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Uninterruptible power supply for oil rig in tropics

18 January 2011

An uninterruptible power supply (UPS) is mandatory on oil rigs: for safety reasons, the signal lighting must be operational at all times. Tasked with designing a UPS solution for an offshore rig off the African coast, R. STAHL was faced with three challenges. The UPS system not only required protective measures against explosive atmospheres, but also had to be adapted to the great heat and maritime climate at the rig's location.

Complete blackouts or malfunctions in the mains power supply, such as frequency fluctuations or overvoltages caused by lightning strikes, must be safely bridged on oil rigs. Above all, functioning signal lights are not only essential for smooth operation on the rig, but also for the safety of all nearby ships and airplanes. Therefore, the position lights must not fail under any circumstances. The platform, which is situated off the African Atlantic coast and whose cavern is currently being used as a temporary store for natural gas, is usually supplied by means of a solar power plant. A battery-powered emergency grid was installed in parallel. In addition to the power supply unit, R STAHL assembled further energy distribution components from its modular kit for UPS system solutions

In addition to explosive atmospheres occurring occasionally (Ex Zone 1), the tropical climate and sea air had to be taken into account. The temperature on the platform often exceeds 35°C, and occasionally reaches peaks of 50°C. At such ambient temperatures, the operation of the charger for the UPS batteries, which generates a great deal of power loss, required special measures. In order to prevent overheating, R. STAHL installed the battery charger in the largest available CUBEx enclosure. Several special features ensure heat dissipation: by means of a patented cold plate technology, the heat is directly evacuated from the protective enclosure. The heat sink is attached to the side of the enclosure, allowing for easy access to the CUBEx unit during maintenance.

A stainless steel fan for hazardous areas with a motion monitoring function, which is installed above the enclosure, facilitates heat dissipation. The fan also had to be adapted for use in Zone 1 under extreme environmental conditions. The forced cooling is controlled and monitored by an intelligent control system: the operating mode of the fan is adjusted depending on the current ambient temperature. Thanks to these measures, a maximum power loss of 1,000 W can be compensated in addition to the amount dissipated through the enclosure surface by default. The considerable amount of exhaust heat generated by charging the batteries can thereby be released to the environment without compromising the operation of the charger.

Like in all other UPS solutions from R. STAHL, the required explosion protection for Zone 1 is ensured by a special feature which is unique on the market: an integrated Ex UPS guard monitors the functions of the battery charger and the batteries, which are also installed within the Ex zone, according to the ATEX directive and according to IEC/EN 60079 ff, thereby allowing for safe operation.

In order to ensure a continuous power supply for the lighting, R. STAHL installed a distribution panel with a safe parallel feed from the standard power supply and the emergency grid. The lighting distribution panel features two interlocked inputs. Additionally, each outgoing line is separately monitored for ground faults. All functions can be activated from the outside, and the status of each outgoing line is clearly indicated by the position of the handle. By installing a combination of CUBEx flameproof enclosures (Ex d) and enclosures with increased safety (Ex e) connection chambers, R. STAHL was able to implement a very compact, weight-optimised system within the limited installation space on the offshore platform. All outside metal parts of the seawater-resistant CUBEx enclosures are made from SS316 stainless steel. This material requires no further surface treatment for use in maritime environments.

UPS solutions component range includes battery chargers, various explosion-protected rechargeable batteries (lead gel, NiCd, NiMH), charge monitoring technology including alarm annunciators, DC power distributors and inverters for various performance classes up to 3,000 VA. Additionally, R. STAHL provides a wide range of explosion-protected enclosures, such as the GUBox series which features Ex e connection chambers. R STAHL also provides modules with user-specific explosion protection types as required.

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