Composite coiled tubing reaching commercial stage

Two joint ventures soon will present latest products

Rick Von Flatern
Technology editor

PART II: Part II of this two-part series on composites involves coiled tubing research. Part I in the June issue focused on drilling and production risers, drillpipe, and facilities piping.

If anything is as dear to the hearts of floating platform designers as space, it is weight. More to the point, saved weight. As production and drilling facilities move into increasingly deeper water, weight saved is being measured not in pounds - but dollars - by contractors eager for ways to trim the massive expense of rig upgrades and newbuilds.

Although estimates vary according to estimator and platform type, most experts agree each extra pound to be buoyed costs builders about $12. As a result, contractors are now more likely than ever to think beyond initial costs to the indirect benefits of such things as weight reduction and product longevity.

Weight-as-cost thinking is most visible in the proliferation of composite material decks and railings on new and upgraded floating facilities. In fact, spurred on by the twin benefits of lower costs and corrosion resistance, a variety of traditional steel components are being replaced with ones made of composite.

Composite drilling and production risers, drill pipe, production platform piping, and other drilling and production staples are in varying stages of implementation or testing throughout the offshore oil and gas industry. Now, special composite material coiled tubing (CT) appears on the verge of commerciality from two US sources - Fiberspar and Hydril.

Fiberspar projects: Fiberspar, a long-time composite product manufacturer now aligned in a joint development project with Conoco and others, is testing extended lengths of 1 1/2-in. strings. They expect to have two commercial jobs ongoing in the North Sea by October 1997. One project is a 1 1/2-in. internal diameter (ID) service string to be used by NAM to do plug and abandonment (P&A) work for Shell. The second project is a 2 1/8-in. velocity string to be installed in a corrosive environment. The P&A work should be of serious interest to industry observers as the work will consist of many runs within a relatively short time. The runs are not deep and the pressures involved are not particularly high, but the work should prove a showcase of composite CT's ability to resist fatigue.
Hydril projects: The second joint effort includes divisions of Amoco, Mobil, Shell, Phillips, Dow, Dowell, Elf, and the University of Houston, and is led by Hydril. Currently in the manufacturing stage of development the joint venture says it will be market-ready by the end of the year. The consortium won matching funds from the US Dept. of Commerce's Advanced Technology Program for a five-year program to develop their product.

Different approaches

While the two groups are obviously pursuing the same market, their technical approaches have been different. Fiberspar has been in a composite spoolable tubing development alliance with Conoco since 1994. In late-1995, Conoco left the project and its licenses and patents to Fiberspar who was shortly thereafter joined by US-based service industry giant, Halliburton.

By mid-1997, the venture had developed a composite CT design with a thermoplastic liner encased in a carbon- or glass-fiber/epoxy matrix. The weight of the composite CT, despite twice the wall thickness, is 33% the weight of steel (14% in water).

Smaller IDs through composite tubulars are the result of composite layering. Unlike isotropic materials which react identically in all directions, composites react differently in each direction. Therefore some composite layers are laid down in a direction to handle axial stresses, while others must be oriented to deal with internal, or hoop, pressures. The result is a thicker wall than comparable steel tubulars and, naturally, a smaller ID.

The reduced ID and its effect on flow rates, however, said Fiberspar vice-president, Hampton Fowler, can be mitigated by reduced friction losses along the liner's smooth interior surface. Too, he said, his company's tubing is designed to withstand pressures such that equivalent or even higher flow rates can be gained through the combination of the lower friction losses and higher pressure capabilities.

Both efforts must employ liners for pressure integrity and Hydril, which uses a nylon liner and a fiberglass and carbon fiber exterior, took a significant step toward marketability recently when it successfully attached the composite outer layer to the no-stick inert liner. The task was accomplished through a proprietary process that resulted in a chemical bond. Their final product, according to Hydril sources, will be about 40% lighter than steel and nearly neutrally buoyant.

Working as they were towards the same goal of a commercially viable coiled tubing, it is not surprising both groups have followed similar paths. It is a matter of dispute, for instance, which one is axially stronger. Both claim considerable overpull capabilities. Both expect their products to have working pressures of about 7,500 psi.

Different approaches

Despite the similarities of each group's product, a very basic difference does exist between the two. Fiberspar has chosen to base its composite work on the braiding technology with which it became familiar as a long-time manufacturer of composite-material sporting goods. In contrast, Hydril uses "filament winding" processes to manufacture its product.

As is to be expected, the superiority of one over the other option is debatable. Proponents of winding contend, for example, that braiding distributes loads transversely along the fibers, while winding distributes them axially. The axial direction, they say, is the most favorable to composite strength.

Those favoring braiding dispute this assertion and counter that wound filaments are difficult to secure within the matrix and can move during use, resulting in performance inconsistency. They also say that winding results in significantly lower compressive strengths, compared to tensile strengths which can result in product damage during such operations as packer setting which require compression loads.

But such arguments may be moot since in such mechanical matters as tensile ratings, compression ratings, torque ratings, and pressure holding capabilities, each product is evincing more than sufficient capabilities for CT-type applications.

Costs and fatigue

Filament winding, all parties seem to agree, can be more difficult than braiding, even while it can be done more quickly. That speed of application, says the winding camp, means it can be done less expensively and the savings will be passed on to the customer.

"I wouldn't disagree that the winding process is probably faster than our braiding," Fowler said. But because the product is a multi-layered one, he does not believe manufacturing, or line, speed is the cost control factor. "Some of the things we have seen is that curing time is more of a controlling process than the actual run time." Final product cost has not been made public by either company, and since both can use the same raw materials in their proprietary methods, cost comparisons will probably be ironed out later in the marketplace.

But just as weight saving is an indirect cost saver on rigs with higher initial costs offset over time, the cost issue in composite coiled tubing is not a function of product cost. In the case of composite CT, the initial higher investment (considerably more expensive per ft than steel) is offset by its longevity, a function of both corrosion resistance and fatigue life.

Fiberspar reports a 1 1/2-in. section of composite CT held beyond 2% strain with 7,500 psi internal pressure, survived 5,000 cycles without failure or degradation of properties. Hydril, too, has done successful four-point bending tests, and while the results are still proprietary, it must be assumed they have been satisfactory since the company is now in the manufacturing mode and will soon be on the market.

Judgment as to the superiority of wound or braided composite CT, or if it makes a difference, will undoubtedly be handed down by the marketplace. It appears the verdict will arrive soon.