Diamond Technology

 

The Development of Open Water-lubricated Diamond Thrust Bearings for Use in MHK Energy Machines

(The Development of Open Water-lubricated Polycrystalline Diamond (PCD) Thrust Bearings for Use in Marine Hydrokinetic (MHK) Energy Machines)



Craig H. Cooley, Michael M. Khonsari, Ph.D., Brent Lingwall

diamond engineering

Introduction

A polycrystalline diamond (PCD) bearing has been developed that will operate successfully in Marine Hydro-kinetic (MHK) machine applications. PCD bearings represent a new class of bearings that have, heretofore, not been extensively studied. This work makes significant contributions to understanding the performance of these bearings.

The study results will be presented in the main body of the report approximately as outlined in the Schedule of Project Objectives (SOPO). This organization will, hopefully, help in understanding the thought and reasoning process as progress toward the final MHK diamond bearing designs was made. (Appendix 11.1 is an abridged summary of the work performed during the project and it is formatted like the quarterly reports)

The environment for MHK machines is difficult and presents challenges to conventional bearing technology. Conventional roller and sliding bearings require protection from the submerged environment and must be provided with lubrication. In addition they are subjected to frequent starting and stopping, contaminants and long time periods between maintenance. These are challenges that are beyond conventional marine bearing technology. The goal of this work was to provide to the MHK community a robust and simple bearing technology that would contribute to the commercial success of MHK by meeting the challenges of the MHK applications.

This work proposes PCD for bearings in the MHK application. PCD is not in the lexicon of materials used daily by most engineers. It is considered exotic, but it should not be. Recent work by US Synthetic (USS) has shown that its range of economic application can and should be greatly expanded. Among the properties that make PCD an excellent bearing material are: thermal conductivity approximately twelve times that of steel, hardness that surpasses any other know material, exceptional strength and resistance to abrasion.

PCD has found use as bearings in the oil and gas drilling environment where it has been used in down-hole drilling tools such as drilling motors, turbines, and steering devices. In this environment PCD has proven to be economical and operate successfully even while being subjected to abrasive lubrication, shock loading, and extended life requirements. It is believed that these attributes that make PCD bearings successful in application involving the oil and gas well bore will also contribute to making it successful in MHK applications.

One key similarity is the cost of bearing replacement in both applications. Replacing drilling equipment requires the drilling rig to stop drilling and extract and replace a long string of drill pipe (referred to as ‘tripping’) which is expensive (off shore it costs over $500,000 per trip) and 2 drives the economics that makes cost effective the use of a more expense material like diamond (PCD) for use in bearings. The same would be true of an MHK application which might require the use of special ships or rigs to extract the machine and replace failed equipment. It is easy to imagine that maintenance of MHK machines would be equivalent to, or even more expensive than, the trip necessary to change down-hole drilling tools.

PCD bearings applied in MHK would make the use of the surrounding water as a lubricant possible. This in turn would allow for simple and robust bearing designs, which minimize maintenance and eliminate the potential of environmental lubricant leaks. Further the hardness and abrasive resistance of PCD would also minimize effects of sediment contamination on bearing life and performance, a potential issue in near shore tidal applications. In addition sediment contamination is less likely to affect bearing performance when PCD bearings are used. Finally, USS has demonstrated that these bearings operate effectively regardless of speed. Slow operation where the bearing surfaces are rubbing can be accommodated as can faster operation where a fluid-film is present (this will be clarified later). The ability to do this makes the start/stop operation of tidal MHK machines a non-problem.

An additional goal of this work was to move the technology readiness level (TRL) of PCD bearing technology forward, specifically from TRL 3 to TRL 4. To accomplish this a project objective and a specific set of tasks were identified and documented in a ‘Statement of Project Objectives’ (SOPO) document that included seven specific tasks to be accomplished. The project objective as stated in the SOPO is: “to demonstrate that a diamond thrust bearing will have the necessary life and operate with the necessary efficiency (lack of friction) under conditions similar to those expected in MHK applications.” To that end the tasks enumerated in the SOPO were completed and the results and conclusions for each task enumerated in the SOPO are reported in this document. The specific results for each task are given in Appendix 11.1.

The work presented shows that PCD bearings submerged in a water environment easily survive the anticipated MHK static and cyclical loads. In addition a simulation of start/stop operations demonstrated an anticipated life that could be equal to at least 11.5 years. In addition a basic understanding of the important performance variables was won, making it possible to make approximate design recommendations based on the bearings primary mode of operation, be it primarily slow running with bearing surfaces rubbing or quicker operation with bearing surfaces separated by a fluid-film.

Finally, USS has made a successful outreach to industry and the R&D community at large. Two papers have been accepted and will be presented over the course of the next year. A patent has been applied for and US Synthetic Bearings has identified at least three potential customers who seem willing to evaluate and perhaps test PCD diamond technology in their prototype MHK machines.


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