StatoilHydro operates a large number of High Pressure/High Temperature (HP/HT) pipeline systems in the Norwegian Sea. These lines connect remote subsea templates to floating processing and storage units. Flowlines are designed for a maximum temperature up to 155 °C and a pressure in the range of 390–500 bar. Design pressure for injection lines are in general 500 bar. In addition to high operating loads, the infield lines are exposed to challenging seabed conditions and a potential for interference with on-bottom trawl gear. The objective of this paper is to present the principles and the methodology used for integrity assessment of the HP/HT lines, which provides the basis for specification of optimum maintenance requirements. The risk of integrity failure, associated with any hot-spot location, is consistently estimated based on safety margins predicted for the actual design criterion and as a function of the pre-defined Safety Class. The developed methodology assumes that the pipeline configuration and associated response parameters are accurately determined for relevant operating conditions. This requires finite element models that are calibrated according to survey data and corresponding operating parameters. A cyclic load history is applied to the post-buckled pipeline model in order to simulate actual operating conditions. Design criteria for relevant degradation and failure modes are established for a wide range of operating configurations and conditions. These criteria are used both to identify potential hot-spots and to estimate the relative utilization. The relative utilization may be estimated for an extreme single event, i.e. a design condition, or due to long term degradation. The risk of integrity failure is then determined as a function of the Safety Class and the relative utilisation, expressing the consequences and the probability of failure, respectively. A risk matrix, configured according to design safety principles, determines the risk of integrity failure. A software interface has been developed to compare and to visualize pipeline simulation results, survey data and corresponding design criteria. This information is used for documentation of the pipeline operating condition, and finally, for specification and follow-up of maintenance measures. The new integrity assessment methodology has been implemented as part of the condition management system for more than 30 HP/HT pipelines operated by StatoilHydro.

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