First concrete gravity unit installed in South China Sea

July 17, 2000
The 91,000-tonne Malampaya concrete gravity substructure (CGS) was recently installed off Palawan Island in the Philippines.

The 91,000-tonne Malampaya concrete gravity substructure (CGS) was recently installed off Palawan Island in the Philippines.

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It is the first such installation in the South China Sea, according to Brown & Root Energy Services (BRES), which was contracted by the operator Shell Philippines Exploration BV (SPEX) to design, procure, install, and commission the gas production platform (Fig. 1).

The CGS is 112-m long and 83-m across. It has a 16-m caisson height. Four 11-m diameter shafts extend 40 m from the top of the caisson roof.

BRES says some 32,000 cu m of concrete, 12,000 tonnes of reinforcing steel, and 750 tonnes of prestressed steel strands were used in its construction.

The CGS design provides storage capacity for up to 385,000 bbl of condensate, recovered from the gas. The storage cycle includes inert gas initially filling the CGS storage compartments. Then, produced condensate will displace the inert gas, and as shuttle tankers periodically offload the condensate through a buoy 3 km away, the inert gas will be replaced.

Plans call for the installation of the 10,500 tonne topsides in March 2001, with gas production starting by Oct. 1, 2001.

Construction site

Construction of the CGS took place at Green Beach, Subic Bay, Philippines, in a specially built graving dock that involved the excavation of 320,000 cu m of earth to create a dock with a floor 12 m below sea level. The excavation started in December 1998 and CGS construction commenced in May 1999.

CGS access to the open seas required the dredging of a channel measuring 150-m wide and 12-m deep from Green Beach to the deep waters of Subic Bay.

BRES says this activity was completed by May 28, 2000, and moved a total of 250,000 cu m of soil through a 750-m floating pipe to a designated disposal site within the bay.

Site preparation

Placement of 361 mounds of crushed rock prepared the location for the installation of the CGS (Fig. 2).

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To provide a level and even seabed off Palawan, the work placed 361 mounds of crushed rock with an average height of 1.4 m (Fig. 2). BRES says these mounds were designed to flatten out during the CGS installation to form an even layer of rock with a minimum thickness of 500 mm, thereby smoothing the seabed.

Placement of the 17,000 tonnes of rock was completed on May 16.

BRES says this operation marked the first time that a seabed had been specially prepared to accept a CGS.

Installation

On May 28, three seagoing tugs, with a combined towing force of 30,000 bhp, began the 4 day, 209-nautical-mile tow of the CGS from Subic Bay to 50 km off Palawan. The towing process included flooding the external cells of the CGS to provide greater stability for the sea tow.

At the installation site, a fourth tug assisted in holding the CGS steady. The tugs formed a star formation around the CGS, one at each corner. To provide greater positional stability, the tugs were connected to prelaid seabed anchors.

The installation process began on June 1. An 11-hr ballasting of the internal compartments with seawater allowed the CGS to descend in a controlled manner. The process entailed preinstalled seabed transponders that helped determine the exact position of the CGS as it moved through its controlled descent.

Iron ore, 75,000 tonnes, provides the permanent ballast (above, right) for the CGS (Fig. 4).
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BRES says adjustments to the CGS's position were made through synchronized changes in pulling forces on each of the four tugs, resulting in the final CGS position to be less than 3 m from its target location, 43 m below sea level.

Rocks, 3,000 tonnes, around the four corners of the base (above, left) protect the CGS from being undermined by wave and tidal action (Fig. 3).

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To complete the installation, the plans call for protecting the CGS from being undermined by wave and tidal action by placing 3,000 tonnes of rock around the four corners of the base (Fig. 3). The CGS will then be permanently ballasted down by placing 75,000 tonnes of iron ore in its open cells (Fig. 4).

BRES says this weight is designed to keep the CGS in its location even during a one in 10,000-year earthquake.