Keywords: creep, damage, failure life, finite element analysis, pressurised pipe, geometric non-linearity.

Plain pipe structures are key elements in the high temperature piping systems of power and chemical plants. One of the main concerns in the design of these pipes is the ability to accurately predict the service life of the pipes, under creep conditions. The normal design rule for both thick and thin walled pressurised pipes is based on the mean diameter hoop stress. Very few pipe failures have occurred in practice, clearly showing the conservatism of the design rules [1]. In this context, there is a need to reappraise the potential creep life of simple pressurised pipe structures based on less conservative methods to achieve the maximum economic benefit whilst ensuring safe operation. Simplified techniques have been developed, such as those based on steady-state reference rupture stresses [2,3], aiming to produce alternative criteria for life assessment or design.

Finite element analyses using Norton's law and damage mechanics constitutive equations have been widely used to study the deformation, stress redistribution and failure behaviour of internally pressurised plain pipes and related components under creep conditions (e.g. [3,4,5]). These analyses are normally based on the assumption that the pipe dimensions remain unchanged during creep (small deformation cases) and the effects of geometry change on the creep behaviour of the internally pressurised pipe components have not yet been investigated; these effects may be significant on the stress distributions and failure life predictions. For example, under constant applied loading (internal pressure and end pressure, say), ignoring the geometry change, stationary state creep solutions can be obtained when Norton's law is employed in the FE analysis, and constant stress distributions with time can be obtained. However, if the change in geometry is considered, the total loading due to the constant pressures (controlled by the pipe system) will change, as a result of the change in pipe diameters and subsequently the cross-sectional area. In this case, the linear relationships between the strain or deformation and time will not exist. It is clearly of interest to understand the influence of geometry changes on the stress distribution and failure behaviour of pipes or related components, such as pipes with circumferential welds or pipe bends.

In this paper, the influence of the geometry change during creep of simple internally pressurised plain pipes is investigated using finite element (FE) creep damage analyses, by including a geometric non-linearity function in the analyses. The validity of the FE analyses is demonstrated by comparing the results obtained using an iterative approach with those obtained from FE solutions in the uniaxial case. Results of the failure life obtained using the material properties for a CrMoV pipe steel at 640C, clearly show a significant life reduction when the geometry change is included. In the range of the pipe diameter ratio and end load ratio investigated, it is found that the failure lives can be reduced to between 60 and 80 % of those obtained from constant geometry cases, indicating that the influence of geometry change may need to be appraised for detailed numerical analysis of pressurised pipes.

1

Aburrow, A.F., Cane, B.J., Carmichael, G.D.T., Dewar, A., Hart, R.V., Heather, C.W., Plastow, B., Williams, J.A. and Womersley, S., "Creep of CrMoV piping systems", Conf. on Pipework Design and Operation, I.Mech.E., London, 1985.

2

Hyde, T.H., Sun, W. and Williams, J.A., "Prediction of creep failure life of internally pressurised thick walled CrMoV pipes", Int. J. Pres. Ves. & Piping, 76, 925-933, 2000. doi:10.1016/S0308-0161(99)00078-2

3

Hyde, T.H., Sun, W. and Williams, J.A., "Steady-state creep reference rupture stresses for internally pressurised pipes with additional end load", Proc. of Creep 7, JSME, Tsukuba, Japan, 1-6, 2001.

4

Hall, F.R. and Hayhurst, D.R., "Continuum damage mechanics modelling of high temperature deformation and failure in a pipe weldment", Proc. R. Soc. London, A443, 383-403, 1991. doi:10.1098/rspa.1991.0054

5

Hyde, T.H., Yaghi, A., Becker, A.A. and Earl, P.G., "Finite element creep continuum damage analysis of pressurised pipe bends with ovality", Proc. of Creep 7, JSME, Tsukuba, Japan, 507-511, 2001.

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