CRISP, Manager, UK Offshore Steels Research Project BASIC FATIGUE TESTS in air and sea water including cathodic protection with constant and variable amplitude loads Tests on Weldments
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ISBN: 978-0-7277-0108-4
Trang 3FATIGUE IN OFFSHORE STRUCTURAL STEELS
I m p l i c a t i o n s o f t h e D e p a r t m e n t o f E n e r g y ' s R e s e a r c h P r o g r a m m e
Trang 5F A T I G U E I N O F F S H O R E S T R U C T U R A L S T E E L S Implications of the Department of Energy's Research Programme
Proceedings o f a C o n f e r e n c e o r g a n i z e d b y t h e Institution of Civil Engineers,
h e l d in L o n d o n o n 2 4 - 2 5 F e b r u a r y 1981
THOMAS TELFORD LTD, LONDON, 1981
Trang 7C o n t e n t s
Opening address H G CRISP 1
1 The design of the UKOSRP basic fatigue programme J G H I C K S 3
2 Constant amplitude corrosion fatigue strength of welded joints G S B O O T H 5
Discussion on Papers 4 and 5 4 5
6 Review of stress analysis techniques used in UKOSRP N M I R V I N E 4 7
7 Stress concentration factors at K and KT tubular joints A G W O R D S W O R T H 5 9
Discussion on Papers 6 and 7 6 7
8 The fatigue strength of tubular welded joints K J M A R S H 71
9 Modes of fatigue crack development and stiffness measurements in welded tubular
joints J G W Y L D E a n d A M c D O N A L D 7 9
1 0 Experimental results of fatigue tests on tubular welded joints A M c D O N A L D a n d
J G W Y L D E 8 9
Discussion on Papers 9 and 1 0 101
1 1 Prediction of crack growth in tubular joints—an alternative design approach
A M C L A Y T O N 1 0 5
Discussion on Paper 11 111
1 2 Summary of current design and fatigue correlation P J FISHER 1 1 3
1 3 Relationship of Guidance Notes and applicability to offshore design J R PETRIE 1 2 3
Discussion on Papers 1 2 and 1 3 1 2 7
Trang 9H G CRISP, Manager, UK Offshore Steels Research Project
BASIC FATIGUE TESTS
in air and sea water (including cathodic protection) with constant and variable amplitude loads
Tests on Weldments WELDING INSTITUTE NATIONAL ENGINEERING LABORATORY
Tests on Steel Plate UKAEA/Harwell UKAEA/RFL
1 TUBULAR JOINT TESTS
in air
Tests on T-Joints with constant and variable amplitude loads
Tests on Large H-Joints NATIONAL ENGINEERING LABORATORY
Tests on T, K and TK Joints with constant amplitude loads WELDING INSTITUTE
1
FRACTURE TESTS
Selection of Fracture Resistant Materials WELDING INSTITUTE
F i g 1 U K O f f s h o r e S t e e l s R e s e a r c h P r o j e c t
Trang 11J G HICKS, MA, FWeldl, MRAeS, Consultant
in Welded Fabrication and Design
The design of the UKOSRP basic fatigue programme
The UKOSRP basic fatigue programme was designed to acquire information on the fatigue life of welded joints in sea water for use in compiling design rules for offshore structures The test programme was designed to examine the variables relevant to offshore platforms; the various test series were chosen to isolate the variables as far as possible and also to facilitate comparison with existing data Although the main aim of the programme was to acquire stress/life data for constant amplitude and variable amplitude loadings a number of tests measured the crack propagation rates with the intention of deriving a generalised fatigue life prediction method based on
computed stresses
INTRODUCTION
The research programme was initially designed
in 1973* At that time there was relatively
little experience of the use of structures in
the water depths, wave height distributions
and temperatures of the northern North Sea
Such structures would be constructed of steels
of greater thickness and with joints of greater
complexity than hitherto The design data in
respect of fatigue at that time had been based
on tests in air of small specimens under con
stant amplitude conditions The commentary on
the rules in the AWS Structural Welding Code
at that time said that "Calculated fatigue
lives based on the proposed curves should be
viewed with a healthy amount of scepticism and
should be used more as design guidance than as
an absolute requirement"
It was apparent then that a complete test
programme would be needed to provide design
data which could be used to design new
structures and to assess the integrity of those
being designed and built at the time (ref. 1 )
The number of variables involved made the idea
of a simplistic test programme covering all
these variables in one series of tests unrea
listic from the point of view of time and cost
Test programmes were accordingly planned to
deal with the selected variables in parallel
tests with sufficient interfaces to provide,
at the end of the work, a satisfactory set of
results with a unified basis
The programme was designed to meet the needs of
the Department of Energy for certifying fixed
offshore structures in accordance with the
legislation, and also, what were seen at the
time to be both the long term and the short
term needs of the industry
DESIGN OF THE TEST PROGRAMME
To achieve these aims the test programme was
planned to take place on two separate but
inter-related levels
The first level consisted of the fatigue
testing of standardised welded joint specimens the results from which would provide data in the traditional form of fatigue life as a function of stress The life was defined as the complete failure of a specimen Series of specimens were to be tested under both constant and variable amplitude stress histories in air and simulated sea water; separate series of specimens in sea water were to be tested under freely corroding conditions and under a
cathodic protection condition
The second level of testing would involve the measurement of crack growth rates throughout the specimen life It was intended that the data acquired here could be used in conjunction with fracture mechanics analyses to develop a generalised method of life prediction for any type of joint for which the detailed stress distribution could be calculated Methods for calculated stress distributions in tubular joints were to be examined and assessed in other parts of the Project The value of existing fatigue test data was not dismissed and the test programme was designed to permit comparisons with that data and minimise new test work as much as possible
The effect of the seawater environment was acknowledged to be a time dependent phenomenon and fatigue testing would be relevant only if the tests in seawater were undertaken at a cyclic frequency approximating that of the load frequency on offshore structures, namely
around the wave frequency Wave frequency varies with wave height but for the purpose of the tests a figure of 0.1 Hz was decided upon
To acquire test results in a reasonable time
it was then necessary to postulate multiple testing of a large number of specimens The necessary test equipment would then be quite extensive requiring a large number of stations each capable of applying a load to its specimen Axial load testing had been the conventional method in standard testing machines but the thickness of the specimens for the offshore
Trang 12The two t y p e s o f s p e c i m e n u s e d were a t r a n s
v e r s e c r u c i f o r m b u t t weld and a non l o a d
c o n s t a n t a m p l i t u d e t e s t s and t h e r e f o r e t h e
b a s i c c o n s i s t e n c y and r e p e a t a b i l i t y o b s e r v e d
i n t h e c o n s t a n t a m p l i t u d e t e s t s c o u l d b e assumed The c o m p a r i s o n s between c o n s t a n t and
b e e n shown t o be much l e s s marked than, would
h a v e "been e j e c t e d from p r e v i o u s work, and
Trang 13The S-N curves for freely corroding joints and for joints alternately immersed in sea water and exposed to air were not significantly different from the results of joints tested in air
At high stress ranges, cathodic protection did not influence the fatigue lives of continuously immersed joints As the stress range was decreased, however, the cathodically protected joints exhibited increasingly longer lives than the freely corroding joints
Grinding the weld toes resulted in only a small increase in the fatigue strength of freely
corroding joints This increase was much smaller than is achieved in joints tested in air
INTRODUCTION
A major problem which faced the designers
of steel platforms now operating in the North
Sea, highlighted in a review of such problems" 1 ,
was the lack of information about the effect
of sea water on the fatigue strength of welded
joints Although it was known that fatigue
strength could be reduced as a result of immer
sion in sea w a t e r 2 , the effect could not be
quantified in the context of North Sea opera
tion Therefore, as part of the United Kingdom
Offshore Steels Research Project, an investi
gation of the influence of North Sea conditions
on the fatigue strength of welded joints was
initiated and this paper describes the work
carried out so far
The critical joints in offshore platforms
are the intersections between tubular members
(nodes) where high local stresses can arise
due to bending of the tube wall An ideal
solution to the problem of obtaining relevant
fatigue data would have been to carry out
fatigue tests on tubular joints under simulated
North Sea environmental conditions However,
it was not considered necessary to test full
scale nodes in sea water and instead tests were
carried out in bending on cruciform joints, in
which the weld detail is similar to that in
tubular connections
Three environmental conditions were in
vestigated:
a) Joints freely corroding in sea water,
to represent a welded joint simply
immersed in sea water
b) Joints cathodically protected in sea
water, to represent the more common
situation in which platforms are
protected against general corrosion
c) Joints alternately freely corroding
in sea water and exposed to air, to represent regions near the water line which are submerged for only a pro portion of the time
Similar specimens have been tested in air to provide data for comparison with the results of the present work
It is known that the fatigue strength of transverse welds in air, such as those in a node can be improved by grinding the weld
t o e 3 , 4 , 5 The welds in tubular joints are often ground to facilitate inspection and if it
is found that the fatigue strength is also in creased in sea water, the designer might be able to take advantage of the benefit obtained However, it is possible that any benefit would
be lost in joints freely corroding in sea water because of the introduction of corrosion pits at the weld toe To investigate this, a series of tests were carried out on specimens with ground weld toes, freely corroding in sea water
This paper, therefore, describes the results of constant amplitude fatigue tests on planar welded joints under environmental con ditions intended to represent an offshore plat form in the North Sea It is based on an earlier p r e s e n t a t i o n ^ ) of the work at the Offshore Technology Conference in 1979
EXPERIMENTAL WORK
a ) Material Steel, similar to that used in offshore installations in the waters surrounding the United Kingdom, was used to fabricate the specimens The transverse plates were made from steel to BS 4360:1972 grade 50D (modified)
Trang 14T a b l e 1
S p e c i f i c a t i o n o f s t e e l t o BS 4 3 6 0 g r a d e 50D
C h e m i c a l Composition (wt %) STEEL
C(max) S i Mn(max) S ( m a x ) P(max)
BS 4 3 6 0 g r a d e 50D 0 2 2 0 1 0 - 0 5 5 1 6 0 0 5 0 0 0 5 0
( a ) c h e m i c a l c o m p o s i t i o n
M e c h a n i c a l P r o p e r t i e s
Y i e l d T e n s i l e E l o n g a t i o n Charpy V n o t c h STEEL S t r e s s (min) S t r e n g t h on 20Qnm
T e m p e r a t u r e 5 ° C - 8 ° C
Trang 17PAPER 2: B O O T H
node quality and the remaining plates from
steel to BS 4360:1972 grade 50D (modified)
The steel specification is summarised in table
1
b) Specimen Design and Fabrication
The specimen configuration is shown in
figure 1 Manual metal arc welding, using
electrodes complying with BS 639:1976 E51 28H,
was used to fabricate the specimens and a pre
heat temperature of 150°C was employed Each
specimen was manufactured individually and each
weld pass was continued onto a run-off tab
which was subsequently machined off
The weld toes of one series of joints were
ground using the technique5 recommended to
achieve an improvement in fatigue strength
This involved grinding to a depth of 0.8mm
beneath the plate surface at the weld toes on
the stressed plate A pneumatic grinder with a
100mm diameter disc with a 36 grit in an epoxy
matrix was used
c) Test Conditions
In air, the fatigue strength of a welded
joint is relatively insensitive to a number of
variables which significantly influence the
fatigue strength in other environments2
Previous work ' has identified four major para
meters which influence the rate of fatigue crack
growth in sea water These are sea water tem
perature, electrochemical potential, loading
frequency and stress ratio (minimum stress/
maximum stress = R ) , with other variables such
as sea water chemistry exerting only a secondary
influence Thus, in order to obtain relevant
S-N curves it was essential to carry out the
tests with values of the four major parameters
appropriate to a platform in the North Sea
The tests were therefore carried out under the
following environmental and stressing con
ditions
i) Environmental Conditions Each speci
men was placed in a sea water cell as shown
diagrammatically in figure 2 The volume of
each cell was approximately 10 litres and the
sea water flow rate was approximately 1 litre/
min
The sea water was prepared according to a
standard specification for substitute ocean
w a t e r9 Stock solution number 3, however, was
not added and thus the sea water did not con
tain heavy metal ions Acceptable ranges for
the pH, chlorinity, bicarbonate ion concentra
tion and salinity of the sea water had been
defined and are shown in table 2 When any
parameter neared the extreme of its allowed
range a fresh mix of sea water was substituted
This occurred approximately every three months
The temperature of the sea water was main
tained within the limits of 5°C to 8°C, which
is representative of the temperature range in
the North Sea
The specimens were tested under three
environmental conditions Firstly, joints were
tested at the free corrosion potential, which was found to be -0.63V with respect to a silver/ silver chloride reference electrode (All potentials were measured with respect to this electrode) Secondly, tests were carried out
on joints cathodically protected at the poten tial recommended"1 0 for immersed steel struc tures of -0.85V This potential was maintained
to ±0.02V using an impressed current system The anode was platinum wire wound onto a nylon framework surrounding the joint Thirdly, tests were carried out on joints which were alternately immersed in sea water at the free corrosion potential for six hours and exposed
to air for six hours This immersion/exposure cycle was selected to simulate tidal zone con ditions
ii) Stressing Conditions As illustrated
in figure 2, each joint was loaded in cantilever bending Two strain gauges were bonded onto the specimen centre line, 15mm from the weld toe as shown in figure 2 The strain gauges were used to establish the initial load but thereafter the tests were carried out under load control The load was applied by a hy draulic actuator of approximately 20kN capacity
Because the main source of dynamic load ing acting on an offshore platform derives from wave action, the tests were performed at a frequency of 1/6Hz, a typical wave frequency
As a result of the low testing frequency, the duration of some of the tests was greater than
a year and some unbroken specimens are still accumulating cycles In order to obtain suf ficient results in a realistic time, a special test rig, shown in figure 3, was built to enable 32 specimens to be tested simultaneously and independently
As a result of the existence of large tensile residual stresses in as welded joints, the effective stress ratio should be large and positive in the vicinity of the joint, irres pective of the applied stress ratio The fatigue crack growth rate, however, appears to
be independent of stress ratio above R = 0 68 and therefore in as welded joints no effect of stress ratio was expected The tests were carried out at R = - 1 , i.e fully reversed loading in order to reproduce the loading likely to occur in real structures Addition ally, tests were carried out at R = 0 to con firm that the levels of residual stress were sufficiently high to result in effective stress ratios greater than 0.6 under both applied stress ratios
PRESENTATION AND DISCUSSION OF RESULTS The specimens failed by fatigue crack growth from the weld toe through the plate thickness A failed specimen, tested at the free corrosion potential, is shown in figure 4
A specimen was assumed to have failed when the maximum stroke of the actuator was reached This corresponded to a crack through approxi mately half the plate thickness By then, how ever, the fatigue crack growth rate would have
Trang 18Fig 5 - Results for Transverse Joints, Stress Ratio = 0
Trang 195 and 6 The r e s u l t s f o r s p e c i m e n s s t i l l under
t e s t , shown a s unbroken i n f i g u r e s 5 and 6 ,
a n t r o l e in t h e d e t e r m i n a t i o n o f t h e c u m u l a t i v e damage summation and s i g n i f i c a n t l y i n f l u e n c e s
i n c r e a s e d f a t i g u e c r a c k growth r a t e which may
Trang 21was t o remove t h e u n d e r c u t and d e f e c t s ^1 2 at t h e
weld t o e and t o p r o d u c e a smooth t r a n s i t i o n
between t h e p a r e n t p l a t e and weld m e t a l , t h u s
I n f o r m a t i o n7*8 a l r e a d y e x i s t s r e g a r d i n g
t h e i n f l u e n c e o f s e a w a t e r on f a t i g u e c r a c k growth r a t e These d a t a , however, a r e g e n e r
a p p l i c a t i o n s , i t h a s been recommended t h a t t h e mean minus two s t a n d a r d d e v i a t i o n s d e s i g n c u r v e
1 The S-N c u r v e s (based on s t r e s s r a n g e )
f o r j o i n t s f r e e l y c o r r o d i n g in s e a w a t e r were not s i g n i f i c a n t l y d i f f e r e n t from
Trang 221 0 B r i t i s h S t a n d a r d s I n s t i t u t i o n : "Code o f
p r a c t i c e f o r c a t h o d i c p r o t e c t i o n , " CP 1 0 2 1 : 1 9 7 3
Trang 23D i s c u s s i o n o n P a p e r 2
DR R P M P R O C T O R , Corrosion and Protection
Centre, University of Manchester Institute of
Science and Technology
Trang 25G S BOOTH, MA, PhD, MWeldl, The Welding Institute, and R HOLMES, BSc, National Engineering Laboratory
Trang 26• Sea wafer, free corrosion
c Sea wafer, infermiffenf
Trang 28Sea waterfree corrosion
Sea water, intermittent
immersion
Sea water,cathodic protection
F i g k. E f f e c t o f
e n v i r o n m e n t at R = ( t r a n s v e r s e j o i n t s )
Constant amplitude
R= 0
• Sea water, free corrosion
Trang 33H G MORGAN, BSc, MSc, PhD, Springfields Nuclear Power Development Laboratories, and T W THORPE, BSc, Atomic Energy Research Establishment, Harwell
Trang 34F i g k S u r f a c e n o t c h e d s p e c i m e n
A l l t h i c k n e s s e s 35mm
A l l d i m e n s i o n s m m
Trang 36r a n g e / P / + 0
/ /
Trang 40c a r r i e d o u t f o r U K O S R P a r e e n t i r e l y i n a g r e e
m e n t w i t h t h e s e o t h e r i n v e s t i g a t i o n s F i g 15
s h o w s t h a t f a i l u r e i n a i r i s a l m o s t e x c l u s i v e l y
d u c t i l e w i t h f a i n t f a t i g u e s t r i a t i o n s b e i n g