Composites are coming

Composite for coiled tubing has been tried before, but the industry wasn’t ready yet. Norway’s Prototech is taking another look.

New software optimizes composite pipe based on typical CT loading conditions. Image from Prototech.

It is well known that composites are being increasingly recognized for their suitability in the most specific and demanding areas across many industries. Advantages of composite materials such as lightweight, resistance to a wide range of fluids (including seawater, aerated water and hydrocarbons) which can attack metals, good thermal insulation, excellent damping and fatigue performance, and high specific stiffness make them ideal candidates for use in the water environment for structural and non-structural applications. These properties combined with the unmatched tailoring of fiber reinforcements along load paths have motivated the industry to promote the use of composites in several critical load-bearing applications: particularly for risers, spoolable tubulars and tethers. Other areas where composites can be used include pipework, paneling, casings and structural repairs.

Furthermore, composites also have very interesting directional properties and are even more unique when high strains are introduced. The applications for flexible composites in high-strain structures are numerous though their implementations have not yet been fully realized due to the complexities in design optimization and characterization. Examples of flexible composites in high-strain structures are: wind and tidal turbine blades, helicopter blades, some aircraft wings, and some tethers. Design of each of these structures is a very multidimensional task which involves optimizing the composite layup based on the structure’s geometry, loading conditions and failure criteria. Loading on aero and hydrodynamic structures is dependent on the structure’s shape, requiring additional understanding to address fluid-structure interactions.

However, despite their undoubted advantages for oilfield systems targeting increasing depths, the introduction of composite materials is a very slow process and they have not received a wide application yet (as it has been successfully done in aerospace industry). The major barriers holding composites back are the lack of appropriate performance information, full scale parametric testing under different types of loading for verification and certification, regulatory requirements, efficient design procedures and reparability issues. Thus, the composites’ ability to stand up to impact and cyclic loading, stability and fatigue performance, resistance to the environment (aggressive fluids, temperature and pressure) are the topics of the current research. In particular, composites subject to high-strain deformations, which can be found in coiled tubing (CT), will be considered.

Analyzing the structure

Tubing materials with high corrosion resistances like carbon fiber reinforced plastics (CFRP) are ideal for transporting various substances and operating in harsh environments. At the same time, a CFRP pipe must be capable of withstanding working internal and external pressure, high temperatures and axial loads.

Researchers from Prototech in Bergen, Norway, and the University of Aberdeen engineering department are collaborating through a grant from Regional Research Fund Vestlandet to explore flexible composite materials for use in CT applications. Several topics from both materials engineering and structural performance viewpoints are being explored. From the materials engineering perspective, optimization-based analysis programs have been developed to calculate composite fiber orientation, ply thickness and stacking sequence.

The optimized tube materials are calculated based on pressure differentials, bending, torsion and axial loads. The analysis tool requires a number of simple user inputs like material properties, tubing diameter and load requirements and produces stresses (and failure coefficients) at any point through the tube wall/composite layup. The software is capable of optimizing for both thick and thin-walled composite tubes, and isometric layers like metallic or plastic can also be include in the optimization algorithm.

Investigating the costs

The overarching goal of the technology is to develop a CT system that improves energy efficiency and has a lower lifetime cost than today’s low-alloy carbon steel CT market. These goals are achieved by relation to two basic properties inherent to the composite material: material density and fatigue resistance. CFRP are roughly five times less dense than steel, and in the high-strain setting, can be superior in fatigue resistance if designed correctly. Fatigue life of CT has a direct effect on the cost; for example, a two-fold increase in cost can be justified by a two-fold increase in allowed installations before decommissioning.

The 5x reduction in material density has numerous secondary effects on the cost of the equipment including transportation costs, logistics, operations, etc. Lower masses also promote reduced energy consumption and emissions during every operation. From a logistical perspective, there exists some surprising cost-savings potential as illustrated by the simple example below. Fully loaded (steel CT) spools weight upwards of 20- to even 30-ton, have typical diameters between 3-4m and widths about 2m. Many standard shipping trailers support these diameters and widths, provide a length dimension of 8m+ and support a maximum load of around 22-ton. The maximum capacity of these trailers however is met with just one fully loaded spool. When taking a conservative approach, the researchers anticipate the CFRP CT with spool to weigh less than half. This provides the advantage to use the remaining space in the length direction of the trailer. Thus, a second spool can be transported on the same trailer and at the same cost as transporting just a single steel CT spool. There are, of course, many other transportation options, but they certainly also come with a cost.

When considering base materials, the cost of steel is about US$0.5/kg and that of CFRP is upwards of $10/kg; however, this doesn’t tell the whole story due to the 5x difference in density. The strength-to-weight ratio for steel is about 50 kNm/kg while CFRP is roughly 390 (for bi-directional loads as is common for CT). Instead of comparing cost per mass, it might be more reasonable to compare the cost per strength since the tubing is a type of structural equipment. When calculating this, we find steel has factor of 100 kN/USD and CFRP has 39, a difference of only 2.5x.

Despite popular belief, material cost may not be a main driver for CFRP flexible tubing, especially when the other benefits are considered. Manufacturing CFRP CT however still represents a major challenge. Fabrication costs for several kilometer-long CRFP tubes would be markedly higher in comparison to the steel CT fabrication method which has several decades of maturity. It is difficult to estimate these costs now and especially how they will improve with time. Nonetheless, there are exciting opportunities ahead to develop these innovative manufacturing techniques.

The potential improvements in fatigue life along with the extensive list of secondary benefits related to lightweight materials are sure to bring the cost of this technology closer to competing with the steel tube solution of today.

Kevin Cox, PhD, is at CMR – Prototech and holds an MSc in mechanical engineering and a PhD in materials engineering. He has more than 10 years’ experience in structural design, analysis and research and development withing the oil and gas, renewables, marine and aerospace industries.

Oleksandr Menshykov, PhD, is at the University of Aberdeen and holds an MSc in applied mathematics and a PhD in solid mechanics. Menshykov has more than 15 years’ experience and has contributed to more than 100 publications.