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
"Smart" composites part of systems monitoring
One area where composites have found a niche is in monitoring systems where they are combined with other materials and sensors. An example is a composite "shape sensing mat" developed by U.K.-based Insensys Ltd. (Hamble, Southhampton, U.K.) for use with a metallic riser system. The flexible mat, which incorporates fiber optics, wraps around a steel riser and enables operators to monitor excessive bending and fatigue life during riser deployment.
"The idea is to incorporate the fiber optics into a structure that can be mounted anywhere on any type of pipe. It's a carrier mat designed to transmit strain data," says Damon Roberts, Insensys' founder and VP. An Insensys mat was installed on the lowest point of a completion-and-work-over riser string on the Enterprise Endeavor, a dynamic positioned drill ship (DPDS) moored in the Gulf of Mexico's Thunder Horse field, during July 2004. The 5.5m/18-ft long mat, affixed to the riser joint by means of straps, extended about 180°, or about half way around the circumference of the pipe. Roberts says the mat successfully reported data to the surface during multiple trips to the sea floor and back during riser deployment (i.e., as the riser was lowered through the water column, joint by joint, then pulled up again), in more than 1,830m/6,000 ft of water.
Source: Insensys
A composite carrier mat with embedded sensors is used to monitor bending strains in a metallic riser system.
Reported data consisted of direct strain measurements, obtained with Bragg grating strain gages. The tiny gages, about 5 mm/0.2 inch long, are placed at various points along fiber-optic cables, themselves only 0.25mm/0.01 inch in diameter. When light pulses are passed through the cables and thus through the gratings, any bending of the pipe changes the wavelength of the light transmitted or reflected by the fiber, in a linearly proportional relationship. By placing the sensors around the circumference of the riser pipe, at different "clock" positions, Roberts explains, it becomes possible to monitor actual pipe strain: "The strain differential shows the magnitude of the bending and hence the strain in the riser string." Real-time light wavelength monitoring is accomplished with an opto-electronic "interrogation unit," also part of the sensing mat.
The benefit of composites for this application is that the fiber optics and sensors can easily be embedded within the layup, to form a flexible yet strong sensing "mesh" in the desired configuration. For the Enterprise Endeavor project, the mat was fabricated with multiple plies of woven biaxial E-glass fabric in an epoxy matrix, using a vacuum-assisted resin transfer molding (VARTM) process in an open, curved composite mold. Fiber-optic cables with Bragg grating sensors were located at three stations along the length of the mat, in the middle of the layup thickness. The biaxial fiber architecture makes the mat flexible longitudinally, enabling it to conform to the pipe shape, while maintaining enough stiffness in the hoop direction to hold the cables and sensors in place and keep them separated properly, says Roberts. At the upper edge of the mat, cables were gathered together in a customized "exit structure," that was potted for water and pressure resistance.
Roberts says the concept has tremendous flexi-bility for many different types of structures: "We could do multiple units on a single long riser string, or a very long mat covering several riser joints." The mats don't need to be directly in contact with the pipe, but can be installed over deepwater insulation units, he continues, as long as such units mimic the general shape of the pipe. They can even be placed onto pipelines remotely, by an underwater ROV (remotely operated vehicle).
While the composite will likely absorb approximately 1 to 2 percent water (by weight), says Roberts, the structural performance knockdown (if any) isn't an issue: "The application isn't structural, it must simply maintain enough strength to hold the sensors in place."
Roberts' company isn't the only one that envisions "smart" composites for offshore applications. SMARTEC SA (Manno, Switzerland) uses the phenomenon of Brillouin scattering — when refracted light changes its path slightly due to variations in density, which can be caused by temperature gradients — in its SMARTape and SMARTProfile sensors. The sensors can be incorporated into the walls of coiled tubing or pipelines. The company is a partner with Smart Pipe Co. LP (Houston, Texas). The latter is developing self-monitoring pipelines that use the temperature gradient changes to detect leaks. One of its applications is a pull-through system that acts as a liner to remediate existing oilfield pipelines — special machinery literally folds the flexible pipe liner into a C-shape for easy installation, says Smart Pipe's Steve Catha. Another partner is Airborne Composites (Leidschendam, The Netherlands), which sells PDT-Coil, a smart downhole coiled tubing complete with power and data transmission capabilities for drilling or workover applications.
Spoolable pipe
The concept of spoolable composite piping has been around since the 1960s. Conoco was the first oil company to push for a commercially viable product in the mid-1980s. Although they were envisioned for high-pressure downhole and offshore uses, composite spoolables, for the present, find greater use in onshore gathering systems, says Mike Feechan, VP of operations at Fiberspar LinePipe LLC (Houston, Texas), owing to their corrosion resistance and the fact that they can be produced in long, continuous lengths that reduce connections.
Spoolable composite pipe consists of a thermoplastic liner overwrapped with a structural laminate of glass or carbon fibers in an epoxy matrix, which is then covered with an outer sacrificial wear layer of either unreinforced or glass-reinforced thermoplastic. In "bonded" spoolables, the thermoplastic liner is directly bonded to the structural laminate, while "unbonded" pipe has multiple discrete and unattached structural layers (composite and/or metallic) over the liner that can slip in relation to each other, allowing higher spooling strain and generally higher pressure capacity.
Fiberspar manufactures its bonded pipe by first extruding the thermoplastic (usually high-density polyethylene or crosslinked polyethylene) in the appropriate size, typically 62 mm to 112 mm (2.5 inch to 4.5 inch) in diameter. The liner acts as a moving production mandrel, pulled through a system of orbital filament winding devices that wind the wet-out glass or carbon fibers around it in a helical fashion. The production rate is typically 3m/10 ft or more per minute. The proprietary fiber architecture, winding angles and wall thickness of the structural laminate have been optimized in order to accommodate high spooling strains and pressure loads, says Feechan.
More than 1.8 million m (6 million ft) of spoolable pipe have been installed onshore in North America in the last five years, in applications such as wellhead production gathering lines, flow lines and injection lines. The composite product is displacing steel primarily because of speed and ease of installation — more than a mile of pipe can be spooled onto a single reel. "Experimental trials are one way to show feasibility and gain acceptance," he says, pointing out that Fiberspar both manufactures and installs its products, using appropriate equipment compatible with the composite. He expects that more than 1.2 million m (4 million ft) of additional spoolable line pipe will be installed this year and reports that the company has tripled its capacity to accommodate high demand. One growing use involves pulling the spoolable pipe through leaking steel pipelines, as a remediation measure, creating an impermeable and corrosion-resistant liner.
Source: Conoco
A CompRiser composite riser joint with titanium liner was successfully testing in the North Sea in 2001. The future of composite risers remains to be seen.
Opportunities for offshore spoolable applications are growing slowly. Statoil (Stavanger, Norway) tried a composite spoolable pipeline project several years ago with mixed results. Bjorn Melve, Statoil's composite champion, says the company qualified an offshore spoolable pipeline for 380 bar/550 psi glycol service at a water depth of 300m/980 ft, in the North Sea's Åsgard field. The now-defunct NAT Compipe AS (Tau, Norway) manufactured the 6.5 km/4 mile-long continuous pipe in the late 1990s. Pipe laying was complicated because of its buoyancy and the use of installation equipment and methods designed for steel, says Melve. But the biggest issue was the pipe's extreme elongation under pressure loading. "The internal pressure caused a 0.41 percent strain," he reports. "The resultant buckling made the pipe jump completely out of the sea floor trench in some areas." Unfortunately, the pipeline was abandoned because of uncertainty regarding its performance. But, says Melve, an appropriate design with the correct winding angle and wall thickness can result in an elongation matching that of a steel pipe of similar size for any type of pressure application.
