New Developments in the Design of Concrete Gravity Substructures
Authors: McNulty A.J.W. , Grigsby A. , Humpheson C. , Duan X. , Willford M.R. , den Velde W. Op 
Published: October 2008

Shell Philippines Exploration selected a Concrete Gravity Sub-Structure to support the central gas processing facilities for its Malampaya Gas to Power Project in the Philippines. Besides supporting the 15,000 te deck, which was installed by floatover, the CGS provides integral dry cell storage for 385,000 bbl condensate.

The contract for the design, construction and installation of the CGS was awarded following a competitive tender to the Malampaya CGS Alliance, a Contractors' Alliance comprising Arup Energy, John Holland and Van Oord ACZ. The contract was awarded in late September 1998 and the CGS was installed 20 months later, exactly 3 months ahead of schedule.

The Concrete Gravity Sub-Structure (CGS) for the Malampaya field was installed on a prepared seabed in 43m water in an area of high seismicity and close to a national maritime park off Palawan Island, Philippines. For environmental reasons the operator, Shell Philippines Exploration (SPEX), required the CGS to provide integral storage of 385,000 bbl condensate using a "dry cell" storage system whereby the condensate is stored at atmospheric pressure under blanketing gas.

This combination of requirements is unique. The Malampaya CGS demonstrated that concrete gravity structures can be safely designed and are economic for installation in:the most highly seismically active areas of the worldareas of uneven and/or rocky seabedsenvironmentally sensitive areas

Storage System Design

Introduction.

There are two fundamental forms of storage system that can be selected for a CGS: wet and dry. In a wet system, oil entering storage displaces ballast water which is then discharged to sea. In a wet closed system, the displaced water is pumped to the topsides for possible treatment prior to discharge. In a wet open system, the ballast water flows directly to sea. With a dry storage system, the oil simply enters an empty tank in a similar manner to an onshore tank. The vapour space above the oil is filled with an inert blanket gas to eliminate the risk of ignition.

In selecting the type of storage system, the process designers must evaluate the environmental impact of any hydrocarbon contamination of the discharged ballast water. In particular they must consider the potential for hydrocarbon droplets to form in the ballast water, for water/oil emulsions to form and for hydrocarbons to dissolve in the ballast water. In the past, designers that could ensure that the ballast water was not contaminated were able to adopt an open system. More typically however, designers adopted closed systems such that topside treatment of the ballast water could be undertaken ahead of discharge. This treatment successfully removed interface emulsions and suspended oil droplets. Recently, a number of offshore installations have also investigated technologies such as solvent extraction to remove dissolved hydrocarbons.

On Malampaya, the rigorous project environmental standards and the proximity of the platform to the Palawan National Maritime Park dictated that a dry storage system was finally selected; (a wet storage system had originally been tendered and was initially assumed for detailed design.