Section 1 Hazardous areas – General
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Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Offshore Units, July 2022 - Part 7 Safety Systems, Hazardous Areas and Fire - Chapter 2 Hazardous Areas and Ventilation - Section 1 Hazardous areas – General

Section 1 Hazardous areas – General

Parent topic: Chapter 2 Hazardous Areas and Ventilation

1.1 Application

1.1.1 Units for oil and gas exploitation, units with production and process plant, drilling plant, pipe-laying system and other units where explosive gas-air mixtures are likely to be present are to be classified into ‘hazardous areas’ and ‘non-hazardous areas’ in accordance with the requirements of this Chapter, or alternatively, with an acceptable Code or Standard giving equivalent safety.

1.1.2 These requirements do not apply to the release of explosive gas-air mixtures as a consequence of an uncontrolled well blow out or catastrophic failure of pipes or vessels.

1.1.3 For special requirements relating to drilling, workover and wirelining operations, see Pt 7, Ch 2, 3 Hazardous areas – Drilling, workover and wirelining operations.

1.1.4 For special requirements relating to units intended for the storage of oil in bulk, see Pt 7, Ch 2, 9 Additional requirements for electrical equipment on oil storage units for the storage of oil in bulk having a flash point not exceeding 60°C (closed-cup test). For special requirements relating to units intended for the storage of liquefied gases in bulk or other hazardous liquids, see Pt 7, Ch 2, 10 Additional requirements for electrical equipment on units for the storage of liquefied gases in bulk and Pt 7, Ch 2, 11 Additional requirements for electrical equipment on units intended for the storage in bulk of other flammable liquid cargoes and Pt 11, Ch 10 Electrical Installations.

1.1.5 The hazardous areas applicable to well testing will be specially considered.

1.2 Definitions and categories

1.2.1 A hazardous area is an area on the unit where flammable gas-air mixtures are, or are likely to be, present in sufficient quantities and for sufficient periods of time such as to require special precautions to be taken in the selection, installation and use of machinery and electrical equipment.

1.2.2 Hazardous areas may be divided into Zones 0, 1 and 2, defined as follows:

Zone 0: An area in which ignitable concentrations of flammable gases or vapours are continuously present or present for long periods.

Zone 1: An area in which ignitable concentrations of flammable gases or vapours are likely to occur in normal operation.

Zone 2: An area in which ignitable concentrations of flammable gases or vapours are not likely to occur, or in which such a mixture, if it does occur, will only exist for a short time.

Non-hazardous areas are those which are not classified as hazardous according to the above definitions.

1.2.3 An enclosed space is considered to be any building, room or enclosure, e.g. cabinet, within which, in the absence of artificial ventilation, the air movement will be limited and any flammable atmosphere will not be dispersed naturally.

1.2.4 A semi-enclosed space is considered to be a space which is adjoining an open area where the natural ventilation conditions within the space are restricted by structures such as decks, bulkheads or windbreaks in such a manner that they are significantly different from those appertaining to the open deck, and where dispersion of gas may be impeded.

1.2.5 When an enclosed or semi-enclosed space is provided with a mechanical ventilation system which ensures at least 12 air changes/hour and no pockets of stagnant air within the space, such a space may be regarded as an open space.

1.2.6 An open space is an area that is open-air without stagnant regions where vapours are rapidly dispersed by wind and natural convection. Typically, air velocities will rarely be less than 0,5 metres per second and will frequently be above 2 metres per second.

1.2.7 Under normal operating conditions, a hazardous zone or space may arise from the presence of any of the following:
  1. Spaces or tanks containing any of the following:
    1. Flammable liquid having a flash point not exceeding 60°C closed-cup test;
    2. Flammable liquid having a flash point above 60°C closed-cup test, heated or raised by ambient conditions to a temperature within 15°C of its flash point;
    3. Flammable gas.
  2. Piping systems or equipment containing fluid defined by Pt 7, Ch 2, 1.2 Definitions and categories 1.2.7.(a) and having flanged joints, glands or other fittings through which leakage of fluid may occur.
  3. Piping systems or equipment containing flammable liquid not defined by Pt 7, Ch 2, 1.2 Definitions and categories 1.2.7.(a), and having flanged joints, glands or other fittings through which leakage of fluid in the form of a fine spray or mist could occur.
  4. Equipment associated with processes such as battery charging or electrochlorination which generate flammable gas as a by-product, and having vents or other openings from which gas may be released.
1.2.8 Release of explosive gas-air mixtures may be categorised into continuous, primary and secondary grades:
  1. Continuous grades of release include the following:
    1. The surface of a flammable liquid in a closed tank or pipe.
    2. A vent or other opening which releases flammable gases or vapours frequently, continuously or for long periods.
  2. Primary grades of release include the following:
    1. Pumps and compressors with standard seals, and valves, flanges and fittings containing flammable fluids if release of fluid to atmosphere during normal operation may be expected.
    2. Sample points and process equipment drains, which may release flammable fluid to atmosphere during normal operation.
    3. Pig launcher and receiver doors, which are opened frequently.
    4. Vents which frequently release small quantities, or occasionally release larger quantities, of flammable gases to atmosphere.
    5. Tanks or openings of the active mud circulating system between the well and the final degasser discharge, which may release gas during normal operation.
    6. Drilling operations in enclosed or semi-enclosed spaces, see Pt 7, Ch 2, 3 Hazardous areas – Drilling, workover and wirelining operations.
  3. Secondary grades of release, include the following:
    1. Pumps and compressors, and valves, flanges and fittings containing flammable fluids.
    2. Vents which release flammable gases intermittently to atmosphere.
    3. Tanks or openings of the mud circulating system from the final degasser discharge to the mud pump connection at the mud pit.
    4. Drilling, workover and wirelining operations in open spaces, see Pt 7, Ch 2, 3 Hazardous areas – Drilling, workover and wirelining operations.

1.3 Documentation

1.3.1 Single copies, unless otherwise stated, of the following documentation on ‘hazardous areas’ are to be submitted for consideration:
  • Hazardous area classification philosophy.
  • Hazardous area classification design specifications.
  • Facility layout plans (plot plans).
  • Hazardous area classification schedule (data sheets), see also Pt 7, Ch 2, 1.3 Documentation 1.3.2.
  • Hazardous area classification plans.
1.3.2 It is expected that the data sheets, referred to in Pt 7, Ch 2, 1.3 Documentation 1.3.1, include, but are not limited to, the following information:
  • Equipment identification.
  • Operating conditions.
  • Media and media properties.
  • Fluid category.
  • Sources of potential release.
  • Grades of release.
  • Venting rates.
  • Hazardous zones determined.
  • Dimension of each hazardous zone.
  • Code or Standard used for reference.
1.3.3 Single copies, unless otherwise stated, of the following plans and particulars on ‘ventilation’ are to be submitted for consideration:
  • Ventilation design philosophy.
  • Ventilation design specifications.
  • Ventilation layout plans.
  • Ducting and instrumentation plans (D & IDs).
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