Dynamics of Offshore Structures

407 Pages·1989·10.195 MB·English

by  Minoo H Patel (Auth.)

Dynamics of Offshore Structures
Minoo Ç Patel,
BSc, PhD, MIMechE, MRINA, CEng
Reader in Mechanical Engineering, University College, London
Butterworths
London Boston Singapore Sydney Toronto Wellington
Dedicated to Irene, Doly and Homi
for their loving support through the years
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to reproduce, transmit or store in a retrieval system any part of this publication
should be addressed to the Publishers.
Warning: The doing of an unauthorised act in relation to a copyright work may
result in both a civil claim for damages and criminal prosecution.
This book is sold subject to the Standard Conditions of Sale of Net Books and
may not be re-sold in the UK below the net price given by the Publishers in
their current price list.
First pubhshed 1989
© Butterworth & Co. (Publishers) Ltd, 1989
British Library Cataloguing in Publication Data
Patel, Minoo H.
Dynamics of offshore structures.
1. Offshore structures. Dynamics
I. Title
627'.98
ISBN 0-408-01074-6
Library of Congress Cataloging in Publication Data
Patel, Minoo H.
Dynamics of offshore structures/Minoo H. Patel
p. cm.
Includes index.
ISBN 0-408-01074-6
1. Offshore structures—Hydrodynamics. I. Title
TC1665.P38 1989
627'.98-<ic 19
Typeset by KEYTEC, Bridport, Dorset
Printed and bound in Great Britain by Butler & Tanner, Frome, Somerset
Dedicated to Irene, Doly and Homi
for their loving support through the years
PART OF REED INTERNATIONAL P.L.C.
All rights reserved. No part of this publication may be reproduced or
transmitted in any form or by any means (including photocopying and recording)
without the written permission of the copyright holder except in accordance with
the provisions of the Copyright Act 1956 (as amended) or under the terms of a
licence issued by the Copyright Licensing Agency Ltd, 33-34 Alfred Place,
London, England WCIE 7DP. The written permission of the copyright holder
must also be obtained before any part of this publication is stored in a retrieval
system of any nature. Applications for the copyright holder's written permission
to reproduce, transmit or store in a retrieval system any part of this publication
should be addressed to the Publishers.
Warning: The doing of an unauthorised act in relation to a copyright work may
result in both a civil claim for damages and criminal prosecution.
This book is sold subject to the Standard Conditions of Sale of Net Books and
may not be re-sold in the UK below the net price given by the Publishers in
their current price list.
First pubhshed 1989
© Butterworth & Co. (Publishers) Ltd, 1989
British Library Cataloguing in Publication Data
Patel, Minoo H.
Dynamics of offshore structures.
1. Offshore structures. Dynamics
I. Title
627'.98
ISBN 0-408-01074-6
Library of Congress Cataloging in Publication Data
Patel, Minoo H.
Dynamics of offshore structures/Minoo H. Patel
p. cm.
Includes index.
ISBN 0-408-01074-6
1. Offshore structures—Hydrodynamics. I. Title
TC1665.P38 1989
627'.98-<ic 19
Typeset by KEYTEC, Bridport, Dorset
Printed and bound in Great Britain by Butler & Tanner, Frome, Somerset
Preface
Exploitation of oil and gas reserves from hydrocarbon reservoirs below
the sea bed has increased rapidly since the early 1960s and in doing so
has stimulated a wide-ranging base of theoretical analysis, model testing
and practical experience in the scientific disciplines that contribute to
the design and operation of offshore structures. These disciphnes are,
however, spread out over the traditional boundaries of the established
physical sciences. The design, construction and operation of fixed and
floating offshore structures requires expertise in subject areas ranging
from meteorology, oceanography, hydrodynamics, naval architecture,
structural and fatigue analysis, corrosion metallurgy, sea bed soil mecha­
nics, mechanical and process engineering, diving physiology and even
marine biology. These disciplines are often combined within the descrip­
tive title of 'ocean engineering'.
The wide range of subject areas within ocean engineering has posed
problems in the teaching of both undergraduate and postgraduate
students because most existing text books are written within traditional
discipline boundaries. Teaching material in ocean engineering is, there­
fore, distributed between many text books and research papers and is
not generally available within a suitable context or with an eye to
application in ocean engineering.
This text book has been written to help remedy this situation by
presenting an integrated treatment of the main subject areas that
contribute to the design, construction, installation and operation of fixed
and floating offshore structures. The book has been prepared with
particular emphasis on the fundamentals of oceanography, basic fluid
mechanics, wave theory, hydrodynamics, naval architecture and struc­
tural analysis to meet the needs of students reading ocean engineering
or naval architecture, at both undergraduate and postgraduate levels.
Basic equations and theoretical results are derived in a rigorous manner
but sections on model testing, full-scale measurements, design and
certification are also induced to ensure that the book is of value to
professional engineers seeking a balanced treatment of fundamental and
practical issues.
A limited number of original source references are cited for further
reading but considerable selectivity has been exercised to ensure that
only those research results relevant to current design practice are
iv Preface
introduced in the text or through the references. Nevertheless, some
recent research advances are included where it is believed that they will
have a strong impact on offshore structures design practice in the near
future.
I am indebted to a number of my colleagues for their help and advice
during the preparation of the manuscript. In particular, the assistance of
Dr David Brown, Dr John Harrison, Mr Farman Seyed and Dr Joel
Witz was invaluable, as was the infinite patience of Miss Vanessa Smith
during the typing of the manuscript. I am especially grateful to Professor
Tom Lambert for his encouragement and guidance over many years.
Minoo Patel
Chapter 1
Introduction
The oceans of planet Earth exercise a dominant influence on its weather
systems and ecology. The oceans are believed to have been the medium
in which life first appeared on the planet and gradually evolved into
land based plant and animal species. The continued existence of land
based life is still dependent on the oceans through their effect on the
planet's weather and the crucial role that they play in global ecology.
Since prehistoric times and up to the early part of this century, however,
man's use of the oceans has been largely restricted to near-shore
transportation and fisheries, although there is evidence to suggest
migrations of people across very large oceanic distances. Since the
fifteenth century, the pace of ocean transportation and deep water
fishing has gradually increased but man's utilization of the oceans has
sfill been restricted to these two activities.
Over the last five decades, however, traditional uses of the oceans
have expanded to include the exploitation of hydrocarbons below the
sea bed and the potential of large-scale mineral gathering and energy
extraction. The development of offshore oil and gas has led the way
with land based exploration and production for crude oil being extended
to the sea bed as early as 1887, in California, when the first ever
exploration drilling of more than a few feet of water was carried out.
Since then the pace of oil exploration and production in shallow water
has gradually increased, with the next major development occurring in
1910, when wells were sunk in Ferry Lake, Louisiana. This was followed
in 1929 by the exploration and production of a large oil reservoir on
Lake Maracaibo in Venezuela. The development of the Gulf of Mexico
as an offshore area began in the 1930s, with oil first being produced in
1938 from a timber platform in 4.3 m (14 ft) of water, on the Creole
Field, which was located 1.6 km (1 mile) off the coast of Louisiana.
Following the Second World War, offshore activity in the Gulf of
Mexico rapidly evolved, with oil exploration and production in deeper
waters until, by 1959, an oil production platform had been installed by
Shell on a block off Grand Isle in 30.5 m (100 ft) of water. At the same
time British Petroleum were developing shallow water oil finds in the
Persian Gulf.
Such oil and gas exploration and development spread to most of the
continental shelf areas of the world from the mid-1950s, commencing
2 Introduction

4 Introduction
first in the Gulf of Mexico and then spreading to the waters off Mexico,
Brazil and Brunei. The map illustrated at Figure 1.1(a) presents an
overview of this worldwide offshore exploration and production activity,
whereas Figure l.l(ft) uses sea bed contours to show the extent of the
continental shelves around all the major land masses. The continental
shelves cover only a small fraction of the total ocean area but they are
potential sites for exploration, or are already known to have large
reserves. The offshore industry embarked upon a technically more
challenging phase when the North Sea was first explored as a potential
offshore area in the early 1960s. Figure 1.2 presents an overview of the
fields developed in the North Sea since the mid-1960s. These fields
range from large gas reservoirs in the relatively calm waters off East
Anglia in south east England to harsh northerly environments in the
vicinity of 60° north where fields such as Snorre, Troll, Oseberg and
Frigg are exposed to some of the harshest wind and wave conditions of
any maritime area in the world. The North Sea is typical of many
offshore areas where the development of technology and structures to
exploit oil and gas fields has had a significant positive impact on the
economies of surrounding countries.
The development of land based and offshore oil and gas deposits has
played an essential role in laying down the foundations of the modern
world with its high living standards. Table 1.1 shows the rapid growth of
total (both land based and offshore) oil production over the last 45 years
and its distribution across major regions of the Earth. The ten-fold
increase in production over this period reflects the increasing demand
for energy throughout the world, although produced oil and gas is also
used as feedstock, and in the chemical industry, leading ultimately to a
multitude of products including drugs, man made fibres, plastics and
industrial chemicals. Table 1.1 shows that production in the Middle East
dominated world oil supplies during the 1970s. The resultant economic
and political leverage derived by Middle Eastern oil producing countries
has played a significant political role in modern times. In particular, oil
price rises caused by excess oil demand over supply in the 1970s has
prompted the development of offshore oil and gas throughout the world
in order to reduce dependence on the Middle East. Figure 1.3 illustrates
the relative volumes of onshore and total oil production over the last
two decades and shows that offshore oil production is now approaching
30% of total worldwide production.
In the long term, however, man's utilization of the oceans as a
resource will sfimulate engineering developments in other areas. The
oceans have the potential to provide worth while amounts of renewable
energy, either from ocean waves of by exploiting thermal gradients
between the surface and deeper layers of the ocean. Surface-deployed
wave energy extraction devices and ocean thermal energy conversion
plants are potentially able to extract such renewable energy. The oceans
can also make a worthwhile long-term contribution to the mineral
wealth of the world. It is known that sea bed material in the form of
nodules is rich in several metals which are in short supply from land
based sources - manganese being one example. Sea bed mining of such
mineral-rich nodules could contribute to the world's mineral supplies.
Introduction 5
" VW \ χί
Figure 1.2. Distribution of fields in a typical offshore location - The North Sea. Only a
representative selection of fields are illustrated for clarity. Key: A - median lines marking
Hmits of neighbouring country's offshore sectors. Names of oil and gas fields: a - Snorre; b
- Troll; c - Oseberg; d - Frigg; e - Heimdal; f - Balder; g - Sleipner; h - South Brae; i -
Piper; j - Tartan; k - Claymore; 1 - Forties; m - Montrose; η - Ula; o - Ekofisk; ρ -
Eldfisk; q - Valhall; r - Innes; s - Clyde; t - Fulmar; u - Auk; ν - Duncan; w - Argyll; χ
- Tyra; y - Dan; ζ - F2-3; 1 - Forbes; 2 - Gordon; 3 - Esmond; 4 - Ravenspurn; 5 -
Cleeton; 6 - West Sole; 7 - Viking; 8 - Vulcan; 9 - Hewett; 10 - Indefatigable; 11 -
Leman; 12 - L4-A; 13 - L7-C; 15 - K15-FA; 16 - P6; 17 - K14; 18 - K-17, 19 - Kinsale
Head; 20 - Morecambe (from the Department of Energy, UK, 1986)
There is also the potential for extracting the extremely low levels of
various metallic compounds that are dissolved in sea water. Furth­
ermore, future development of the world's food resources is going to
require increasingly stringent conservation of fisheries and a higher level