Conceptual Comparison of On-Platform Versus Onshore Gas Dehydration
Authors: Ungs Ronna T. , Inman Paula R. 
Published: October 2008

The presence of a gas interval detected during drilling of an oil production well on Forest Oil Corporation's Osprey Platform created a dilemma regarding effective management of this unexpected resource. A screening study was conducted to analyze design, operating, and cost issues associated with onshore and on-platform dehydration of the produced gas for use as fuel at Forest's Kustatan Production Facility. Although on-platform dehydration using a desiccant was initially preferred to minimize concern for hydrate formation and corrosion, subsequent technical and practical considerations altered the outcome in favor of transporting wet gas some 1.8 miles under the Cook Inlet, and dehydrating onshore. This paper presents a qualitative look at key technical and economic issues affecting selection of onshore versus offshore gas dehydration. Although the scale of this single-well operation was small (4 MMSCFD), the issues evaluated apply to many other types of platform production and export systems. These include the relationship between process conditions and dehydration system effectiveness, spatial constraints and structural concerns, water slugging and wet gas transportation considerations, pressure profile and freezing concerns, system relief and venting requirements, potential onshore and offshore operating problems, and pipeline corrosion prevention.

Background

Forest Oil Corporation's Osprey Platform was built to support drilling and production for up to 28 wells to be placed in the Redoubt Shoal development. The platform was constructed in concert with the Kustatan Processing Facility (KPF), situated on-shore approximately 1.8 miles distant. KPF was designed to incorporate two fluid trains for produced oil phase separation, and included three turbine-generator sets for power supply to both the on-shore facility and platform.

Platform.

Osprey Platform, a pile-supported exploratorydrilling structure, was built in Korea in 1998 and 1999, and was installed in the Cook Inlet near the West Forelands Peninsula during the summer of 2000. The platform layout provided for four well rooms corresponding to the platform legs, each provided with slots for a combination of up to 7 production or injection wells. A cantilever was added to the platform in the summer of 2001 to allow for electrification of electrical submersible pumps (ESPs) for the oil production wells. The platform was connected to shore via three pipelines (one for oil, one for test/gas, and the third for water injection) and a power/communication cable bundle.

Figure 1 - Osprey Platform Location (Available in full paper)

A study was conducted prior to platform design to determine the extent of on-platform processing, mainly, whether produced fluids would be transferred as multiple or single phase. The approximate cost to build a platform with separation capabilities ($300-500 million) overwhelmed that for a small exploratory platform, such as the one that was finally built and installed. The small platform was also suitable for installation in the depths of the Cook Inlet. Operating philosophy of the new platform was to send all produced liquids to shore at conditions above the bubble point pressure (1450-1600 psig).