With an increasing percentage of subsea production expected to occur in ultra deepwater, suppliers continue to push the development of technology to make ultra deepwater production more economically viable with less risk. One solution is carbon fiber rod enhanced deepwater umbilicals.
At the end of July 2008, BP contracted Aker Solutions to supply 48 km (30 mi) of subsea umbilicals for the PSVM ultra deepwater field development offshore Angola. The dynamic section of the umbilicals would be made using the company’s patented carbon fiber rod technology developed specifically for deepwater and ultra deepwater.
This will be the first time carbon fiber rod technology is used in African waters, but the technology has been tested and proven elsewhere. The first project to use the combination of stainless steel tubes and carbon fiber rods was the ultra deepwater Independence Hub in the Gulf of Mexico. There, carbon fiber rod umbilicals were installed in up to 2,900 m (9,514 ft) of water, a world record water depth for umbilicals.
Carbon fiber versus steel
Arild Figenschou, senior specialist engineer based in Aker Solutions’ headquarters in Oslo, Norway, led the research and development that culminated in commercial application of carbon fiber rods and stainless steel tubes for umbilicals.
“Traditional thinking for umbilical design is to have one system to take the force of the load (a steel umbilical tube) and to have inside it – cable bundled together,” Figenschou says. “But in water depths of 2,000 to 3,000 m (6,400 to 9,600 ft), where greater axial strength is required by the umbilical, carbon fiber rods add stiffness without adding weight.”
Illustration of the carbon fiber umbilicals that are installed in 2,700 m (8,858 ft) water depth for the Independence Hub project in the Gulf of Mexico.
Another important element in this is the use of polyvinyl chloride (PVC) in umbilicals. Instead of bundling the internal components of an umbilical then surrounding them with steel tubes of greater diameter, Figenschou and his R&D colleagues put each element in a separate conduit within a smaller diameter PVC core inside the umbilical tube. In this patented design, the squeeze load an umbilical experiences in ultra deepwater is distributed equally to all elements.
According to Figenschou, these design enhancements mean there is no need for additional armoring because the stainless steel tube acts like an armor system. “In special cases, like extremely deep water, there is greater load on the umbilical from its own suspended weight. The umbilical becomes so heavy, steel tubes alone cannot carry the load,” he says. “Traditionally, steel armoring was added to the exterior of the umbilical. This created major installation problems. Then we discovered that carbon fiber rods added to components of the umbilical provided all the axial strength needed. The density of carbon fiber rods is 1.6 compared to the 7.8 of steel.”
Technology milestone
Initially Anadarko Petroleum and Dominion Oil and Gas, and later Hydro Gulf of Mexico, contracted Aker Solutions to manufacture and deliver subsea umbilicals for the Independence Hub development. At 180 km (112 mi) total, this was the largest individual umbilical contract awarded ever. Moreover, it would be the first project to use the patented carbon fiber rod technology.
The production line of Aker Solutions’ umbilical manufacturing center in Mobile, Alabama.
The delivery consisted of 15 umbilicals to connect gas production wells to theIndependence Hub, a deep draft semisubmersible platform installed in 8,000 ft (2,500 m) of water in Mississippi Canyon block 920, approximately 123 mi (193 km) southeast of Biloxi, Mississippi. First gas flowed to the platform on July 19, 2007.
Umbilicals connect subsea wells from 10 anchor gas fields – San Jacinto, Spiderman, Q, Merganser, Mondo NW, Atlas NW, Atlas, Vortex, Jubilee, and Cheyenne – to the hub. The Cheyenne gas field has the world’s deepest subsea production tree, which lies in 10,233 ft (3,198 m) of water. The other production trees are in water depths ranging from 7,900 to 8,800 ft (2,469-2,750 m). To reach these depths, umbilicals had to resolve issues exacerbated by water temperature, pressure, and depth.
Leo Caffrey, technical manager at Aker Solutions’ umbilical manufacturing facility in Mobile, Alabama, explains the innovative design approach: “We use carbon fiber rods in umbilicals deployed at 7,580 ft (2,453 m) or more of water to provide greater axial stiffness to keep the umbilical from experiencing excessive strain (i.e., elongation), which otherwise would cause the steel tubes to yield and damage electrical cables inside the umbilicals during installation and operation. Carbon fiber rods add significant axial stiffness but just a fraction of the weight that would be incurred by adding armoring.
“Before carbon fiber rods, if you made umbilical tubes of thicker steel to give them greater strength, the added weight offset the added strength,” Caffrey says. “Eventually, you get to a point of diminishing returns.”
The umbilical for Merganser was the first to incorporate carbon fiber rods. “We knew the Merganser umbilical would lie in about 8,100 ft (2,531 m) of water,” Caffrey says. “Our clients knew this was an ultra deep location, so they anticipated a new design approach would be required for the umbilicals. They knew traditional design would not do, but of course, they also required that any new design be tested and qualified to ensure a successful project. Subsea production equipment is never cheap, but subsea production in ultra deepwater is a high-dollar, high-risk endeavor. We knew the more risk we could eliminate by building, testing, redesigning, and retesting prototypes, the greater the margin of safety we would build into the final product.”
Qualification testing
Under Figenschou’s direction, the umbilical R&D team used the Umbilical Stress Analysis Program (USAP), a tool developed jointly by Aker Solutions and the Marine Technical University.
“USAP can simulate a whole subsea system during operation of a dynamic umbilical,” Figenschou says. “It calculates the forces of friction in an umbilical, which is important for determining how the design will react to fatigue. USAP also can analyze different temperatures in different tubes within the same umbilical. This is critical because the mechanical characteristics of elements within the umbilical (such as tubes and cables) can change when their temperatures change, affecting their ability to carry loads.”
Aerial photo of Aker Solutions’ umbilical factory in Moss, Norway, where the block 31 umbilicals are scheduled for production.
Another design challenge required Figenschou and his team to determine how to anchor the individual carbon fiber rods at each end of the umbilical. Whatever anchoring method they developed would have to be tested and qualified before manufacture of umbilicals for the Independence project.
Such analyses are performed only in Oslo, where the majority of the design of the umbilical layout occurred. When final umbilical cross-section design was completed in Norway, Caffrey and his colleagues in Mobile faced the challenge of translating design into prototype.
“We made a full-size, sample-length prototype, and then cut it into test sections for the qualification tests described in the ISO 13628-5 standard specification for subsea umbilicals,” Caffrey says. “A whole series of qualification tests were conducted by Tension Member Technology (TMT) at its facility in Huntington Beach, California. The same tests were performed on the prototype designed and manufactured for each of the umbilicals in our contracts, beginning with Merganser.”
Qualification tests validate the accuracy of design. In this case, qualification tests included tensile and fatigue tests. “The fatigue test sets up a length of the umbilical prototype in a machine that simulates 100 years of normal wave motion and 100 years of hurricane wave motion,” Caffrey says. “The crush test determines whether an umbilical containing carbon fiber rods could be handled by mechanical tensioners and the lay system without failing during installation. Bend tests measured the stiffness properties of the prototype design, which are critical for successful installation.”
The first umbilical containing carbon fiber rods was delivered in March 2007. Delivery of the last Independence project umbilical was in July 2007.
In October 2007, Petrobras Americas Inc. awarded Aker Solutions a contract for design, manufacture, and qualification of 44 mi (70 km) of high-voltage power cables, as well as static and dynamic steel-tube umbilicals, for the Cascade-Chinook subsea development in the Walker Ridge area of the GoM. Both the high-voltage power cables and umbilicals will be installed in water depths up to 8,800 ft (2,700 m). The umbilicals will be made with carbon fiber rod technology. Delivery of the power umbilicals is expected by the end of 2009.
Then, in July 2008, Aker Solutions signed a contract with BP to supply carbon fiber rod enhanced umbilicals to its PSVM ultra deepwater field development offshore Angola. Delivery is expected in 3Q and 4Q 2010.
