DOT 2010: MEG conditioning speeds up Frade pipeline check

Offshore staff

HOUSTON – Acergy/Halliburton adopted a novel approach for pre-commissioning of the 42.6-km (26.5-mi) Frade flexible pipeline offshore Brazil, according to a presentation at DOT International by Sean Barr, Victor Birch and Jean-Francois Saint-Marcoux of Acergy.

The 8-in. (20.3-cm) gas import/export (I/E) line runs between the field’s FPSO, in 1,100 m (3,609 ft) water depth, and a subsea pipeline end termination (PLET) in 106 m (348 ft) of water. Pre-commissioning requirements included flooding, testing, de-watering, and drying. The dryness criterion was 51 lb/MMcf (equivalent dew point of -20º C, or -4º F) in order to prevent hydrates forming once the gas was introduced at high pressure into the system.

According to the authors, the options for achieving this criterion were drying or conditioning. For a high-volume line, supplying nitrogen in large quantities for drying would introduce complex logistical, pressure maintenance, and safety issues, and the drying process would take too long. Also, there was no experience to draw on for drying a flexible gas line of this volume. Therefore, conditioning was seen as more practical.

With conditioning, not all the water is removed from the system. Hydrate inhibitor (i.e. methanol or glycol) is added to the water that remains, sufficient to modify the temperature and shift the hydrate formation curve beyond the maximum operating conditions.

After evaluating options, Acergy and Frade operator Chevron settled on MEG conditioning as the best method to prevent hydrate formation. Based on the volume of water to be left in the line, and Chevron’s stipulation of 55% MEG concentration in the MEG/seawater mix (to safeguard against corrosion), the quantity of MEG required for this operation was calculated at 135 cu m (4,767 cu ft).

To manage the dewatering and conditioning program, the Acergy Discovery was equipped with a pumping spread, for pig train loading; and nitrogen spread, comprising two nitrogen membranes and two booster pumps; and the MEG spread. The Skandi Salvador was chartered for subsea valve operations, while the FPSO would store all fluids returning from the pipeline system.

Although the 41-km (25.5-mi) static section of the gas I/E line was installed dry, the riser connection to the deepwater PLET made it necessary for the riser to be installed flooded. The gas I/E line therefore had to be flooded with filtered untreated seawater in order to open the deepwater PLET gate valve.

This was achieved by pumping seawater into the line from the shallow water PLET side. The Acergy Discovery performed this task using a single pig to separate untreated seawater in the line from treated water used for leak testing, the latter task being managed from the FPSO.

After leak testing was completed, the pipeline was bulk de-watered and conditioned with MEG from the subsea end of the flowline back to the FPSO, using a train of six bi-directional pigs, traveling at an average of 0.18 m/sec (0.6 ft/s). To determine the percentage of water versus MEG retained in the pipeline, an MEG sample point was installed on the FPSO pig launcher/receiver. Return fluids were measured using a hydrometer.

Thereafter, nitrogen pressure in the pipeline was vented down to 13 bar, leaving it in an inert state. Divers then were able to remove the multiple pig launcher from the shallow water PLET. Pre-commissioning was completed in just under 14 days, much shorter than using conventional drying methods, the authors claim.

02/03/2010