A high flowrate test was undertaken on the Shell/Esso FLAGS (Far North Liquids and Associated Gas System) Pipeline, located-in the Northern North Sea,- with the aim of acquiring sufficient operational data to enable prediction of dynamic pipeline behaviour at high throughputs. Based on the data obtained, a simple transient pipeline model was developed, results from which showed good agreement with the data gathered and which has demonstrated the suitability of steady-state equations for modelling transient behaviour.
The 36 inch diameter, 447 km FLAGS Pipeline is one of the longest deep water offshore pipelines in the world, with water depths up to 163 m. The line is an integral part of the FLAGS System which was developed for the utilisation of associated gas from oil fields located in the northern sector ofthe North Sea, by the U.K. market.
While designed for two-phase operation, the pipeline operates for most of its length in the single-phase region with retrograde condensation normally taking place in the last 70 km, as the system pressure falls. Should excessive liquid be produced from the line, either as steady flow or more significantly as slugs, then this could overload the capacity of the slug catcher and liquid handling facilities at the onshore treatment plant. This is a major system constraint that is reflected in the original computer simulation program which is used for day-to-day operational control of the pipeline.
Gas nominations can vary daily, whilst the line has a transit time of 2 to 3 days and hence, line-pack is used to balance sales and production in the short term. This operational situation obviously gives rise to different steady-state pressure profiles for any specific flow, as well as complex transient pressure profiles whilst line-pack is reduced or replaced. Indeed, for the majority of time, the line operates in the transient condition.
The original computer simulation model was based on steady-state behaviour with pipeline capacity being based on assessments of liquid holdup and maximum potential slug size; this has proven adequate for previous throughputs.
However, since the FLAGS Pipeline was commissioned and brought on stream in 1982, utilisation of the line has steadily increased due to the incorporation of additional gas sources into the offshore network, increased production from the existing fields and completion of the onshore gas treatment facilities.
The increase in maximum potential gas throughput, combined with an increased range of gas composition and associated pressures, must take account of the possibility of high rates of retrograde condensation which could overload the liquid handling facilities.
Operationally, two requirements were essential to ensure reliable operation and maximise throughput potential:
Incorporate into the computer simulation model transient as well as steady-state routines, such that an accurate picture of liquid condensation and holdup existing at all times can be assessed, together with the potential impact of flowrate changes