September 2020

43 www.drivesncontrols.com September 2020 SERVICE AND REPAIRS n Compressor motor repair reveals design flaw H igh-pressure gas compressors are vital pieces of equipment for reducing the volume of the gas from floating production storage and offloading (FPSO) vessels before it is transferred to shore. When one such system, operating off the coast of Ghana, was inspected, a long-term defect was discovered in the compressor motor that needed to be resolved. Initially the operator contacted the OEM and aked them to repair the motor’s connections, but as soon as the motor was restarted, it failed immediately. The OEM said that it would be impossible to rewind the motor offshore, while the platform operator did not think it would be possible to remove the motor for an onshore repair. The operator contacted the repairs specialist Sulzer, which offered to investigate the issue and sent engineers to the FPSO to carry out an initial inspection. They tested the motor’s windings and found very low insulation resistance readings. The motor would need to be dismantled to locate the source of the problem precisely. Sulzer’s initial suggestion was to perform a complete rewind onboard the vessel. But once the motor had been separated into its component parts, the engineers discovered that it would be possible to remove it and transfer it onshore. This offered a number of potential advantages, including the ability to carry out a full-load test on the motor once it had been repaired. It transpired that the compressor had been modified to boost its output and this had required additional power from the motor. In fact, rather than operating at 8.5MW as designed, the motor was required to supply 9.3MW, so the ability to perform full-load testing would increase confidence in the future reliability of the motor. Once the decision had been taken to complete the rewind onshore, Sulzer’s field service engineers took winding measurements and sent them to the company’s service centre in Falkirk, Scotland, before the motor itself was air-freighted to the centre to be rewound. “In these applications, speed of response is very important; downtime on such vital pieces of equipment can run into millions of dollars,”points out Marc Stuart, who manages the Falkirk centre.“We fully expected to complete this project within the 45 days that had been agreed with the customer and we worked around-the-clock to get the stator rewound and the motor reassembled.” The new windings were designed and manufactured in Birmingham using the latest in insulation material technology, which has improved since the coils were originally manufactured. The result is that the copper content has been increased by 1.82%, improving the motor’s efficiency and reducing the coil operating temperatures. For projects of this size, Sulzer works with Norwich-based ATB Laurence Scott, which has one of the largest motor testbeds in the UK. This facility was used to carry out a full-load test on the repaired motor, which revealed a cooling issue with the original design. The air flows were unbalanced, with the non-drive end receiving air at 13m/s, while the drive end air flow was negligible. An investigation was instigated and return of the motor was delayed until an answer could be found. Design engineers from Sulzer held discussions with both ATB Laurence Scott and the FPSO operator to examine the options. Making any alterations to the motor design had to be considered carefully because the machine needs to rotate at 1,800 rpm in a potentially explosive atmosphere. New parts were designed that could withstand the operating stresses. Once complete, the new parts were installed and the full-load test repeated. This time there were no anomalies and the air flows were balanced. The motor was returned to Falkirk, where it was dismantled before being airfreighted back to Ghana. The machine has now been returned to the FPSO where it is operating reliably. The customer is now looking to have similar work done on five more motors, which could have the same design flaw. n When repairing a powerful compressor motor from an offshore gas production vessel, UK engineers discovered a problem with the motor’s design that meant it was not being cooled properly. The damaged motor’s rotor was balanced dynamically before being reassembled. Work on the stator rewind continued around-the-clock to get the motor back into service as quickly as possible