Extending knowledge of composites in subsea applications
Dan Jackson
DeepSea Engineering & Management
Composites technology is among the latest advances being discussed in the oil and gas industry. The potential this technology can offer is currently limited, however, by the fact that it is viewed in terms of its benefits over steel, but designed in the same way. Composites offer advantages, but a greater understanding and new thinking is required if the benefits as an enabling technology to the deepwater oil and gas industry are to be fully realized.
The benefits composites offer include weight savings, high strength, and non-susceptibility to corrosion. Composites become attractive where deep and ultra deepwater technical solutions are needed to reduce payloads and component mass while achieving high strengths. Perhaps the most significant advantage is as an enabling technology, allowing new approaches and designs that would not be possible or viable using traditional materials.
The deepwater industry can learn from aerospace in this respect. For years aircraft engineers have used composites in secondary structural roles within the wings and fuselage and gained commercial benefit by simply switching out components using the same design approach as aluminum.
Design certainty and industry confidence is required for similar progress in the oil and gas industry. Operating depths to 10,000 ft and maintenance-free 25-year design life are prerequisites for many applications, as production moves into deep and ultra deepwater, representing significant technical challenges.
CFRP lines
Carbon fiber reinforced plastic (CFRP) lines provide one example. Current CFRP line technology is largely centered on stressed members - rods, tendons, and stranded cables - for civil engineering applications. In the offshore oil and gas industry, two company consortia (Aker Kvaerner/ConocoPhillips and Freyssinet/Soficar/IFP/Doris Engineering) have undertaken some development of this application and applied it to the taut mooring requirements of TLPs. DeepSea Engineering, on the other hand, has focused on the deep and ultra deepwater mobile offshore drilling unit (MODU) market as a first step to bring carbon composites to the offshore sector. This market was previously the proving ground for polyester lines, and CFRP is a lower cost/lower risk component.
As published at Offshore Technology Conference (OTC 2005) and updated at the Deep Offshore Technology Conference and Exhibition (DOT 2005), DeepSea and Petrobras are undertaking a cooperative development program of pultruded CFRP rods for this purpose.
Primary advantages will include reducing the vessel watch circle due to the CFRP line’s stiffer properties (thus extending the riser’s operational capability) and matching the strength-to-diameter ratio of existing steel lines. This second advantage extends the existing fleet capability, one of the key aims being to use existing vessels equipped to handle steel lines with minimal modification.
DeepSea has developed analytical techniques to model a CFRP rope design and configuration for catenary, taut, or semi-taut deepwater moorings, assuming that each layer is orthotropic and homogeneous, with each orthotropic layer comprising individual rods. To date, the prototypes have been through cycles of design, built and tested for tensile and tensile fatigue loading with greater-than-expected success. Next stages include further prototype cycles, leading to field trials on MODU units.
In addition to primary tensile performance, mooring lines see highly complex loading regimes in the sockets involving lateral compression and bending. The ability to model this loading regime successfully within a complex structure has progressed over the last five years with many CFRP virtual modeling, prototyping and testing research cycles investigating progressive failure criteria. The result was rapid development of an acceptable solution removing the need for expensive bespoke terminations.
While carbon composites display very little water uptake at ambient pressures, there is even less uptake at higher pressures such as those experienced subsea.
In deepwater, most composite applications see compression to a varying degree, so they are not under a simple tension regime. For risers or coiled tubing applications, compression can be observed within the joint structures, whereas for non-tensile structures like pressure chambers, the whole structure is dominated by compressive loading. It is the detailed understanding of composites behavior under compression that is a missing link in attaining full design certainty and securing industry confidence.
Composites in compression
The dual element buoyancy unit (DEBU) project seeks to understand compression’s effect on composites in deepwater. This program has expanded into a large, in-depth technical project, and was published for the first time at the Fourth International Conference on Composite Materials and Structures for Offshore Operations (CMOO-4). It was undertaken by DeepSea with a major oilfield service provider.
The DEBU technology has many potential product lines, including ultra deepwater buoyancy systems where it can demonstrate significant technical and commercial advantages by providing efficient buoyant units at great depth. Among various applications, the first envisaged is to address steel catenary riser fatigue at the touchdown point by adding cost-effective buoyancy in the form of multiple large composite pressure vessels designed to withstand the huge external pressures in deepwater.
Various configurations for subsea lines. Due to the tortuous bending expected for some subsea lines, the modeling data for these systems must be detailed to understand how a composite would perform.
To date, the DEBU program has completed extensive engineering design, material testing, manufacturing process control, and scale testing. Multiple tests at an intermediate stage (circa 15 ft long and 4 ft diameter) have been undertaken and, with next steps to include further validation tests to pressures equivalent to 13,000 ft (4,000 m) water depth, the project has progressed well through the development phase.
The DEBU technology has additional potential for a wide range of E&P applications where it can add considerable benefit. For example, DEBU installations distributed through the water column can be used to mitigate the weight of deepwater risers and steel tendons, extending TLP and spar operational depths to 10,000 ft (where previously water depths of 5,000 ft were considered the limit). Composites technology provides low-weight and relatively low-cost high volumes of buoyancy for use at these depths to reduce pipe tensions in deepwater and increase the operating envelopes of top tension riser and tendon systems.
The DEBU project can enable deepwater gravity-based separation tanks for subsea processing to become a reality by reducing the weight of the separator tank so that this proven technology can be deployed at depths where steel would make the weight technically and commercially prohibitive. Capable of withstanding 400 bar external pressure, its ability to be positively buoyant eliminates the need for specialist heavy lift equipment and makes it easily retrievable using a small field maintenance vessel.
In subsea processing applications, the DEBU technology takes the form of a large diameter, thick-walled CFRP pressure chamber with sandwich and monolithic components, but the structure can potentially be further developed to include internal liners, porting, monitoring, and extensive cyclical loading of internal pressure and temperature with multiphase hydrocarbons.
With knowledge of composites in compression now developing, the final gaps in composite materials knowledge as applied to deepwater applications are being filled. This will enable the next step to be taken, to apply composites as an enabling technology.•
