Composites Alive And Well In Offshore Oil Applications
The petroleum industry is moving forward with many interesting projects that will increase composite demand.
By Sara Black, Technical Editor | March 2006
For the near future, at least, oil and natural gas will continue to be critically important sources of energy worldwide. As easily tapped land and shallow coastal water reserves continue to decline, deepwater exploration and production is growing and with it, demand for strong yet lightweight materials able to stand up to incredibly harsh subsea environments.
Source: Conoco
Offshore opportunities for high-performance composites abound, especially as oil prices stay high and deepwater reserves become more important.
At the recent Fourth International Conference on Composite Materials and Structures for Offshore Operations (CMOO-4), sponsored by the University of Houston's Composites Engineering and Applications Center (CEAC), offshore experts were cautiously upbeat about the future of offshore composites and cited a number of new applications — in progress and planned. During introductory remarks, University of Houston vice chancellor R. Arthur Vailas pointed out that new materials, including composites, are almost a necessity in the deepwater environment, one of the most challenging next to space. Dr. Howard Hwang of Shell Exploration and Production described a few of those challenges, which include high hydrostatic pressure, low temperatures, vortex-induced vibration (VIV), platform designs that are maxed out with regard to payload capacity and scattered fields containing low volumes of oil or gas, which make drilling and gathering of oil increasingly difficult: "Composites can reduce the weight and size of platform structures and therefore reduce infrastructure costs," he explains.
Composites have established some real footholds in offshore apps where their properties enable specific performance attributes, especially when combined with other materials — umbilicals, tethers, spoolable pipe, topside platform pipe, grating, "smart" monitoring systems and new concepts for natural gas transport are just some success stories. Composite material proponents at major oil companies say that, though it may take some time, a few recent setbacks in the areas of risers and gathering pipe can be readily overcome.
Umbilicals and tethers
Although the partnership of ConocoPhillips (Houston, Texas) and Aker Kvaerner (Oslo, Norway) — the group that developed the CompRiser and the CompTether during the 1990s — has been dissolved, Aker Kvaerner Subsea AS has continued work on composites applications. The company now is commer-cializing a new umbilical concept, a direct outgrowth of CompTether: the carbon rod dynamic umbilical, the winner of one of 14 "Spotlight on New Technology" awards given by the Offshore Technology Conference in 2005. Turid Storhaug, department manager with Deepwater Composites, a department within Aker Kvaerner Subsea, reports that umbilicals certified by Det Norsk Veritas (DMV, Oslo, Norway) are currently being assembled at Aker's facility in Mobile, Ala. and will be installed this summer in 3,000m/9,842 ft of water on Kerr-McGee's Merganser and Independence Hub MC-920 platforms in the Gulf of Mexico.
Offshore umbilicals are critical to operation of subsea systems in which a host production platform connects to wells scattered over long distances on the sea floor. Typically more than a mile in length and 140 mm to 175 mm (5 inches to 8 inches) in diameter, umbilicals are essentially large hoses or pipelines containing a cluster of smaller, high-pressure hydraulic hoses as well as electrical and optical cables that connect to subsea wellheads for remote operation of valves. Early models were simple thermoplastic tubes bound together, but with increasing water depths, Aker Kvaerner Subsea turned to a design that places the hoses and cables within free-floating stainless steel tubing cradled within plastic profiles, all covered with an extruded thermoplastic overwrap. This arrangement better resists the tensile loads induced by long, free-hanging catenary configurations. But at depths greater than about 2,150m/7,000 ft, the steel elongates under the extreme tensile loading, exceeding the tensile strength of the contained electrical cables, which can short out wellhead connections. "The large tensile loads produced in this type of dynamic service required further axial reinforcement," says Storhaug. "We found that the Merganser/MC-920 environment had surpassed the limits of our patented steel tube product."
Increasing the thickness of internal steel tubes within the umbilical cluster, or adding even more steel, would have increased umbilical weight to an unacceptable level. The company instead has enhanced axial stiffness with carbon fiber rods, nearly as stiff as steel but 80 percent lighter.
Thin carbon rods are pultruded by Vello Nordic AS (Skodje, Norway) using Zoltek Corp.'s (St. Louis, Mo.) Panex 35 commercial 48K tow (produced at Zoltek's Hungarian facility) and vinyl ester resin from Reichhold (Research Triangle Park, N.C.). The 6.5-mm/0.25-inch rods, with a tensile modulus of 150 GPa and tensile strength of 1,730 MPa, are made in 3,050m/10,000-ft continuous lengths. For the Merganser/MC-920 platform project, a total of 384 rods (about 60 tons of fiber) were required for the four-umbilical set. The rods are spooled at the Vello facility onto 1.8m/6-ft diameter reels, a size that permits easy incorporation into the manufacturing line without exceeding the material's strain capacity.
Source: Aker Kvaerner Subsea
An umbilical incorporating carbon fiber rods is being manufactured in Mobile, Ala. for a deepwater Gulf of Mexico platform.
As shown in the figure on p. 36, rods are placed along the outer circumference of and within the umbilical structure, held in place by designed indentations in the extruded internal plastic profiles that cradle the multiple stainless steel tube conduits. The rods are incorporated into the umbilical during manufacturing just like the other elements, at the same helical "lay angle," explains Storhaug. "This is an elegant, cost-effective design that has the benefit of eliminating bulky added buoyancy elements, which greatly complicate installation and add considerable project cost."
Storhaug reports that there is a lot of enthusiasm about offshore composite materials, not only among customers but even within her own company. "We get a lot of phone calls from internal groups who want to learn more. This umbilical project has been very important for carbon fiber in deepwater structural elements — we think that this is the start of something quite big," she concludes.
Another application for thin, pultruded carbon rods is reported by offshore project integrator Deepsea Engineering & Management (Epsom, Surrey, U.K.). Dan Jackson, Deepsea's commercial director, says that his company is investigating carbon fiber tethers as an alternative to polyester mooring ropes for anchoring mobile offshore drilling units (MODUs), essentially ships or moveable semisubmersible structures that drill offshore wells. Multiple tethers are needed to keep the MODU "on station," or centered exactly above the hole being drilled, typically thousands of feet below.
Source: Aker Kvaerner Subsea
The cross-section of the Aker Kvaerner Subsea umbilical shows the plastic profiles and stiffening rods that increase tensile strength.
Jackson says that the carbon tethers will perform much better than polyester because they're far stiffer at one-half the diameter. Their stiffness reduces the "watch circle" or the distance tethers allow the MODU to drift from center. The tethers are essentially carbon fiber ropes made from pultruded rods. They are manufactured by Oceaneering Multiflex & Marine Production System (Houston, Texas), an umbilical supplier. The 6mm (almost 0.25 inch) rods are bundled, then multiple bundles are pulled together to form a helical rope. The rope is encased in an outer armor layer made of steel. The overall reeling diameter is a manageable 3m/10 ft.
The 500m/1,640-ft long tethers, produced for Petrobras, are scheduled for a full-scale offshore field trial at the end of 2006, Jackson reports, adding, "Petrobras is very happy so far, with the preliminary test results."
