Conoco Norway Inc. wrote this article at the request of Oil & Gas Journal.
The world's first concrete tension leg platform (TLP) is being assembled offshore Norway for development of Heidrun field by Conoco Norway Inc. to meet a target for first oil production on Aug. 1, 1995.
Development of Heidrun field is in many ways a frontier project. It lies in 350 m of water in the Norwegian Sea off central Norway and is the most northerly development in Norwegian waters.
It sees introduction of the concrete floater and the use of titanium drilling risers. It is the first development offshore Norway to utilize offloading of produced oil directly into shuttle tankers without storage.
Installation methods for Heidrun's topsides and foundations were designed to avoid the requirement for heavy lift vessels. And concrete was used for the first time to make module support beams in the TLP hull.
Heidrun field is on Haltenbanken, 175 km off the coast of mid-Norway and 270 km from Trondheim. The field lies mainly in Block 6507/7, awarded under production license 095, with Conoco as operator, in the eighth licensing round in 1984.
About a quarter of the reserves are located in the adjacent Block 6507/8. This was awarded under production license 124, with Den norsk stats oljeselskap AS (Statoil) as operator, under licensing round 10B in 1986.
Heidrum field license interests are: development operator Conoco 18.125%, production operator Statoil 76.25%, Neste Petroleum AS 5%, and Norminol AS 0.625%. Total development costs are estimated at 25 billion kroner ($3.4 billion).
EXPLORATION HISTORY
The discovery of Heidrun was made in June 1985. In 1987 Storting, the Norwegian parliament, decided that Statoil would take over operatorship at the start of production.
Discovery well 6507/7-2 was completed on June 11, 1985, and encountered a 137 m oil column in the Lower Jurassic Tilje formation. In addition, the upper part of the Tilje formation and the Middle Jurassic Fangst formation were found to be gas bearing.
Heidrun is an oil field with associated gas and a gas cap. It covers an area of 38 sq km (9,000 acres). The reservoir is relatively shallow at 2,300 m below the seabed.
Estimated reserves are 750 million bbl of oil, 9.35 billion cu m of associated gas, and a gas cap of 37 billion cu m. There are three main producing reservoirs: the Fangst group, the Tilje formation, and the Are formation.
The field is located on the southernmost extension of the predominantly southwest-northeast running Nordland Ridge, which is an intensely faulted high that separates Helgeland Ridge from the more westerly Voring basin.
South of the Nordland Ridge, the area plunges into the less faulted Halten Terrace, where numerous oil and gas fields including Heidrun have been discovered.
A statement of commerciality for Heidrun was accepted by the license owners and submitted to the authorities on Dec. 18, 1986, followed by a plan for development and operations (PDO) on Nov. 24,1987. In December 1989 a PDO update was submitted to the authorities which was approved by Storting in May 1991.
TLP DETAILS
Before Heidrun, Conoco had already completed two developments using steel TLPs: Hutton field in British waters and Jolliet field in the U.S. Gulf of Mexico, where water depth is 536 m.
The Heidrun substructure consists of the concrete hull, concrete module support beams, concrete foundations, and steel tethers. The hull, which is quite different from any offshore construction before it, is built from aggregate lightweight concrete.
The hull comprises four cylindrical columns and four square pontoons connected to the base of the columns. The measurements are: distance between column center lines 80 m, column diameter 31 m, pontoon width 15.95 m, pontoon height 13 m, hull draft 77 m, concrete volume 63,000 cu m, and displacement 288,000 metric tons.
Distance between the two module support beams is 48 m. The beams are 147 m long, 7.7 m high, and 7 m wide, with a concrete volume of 6,000 cu m each and combined weight of 16,000 metric tons.
Below each comer of the TLP on the seabed will be a 21,000 metric ton concrete foundation. The four foundations will be attached to the TLP by a total of 16 tethers. Each foundation consists of 19 closed cells with skirts penetrating 5 m into the seabed. The tether connection porches rise 6 m above the cell tops in the center of the foundation. Total height of each foundation is 27 M.
Module weights are drilling module 4,650 metric tons; drilling rig 2,350 tons; well bay module 8,500 tons; accommodation module 5,500 tons; flare, lifeboat, and infill areas module 4,450 metric tons; and utilities module 9,050 metric tons.
The platform is designed to produce a maximum of 220,000 b/d of oil and 4.7 million cu m/day of gas. Oil production is expected to last 21 years.
Sales of associated gas on the order of 650 million cu m/year will commence in late 1996. Export gas will be transported through Haltenpipe to the Conoco/Statoil owned methanol plant at Tjeldbergodden.
The gas handling system is designed to reinject up to 4.2 million cu m/day. The produced water treatment system is designed to handle up to 155,000 b/d. The platform will include systems to inject up to 330,000 b/d of filtered, de-oxygenated seawater to maintain reservoir pressure. Reinjection will take place from start of production.
The drilling riser will be made from titanium to provide the same strength as conventional risers for a lower weight. Pipes in inaccessible areas have also been made in titanium.
MARINE OPERATION
The Heidrun marine operation is probably the largest for any field development in the world so far. This summer activities peaked when more than 500 people working on 10-12 vessels carried out comprehensive offshore construction work at Haltenbanken.
Two water injection templates were installed, together with flow lines and umbilicals. One gas export pipeline has been installed. This is the start of the gas pipeline to the methanol plant currently being constructed at Tjeldbergodden. Two export lines to the oil offloading system have also been installed.
In July the four 21,000 metric ton concrete foundations, which have been built by Kvaerner Concrete Construction at Hanoytangen outside Bergen, were towed out to the field and installed.
The foundations were towed to the field empty and lowered into position by flooding their central cells, with the operation controlled via an umbilical to the surface. This avoided the need for expensive heavy lift vessels.
Heidrun platform's concrete hull and giant concrete module support beams were mated in early June. Conoco is now preparing for mating of all the topsides modules and the hull.
Mating will begin Sept. 1, out in the Gandsfjord, close to the Norwegian Contractors AS construction site near Stavanger. The modules are waiting in Stavanger harbor after being loaded out from the yards during June and July.
The six modules on barges will be floated out to the hull and guided between the two module support beams. Then they will be lowered into position by ballasting the barges. Again, this avoids the need to use heavy lift crane vessels for installation.
The hook-up and commissioning contract has been awarded to Kvaerner Rosenberg AS, Stavanger, and the organization has been built up to start physical work on the platform in Gandsfjorden inside Stavanger in October. Up to June 1, 1995, when the Heidrun TLP will be towed out to the field and installed, 1,000-1,500 people will be on board the platform.
Brown & Root Highland Fabricators Ltd. of Nigg, Scotland, and LMI Gullvika AS are assembling the 275 in long tethers at the new Gullvika yard outside Trondheim. These tethers will connect the platform to the foundations, and together they will serve as the platform anchoring system. The 16 steel tethers will be towed out in one piece and installed in April-May 1995, awaiting the TLP's arrival in early June. A tether dummy has been assembled and tested in towing and upending simulations.
Finally, the two oil direct shuttle loading systems (DSLs) have been installed. This DSL system was chosen because it saved 700 million kroner ($95 million) compared with conventional shuttle systems and will simplify the offshore loading operations considerably.
Oil is produced directly via the platform to the flexible riser and through a cone which is sucked into a hole in the bottom of the shuttle tankers. When a tanker arrives, it picks up a cone about 90 in below the sea surface. The cone serves as a turret and production buoy enclosed within the tanker hull. Use of two DSL systems ensures that tanker loadings can overlap, thereby ensuring continuous production.
Throughout the marine operations the Transocean 8 drilling rig has been drilling wells. The rig completed the predrilling program of nine wells in April, as well as batch setting 30 in. and 20 in. casing in all 56 slots of the subsea drilling template. The operation was completed 5 months ahead of schedule and with a cost saving of 200 million kroner ($27 million). This summer Transocean 8 has been busy drilling the six water injection wells through the two templates.
The verification process has been comprehensive throughout the project. Immediately after Norwegian Contractors and the concrete TLP were chosen, Conoco brought the concept into the test tank at Marintek, Trondheim.
Since then, every step has been tested, verified and retested to ensure that the frontier technologies are safe and completely acceptable. During the tests it was verified that the hull dimensions needed to be changed, with subsequent strengthening of hull and modules. All marine installation procedures were also tested at Marintek.
CONTRACTS
The Heidrun project has had a considerable impact on employment in Norway. During the peak construction period a total of 8,000 people were working on a large number of construction sites.
Since development of Heidrun and Draugen, which last year became the first Norwegian Sea field to begin production, required a move of petroleum industry activities into a new Norwegian region, the government asked Conoco to ensure that the industry in mid-Norway would get a fair chance to compete for construction contracts.
A large network of companies was built up. Some companies got help to improve their procedures, enabling them to bid. During international bidding rounds, the regional companies were awarded 10.2% of the contracts at a value of 1.5 billion kroner ($200 million).
Conoco estimates it saved 70-100 million kroner ($9.5-14 million) by using mid-Norwegian companies. No subsidies were involved. The contracts were awarded on strictly commercial terms.
The Heidrun project is more than 80% Completes It is on schedule and within budget. Heidrun may be the last field development on the Norwegian continental shelf using giant concrete platforms. The trend is now for subsea or floating systems. However, Heidrun TLP is an important third generation TLP platform, which paves the way for similar structures on a smaller scale.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
