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Chapter 15

Residual Strength of Dented Pipes with Cracks

15.1 Introduction

With the increased use of pressure vessels, pipelines and piping systems, more and more pipes
are being put into use. Mechanical damages to pipes occur frequently. These damages are
mainly caused by operation activities, fabrication errors etc. Leakage of gas and oil from
pipes due to structural failure may lead to reduced operating pressure or stopped production,
human and environmental hazards and the heavy economic loss consequently. Since the
existence of dents especially at weld seams is one of the causes of leakage, it is important to
arrive at a basis for assessing the structural integrity of dented pipe with cracks.

In this Chapter the existing criteria based on the knowledge of linear elastic fracture
mechanics are reviewed. The existing criteria modified using the latest advances in the
fracture mechanics. In addition, safety factors are calibrated considering safety philosophy,
consequence of failure and uncertainties through reliability analysis.

Due to the uncertainties involved in loading, strength and modeling of design and assessment,
it is necessary to carry out the pipe integrity assessment based on reliability theory accounting
for those uncertainties rationally. While a conservative approach to these uncertainties leads
to excessive costly structures, an unconservative approach will make the structure unsafe. A
probabilistic approach - i.e. reliability analysis, needs to be introduced towards a design with
balanced consideration of safety and economy @ai and Song, 1997).

The first part of the chapter deals with the burst strength criteria of dented pipes with
longitudinal and circumferential cracks. Subsequently, fracture assessment of damaged pipes
is studied. Uncertainties involved in loading, strength and modeling are assessed. In the third
part of the chapter, fracture reliability model of dented pipes with cracks is developed, a new
design equation for dented pipes with cracks in operation with respect to fracture criterion is
derived, reliability-based calibration of safety factor and uncertainty modeling is performed
considering the target safety level. To verify the presented model, a design example is made
based on an existing pipe and numerical analysis is carried out. Predicted burst strength based
on the formulae presented in the chapter agrees with test results (0lberg et al. (1982)).
Reliability-based fracture assessment and detailed parameter studies are performed for a
damaged pipe. Conclusions are made and suggestions of further work are also outlined.