- Statoil's proposed SWATH design could not handle the heavy loads without costly strengthening and loss of some seakeeping advantages, so the company is looking at options.
Specialized liners find place among alloys on surface, downhole
An entire technology is developing around the lining and cladding of wellheads, flowlines, injection lines, and pipelines that transport production or solutions and gases injected into the reservoir. Traditionally, producers have simply resorted to solid alloy castings and forgings to deal with fluid and gas streams loaded with corrosive and erosive components. Even injection lines had to be able to withstand halide solutions. Alloys are expensive, especially for entire fields with hydrogen sulfide, carbon dioxide, high temperatures, and other troubling conditions.
That scene is changing today. Alloys are still popular, but producers increasingly are looking at lining technology as a less expensive option, especially now that the process of holding the liners in position has improved dramatically. The mechanism of holding them in place is critical, since joint leaks can produce catastrophic conditions, or at the least, send the stripped off cladding downstream to be sucked into turbine or pump blades. Refinery and process operators are sensitive to this condition and conduct many measurements to determine if cladding or lining materials are weakening. Offshore, such measurements are not as easily conducted, nor is the pipe or equipment so easy to replace, hence the preference for alloys when economic.
Liners can be made of materials ranging from low and high density polyethylenes and polyurethanes up to tough inert metals. They are inserted, pressed, molded, swaged, or welded to the inside of production equipment and pipelines. Even trees with 6-8 different diameters, hanger surfaces, sensor probes, and valve holes can be lined effectively.
Although cladding of production trees and equipment is done more often in Houston than elsewhere, much of the development of pipeline lining is taking place in the North Sea, where producers are facing difficult conditions on very commercial fields.
McDermott Marine has used new technology from British Gas and British Steel to install lined pipelines. The firm used swaging (called swagelining by British Gas, which developed it) to seal an inserted polyethylene liner within a carbon steel pipe for the Foinaven injection lines (See article on this project in this issue). Basically, the polyethylene liner is reduced in size temporarily, pulled through the carbon steel pipe, and then swaged (high pressure heat pressing) to the ID of the steel pipe. British Gas and many other companies today use the process to replace damaged and aging distribution lines onshore. Offshore, the difference is the use of new compression fittings to ensure sealing during pipelaying and after many years of operation.
The liner process is being used for insulation purposes as well, since less attractive fields with heavy crude are being targeted for development. British Steel manufactured a Hydrotherm insulated pipe-in-pipe flowline for Texaco's Erskine gas field in the North Sea. The objective was to keep production at 150 degrees C through the length of the line.
A 16-in. OD flowline was inserted in a 24-in. OD sleeve pipe. Aluminum silicate microspheres were inserted and compacted between the flowline and sleeve for thermal insulation. Polyurethane-lined stops, developed by PPL Polyurethane, were inserted to hold the flowline to the outer sleeves during pipelaying, which was done in an S-Lay fashion. While a seal was not as critical to the line, ensuring insulation capacity was, and the pipelay process was most critical to preserving position of the liner. McDermott and ETPM were the contractors for that project.
Load bearing problem for SWATH vessels in wellhead work
The advantages to single water plane twin hull (SWATH) vessels are their all-weather seakeeping abilities and speed in transit. A disadvantage is that the hull is weight sensitive with respect to seakeeping and cannot easily support or suspend a heavy load on the centerline. That disadvantage played a role in the model tank testing of Statoil's proposed SWATH vessel for well completion and intervention work in the North Sea.
In concept, the Statoil SWATH was an efficient vessel and could replace more expensive semisubmersible drilling units for completion and workover activities. In model testing, with full deckloads (drilling package) and suspended loads (production tubing), the design was insufficient. In testing of the Statoil vessel and others previously, it was found that seakeeping was lost in some cases and in others, the stresses on the deck crossmembers linking the twin hulls exceeded what would have been safe.
To remedy the stresses, designers would have needed a larger SWATH or to install cross bracing close to the waterline and bulk up the crossmember density on the current model. With the latter remedy, the SWATH cross-framing would come to resemble a box, which is exactly the design strength of a semisubmersible, which uses both box deckframing and crossmembers below the waterline.
However, a larger or bulkier SWATH design would cancel some of the hull advantages and push vessel costs into the small semisubmersible range. Statoil is looking at either a monohull vessel, which can better support a drilling package, or a small semisubmersible.
Elsewhere, Global Industries is completing a 200-ft long SWATH vessel, which will be used for subsea and topside construction support. Lifting will be done with a light crane over the center or one positioned over one of twin hulls where support is available.
Resistivity measurement used to evaluate composite strength
Composite materials are advantageous to use offshore because they are half the weight of steel and easy to mold, machine, and fasten. Until now, however, there has been no reliable method of measuring bond strength in composite materials, and thus no way to predict loss of strength or structural integrity over time.
Enter resistivity measurement, that 80-year technology that has served many industries, including well logging, over the years. Researchers D. I. Chung and Xuli Fu of the State University of New York at Buffalo have extended a process used to measure bond strength in steel and carbon fiber reinforcement in concrete to composite materials.
Previously, the ability to pull out wire fibers from concrete provided a measure of internal bond strength. However, the method has some drawbacks. Recently, the use of resistivity measurement between selected fibers, repeated from time to time, was found to provide an equally good measurement without damaging the component.
Researchers now attach wires to carbon fibers in composites and measure the amount of resistivity to current movement. The resistivity correlates accurately to bond strength and can sense the beginning of bonding loss.
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