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Thunder Horse
How copper/nickel is an essential component of BP's latest oil and gas production
platform
By Carroll McCormick
Nickel Magazine, July 2005 -- The world's largest delivery of
copper/nickel alloy
C70600 (90/10 copper/nickel) to date for an offshore oil and gas production platform is doing duty at
Thunder Horse, the huge platform off the coast of Texas.
KM Europa Metal supplied 11,000 metres, or 700 tonnes, of seamless and seam-welded pipe and fittings in
diameters ranging from 2.54 to 91.54 centimetres (cm), with wall thicknesses of 1.5 to 19 millimetres (mm).
The applications consist of fire-fighting, cooling water for equipment such as compressors, turbines and
pumps, and bilge and ballast systems.
Towed to its drill site in the Gulf of Mexico, 200 kilometres southeast of New Orleans, in 1,800 metres of
water over the Thunder Horse oil field, the same-named semi-submersible rig will tap into reserves of 1 to 3
billion barrels. The rig is 156 metres (m) long, 114 m wide and 132 m high, and displaces 130,000 tonnes. It
will process 250,000 barrels of oil and 200 million cubic feet of natural gas per day.
The water temperatures in the Gulf of Mexico exceed the combination of service temperature and chlorine
concentration recommended for super duplex and 6% molybdenum stainless steels (that is, from 10ºC and 200
parts per million maximum free chlorine to 40ºC and 0.7 ppm maximum free chlorine).
Also, C70600 is easier to fabricate than other alloys, says Wilhelm Schleich, a technical advisor for KM
Europa Metal. "With a higher-alloy material, you have to control the heat precisely. If you have a defect in
the weld, it is more difficult to repair it. You may have to remove that section and re-weld it."
The awareness of problems associated with glass-reinforced plastic (GRP) tubes (for example, mechanical
damage, leakage and burst tubes) were encountered during construction of the platform at Daewoo Shipbuilding
& Marine Engineering (DSME) in Korea. This led to the recommendation that BP use C70600 for seawater
applications. BP is the rig owner and operator, with 75% equity ownership; Exxon is BP's partner, with 25%
equity ownership. "There were large sections of Thunder Horse that were equipped with GRP which had to be
pulled and replaced with copper-nickel," says John Shay, president of KME America Inc.
"We've introduced specs on pipes, fittings and flanges in one book," he adds, "which companies are using
as their guide."
There is another reason for choosing C70600: it is highly resistant to corrosion, which in turn allows for
a low corrosion allowance. The Engineering Equipment & Materials Users Association suggests a corrosion
allowance of 0.5 mm for piping systems designed for a service life of 20 years.
Copper/nickel, or "cupronickel" as it is sometimes called, is also highly resistant to uniform corrosion
at ambient temperatures, and the average depth of pit penetration does not increase continuously over time.
Cupronickel is less resistant to erosion and corrosion than other materials at higher seawater velocities,
but those same corrosion- and erosion-resistant materials are often susceptible to localized corrosion in
standing seawater. Cupronickel is resistant to stress corrosion cracking as well.
In addition, Cupronickel is more resistant to "bio-fouling" than nearly any other industrial material.
Marine organisms are intolerant of the cuprous ions cupronickel releases, and because bio-fouling adheres
only weakly to the passive film that develops on the metal's surface, it sloughs off.
In unchlorinated service, such as the bilge and ballast systems, bio-fouling can both increase
corrosion and produce a pressure drop as the inside surface of pipe becomes rough (in one rig in the Gulf of
Mexico, bio-fouling in 20.32-cm-diametre pipe reduced the inside diameter to just 13.97 from 19.37 cm), as
well as reduce the heat transfer of heat exchangers.
Anti-bio-fouling treatments such as chlorination can enhance the susceptibility of stainless steel to
localized corrosion.
Maintenance costs of cupronickel are lower than those associated with steel. Many of these
savings are due to lower downtime. On the far end of a platform's life, copper/nickel is a desirable scrap
metal because of its valuable copper and nickel content.
Carroll McCormick is a Montreal-based freelance writer.
PHOTOS: Associated Tube Industries
"Textbook" discussion of CuNi was drawn from "Biofouling Resistance of Cupronickel - Basics and Experience",
by W. Schleich, K. Steinkamp, KM Europa Metal AG, Division Marine Applications, Germany. |
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