Summary and Conclusions

A series of fourteen industrial failure case studies have been presented, including pressure vessels, ships, bridges, storage tanks and an offshore rig. For each case study, the failure events have been described, together with an account of the main

contributing factors and failure mechanisms, and the consequence of the failure. A range of issues have been identified which have significantly contributed to the structural failures described in this report:

i. Material properties The strength of a component is dictated by the geometry of the structure, stresses within the structure, and the properties of the materials which comprise the structure. Important material properties include the yield and tensile strength of the material, together with fracture toughness. Fracture toughness is of particular importance for welded fabrications, were fracture toughness is dependent on microstructure, joint configuration, loading rate and temperature. Low fracture toughness was a factor which contributed to most of the failures which have been discussed in this report. Careful consideration should be given to any factors which might reduce the fracture toughness of the materials in critical structures, such as reductions in temperature or strain ageing embrittlement (see Fawley and Ashland storage tank failures, for instance).

ii. Welds

All of the failures which have been described in this report were associated with defective welds. For fracture to occur, there must be a detrimental combination of stresses, flaws and fracture toughness. Weldments are associated with a higher risk of fracture due to the combination of complex metallurgy, welding residual stresses and stress concentrations and higher constraint associated with the joint configuration, together with the inherent flaws which are present in all welds. To minimise the risk of fracture in critical structural components, special consideration must be given to welding during design, fabrication and operation. Additional consideration should be given to repair welds, were control of the welding process and post-weld heat treatment may be difficult. Attachment welds are also important, as running fractures can progress into the main structure (for example, the Alexander L Kieland platform and M V Kurdistan tanker).

Many of the failures occurred at relatively low temperatures (-20 to 13°C), including the Hasselt and Kings bridges, Fawley and Ashland storage tanks, Schenectady and M V Kurdistan tankers, Typpi Oy ammonia water coolers, together with the Sizewell, John Thompson, Cockenzie and Robert Jenkins pressure vessels. Ferritic steels undergo a transition from ductile behaviour at higher temperatures to brittle behaviour at lower temperature. The decrease in fracture toughness associate with this transition can be significant, over a fairly small temperature range. The absolute value of this fracture toughness transition temperature varies, depending on the steel composition, joint geometry, environment and loading rate. For these reasons, Charpy impact energy requirements in fabrication codes are typically specified at specific temperatures relative to the service temperature, to ensure that the material toughness is sufficient.

iv. Proof testing

Many of the failures occurred during hydrotest, including the Fawley storage tank and the Sizewell, John Thompson, Cockenzie and Robert Jenkins

pressure vessels. It could be maintained that these hydrotests were successful, in that they prevented potential catastrophic failure in service. However, hydrotest failures are expensive, and appropriate lessons must be learnt to ensure procedures are modified to reduce this risk (i.e. restrictions on minimum temperature for testing, maximum pressurisation rates, improved inspection, etc., to facilitate repair prior to hydrotest). Nevertheless, as the Typpi Oy failure demonstrated, the benefit of a proof test may be removed by stress corrosion and other forms of crack extension in service.

v. Environment/service conditions

The environmental/service conditions to which critical structures are exposed is important. This includes any factors which could lead to embrittlement of the component during its anticipated lifetime (for example, the Union Oil amine absorber tower and Typpi Oy ammonia plant water coolers, were hydrogen embrittlement was a major contributing factor, and the Sizewell pressure vessel and World Concord ship, where dynamic loading was considered to be a factor.

vi. Maintenance/inspection/quality assurance

The effective management of fracture control in critical structures implies an on-going commitment for effective maintenance, with regard to issues relating to management of fracture risk (i.e. identification of critical components and joints, regular inspection, etc.). Poor maintenance was identified as a major contributing factor to the amine absorber pressure vessel failure. Particular attention should be given to these issues if a change of service conditions of life extension is planned.

Table 1 indicates which of these six factors played a role in each of the failure cases described in this report.

The lessons which are learnt from structural failures influence the industrial and national codes of practice for design, fabrication and operation of critical plant. For instance, the amine absorber tower failure resulted in widespread changes to industrial codes for plant maintenance, while the Cockenzie boiler drum failure resulted in the requirement for inspection after post-weld heat treatment, and the specification of minimum temperatures for hydrotesting. The failure of the Fawley storage tank, and the Schenectady and World Concord tankers resulted in extensive research into fracture mechanics, leading to the requirements for adequate notch toughness in critical fabrications.

Structural failures have also resulted in the development of 'fitness-for-purpose' assessment methods, such as BSI PD 6493:1991. These methods are based on fracture mechanics principles, and allow the significance of weld flaws to be assessed in terms of structural integrity assessment. PD 6493-type methods are used extensively, on an international basis, for many applications, including pressure vessels, pipelines, storage tanks, ships, bridges, buildings and other structural components.

16.1 References

Author Title

BSI PD 6493:1991: 'Guidance on methods for assessing the acceptability of flaws in fusion welded structures', British Standards Institution, London, 1991.

Challenger N V, Phaal R and Garwood S J

"Appraisal of PD 6493:1991 fracture assessment procedures. Part III: assessment of actual failures", TWI Research Report 512/1995.

Table 1: Summary of factors contributing to failures

Failure Case Factors Contributing to Failure

Hasselt Bridge i, ii, iii

Schenectady T2 Tanker i, ii, iii

Fawley Crude Oil Storage Tank(s) i, ii, iii, iv

World Concord Tanker i, ii, iii, v

Kings Bridge i, ii, iii, vi

John Thompson Pressure Vessel i, ii, iii, iv Cockenzie Power Station Boiler Drum ii, iv, vi Typpi Oy Ammonia Plant Water Coolers i, ii, iii, v, vi Robert Jenkins Pressure Vessel i, ii, iii, iv, v

MV Kurdistan Tanker i, ii, iii, v, vi

Alexander L Kielland Accommodation Platform ii, v, vi Union Oil Amine Absorber Tower i, ii, v, vi

Ashland Storage Tank i, ii, iii

17. Acknowledgements

The work described in this report was carried out within the TWI Core Research Programme, funded by the Industrial Members of TWI.

The photograph in Fig. 22 is reproduced courtesy of Battelle Memorial Institute and Ashland Petroleum Company.

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