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Chapter 14

Remaining Strength of Corroded Pipes

14.1 Introduction

Marine pipelines are usually designed so that they can withstand some corrosion damages,
because of economic considerations. However, these damages must be controlled in order to
prevent failures of the pipelines. Such failure can and has resulted in huge economic and
environmental loss. Any economic savings made possible by allowing corrosion damages
should be reconciled with potential failure of the pipeline. Therefore, proper selection of
design criteria of corroded pipelines, is an important and complex task, and current design
code recommendations only offer very general guidelines.

A number of studies on corroded pipelines have been conducted to develop better fitness for
purpose criteria (eg Bai et al. (1994, 1997)). It is not clear how much confidence an engineer
can place in them in a design or requalification process, because of the variety of such studies
and the many assumptions involved in their developments.

The purpose of this chapter is to develop reliability-based design and requalification criteria
of corroded pipelines. Following a brief summary of available criteria, the technical
developments and problems of corroded pipelines are illustrated by evaluating the reliability
of ASME B31G dcsign critcria. This evaluation focuses on three interrelated issues:

1) corrosion mechanism;
2) the accuracy of the maximum allowable corrosion length, the safe maximum pressure
level;
3) practical problems excluded in the B31G criteria.

Based on this evaluation, new criteria are proposed to help overcome these problems.
Reliability-based safety factor calibration is conducted to assure that the safety level in the
new criteria is the same as that in the existing criteria. The new criteria are then applied in the
requalification of aging corroded pipelines.