Garden Banks 388 Abandonment - Technical Aspects Covering Pigging and Flushing Operations
Authors: MacLeod R.B. , Pierson J.M. 
Published: October 2008

EEX Corporation decommissioned their Cooper Field Development during the summer of 1999. As part of the decommissioning activities, the subsea flowlines, gathering pipelines and export pipelines were pigged, flushed with inhibited seawater, and removed from service. All pumping operations were performed from the Floating Production Facility (FPF) "Enserch Garden Banks", which was moored at the host template in Garden Banks Block 388, in 2,200 feet of water. Unique to this phase of the decommissioning project was the use of a gel pig in conjunction with foam pigs to clear and flush the oil gathering pipeline.

Fig. 1-Cooper Field Deepwater Development (Available in full paper)

Introduction

Production from the Cooper field flowed from seven subsea wells: Three wells were located on the "Satellite B" ("SB") template in Garden Banks Block 387, approximately 3 miles from the "Host A" ("A") template, and flowed to the "A" template via a 28-inch flowline bundle consisting of six 4 ½-inch flowlines; Three wells were located on the "Satellite A" ("SA") template in Garden Banks Block 388, approximately 100 feet from the "A" template, and flowed to the "A" template via six individual 4 ½-inch flexible and hard flowlines; And one well was located on the "A" template in Garden Banks Block 388. Production from the seven subsea wells boarded the FPF via dual flowlines (one annulus and one production) that were contained in the Free Standing Production Riser (FSPR).

The FPF provided initial separation of water and gas from the Cooper Field crude production. The natural gas was dehydrated, and the resulting sales quality gas and reduced BS&W content crude oil were transported to the Eugene Island 315 Shallow Water Facility (SWF).

In addition to the incoming flowline risers, the FSPR also housed the 8 5/8-inch departing gas gathering pipeline riser and the 12 ¾-inch departing oil gathering pipeline riser. The gathering risers tied in subsea to the 12 ¾-inch gas and 12 ¾-inch oil gathering pipelines, which transported the respective products to the SWF, approximately 52 miles away. At the SWF, the incoming natural gas was metered for sales purposes, and was transferred to the gas transmission pipeline via a 1/3-mile long 12 ¾-inch export pipeline. The incoming crude oil was processed to pipeline quality, metered, and transferred to the oil transmission pipeline via a 2-mile long 8 5/8-inch export pipeline (Figure 1).

Methodology

Flowlines:

During normal operations, all well flowlines were pigged on either two or three week intervals, depending upon the paraffin tendencies of the individual well. Normal pigging operations consisted of pumping a 3.2-inch O.D. poly disc pig from the FPF to the subsea well (via the annulus flowline) and allowing the well's energy to return the pig to the FPF (via the production flowline).

For decommissioning operations, this program was modified slightly to incorporate injectivity testing during the pigging routine. The objective was to pig and flush the flowline with filtered inhibited seawater, confirm receipt of the pig at the FPF, and then confirm wellbore injectivity utilizing filtered inhibited seawater.