ISO 15138:2000

INTERNATIONAL ISO
STANDARD 15138
First edition
2000-11-01
Petroleum and natural gas industries —
Offshore production installations —
Heating, ventilation and air-conditioning
Industries du pétrole et du gaz naturel — Installations en mer —
Chauffage, ventilation et climatisation
Reference number
©
ISO 2000
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ii © ISO 2000 – All rights reserved

Contents Page
Foreword.iv
1 Scope .1
2 Normative references .1
3 Terms and definitions .1
4 Abbreviated terms .2
5 Design .3
5.1 Introduction.3
5.2 Development of design basis.6
5.3 System design — General .10
5.4 Area-specific system design .11
Annex A (informative) Guidance to clause 5 Design.14
Annex B (informative) Equipment and bulk selection.41
Annex C (informative) Installation and commissioning guidance .61
Annex D (informative) Operation and maintenance.66
Annex E (informative) Data sheets .69
Annex F (informative) Flange standard.101
Bibliography.104
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights.
International Standard ISO 15138 was prepared by Technical Committee ISO/TC 67, Materials, equipment and
offshore structures for petroleum and natural gas industries, Subcommittee SC 6, Processing equipment and
systems.
Annexes A through F of this International Standard are for information only.
iv © ISO 2000 – All rights reserved

INTERNATIONAL STANDARD ISO 15138:2000(E)
Petroleum and natural gas industries — Offshore production
installations — Heating, ventilation and air-conditioning
1 Scope
This International Standard specifies requirements and provides guidance for design, testing, installation and
commissioning of heating, ventilation, air-conditioning and pressurization systems and equipment on all offshore
production installations for the petroleum and natural gas industries which are:
� new and existing;
� normally occupied by personnel and not normally occupied by personnel;
� fixed or floating but registered as an offshore production installation.
NOTE For installations that could be subject to "Class" or "IMO/MODU Codes & Resolutions", the user is referred to HVAC
requirements under these rules and resolutions. Should these requirements be of a lesser degree than those being considered
for a fixed installation, then this International Standard, i.e. requirements for fixed installation, should be utilized.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of
this International Standard. For dated references, subsequent amendments to, or revisions of, any of these
publications do not apply. However, parties to agreements based on this International Standard are encouraged to
investigate the possibility of applying the most recent editions of the normative documents indicated below. For
undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC
maintain registers of currently valid International Standards.
ISO 8861, Shipbuilding — Engine-room ventilation in diesel-engined ships — Design requirements and basis of
calculations.
IEC 60079-10, Electrical apparatus for explosive gas atmospheres — Part 10: Classification of hazardous areas.
3 Terms and definitions
For the purposes of this International Standard, the following terms and definitions apply.
3.1
displacement ventilation
movement of air within a space in piston- or plug-type motion
NOTE No mixing of room air occurs in ideal displacement flow, which is desirable for removing pollutants generated within
aspace.
3.2
fixed offshore installation
all facilities located and installed on fixed offshore structures, which are provided to extract oil and gas
hydrocarbons from subsea oil and gas reservoirs
3.3
fixed offshore structure
structure permanently fixed to or located on the sea bed, including moored ships and hulls, which is held in position
by anchors or tensioned cables and is provided to (structurally) support topsides facilities
NOTE Vessels and drilling rigs, etc. which are in transit or engaged in exploration and appraisal activities are specifically
excluded from this definition.
3.4
fugitive emission
emission which is always present on a molecular scale from all potential leak sources in a plant under normal
operating conditions
NOTE As a practical interpretation, a fugitive emission is one which cannot be detected by sight, hearing or touch but may
be detected using bubble-test techniques or tests of a similar sensitivity.
3.5
open area
area in an open-air situation where vapours are readily dispersed by wind
NOTE Typical air velocities in such areas should rarely be less than 0,5 m/s and should frequently be above 2 m/s.
3.6
temporary refuge
TR
place where personnel can take refuge for a pre-determined period whilst investigations, emergency response and
evacuation pre-planning are undertaken
[ISO 13702:1999, definition 2.1.52]
4 Abbreviated terms
AC Alternating Current
AC/h Air Changes per hour
AHU Air Handling Unit
AMCA Air Movement and Control Association Inc.
API American Petroleum Institute
ASHRAE American Society of Heating, Refrigerating and Air-Conditioning Engineers
BS British Standard
CCR Central Control Room
CFD Computational Fluid Dynamics
CIBSE Chartered Institution of Building Services
CMS Control and Monitoring System
CVU Constant-Volume Terminal Reheat Unit
DC Direct Current
2 © ISO 2000 – All rights reserved

DE Driven End
DX Direct Expansion
EN European Standard
ESD Emergency Shutdown
F&G Fire and Gas
HAZOP Hazard and Operability Study
HSE Health, Safety and Environment
HVAC Heating, Ventilation and Air Conditioning
HVCA Heating and Ventilating Contractors' Association
IEC International Electrotechnical Commission
IMO International Maritime Organization
IP Institute of Petroleum
IP Integrity Protection
LFL Lower Flammable Limit
LQ Living Quarters
MODU Mobile Offshore Drilling Unit
NDE Non-driven End
NFPA National Fire Protection Association
NS Norsk Standard (Norwegian Standard)
QRA Quantitative Risk Analysis
r.m.s. Root mean square
5 Design
5.1 Introduction
Clause 5 together with annex A provide requirements and guidance on all aspects of the design of heating,
ventilation and air-conditioning (HVAC) systems for offshore installations for the petroleum and natural gas
industries. The HVAC systems form part of the safety services of the installation. The safety goals are to:
� prevent, through pressurization, the ingress of potentially flammable gas-air mixtures into all designated
nonhazardous areas;
� prevent the formation of potentially hazardous concentrations of flammable gaseous mixtures in hazardous
areas by provision of sufficient ventilation for the dilution, dispersion and removal of such mixtures;
� maintain ventilation to all equipment and areas/rooms which are required to be operational during an
emergency when the main source of power is unavailable;
� provide a controlled environment in which personnel, plant and systems can operate effectively, including
smoke control.
These high-level goals are supported by lower-level goals of a functional nature which are stated later in the
appropriate sections of this document.
Subclause 5.2 concentrates on functional requirements in the development of a basis of design for either a new
project or major modification to an existing installation. The requirements are related to:
a) platform orientation and layout;
b) hazard identification and hazardous area classification;
c) environmental conditions;
d) choice of natural or mechanical ventilation systems;
e) development of the controls philosophy;
f) material selection;
g) design margins and calculations;
h) design development and validation using wind tunnel testing or Computational Fluid Dynamics (CFD).
Ventilation may be natural (i.e. the wind) or mechanical or a combination of both. Throughout this International
Standard, the use of the term "ventilation" should be taken to include either natural or mechanical ventilation, as
appropriate.
Natural ventilation is preferred over mechanical ventilation where practical, since it is available throughout gas
emergencies, does not rely on active equipment and reduces effort required for HVAC maintenance.
For new designs, the development of a design basis may be progressed using the guidance and examples of good
engineering practice that are identified in this document, though it should be recognized that it involves a process
of iteration as the design matures and does not take place as the sequential series of steps used in this document
to facilitate presentation. The processes outlined here are equally applicable to major redevelopments to existing
installations, but some compromise may need to be made as a result of historical decisions regarding layout,
equipment selection and the prevailing level of knowledge at the time. The challenge of providing cost-effective
solutions in redevelopment may be significantly greater than for a new design.
The finalized basis of design may be recorded on data sheets such as those provided in annex E.
The completed design should be subject to hazard assessment review. The Hazard and Operability Study
(HAZOP) technique may be used for this.
In 5.2, objectives are identified which establish the goals. Functional requirements are outlined which will enable
the objectives to be achieved. The functional requirements are supported by technical guidance given in annex A,
which discusses the suitability of different techniques for different applications and identifies examples of good
engineering practice or cost-effective solutions that have been used in some parts of the world. The functional
requirements may be satisfied by other methods not identified in this document, but it is the responsibility of the
user to assess whether the method is technically acceptable and acceptable to the local regulator.
Subclause 5.3 addresses the fundamental choice in system design, i.e. between natural and mechanical methods
of ventilation.
4 © ISO 2000 – All rights reserved

Subclause 5.4 gives functional requirements associated with the design of HVAC systems for different areas of a
typical offshore installation which require particular technical considerations due to their location and/or their
function.
Figure 1 is intended to illustrate the processes undertaken at various stages of the installation life cycle and to
identify reference documents and the appropriate clauses of this International Standard which provide the
necessary guidance.
Figure 1 — Application of this International Standard to a project life cycle
5.2 Development of design basis
5.2.1 Orientation and layout
5.2.1.1 Objective
To provide input into the early stages of design development so that areas and equipment that may have a
requirement for HVAC, or be affected by its provision, are sited in an optimum location, so far as is reasonably
practicable.
5.2.1.2 Functional requirements
Installation layout requires a great deal of coordination between the engineers involved during design and the
operation, maintenance and safety specialists. Attention shall also be paid to the minimization of construction,
offshore hook-up and commissioning. It is not the intention of this International Standard to detail a platform-layout
philosophy, but to identify areas where considerations of the role of HVAC, and requirements for it, might have an
impact in the decision-making surrounding installation orientation and layout.
Each installation should have a temporary refuge (TR). The TR should in almost all cases be the LQ, where they
are provided. The survivability of the TR, which is directly related to the air change rate, may introduce
consideration of active HVAC systems for pressurization of the Living Quarters (LQ) or enclosed escape and
evacuation routes. Active systems require detailed risk assessment exercises to be undertaken as part of the
design verification, and passive systems are generally preferred since they do not rely on equipment functioning
under conditions of emergency.
Hazardous areas, particularly those containing pressurized hydrocarbon systems, should be located as far as
practicable from the TR so that any gas leaks will be naturally dispersed.
The layout shall include correct positioning of ventilation inlets and outlets, engine inlets and exhausts, vents and
flares to allow for safe operation, particularly of the TR. Hot exhausts shall not interfere with crane, helicopter,
production or drilling operations or the LQ, and shall be directed so as not to be drawn into gas turbine air intakes.
Air intakes to hazardous and nonhazardous areas shall be located as far as is reasonably practicable from the
perimeter of a hazardous envelope and not less than the minimum distance specified in the prevailing area
classification code.
For guidance, reference is made to clause A.1.
5.2.2 Hazardous area classification and the role of HVAC
5.2.2.1 Objective
To adopt in the design and operation processes a consistent philosophy for the separation of hazardous and
nonhazardous areas and the performance of ventilation in those areas.
5.2.2.2 Functional requirements
IEC 60079-10 shall be used for classification of a hazardous area. The choice of hazardous area code determines
the choice of equipment to be used in particular areas of the installation and also provides input to the performance
standards for HVAC systems in those areas.
For guidance, reference is made to clause A.2.
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5.2.3 Environmental conditions
5.2.3.1 Objective
To determine an environmental basis of design that enables HVAC systems to be designed in order to meet the
objectives for HVAC.
5.2.3.2 Functional requirements
External and internal environmental bases suitable for the location of the installation shall be established for the
design.
For guidance, reference is made to clause A.3.
5.2.4 Natural/mechanical ventilation
5.2.4.1 Objective
To select a means of providing ventilation to any hazardous or nonhazardous area of an installation.
5.2.4.2 Functional requirements
Provide ventilation to any area which may require it, giving consideration to:
a) meteorological conditions, particularly prevailing wind and its strength, external temperature, and precipitation;
b) risk-driven segregation of hazardous areas;
c) heating and cooling design loads;
d) life cycle costs of the purchase and maintenance of mechanical HVAC and associated Emergency Shutdown
(ESD) systems;
e) environmental considerations, such as personnel comfort, particulate control, and noise;
f) weather integrity of instrumentation and controls;
g) need for structural integrity;
h) control and recovery from hydrocarbon loss of containment;
i) process heat conservation.
NOTE Many of these factors are controlled by local legislation, which should be consulted for implications.
For guidance, reference is made to clause A.4.
5.2.5 Selection of controls philosophy
5.2.5.1 Objective
To provide a system for controlling HVAC systems from a frequently manned location that provides the operator
with essential information on the status of the plant and is integrated with the installation fire and gas (F&G) and
ESD systems, so that actions in an emergency minimize the risk to personnel.
5.2.5.2 Functional requirements
The control and monitoring system shall:
a) provide the operator with the status of the HVAC plant;
b) provide the minimum necessary controls for the plant consistent with the operation and maintenance
philosophies;
c) provide a link to the installed F&G and ESD systems, if required;
d) comply with the installation smoke and gas control philosophy.
For guidance, reference is made to clause A.5.
5.2.6 Operating and maintenance philosophy
5.2.6.1 Objective
To provide an HVAC design which provides as high a degree of operational availability, so far as is reasonably
practicable, within the constraints imposed by installed cost, maintenance resources and the consequences of
failure.
5.2.6.2 Functional requirements
The design shall include the necessary standby arrangements, plant operating margins, access provisions and
requirements for routine maintenance to enable a specified operational availability to be achieved at minimum cost
over the lifetime of the installation.
For guidance, reference is made to clause A.6.
5.2.7 Materials and corrosion
5.2.7.1 Objective
To specify materials and protective coatings for equipment and components that minimize, as far as is reasonably
practicable, life cycle costs for the installation and potential harm to personnel who may be affected by their
operation.
5.2.7.2 Functional requirements
The design shall recognize the saliferous atmosphere and relative humidity that will be present throughout the
installation life.
Non-combustible, non-toxic materials shall be used throughout; such materials, when heated, shall not emit toxic
fumes.
The design shall recognize that operation in conjunction with flammable atmospheres may be required for some
components.
For guidance, reference is made to clause A.7.
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5.2.8 Design margins and calculations
5.2.8.1 Objective
To ensure that design integrity is demonstrated in the provision of cost-effective HVAC systems by calculations
which take due account of the accuracy of HVAC system data and extremes of design environmental conditions.
5.2.8.2 Functional requirements
The design shall be documented in accordance with suitable industry standards, i.e. those of ASHRAE, CIBSE or
similar recognized authorities.
Specification of equipment shall recognize the maturity of the design and the level of information provided by other
disciplines in the design process.
For guidance, reference is made to clause A.8.
5.2.9 Wind tunnel and CFD modelling
5.2.9.1 Objective
To undertake a modelling programme that reproduces installation conditions within a reasonable accuracy, so that
design options may be consistently evaluated and the chosen option optimized with a high degree of confidence
that the design performance will be replicated by actual measurements.
5.2.9.2 Functional requirements
If required, a modelling programme shall be undertaken to predict
� natural ventilation rates and frequencies;
� wind pressure distribution around the installation to fix air inlet and outlet positions;
� requirements for secondary ventilation;
� gas build-up inside hazardous modules;
� helideck configurations and operating envelopes;
� hot plume and contaminant (noxious exhaust and hydrocarbon) smoke or gas flows around the installation.
For guidance, reference is made to clause A.9.
5.2.10 Performance standards
5.2.10.1 Objective
To define performance standards for HVAC systems which may be used as a basis for managing risk throughout
the life of the installation.
5.2.10.2 Functional requirements
Performance standards are statements which can be expressed in qualitative or quantitative terms of the
performance required of the system, item of equipment, person or procedure, and which are used as a basis for the
management of risk throughout the installation life. They shall be set commensurate with the magnitude of the risk
to be managed and shall clearly define the level of performance required for compliance.
For guidance, reference is made to clause A.10.
5.3 System design — General
5.3.1 Natural ventilation
5.3.1.1 Objective
Natural ventilation shall, wherever possible, be provided to
� dilute local airborne concentrations of flammable gas due to fugitive emissions;
� reduce the risk of ignition following a leak by quickly removing accumulations of flammable gas.
5.3.1.2 Functional requirements
It is important to note that the distribution of air within an area/module is considered to be at least as important as
the quantity of air supplied. As a consequence, compliance with the following basic requirements is necessary if
ventilation of an area/module by natural means alone is to be considered sufficient:
� minimum ventilation rate shall be provided throughout the area;
� minimum ventilation rate shall be as stated for mechanical ventilation.
Consideration shall be given to the working environment by the adoption of a natural ventilation philosophy.
For guidance, reference is made to clause A.11.
5.3.2 Mechanical ventilation
5.3.2.1 Objective
To provide mechanical ventilation when ventilation by natural means is unable to satisfy requirements.
5.3.2.2 Functional requirements
The HVAC systems shall be designed to prevent contamination between areas and maintain acceptable working
and living environments for personnel and non-destructive conditions for equipment.
For practical reasons, systems may be separated for the following areas:
a) nonhazardous areas;
b) hazardous areas;
c) living quarters;
d) areas to be in operation during emergency situations;
e) auxiliary systems for naturally ventilated areas;
f) drilling areas;
g) substructure;
h) areas with contaminated air (separate extract).
The minimum ventilation air volumes shall be documented.
For guidance, reference is made to clause A.12.
10 © ISO 2000 – All rights reserved

5.3.3 Secondary ventilation systems
5.3.3.1 Objective
To provide a system to supplement natural or mechanical ventilation when the distribution of air is not adequate.
5.3.3.2 Functional requirements
Stagnant areas formed by structural steelwork, decking plates, sumps and equipment, etc. shall be assessed and
ventilated accordingly.
Consideration shall be given to the dilution of fugitive hydrocarbon emissions and dissipation of internal heat gains.
A uniform ventilation pattern shall be provided between primary supply and extract points.
For guidance, reference is made to clause A.13.
5.4 Area-specific system design
5.4.1 Process and utility areas
5.4.1.1 Objective
To provide mechanical ventilation when ventilation by natural means is unable to satisfy requirements.
5.4.1.2 Functional requirements
Systems provided for hazardous areas shall be entirely separate from those serving nonhazardous areas. The
system design shall include a fan-powered supply plant which draws 100 % of its outside air from a nonhazardous
area and supplies it to the module.
Airflow in terms of air changes per hour for a nonhazardous area shall be sufficient to meet the pressurization
requirements determined by the local regulation or hazardous area code classification adopted, free-cooling and
personnel needs. Specific exhaust requirements, e.g. fume cupboards, welding benches/booths, etc., may also
require consideration.
At outside maximum and minimum design temperatures, areas shall not exceed the temperature set by local
regulation, applicable codes of practice or company standards. Refer to Table A.2 in A.3.4.
Air change rates determined in 5.2.2 shall be applied, but at a rate sufficient to dilute fugitive hydrocarbon
emissions. Any free-cooling requirements for the area shall also be met.
For guidance, reference is made to clause A.14.
5.4.2 Living quarters
5.4.2.1 Objective
To provide a controlled environment for personnel.
5.4.2.2 Functional requirements
HVAC systems shall be designed to maintain internal-air conditions defined by local legislation or codes of practice
at maximum and minimum outside air conditions, taking into account detectable and latent loads from lighting,
personnel and other sources.
Ventilation rates (in terms of air changes per hour) shall be established from requirements for heating or cooling
outdoor air and the need for pressurization.
The air change rates for galley and laundry shall include consideration of the cooling load requirements imposed by
the equipment and the efficiency of extract hoods and systems.
The design of supply and extract systems shall include adequate protection to prevent cross-contamination or the
circulation of odours through the LQ on partial or total system failure.
For guidance, reference is made to clause A.15.
5.4.3 Drilling and drilling utility areas
5.4.3.1 Objective
To provide mechanical or natural ventilation to satisfy requirements.
5.4.3.2 Functional requirements
The HVAC system shall be designed to reduce the exposure of personnel to dust, fumes, heat, etc. to the level
specified by local legislation or industry codes.
Any special requirement for the ventilation of equipment in normal or emergency operation shall be incorporated in
the design.
For guidance, reference is made to clause A.16.
5.4.4 Gas turbine enclosures
5.4.4.1 Objective
To provide sufficient ventilation to satisfy requirements.
5.4.4.2 Functional requirements
The ventilation system shall be designed to remove heat from machinery and to dilute flammable gas.
Pressurization shall be provided in accordance with the hazardous area classification philosophy adopted for the
area.
For guidance, reference is made to clause A.17.
5.4.5 Emergency plant ventilation
5.4.5.1 Objective
To provide sufficient ventilation to equipment that may be required to operate in an emergency under the conditions
that may be present at the time.
5.4.5.2 Functional requirements
HVAC systems serving emergency plants shall be designed to ensure the equipment continues to function under
the conditions prevailing when it is called upon to work.
For guidance, reference is made to clause A.18.
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5.4.6 Battery and charger rooms
5.4.6.1 Objective
To provide sufficient ventilation to satisfy requirements.
5.4.6.2 Functional requirements
The HVAC system serving battery and charger rooms shall be sized for the removal of all noxious and flammable
products and any heat generated.
For guidance, reference is made to clause A.19.
5.4.7 Laboratories
5.4.7.1 Objective
To provide sufficient ventilation to satisfy requirements.
5.4.7.2 Functional requirements
Laboratories shall be provided with a dedicated fan-powered extraction system to prevent personnel exposure to
exhaust fumes.
For guidance, reference is made to clause A.20.
5.4.8 Purge air systems
5.4.8.1 Objective
To provide sufficient ventilation to satisfy requirements.
5.4.8.2 Functional requirements
HVAC services for air supplies to components and plant requiring to be kept purged, such as draw-works and
rotary table motors, shall at all times have outside air drawn from a nonhazardous source.
For guidance, reference is made to clause A.21.
5.4.9 Rooms protected by gaseous extinguishing agents
5.4.9.1 Objective
To provide sufficient ventilation to satisfy requirements.
5.4.9.2 Functional requirements
Where rooms are provided with gaseous extinguishing systems, ventilation supply and exhaust dampers and fans
shall be interlocked with the F&G control to shut prior to gas release.
For guidance, reference is made to clause A.22.
5.4.10 Engine-room ventilation in diesel-engined ships
Design requirements and basis of calculation shall be in accordance with ISO 8861.
Annex A
(informative)
Guidance to clause 5 Design
A.1 Guidance to 5.2.1
Results of wind tunnel model tests or CFD calculations on the installations should be considered in order to
determine the external zone(s) of wind pressure in which to locate the intake(s) and outlet(s) for the HVAC
system(s). Particular care should be taken in locating air intakes and discharges with regard to the location's
coefficient of pressure and its subsequent effect on fan motor power.
The underside of a platform can be a convenient location for HVAC inlets and outlets because a large proportion of
the below-platform zone may be classified as nonhazardous and have stable wind conditions. However,
consideration should be given to the effects of the wind and waves and the location of items such as dry powder
dump-chutes and cooling water discharges, when locating the outdoor air intakes and extract discharges below the
platform.
Air intake and discharge from the same system on conventional installations should, where reasonably practical, be
located on the same face of the installation or in external zones of equal wind pressure. Particular care should be
taken in orienting air intakes and discharges on systems serving adjacent hazardous and nonhazardous areas,
such that whilst the wind may affect the absolute values of pressurization in each area, the differential pressure
requirements between them will not vary to a significant degree. For floating production systems (FPS) however,
the downwind area may provide an appropriate intake location but it will be necessary to show that it is positioned
to avoid ingestation of smoke or contaminants and capable of operation in adverse weather.
Air intakes should be located to avoid cross-contamination from
� exhausts from fuel-burning equipment;
� lubricating oil vents, drain vents and process reliefs;
� dust discharge from drilling, dry powders;
� helicopter engine exhaust;
� flares;
� supply and support vessels.
The siting of gas turbines and generators usually poses a challenge to layout designers. They should be located in
a nonhazardous area and with consideration to the following points.
a) The air intake should be sited the maximum possible distance from hazardous areas and as high above sea
level as possible to avoid water ingress (an absolute minimum of at least 3 m above the 100-year storm wave
level). If enclosed, the intakes should be located such that powder and dust is not ingested. Since most
particulate matter in the air is generated on the platform from drilling operations and grit blasting, the preferred
arrangement is for air intakes to be located above the upper-deck level.
b) Recirculation from the exhaust back to the inlet should be avoided and be demonstrated by wind tunnel tests
or CFD. These tests should also show that exhaust flue emissions do not interfere with helicopter, production,
drilling and crane operations.
14 © ISO 2000 – All rights reserved

In the absence of any performance standards set by the local aviation authority, a maximum air temperature rise
above the surface of the helideck should be agreed for helicopter operation.
Computer models are available to model hot and cold plume dispersion patterns and may be used to establish
outlet positions but the final layout/model should be wind tunnel tested at an early stage in platform design
development.
A.2 Guidance to 5.2.2
The application of a recognized hazard identification and assessment process may identify a role for the separation
and segregation of inventories on an installation. Area classification codes specify separation distances between
hazardous and nonhazardous areas in order to avoid ignition of those releases which inevitably occur from time to
time in the operation of facilities handling flammable liquids and vapours.
All area classification codes should be interpreted in a practical manner. They offer only best guidance and often
the particular circumstances require a safety and consequence review and the subsequent application of "as safe
as is reasonably practicable" approach to location of classified area boundaries and potential ignition sources
nearby. In order to correctly and consistently establish area zoning, historical data from similar plant operating
conditions may be used as a basis for assessment.
Ventilation impacts upon hazardous area classification and provides a vital safety function on offshore installations
by
� diluting local airborne concentrations of flammable gas due to fugitive emissions;
� reducing the risk of ignition following a leak by quickly removing accumulations of flammable gas.
The quantity of ventilation air to maintain a non-flammable condition in areas with fugitive emissions can be
calculated from data in API 4589 [29], using the methodology given in API RP 505 [28].
It is recommended that areas be classified using the general guidance of IEC 60079-10. Specific guidance for
classifying petroleum facilities can be found in documents such as The Institute of Petroleum Area Classification
Code for Petroleum Installations, Model Code of Safe Practice Part 15 (IP Code Part 15) and API RP 505 [28].
It should be recognized that a higher level of ventilation than the default lower limit of acceptable ventilation given
in the hazardous area codes might be required to
� provide a suitable atmosphere for personnel and equipment;
� remove excess heat;
� provide enhanced rate of ventilation to mitigate against the build-up of potentially explosive atmosphere.
A.3 Guidance to 5.2.3
A.3.1 External meteorological conditions
In the absence of local regulations, the need for shelter should be evaluated and may reveal a subsequent need for
an HVAC system.
The design of the HVAC systems should be based on local regulations or design codes. Conservative selection of
criteria may carry a cost, weight and power penalty.
Seasonal extremes of temperature, humidity and wind speed vary widely throughout the world, and local
regulations governing working conditions may also dictate the allowable extremes in occupied or unoccupied
spaces. Local environmental information should be specified in the basis of a design that does not result in
additional capacity being installed to cater for a small proportion of the year during which meteorological extremes
are encountered.
Sub-local effects on the external environmental conditions should be considered for design purposes in case they
have any influence on the design, such as heating of the air before the air reaches the intakes, intake
contamination, shading of solar radiation, reflection of solar radiation from the sea surface, changes in wind speed
and direction and consequently wind pressure.
Effective temperatures, resulting from wind chill or heat loading, should be determined to establish the effects on
personnel operating efficiency (where personnel are required to work in thermally uncontrolled areas) and
equipment, and consequently the extent of any required protection. In determining operating efficiency,
consideration should be given to the nature of the work (sedentary or physical) being undertaken.
There are various agencies that can provide meteorological information. Most of these contribute to a worldwide
database that can be accessed by local meteorological services, but there are also individual databases. Those
data sets based on observations from passing ships are likely to be extensive, with many observations over a long
time period for those locations near to shipping lanes. Satellite measurement is increasing in terms of history, detail
and quality, and some agencies may provide data from this source for areas where ship data is not statistically
significant. A third alternative, but probably the least reliable, is the extrapolation of data from nearby onshore sites.
The following provides typical data that could be used to establish an environmental basis of design in an area
where microclimate is not an important factor and variations in any month follow a normal distribution:
Maximum temperature: 2 % probability of exceeding the all-year average
Minimum temperature: 2 % probability of exceeding the all-year average
Design wind speed: 1/12th year-1 h mean velocity at a reference height of 10 m
Maximum wind speed: maximum 1/12th average 3 s gust at the height of equipment
NOTE The 1/12th year mean condition is that which on average is exceeded 12 times a year.
Wind velocity data are usually reported at a standard 10 m height, but may be recorded at a different height on an
installation. The corrections factors in Table A.1 should be applied to the commonly reported 1-h mean wind
velocities.
Table A.1 — Wind corrections factors
Height above mean
Duration of gust Sustained mean wind duration
sea level
m 3s 15s 1min 10min 1h
10 1,33 1,26 1,18 1,08 1,00
20 1,43 1,36 1,28 1,17 1,09
30 1,49 1,42 1,34 1,23 1,15
50 1,57 1,50 1,42 1,31 1,22
60 1,59 1,52 1,44 1,34 1,25
80 1,64 1,57 1,49 1,39 1,30
100 1,67 1,60 1,52 1,42 1,33
120 1,70 1,63 1,55 1,46 1,36
150 1,73 1,66 1,58 1,49 1,40
Exponent (n) 0,100 0,100 0,113 0,120 0,125
EXAMPLE 1 Given a 1-h mean wind velocity of 24 m/s at 10 m height, the maximum 1-min sustained wind velocity at a
heightof50m is estimatedtobe24 m/s � 1,42 = 34 m/s.
Wind velocity factors at other heights can be obtained from the reference value at 10 m using the power law profile
in equation (A.1):
16 © ISO 2000 – All rights reserved

n
vv��h/10 (A.1)
� �
h 10
where
v is the velocity at 10 m above sea level;
v is the velocity at height h above sea level;
h
n is the power law exponent.
EXAMPLE 2 A wind measured at an average velocity of 7 m/s at deck level 50 m above mean sea level would have a
velocity at the 10 m base of
v��7 m/s 1,00/1,22� 5,77 m/s.
� �
In areas where there are high seasonal fluctuations from an average, such as in monsoon, typhoon and tropical
regions, it may be necessary to consider setting design criteria based on the number of days or hours of
exceedance if data is available for analysis in this form.
Where there is a significant microclimate, it may be necessary to analyse data under additional criteria for which
the following guidance is appropriate.
A.3.2 Maximum sea temperature
The maximum monthly average water temperature during the warmest month at the depth of abstraction, which
may be extrapolated from surface temperature measurements.
A.3.3 Direct and diffuse solar radiation intensities
For detailed design calculation, hourly radiation data for a period of clear days in the warmest month is necessary.
The period is considered to coincide with a period in which the maximum temperature and the coincident relative
humidity occur. The traditional method of designing structures assumes that the maximum room-cooling loads and
the maximum refrigeration load for air-conditioning occur simultaneously, but it should be noted that maxima of
room-cooling load may actually occur in a period which is not coincident with maximum outside temperature.
In the absence of solar radiation data for the location, data may be taken from a similar locality at the same latitude.
In the absence of collected data, calculated values can be applied from reference [32] or a similar reference.
The reflection from the sea surface may be taken as 20 % of the total radiation intensity.
Radiation heat gains from flare stacks should also be considered.
A.3.4 Internal environmental conditions
Two approaches may be used for the specification of internal environmental conditions. The traditional approach
relies on the specification of absolute values established by experience or local regulation. An alternative approach
based on a measurement of population acceptance is
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