UPS systems provide last line of defense for platform power grids
When it comes to power on an offshore oil rig, there is no on/off switch for crew shift changes. For safety as well as operational and productivity reasons, the flow of energy must be 24/7 and completely protected from possible blackouts and malfunctions in the main power supply caused by frequency fluctuations or lightning strike over-voltages.
Distributed control systems and other monitoring equipment are used to ensure smooth operation of offshore platforms. Navigational aid signal lights, running day and night, are essential to the safety of nearby ships, aircraft and the platform itself, which is why every rig is designed to accommodate customized, ruggedized uninterruptible power supply (UPS) systems.
A UPS system consists of one or more battery chargers, rechargeable batteries (lead acid or nickel cadmium, either flooded or valve regulated) usually installed in a dedicated room, battery health monitoring technology that includes alarm annunciators, DC and AC power distribution, along inverters for various critical loads on the platform. They are also equipped with special safeguards that will signal platform personnel when a malfunction occurs.
Since every platform is different, and every company has its own criteria for how a platform is designed and constructed, the UPS system is custom-designed to meet the project requirements. Special needs of the application, such as space, weight, and load requirements, are taken into account. Much depends on where the platform is going to be located. For example, fixed support legs are used in shallow-water areas to secure the platform to the seabed. Tethered and mobile drilling platforms used in deepwater areas have additional requirements. The cabinet containing the UPS system needs to be more robust, and greater attention is paid to corrosion prevention for the cabinet as well as all connected cables and electrical wiring. Combining flameproof enclosures with extra safe connection terminals, the UPS system is able to operate in a compact, weight-optimized installation space on the platform.
Particularly critical is the UPS’ ability to withstand the most severe environments imaginable for as long as possible. These environmental considerations, when combined with the possibilities for corrosion, dust, and atmospheric gas, electrical failures, and other unforeseen problems, can contribute to power glitches in the power grid, including the systems themselves, their batteries, and battery chargers.
To ensure a continuous power supply for the lighting of the platform, each UPS system uses a distribution panel with a parallel feed from the standard power supply and the emergency power supply.
To withstand the increased charging power while preventing corrosion and keeping safety measures at their highest levels, the UPS systems use ABF-approved epoxy coatings or nickel-plated copper bus bars, special anti-corrosion terminal connections, and acid resistant metal surface coatings. In extreme cases, the cooling air to the UPS cabinets is brought in from a clean (or scrubbed) source at a positive pressure and exhausted out of the cabinet tops to keep the salty air and high humidity from attacking the circuitry. In addition, all terminals to the UPS enclosure and to the cables that take the power to the rest of the platform must be secured and protected.
The service life of any offshore UPS system is reduced by operation in high ambient temperatures. The battery is most affected by high ambient temperatures; other internal components, such as DC bus filtering capacitors, may have their service life shortened by high ambient temperatures unless special high-temp components are selected.
That is why these offshore systems have built-in design margins to preserve the operation life in tougher environments. In addition, the equipment also will be accompanied by predictive parts replacement programs to insure high UPS mean time between failures (MTBF).
A key component of all UPS systems is a hybrid or “wrap-around” static switch. Only the bypass pole of the static switch has the inverse-parallel silicon controlled rectifier (SCR) pair for electronic power switching. The inverter side of the hybrid static switch uses a power relay with normally open contacts to disconnect the inverter from the bypass during the normal static switch critical load transfer operation.
Under critical load fault, downstream of the UPS output, the short circuit current is limited only by the impedance of the bypass source. The high fault current levels can weld the contactor’s electrical contacts closed. If the UPS hybrid static switch contactor contacts fail to open, the UPS output will be connected continuously to the bypass source. UPS inverters, especially some PWM inverters, are not designed to have their AC outputs back-fed from the bypass for more than 30-50 milliseconds.
UPS systems in offshore and other heavy-duty applications use inverse-parallel SCR devices on both the inverter and bypass poles. This modification to the hybrid static switch is a more expensive design feature, but the elimination of the contactor increases the overall static switch reliability. The MTBF of the UPS is directly linked to the reliability of the static switch, which is in the power path between the inverter and critical load.
CPI introduces products for compressor condition monitoring
Compressor Products International (CPI) has introduced the new Proflo USB-IR Adaptor and Proflo Assist Software for condition monitoring of the critical reciprocating compressor lubrication equipment used on offshore drilling platforms and floating production storage and offloading (FPSO) vessels.
The Proflo USB-IR Adaptor and Proflo Assist Software read and transfer information from the Proflo PF1 monitoring device, which records the average cycle time for each 30-minute block of lubrication system operation to a Microsoft Windows-based laptop, tablet or other USB equipped device through the use of infrared technology. The accompanying Proflo Assist Software presents the cycle time trend data to operations and maintenance personnel, delivering powerful, convenient condition-based maintenance capabilities.
The Proflo PF1 monitors a compressor lubrication system, alerting an operator or engaging shutdown protection based on the cycle time of the system. Real-time access to this information can help technicians intervene and prevent a range of problems, including the extrusion of packing and piston rings, locking of the packing case, valve stiction and more.
“Our Proflo product line has long been one of the most powerful weapons in the battle against over- and under-lubrication,” Robert Spadt, CPI Product Manager for Jubrication Products, said. “Real-time operational data is a prerequisite for any condition-based or predictive maintenance program. And the valuable condition monitoring data the system collects is now even easier to access thanks to this USB receiver and software interface.”
The software will operate on Windows versions 2000, XP, 7, 8 or 10. Each USB-IR adaptor can be used with multiple PF1 monitoring devices. Included are the USB-IR Adaptor, instructions and a download link for the Proflo Assist Software and a 6-ft USB male-to-female cable.
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