Another Day at the Gear Hospital
Over the past 40 years, we have had many gearboxes come to our door that have had chronic, operational issues in need of remedial treatment. Sometimes, the fix is straightforward, and sometimes it’s pretty complex.
This last year, The Gear Works became involved in a gearbox repair and upgrade project that pushed the limits of our gear manufacturing and testing capability, requiring the combined forces of some of the most respected engineers in our industry. The decision to take on the project was based on a desire to increase our experience and capacity in repairing high-speed, high-horsepower gear drives, while recognizing it had the potential of becoming the dreaded job from hell. Just like many gearbox projects, the customer had a painful history of failures and unsatisfactory performance before The Gear Works was finally contacted.
The saga began over ten years ago after a major oil company commissioned a large Floating, Production Storage and Offloading (FPSO) vessel to pump crude oil from submerged oil wells located in the North Atlantic. The ship is one of the largest ever built, approximately the size of three football fields with the capacity to store 960,000 barrels of oil. From a network of flexible flowlines protruding from a rotating collar at the bottom of the hull, oil is pumped into onboard holding tanks and then offloaded onto shuttle tankers for shipment to refineries. The vessel is also equipped with a sophisticated global positioning system, plus bow and stern thrusters, so its heading can be maintained into the prevailing seas. The ship was designed and built to operate 24 hours a day for many months at a time. A centralized onboard electrical power station provides all of the electricity required to power the production equipment and other ancillary demands of the vessel. The ship is equipped with two enormous generators, each driven by a Frame 6 gas turbine engine through a 1.42 / 1 reduction gearbox. Each system can produce up to 54 megawatts, or enough onboard capacity to power over 50,000 average households. Figure 1
Bring in the Experts
Not long after the ship began full operation, performance issues arose with the gearboxes resulting in multiple gear tooth failures. After several unsuccessful attempts by the original equipment manufacturer to correct the problems, the owners decided to contact third-party experts. The firm of Artec Machine Systems of North Brandford, CT. was hired to perform onboard inspections, technical field support, an engineering analysis and logistical assistance for replacement gear elements. In addition, Bob Errichello and Jane Muller from GEARTECH of Townsend, MT. were contracted to perform a failure analysis of the failed gear sets and conduct a forensic review of the available inspection reports, metallurgical lab results, and all original manufacturing documents. GEARTECH’s investigation revealed that there were several failure modes. The most prevalent root cause was the result of material transformation of the gear teeth, which over time negatively affected the load distribution across the tooth mesh. This phenomena coupled with an imprecise helix modification severely reduced the tooth contact area. In another incident, grind temper was discovered as the cause of failure. Artec was contracted to procure a new replacement gear set from The Gear Works made to an upgraded design under the quality procedures and requirements as directed by GEARTECH. The gears would be installed in a spare gear case, spin tested and delivered to the customer before the end of the 2013 summer and the oncoming rough seas. Figure 2
This Isn’t Going to be Easy
Transmitting 54 megawatts, or 72,000 horsepower, through one gear mesh at a pitch-line velocity of 29,000 feet per minute (147 m/s) is not child’s play. Unforeseen consequences can occur under these critical conditions. Our advantage was that we knew from past experience what didn’t work and that the end customer was committed to doing the job right. Reliability of the gearbox trumped low bid.
The first task was to redesign the gears to mitigate the thermal effects operating along the 20.5” tooth width. Because of the high RPM and the lower helix angle, the axial meshing velocity was in excess of Mach 3 causing churning, shearing and heating of the oil and air vapor as they were pumped along the face width of the gear. The original O.E.M. unsuccessfully attempted to deal with this issue by providing a 2” relief gap, centered mid-face. Using decade’s old research by the Maag Gear Wheel Company and years of field observations of single helical, high speed gearing contact patterns, Artec modified the pinion lead to compensate for the non-linear thermal distortion and the torsional/bending deflections. The coast flanks were kept straight to facilitate alignment in the field. The helix angle was increased from 8 degrees to 10 and the modular pitch was slightly reduced to maintain the original center distance. Tip relief on the gear and pinion was added by GEARTECH. The gear geometry changes resulted in a 20% reduction in the axial mesh velocity and eliminated the need for the relief gap. Figure 3
The second task was to develop a set of quality procedures that would dictate the manufacturing requirements and documentation needed to assure the end customer that past quality issues could be avoided. Several pre-production meetings were held at The Gear Works with a cadre of seasoned metallurgists, mechanical and test engineers, field technicians, and production experts to add input to the quality procedures being compiled by GEARTECH. The result was a phonebook-sized quality procedure manual, running several hundred pages, citing dozens of specifications and standards from organizations such as AGMA, ISO, API, ASTM, ASM and SAE. The manual became the gospel guiding The Gear Works in manufacturing, quality assurance, and product testing throughout the project. The scope of work also included a factory spin test of the new gear set before final shipment.
Let’s Make Chips
The manufacturing process began with procurement of special steel forgings (18-CrNiMo-7-6) from Europe for the new gear and pinion. Strict chemical composition and cleanliness specifications had to be held along with a forging reduction requirement of 5:1 minimum. The forgings were rough turned, magnetic particle and ultrasonic inspected for cracks and inclusions before shipment to the U.S. with the certification of Lloyd’s Register. Upon receipt of the material in the plant, customer pressure was high to keep the project on schedule through the machining and gear cutting operations despite the mountain of technical challenges. Figure 4
To assure proper case hardening, distortion control and optimal stock removal, a sacrificial test gear was made with the same alloy and gear geometry as the redesigned low speed gear. The 4,000 pound test gear was hobbed, tooth charted, and then case carburized by a pre-approved vendor. Subsequently, the test piece was re-charted for distortion analysis and then sectioned and inspected for surface and core properties at a metallurgical testing laboratory. Those results helped predict the reaction of the alloy to a specific oven using a predetermined carburizing and quench recipe. When the actual replacement gears were case hardened, the microstructure and hardness results were accurately forecasted and within the specified requirements. Gear tooth distortion was held within acceptable limits and mirrored the test gear fairly closely.
After post heat treat blank preparation, the tooth flanks were finished ground on a Gleason-Pfauter P1600G gear grinding machine to AGMA-A4 accuracy tolerances. To minimize the risk of grind temper (the gear man’s curse), CBN grinding wheels were employed in lieu of dressable vitrified wheels. Enhanced, pre-grind tooth mapping to identify the locations of maximum positive grind stock was also used. After the final grind and application of the topological lead modifications to the pinion flanks, the results were independently verified on a CNC gear analyzer in the Company’s temperature controlled metrology laboratory. Additionally, both gear elements were mounted in a roll stand to check for contact patterns on both loaded and unloaded gear flanks. These contact patterns were later used to confirm proper mounting and alignment in the gear case. The gear case was also inspected for bore alignments and sizing on a horizontal boring mill. Figure 5
To check for grind temper, the gears underwent a nital-etch inspection per ISO-14104 under the watchful eye of Bob Errichello. Within four hours of completing the inspection, the gears were then oven baked at 325 degrees F. to eliminate the risk of hydrogen embrittlement. After some final machining, burnishing of proximity probe diameters and dynamic balancing to API 613, the 8600 pound gear set was ready for assembly.
Sleepless in Seattle
The scope of work required that the gearbox be assembled and tested at The Gear Works per an extensive procedure provided by Artec. Because of its overall weight of 25,000 pounds, the gearbox was assembled on riser blocks mounted right on the test bench. In addition to the upgraded gears, the assembly required a replacement quill shaft, a new thrust retaining hub, new tilting pad radial bearings for the pinion and new journal bearings for the gear. Two new oil spray manifolds for the gear mesh were also designed and shop tested. Of the 65 gpm of turbine oil supplied to the gears, 20% was directed to the input mesh and 80% to the exiting mesh. Another 210 gpm of oil was provided to the bearings via a network of internal passages and external plumbing. The gearbox was also outfitted with 13 proximity probes and accelerometers plus 7 thermocouples. Throughout the assembly process, Artec provided invaluable, hands-on technical direction. Figure 6
The no-load spin testing proved to be challenging, and expensive. To meet API testing requirements, the gearbox needed to be incrementally ramped up to the operating output speed of 3600 rpm, held constant for 4 hours and then pushed to a 10% over-speed condition for 10 minutes. Easy to say, tough to do. Not looking for yet another reason to invest more hard-earned capital into the business, The Gear Works forged ahead anyway with a major upgrade to its test bench. To manage the oil distribution system, the company procured a 950 gallon reservoir, 6 large air-to-oil heat exchangers, and 3 large pump packages at 50 H.P. each to supply, scavenge, circulate and filter the oil at a rate of 300 gallons per minute. To drive the gearbox, a 1000 HP, 4160 volt motor and a large variable frequency drive to control the speed was also purchased. Because all this equipment would be operating at the same time, special permission was granted from the local utility to consume up to 4 megawatts of power available to The Gear Works’ assembly facility.
After several trial runs, official testing took place in late summer of 2013 under the witness of Lloyd’s Register. Artec took responsibility to install and monitor the instrumentation while The Gear Works’ test engineer and support staff manned the motor controls and cooling system. As the RPM was slowly increased to operational speed, ambient noise levels in the test room and heat exchanger bay reached levels where ear protection was mandatory. To mitigate complaints from the neighboring businesses, testing took place in the evening. Despite conservative estimates to size the test motor, 100% of motor rating was reached at 3600 rpm, and pushed to 130% to meet the 10% over-speed condition. Lesson learned: size the test motor at twice estimated load. Notwithstanding the challenges, the gearbox operated very smoothly and within the specified vibration limits. After the test, the gearbox was partially disassembled and inspected for bearing wear and gear tooth contact. Everything looked great. The gearbox was then shipped and successfully installed on the vessel with the oversight of an Artec engineer before the onslaught of inclement weather. Figure 7
Break Out the Champagne?
Six months after installing and operating the upgraded gearbox, a thorough inspection of the rotating elements was conducted by Artec onboard the FPSO vessel. To everyone’s delight, the vibration and temperature records revealed no indication of problems and the gears looked beautiful, showing no signs of distress. We are now looking forward to refurbishing a second gearbox to serve as a backup to the two in service. None of this would have been possible without the collaboration, coordination and close relationship between our company and the engineers at Artec Machine Systems and GEARTECH. I also owe a debt of gratitude to Mr. Richard Meredith of D.C. Energy who guided The Gear Works through the design and development of the test bench, and Mr. Rainer Eckert of Northwest Laboratories who rendered invaluable metallurgical expertise. With this close alliance and all this experienced talent, I see more extraordinary projects and sleepless nights ahead.