Construction of Offshore Concrete Structures in Developing Regions The Malampaya Concrete Gravity Sub-Structure
Authors: Chudacek J. , Holland John , Roberts J. , Visser L. 
Published: October 2008

Offshore concrete structures can be constructed in almost any country in the world using indigenous labour and materials. This can help oil companies meet agreed targets for the local content of a field development and in relation to technology transfer. However, construction in a developing region may be of concern particularly in terms of meeting the delivery schedule.

The Malampaya concrete gravity substructure (CGS) project has demonstrated that offshore concrete structures can be constructed to the highest standards of safety and workmanship. The CGS was constructed in a remote part of the Philippines under a fixed price, lump sum contract faster than any previous similar project. During construction of the CGS there were no lost time incidents in 5 million man-hours worked.

The paper describes those factors which contributed to this success. Data from the Malampaya project is used to provide guidance on how similar structures can be built in other developing regions around the world with confidence that the operator's objectives will be met.

Background

Attractions of Concrete Offshore Structures.

Concrete gravity sub-structures (CGSs) have been in use for almost 30 years since the first platform, the Ecofisk Tank, was installed in the North Sea. The performance of the structures has been exemplary. Concrete has proved itself to be a highly durable, maintenance-free material suitable for the harshest of environments including ice-infested and highly seismic regions1.

Dry-build CGSs are a more recent development 2. These structures are completed and pre-commissioned in an existing construction dock, a ship repair dry dock or a purpose-built casting basin, usually a simple "hole-in-the-ground". In general they have proved to be a more economical solution than jackets or floating systems in water depths up to about 150m especially when one or more of the following conditions applies:the field is remote and there are no existing pipeline export systems;in-field storage of oil or condensate is required;the platform is required to support heavy topsides;ground conditions are soft or unsuitable for piling.

The structural form of CGSs is inherently suited to installation of the deck after emplacement of the CGS at the offshore location. A transportation barge supporting the deck is moored between the shafts and ballasted down until the load is permanently transferred to the support points on the top of the concrete shafts (Figure 1). This "float-over" installation technique has proved a cheaper alternative than either mating the deck and CGS in sheltered inshore water or using a heavy lift vessel at the offshore location.

A more recent development is the concrete gravity tank (CGT) for offshore LNG facilities either for liquefaction or regassification (Figure 2). A number of schemes are now being actively considered for developments in Europe, West coast of Africa, Southeast Asia, Australia and North America.

Concrete sub-sea storage tanks (CSSTs) are also under development as an economical, unmanned alternative to floating storage and offtake systems (Figure 3).