Composites cut riser weight by 30-40%, mass by 20-30%

Composite drilling risers blazing trail
for production and tensioner use

Leonard Le Blanc
Editor

The only metal components on a composite drilling riser would be the two flanged end sections and thin metal liners inside the central drilling core body and the choke and kill lines on the outside. All other components should be composites (Drawing by ABB Vetco Gray). [18,965 bytes]
Design features of a composite drilling riser being manufactured for the Heidrun drilling program in the North Sea (Source - Murali et al). [38,073 bytes]
This ABB Vetco Gray composite drilling riser joint will be tested offshore later this year. [11,604 bytes]
Three types of mechanical interlocking methods (Source - Murali et al). [28,829 bytes]

PART II: This is Part II in a multi-part series on the challenges confronting conventional drilling risers and prospective solutions. Part I, featured in December, dealt with riserless drilling. The series will continue in subsequent issues.

The riser linking the drilling vessel with the blowout preventer stack on the seafloor becomes increasingly important in terms of downtime and rig day rates as water depths climb past the 5,000 ft mark. The weight of the riser string with drilling fluid surpasses 2,000 tons, mandating the use of larger drilling units and reducing the deckload capacity needed for consumables (pipe, mud, etc).

To reduce tensioning required to support the riser at the surface, engineers design in buoyancy along the length of the riser. Unfortunately, buoyancy adds cross-section and mass to the riser. These additions, coupled with riser length, result in amplification of two riser responses:

Larger lateral excursions, proportional to current speeds and depth of occurrence.
Larger dynamic response to heave-induced axial motion, usually in hang-off mode, but also present during large variations or cycling in tensioner loads.

As currents manipulate riser mass, landing the string on the wellhead becomes difficult, and even after landing, high current speeds (1-3 knots) bow out the riser string. Unlike risers used for production or tensioning, drilling risers must accommodate the vertical and rotating motion of the contained drillstring and tool assemblies. With increasing bow in the riser, continuing contact between the riser ID and drillstring and tools can turn into wear and damage. The antidote is to straighten the riser by increasing surface tension, which consumes more deckload weight capacity, which requires a larger rig.

Weight reduction

To halt the riser weight-mass-rig-cost spiral as water depths surpass 7,000-8,000 ft, one solution is to reduce the weight of the riser, in turn reducing deckload at the surface and riser mass in the water column (less buoyancy needed). A riser with less mass and a smaller overall diameter is less subject to lateral excursion, to bowing out in currents, and is easier to handle in hang-off mode.

To reduce riser weight, the steel walls in the joints must be replaced entirely or partially by lighter materials. Titanium is presently being used for at least one drilling riser application, but the cost is high and the weight savings is not as dramatic as a combination of titanium/composites or steel/composites/titanium. Molded and cured composite laminates are among the lightest materials capable of withstanding large external and internal pressures, while retaining the flexibility needed to withstand tension, compression, and torsional forces.

Ceramics are almost as light, but they lack the flexibility of composites. Some ceramic composite materials have potential for use as riser joint mating surfaces, but they are far back on the development curve and could be costlier to produce in the required shapes and forms.

At present, steel or titanium, are the only materials considered for the mating sections of riser joints. Composites are not capable of dealing with the forces present at the joints, but other lightweight materials may become an option in the future if raw material costs come down.

Calculations show that composite use in place of steel in the mid-sections of riser joints can result in a 30-40% weight savings. In addition, when stacked, a riser with less cross-section occupies less space. The combination can allow a generation reduction in the class of the mobile offshore drilling rig, with a commensurate day rate savings of $20,000-40,000/day.

Longevity

Just as when they were first introduced for aerospace use, the longevity of composite materials on riser joints remains a central concern. Not only is the drilling riser exposed to physical abuse, but exposure to an environment loaded with chemical extremes - halides externally, and caustic or acidic chemicals internally.

Thus far, composite riser joints have survived separate stress and immersion tests. However, the variety of forces acting on a drilling riser cannot be replicated by a test stand. Physical or chemical stresses could weaken the laminations, if not the impregnated materials themselves.

This is the major reason drilling contractors and operators are moving through the test regime for composite-wrapped riser joints at a measured pace. Exposure of the joints to full drilling stresses internally and externally in deepwater will be the true test, and this is expected to come later this year or early in 1999. Drilling risers for both low pressure (blowout preventer stack on seabed) and high pressures (blowout preventer stack on surface vessel) will be tested.

Production first

For deepwater producers and riser manufacturers, the initial investigation of composite risers was to replace steel in production risers, not drilling risers. For tension-leg platforms, riser weight savings was a major target for researchers, and the source of early interest in composites.

Each TLP or other weight-sensitive surface vessel requires a number of production and sales risers, and sometimes separate risers for injection and pigging loops.
The cross-sectional geometry of each riser simplifies manufacturing with composites and minimizes costs.

Depending on water depth and diameter, each steel riser represents a deckload of 100-300 tons. Each ton of weight saved means a reduction of $8,000-10,000 on the investment end. A study conducted by the University of Houston, Shell, and BP estimated that the cost of producing 18 composite 4,000-ft-long production risers would exceed that of steel by an estimated $40,000 per riser, yet yield $14.8 million in weight savings.

Other than titanium joints in critical stress areas (tapered stress joint), no thought was given to replacing steel's low cost and toughness for drilling riser purposes. The conventional thinking on drilling in greater water depths was to build drilling equipment bigger and heavier (larger drilling vessels, greater deckload, larger risers). This strategy might have succeeded all the way down to 10,000-12,000 ft water depths, were it not for one unrelenting factor - seafloor currents.

In a series of wells in 5,000-7,000 ft water depths, drilling contractors realized they might not be able to rely upon riser weight and inertia to overcome current forces. Certainly, for operators, the cost of pushing heavier iron to solve the problem might exceed what they had hoped to spend in ultra-deepwater.

Despite being envisioned as the solution for production risers, composites will experience much of the initial testing in risers as part of the drilling phase of oil and gas exploitation.

Interface technology

One of the most import elements in the use of composites for risers, especially drilling risers, is the transition from metal joint sections to the layered composite-liner segments that make up the center of the joint. The joint is required to serve two functions - load transmission between the two materials and internal and external sealing. Some types of joint configuration are better at transferring a tensile load, and others are better at carrying compression or torsional loads.

Two types of mechanisms are used to transfer loads at the interface - mechanical interlocking (pins, groove-hoop systems, taper-curve sets) and adhesive bonding. Most frequently, and certainly for drilling riser applications, a combination of methods will be used to assure integrity under various combinations of maximum stress.

O-rings and seals are the most familiar of sealing type mechanisms, but these were rejected early as too troublesome. For deepwater applications, 40-100 joints would have to carry such seals on two inferfacings. A leak in only one would require pulling the riser string until the joint was reached. Manufacturers favored the use of adhesives over which the composite layers could be tightly wound as the least costly and most trouble-free interface possible.

For most metal-composite interface designs, the composite layers are woven over the steel end joints and transition elements and cured. One or two designs mate the end joint after the composite tube is manufactured. Drilling riser designs tend to be different than production risers.

Production riser interfacing design tends to focus on axial and internal and external pressure loadings, while drilling risers are usually designed for tension, compression, and torsional loadings.

The aerospace industry has had to deal with similar transitions and some of these have been copied for use in composite risers. The more popular transitions include the following:

Geometric trap: Separate metal bulges are formed over the end joints and fiber is wound in two directions over the bulge, after which the windings are cured and stiffening hoop, sleeves, or compression rings are placed over the bulges to prevent separation in tension or compression.
Traplock: This design uses grooves or constrictions in the OD of the joint barrel, into which the composite layers are wound. Metal hoops or rings are inserted into the constrictions and compressed in place. The traplock, which uses one to four constrictions, is a popular joining mechanism for aerospace applications, but not especially resistant to torsional loading, according to some literature on the process.
Pinned joint: Insert pins are used to join the composite layers and the end joints, along with retaining sheaves for stability and sealing purposes. Layers of composite material are then woven over the pins and cured.
Pin-trap combination: This design, probably the most complex, incorporates both insert pin arrangements with 1-3 trap constrictions on the joint barrels. In exchange for a more difficult manufacturing process, this design greatly reduces the risk of interface separation.
Sheaves: This design, similar to the geometric trap, employs sheaves instead of bulges. Composite layers are woven over the sheaves and cured to solidify the interface

Liner protection

Early on in testing risers made of composite materials, researchers realized there was no chance of using composites to line the riser tubes. Not only was the ID subject to physical abuse from rotational and vertical movement of drillstring and tool assemblies, but also from various chemicals used in drilling and completion fluids. These chemicals, which could attack and weaken the composite laminates, included highly caustic or acidic fluids, biocides, chlorides, corrosion and scale inhibitors, and demulsifiers.

Hence, many of the composite riser joints being designed today incorporate two layers into the inner barrel surface to buffer internal physical abuses and resist chemical damage:

A thin metal liner, typically titanium for minimum weight and hoop strength, which will ensure pressure integrity.
An elastomeric liner, to protect against physical and chemical abuses.

An example of maximum protection is the drilling riser being designed to replace the all-titanium riser used for Heidrun drilling in the North Sea. The composite being designed will incorporate a 3-mm hydrogenated nitrile rubber liner on the inner ID, which will be adhesively bonded to a titanium liner just outside. The titanium will then be wrapped with composite materials.

Two major projects

The earliest work on the actual testing of composites in risers appears to have been done in 1983. Testing by Institut Francais du Petrole (IFP) and Aerospatiale indicated that riser weight could be reduced by 24%, helping to reduce dynamic forces on the riser. The two firms built choke and kill lines out of composites and tested the lines in a North Sea drilling campaign. The lines held up under the pressures and resisted deterioration.

In 1985, the two firms tested a 9-in. production riser up to 15,000 psi burst, 5,400 psi collapse, and an axial pressure of 450 tons. Later, Lincoln Composites (formerly Brunswick), Coflexip, IFP, and Aerospatiale conducted studies in strengthening the composite-steel bonding and reducing the cost of manufacture.

At the present, there are at least two major projects underway to investigate, design, and test the use of composites in riser joints:

(1) Conoco, Statoil, Kv?rner: The three companies plan to test a 22-in. diameter high pressure composite drilling riser on the Heidrun platform in the North Sea in 1999. The riser is high pressure, because the blowout preventer will be located at the surface instead of the seabed.

The joint will have a carbon-epoxy composite body, a titanium internal liner, an elastomeric external liner, and titanium flanges. The test joints will replace the current titanium (Ti-6Al-4V) riser joints. Additionally, the composite riser joints are expected to withstand an external impact of 250- kJ, in order to protect it from falling objects when stacked or hung off.

(2) NIST, ATP, Northrup Grumman, ABB Vetco Grey, Reading & Bates, Deepstar, OTRC: The joint industry project program calls for deployment of composite test joints in a low-pressure drilling riser (blowout preventer on seabed) system shortly. The joints consist of 20-in. diameter main body with 15,000-psi choke and kill lines. Two 25-ft long prototypes have been manufactured, with an air weight of 50% that of steel.

The group plans to produce 79 joints, which along with a telescopic joint, four pup joints, a tensioning ring, and syntactic foam, will make up the complete system for 6,000-ft water depths. The composite uses carbon and glass fibers embedded in an epoxy matrix with internal and external liners.

Other smaller internal investigations are underway at most major oil producers that expect to need them in deepwater. Other operators and manufacturing companies have investigated composite technology, and have chosen not to participate, in favor of other means of reducing riser mass and weight, or have been excluded from participation for various reasons.

References

Murali, J., Salama, M., Jahnsen, O., Meland, T, "Composite Drilling Riser - Qualification Testing and Field Demonstration," Proceedings, CMOO-2 Conference, University of Houston, Houston, October, 1997.

Anderson, W., Sweeney, T.,"Advanced Composite Drilling Riser System," Proceedings, CMOO-2 Conference, University of Houston, Houston, October, 1997.

Vennett, R, Williams, J, Lo, K-H, Ganguly, P., "Economic Benefits of Using Composites for Offshore Development and Operations," Proceedings, CMOO-2 Conference, University of Houston, Houston, October, 1997.

Fischer, F., Salama, M., "Emerging and Potential Composite Applications for Deepwater Operations," Proceedings, CMOO-2 Conference, University of Houston, Houston, October, 1997.

Baldwin, D., Newhouse, N., Lo, K., "Composite Production Riser Development," Proceedings, CMOO-2 Conference, University of Houston, Houston, October, 1997.