October 9, 2008

The new Hebron oil platform will use a concrete base structure fixed to the ocean floor through gravity alone.

Next Hibernia oil platform will employ concrete gravity base structure (CFBS) technology

PETER KENTER

correspondent

ST. JOHNS, NFLD

The details of the recent $20-billion offshore oil deal signed between Newfoundland and Labrador and an oil consortium including ExxonMobil, Chevron Resources, Petro-Canada and Norsk Hydro have been widely disseminated.

Little is known, however, about the specific design of the new Hebron oil platform — except that it will probably be a little smaller than the Hibernia platform, and require a lot of concrete.

The design will employ a concrete gravity base structure (GBS), a weighted base that remains fixed to the ocean’s floor through gravity alone.

“We know the gravity base will be about two-thirds the size of the Hibernia GBS,” said Tim Murphy, external affairs manager, Chevron Canada Limited.

The Hebron platform will be located 310 kilometres southeast of St. John’s and calls for a GBS to be built at Newfoundland’s Bull Arm construction site, where Hibernia’s GBS was built more than 10 years ago.

The original Hebron oil platform.

Work on the concrete GBS alone is expected to generate over four million person-hours of construction employment.

“The GBS is built on dry land in an area along the shore that’s closed off to the ocean,” said Keith Ryan, chair of the Newfoundland and Labrador Construction Association.

“When the work is done, they flood the area and float the platform out to sea.”

Ryan estimated the cost of the structure will be between $4 and $6 billion.

“The quarry material was trucked to the site from a quarry some 160 kilometres away from Bull Arm,” said Fraser Edison, executive vice-president and director of Rutter Inc., who worked on the Hibernia project. Cement was supplied locally by St. Lawrence Cement.

Because concrete can’t contain seawater, it was mixed on site using fresh water.

“The concrete was pumped in place or placed by tower cranes depending on the area,” said Edison.

“We used approximately 160,000 cubic metres of concrete on the project.”

The Hibernia GBS was built using high-strength concrete reinforced with steel rebar and built through slip-forming, the likely method for constructing the Hebron GBS.

Because the platform will sit in an area of the ocean known as Iceberg Alley, the concrete structure will require extra protection.

The Hibernia platform features an ice belt that is 15 metres thick, including an external ice wall which is 1.4 metres thick.

A series of 16 sharp teeth around the Hibernia GBS ice wall are designed to distribute the force of an iceberg over the surface structure, although an ice management strategy is designed to avoid contact with icebergs.

The Hibernia GBS is 111 metres high, and the platform weighs 600,000 tonnes, with more than 400,000 tonnes of dry ballast added to the platform after it was installed, to help stabilize the structure.

The GBS contains four shafts, each 17 metres in diameter, inside the ice wall:

• The utility shaft, which houses the mechanical system required to operate the platform, including pipes, heating and air conditioning and electrical controls;

• The riser shaft, which contains four crude oil risers to export oil from the GBS to the offshore loading system, and fire water pumps; and

• Two drill shafts, which house 32 drill slots to accommodate the wells more than 3,700 metres below sea level.

Construction of the GBS is expected to begin in 2012.

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