Thermoplastic composites are offering potential solutions to the challenges of extracting deepwater oil and gas.

The insatiable demand for oil and gas has driven the exploitation of offshore resources, which are deeper and more difficult to drill and develop. The risers, pipelines, fittings and other piping systems operating on the seabed have to cope with greater challenges in terms of loads, pressures and environments. At the same time, the weight of the pipelines must be as light as possible to stay within the rated load capacity of floating drilling platforms and storage platforms.

In general, material needs have begun to exceed the mining capacity that can be achieved with current pipeline technology based on metal as the main material. Reinforced plastics, especially reinforced thermoplastic composites (PEEK + carbon fiber), may be the answer.

Why Thermoplastics

According to Dr. Rod Martin and Dr. Morris Roseman of Element Hitchin (MERL, formerly Materials Engineering Laboratory), composites are key to the industry’s ability to meet the energy security challenges facing the nation. Composites enable large-bore pipes to have the required combination of strength, toughness, lightness, high heat and pressure resistance, low flow resistance, high resilience and durability, surpassing the performance of current mainstream metal pipes, even though they are often lined with polymers.

Experiments have shown that conventional thermoset composites are not good substitutes for metal materials (such as Compipe, which was born in Norway in the late 1990s and consists of reinforced epoxy resin pipe and unreinforced thermoplastic liner), because the manufacturing process of pipe is a continuous process, and its curing dynamics are a major obstacle to this process. In contrast, thermoplastics can adapt to continuous production because the consolidation and hardening problems of thermoset materials are not a problem for thermoplastics.

Element Hitchin is part of a collaborative group evaluating the suitability of various thermoplastic composites for use in oil pipelines. Glass-fiber-reinforced polyoxymethylene (POM) and polypropylene (PP) perform well in seawater, while carbon fiber-reinforced polyetheretherketone (PEEK) has proven well suited for sour oil and gas environments. Carbon fibers in polyphenylene sulfide (PPS) perform well in sour oil and gas environments and hydrocarbon gas concentrates when the ambient temperature rises. (Note: Sour oil and gas environments are oil mixed with water, various chemicals, reactants, hydrocarbons and byproducts, and sand, which can affect the permeation resistance and service life of some thermoplastics.)

Thermoplastic composites can be produced into long, continuous, spoolable pipes using tape laying or winding processes. Many companies are now conducting research and development work on the processing performance of the main structure of oil and gas pipelines.

For example, Airborne International Ltd. in the Netherlands has begun using what it claims to be the world’s first high-end all-thermoplastic composite pipe continuous production equipment. Two production lines were put into operation in its 9,000 square meter Ijmuiden plant the year before last.

Airborne’s pipe products feature two layers of glass or carbon fiber, combined with a thermoplastic polymer in the molten state to create a fully bonded solid-wall pipe. These products are lighter and softer than metal pipes, while also being easier to spool and transport, thus requiring smaller vessels than traditional pipe deployment. The pipes are extremely strong and have good resistance to internal pressure. They are non-corrosive, resistant to many different chemicals, and will not gradually degrade in water. The pipes have smooth bores that promote fluid flow, and they are highly resistant to heat, fatigue, and durability. In fact, the ductility (rather than brittleness) exhibited by the material extends its service life. By using the same thermoplastic polymer to produce the liner, composite layer, and pipe outer jacket, a single-material solution is provided for heavy-duty pipes.

“In general, for high-pressure, high-temperature risers operating in deep water, this is the answer,” asserts Martin van Onna, commercial director of Airborne’s oil and gas division.

Airborne’s award-winning technology has won the favor of Petrobras, which has selected the company for its offshore oil pipeline. Petrobras is working with Saipem SA on a certification project in Brazil that includes a 2.5-kilometer subsea pipeline for the Guara and Lula Northeast oil and gas fields.

Closer to home, in the United States, Oceaneering has produced kilometers of thermoplastic umbilicals for Petrobras to operate deepwater oil and gas fields in the Espirito Santo Basin. Oceaneering’s hoses range from 3 to 16 inches in bore diameter and are suitable for pressures from 3,000 to 15,000 psi. Thermoplastics with high collapse resistance (HCR) can be used for injection molding of low molecular weight chemicals in the deep sea.

The UK solution

Meanwhile, in Portsmouth, UK, Magma Global Ltd. is about to trial new capacity to produce carbon fiber reinforced PEEK thermoplastic composite pipes under new conditions. So far, the company has produced pipes up to 25 meters long for testing and development, but the new line to be trialed at its Portsmouth plant will begin producing continuous lengths of pipe for spooling and subsequent deployment at oil and gas fields.

The company was founded in 2010 by Martin Jones and Damon Roberts, who are the company’s CEO and technical director respectively. They had already been involved in the oil and gas sector when they developed sensitive fiber optic equipment to test the strain of masts and other structures at their previous company Insensys (later acquired by Schlumberger). The pair had experience in the oil and gas industry and understood the load conditions of pipelines and knew that significant improvements in pipeline technology would be necessary for future oil and gas field development. Their experience at Insensys gave them specialist composites knowledge, which was derived primarily from the design and production of advanced composite masts, often with embedded fiber optic strain sensing, for high performance superyachts. This business continues under the Magma umbrella.

From the outset, Magma determined that a high-end product based on carbon fiber-PEEK composites would be the best solution, despite the high costs of both raw materials.

As Roberts told Reinforced Plastics, “We saw a clear need for pipe products for deepwater applications, and existing materials were not adequate for the high pressures and sometimes high temperatures that are present. So low-cost feedstock and low-cost manufacturing were not an option. We needed a high-specification solution that could be manufactured with precision, repeatability and consistency. Carbon fiber-PEEK composites not only met those requirements, but were also extremely strong and tough, so we felt it was the right choice.”

The glass transition temperature Tg of PEEK material is 143℃, and it can maintain good structural properties at 200℃. Magma typically uses a composite material, which uses PEEK resin from Victrex as the base material and adds high-performance carbon fiber (from Toray, with a tensile strength of 4.9 MPa) for reinforcement. Although high-strength medium modulus carbon fiber is the main reinforcement material, other medium modulus or even high modulus grades of carbon fiber can be used in situations where high strength and toughness are required.

Roberts and his team have developed a patented manufacturing process that can combine the two materials to produce continuous lengths of more than 2 kilometers of pipe. The process is described as: automated, robotically operated, scalable and monitorable. The continuous production process can achieve high product quality and production consistency.

While the base material costs are high, Roberts argues that the cost of deploying the pipeline is becoming increasingly competitive with other technologies, especially with steel-based pipelines.

As he points out: “Our pipe is more flexible, more coilable and lighter than steel. This means that it can be deployed using smaller vessels and is easier to manage. In water, it weighs only one-tenth of a steel pipe. In addition, reliability issues with existing technologies mean that assembly and commissioning costs for steel pipes always exceed budgets. All these factors affect relative costs. Moreover, the deployment cost of the pipe is only one thing, if you consider the life cycle cost, our odds increase further because our material is flexible and durable.”

Specific pipe products, including m-pipe and s-pipe, are available in either continuous spoolable or discontinuous lengths. Each length is assembled with pipe fittings and flanges and threaded connections where required. m-pipe is available in diameters ranging from 2 inches to 2 feet for a variety of risers, pipelines and umbilicals. It is lightweight yet has high strength and high strain characteristics to aid in pipe installation.

Although the material of s-pipe is similar to that of m-pipe, it is positioned for small-diameter pipelines and pipeline interventions, and the largest diameter commonly used is 3 inches. It is reported that this pipe is ideal for challenging pipeline intervention and pipeline renovation applications, and can be purchased in specifications of 5, 10, 15 and 20ksi strength. This product was launched at the Offshore Technology Conference (OTC) held in Houston, Texas two years ago and attracted great attention.

(Pipeline) Terminal Solutions

One particular strength of Magma is its ability to solve a problem that has hampered the adoption of composite pipes for two decades: designing and manufacturing pipe joints that last. To match the oil and gas industry’s standard metal interfaces, hybrid metal/composite terminations are required, and the challenge has been to develop a product design that accommodates fundamental differences between the materials, such as stiffness and thermal performance, while providing adequate structural strength and sealing integrity over the normal lifecycle. This reliability is a must for composite pipes to succeed in this demanding offshore market.

Many solutions have been proposed and tried over the years. In the Traplock method, the composite material at the end of the pipe is wound onto the grooves on the surface of the lined metal mandrel, i.e., the traplock. The axial load is transferred through the winding and the grooves. During the process, the fibers are wound under tension to provide a preload to account for the creep that the composite material may experience over time under tension. However, the preload tends to gradually decrease and lead to failure, especially at elevated temperatures.

Another disadvantage is that the bond between the end fitting and the pipe liner may not be reliable enough, and even the elastomeric seal may fail.

Another design clamps the composite pipe end between a metal mandrel and an outer element. The inner mandrel has a serrated profile that provides an interference fit with the pipe bore and a tight sliding fit with the outer sleeve. An O-ring seals the joint. The difficulty with this approach is that the pipe bore is constrained by the inner mandrel and the rigid metal component and composite material create pressure under bending loads. Thickening the pipe end helps relieve this pressure.

This “swaged end” approach is considered more attractive for small bore hose products than for large bore high pressure and dynamic applications where the load required for the reducer becomes greater.

Because of these challenges in designing viable pipe terminations, some designers have advocated a hybrid solution where the entire pipe consists of a composite sleeve and a metal liner. A thin steel or titanium liner is welded to a metal joint and is continuous with the liner, eliminating the need for a sealing assembly. Axial loads are carried by the metal liner, and hoop (burst) loads are carried by the fibers of the outer composite sleeve. Disadvantages of this compromise approach include the weight and cost of the added metal, as well as restrictions on bend radius, and the solution relies on the integrity of the weld at the joint. The corrosion allowance required for the metal liner makes the pipe thicker, reducing the weight advantage that this solution might have.

Magma Global’s solution is different from many of the above approaches. Its design separates the structural and sealing functions, exploiting the unique advantages of thermoplastics. While most of the solutions proposed use thermoset materials, the end of the pipe is locally thickened by combining carbon fiber-PEEK composites and Magma’s manufacturing technology developed for this material. This process can be used both in the initial manufacturing stage and in secondary processing. The thickened end is machined into a tapered shape, which is designed to match the inner profile of a metal ring that is clamped around the end of the pipe by a hydraulic tool. The structure locks the pipe with a steel joint that fits tightly with the oil and gas pipeline joint to be used.

Engineers carefully calculated the taper angle to better distribute the large loads, minimize axial movement, create radial preload, and minimize interlaminar shear stress in the composite. The interface surfaces are designed for optimal clamping and are precisely machined to avoid excessive local pressure. PEEK or stainless steel bore seals placed between the inner surface of the pipe and the steel fitting ensure the integrity of the seal.

The advantages of this design include a smooth continuous bore, high structural strength, fatigue resistance and seal integrity. The thickened pipe ends can withstand large bending loads. Because the design does not rely on a bonding surface, it can accommodate some minor relative movement between the steel ring and the composite pipe, even when the joint becomes hot.

Magma’s terminal systems are relatively light and affordable. When it becomes necessary to replace a part, the terminal joints are easy to disassemble, inspect and assemble. Creep issues, a key design issue for composites under tension, are reduced by the nature of the composite-metal interface, which is able to distribute stresses over a large surface area. In any case, PEEK has high creep resistance, even at high temperatures. Carbon fiber reinforcements have negligible creep properties, which further stabilizes the polymer.

Extensive physical and finite element analysis tests confirm the effectiveness of Magma pipe/terminal combinations. Pipes of different sizes are selected and fitted with Magma terminal fittings and PEEK bore seals, which are subjected to a range of load conditions, and also require evaluation of heat, pressure, bending and fatigue resistance. During the oil and gas installation process, the evaluation samples are in use under operating conditions.

Magma is confident it has solved the end-joint problem that has always been associated with composite pipe solutions. The company believes its carbon-PEEK thermoplastic pipe can be a key enabler for the future of the oil and gas industry, where metal pipe technology is struggling to meet project requirements. From late 2013, its Portsmouth factory has been ready to meet the needs of the market.