Hibernia, Jeanne d'Arc Basin, Canada

Hibernia is located in the Jeanne d'Arc Basin, 315km east of St John's, Newfoundland, in water 80m deep. The field consists principally of two early Cretaceous reservoirs - Hibernia and Avalon - located at average depths of 3700m and 2400m, respectively. Hibernia oil is a light sweet crude, with a density of about 32° to 34° API and a sulphur content, by weight, of 0.4-0.6%. The field contains approximately three billion barrels of oil in-place, and recoverable reserves are estimated to be at around 615 million bbl.

DEVELOPMENT OF HIBERNIA

It was decided that the Hibernia field would be developed using a special gravity-base structure, strong enough to withstand a collision with a one-million-tonne iceberg (expected to occur once every 500 years) and a direct hit from a six-million-tonne iceberg (expected just once every 10,000 years).

GRAVITY BASE STRUCTURE

In September 1990, HMDC awarded the gravity base structure (GBS) contract design to Newfoundland Offshore Development Constructors (NODECO). The detailed design was subcontracted to Doris Development Canada (DDC).

The Hibernia's novel 450,000t gravity base structure design consists of a 105.5m concrete caisson, constructed using high-strength concrete reinforced with steel rods and pre-stressed tendons. The caisson is surrounded by an icewall, which consists of 16 concrete teeth. Structurally, the 1.4m-thick icewall is supported by a system of X and V walls, which transmit the loads to the interior tiewall. The X and V walls have a thickness varying from 0.7m to 0.9m and the tiewall has a thickness of 0.9m. Put together, these walls form the icebelt. The caisson is closed at the bottom and top by horizontal slabs and the base slab has a diameter of 108m. The upper top-surface slab is about 5m above sea level.

Inside the gravity structure are storage tanks for 1.3 million bbl of crude oil.

Four shafts run through the GBS from the base slab to support the topsides facilities: namely the utility shaft, the riser shaft and two drill shafts. Each of the shafts are 17m in diameter and extend to a total height of 111m.

The utility shaft houses the mechanical outfitting required to operate the GBS system. It includes pipework, heating and air-conditioning, and electrical controls. The two drill shafts each house 32 drill slots to accommodate the wells, which will reach depths of more than 3700m below sea level, down into the oil reservoirs.

TOPSIDES

The topsides have a design capacity of 23,900m³/d (150,000 b/d), based on the 98 million m³ (615 million barrel) estimate. The topside facilities consist of five super-modules (processing, wellhead, mud, utilities, and accommodation for 185 people) as well as seven topside mounted structures (helideck, flareboom, piperack, main and auxiliary lifeboat stations, and two drilling modules).

The wellhead module for Hibernia was fabricated at Bull Arm, while the remaining components were made in construction sites located around the world - two in Italy and the remaining two in South Korea. Four of the topside mounted structures (flareboom, helideck, main and auxiliary lifeboat stations) were also fabricated at Bull Arm. The other three topside mounted structures (components of the two drilling rigs and the piperack) were fabricated in Newfoundland and New Brunswick, with some of the components being built in Alberta.

MATING

The 37,000t integrated topsides facility was transported by barges to the Hibernia deepwater site and positioned above the partially submerged GBS shafts to form the completed 600,000t production platform. This was then towed to its final site and 450,000t of solid ballast was added to secure it in place.

OFFSHORE LOADING SYSTEM

Oil stored in the GBS will be exported by means of an offshore loading system (OLS) consisting of subsea pipelines, a subsurface buoy and flexible loading hoses, feeding a purpose-built shuttle tanker.