Dewatering And Commissioning The Uk Central Area Transmission System (CATS) Pipeline
Authors: Langford G.L. , Krisa T.E. , McCrary M.L. 
Published: October 2008

ABSTRACT
A monoethylene glycol swabbing technique was used to successfully dewater the 36" diameter, 410 kilometer, Central Area Transmission System (CATS) gas pipeline in the United Kingdom, and condition it for hydrate inhibition. The chronicle and analysis of the operation provides insightful information for design of future dewatering and commissioning operations. The process worked so effectively that the pipeline was not only conditioned, but was dried such that target product water dew points were met shortly after start-up. The method was flexible and integrated well with other associated project construction work. The duration of the actual dewatering/conditioning operation was significantly less than that for the commonly used vacuum drying method. The keys to the success were the excellent sweep efficiency of the pig/glycol train and the aggressive and flexible execution philosophy.

INTRODUCTION
The Central Area Transmission System consists of an offshore riser platform in the Central Graben Area of the North Sea, a 410 kilometer long 36" diameter high- pressure subsea gas pipeline, and an onshore gas receiving and metering terminal at Teesside on the northeast coast of England. The Central Graben Development, comprising the installation of the North Everest and Lomond production platforms, and the CATS project, were executed concurrently. Please refer to ' Figure 1(Available in full paper) for a schematic diagram of the production and pipeline facilities and their relative locations. The Riser Platform, located adjacent (and bridge connected) to North Everest, receives dry gas from the North Everest and Lomond platforms and transfers the production do the CATS pipeline. The CATS pipeline has capacity to transport additional gas production from future platforms inthe central North Sea.
Referring again to Figure 1, the CATS pipeline runs from a pig launcher on the Riser Platform to landfall at Teesside, 402 kilometers away. From there, the onshore portion of the pipeline traverses a beach valve station, runs under the River Tees in a flooded tunnel, and terminates at a, pig receiver at the gas terminal. The onshore pipeline is approximately eight kilometers long.
The North Everest and Lomond platform and CATS pipeline projects were executed with a "fast-track" approach, in order to deliver gas by April 1, 1993, as required by contractual agreements. Detailed engineering kicked-off in April 1990. The pipeline was completed and ready for service on schedule.
To meet the schedule, glycol swabbing, rather than vacuum drying, was used to condition the pipeline. The glycol swabbing technique used to dewater the CATS pipeline was a major contributor to project success. The dewatering and commissioning objectives were to dewater, condition to avoid hydrates, and fill the pipeline with sales quality gas from shore, prior to platform completion, start-up, and gas production. Despite numerous equipment problems and delays due to both conflicting operations and bad weather, this method allowed these objectives to be achieved.