Offshore Electrical Engineering

299 Pages·1992·6.487 MB·English

by  G. T. Gerrard (Auth.)

Offshore Electrical
Engineering
G. T. Gerrard
■ l U T T E R W O R TH
E I N E M A N N
Butterworth-Heinemann Ltd
Linacre House Jordan Hill Oxford OX2 8DP
, ,
*fg PART OF REED INTERNATIONAL BOOKS
OXFORD LONDON BOSTON
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TOKYO TORONTO
First published 1992
WELLINGTON
© Butterworth-Heinemann Ltd 1992
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British Library Cataloguing in Publication Data
Gerrard, G. T.
Offshore electrical engineering.
I. Title
621.38209163
ISBN 0 7506 1140 5
Library of Congress Cataloging in Publication Data
Gerrard, G. T. (Geoff T.)
Offshore electrical engineering/G. T. Gerrard.
p. cm.
Includes bibliographical references and index.
ISBN 0 7506 11405
1. Electrical engineering. 2. Offshore structures- -Electrical
equipment. I. Title.
TK4015.G37 1992
627'.98—dc20 91-31333
CIP
Typeset by TecSet Ltd, Wallington, Surrey
Printed and bound in Great Britain
Preface
I hope that many of the electrical engineering lessons which have been
learned by experience during the last 20 years or so of North Sea oil
endeavour are covered in this book in a way that people in all walks of life
will find interesting.
Nevertheless, may I apologize in advance to those who may find the
coverage lacking in some way. Please write to me if you have constructive
comments to make and I promise I will bear them in mind when
considering any future revision of the book. I cannot promise, however, to
reply to all correspondence.
During the several years that this book has taken shape, many significant
events have taken place within the oil industry; some have been good and
others disastrous. If there is one precept worth adopting, it is the need to
consider everything and every situation as new and unique. Be warned that
blindly or rigidly applying regulations may cost lives.
Geoff Gerrard
IX
Acknowledgements
The author wishes to thank and gratefully acknowledge all those who
provided material and advice for the production of this book, particularly
the following:
Stephen Rodgers, John Brown Engineering Ltd., Clydebank
Ian Stewart and Arlene Sutherland, BP Exploration Ltd., Aberdeen
Andrew White, Andrew Chalmers and Mitchell Ltd., Glasgow
David Bolt, Ewbank Preece Ltd., Aberdeen
Lynn Hutchinson, Ferranti Subsea Systems Ltd., Victoria Road, London
W6
Hamish Ritchie, Geoff Stephens and John McLean, Foster Wheeler Wood
Group Engineering Ltd., Aberdeen
Gordon Jones, G. E. C. Alsthom Large Machines Ltd., Rugby, Warwick-
shire
Mr P. G. Brade, G. E. C. Alsthom Measurements Ltd., Stafford
Mrs M. Hicks, Publicity Department, G. E. C. Alsthom Installation
Equipment Ltd., Liverpool
Pat Dawson, Hawke Cable Glands Ltd., Ashton-under-Lyne, Lanes.
Gordon Shear, Hill Graham Controls Ltd., High Wycombe, Buckingham-
shire
Richard Crawcour and Mr K. M. Hamilton, P & B Engineering Ltd.,
Crawley, Sussex
Sue Elfring, Crest Communications Ltd., for: Rolls-Royce Industrial and
Marine Ltd., Ansty, Coventry
John Day, formerly with Shell UK Exploration and Production, Aberdeen
Jim Bridge and Keith Stiles, SPP Offshore Ltd., Reading
Ian Craig and Graham Sim, Sun Oil Britain Ltd., Aberdeen
Prof. John R. Smith, University of Strathclyde, Glasgow
Stephen Rodgers, John Brown Engineering Ltd, Clydebank
John Baker, GEC Alsthom Vacuum Equipment Ltd.
Mr John Hugill, Thorn Lighting Ltd., Borehamwood, Herts
X
Chapter 1
Introduction: offshore power
requirements
Designing for provision of electrical power offshore involves practices
similar to those likely to be adopted in onshore chemical plants and oil
refineries. However, other aspects peculiar to offshore oil production
platforms need to be recognized. It is suggested that those unfamiliar with
offshore installations read the brief guide in Appendix A before continuing
further.
The aspects which affect electrical design include the following:
1. The space limitations imposed by the structure, which add a three-
dimensional quality to design problems, especially with such concerns
as:
(a) hazardous areas;
(b) air intakes and exhausts of prime movers;
(c) segregation of areas for fire and explosion protection;
(d) avoidance of damage to equipment due to crane operations.
2. Weight limitations imposed by the structure, which require:
(a) the careful choice of equipment and materials in order to save
weight;
(b) the avoidance of structurally damaging torques and vibrations
from rotating equipment.
3. The inherent safety hazards presented by a high steel structure sur-
rounded by sea. Such hazards often require:
(a) particular attention to electrical shock protection in watery envi-
ronments;
(b) good lighting of open decks, stairways and the sea surrounding
platform legs.
4. The corrosive marine environment.
1.1 Hazards offshore
1.1.1 Marine environment
Wave heights in the North Sea can exceed 20 metres, and wind speeds can
exceed 100 knots.
1
2 Offshore electrical engineering
1.1.2 Gas
Accumulations of combustible gas can occur on an offshore installation
from various sources, including the following:
(a) equipment and operational failures such as rupture of a line, flameout
of an installation flare, a gland leak etc.;
(b) gas compressor vibration causing failure of pipe flanges, loss of
compressor seal oil etc.;
(c) drilling and workover activities;
(d) in concrete substructures, the buildup of toxic or flammable gases due
to oil stored in caisson cells.
1.1.3 Crude oil and condensates
Equipment and operational failures, such as the rupture of a line or a gland
leak, can release oil and condensates. The high pressures involved in some
cases cause spontaneous ignition due to electrostatic effects.
1.1.4 Operational hazards
Apart from the fire and explosion hazard of process leaks, there is a hazard
to personnel purely from the mechanical effects of the leak jet and the
sudden pressure changes caused by serious leaks in enclosed compart-
ments.
Care must be taken in the siting of switchrooms, generator sets and
motor drives to minimize the risk of damage due to crane operations,
especially if cranes are sited near drilling equipment areas where heavy
pipes and casings are being frequently moved.
1.2 Electrical system design criteria
The purpose of any offshore electrical supply system is to generate and
distribute electricity to the user such that:
1. Power is available continuously at all times that the user's equipment is
required to operate.
2. The supply parameters are always within the range that the user's
equipment can tolerate without damage, increased maintenance or loss
of performance.
3. The cost per kilowatt hour (kWh) is not excessive, taking into consider-
ation the logistical and environmental conditions in which generation
and distribution are being effected.
4. Impracticable demands are not made on the particular offshore infra-
structure, i.e. such as those for fuel or cooling medium.
5. The safety requirements pertinent to an offshore oil installation are
complied with, in particular those associated with fire and explosion
hazards.
Introduction: offshore power requirements 3
6. The weight of the system is not excessive for the structure on which it is
installed. In the case of rotating machinery, the effects of vibration and
shock loads must be taken into account.
A single-line diagram of a typical offshore electrical system is shown in
Figure 1.1.
1.3 Main prime movers
With the obvious availability of hydrocarbon gas as a fuel, and the
requirement for a high power-to-weight ratio to keep structural scantlings
to a minimum, gas turbines are the ideal prime movers for power
requirements in excess of 1 MW. Below this value, reliability and other
considerations (dealt with in Chapter 3) tend to make gas turbines less
attractive to the system designer.
Owing to the complexity and relative bulk of gas turbine intake and
exhaust systems, the designer is urged towards a small number of large
machines. However, he is constrained by the need for continuity of supply,
maintenance and the reliability of the selected generator set to an optimum
number of around three machines.
A variety of voltages and frequencies may be generated, from the
American derived 13.8 kV and 4.16kV 60 Hz to the British 11 kV, 6.6kV
and 3.3 kV 50 Hz. Many ships operate at 60Hz, including all NATO
warships, and there is a definite benefit to be gained from the better
efficiencies of pumps and fans running at the 20% higher speeds.
1.4 Key services or submain generators
On most platforms, smaller generators are provided to maintain platform
power for services other than production. These are also normally gas
turbine driven and can provide a useful blackstart capability, especially if
this is not available for the main machines.
1.5 Medium-voltage distribution
The design of the distribution configuration at the platform topsides
conceptual stage is very dependent on the type of oil field being operated
and the economic and environmental constraints placed on the oil compa-
ny at the time. The older platforms originally had few or no facilities for
gas export or reinjection, and therefore the additional process modules
installed when these facilities were required have their own dedicated
high-voltage switchboards. This is also the case if the power requirement
for such a heavy consumer as sea water injection is underestimated at the
time of construction.
In general, however, it is better to concentrate switchrooms in one area
of the platform in order to avoid complications with hazardous areas,
ventilation etc., as discussed in Chapter 2.
4
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o
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introduction: offshore power requirements 5
With such relatively high generation capacities and heavy power users
within the limited confines of an offshore platform, calculated prospective
fault currents are often close to or beyond the short-circuit capabilities of
the MV switchgear designs available at the platform topsides design phase.
Currently, fault ratings of 1000 MVA are available, and with careful study
of generator decrement curves etc. it is usually possible to overcome the
problem without resorting to costly and heavy reactors.
All the available types of MV switchgear are in use offshore. The use of
bulk oil types, however, is questionable owing to the greater inherent fire
risk.
Unlike land based switchboards, there has been found to be a significant
risk of earth faults occurring on the busbars of offshore switchboards, and
so some form of earth fault protection should be included for this.
The platform distribution at medium voltage normally consists of
transformer feeders plus motor circuit breaker or contactor feeders for
main oil line (MOL) pumps, sea water lift and water injection pumps, and
gas export and reinjection compressors. Depending on process cooling
requirements, cooling medium pumps may also be driven by medium-
voltage motors.
Operating such large motors on an offshore structure (i.e. on the top of a
high steel or concrete tower) can lead to peculiar forms of failure owing to
the associated vibration and mechanical shock, almost unheard-of with
machines securely concreted to the ground. This has led to offshore
platform machines being fitted with more sophisticated condition monitor-
ing than is usually found on similar machines onshore.
Another problem, which will be discussed in more detail in Chapter 9, is
the transient effect on the output voltage and frequency of the platform
generators with such large motors in the event of a motor fault, or for that
matter during the normal large-motor switching operations. Computer
simulation of the system must be carried out to ensure stability at such
times, both at initial design and when any additional large motor is
installed. Facilities such as fast load shedding and automatic load sharing
may be installed to improve stability and also make the operator's task
easier. This subject is discussed in Chapter 5.
1.6 Low-voltage distribution
Using conventional oil or resin filled transformers, power is fed to the
low-voltage switchboards via flame retardant plastic insulated cables.
Cabling topics are covered in Chapter 7.
Bus trunking is often used for incoming low-voltage supplies from
transformers. Owing to competition for space, this is just as likely to be due
to bending radius as to current rating limitations of cables, since bus
ducting may have right angle bends.
The type of motor control centre switchboard used offshore would be
very familiar to the onshore engineer. However, the configuration of the
low-voltage distribution system, to ensure that alternative paths of supply
are always available, is usually much more important offshore. This is
6 Offshore electrical engineering
because, although every effort is put into keeping it to a minimum, there is
much more interdependence between systems offshore.
A few examples of small low-voltage supplies which are vital to the safe
and continuous operation of the installation are as follows:
(a) safe area pressurization fans;
(b) hazardous area pressurization fans;
(c) generator auxiliaries;
(d) large-pump auxiliaries;
(e) large-compressor auxiliaries;
(f) galley and sanitation utilities for personnel accommodation;
(g) uninterruptible power supply (UPS) systems for process control and
fire and gas monitoring;
(h) sea water ballast systems on tension leg platforms and semisubmer-
sibles.
The topics of maintenance and availability are covered in Chapter 12.
1.7 Emergency or basic services switchboard
As a statutory requirement, every British Sector installation must have a
small generator to provide enough power to maintain vital services such as
communications, helideck and escape lighting, independently of any other
installation utility or service. In the event of a cloud of gas enveloping the
platform owing to a serious leak, even this may need to be shut down as a
possible ignition source.
1.8 Fire pumps
Again as a statutory requirement, every installation must be provided with
at least sufficient fire pumps with enough capacity to provide adequate
water flow rates for fighting the most serious wellhead, pipeline riser or
process fires. The numbers and capacities of these pumps have to take into
account unavailability due to routine maintenance and failure. These
pumps may be submersible electric, hydraulically powered, or directly
shaft driven from a diesel engine.
Typically, one pump arrangement could have an electrically driven
100% capacity pump supplied from a dedicated diesel generator set which
is directly cabled to the pump, i.e. with no intervening switching or
isolating devices. This has the advantages of increased reliability due to
fewer components and soft starting of the motor. This kind of pump runs
up to operating speed with the generator, in the same manner as a
diesel-electric railway locomotive would accelerate from start. The second
100% capacity pump could again be electric but supplied from the platform
distribution system in the conventional way. The purpose of this arrange-
ment is to avoid failure of both pumps owing to a common operational
element, i.e. common mode failure. A third 100% capacity pump would be
required to allow for maintenance downtime. Details on the electrical
design of diesel-electric fire pump packages are given in Chapter 4.