Operating design parameters are normally established at an early stage of pipeline development projects based on transport capacity calculations covering the entire system lifetime. Uncertainties related to such calculations cannot be fully accounted for in the selection of operating design parameters. During operation the actual utilisation of the pipeline may be found to be lower than predicted from design, providing a potential for upgrading of the system. However, this potential has to be verified by inspection data and parameters monitored during operation. In addition, this information can be used to increase the reliability of input parameters defining governing failure modes. This paper presents typical steps associated with an upgrading design methodology, which is further illustrated by actual case studies.
As an operator of several transport systems in the Norwegian and North Seas, Statoil has considered the possibility of upgrading individual lines, in order to increase the flexibility in the entire transport network. Due to future changes in operating parameters and transport capacities, upgrading studies have been performed for the Zeepipe IIA and Zeepipe liB pipeline systems, which are two major lines in the Norwegian Gas Transport Network. An upgrading study has also been carried out for the pipeline Troll Oljeror 1, which transports oil from the offshore platform Troll B to an onshore refinery. This paper outlines typical stages in the verification and upgrading process, and shows how the approach has to be adjusted for each individual system. System bottlenecks are identified and suitable inspection tools are selected in order to verify the normal operating condition prior to upgrading. This also includes calibration of the process simulation model as well as the structural analysis model. An updated structural model of the pipeline is then used to define governing limit states.
