FPSO Handbook by Angus Mather Printed and bound in Great Britain by Bell & Bain Ltd, Glasgow Published in 2009 by Witherby Seamanship International Ltd 4 Dunlop Square, Livingston, Edinburgh, EH54 8SB, Scotland, UK Tel No: +44(0)1506 463 227 Fax No: +44(0)1506 468 999 Email: info@emailws.com Web: www.witherbyseamanship.com First edition published 2009 ISBN - 978 1 905331 57 4 © Witherby Seamanship International Ltd, 2009 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Notice of Terms of Use All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publishers. While the advice given in this book, FPSO Handbook, has been developed using the best information currently available, it is intended purely as guidance to be used at the user’s own risk. WSIL accepts no responsibility for the accuracy of any information or advice given in the document or any omission from the document or for any consequence whatsoever resulting directly or indirectly from compliance with or adoption of guidance contained in the document even if caused by failure to exercise reasonable care. This publication has been prepared to deal with the subject of FPSOs. This should not, however, be taken to mean that this publication deals comprehensively with all of the issues that will need to be addressed or even, where a particular issue is addressed, that this publication sets out the only definitive view for all situations. The opinions expressed are those of the author only and are not necessarily to be taken as the policies or views of any organisation with which he has any connection. iii About the Author The author, Angus Mather, commenced his career with the Ministry of Defence at Her Majesty’s Dockyard, Devonport, in 1970 as a Technician Apprentice. This was followed by 10 years’ service as an Engineer Officer in the British Merchant Navy prior to entering the oil and gas industry. The next 25 years were spent employed both in the contracting industry as Engineering Manager of Kvaerner Oil and Gas Services, and as a Classification Society surveyor, involved with oil and gas related projects, both on and offshore, in the UK, the Middle East, the Gulf of Mexico and West Africa. Author’s Introduction The exploitation of oil and gas reserves is a relatively new industry when compared to the marine industry which has been around in one form or another for centuries. In an FPSO, the two industries are married together in a very effective union that facilitates the exploitation of oil fields that are located either in waters beyond the reach of a conventional steel jacket structure, or where the field size would not justify the cost of such a permanent installation. This book, The FPSO Handbook, describes the marine equipment, the oil and gas processing arrangements, and the legislation pertaining to both industries, including marine classification issues. It has been written as a source of reference for marine and offshore professionals and it is envisaged that it will be of value to those involved in the design, construction, operation, inspection and maintenance of FPSOs, whether they are based onshore or offshore. Angus C. Mather Suffolk June 2009 Contents About the Author .............................................................................................................iii Author’s Introduction ......................................................................................................iii Acknowledgments ...........................................................................................................iv 1 The Hull ....................................................................................................................3 1.1 The Concept ................................................................................................................................................................................3 1.2 Construction ...............................................................................................................................................................................4 1.2.1 The Hull ........................................................................................................................................................................5 1.2.2 Topsides .......................................................................................................................................................................5 1.3 Confirmation of Design ...........................................................................................................................................................7 1.4 Conversion Versus Newbuild ................................................................................................................................................8 1.5 Hull Layout and Regulations ...............................................................................................................................................10 1.5.1 Topside Layout ........................................................................................................................................................10 1.5.2 Hull Layout ................................................................................................................................................................12 1.5.3 SOLAS Regulations ................................................................................................................................................14 2 Gathering Systems ................................................................................................19 2.1 Mooring Systems .....................................................................................................................................................................19 2.1.1 Mooring Arrangements .......................................................................................................................................19 2.1.2 Mooring Design ......................................................................................................................................................20 2.1.3 Mooring Components ..........................................................................................................................................22 2.1.4 Installing the Mooring System ..........................................................................................................................25 2.1.5 Planned Maintenance and Inspection ............................................................................................................25 2.2 Fluid Transfer Systems ...........................................................................................................................................................25 2.2.1 Turrets and Swivels ................................................................................................................................................25 2.2.2 CALM Buoy ...............................................................................................................................................................28 2.3 Pipeline Systems and Risers .................................................................................................................................................32 2.3.1 Pipeline Systems .....................................................................................................................................................32 2.3.2 Environmental Considerations ..........................................................................................................................33 2.3.3 Risers ...........................................................................................................................................................................39 2.3.4 Flexible Pipes ...........................................................................................................................................................42 3 Regulations ............................................................................................................49 3.1 Regulations – Statutory Legislation ..................................................................................................................................49 3.1.1 Section One - Statutory Regulations ...............................................................................................................51 3.2 Regulations – Ship Classification .......................................................................................................................................65 3.2.1 Class ............................................................................................................................................................................65 3.2.2 Surveys .......................................................................................................................................................................66 3.3 Regulations – Offshore Legislation ...................................................................................................................................73 3.3.1 Certification ..............................................................................................................................................................73 3.3.2 Verification – Overview ........................................................................................................................................79 3.3.3 Verification Schemes .............................................................................................................................................79 3.3.4 The Verification Process .......................................................................................................................................80 3.3.5 Summary of Applicable Legislation ................................................................................................................81 4 FPSO Personnel and Training ...............................................................................85 4.1 Personnel ....................................................................................................................................................................................85 4.1.1 Offshore Installation Manager (OIM) ...............................................................................................................85 4.1.2 Production Superintendent (FPSO) .................................................................................................................86 4.1.3 Control Room Operator (CRO) ...........................................................................................................................86 4.1.4 Medic/Safety Officer .............................................................................................................................................86 4.1.5 Electrician ..................................................................................................................................................................86 4.1.6 Production (Process) Operators/Technicians ...............................................................................................87 4.1.7 Deck Foreman .........................................................................................................................................................88 4.1.8 Marine Superintendent ........................................................................................................................................88 4.1.9 Marine Supervisor/Operator/Technician .......................................................................................................88 4.1.10 Helicopter Landing Officer (HLO) .....................................................................................................................89 v vi • FPSOs 4.1.11 Instrument Technician ..........................................................................................................................................89 4.1.12 Maintenance Superintendent ...........................................................................................................................89 4.1.13 Maintenance Technician ......................................................................................................................................89 4.1.14 Technical Clerk ........................................................................................................................................................90 4.1.15 Chemist .....................................................................................................................................................................90 4.2 Training Requirements ..........................................................................................................................................................90 5. Cargo Handling Systems .......................................................................................95 5.1 Cargo Handling Systems - Cargo and Ballast Systems ...............................................................................................95 5.1.1 Crude Oil Storage ...................................................................................................................................................95 5.1.2 Crude Oil Loading and Discharge Arrangements ......................................................................................96 5.1.3 Ballast Water System .............................................................................................................................................98 5.1.4 Crude Oil Washing (COW) ...................................................................................................................................99 5.1.5 Slop Tanks ..............................................................................................................................................................100 5.2 Cargo Tank Vent and Inert Gas Systems .......................................................................................................................101 5.2.1 Flare and Vent System .......................................................................................................................................101 5.2.2 Cargo Tank Vent and Pressure Relieving Systems ...................................................................................104 5.2.3 Inert Gas Systems ................................................................................................................................................106 6. Oil and Gas Production .......................................................................................117 6.1 Oil and Gas Production ......................................................................................................................................................117 6.1.1 Oil Production ......................................................................................................................................................117 6.1.2 Produced Water System ....................................................................................................................................121 6.1.3 Gas Production .....................................................................................................................................................122 6.1.4 Chemical Injection ..............................................................................................................................................127 6.1.5 Additional Processing Arrangements ..........................................................................................................128 6.2 Enhanced Oil Recovery and the Oil Drive Mechanism ...........................................................................................132 6.2.1 The Oil Drive Mechanism .................................................................................................................................132 6.2.2 EOR Techniques ...................................................................................................................................................132 6.2.3 Minox De-Oxygenation System .....................................................................................................................135 7. Pumping and Piping Systems .............................................................................139 7.1 Pumping and Piping Systems ..........................................................................................................................................139 7.1.1 Marine Systems ....................................................................................................................................................139 7.1.2 Utility Systems ......................................................................................................................................................143 7.1.3 Construction of Piping Systems .....................................................................................................................148 7.1.4 Piping Systems and Pressure Vessel Design ..............................................................................................156 8. Machinery, Equipment and Safety Systems ......................................................161 8.1 Safety and Emergency Systems ......................................................................................................................................161 8.1.1 Machinery and Electrical Installations .........................................................................................................161 8.1.2 Emergency Support Systems .........................................................................................................................163 8.2 Emergency Support Systems (ESS) ................................................................................................................................165 8.2.1 Emergency Shutdown System (ESD) ............................................................................................................166 8.2.2 Process ESD Valves ..............................................................................................................................................166 8.2.3 Gas Detection .......................................................................................................................................................167 8.2.4 Fire Detection .......................................................................................................................................................167 8.2.5 Containment Systems .......................................................................................................................................168 8.2.6 Fire Loop .................................................................................................................................................................168 8.2.7 Emergency Shutdown Valves – Pipelines ...................................................................................................169 8.2.8 Guidance Notes and Reference Standards ................................................................................................169 8.3 Process and Marine Machinery .......................................................................................................................................169 8.3.1 Process Machinery ..............................................................................................................................................169 8.3.2 Marine Machinery ...............................................................................................................................................171 9. Hazardous Areas, Ventilation and Hazardous Area Equipment ......................181 9.1 Hazardous Areas and their Determination .................................................................................................................181 9.1.1 Hazardous Areas ..................................................................................................................................................181 9.1.2 Determination of Hazardous Areas ..............................................................................................................182 vii 9.1.3 Enclosed and Semi-Enclosed Spaces ...........................................................................................................185 9.1.4 Hazardous Enclosed Spaces Located in Safe Areas ................................................................................187 9.1.5 Hazardous Enclosed Spaces Located in Hazardous Areas....................................................................188 9.1.6 Access to Hazardous Areas from Enclosed and Semi-Enclosed Non-Hazardous Areas ............188 9.2 Ventilation Systems and Machinery in Hazardous Areas .......................................................................................189 9.2.1 Ventilation Systems ............................................................................................................................................189 9.2.2 Ventilation of Gas Turbine Powered Enclosures .......................................................................................189 9.2.3 Machinery in Hazardous Areas .......................................................................................................................190 9.3 Hazardous Area Equipment ..............................................................................................................................................191 9.3.1 Selection of Equipment ....................................................................................................................................191 9.3.2 Equipment Protection .......................................................................................................................................191 9.3.3 Certification and Marking ................................................................................................................................194 9.3.4 Commissioning, Inspection, Maintenance, Testing and Repair of Electrical Equipment ..........195 9.3.5 Records ...................................................................................................................................................................195 9.3.6 Alternative Equipment Testing Authorities ...............................................................................................195 9.3.7 Guidance Notes and Reference Standards ................................................................................................196 10. Fire Protection, Fire Detection and Fire Extinction ..........................................199 10.1 Application, Basic Principles and Definitions .............................................................................................................199 10.1.1 Application ............................................................................................................................................................199 10.1.2 Basic Principles .....................................................................................................................................................199 10.1.3 Definitions .............................................................................................................................................................200 10.2 Active Fire-Fighting Systems ............................................................................................................................................200 10.2.1 Fire Control Plans ................................................................................................................................................200 10.2.2 Fire and Gas Detection ......................................................................................................................................200 10.2.3 Fire Main .................................................................................................................................................................202 10.2.4 Fire Pumps .............................................................................................................................................................204 10.3 Fire Tests and Material Certification ...............................................................................................................................205 10.3.1 Passive Fire Protection .......................................................................................................................................205 10.3.2 Fire Tests and Fire Divisions – Designations ..............................................................................................205 10.3.3 New Developments ............................................................................................................................................207 10.4 Construction and Passive Fire Protection Measures ................................................................................................207 10.4.1 Location and Separation of Spaces ..............................................................................................................207 10.4.2 Structural Arrangements ..................................................................................................................................208 10.4.3 Requirements for Helicopter Facilities .........................................................................................................208 10.4.4 Fire Integrity of Bulkheads and Decks .........................................................................................................208 10.4.5 Means of Escape ..................................................................................................................................................209 10.4.6 Ventilation Systems ............................................................................................................................................209 10.4.7 Penetrations in Fire-Resisting Divisions ......................................................................................................210 10.4.8 Restricted Use of Combustible Materials ...................................................................................................211 10.4.9 Furniture and Furnishings ................................................................................................................................211 10.5 Fixed Fire-Fighting Systems ..............................................................................................................................................211 10.5.1 Fixed Fire-Fighting Systems – Machinery Spaces ....................................................................................211 10.5.2 Fixed Foam Systems ...........................................................................................................................................211 10.5.3 Fixed Gas Fire-Extinguishing Systems .........................................................................................................212 10.5.4 Fixed Water Spray Systems ..............................................................................................................................212 10.5.5 Fixed Deck Foam Systems – Cargo Tank Protection ...............................................................................213 10.5.6 Deluge Systems ...................................................................................................................................................214 10.5.7 Paint Lockers .........................................................................................................................................................215 10.5.8 Accommodation Sprinkler Systems .............................................................................................................215 10.6 Miscellaneous Fire-Fighting Items .................................................................................................................................216 10.6.1 Portable Extinguishers ......................................................................................................................................216 10.6.2 Portable Appliances ...........................................................................................................................................217 10.6.3 Remote Stops and Closing Devices ..............................................................................................................217 10.6.4 Fire Drills .................................................................................................................................................................218 10.6.5 Firemen’s Outfits ..................................................................................................................................................218 10.6.6 Provision, Maintenance and Availability of Equipment ........................................................................219 10.6.7 Guidance Notes and Standards .....................................................................................................................219 viii • FPSOs 11. Lifesaving Appliances and Arrangements ........................................................223 11.1 Regulatory Requirements .................................................................................................................................................223 11.1.1 Application and Exemptions ...........................................................................................................................223 11.1.2 General ....................................................................................................................................................................223 11.1.3 Evaluation, Testing and Approval of Lifesaving Appliances and Arrangements ..........................223 11.1.4 Manufacture and Certification .......................................................................................................................223 11.1.5 Survey and Examination ...................................................................................................................................223 11.1.6 Lifesaving Appliance (lSA) Plan ......................................................................................................................224 11.1.7 Temporary Refuge and Means of Escape ...................................................................................................224 11.2 Communications ..................................................................................................................................................................224 11.2.1 Public Address System and Emergency Alarm System ........................................................................224 11.2.2 Communications .................................................................................................................................................224 11.2.3 Distress Signals.....................................................................................................................................................224 11.3 Personal Lifesaving Appliances .......................................................................................................................................225 11.3.1 Lifebuoys ...............................................................................................................................................................225 11.3.2 Lifejackets ..............................................................................................................................................................225 11.3.3 Immersion Suits and Anti-Exposure Suits ..................................................................................................226 11.3.4 Grab Bags ...............................................................................................................................................................226 11.4 Survival Craft ..........................................................................................................................................................................226 11.4.1 General Requirements .......................................................................................................................................226 11.4.2 Requirements for Fully Enclosed Lifeboats ................................................................................................227 11.4.3 Liferafts ...................................................................................................................................................................230 11.4.4 Rescue Boats .........................................................................................................................................................231 11.5 Training, Inspections, Maintenance and Drills ...........................................................................................................233 11.5.1 Operational Readiness and Inspections .....................................................................................................233 11.5.2 Emergency Training and Drills ........................................................................................................................233 11.5.3 Abandon Ship Drill .............................................................................................................................................233 11.5.4 Training Manual and Onboard Training Aids ............................................................................................233 11.5.5 Maintenance and Inspections ........................................................................................................................233 12. Standby Vessels (SBVs) and the Safety and Response Equipment they are Fitted with ........................................................237 12.1 The Ships .................................................................................................................................................................................237 12.2 The Equipment ......................................................................................................................................................................238 12.2.1 Fast Rescue Craft (FRCs) ....................................................................................................................................238 12.2.2 Daughter Craft......................................................................................................................................................239 12.2.3 FRC Davits ..............................................................................................................................................................239 12.2.4 Power Assisted Recovery Method .................................................................................................................239 12.2.5 Rescue Basket .......................................................................................................................................................240 12.2.6 Line Throwing Apparatus and Lifebuoys ....................................................................................................241 12.2.7 Helicopter Winching Areas ..............................................................................................................................241 12.2.8 Protective Spray System ...................................................................................................................................241 12.2.9 Survivors’ Facilities ..............................................................................................................................................241 12.3 Communications ..................................................................................................................................................................241 12.3.1 Air Communications ..........................................................................................................................................241 12.4 Responsibilities .....................................................................................................................................................................241 12.4.1 The Master .............................................................................................................................................................242 12.4.2 The Offshore Installation Manager (OIM) ...................................................................................................242 12.4.3 The ERRV Operator and the Operator of the Installation .....................................................................242 12.5 The Crews ................................................................................................................................................................................242 12.5.1 Manning .................................................................................................................................................................242 12.6 The Training ............................................................................................................................................................................242 12.6.1 Initial Training in Shipboard Operations .....................................................................................................242 12.6.2 Advanced Medical Aid (AMA) .........................................................................................................................243 12.6.3 CAA Certificate .....................................................................................................................................................243 12.6.4 FRC Boatman’s Course .......................................................................................................................................243 12.6.5 FRC Coxswain (cox’n) .........................................................................................................................................243 12.6.6 Onboard Training and Exercises ....................................................................................................................243 ix 12.7 The Tasks ..................................................................................................................................................................................243 12.7.1 Man Overboard (MOB) ......................................................................................................................................243 12.7.2 Helicopter Ditching ...........................................................................................................................................243 12.7.3 Installation Escape to the Sea ........................................................................................................................243 12.8 Operations ..............................................................................................................................................................................243 12.8.1 The State of the Equipment ............................................................................................................................244 12.8.2 The State of the Weather ..................................................................................................................................244 12.9 Additional Duties ..................................................................................................................................................................244 12.9.1 Guard Vessel ..........................................................................................................................................................244 12.9.2 Fishing Boats .........................................................................................................................................................245 12.9.3 Pollution Control .................................................................................................................................................245 12.10 Operation Jigsaw ..................................................................................................................................................................245 Appendix I Contents of the consolidated text of the Annex to the 1974 SOLAS Convention .................................................247 Appendix II Contents of the International Life-Saving Appliance Code ......................................................................................249 Appendix III Contents of the Code for the Construction and Equipment of Mobile Offshore Drilling Units ......................................250 Index ..............................................................................................................................253 100 • FPSOs It is very important that the atmosphere in the cargo tanks is permanently maintained in an inert condition while tank cleaning, particularly when using water, as the water spray generates static electricity which when released can easily ignite a gaseous mixture. Explosions in cargo tanks were a regular occurrence in the 1960s, prior to the introduction of inert gas systems. Discharges into primary slop tank Primary slop tankSecondary slop tank Discharge overboard Figure 5.14 – Slop tank decant system Figure 5.13 – Slop tank schematic Warm oily slop water provides a breeding ground for bacteria. The bacteria produce hydrogen sulphide (H2S) which can be very corrosive to the slop tank. It is important to regularly monitor the H2S level as levels of 200-300 ppm H2S can rapidly occur. However, the bacteria are easily eradicated by specific biocides. 5.1.5 Slop Tanks Slop oil tanks can best be described as large capacity holding tanks where the drains from the crude oil processing system, cargo handling system, tank 101 cleaning, open and closed drains and bilge systems are collected. Produced water may occasionally be routed into these tanks if its quality is not suitable for overboard discharge. The slop oil tanks are primarily designed to enable oil and drainage water to be separated prior to discharge into the sea. They may be designated as either clean or dirty tanks. To comply with MARPOL regulations, at least two slop tanks must be provided and these are normally located behind or at the side of the aft cargo oil tanks. The regulations require the capacity of the slop tanks to be at least 1.5% of the capacity of the cargo tanks. This means that, on a large FPSO, the slop tanks may have a combined capacity in the region of 10,000 m³. 5.1.5.1 Oil/water separation process The dirty slop oil tank receives water from the various drain systems and is where the main oil and water separation process takes place. As the settling process progresses, the water gravitates to the bottom of the tank, where it can flow through a decanting line into the clean slop tank. The decanting line is directed upwards inside the clean slop tank to encourage any remaining oil to move to the surface. This oil can be returned to the dirty slop tank by adding seawater to raise the oil bearing surface of the liquid up to a skimmer pipe. The oil will then gravitate back into the dirty slop tank. The oil remaining in the dirty slop tank will gradually float to the surface. It is then removed by an oil skimming pump and deposited into a cargo tank. Alternatively, the oil discharged from the dirty slop tank can be directed into the cargo oil transfer manifold or the flare scrubber, where it can be routed into the low pressure (LP) separator. A requirement of MARPOL regulation 15 is that, as water is discharged from the slop tanks to the sea, it must pass through an oil discharge monitoring and control system that has been approved by the Administration. This will automatically stop the water being discharged if the oil content exceeds a limit of 15 ppm. A continuous record of the discharge, in litres per nautical mile, and total quantity of oil discharged or oil content and rate of discharge must be produced and retained onboard for a period of 3 years. 5.1.5.2 Tank equipment Slop tanks are normally provided with high capacity (400 m³/hour) submerged pumps for transferring the tank contents to either the cargo oil transfer header or the COW header for seawater tank cleaning. A smaller capacity skimming pump (100 m³/hour) provides the way for oil to be routinely removed from the surface of the water in the dirty tank and pumped into a cargo tank when the oil/water interface is at, or below, 70%. Both slop tanks are fitted with heating coils to assist in separating water from oil and also to provide hot seawater for tank cleaning. The heating coils may be heated by steam or by a heating liquid medium. The tanks themselves are fitted with fixed tank cleaning machines. To maintain a safe atmosphere within the slop tanks, they are kept at a slightly positive pressure to the atmosphere by inert gas. Tank pressure is controlled by venting excess inert gas through the inert gas header control system. 5.2 Cargo Tank Vent and Inert Gas Systems 5.2.1 Flare and Vent System The flare and vent systems facilitate the disposal of large quantities of vapour and gas. They are two of the largest and most complex piping systems on the installation and, while they both terminate within the flare stack, it should be remembered that they are two completely separate systems. The flare is associated with oil and gas production activities while the vent system, in conjunction with the inert gas system, is used in cargo storage and handling operations. 5.2.1.1 The flare stack Structural aspects Figure 5.15 – Flare stack The flare stack is one of the most distinctive features on an FPSO, stretching out over the sea some 70-90 m. It is a large steel framework that supports the flare and vent headers that come from the oil and gas processing facilities. The vent headers terminate approximately half way up the flare stack while the flare headers run the full height of the stack. The headers are constructed from low temperature steel to cope with the cooling effect that occurs as high pressure gas is vented. A flame arrestor is fitted at the foot of each header to prevent the flames travelling back down. 102 • FPSOs Flare tip features Figure 5.16 – Flare tip Typically, there are two flare tips at the top of the flare stack, the atmospheric flare tip and the relief flare tip. Pilot burners at the flare tips, ignited by high energy spark igniters that burn continuously, provide a constant ignition source. They are fuelled with gas from the blanket gas header system. Gas diffuser nozzles create high velocity turbulent flow conditions as the gas leaves the header, which both improves combustion and reduces thermal emissions. A windshield surrounds the outlet nozzles and the headers are fitted with mechanical air lock seals to prevent the air backflow into the headers. The bases of the flare headers have non-return valves to allow any accumulation of condensed liquids or rainwater to drain. Figure 5.17 – Flare tip igniters Flare and vent headers A header is essentially a collection pipe into which various pressure vessels and tanks are connected, either through relief valves, automatically operated ESD blowdown valves or atmospheric vents. The headers run the length of the ship and terminate in either the flare scrubber vessel or at the flare stack. A number of flare and vent headers are needed as the condition of the fluids released differs considerably in temperature, pressure and volume, depending on the system they have been released from. 5.2.1.2 Flare system Gas flaring was previously carried out indiscriminately as gas was considered to be more of an inconvenience to the oil processing operation Figure 5.18 – Flare and vent arrangements 103 than a useful commodity. Flaring has been discouraged in recent years. Anti-flaring legislation has been introduced and countries wish to reduce the pollutants that are being pumped into the atmosphere. The flare is now used primarily during start-up operations until steady state conditions can be maintained, and during emergency shutdown situations when the process gas has to be disposed of quickly and safely. For the rest of the time, the flare is used to burn off either toxic products that would constitute a hazard onboard the FPSO or gases that are not commercially viable to sell. Figure 5.19 – Routine flare operation Flare headers In accordance with the requirements of API RP 521, ‘Guide for Pressure-Relieving and Depressuring Systems’, and to provide protection against over- pressurisation, relief valves must be provided on all process pressure vessels. API 521 states that all vessels that are less than 8 metres above a potential pool fire location, and as a result at a greater risk due to high heat inputs, should have relief valves that take this situation into account. Depending on their location, these relief valves are connected to the flare stack through one of four header systems: Production relief header - process pressure •control valves, relief valves, ESD blowdown valves and bursting discs discharge into the production relief header subsea blowdown header - the production •subsea header is used to dispose of hydrocarbon fluids from the relief and blowdown lines on the pig launchers/receivers and heat exchangers that service the subsea flow lines compression flare header - this is used for •disposing of high pressure gas released from the relief valves and bursting discs on the gas compression and dehydration equipment, gas metering and enhanced oil recovery gas lift equipment test separator header - when well testing is in •progress, the gases released from the well test separator are measured and then released to flare through a dedicated flare header. Figure 5.20 – Flare during blowdown operations The production relief, compression flare and subsea blowdown headers are continually purged with a blanket gas supply at a rate of 10-50 kg/hour. This must be maintained so that air is excluded from the flare. The header’s contents are routed through the flare scrubber, where they exit through a single line that terminates at the relief flare tip. The atmospheric flare tip is used to burn off gas from the process pressure vessels that run at atmospheric or near to atmospheric pressure. Flare scrubber The purpose of the flare scrubber is to remove any residual liquids present in the gases being vented so that liquid is not released at the tip of the flare, causing disastrous results. Gases enter the scrubber through a deflector box that aids the separation of liquids from the gas. As the gases leave the scrubber, they pass through a cyclonic filter arrangement designed to remove any remaining liquids. Liquids removed from the flare gases are pumped back into the oil process LP separator. If the build-up of liquids exceeds the capacity of the flare scrubber, a large diameter relief line is opened, allowing the liquids to be dropped directly into either the slop tanks or a cargo oil tank. 5.2.1.3 Vent system The term venting refers mainly to the removal of air, inert gas, hydrocarbon gases or a combination of these gases from inside the ship’s cargo tanks during cargo handling and gas freeing operations. It includes arrangements for pressure protection of the tanks. A comprehensive vent system is required to deal safely with this huge volume of constantly changing vapours. This system must be entirely separate from the air pipes and ventilation systems that service other parts of the hull and the oil and gas processing systems.