HSEHealth & Safety
Executive
Assessment of strengthening clampfrom the Viking offshore platform:
Phase II
Prepared by theHealth and Safety Laboratory
for the Health and Safety Executive
OFFSHORE TECHNOLOGY REPORT
2000/058
HSEHealth & Safety
Executive
Assessment of strengthening clampfrom the Viking offshore platform:
Phase II
Health and Safety LaboratoryEngineering Control Group
Broad LaneSheffield
S3 7HQUnited Kingdom
HSE BOOKS
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© Crown copyright 2002Applications for reproduction should be made in writing to:Copyright Unit, Her Majesty’s Stationery Office,St Clements House, 2-16 Colegate, Norwich NR3 1BQ
First published 2002
ISBN 0 7176 2279 7
All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmittedin any form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.
This report is made available by the Health and SafetyExecutive as part of a series of reports of work which hasbeen supported by funds provided by the Executive.Neither the Executive, nor the contractors concernedassume any liability for the reports nor do theynecessarily reflect the views or policy of the Executive.
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SUMMARY OBJECTIVES The Viking platform was decommissioned recently and one of the strengthening clamps was obtained by HSE for assessment. The objectives of the second phase of this assessment were:
• to dismantle one half of the clamp
• to carry out compression tests on samples of grout
• to assess the residual fatigue life of the studbolts
• to measure the fracture toughness and impact toughness of parent plate, weld metal and heat affected zone (tubulars)
• to measure the fatigue crack growth rate and threshold stress intensity in the
parent plate (tubulars) MAIN FINDINGS One half of the clamp was dismantled successfully; there was no indication of any significant deterioration. The residual fatigue life of the studbolts was consistent with what would be expected from new bolts. The fracture toughness and impact toughness of the tubular material were within the range normally expected for 50D steel. The fatigue crack growth rate and the fatigue crack threshold stress intensity were consistent with values reported previously for this material. The grout annulus was in relatively good condition but the presence of a layered structure within the grout material may have had an effect on the strength of the clamp. MAIN RECOMMENDATIONS Not applicable
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CONTENTS
Page
1. INTRODUCTION 1 2. DISMANTLING OF CLAMP 1 3. COMPRESSION TESTS ON GROUT 2 4. RESIDUAL FATIGUE LIFE OF STUDBOLT 2 5. FRACTURE TOUGHNESS OF TUBULAR MATERIAL 3 6. FATIGUE PROPERTIES OF TUBULAR MATERIAL 3 7. CONCLUSIONS 4 8. REFERENCES 4
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1. INTRODUCTION Strengthening clamps are used in the North Sea for the reinforcement of the joints of offshore platforms. Such clamps do not have a specified design life, some have been in service for many years and are expected to continue for many more. HSE needs to take a view of the structural integrity of these items in order to ensure their continued safe use. The Viking platform was decommissioned recently and one of the strengthening clamps was obtained by HSE for assessment. This clamp is the first of its kind to become available for examination and it is opportune, therefore, to extract as much information as possible concerning its condition after twelve years in service. OSD asked the Health and Safety Laboratory to carry out this examination and, for this purpose, the clamp, weighing approximately 12 tonnes was transported to HSL1s Buxton Site. The second (final) phase of the project consisted of the following tasks:
• dismantle one half of the clamp • carry out compression tests on samples of grout
• assess the residual fatigue life of the studbolts • measure the fracture toughness and impact toughness of parent plate, weld
metal and heat affected zone (tubulars) • measure the fatigue crack growth rate and threshold stress intensity in the
parent plate (tubulars) This report should be read in conjunction with the report on phase 1 of the project (1) and gives details of the findings of each task. 2. DISMANTLING OF CLAMP The upper end of the clamp was used in Phase 1 for the corrosion assessment and studbolt load measurements and hence this end was selected for dismantling. This is shown in the foreground in Figure 1 and the side uppermost, to which the lifting tackle is attached, would have been the left-hand side when the clamp was in use. The dismantling procedure was as follows: a) the left-hand strongback (the upper one in Figure 1) was severed by oxy-acetylene cutting b) the twelve studbolts at the upper end were removed; some were unscrewed so that they could be used in the fatigue tests (see section 4 of this report), others were cut c) the grouted connection was broken using a hydraulic jack placed, at various positions in sequence, between the two halves of the upper end of the clamp.
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d) the left-hand half of the upper end of the clamp was lifted off using the lifting tackle attached to the overhead crane The upper end of the clamp is shown, with the left-hand half removed, in Figure 2. The exposed su rface of the tubular appeared to be coated with a grey/black layer which had effectively prevented any corrosion. The inside of the strongback, although unprotected, was also free from significant corrosion. 3. COMPRESSION TESTS ON GROUT Visual examination and mechanical testing of the grouted annulus between the clamp body and the tubular was carried out by Advanced Structures Technology Ltd and a separate report was produced (10). This showed that the annulus was complete, relatively uniform in thickness and there were no large voids present. The bond between the clamp body and the grout material was very good despite the presence of an unidentified crystalline material at the interface. On the other hand, there was little or no adhesion between the grout and the tubular and a black powdery substance was present at the interface. This was not investigated further being beyond the scope of the project. The grout had a layered or “slated” appearance, this effect has been observed previously in this material and is thought to be due to “early age cycling". Tests showed that the compressive strength of the grout material met the design requirement but that the shear properties were anisotropic, being influenced by the layered structure such that the shear strength along a layer was approximately one third of that across a layer. These observations and results could have implications for the strength of the clamp which is dependent, inter alia, on the bond between the grout and the tubular and the mechanical properties of the grout material itself. 4. RESIDUAL FATIGUE LIFE OF STUDBOLTS The aim of this part of the project was to establish whether the fatigue life of the studbolts had been affected by the period in service. As stated in the report on phase 1(1), the studbolts had been made from a low alloy steel, probably grade L7 in AISI 4140, rather than monel as specified on the drawings. The tensile strength of the studbolt material was found to be 930 MPa, with 0.2% proof stress of 825 MPa, and thus the tensile properties are equivalent to those of a grade 8.8 bolt. A comprehensive assessment of marine bolting materials has been carried previously (2) on behalf of OSD and it was thought initially that the data on grade L7 bolts from this project could be used as a basis for comparison. However, this was not possible as this project used studbolts of a different size and none of the tests on the grade L7 material were carried out in air. As an alternative, the approach outlined in BS 7608:1993 “Code of practice for fatigue design and assessment of structures" was adopted because it deals specifically with the fatigue assessment of axially loaded bolts (section 3.8). Two tests samples, each approximately 0.5m in length were cut from the studbolts removed from the upper end of the clamp, no additional machining was carried out. Each sample end was screwed into a cylindrical threaded adaptor and held in position with one of the studbolt nuts. Fatigue tests were conducted individually in a Dartec 2000kN servohydraulic test machine operating at a frequency of approximately 2 Hz. The threaded adaptor at each end of the sample was clamped in the machine using hydraulic grips.
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For the purposes of the fatigue test, the mean stress was chosen to be equivalent to the stress induced by the specified pre-tension in each studbolt (88,000 psi) and the stress range was taken as 20% of the pre-stress value in accordance with BS 7608. It should be noted that the measured pre-tension (1) was close to the specified value. Each studbolt sample was thus subjected to a force range of 400±40 kN and testing was allowed to continue until failure occurred. In both tests the studbolt sample fractured transversely at the junction between one of the nuts and a cylindrical adaptor; fatigue lives of 754,500 cycles and 798,240 cycles were recorded for tests 1 and 2 respectively. An S/N curve for the axial tensile loading of bolts is given in Figure 10 of BS 7608 in which the stress range is rationalised with respect to the tensile strength of the bolt material. For the studbolts, the applied stress range/tensile strength is approximately 0.1 and the mean equivalent fatigue life, read off the graph, is 900,000 cycles. The mean minus two standard deviations fatigue life, above which 97% of results would be expected to lie and on which design is often based, is 400,000 cycles. Thus the results for the studbolts are considered to be acceptable and are, in fact, very close to the mean value in BS 7608 when expressed logarithmically. Although it cannot be stated with certainty, it is probable that the residual fatigue life of the studbolts has not been reduced by the period in service. 5. FRACTURE TOUGHNESS OF TUBULAR MATERIAL Fracture toughness specimens (three point bend) were manufactured from parent material, heat affected zone and weld metal of the tubulars and these were tested in accordance with BS 7448 (3) at a temperature of approximately 00C, the minimum operating temperature. The results, expressed in terms of crack tip opening displacement (CTOD), are shown in Table 1. Charpy data at a similar temperature are shown in Table 2. Both the fracture toughness and impact toughness of the tubular material is within the range normally expected for 50D steel. 6. FATIGUE PROPERTIES OF TUBULAR MATERIAL The threshold value of the stress intensity factor range (∆Kth) and the fatigue crack growth rate (da/dN) in air were determined using three point bend fracture toughness specimens tested in accordance with the methods given in references (4) and (5) respectively. The results indicated that ∆Kth was approximately 3.5 MPam 1/2 at an R ratio of 0.35. The fatigue crack growth rate data are given in Figure 3, the Paris Law constants m and C were found to be 3.78 and 5.89 x 10 –13 respectively. Published fatigue threshold data on this material, grade 50D in BS 4360, (6) show a wide range of values and the present result falls into the lower end of this range. A comprehensive test programme was carried out by the British Steel Corporation in the late 1980s (7), the HSL result is comparable, although slightly lower than the BSC data. The fatigue crack growth rate data are very' similar to those determined by HSL in previous investigations (8, 9). As with the studbolts, it is reasonable to conclude that the fatigue properties of the tubulars have not decreased during the life of the clamp.
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7. CONCLUSIONS 7.1 One half of the clamp was dismantled successfully; there was no indication of any significant deterioration. 7.2 The residual fatigue life of the studbolts was consistent with what would be expected from new bolts. 7.3 The fracture toughness and impact toughness of the tubular material were within the range normally expected for 50D steel. 7.4 The fatigue crack growth rate and the fatigue crack threshold stress intensity were consistent with values reported previously for this material. 7.5 The grout annulus was in relatively good condition but the presence of a layered structure within the grout material may have had an effect on the strength of the clamp. 8. REFERENCES 1) HSL Report MM/99/08 "Assessment of strengthening clamp from the Viking offshore platform: Phase 1", issued 18 February 2000. 2) AERE G - 4998, ref AIIR/715, "Marine Bolting Materials Project - Final Report", Harwell. 3) BS 7448 "Fracture mechanics toughness tests".
Part 1:1991 '1Method for determination of K IC, critical CTOD and critical J values for metallic materials"
Part 2:1997 "Method for determination of K I C, critical CTOD and critical J values of welds in metallic materials"
4) BS DD 186:1991 "Method for determination of threshold stress intensity factor and fatigue crack growth rates in metallic materials". 5) BS 6835:1988 "Method for the rate of determination of fatigue crack growth in metallic materials"
6) Taylor D, "A compendium of fatigue thresholds and growth rates", EMAS, Cradley Heath, 1985. 7) CEC Report EUR 12204 EN, "The fatigue and stress corrosion properties of conventional and higher strength steels for use on offshore environments", 1989. 8) Shuter D M & Geary W, "The effects of variable amplitude loading of the fatigue performance of engineering components. Part 1: The influence of specimen thickness on fatigue crack growth retardation following an overload", HSL Report MM/94/12, May 1994.
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9) Shuter D M, "An investigation of fatigue crack growth from single sur face defects", HSL Report MM/94/4, January 1994. 10) AST Ltd "Viking Platform. Investigation of Node 29 Repair Clamp", March 2000.
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Table 1: Results of Offshore Clamp Fracture Toughness Tests
Material Test Temperature
C
CTOD
mm
Parent 1 -3.0 0.315 Parent 2 1.4 0.350 Parent 3 -0.8 0.356
HAZ 1 1.2 0.342 HAZ 2 -0.1 0.479 HAZ 3 0.0 0.515 Weld 1 -2.4 0.382 Weld 2 -0.2 0.297 Weld 3 0.7 0.297
Table 2: Results of Offshore Clamp Charpy Tests
Material Test Temperature
C
CHARPY ENERGY
J
Parent 1 -0.3 116.5 Parent 2 -0.4 103.5 Parent 3 -0.1 99
HAZ 1 -0.1 110.5 HAZ 2 -0.1 120 HAZ 3 -0.3 113.5 Weld 1 -0.1 86 Weld 2 -0.1 77 Weld 3 -0.1 74
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FIGURE 1: Upper end of clamp before dismantling
FIGURE 2: Upper end of clamp after dismantling
Printed and published by the Health and Safety ExecutiveC0.35 02/02
Printed and published by the Health and Safety ExecutiveC30 1/98
Printed and published by the Health and Safety ExecutiveC30 1/98
OTO 2000/058
£7.50 9 780717 622795
ISBN 0-7176-2279-7
