Risk-Based Maintenance Planning for Offshore Pipelines
Authors: Nessim Maher A. , Stephens Mark J. , Zimmerman Tom J.E. 
Published: October 2008

Abstract
The paper describes a quantitative model for risk-based integrity maintenance planning for offshore pipelines. The model calculates the risk level as the product of failure consequences and failure probabilities. Failure consequences considered include financial aspects relating to repair, interruption and restoration; safety aspects related to platform and vessel occupants; and environmental aspects associated with spills. Distinction is made between small leaks, large leaks and ruptures, and the major failure causes (namely, corrosion and mechanical damage) are considered. The model quantifies the benefits associated with different maintenance alternatives by calculating their impact on the probability of failure and risk level. A formal decision analysis approach is then used to evaluate the costs and benefits associated with each alternative, and identify the best overall maintenance strategy.
This model differs from previous work in risk-based planning in that it:utilizes structural pipe failure models and reliability analysis techniques to calculate failure probabilities;quantifies the impact of maintenance on risk; andutilizes a methodology based on decision theory to identify optimal choices.
It is applicable to various aspects of maintenance planning for offshore pipelines such as the selection of inspection tools, inspection intervals and defect repair criteria; and the optimization of damage prevention activities including mechanical protection, cover monitoring and public awareness programs.
The paper will include the results of simulations dealing with maintenance optimization for corrosion and mechanical damage. These results demonstrate that risk-based planning is a powerful tool for minimizing risk by maintaining high reliability at the lowest possible cost. The most significant aspect of this work is that it provides a novel approach to managing pipeline integrity in a cost-effective manner.

Introduction
The goal of an effective pipeline integrity maintenance program is to ensure that the limited resources available are utilized most effectively in reducing the risks posed by pipeline failures to life, the environment and financial assets.
The uncertainties associated with making such an assessment has led to a recognition of risk analysis as a sound basis for making decisions on integrity maintenance. Risk, in this context, is defined as the probability of line failure multiplied by a measure of the associated adverse consequences. The risks associated with a pipeline system and its individual segments, and the potential risk reductions resulting from different maintenance activities can be used as a basis for evaluating different alternatives and developing an optimal maintenance plan.
This paper describes a comprehensive methodology for risk-based integrity maintenance optimization of offshore natural gas, crude oil and petroleum product pipelines. The methodology has been developed and implemented in a software package called PIRAMID?, (Pipeline Risk Analysis for Maintenance and Inspection Decisions) under a joint industry program sponsored by a number of transmission companies and government agencies (see Acknowledgements). The types of decisions addressed by the methodology include the choice of inspection methods (e.g., cover depth survey, in-line inspection) and inspection intervals, as well as the choice of maintenance actions (e.g., corrosion inhibition, hydro-test, coating reconditioning and replacement).