the RFO tasks (Ready For Operation).