Ultrahigh Conductivity Umbilicals: Polymer Nanotube Umbilicals
Authors: Dyke Christopher 
Published: May 2013

The technology under development and discussed in this paper, a carbon nanotube (CNT) wire with polymer, is designed to replace copper power transmission lines used in subsea applications. The difficulties with copper include a) corrosion, particularly at wire terminations, and b) strength, in some applications the weight of copper exceeds its tenacity. A technical solution to these difficulties is a carbon-based wire with similar conductivity to copper at a fraction of the weight. To this end, the resources provided by the Research Partnership to Secure Energy for America (RPSEA), contract number 10121–4302–01, are being used to continuously produce CNTs in wire form and jacketed with a polymer. The material is termed polymer nanotube umbilical (PNU®) cable. Since the wire is carbon-based, we anticipate minimal salt water corrosion and a conductor having 1/6th the weight of copper. Also, the CNT conductor shall have greater tensile strength than copper and higher current carrying capacity. Improved conductivity is the chief aim of the development effort discussed here. NanoRidge Materials, Inc., located in Houston, Texas, is the prime contractor and has subcontracted Rice University and DUCO, Inc. to participate in this endeavor. Cambridge University, UK, is a consultant on the contract. Representatives from Shell, Baker Hughes, Total, and DUCO, the project's cost share partners, comprise the Steering Committee and Working Project Group. Described in this paper are 1) our previous effort, 2) aspects of the current state of the art, 3) our technical approach, and 4) the processes to form the bare and jacketed conductor.

Introduction

Since their discovery in 1991, carbon nanotubes have attracted considerable attention (O'Connell, 2006). They are comprised exclusively of sp2-hybridized carbon (same for graphite) in cylindrical form. Depending on the variety, these cylinders, or tubes, are between one nanometer to a hundred nanometers in diameter. A single-walled carbon nanotube (SWNT) is a hollow cylinder with the wall being one carbon atom thick, whereas, double-walled carbon nanotubes have a tube-in-a-tube morphology. Metallic SWNTs have a reported current-carrying capacity of 109 A/cm2 (Yao, 2000); this is 1,000 times greater than copper wire, 106 A/cm2. It is important to note, however, that bulk production of SWNTs yields a mixture of chiralities with their electronic properties being chirality-dependent (Jarosz, 2011). Therefore, as-produced SWNT consists of metals and semi-conductors; i.e., consists of a mixture of chiralities. Although progress has been made in selective production of metallic SWNT (Sundaram, 2011), the mixture of chiralities has complicated efforts to obtain 109 A/cm2 in macroscopic carbon nanotube materials. DWNT have a current-carrying capacity equally as high if not higher than SWNT irrespective of chirality (Moon, 2007). A plausible explanation for the higher current-carrying capacity is the mitigation of electron-phonon scattering due to intershell coupling. In addition to the noteworthy electronic properties, carbon nanotubes have exceptional mechanical properties. The tensile strength of an individual nanotube is approximately 22 GPa (Li, 2000); the tensile strength of copper is 0.25 GPa. Although translation of individual CNT measurements to bulk properites is a work in progress, carbon nanotube fibers have been formed with 6 GPa strength (Koziol, 2007). Tensile strength of fibers is a function of percent defects and voids. A key aspect of our effort is to minimize defects and densify the as-produced fiber.