Thermoplastic composite solution for deep oil and gas reserves – Part 2

Part 1 of this feature looked at why thermoplastic composite pipe is being considered for this application and outlined two pipe designs. In Part 2 we consider the critical pipe end fittings.
 

Termination issue

A particular Magma strength is that it has, as it claims, managed to solve a problem that has hindered the adoption of composite pipe for two decades, namely that of engineering effective and durable pipe end fittings.

To mate with O&G-standard metal interfaces, hybrid metal/composite terminations are needed and the difficulty has been to develop a design that accommodates the basic differences between the materials, in stiffness and thermal properties for instance, while providing adequate structural strength and leak-tight integrity over a normal service lifetime. Such reliability is essential if composite pipe is to succeed in this tough offshore market.

Various solutions have been proposed and tried over several years. In one, the traplock approach, composite material at the end of the pipe is hoop wound into grooves machined into the surface of an inner metallic mandrel, or traplock. Axial loads are transferred via the windings and grooves. During manufacture fibres are pre-loaded by winding them under tension, in order to combat the creep that can occur over time in composites under tension. However, the applied pre-loading tends to reduce progressively, especially at elevated temperatures, and this can ultimately lead to failure.

Another vulnerability is that the adhesive bond normally made between the end fitting and the tube liner can be insufficiently reliable and even an elastomeric seal can fail.

An alternative design relies on tightly sandwiching the end of the composite pipe between metallic inner and outer elements. The inner mandrel has an interference fit with the pipe bore, via a serrated profile, while the outer sleeve is a close sliding fit. An O-ring seal is incorporated to make the joint leak tight. Difficulties with this approach include restriction of the pipe bore by the inner mandrel and stresses induced between the stiff metal parts and the composite under bending loads. Thickening the pipe end helps combat the stresses.

This ‘swaged end’ approach is considered more attractive for smaller diameter hose products than for large diameter high-pressure and dynamic applications. Loads required for swaging become prohibitively large for the latter.

Due to the difficulties of engineering viable pipe terminations, certain designers have advocated a hybrid material solution in which the entire pipe is made up of a composite outer and a metallic inner. The thin steel or titanium liner is welded to the metal end fitting and is therefore continuous with it, avoiding the need for a seal assembly. Axial loads are accommodated by the metal liner while hoop (burst) loads are resisted by the fibres in the outer composite tube. Objections to this compromise approach include the added weight and cost of the metal, the restriction on bend radius it imposes, and the solution’s absolute dependence on the integrity of the weld at the end fitting. Corrosion allowances that have to be made for the metal liner make the pipe thicker, reducing any weight advantage this answer might have had.

A different solution

Magma Global’s solution is different to all of the above. Its design separates structural and sealing functions and exploits particular advantages of thermoplastics, whereas the solutions already outlined have mostly been trialled with thermosets. The combination of carbon-PEEK and the manufacturing technology Magma has developed for it allows pipe wall thickness to be built up locally at the pipe end, either during initial manufacture or as a secondary process. This thickened end is machined to a taper designed to match the internal profile of a metal collar that is tightly clamped round the pipe end using a hydraulic tool. This locks the pipe over a steel end fitting that will mate with the intended O&G fitting.

Engineers have carefully calculated the taper angle to best spread peak loads, minimise axial movement, generate radial preload and minimise interlaminar shear stresses in the composite. The interface surfaces are engineered for best grip and are accurately machined to avoid high local stresses. A bore seal of either PEEK or stainless steel positioned between the inner face of the pipe and the steel fitting ensures leak-tight integrity.

Advantages of the design include a smooth uninterrupted bore plus high structural strength, fatigue tolerance and seal integrity. The thickened pipe end resists high bending loads. Because the design does not rely on bonded surfaces, it can accommodate some minor relative movement between the steel collar and composite pipe, even when the end fitting is hot.

Magma’s terminations are relatively light and affordable. End fittings are readily disassembled, inspected and reassembled, if necessary with replacement parts. The problem of creep, a key design issue with composites under tension, is minimised by the nature of the composite-metal interface which distributes stresses over a large surface area. PEEK has, in any case, a high inherent creep resistance, even at elevated temperatures. The carbon reinforcement, which also has a negligible creep response, further stabilises the polymer.

Extensive tests, both physical and in FEA simulation, have demonstrated the effectiveness of Magma’s pipe/termination combination. Sections of pipe of various sizes terminated with Magma fittings and PEEK bore seals have been subjected to a range of load cases and assessed for thermal, pressure, bend and fatigue tolerance. Evaluation specimens are in use under service conditions at O&G installations.

Magma, confident that it has solved the end fitting problem that has dogged previous composite pipe solutions, believes that its carbon-PEEK thermoplastic pipe can now be an important enabler for future O&G developments where metallic pipe technologies struggle to meet project requirements. From late this year, its Portsmouth factory will be poised to meet the need. ♦
 

Part 1 of this feature is available here.

This article was also be published in the July/August issue of Reinforced Plastics magazine.

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