Power Electronics

UPS System for Offshore Platform Requires More Safeguards

For safety reasons, an Uninterruptible Power Supply is mandatory on oil rigs to ensure that signal lighting is operational at all times. And, the UPS must be able to operate properly in a difficult, hazardous environment.

Design of a UPS solution for an offshore rig off the African coast, faces three challenges:

  • Protecting the UPS against a potentially explosive environment
  • Ability to withstand high temperatures
  • Able to survive in the 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 (Fig. 1).

BEAUTIFUL WEATHER, RUGGED ENVIRONMENT

Besides explosive atmospheres occurring occasionally, 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, the battery charger was installed 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 easy access to the CUBEx unit during maintenance. CUBEx is an R. STAHL brand name for a series of enclosures that are suitable for use in hazardous areas. These types of cabinets feature various special protective measures.

A Zone classification is used in Europe and other parts of the world in a similar way as the Division/Class classification is used in North America. Zone 0/20, 1/21, and 2/22 are differentiated by the likelihood of an explosion. Zone 1 can be equivalent to Division 1, but this is a broad generalization.

A stainless steel fan for hazardous areas with a motion monitoring function is installed above the enclosure to facilitate heat dissipation (Fig. 2). 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.

The required explosion protection for Zone 1 is ensured by a special unique feature: 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. Ex Zone is a term commonly used in Europe to designate hazardous areas with a significant risk of explosions.

Integrated Ex UPS guard (Fig. 3) is the name R. STAHL uses for specific technical functions. They can't divulge any design specifics regarding the actual implementation (wiring/components), but this is a protection mechanism that monitors the functions of the battery charger and the batteries. So if there is any kind of malfunction, not to mention a total failure of the charger or battery, the Guard function will signal that to alert the users/plant operators. ATEX clauses and the IEC/EN 60079 et seq. international standards specify certain functional/technical requirements for such a guard feature, and R. STAHL's solution complies with these requirements.

ATEX is an acronym derived from the French term “atmosphures explosibles,” i.e. “explosive atmospheres.” It's the name of the European Union Directive that applies to hazardous areas, i.e. areas where dust or atmospheric gas is present that can be ignited by e.g. sparks. Hence, any electrical equipment that is operated in these areas must be properly protected. An EU Directive is basically a sort of legal template or framework: once established, the stipulations laid down in the Directive are adapted by all European Union member states into their various national laws. To say that some component or system “complies with the ATEX Directive” is shorthand for saying, “is designed for use in hazardous areas in the way legally required by the various applicable national laws/standards that are based on and reference the ATEX Directive throughout the European Union.”

CONTINUOUS LIGHTING WITH A DOUBLE POWER SUPPLY

To ensure a continuous power supply for the lighting, a distribution panel was installed with a safe parallel feed from the standard power supply and the emergency grid (Fig. 4). 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, the UPS was able to implement a very compact, weight-optimized 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.

These are methods of protecting an atmosphere containing dangerous gases or particulate matter (dust) from ignition sources by specific technical measures defined in the IEC 60079 standard. For all intents and purposes, it suffices to say that an “Ex d” enclosure is a “flameproof” casing, and an “Ex e” cabinet is one featuring “increased safety.” Both shield the surrounding atmosphere from electrical equipment contained within the enclosure. Since the entire field of explosion protection for electrical equipment is originally a domain of German engineering, these letters come from German terms, namely, d from “druckfest gekapselt,” and e from “erhuhte Sicherheit.” The latter literally translates to “increased safety.” The former actually means “pressure-proof encapsulation,” but in this case “flameproof” has been established as the verbatim English term in the standards.

100% PROCESS SAFETY IN HAZARDOUS AREAS: MODULAR UPS SOLUTIONS IN DETAIL

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, the UPS 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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