Current and Potential Use of High Strength Steels in Offshore Structures

55 Pages·1995·4.974 MB·55\55

by  Billingham, J., Healy, J., Spurrier, J.

THE MARINE TECHNOLOGY
DIRECTORATE LIMITED
CURRENT AND POTENTIAL USE
OF HIGH STRENGTH STEELS
IN OFFSHORE STRUCTURES
J.Billingham, J.Healy and J.Spurrier
PUBLICATION 95/102
The authors wish to acknowledge the sponsors of the Managed Programme of University
-
Research, High Strength Steels in Offshore Engineering, 1992 94, at Cranfield University, for
their permission to publish this document. Sponsors of the Programme include MTD Ltd acting
on behalf of EPSRC, and AMEC, Amoco, BOC, BP, DRA, ESAB, HSE, NPD, Shell Expro,
Texaco, Total.
We are also grateful to the following organisations for information contributed to this study and
helpful discussions:
Billington Osborne-Moss Engineering Ltd, Maidenhead
British Steel Corporation
John Brown Engineers and Constructors Ltd, London
Dillinger Hiittenwerke
Fabrique de Fer de Charleroi S.A.
GTS Industries, Dunkerque
Kvaerner Earl and Wright, London
McDermott Engineering, London
SSAB Sweden
Sumitomo London
Swedish Steel UK Ltd
Published by MTD
The Marine Technology Directorate Limited
Registered in England
Registered Office
19 Buckingham Street
London WC2N 6EF
Registered as a Charity under the Charities Act 1980
Registered Charity No 295576
0 MTD 1995
ISBN: 1 870553 24 1
2 MTD Publication 951102
CONTENTS
Page No.
1. General introduction 7
2. Design with high strength steels 8
-
3. High strength steels a number of areas of concern 13
4. Performance of modern high strength steel 14
4.1 Mechanical properties 14
4.2 Plate weldability 20
4.3 Requirement for high strength weld metals 28
4.4 Fatigue performance 30
4.5 Static strength, buckling and Code requirements 32
5. Special design requirements 41
5.1 Buckling 41
5.2 Leakage 41
5.3 Ductile failure 41
5.4 Brittle fracture 41
5.5 Fatigue and corrosion fatigue 42
5.6 Weld metal properties 43
5.7 Cathodic protection levels 43
6. Summary of current steel performance 44
7. Conclusions 46
8. References 47
Offshore use of high strength steels 3
1. GENERAL INTRODUCTION
High strength steels have been available for many years, but their use in offshore engineering has
been severely restricted except in specialised applications. This is largely because, in general,
satisfactory performance can be achieved with cheaper, more readily available, lower strength
steels. Additionally, as strength increases, not only does the cost increase but the ductility and
weldability generally decrease. In major structural applications such factors have significant
influence. Thus most steels used in offshore structural applications have yield strengths in the
range 250 to 350MPa. Indeed, there are restrictions in many codes which militate against using
steels with yield strengths greater than 460MPa(').
The major requirement offshore is for tubular construction practice which utilises steel plate as
the raw material. The ferrite pearlite steels used must be readily available in tonnage quantities
and in a range of thicknesses up to 1OOmm. The conventional steel production route is
normalising in order to produce satisfactory properties in the thicker section components. A
primary requirement for this production route is adequate weldability, which is provided by
limiting the steel hardenability through compositional limitations usually imposed through
restrictions in allowable carbon equivalent values? This factor, combined with the need for
).
good notch ductility, has led to a continual reduction in carbon levels in such steels over the past
two decaded3).
In contrast, over the same period, the pipeline industry has successfully exploited large tonnages
of higher strength steels in marine environments. Thus, X70 grade (480MPa) steels are
commonly used in subsea pipelines. Such steels are ferrite pearlite steels which are micro-alloyed
to produce a fine grain size, to give both strengthening and resistance to low temperature brittle
fracture. They also have low carbon contents to ensure good weldability and toughness, and low
levels of impurities to ensure resistance to ductile failure processes, which might occur in gas
pipelines as a result of overpressurisation. Pipelines utilise much smaller section thicknesses
(usually 19mm or less), and they can therefore take advantage of alternative steel production
routes such as controlled rolling or TMCP to develop the necessary fine grained microstr~cture(~).
Even higher strength steels (700MPa yield strength) have been used for some time in jack-up
platfc>rms('). Such steels have usually had a relatively high carbon content, because good abrasive
resistance is required in the jacking operation, and weldability is not a prime requirement@.
Because of the periodic docking arrangements which allow easier inspection capabilities, fatigue
is also not seen as a major design limitation. More recent proposals to use such structures on
much longer term operational schedules have insti.gated a more widespread interest in the likely
fatigue performance of the type of steels used(>.
Another traditional application for higher strength steels in marine environments has been
submarines('), where higher strength (700MPa) steels are utilised to reduce section thicknesses
because of their excellent mechanical property combinations in terms of strength and toughness(9).
However, such steels are generally much more highly alloyed, making them more expensive and
more difficult to weld. In turn, this often necessitates lengthy and costly pre-, and sometimes
post-welding treatments, which extend and complicate fabrication procedures, and introduce
considerable additional costs(''). Similar steels have also been used in mooring applications such
as the tethering attachments in tension leg platforms, where their high strength and good resist-
ance to fatigue can be utilised. In this application, the steels are not usually used in the welded
condition, screwed connectors being used for example, so that their excellent resistance to fatigue
initiation or sudden overloads can be utilisedc"). However, a recent example used welded con-
nections on a 38-mm thick lower strength X70 (485MPa) tether string for the Heidrun project('*).
Offshore use of high strength steels 7
Most current fixed platform structures contain considerable quantities of structural steel of yield
strengths up to 350MPa. Some in-depth studies over the period 1985 to 1990 have illustrated the
'' .
weight saving potential of replacing such steels .with modern higher strength steels(6v For
'O 19)
example, in topside structure design, by substituting a SOOMPa steel in place of a conventional
350MPa steel, a 25 to 30% reduction in weight is predicted("). Indeed, high strength steels,
typically Grade 460, are widely utilised in such areas where fatigue is not a design consideration.
This topside slimming also brings about a progressive saving in sub-structure design(14 The
to ").
price premium on a modem Grade 460 steel compared with a Grade 355 steel is typically of the
order 10 to 15%, which is more than offset by the reduced steel tonnage consumed('9) and the
reduced costs of fabrication.
2. DESIGN WITH HIGH STRENGTH STEELS
For offshore oil and gas exploration and production, the selection of structure type depends on
three primary factors; location, water depth and number of welds required to exploit the field(zo~zl).
Together, these dictate the expected environmental conditions (wave and wind loading), topside
loads, and overall size, shape and weight of structure. The main driving force in the development
of higher strength steels has been the desire for lighter weight structures. The demands made by
the offshore industry have prompted most work in this area, and because of the immense size of
offshore structures and mobile drilling rigs, there is considerable potential for weight saving (and
hence cost benefits). This has brought about considerable efforts to minimise structural weight by
optimising structural design, platform layout and topside equipment(13 "I.
lo
Fixed jacket structures are the most widely used form of structure currently used offshore and, as
indicated in Figure 1, are a cost-effective alternative to other structure types at depths of up to
about 300m("). Over the past 5 years, fixed platfhrms have been deployed in the North Sea in
water depths ranging from about 60 to 140m, with jacket weights in the range 5000 to 9500
tonne. According to designers, the application of high strength steels in such structures, typically
Grades 420 and 460, presented a number of direct routes to save structural weight and reduce
total project costczoto z5). Reducing jacket weight has enabled projects to meet critical lift limits
.
for direct crane installation(z6) Alternatively, use of higher strength steels can minimise the
.
reinforcing steel requirements for launching supports(" to 1'' Mc> re specifically, the use of higher
strength steels can lead to reductions in piling requirements, and it can reduce member sizing
(and thus reduce wave loading). Table 1 shows a breakdown for a typical Gulf of Mexico jacket
weight by percentage, and it is notable that 80% of the jacket weight is in the legs and braced").
Similarly, for North Sea fixed structures, the nodal joints represent only 15 to 25% of total jacket
weight, depending on overall jacket size and working environment("). This highlights the area
where optimum material and member size selection may lead to significant savings in materials,
and it indicates a prime area for utilising higher strength steels as demonstrated in the recent
Enterprise/Shell Nelson Jacket, where 450MPa steel was used in the interconnecting tubulars,
while 350MPa steel was retained in the fatigue critical nodal joints(14).
Additional benefits of using modern higher strength steels can be derived in the fabrication yard.
A recent review indicated that fabrication (forming and welding) represents approximately 57% of
total project cost, so any positive impact in this area could yield significant savings(14). As a
consequence of changes in steel alloying and production routes, modern higher strength steels
offer improved weldability, which is reflected by reported fabrication yard experiences, where no
difficulties were encountered in comparison to fabricating with the lower strength Grade 355
.
steels(14, 2 7 ,to~ 30 ) e use of higher strength steels enables thinner sections to be utilised. This
8 MTD Publication 951102
Most current fixed platform structures contain considerable quantities of structural steel of yield
strengths up to 350MPa. Some in-depth studies over the period 1985 to 1990 have illustrated the
'' .
weight saving potential of replacing such steels .with modern higher strength steels(6v For
'O 19)
example, in topside structure design, by substituting a SOOMPa steel in place of a conventional
350MPa steel, a 25 to 30% reduction in weight is predicted("). Indeed, high strength steels,
typically Grade 460, are widely utilised in such areas where fatigue is not a design consideration.
This topside slimming also brings about a progressive saving in sub-structure design(14 The
to ").
price premium on a modem Grade 460 steel compared with a Grade 355 steel is typically of the
order 10 to 15%, which is more than offset by the reduced steel tonnage consumed('9) and the
reduced costs of fabrication.
2. DESIGN WITH HIGH STRENGTH STEELS
For offshore oil and gas exploration and production, the selection of structure type depends on
three primary factors; location, water depth and number of welds required to exploit the field(zo~zl).
Together, these dictate the expected environmental conditions (wave and wind loading), topside
loads, and overall size, shape and weight of structure. The main driving force in the development
of higher strength steels has been the desire for lighter weight structures. The demands made by
the offshore industry have prompted most work in this area, and because of the immense size of
offshore structures and mobile drilling rigs, there is considerable potential for weight saving (and
hence cost benefits). This has brought about considerable efforts to minimise structural weight by
optimising structural design, platform layout and topside equipment(13 "I.
lo
Fixed jacket structures are the most widely used form of structure currently used offshore and, as
indicated in Figure 1, are a cost-effective alternative to other structure types at depths of up to
about 300m("). Over the past 5 years, fixed platfhrms have been deployed in the North Sea in
water depths ranging from about 60 to 140m, with jacket weights in the range 5000 to 9500
tonne. According to designers, the application of high strength steels in such structures, typically
Grades 420 and 460, presented a number of direct routes to save structural weight and reduce
total project costczoto z5). Reducing jacket weight has enabled projects to meet critical lift limits
.
for direct crane installation(z6) Alternatively, use of higher strength steels can minimise the
.
reinforcing steel requirements for launching supports(" to 1'' Mc> re specifically, the use of higher
strength steels can lead to reductions in piling requirements, and it can reduce member sizing
(and thus reduce wave loading). Table 1 shows a breakdown for a typical Gulf of Mexico jacket
weight by percentage, and it is notable that 80% of the jacket weight is in the legs and braced").
Similarly, for North Sea fixed structures, the nodal joints represent only 15 to 25% of total jacket
weight, depending on overall jacket size and working environment("). This highlights the area
where optimum material and member size selection may lead to significant savings in materials,
and it indicates a prime area for utilising higher strength steels as demonstrated in the recent
Enterprise/Shell Nelson Jacket, where 450MPa steel was used in the interconnecting tubulars,
while 350MPa steel was retained in the fatigue critical nodal joints(14).
Additional benefits of using modern higher strength steels can be derived in the fabrication yard.
A recent review indicated that fabrication (forming and welding) represents approximately 57% of
total project cost, so any positive impact in this area could yield significant savings(14). As a
consequence of changes in steel alloying and production routes, modern higher strength steels
offer improved weldability, which is reflected by reported fabrication yard experiences, where no
difficulties were encountered in comparison to fabricating with the lower strength Grade 355
.
steels(14, 2 7 ,to~ 30 ) e use of higher strength steels enables thinner sections to be utilised. This
8 MTD Publication 951102
TABLE 1 Breakdown of Gulf of Mexico jacket weight
Item Percentage of total jacket weight
39.9%
Legs
Cans 14.5%
Other 25.4%
Braces 40.8%
Vertical diagonals 19.1%
Horizontals (including cans) 13.4%
Horizontal diagonals (including cans) 8.3%
Other fianiing 9.8%
Conduc.totor framing 2.9%
Launch trusses and runners 6.7%
Miscellaneous framing 0.2%
Appurtenances 9.5%
Boat landing 2.3%
Barge bumpers 2.4%
Anodes 1.8%
Walkways 1.3%
Mudiiiiits 0.4%
Padeyes 0.2%
Closure plates 0.2%
Flooding systeiii 0.3%
Miscellaneous 0.3%
TOTAL 100.0% 100.0%
60
BBUULLLLWWIINNKKLLEE
50 FFIIXXEEDD CCOOMMPPLLIIAANNTT TENSION LEG
PPLLAATTFFOORRMMSS TTOOWWEERRSS PLATFORMS
40
Y
J
w
3 3 c J
20
FFLLOOAATTllNNGGllSSUUBBSSEEAA SSYYSSTTEEMMSS
i a I II II II II
II 330000 660000 990000 1122 0000 11550000 11880000
WWAATTEERR DDEEPPTTHH,,MM
FIGURE 1 Influence of water depth and number of wells on selection of platform type
Offshore use of high strength steels 9
leads to reduced weld volumes, and because resultant weld volume is proportional to the square
of the thickness, this change in thickness has a significant effect on the required quantity of weld
metal and welding time, and also presents the prospect of reducing or avoiding PWHT in some
cases.
A number of major offshore designers, operators and steelmakers have been surveyed to establish
the trends in high strength steel usage offshore("). Firstly, it must be recognised that high
strength steels may not be the' best alternative for every offshore application, but their suitability
will be'judged on the nature of the structure (e.g. type, size, location, member design, etc.). Over
the period 1988 to 1995, the use of higher strength 'steels (Grade 460) in structures, where weight
has been a primary consideration for the lift installation procedure, has increased from
approximately 8% to typically 40% as illustrated in Figure 2 and Table 2. Initially, such steels
were only used on topsides, but now they are also frequently used in jacket legs and bracing
members not subject to fatigue loading, particularly in jackets installed in UK waters. It is
notable that a number of recent Norwegian structures comprised approximately 95% higher
strength Grade 420 steels in the jacket. This is considered to be the result of the less restrictive
design rules and procedures, particularly with respect to their application in fatigue critical areas,
for example nodal joints(23).
Many designers believe that in principle there appears to be little in the way of special design
modifications needed for the application of the current range of Grade 450 type steels. However,
a number of perceived problems do require further consideration. Fatigue behaviour is a primary
factor, particularly corrosion fatigue, and the application of cathodic protection. The current
fatigue database is generally based on SOD steels (-355MPa), with little or no data for higher
strength steels. Consequently, fatigue design with higher strength steels is based on the same
-
S N data, offering no advantage in fatigue. However, laboratory crack growth data indicates the
-
satisfactory behaviour of high strength steels, and there is a need to genkrate large-scale S N
fatigue data on such steels to resolve these issues. Buckling resistance, dictated by both yield
strength, member slenderness ratio and diameter to thickness ratio, is another important
consideration and will be discussed later in detail.' Other concerns relate to steel performance and
availability (in sufficient quantities and at a competitive price), weldability, weld strength
matching requirements and weldment toughness. These separate factors are addressed in this
report. A further general comment from some designers was that current offshore platform
guidance notes do not clearly state all the requirements for higher strength steels (e.g. fatigue, CP
levels, Charpy, CTOD and PWHT requirements).
10 MTD Publication 95/102
C
v,
CD
%
U
di'
U
r
ii:
I
03
U TABLE 2(a) Amounts of high strength steel (yield strength 420 to 450MPa) used in recent offshore projects
v,
3
",
v, Manufacturer 1:
Manu fact urer 2:
Opwn tor: BP BP sit ell Shell Arncmda Has Afarorhon Chavmn
Projccl: hlilitr Bruce (;a?fttel i\'ClSOR Scdf E d B rat A h
Tonne: 11000 13300 3500 9000 5000 4200 10000
TABLE 2(b) Amounts of high strength steel used in recent offshore projects
Proporiion of total
tonnage
1992 BP/Bruce 1600 18%
BP/Uni ty 1000 10%
ShelVGannet 100 1%
1993 En terprise/Nelson 8000 30%
Amerada Hess/Scott 2 jackets 50% Grade 420
-25000
Chevron/Alba 15000 15%
Phillips/Judy 9000 40 %
1994 To taVDun bar 10000 40 %
Statoil, Sleipner, 30000 95% Grade 420
Kvaernedfor
Norwegian section
+
2 jackets topsides
1995 Philips/Ekofisk 33000 95% Grade 420
+
2 jackets topsides
8
Year of supply. Year of supply
FIGURE 2 Increasing use of high strength 450MPa steel over recent years observed
from supply records of one major steel manufacturer(””
339 44)
12 MTD Publication 951102