Table Of ContentDesign and Construction of
OFFSHORE STRUCTURES
Proceedings of the Conference held on 27-28 October 1976
THE INSTITUTION OF CIVIL ENGINEERS LONDON 1977
Conference sponsored jointly by the Institution of Civil Engineers, the Institution of Structural Engineers and the Society
of Underwater Technology
ORGANIZING COMMITTEE
C. J. Antonakis (Chairman)
R. W. Bishop
J. A. Derrington
F. J. Hansen
D. J. Lee
Professor D. M. McDowell
J. F. O'Hara
W. J. Shirley
EDITORS
J. P. Blanc and Mary Monro
ISBN 0 7277 0041 3
© 1977 The Institution of Civil Engineers
All rights, including translation, reserved. Except for fair copying, 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 written permission of the Institution of Civil Engineers.
The Institution of Civil Engineers as a body does not accept responsibility for the statements made or the opinions expressed
in the following pages.
Published by Thomas Telford Ltd, for the Institution of Civil Engineers.
Made and printed in Great Britain by William Clowes & Sons Limited, London, Beccles and Colchester
Contents
Opening address. Sir
NORMAN ROWNTREE V
1. Design of the deck structure for the Sea Platform Constructors (Scotland) Ltd gravity
production platform, p. and 1
J. CLARK J. M. DAWSON
2. Geophysical and mechanical platform instrumentation, and p. 9
D. BOURGEOIS J. LEROY
Discussion on Papers 1 and 2 17
3. The Abu Dhabi marine areas development project, w. s. and s. 23
MCPHEE K. HUSSEIN
4. Design, construction and installation of the Loango steel gravity platforms, 31
D. LALLI
Discussion on Papers 3 and 4 39
5. Representative sampling for sea bed foundation design,
B. DENNESS, D. M. MCCANN
and J. T. LAMBERT 43
6. Studies of offshore caissons founded on Oosterschelde sand. and w.
P. W. ROWE H.
CRAIG 49
Discussion on Papers 5 and 6 57
7. North Sea platform piling—development of the Forties Field piles from West Sole
and Nigg Bay experience and tests, and 61
D. A. FOX, V. J. R. SUTTON Y. OKSUZLER
8. Effect of cyclic loading on clay behaviour, s.
K. H. ANDERSEN, F. BROWN, I. FOSS, J. H.
POOL and w. F. ROSENBRAND 75
Discussion on Papers 7 and 8 81
9. Risk analysis for offshore structures—the aims and methods. and
A. R. FLINT
M. J. BAKER 87
Discussion on Paper 9 91
10. Maritime operations relative to construction of large concrete offshore structures.
K. WERENSKIOLD 97
11. Liquefied gas floating facilities for offshore service, and c c 107
F. MASCARO JANSKY
Discussion on Papers 10 and 11 111
12. Experience in prestressing grouting concrete offshore structures. 115
J. E. LONG
13. Construction of McAlpine/Sea Tank gravity platforms at Ardyne Point, Argyll, j. A.
DERRINGTON 121
Discussion on Papers 12 and 13 131
14. The case for a hybrid, and 135
F. J. HANSEN L. C F. INGERSLEV
15. The time-temperature dependence of stresses in offshore concrete structures, B.
RICHMOND 143
Discussion on Papers 14 and 15 151
16. Repair and protection of offshore structures, 157
K. N. TUSCH
17. Compliant drilling and production platforms, p. s. 161
GODFREY
Discussion on Papers 16 and 17 171
Closing address, 177
PETER MASON
Opening address
Sir Norman ROWNTREE, BSC (ENG), DSC, President, Institution of Civil Engineers
Offshore engineering has developed rapidly and is good applied research and development, calling
already an exciting business. There are some people upon the work done in other sciences are very great if
who have been interested for many years, but the rate this is applied correctly.
of expansion of the work has created an entirely new At the present time one finds considerable govern
situation. North Sea offshore engineering has ment resources going into individual research and
opened up for UK civil engineers a challenging new development in the pure sciences because these pro
field of activity where their enthusiasm has been jects are easier to identify. But other than in the
stimulated by the combination of operations in very government laboratories such as the Building
unfavourable conditions with urgency. Research Establishment and the Transport and Road
It is satisfactory to the Institution of Civil Engin Research Laboratory, I do not think machinery has
eers that this conference is jointly sponsored by the been properly developed for financing research and
Institution of Structural Engineers and the Society development carried out in the course of one's work.
for Underwater Technology. Each of these organi It is a major problem in order to ensure that know
zations has specific interests and responsibilities ledge is applied in the best possible way. This must
which need to be brought together to ensure that be something which is of great concern in the off
the subject of offshore engineering is adequately shore field of activity, since here brand new tech
examined. The structural engineers have obvious nologies have had to be developed quickly with the
specific interest in the structures themselves; the civil urgency of big public pressures behind them and
engineers bring in a wide variety of disciplines con considerable financial reward if they succeed.
cerned with related problems, such as hydraulics, sea The engineering Institutions must continue their
bed foundation strengths, and many of the logistical activities of publicizing knowledge and exchanging
problems. The Society for Underwater Technology information on the widest possible scale. In this way
has the great advantage of interdisciplinary interest major steps forward can continue to be made, lead
which all major civil engineering efforts require to ing to the efficient and safe development of those new
call upon, and none more than offshore engineering. techniques obviously required in all forms of off
It is therefore a team activity, and this again em shore engineering, and particularly for those on the
phasizes the special quality which the civil engineer UK's own doorstep in the North Sea. The UK is
is encouraged to develop from his youth onwards: particularly fortunate in having these major prob
an understanding of a wide range of subjects, lems to solve so close at hand, facilitating the build
enabling him to bring together the expertise in these ing up of local expertise which is likely in the future
fields to make major contributions for the benefit of to be of much more general service throughout the
mankind. This is a subject about which I feel world.
strongly. Traditionally the civil engineer has known Unfortunately, North Sea engineering is carried
a bit about everything. There has been a post-war out in the glare of publicity and interest. Such
tendency to overspecialize. The offshore engineer widespread publicity can lead to bias in decision
ing problems have indicated again how this profes making, particularly where large government in
sion of civil engineering has to be prepared for all terests are involved. Highly professional discus
sorts of eventualities. sions, such as those to take place at this conference,
It is a special concern to the civil engineer that each will help to reinforce acceptance of the best designs
major new structure tends to be unique, and it is for the future. The results may well contribute to a
therefore axiomatic that an element of research and more economic development and to more authority
development is involved in each case. In industry to the engineers in the shaping of those develop
generally the research and development can be ments.
looked upon as an introductory phase to long pro I hope that discussion here will encourage the
duction runs and continuing profits, if successful. three Institutions to continue their active interest in
In civil engineering, the research and development offshore engineering and increase still further the
has usually to be related to a very limited number of value of the technological contributions that mem
units of construction. Nevertheless, savings from bers make in this field.
Paper 1: The design of the deck structure for the Sea Platform Constructors
(Scotland) L td gravity production platform
P. J. CLARK, BSC, ACGI, FICE, Partner, Rendel, Palmer and Tritton
J. M. DAWSON, MSC, ACGI, MICE, Senior Design Engineer, Rendel, Palmer and Tritton
The Paper covers the design of the integral deck and upper steel leg section of a concrete gravity platform for 150 m in the North
Sea. It deals first with the principles which have governed the design conception and final selection of a deep plate girder solu
tion. Key objectives have been almost exclusive use of easily available steels of conventional structural grades and thicknesses,
the virtual elimination of stress relieving, minimum welding, and minimum shop and site handling. The Authors deal with the
alternative attractions and problems associated with box and truss decks, and give their reasons for adopting a deep plate girder
solution. The steel deck design was carried out by the Consultants in parallel with their drafting of the new UK Steel Bridge
Code. The way in which this has influenced the design of particular elements of the deck is reported (and briefly, as a matter of
interest, the way the new code has been influenced by experience with the deck design). The Paper covers the application of limit
state concepts where applicable, and discusses the place of both load factor and safe stress approaches in the design. Design
against fatigue is dealt with in considerable detail. Several features of special interest have been selected for detail reporting.
The webs of the plate girders have been designed to incorporate safely extensive holes and large cut-outs in anticipation of prob
able requirements, and the various stiffening systems used have been described. Solid extruded steel sections have been used to
eliminate fatigue problems at web intersections.
In common with other gravity production platforms Selection of plate girder solution
for the North Sea, the SPC(S) design is adaptable to 3. Perhaps one of the most interesting features
allow it to meet a broad range of requirements of the of the gravity platform deck designs is the very wide
oil companies in respect of water depths and pro variety of solutions which different design teams
duction facilities. The structure consists of a have produced to what is essentially the same prob
gravity base which is designed to support three or lem. For this reason alone it is perhaps valuable to
four concrete portal legs. The concrete legs are explain the thinking behind the solution adopted for
connected at the top to short steel legs which frame the SPC(S) design.
into a steel deck structure (Fig. 1). This Paper deals 4. The deck structure is both the major element
with the design of the steel deck and with the dyna carrying the deck payload and also the portal beam
mic analysis of the whole structure for the calculation which helps to resist the dynamic wave induced sway
of fatigue life. forces on the structure overall. In this latter capac
ity it is subjected to a potentially damaging fatigue
regime. Both the magnitude of the static loads and
the headroom requirements of the cellar deck deter
DESIGN
mine that the deck members should have relatively
Layout of the deck structure high depth to span ratios. This in turn defines the
2. The primary structure of the deck consists of deck as a relatively stiff beam, so that it attracts
eight deep plate girders which contain the cellar heavy shear loads when the structure deforms in the
deck, and which directly support the modules on the sway mode.
production deck. The cylindrical steel upper legs 5. Three structural forms were considered for the
extend through the depth of the deck and finish at SPC(S) structure: truss, box girder and plate girder.
the level of the production deck. The plate girders A truss solution was rejected, primarily on the
transmit loads directly into the legs. The overall grounds that welded trusses are inherently prone to
area of the deck is increased locally by the addition fatigue damage. It was felt that whilst a combina
of relatively short cantilevers. Fig. 2 illustrates a tion of expensive details and relatively low working
typical layout with a deck area of 5000 m2. This stresses could be employed to obtain specified fatigue
particular deck layout includes a ninth central plate life, the best strategy would be to avoid truss con
girder to suit a particular module specification, but struction itself. Other factors also seemed to point
for several other specifications this girder has not in the same direction. The high shears to be carried
been required. presented problems for trusses and the continuous
cylindrical legs offered no natural nodes for the
Design and construction of offshore structures. Institution of Civil Engineers, London, 1976 1
PAPER 1: CLARK AND DAWSON
and the structure tends to be less efficient for these
reasons. However, narrow boxes are not well
adapted for framing into the main legs at the knees,
and tend to waste the space they enclose. In most
structures the torsional stiffness of box members
confers real compensating advantages in terms of
stability and ability to carry moment forces by
torsion, but in a gravity deck these advantages can
not in practice be usefully employed.
7. Plate girders seemed to offer several clear
advantages over other support systems. They can
be positioned to provide a widely spaced support
grid on which the modules can be supported directly
without the need for significant secondary steelwork.
The resulting broad grid spacing lends itself well to
an efficient arrangement of members at the nodes.
Plate girders are cheaper to make than box girders,
and are usually to be preferred for this reason alone,
other factors permitting.
Overall design considerations
8. As has already been indicated, Seadeck is
structurally a portal frame and, as in any portal, a
key consideration in the design is the efficiency of
Fig. 1. Seadeck platform
the knee joint in transmitting the flange and shear
forces out of the portal beams. The correct con
ception of this joint was one of the first requirements
in determining the overall layout. Other overriding
principles governing the main details were the need
to eliminate potential fatigue or fracture situations,
the requirement for special steels, and the need for
stress relieving. Thus although the precise fatigue
stress spectrum could not be known at the prelimin
ary design stage, it was established that design life
would be maximized for any spectrum which might
apply.
Selection of the delta flange arrangement for plate
girders
9. The delta flange arrangement selected for the
girders and illustrated in Fig. 3 has two main ad
vantages in this application. Firstly, it permits a
large flange area to be achieved without recourse to
plates thicker than about 35 mm. Secondly, the
delta arrangement produces a flange with enhanced
stiffness in both lateral and vertical bending, and in
torsion. Recent research has confirmed the im
portance of the stiffness of the flange and associated
web, in the vertical plane, to provide a stiff boundary
restraint for the web. Without such a boundary
the structural efficiency of the web tends to be di
minished. The addition of a diagonal plate to form
Fig. 2. Plan on seadeck
a delta corner to a box girder has been recommended
by various authorities to provide torsional stiffness
at the web/flange junction. The IDR(1) specifically
connexion of diagonal shear members. The con makes provision for this design detail. Although
centration of large forces into the discreet members the torsional stiffness is not essential in a plate girder,
of trusses tends naturally towards thick and there it gives some extra strength to the web and vertical
fore special steels, which was directly contrary to the stiffeners, and is a bonus to the design.
basic philosophy of the designers. Finally, although 10. The arrangement of the primary structure
an open web truss appears to be ideal in providing makes it possible to use the main plate girders as skid
for ducting and pipe-runs through the webs it is in beams, and a further bonus conferred by the delta
fact less flexible in this respect than a solid web girder, arrangement is an inherent resistance to transverse
which has no defined web member positions and can bending moments from eccentric rolling loads.
therefore in principle be penetrated at any section. 11. The cost and time for fabrication was con
6. Box girders, which are attractive in so many sidered to be fairly closely related to the amount of
structural applications, were found to have several welding, particularly hand welding, required. A
clear disadvantages for a gravity deck. Wide boxes preliminary design for the webs was prepared (Fig. 3)
present severe problems in shear lag in the flanges which used no horizontal stiffening and could be
near the supports, with the need perhaps for extra fabricated without turning the girder over. This
webs leading to problems of stress concentrations, very simple concept carried a weight penalty because
2
DESIGN OF DECK STRUCTURE FOR GRAVITY PRODUCTION PLATFORM
the web had to be designed as load shed(1) and the 1800
final design uses horizontal stiffeners. These de
velop the full bending strength of the web, and also
frame the major penetrations.
35 flange
1
Deck to leg knee joint
12. Study of deck designs suggested that this
detail has presented more problems than any other
part of the deck. The Authors' own preliminary
work on several possible configurations showed that 30 web J
both steel and weld metal could easily build up in t
this area, and in some cases this was associated with 2500
thick plates, through thickness stressing and complex
fabrication. The solution evolved for Seadeck was
conceived to minimize the problems inherent in this
very large 'node' connexion.
13. The layout is designed to give the shortest
possible load paths. Flange forces from the deck
beams are transmitted to the leg cylinders by a pair
of external horizontal diaphragms. The shear webs Scheme I. Vertically stiffened webs
do not surround, but pass directly into, the leg
cylinders, so that the cylinders themselves form part r -
of the overall shear path through the knee. All the
webs change direction before meeting the cylinder, 35 flange
and this separation of the lines of the webs from the
main lines of action of the flange forces is a signifi
cant feature of the design. The detari arrangement,
which is shown in Fig. 4, was conceived primarily to
avoid the need for a special cylinder-to-square
transition section with the associated penalty of © z 3750
extra weight and fabrication complexity. The 4 dia oo
phragms ' are external to the legs, and this allows a -
clear uninterrupted space for pipework and access 30 web _
inside the legs. t
14. This arrangement has the further advantage
that the diaphragms are a simple extension of the
main flange system which thus flows smoothly and
without interruption round the cylinder. Plate
thicknesses are relatively low and there are no joint
problems, no through-thickness stressing, and no Scheme 2. Vertically and horizontally stiffened webs
fatigue problems arising from intersecting flanges.
15. It will be noted that the delta section of the
designed to permit only the transmission of shear Fig. 3. Details of main deck
plate girder flanges is tapered out just before the
stress. Provision is made for a smooth flow of axial beams (dimensions are typ
knee. This is done to avoid the problem of carrying ical only)
stress from the diaphragms to the webs, again with
axial loads through the webs at the intersection
the object of eliminating fatigue situations. In
point. Over the section where the diagonal plate is
addition, by locating the web 'curtailments' in
phased out, the flange plate is increased to as much
regions of lower bending stress, the fatigue situation
as 50 mm. This is the only location in the deck
is further improved.
where any plate thicker than 40 mm is used. The
thicker plate extends locally over the Y section of the
Penetrations
web where the flange can be stressed biaxially, and is
then spliced back into the 35 mm plate which serves 18. At an early stage the accommodation of
for most of the stiffened diaphragm plating. penetrations was recognized as a primary considera
16. The webs of the internal plate girders meet at tion in the design of the deck members. The re
a Y shaped welded joint, and the designers were quirements for penetrations vary widely, depending
anxious to avoid the risk of fatigue at this potentially to a large extent on the equipment that the client
unfavourable location. The solution adopted has requires to be installed in the cellar deck. However,
been the introduction of a small purpose-made ex typical requirements are man access penetrations
truded steel section. By this means the unfavourable 2-2 m high in all webs and bulkheads, major pene
fatigue features of a welded joint occurring at a trations for pipes and heating and ventilating ducts
geometrical stress raiser were avoided, the welds up to 1-3 m diameter and minor penetrations for
being moved away from the intersection point which cables, ducts and small pipework.
now occurs in high quality unwelded material. The
extrusion process has two advantages in this applica Investigation of penetration reinforcements
tion. The capital cost involved in making simple 19. Three factors were considered in the design
relatively low tolerance dies for the one-off section is of suitable reinforcement for penetrations, these
relatively very small, and the mechanical properties being buckling, ultimate strength and fatigue. Of
of the product are very good. these the first, buckling, is the easiest to deal with
17. At this Y joint in the web, axial bending as the standard web stiffening can normally be di
stresses in the webs are carried through the joint, but verted or augmented to prevent instability adjacent
the remaining web connexions are specifically to a hole. As such stiffening is not part of the
3
PAPER 1: CLARK AND DAWSON
primary load paths the sections required are not perties of the various forms of reinforcement are
heavy. The second factor, ultimate strength, can be summarized in Table 1.
difficult to calculate. Linear elastic analysis (for 22. As a result of the investigations it was de
example by finite elements) tends not to be informa cided that, in general, holes in the webs up to about
tive as it makes no allowance for elasto-plastic 1 m diameter could be left unreinforced except for
redistribution of stress at ultimate load and the light stiffening, if required, to prevent buckling.
theoretical elastic stresses are invariably above the Such stiffening would be one sided and kept clear
nominal 'allowable stress' limits. Approximate of the edge of the penetration. For larger holes
analyses, such as the Virendeel analogy, can be used insert plates would be used together with ring
in some cases but the final judgement of the strength stiffening. This solution tends to even out the stress
of the reinforcement system should be based on a level, since the reinforcement matches the tendency
clear identification of the load paths for the primary for stress to concentrate towards the edge of the
system of moments and shear. Particularly for the hole. Hence fatigue and strength properties are
larger penetrations, consideration needs to be given good.
to the extent of the disturbance of the stress field
away from the hole, as well as the position of the
Location of penetrations
maximum stress concentration which frequently
does not occur at the section through the centreline 23. As mentioned in § 5 the plate girders have the
of the penetration. For the third factor, fatigue, the advantage that there are no absolute restrictions on
tool of linear elastic analysis is clearly appropriate the location of penetrations within the depth and
for the calculation of fluctuating fatigue stresses. length of the webs. Web stiffening can easily be
The determination of overall fatigue loading is adjusted to suit the preferred pattern of penetrations.
covered later in this Paper. By calculating the stress For the leg cylinders that extend through the cellar
spectrum and identifying the appropriate fatigue deck to module deck level, there are again no abso
class and associated cut-off stress level, the fatigue lute restrictions but the large man access penetrations
life of all penetrations can be found. However, in have been located in the more lowly stressed upper
practice, this procedure can be very significantly half of the cylinder.
simplified by reducing fatigue loading for each sec 24. All penetrations are regarded as undesirable
tion to a single value of moment (or shear) with an by the structural designer and the optimum com
equivalent number of cycles and using the design promise between structural and plant requirements
method explained by Gurney(2). Hence only for can only be achieved by means of a timely dialogue
critical areas will the full stress spectrum for details between the client and the deck designer. Notwith
adjacent to a penetration be required and for such standing this, it is clearly highly advantageous for
cases the calculations will include consideration of the deck member design to be such that the minimum
cut-off stress levels. overall restrictions are imposed on the plant de
20. For the SPC(S) deck, the types of penetration signer.
reinforcement that have been investigated in detail
include:
Design rules and codes of practice
(a) no reinforcement—the bare hole; 25. A paper dealing with a plated steel structure
(b) patch plates welded on to the web plates; the size of a gravity platform deck must be incom
(c) thicker insert plates; plete if no reference is made to the codes and rules
governing the design. The Seadeck design incorpor
(d) ' boxing' of the hole with stiffeners;
ates only conventional welding details and plate
(e) ring stiffeners in a circular hole;
thicknesses, hence materials and welding require
(/) combinations of {d)-(e).
ments are adequately covered by current rules.
21. To establish basic principles theoretical However, it is common in the evolution of new types
solutions for stress patterns were used where avail of structure for design development to move ahead
able, coupled with axisymmetric finite element of current codes of practice, and this is certainly
analysis for reinforced circular penetrations. Fur true for North Sea structures. For example, there
ther results for large holes plus stiffening in finite is no national code of practice which gives adequate
depth beams were obtained by two-dimensional guidance for the design of stiffened plates and shells,
finite element analysis. It was soon established that although for the former the work of the Merrison
to keep stress concentrations down to an acceptable Committee, and the massive programme of associa
level from the point of view of fatigue it was highly ted research, has gone a long way to remedying this
desirable that all penetrations should be kept as inadequacy. The Merrison Interim Design Rules
nearly circular as possible. In all cases considered (IDR) were therefore taken as the primary guide in
an oversize circular hole was found to be preferable this area. These rules were developed very quickly
to a square hole with rounded corners. (This con to meet the urgent needs of the DoE, and their interim
clusion might not be valid if fatigue were not im nature and the fact that they were conceived in the
portant, or if the shear stresses in the web were much context of long-span bridges together present con
less important than the bending stresses.) It was siderable problems when they are applied in some of
found that for major penetrations in the webs of the the situations encountered in offshore decks.
main girders the stress concentrations due to the 26. The Seadeck designers were in the fortunate
shear loads were dominant in comparison with the position that they were engaged in preparing the
effect of the bending moments. A further general contract draft for the Steel Design Section of the
conclusion was that stiffening designed to replace cut new UK Bridge Code concurrently with their work
away material, to maintain ultimate strength, was on Seadeck. The brief for the preparation of the
generally very heavy and had to be made symmetrical code draft required that it be based both on the best
about the web plate to act effectively. Hence pri of the IDR and BS153 and on the currently available
mary strength was found to be more easily increased results of research into plated structures. The
by supplementing the web plate itself. The pro background information available as a result of this
4
