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.

Functioning signal lights, which are used to ensure smooth operation on the platforms, run day and night and are essential to the safety of nearby ships, aircraft and the platform itself. That is why every rig is designed to accommodate customized and ruggedized uninterruptible power supply (UPS) systems.

Particularly critical to the offshore site is the ability of the UPS system to withstand the most severe environments—storms, salty air, intense vibration, high heat or extreme cold. All this plus corrosion, dust, atmospheric gas, electrical failures and other unforeseen problems can contribute to glitches in the power grid.

Offshore UPS systems must meet strict national and international industrial standards, including those required by the International Association of Classification Societies, American Bureau of Shipping, Lloyd’s Register and others.

UPS system engineers first determine the installation needs. How much space is available for the UPS system? Platform real estate is valued at about $50,000/sq ft. What amount of battery charger capacity is needed? How much power load can the UPS system support during backup? How much will the system weigh? Where will the platform be located?

Other factors include the ability of the equipment to operate when the platform is moving. It must be able to withstand pitch and yaw. The cabinet containing the UPS system must be more robust with particular attention paid to preventing corrosion and overheating. To ensure a continuous power supply for the lighting of the platform, each UPS system uses a distribution panel with a safe parallel feed from the standard power supply and the emergency grid.

An UPS offshore power room diagram is shown. (Image courtesy of AMETEK Solidstate Controls)

Maintaining the UPS system

Maintaining an offshore UPS system is a delicate balancing act. When a platform is abandoned for safety reasons, the navigation beacons must stay on even though the batteries could be drained completely after the four-hour backup capacity has been used up and power for the generators has been depleted. That means the UPS system must increase its charging power.

The critical UPS component most affected by high ambient temperatures is the UPS battery, but other internal UPS components, such as DC bus filtering capacitors, might have their service life shortened by high-ambient temperatures unless special high-temperature components are selected.

Hybrid static switches

Another key component of all UPS systems is a hybrid static switch. Only the bypass pole of the static switch has the inverse-parallel silicon-controlled rectifier 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.

If the UPS hybrid static switch contactor contacts fail to open, the UPS output will be connected continuously to the bypass source. The mean time between failure of the UPS system is directly linked to the reliability of the static switch—the power path between the inverter and critical load.

UPS battery

The UPS systems employed on the oil-drilling sites use a much lower DC link voltage (60-cell, 125 voltage per cell), which has the advantages of using a UPS battery with fewer inter-cell connections and increased reliability. Typically, the charger capacity has to be robust because the battery support times can range from 60 minutes to eight hours or more. Also, it is important that each UPS system has enough battery recharge capacity built into the charger.

Unlike their commercial counterparts, offshore UPS installations have longer battery support times and use a higher end voltage of 1.75 voltage per cell.

Overall, the life expectancy of an offshore UPS system can be anywhere from 10 to 15 years. The equipment, including the cooling fans and DC filter capacitors, are designed to have more than 100,000 hours of mean time between failure under normal operation.

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