HEIDRUN —A Breakthrough For Concrete Technology

In December last year, Norwegian Contractors was awarded an EPCI contract for the concrete hull of Conoco's Heidrun TLP (Tension Leg Platform).

The contract includes engineering, procurement and construction of the hull and two module support beams, as well as field installation of the completed platform.

Heidrun is the world's first TLP with a concrete hull, and represents a breakthrough for concrete as a building material in floating offshore platforms. The combination of the TLP concept, originally developed by Conoco, and the use of concrete will give Heidrun a long lifetime with favorable operating and maintenance characteristics. A particular light aggregate concrete will be utilized to reduce weight.

In the hull a considerable amount of mechanical equipment will be installed for topside and hull systems.

Utilization of special materials as titanium and GRP-piping is required to satisfy the 50 year life time of the platform.

Large Platform The Heidrun TLP is a large platform with topside weight of 43,200 tons. The total platform displacement is 288,200 tons. This is four times the size of the Hutton TLP in the British sector of the North Sea and 15 times as much as the Jolliett TLP in the Gulf of Mexico—both of which are operated by Conoco.

The bottom section of the hull will be built in a drydock at Norwegian Contractors'yard in Stavanger, and towed out into the Gandsfjord in mid-October 1993. Slipforming of the four platform legs will start immediately after and be followed by a hectic installation phase for the mechanical outfitting in the top of the four columns. Eight decks will be located—2 in each of the 4 columns.

The module support beams will be built in parallel with the hull and start of construction will take place in the summer of 1993. Mating of the beams and the topside modules to the hull will take place during summer of 1994, immediately followed by hook-up and commissioning.

Extensive Marine Operations The Heidrun platform represents a unique project for Norwegian Contractors (NC). The amount and complexity of the mechanical engineering involved is actually greater than the engineering of the structure itself.

In addition the marine operations will be the most extensive that NC has ever performed. Numerous mating operations are planned, as are complicated and challenging operations in connection with the installation of the tether foundations and the platform itself.

After the foundations have been installed, the tethers will be towed out and connected horizontally.

Then follows the towout of the platform itself, performed by a fleet of ten tugs, for installation of the tethers.

This will give the company considerable experience and quite a reference.

Mechanical Installations The mechanical installations are as complicated as they are extensive.

The concrete hull contains storage tanks for crude oil and ballast water, as well as fire-water and sea water pumping systems. In addition, much of the instrumentation is to be installed inside the hull.

The Merits Of Concrete The Heidrun contract represents a breakthrough for concrete as a building material for the demanding offshore industry. Concrete has successfully competed with steel for more than 20 years as a building material for seabed-installed structures, but this is the first time that concrete has been preferred for use in a floating platform.

A TLP the size of Heidrun would have been extremely difficult to fabricate in steel, due to the necessary thickness of the steel plates and the resulting welding problems and expenses.

Concrete, on the other hand, is a relatively inexpensive building material that offers flexible solutions and a virtually maintenancefree structure.

Production Start 1995 Installation of the seabottom foundations also in concrete will take place during the summer of 1994. Field installation of the TLPplatform is scheduled for June 1995, after completion of tether installation.

Tension Leg Platforms A tension leg platform—TLP—is a floating structure consisting of a hull with topsides and integrated production facilities, permanently anchored to the seabed by means of tensioned steel tethers. The tethers firmly hold the floating structure in a stable, partially submerged position.

This eliminates nearly all vertical movement of the platform. NC has developed concepts for concrete TLPs for the North Sea, for arctic regions and for areas such as the Gulf of Mexico.

Conoco Norway Inc.

Conoco is an international oil company with more than 20,000 employees in the U.S., Europe, Africa and Asia. The company was established in 1875, but has since 1981 been part of the Du Pont corporation.

Du Pont is one of the world's largest chemical companies, with 140,000 employees. Conoco Norway Inc. has been actively exploring for oil and gas since it was founded in 1971.

Conoco is part-owner of several fields on the Norwegian continental shelf, such as Statfjord, Murchison, Ula, Gyda and Troll.

For further information contact: Norwegian Contractors A/S, An Aker Company, Holtet 45, N-1320 Stabekk, Norway.