07121-1302

Goal
The goal of this project is to develop an ultra-high conductivity power cable suitable for use in undersea umbilicals. The overall objective is to design, build, and test an engineering prototype of a working ultra-high conductivity cable that could in later stages be incorporated into an umbilical exceeding 100 miles in length and called upon to deliver up to 10 MW at up to 36 kV with operating temperatures up to 250°F and pressures up to 4500 psi.

Performers
NanoRidge Materials, Houston, TX 77023
Technip USA, Inc., Sugar Land, TX 77478
DUCO, Inc., Houston, TX 77015-6542
Rice University- Department of Mechanical Engineering and Materials Science, Houston, TX 77005

Background
When considering high power requirements and long umbilical tie-back distances, there is a need for new technologies to enable power delivery to the seafloor. Carbon nanotechnology is one such new technology that could enable efficient power transfer over long tie-back distances where light weight yet durable cable is required. One concept for a new high current density electrical wire is a polymer-nanotube umbilical (PNU) based on single walled carbon nanotubes (SWCNTs) dispersed in a polymer binder. Such a wire would have the ability to be produced in long segements with connections that could be made at numerous points along the length. An SWCNT typically has a diameter that is close to 1 nanometer, and a length that may be many thousand times longer. The way the carbon material is wrapped to form the cylindrical tube can vary.

A low current loss wire could be bundled into an umbilical to provide power for communication lines and to operate pumps and other subsea equipment. The umbilical might include hydraulic lines, communication electrical wiring, and power lines for pump operation.

There are three main deliverables for the project: 1) produce a one foot long conductor that works at room temperature and can carry at least 500 amps while being one half the diameter of a pure copper conductor carrying the same current at the same voltage; 2) deliver a final report that documents the results of the demonstration as well as a wide range of physical properties associated with the conductor (weight, flexibility, conductivity, etc.); and 3) conduct a workshop in Houston, Texas to present and discuss results.

Potential Impacts
The envisioned cable will have an electrical conductivity that will be about four times that of copper in the same cross-sectional area, allowing for power transmission for much greater distances than is currently possible. The cable will also be much lighter and will be close to neutral buoyancy, thus allowing for easier installation. Also, polymer cables will be more fatigue resistant than any metallic conductor. While the purchase cost of the cable could initially be higher than copper, the projected future cost of nanotubes is expected to decrease to the point where in just a few years a nano-polymer cable will be less than copper. A low cost, high capacity, durable electrical cable will enable more robust subsea development in deeper water, enabling the economic development of portions of the ultra-deepwater resource that would otherwise remain inaccessible.

Accomplishments
The Project Management Plan and the Technology Status Assessment were completed. The work breakdown structure concisely addressed the objectives and approach for each task with all major milestones and decision points listed.

NanoRidge conducted experiments with polystyrene to enhance conductivity through melt annealing. Experiments included changing the solvent type, increasing the weight percentage of multi-walled nanotubes used, and molding samples using a vacuum oven. Experiments were also conducted using low and high density polyethylene to determine solvent compatibility. Samples were produced using medium density polyethylene (MDPE) and multi-walled nanotubes (MWNTs) using different techniques including autoclave, annealing samples, and dry and melted samples. The extruder was used to produce long samples of wire utilizing up to 20 wt% multiwall nanotubes in polyethylene. Samples of HDPE/SWNT and MWNT/MDPE were produced, characterized, measured for resistivity, and were further processed using electric fields.

SEM images were produced and nanotube diameters were measured. Data was gathered at various voltages and settings. Electric field testing and optimization were also performed. Doping was used as a technique to try to enhance conductivity. Materials from this research were prepared for and presented at the Offshore Technology Conference in May of 2009.

Throughout this project, the most ideal form of the conductor was not achieved. Work completed during the project year dictates a continuation of effort to achieve the ultimate goal of a PNU in an offshore application. Key findings of the project include:

• Produced polymeric conductors with nanotube concentrations up to 90 wt%; however, the primary focus of the program was directed toward low concentration (10%) samples.
• Reached a minimum resistivity (inverse of conductivity) value of 2x10-2 O.cm in the melt state, versus the ideal goal of 1x10-6 O.cm in a solid wire.
• Identified several new steps for lowering resistivity that should be evaluated in future research projects.
• Workshop was conducted at Rice University on December 10, 2009 and was well attended by over 70 industry and academic representatives.
• The final project report was prepared and is listed below under "Additional Information".

Current Status
The key tasks that were undertaken are outlined below.

Obtain SWCNTs and m-SWCNTs (metallic SWCNTs). Several sources were identified for purchasing purified SWNTs. Purification was conducted by Rice and NanoRidge to assure high quality, undamaged nanotubes. SWNTs enriched in metallic content were purchased from outside vendors for evaluation for the nanoUmbilical project. Multiwall nanotubes and purified SWNTs were used for making preliminary PNU wire to optimize the processing steps.

Produce wires. Wire was produced primarily using three densities of polyethylene. These wires were made both with and without nanotubes. Key factors for the PNU included nanotube dispersion, nanotube alignment, and alignment using electric fields.

Test the properties of the wires. The wires were tested at Rice and NanoRidge for electrical conductivity. Steady state and overload conditions were evaluated.

Conduct SWCNT-SWCNT connection study. Rice conducted a SWNT-SWNT connection study. Nanotubes in the polymer were isolated to evaluate conduction and degree of connectedness using TEM microscopy. Metal nanoparticles were added to the polymer/nanotube extrusion runs to evaluate their effectiveness on electrical conductivity, with no improvement shown.

Disperse SWCNTs. Agglomerated nanotubes limit the degree of conduction that can occur; therefore, the nanotubes were well dispersed in the polymers prior to extrusion and alignment. Since thermoplastics were used, the incipient wetting approach developed by Barrera was used. This approach disperses nanotubes on polymer powder to foster dispersion during the melt processing. This is a patented process that has been proven on several polymer systems including polyethylene, polypropylene, ABS, and epoxy resin. This method showed limited improvement in electrical conductivity in this application.

Electric field use. While dispersion and alignment have been achieved by mechanical mixing and extrusion, further alignment and nanotube connectedness were achieved by use of electric fields.

SWCNT wires characterization. Rice and NanoRidge characterized the SWNT containing wires. These tests were used throughout the project to evaluate the electrical properties of the wires. TEM and SEM were used to evaluate nanotube alignment.

Physical tests. Since the electrical conductivity of the prototype wires did not meet the ideal goals, rigorous physical testing was not conducted.

Connector study. Rice University utilized both solid core and stranded copper wire in their test fixture. Both forms proved to be acceptable.

Project Start: December 5, 2008
Project End: December 4, 2009

DOE Contribution: $ 448,000
Performer Contribution: $ 112,000

Contact Information:
RPSEA – Art Schroeder (aschroeder@rpsea.org or 713-372-2817)
NETL – Jay Jikich (Sinisha.Jikich@netl.doe.gov or 304-285-4320)
NanoRidge Materials – Dean Hulsey (wdhulsey@nanoridge.com or 713-928-6166 ext 17)

Additional Infomation

Final Project Report [PDF-18.0MB]