OTH 92 368

FATIGUE LIFE ENHANCEMENTOF TUBULAR JOINTS BY GROUT

INJECTION

Prepared by

Baker JardineEngineering Consultancy & Software

Whitworth BuildingNational Engineering Laboratory

East KilbrideGlasgow G75 0QU

HSE BOOKS

Health and Safety Executive - Offshore Technology Report

© Crown copyright 1993Applications for reproduction should be made to HMSO:

First published 1993ISBN 0-7176 0613 9

This report is published by the Health and Safety Executive aspart of a series of reports of work which has been supported byfunds provided by the Executive. Neither the Executive, or thecontractors concerned assume any liability for the report nor dothey necessarily reflect the views or policy of the Executive.

Results, including detailed evaluation and, where relevant,recommendations stemming from their research projects arepublished in the OTH series of reports.

Background information and data arising from these researchprojects are published in the OTI series of reports.

CONTENTS

25REFERENCES10.

24ACKNOWLEDGEMENTS9.

23CONCLUSIONS8.

19191919191919202020202122

DISCUSSION7.1 Repaired Condition7.2 Grouting7.3 Static Stress Distribution

7.3.1 Axial static stress distribution 7.3.2 In-plane bending static stress distribution 7.3.3 Out-of-plane bending static stress distribution 7.4 Fatigue Results

7.5 Failure Mode 7.5.1 Crack initiation 7.5.2 Crack growth 7.5.3 Local joint flexibility 7.6 General

7.

1010141517

RESULTS 6.1 Static stress Distribution

6.2 Spate Analysis 6.3 Fatigue Results

6.4 Crack Growth Results

6.

88101010

EXPERIMENTAL PROCEDURE5.1 Static Stress Distribution5.2 Fatigue Test

5.2.1 Initial static check 5.2.2 Fatigue test

5.

55568

EXPERIMENTAL EQUIPMENT4.1 Test Rig4.2 Test Equipment4.3 Strain Gauging4.4 Spate Analysis

4.

11233

SPECIFICATION OF TEST PARAMETERS3.1 Specimen Design3.2 Material3.3 Repair3.4 Grout

3.

1OBJECTIVES2.

1INTRODUCTION1.

ivSUMMARY

PAGE

SUMMARY

Static stress analysis and fatigue tests on repaired and fully internally grouted tubularwelded T joints have been carried out at the National Engineering Laboratory.

Two fatigue damaged 914 mm chord diameter tubular T joints originating from the UnitedKingdom Offshore Steels Research Project Phase II[1] have been repaired and internallygrouted. Extensive static stress analysis was carried out on each specimen to determine theinfluence of full internal grout on the stress and strain concentration factors generatedunder three load conditions, axial loading through the brace member, in-plane bending andout-of-plane bending.

Each specimen was then subjected to axial fatigue loading through the brace embers toevaluate the effect on fatigue performance and failure modes due to the introduction of thistype of stiffening. Throughout each test crack initiation sites and crack propagation dataalong with joint flexibility data were recorded.

The results, presented in the form of stress concentration factors and stress-endurancecurves, suggest that the technique may be applied to existing nodes to extend their fatiguelives by reducing the hot spot stresses around the chord/brace intersection due to the loadswhich the nodes are subject to in service. In addition to the possible extension in fatiguelife, crack propagation and local joint flexibility data indicate that failure modes have notbeen changed from those exhibited by conventional nodes.

1. INTRODUCTION

The current status of the North Sea oil industry in terms of technological advances in oilrecovery techniques, and market forces, has lead to a requirement for offshore platforms toremain operational for longer than original estimations. The economics of such anextension to the service life will be more favourable if action is taken to enhance the fatiguelife prior to deterioration of the integrity of the structural components, that may be expectedas the structure approaches the end of its original design life.

One such fatigue life enhancement technique which may be of considerable benefit is theintroduction of grout into critical tubular noes of the structure. The result would give aconsiderably stiffened node with much reduced stress concentration values, due to the groutpreventing significant bending of the chord walls such as occurs in conventional tubularnodes. Experience suggests that these reduced Stress Concentration Factors (SCFs), andhence the hot spot stress range to which the critical areas will be subjected, may enhancethe fatigue life of the node provided the mode of failure is not modified as a result.

In order to assess the overall viability and benefit of such a technique the influence of thegrout on the stress field around the tubular intersection and the subsequent fatiguecharacteristics must be investigated experimentally.

2. OBJECTIVES

The objective of the research programme was to determine the influence of a solid groutplug on the stress field around a tubular welded T-joint and establish its fatiguecharacteristics by:

w Determining the stress distribution around the test specimen prior to grouting.

w Determining the stress distribution around the test specimens after grouting.

w Determining the fatigue lives of tubular T-Joints containing fully grouted chordmembers.

w Investigating the failure modes resulting from the application of dynamic stressesto such nodes.

w Comparing fatigue crack growth characteristics of grouted T-joints withconventional tubular T-joints.

3. SPECIFICATION OF TEST PARAMETERS

3.1 SPECIMEN DESIGN

The joints investigated were of a single T configuration as shown in Figure 1 with chorddiameters of 914 mm and nominal wall thickness of 32 mm. The brace to chord diameterratio, beta, and the brace to chord wall thickness ratio, tau, were both 0.5. Full geometricparameters are presented in Table 1.

1

Figure 1 T-joint

Table 1Specimen geometric parameters

0.514.280.55.01645732914

taugamma betaalphaThickness

Diameter

Thickness

Diameter

BraceChord

nominal dimensions, mm

alpha = 2 x Chord length / Chord diameter (2L/D)beta = Brace diameter / Chord diameter (d/D)gamma = Chord diameter / 2 X Chord wall thickness (D/2T)tau = Brace wall thickness / Chord wall thickness (t/T)

3.2 MATERIAL

The chords from the United Kingdom Offshore Steels research Project phase II (UKOSRPII) specimens were formed from BS 4360 Grade 50D steel plate rolled to form tubularsections and then butt welded by the submerged metal arc process. The braces were formedfrom hot finished seamless steel tube to API 5XL Grade 52, 1982. A summary of themechanical properties and chemical analysis of the materials is presented in Table 2 andTable 3.

2

Table 2Mechanical properties of materials

184 @ -40oC42551400Brace111 @ -20oC28557387Chord

Charpy V Notch energy

(j)

Elongation

(%)

TensileStrength(MPa)

Yield Stress

(MPa)

Material

Table 3Chemical analysis of materials

0.010.050.0110.0131.40.410.19Brace--0.0050.0141.330.40.15ChordMoCrSPMnSiCMaterial

chemical analysis %

3.3 REPAIR

Two specimens, numbers T208 and T215 from UKOSRP II, were repaired by gouging outfatigue damaged areas, filling with weld material to the original plate thickness and finallyre-welding to API RP2A specifications. All repair welds were inspected using the dyepenetrant method. Figure 2 shows the extent of weld repair on each specimen.

3.4 GROUT

The grouting procedure was carried out with the specimens submerged in 1.5 meters ofwater. Two types of cement were used, Ordinary Portland cement and Oilwell B sulphateresistant cement, one type for each specimen. The cement was injected into spherical bagseals inserted through 50 mm holes in the chord walls, allowed to cure for twenty-fourhours after which the void created by the seals was filled from the lowest point in eachspecimen to displace the trapped water. Seal bag locations are presented in Figure 3. Oncompletion of grouting the specimens were left submerged for twelve days, removed fromthe tank and allowed to cure for a further sixteen days giving a total curing period oftwenty-eight days. The grouting trials were conducted in a water tank to highlight possibleproblems which may be encountered during installation offshore.

Each specimen maintained its original specimen number but with the suffix G to donate"grouted".

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Figure 3Seal bag location

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4. EXPERIMENTAL EQUIPMENT

4.1 TEST RIG

A universal test rig at the National Engineering Laboratory's Structural Center, Figure 4,was employed to load the specimens in each of three modes of loading, axial, in-plane andout-of-plane bending, for which the load range capacities are 5.0 MN, 500 kN and 500 kNfor the axial, in-plane and out-of plane modes respectively.

4.2 TEST EQUIPMENT

The test rig employed a closed loop servo-hydraulic control system capable of operating inone of three control modes (load, displacement or strain). All three feedback signals couldbe monitored simultaneously. The test specimens were protected against overload byupper and lower limit detectors on all three feedback modes. Signals were measured bypeak reading digital voltmeters and computer controlled data logging systems recordedstrain gauge readings. Surface crack length and crack depth data were obtained using eddycurrent crack detectors and ac potential drop systems respectively.

5

The eddy current crack detector consists of a handheld probe in which a coil is woundround a ferrite core. When a current is passed through the coil a magnetic field isintroduced at the end of the probe. If the probe is placed on the surface of a ferromagneticmaterial the characteristics of the magnetic field change. Should the probe pass over asurface breaking crack the characteristics of the magnetic field will change again. Thischange is indicated on an analogue meter on the front panel of the instrument.

The crack depth measurement system consists of two current input probes held in place bypermanent magnets approximately 400 mm apart on the surface of the specimen,equidistant from and perpendicular to the weld toe. The alternating current flows along thesurface of the specimen between these probes and the potential difference is measuredadjacent to and across the cracks using a handheld probe. The increase in potentialdifference between the first and second measurement is proportional to the crack depth.

4.3 STRAIN GAUGING

Strain gauges of the single, two (0o, 90o) and three (0o, 45o, 90o) element type were installedin pairs on both the chord and brace parent material at various intervals from the crown tothe saddle on one quadrant. The gauges were installed within the linear stress region asagreed by the European Coal and Steel Community technical working party on tubular jointtesting and can be found in Department of Energy's "Background to new fatigue designguidance for steel welded joints in offshore structures"[2]. A summary of these linear stressregions is presented in Figure.5. The fully gauged layout around the chord and brace isshown in Figure.6.

Figure 5Regions of stress linearity

6

Figure 6Fully gauged layout

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4.4 SPATE ANALYSIS

Stress Pattern Analysis by Thermal Emission (SPATE) equipment was employed to scanthe saddle position of T215 before and after grouting, under the influence of a small axialdynamic load.

The basic theory behind SPATE is the detection of minute changes in surface temperaturedue to the pseudo adiabatic response of a material under stress. Through an infrareddetector, scanning the surface of a given material, relative changes in temperature are fedto a computer system for correlation and finally presented as a pictorial colour image of thestress pattern over the scanned area. These pictures can be interrogated further to obtainstress values at any given point. As the stresses in the three principal planes contribute tothe overall temperature change stress values obtained are a summation of the principalstresses generated by the dynamic loading in each plane.

5. EXPERIMENTAL PROCEDURE

5.1 STATIC STRESS DISTRIBUTION

For comparison, the repaired specimens were loaded several times, to the same shake download employed during the initial static investigations in UKOSRP II, to release any stressesin the strain gauge installations. After this shake down procedure, all strains were recordedat increments up to the shake down load both in tension and compression. This procedurewas repeated for each load condition.

After grouting the procedure was repeated to loads calculated from the "Recommendationsfor static strength of simple joints" in the UEG "Design of tubular joints for offshorestructures"[3] using 0.4 of the characteristic yield stress of the chord member.

Maximum principal stresses were calculated and the principal stresses were linearlyextrapolated into the weld toe to obtain the hot spot stress. The stress concentration factors(SCFs) were obtained by dividing hot spot stress by the theoretical nominal brace stress,calculated using actual brace wall thicknesses. Linear extrapolation, Figure 7, of theprincipal stresses was adopted in order to obtain the geometric stress at the weld toe, thatis, the stress which has not been influenced by the local notch effect resulting from the lastweld bead geometry, undercut or defects.

8

Figure 7Linear Extrapolation of Stress and Strain

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5.2 FATIGUE TEST

5.2.1 Initial static check

Prior to fatigue testing each specimen was loaded incrementally to the load required toachieve the hot spot test stress. The nominal strain in the brace and the strain at thequadrature points on the brace and chord, adjacent to the weld toe, were monitored to checkfor axiallity of loading. When axial loading had been confirmed the specimen was cycledten times to the test load to relieve any residual stresses in the gauge installations. Afterthis shake down procedure all gauges were re-calibrated and the strains recorded at zeroand at the positive and negative peak loads.

5.2.2 Fatigue test

The fatigue tests were carried out under constant amplitude sinusoidal axial loadingthrough the brace member with a stress ratio R equal to -1, ie. the ratio of minimumapplied stress over the maximum applied stress. The maximum and minimum load andstroke peaks were monitored and recorded throughout the test. The control systemparameters were verified periodically throughout the duration of each test.

Each specimen was examined using visual and NDT techniques at regular intervalsdepending on loading frequency, stress level and stage of the test. This procedure wascarried out in order to detect crack growth as early as possible in the tests. As crackingdeveloped the fatigue tests were stopped at regular intervals for full in-depth examinationsto quantify crack length and depth. When a marked reduction in specimen stiffness wasmeasured, indicating rapid crack growth and nearing severance, a final in-depthexamination was carried out.

6. RESULTS

6.1 STATIC STRESS DISTRIBUTION

SCFs generated on the control specimen from UKOSRP II[4], specimen number T211, andfrom the repaired and grouted specimens are presented graphically in Figure 8 to Figure13, for comparison, Table 4 contains stress concentration factors obtained form parametricequations.

Table 4SCF values from parametric equations

4.55.62.52.46.08.0Wordsworth

5.54.42.21.775.4Kuang

5.15.22.22.26.76.4Gibstein

BraceChord BraceChordBraceChord

Out-of-Plane BendingIn-Plane BendingAxial

10

Figure 8Axial SCF values on chord

Figure 9Axial SCF values on brace

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Figure 10In-plane bending SCF values on chord

Figure 11In-plane bending SCF values on brace

12

Figure 12Out-of-plane bending SCF values on chord

Figure 13Out-of-plane bending SCF values on brace

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6.2 SPATE ANALYSIS

Full field scans were taken at the saddle positions on T215 after repair and again aftergrouting, covering an area of approximately 40 cm square. Line scans were also takenthrough the centre line of each field to obtain the stress topography over the welds. Figure14 to Figure 17 present the stress profiles over the scanned areas.

Figure 14Saddle area full field scan (pre-grout)

Figure 15Saddle centre line line scan (pre-grout)

14

Figure 16Saddle area full field scan (post-grout)

Figure 17Saddle centre line line scan (post-grout)

6.3 FATIGUE RESULTS

The fatigue data are presented in tabular form in Table 5 and graphically in Figure.18, inthe conventional stress-endurance format for tubular joints where stress is the hot spotstress range, calculated using the stress concentration factor derived from the initial staticloading test for each specimen, and endurance is the number of cycles achieved before thefollowing criteria were reached.

N1 First discernible surface cracking as observed by any method available. This stageis considered to have passed if the initial surface crack length is found to begreater than 20 mm.

N2 Intermediate surface cracking. Detected by visual examination without the use ofcrack enhancement fluids or optical aids. However if NDT techniques indicate asurface crack length of 30 mm this stage is considered to have been reached.

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N3 First through plate thickness cracking. This is not easily defined due to theexperimental conditions prevailing. Where access for internal examination isrestricted NDT crack depth assessment techniques must be used and theirlimitations accepted. Where available both the N3d detected internally and N3a

anticipated values should be quoted.

N4 End of test. Complete severance of the brace member or extensive through platethickness cracking leading to loss of specimen stiffness or load symmetry resultingin unacceptable actuator stroke or side load on the bearings.

The results are plotted against the T curve and compared with results from 33 mm thickchord wall specimens of the same geometry tested in UKOSRP II, Task 2.1[4] in theungrouted condition.

Table 5Fatigue results

0.3880.29------20058.311,277T215G1.150.850.245---12335.29782T208G

N4x106

N3ax106

N2x106

N1x106

Hot SpotStressMPa

NominalStressMPa

Load kNSpecimenNo

Figure 18Fatigue results

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6.4 CRACK GROWTH RESULTS

Typical crack growth data obtained from each test is presented graphically in Figure 19 andFigure 20. Figure 21 presents the orientation and pitch of the measurement positionsreferred to in the Figure 19 and Figure 20.

Figure 19Crack development on T208G

Figure 20Crack development on T215G

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Figure 21Crack measurement positions

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7. DISCUSSION

7.1 REPAIRED CONDITION

The stresses generated in the specimens after repair of the welds are of similarmagnitude as those obtained from joints examined prior to testing in UKOSRP II.UKOSRP II axial stress concentration factors[4[ ranged between 5.3 and 6.7 andtherefore values obtained from the joints in the repaired condition can be accepted asvalid data.

7.2 GROUTING

The grouting trials highlighted problems with the seal bag location and the requirement forreinforcing collars on the neck of the bags to prevent rupture. The location problem wasdue to the buoyancy of the bag material, as the grouting procedures were conducted fromthe outside during bag insertion and inflation no intervention relating to bag positioningtook place from the access points at the ends of the specimens. Therefore as the bags wereinserted they had a tendency to float instead of hanging vertically down, this resulted in thebags moving off from their intended sealing position during inflation.

A simple solution to the location problem for future grouting would be to use the groutlance as both lance and locating device by increasing its length to the full diameter of themember being grouted thereby pinning the bag in two locations.

Reinforcing collars were required to prevent the bags being ripped by burrs on the inneredge of the access holes produced during the drilling.

7.3 STATIC STRESS DISTRIBUTION

7.3.1 Axial static stress distribution

The dominant stresses at the brace/chord intersection in a tubular joint under axial loadingare generated by bending, due to global deformation of the chord. Any modification tothese stresses by reduction in chord deformation will have a major influence on the SCFvalues. Introducing grout into the chord member effectively restrains the chord fromdeforming thereby reducing a major portion of the bending stresses generated by theapplied load.

Under axial conditions reductions in maximum SCF values of 40% can be achieved at thehot spot area of the chord as shown in Figure 8. With the chord member under suchrestraint, stresses are redistributed more evenly as indicated by increases in concentrationfactors towards the crown positions on the brace. The combination of overall reduction inSCF's and the redistribution of load suggest greatly enhanced static strength properties.

7.3.2 In-plane bending static stress distribution

Under in-plane bending conditions the stresses at the brace/chord intersection are generatedby very local bending of the chord member. Therefore introduction of grout should havelittle effect on the SCF values generated, as the major stiffening occurs in the hoopdirection. Figure 10 confirms that the SCF values have not been greatly affected.

7.3.3 Out-of-plane bending static stress distribution

The out-of-plane bending condition approaches the axial condition where globaldeformation is the major stress generating parameter at the intersection of brace and chord.As in the axial case, any reduction in global deformation results in reduction of SCFvalues. The SCF values presented in Figure 12 show that under out-of-plane bending

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conditions a reduction of up to 30% on maximum SCF values is possible. This reduction inSCF values, due to the introduction of internal grout, indicates that similar gains arepossible on the static strength under out-of-plane bending conditions as can be achievedunder axial conditions.

7.4 FATIGUE RESULTS

When compared with the fatigue results from nodes of the same geometry in the ungroutedcondition, Figure 18, the grouted data are a lower bound for this geometry but are belowthat expected, with the N3 points lying very close to the design T curve. One possiblereason for the early crack initiations, missed on T215G, may be due to the extent of therepair welding, Figure 2, and that balanced welding could not be achieved. Although it isaccepted that residual stresses in welded joints can be up to yield, the extent of the repairsmay have induced high residual stresses globally in the joints. Repair may not haverestored the joints to their virgin condition.

The results indicate that the fatigue performance is not appreciably altered therefore thetechnique if applied to an as yet uncracked node in a structure could extend its fatigue life.For example assuming a node is subjected to a hot spot stress of 200 MPa with a SCF of6.5[5], such a node would have an expected fatigue life in excess of 2x105 cycles. If thengrouted the hot spot stress will be reduced by the ratio of the ungrouted SCF to the groutedSCF (6.5 to 3.4) of 1.9, ie. a hot sot stress of 105 MPa. Therefore the node in the groutedcondition should now have an expected fatigue life in excess of 1.2x106 cycles.

7.5 FAILURE MODE

7.5.1 Crack initiation

The typical failure mode of conventional unstiffened nodes of the geometry investigated inthis programme, is by multiple crack initiation at the chord weld toe, in the location of thesaddle hot spot stress areas. These small cracks eventually coalesce and propagate throughthe thickness of the chord plate. In many occasions cracking is balanced on both saddlepositions.

The failure mode of the grouted joints did not visibly differ from that of conventional joints.However, in addition to the primary cracks initiating in the chord weld toe, secondarycracks did initiate in the brace weld toe. On specimen No T215G, one of the brace cracksreached through thickness before arresting. However, this alteration was not totallyunexpected as the welds on the brace side had not been repaired, and a proportion of thefatigue life had been taken up in the previous programme of work.

7.5.2 Crack growth

The crack growth rate on conventional joints of this geometry is near linear through theplate thickness, once crack coalescence has occurred.

When crack growth through the plate thickness of the grouted joints is examined thisnormal pattern of linear growth rate is revealed. This can be seen when crack growth fromconventional and grouted joints is compared, with the data normalised to both platethickness and the through thickness N3 cycles for each specimen, Figure 22. Unfortunatelyinitiation and early crack growth data on specimen T215G was missed as initiationoccurred much earlier than expected.

20

Figure 22Crack growth

7.5.3 Local joint flexibility

Local joint flexibility was evaluated by examining the cyclic displacement during thefatigue test by means of a displacement transducer housed in the servo-hydraulic actuator.The displacement history was recorded as maximum and minimum peak displacementthroughout the fatigue test, Figure 23. The "max" and "min" traces indicate the change inflexibility as cracks propagate around the chord/brace weld toe, the "dynamic" traces arethe actual dynamic changes in stiffness of each joint. Comparison of joint flexibilitybetween ungrouted and grouted nodes, Figure 23 and Figure 24 respectively, indicates thatthe joint flexibility has not been modified.

Figure 23Displacement history (grouted joints)

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Figure 24Displacement history (ungrouted joints)

7.6 GENERAL

If it is accepted that the early initiation of cracking in each node is a result of theinfluence of the heavy weld repairs then with the above findings and points taken intoconsideration, the introduction of grout into tubular nodes of the geometryinvestigated in this programme does not appear to alter their fatigue performance interms of failure mode and joint flexibility.

However it must be reiterated that the findings of this programme of work are based on theresults of two repaired nodes where the level of interaction between the weld repairs andfatigue performance cannot be quantified.

The benefits of this technique are that the existing S-N approach to the re-analysis ofstructures incorporating such nodes is not affected, load shedding is typical of thatdisplayed by conventional nodes and present inspection techniques are still applicable.

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8. CONCLUSIONS

The following conclusions have been drawn from the pilot investigation carried outinto the influence of introducing grout into tubular joints on the stress concentrationfactors and fatigue performance of two grout injected tubular nodes subjected to axialloading through the brace.

a) Introduction of grout considerably reduces the stresses generated under axial andout-of-plane bending conditions thereby reducing the stress and strain concentrationfactors around the brace/chord intersection weld.

b) Introduction of grout would appear to have the potential to increase the static capabilityof joints but was not investigated in the programme.

c) The fatigue lives of two grout stiffened nodes were less than predicted by the mean Tcurve but are within the scatter band of that displayed by conventional nodes.

d) Failure modes of grout stiffened nodes of the geometry investigated in this programmeof work have not been altered from that of conventional nodes.

e) Fatigue lives of existing undamaged nodes may be extended by the introduction of groutprovided the dominant load condition is axial.

f) The existing S-N curve approach to structural analysis is still applicable for groutstiffened nodes.

g) Load shedding of a grouted node is similar to that of conventional nodes.

Within the limitations of this study and considering the reduced SCF values achievedduring the static loading programme, the degree of fatigue life enhancement expected wasnot demonstrated. The results are encouraging from a qualitative viewpoint regarding thebeneficial influence of grout. However, a larger investigation would be required to confirmand quantify the benefits to an acceptable level.

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9. ACKNOWLEDGEMENTS

This research project was funded jointly by Elf Enterprise Caledonia, the Department ofEnergy and the National Engineering Laboratory. The author would like to thank the staffof Elf Enterprise for their technical assistance during the research programme.

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10. REFERENCES

1 LONG, D.Description of the Programme of Work for Phase II of UKOSRP.Presentation to Working Group 2 of ECSC Marine Technology Executive,National Engineering Laboratory, East Kilbride, Scotland, 1981.

2 DEPARTMENT OF ENERGYBackground to new fatigue design guidance for steel welded joints in offshorestructures.HMSO, London, 1986.

3 UEG OFFSHORE RESEARCHDesign of tubular joints for offshore structures.UEG Publication UR33, 1985

4 DEPARTMENT OF ENERGYUnited Kingdom Offshore Steels Research Project - Phase II Project Task ReportsHMSO, London 1989, Publication No OTH 89 310

5 BROWN, G.M., HOLMES, R., KERR, J.Fatigue life enhancement of welded tubular joints by injection of grout.In Proc. of Int. Offshore Conf. on Behaviour of Offshore Structures (BOSS '88)Tapir Publishers, Trondheim, Norway, Volume 3 Pages 1081-1095, 1988.

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