Integrity Assessment of HP/HT Infield Pipelines: Experiences With a New Methodology Applied in the Norwegian Sea

Abstract

The subsea pipeline development for the A˚sgard and Midgard fields in the Norwegian Sea has been challenging due to high operating pressure and temperature (HP/HT pipelines), uneven seabed conditions and the potential for trawl gear interference. A general experience from the first years of operation is that it is not easy to use design information as basis for an integrity assessment of the lines. This is mainly due to the complexity of the global buckling process and the significant load fluctuations applied to the lines. As a consequence of this, analysis models established during design may not represent the actual pipeline behaviour properly, and established design limits do not fit intermediate operational load conditions and configurations observed during surveys. StatoilHydro has developed an integrity assessment methodology where analysis models are calibrated according to the as-surveyed condition, and then exposed to operational cyclic loads in order to predict both intermediate long term conditions and a final design condition. In the assessment of long term fatigue accumulation, process parameters monitored during pipeline operation are used as input. The integrity condition of the HP/HT pipelines is assessed based on a staged approach, depending on the criticality of the considered failure mode. The first level is used for screening and initial ranking. At level two the risk of integrity failure is quantified based on general design criteria, covering relevant operating conditions and the most important input parameters. If the uncertainty related to the assessment of an individual hot-spot location is assumed too high, a detail level three assessment may be specified. The operating condition of the pipeline system is expressed as the risk of failure defined by a limited number of hot-spot locations. The risk matrix concept used for the HP/HT pipelines, provides for a consistent comparison between individual failure modes, between different locations and sections, and between different pipeline systems. StatoilHydro has worked in close cooperation with DNV to develop software tools required to implement this integrity assessment methodology. These tools are now used for integrity assessment and follow-up of all HP/HT pipelines operated by StatoilHydro in the Norwegian Sea. The objective of this paper is to show how the methodology is used in practice, discuss major results and findings, and give general recommendations with respect to operational integrity assessment of HP/HT pipelines.