Carbon fibre – a riser system enabler

November 23, 2012

Aircraft builder Boeing will employ composite materials in large quantity in its new Dreamliner 787 and use them for the main structural elements. Is the offshore sector ready to follow suit at last? wonders riser engineering specialist Steve Hatton, the former 2H Offshore director now with Magma Global.

Carbon fibre has been proposed as a potential offshore riser material for over 20 years. In its support there have been strong claims of light weight, high strength and fatigue resistance and yet to-date there remains only a handful of applications in the offshore market.

In such a conservative industry this lack of application is considered to be due to the relatively low technical maturity of composite technology, the lack of applicable test data and established design codes. Additionally, there have been continuous incremental developments with steel and titanium alloys, and also with non-bonded riser pipe, that have largely kept step with market requirements. Furthermore, for the newbuild higher specification drilling, installation and production vessels, weight saving until recently has not been a critical selection driver. Another commercial factor has been the high investment by some SURF contractors into non-bonded flexible technology that results in strong inertia to consider new technologies.

Boeing plans to use composites for the main structural elements of its new Dreamliner 787.

The most notable carbon fibre application has been in its increased use in non-bonded flexible riser pipes as a replacement for the heavy and costly steel elements. This design approach also tackles corrosion concerns with metallic armour wires from seawater ingress through damaged outer carcass or production fluid permeation into the annulus through the pressure barrier. As water depths have increased and service conditions become more demanding, all non-bonded flexible pipe suppliers are offering or developing pipe cross sections with carbon fibre elements to tackle these issues.

But whilst such applications of carbon fibre are important to the offshore industry overall the volumes of carbon fibre are rather limited. There is, however, an increasing expectation for more extensive use of carbon fibre at higher volumes. Looking forwards, those close to the industry believe carbon fibre will be an important enabling technology that will help to unlock the potential of deep and ultra-deep discoveries.

Magma opted to use only high end materials in its carbon fibre m-pipe.

The application of greater carbon fibre volumes in the offshore industry tracks the aerospace industry where its use has seen increasing quantities in military and commercial planes over many years. The Airbus A320 now uses almost 28% by weight of composite materials in structures such as fuselage belly skins and fairings, wing fixed leading and trailing-edges, access panels, trailing edge flaps, spoilers, ailerons, carbon brakes and much more.

Recently, Boeing has selected composite materials for its new Dreamliner 787 and unlike Airbus is using larger quantities and using them for the main structural elements. The aircraft is 80% composite by volume and listed by weight its materials are 50% composite, 20% aluminium, 15% titanium, 10% steel, and 5% other materials. Each 787 contains approximately 32t of carbon fibre reinforced plastic (CFRP) and uses a total 23t of carbon fibres.

Helicopter blades, working under high cyclic bending load conditions, are today practically all composite for precisely the same reasons, high resistance to fatigue, high strain to failure and good impact resistance.

The reasons for this increased use of carbon fibre in both aerospace and offshore are:

  • advances in the specification and capabilities of carbon fibre and polymer materials;
  • improved methods of high-volume manufacturing;
  • improved methods of quality control;
  • development of design codes;
  • increased material availability;
  • lower material costs;
  • requirement for higher safety factors under repeated cyclic loading;and
  • more demanding applications.

complex and where failure is generally catastrophic. Both structures need to be well optimised to accommodate the extreme loading that occurs with a low probability event, such as the 100-year storm, and both are highly fatigue-intensive applications. These structural requirements lead to the need to apply sophisticated design processes and high quality manufacturing methods with highest levels of quality control. But risers cannot be readily inspected and repaired and so they typically require 20-30 years continuous service. This makes riser design one of the most demanding engineering challenges.

The benefit that carbon fibre brings to this design challenge is that when compared to steel pipe and non-bonded flexible pipe it has significantly less weight , dramatically improves fatigue performance and offers high corrosion resistance to sea water, CO2 and H2S. Resistance to H2S is a very major advantage; in steel pipes H2S causes serious embrittlement of welds and fatigue life knockdown, which can be in the range of 10 to 40 times lower. Without expensive CRA liners this can be a feasibility issue for many applications.

A further benefit of carbon fibre is that it can operate at much higher strain levels than steel, operationally up to 1%, and this allows large deflections to be accommodated and/or lower stresses, which at relatively low structural stiffness greatly reduces interface loads.

Non-destructive ultrasonic testing of m-pipe at Magma's Portsmouth, UK, manufacturing facility.

The carbon fibre pipe developed by Magma is not the same as that developed by others in that it is not simply the replacement of steel pipe elements with carbon fibre elements. Magma’s product involves the manufacture of a monolithic wall pipe that has continuous polymer and fibres from the ID to the OD and contains only two materials, the polymer and the carbon fibre.

An early Magma decision was to select only high quality, top end materials leading to the selection of Victrex PEEK as the polymer and Toray T700 for the fibre. These are unashamedly high end, but due to Magma’s highly automated production process the cost of the end product is directly comparable to existing non-bonded flexibles, making m-pipe a highly cost effective riser solution. PEEK offers significantly improved performance over the more typically used polymers in non-bonded flexible pipe and other composite pipes such as PA and PVDF. PEEK has chemical resistance, permeation and explosive decompression capabilities in orders of magnitude better than these lower grade polymers.

In the case of Magma, pipe manufacture is a simple, repetitive, robotically controlled process, making pipe quick and cost effective to manufacture. The light weight of the pipe, coupled with its strength and general robustness to handling, allows it to be installed from smaller, lower day rate vessels, making the installed cost even more attractive. Additionally, selection of high-end materials allows high confidence in long-term reliability and minimal degradation, even under aggressive operating conditions, to further reduce operational concerns and risks.

The manufacturing process used by Magma is considered a ‘step-up’ on even the latest aerospace applications. It is a thermoplastic process that is very different to most people’s image of carbon fibre construction; which is commonly considered to include the use of complex moulding, consolidation and oven curing processes that are difficult to fully control. This is the thermoset process. Magma’s process is a thermoplastic process: a melting process, which importantly is very controllable, repeatable and recordable.

Magma’s m-pipe can be manufactured in continuous lengths and configured in free standing riser geometries such as SLORs and hybrids, in catenary configurations or manufactured in discrete joint lengths and used as kill and choke lines, top tensioned risers for spars and TLPs or for completion and workover risers. Further applications include rigid jumper spools connecting between subsea components such as PLEMS, PLETS, trees and manifolds.

An important requirement is the ability to reliably confirm the pipe quality at every stage through the manufacturing process. This starts with the selection of qualified material suppliers, extends through the pipe manufacturing process (where process parameters are tightly controlled within a predetermined window and recorded up to 50 times per second) and through to non-destructive ultrasonic testing that can detect applicable defects. Linked with this are the criteria for acceptable defects and the process for repair.

When used in continuous lengths the pipe is spooled onto reels or carousels in much the same way as for other reeled pipe product. The diameter of these carousels is larger than that for an equivalent steel pipe, which has the benefit of plastic deformation, but is well within the practical limits of what can be handled onshore and offshore. For a 6in pipe the reeling diameter is 20m and for a 10in pipe, 35m allows such reels to be located on the back deck of a typical pipelay vessel or even a barge. Importantly the pipe is very light and so the structural design of such carousels and their interface with the vessel is relatively simple compared to those carousels designed for non-bonded flexible and steel pipe.

Evaluation of offshore installation options and procedures confirms that relatively small construction vessels will be suitable for deployment as the payload is greatly reduced. Acceptable motion envelopes and weather windows are also much greater for carbon/polymer pipe than steel pipe.

The result is lower day rates and quicker installation cycles and thus installed costs are greatly reduced.

In conclusion, carbon fibre manufacturing technology has changed radically in recent years and the combination of these technological advances, availability of codes, greater acceptance, more demanding load cases and higher required safety factors, make it highly desirable for riser applications. Whilst it is a high end material, the installed costs and even the life of field cost is the important criteria and this is where the technology is believed to be a winner. OE 

Best known for founding and building 2H Offshore, Steve Hatton joined Magma Global as commercial director in August 2011 from the Acteon Group, where he was vice president and principal director of its riser engineering company 2H. He has almost 30 years’ experience in subsea equipment and riser systems design. After graduating from Newcastle-upon-Tyne University, he worked for Vickers Offshore, John Brown and Cameron Offshore Engineering where he was predominantly involved in the detail design of riser systems and riser EPC contracts. In 1993 he was jointly responsible for setting up 2H and subsequently drove the business in riser system and subsea hardware design and analysis.

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