One of only two such facilities in the U.S., the Delaware City refinery project represents the first decommissioning of a frozen earth storage (FES) facility in the country. Commissioned in 1966 as a refrigerated cavern for liquid propane storage, precise planning of the decommisioning process proved critical, as the project held the potential for flammable gas conditions, unstable ground and nitrogen atmospheres.
The FES consisted of a 165-ft-dia by 130-ft-deep frozen earthen wall cavern. Prior to cavern backfilling operations, the system was purged of propane and inerted for several months to restore it to atmospheric conditions. To reduce the risk of losing frozen wall conditions, the project was planned for the coldest months of the year.
Crews had to re-create an impermeable layer to isolate the facility from two regional aquifers. They placed 42,000 cu yd of low-strength, low-permeability grout at approximately 1,000 cu yd per day. A sand-cement-bentonite grout was engineered for adequate strength and permeability characteristics. The team also needed to be able to pump the grout over long distances and into frozen earth conditions.
During grouting, crews used freeze rings in cold nitrogen. Due to the hazards of nitrogen and the concerns about adjacent grade stability, the team established a 75-ft exclusion zone.
After grouting, crews were able to backfill soil through square openings cut into the dome metal skin in order to fill the remaining 50-ft height in the FES.
As this was a new decommissioning process, the design eliminated conventional methods using a multidiscipline engineering approach, says Fred Rappold, vice president, MWH, Malvern, Pa. Civil engineering, mechanical engineering, chemical engineering, geology, hydrology, commissioning planning, construction management and safety were incorporated.
"The MWH and [geotechnical engineer] Hayward Baker team demonstrated how hydrology, geotechnical and chemical engineering skill sets can combine to successfully complete a unique project," Rappold adds.
Key Players: Premcor Refining Group Propane Frozen Earth Storage Decommissioning Delaware City, Del.