Development of a Risk Based Underwater Inspection (RBUI) Process for Prioritizing Inspections of Large Numbers of Platforms
Authors: DeFranco Sam , O&apos;Connor Pat , Tallin Andy , Roy Raja , Puskar Frank 
Published: October 2008

Abstract
Amoco has several hundred platforms located worldwide that require underwater inspections on a regular basis. Because of the high cost of underwater inspections, Amoco has developed a risk-based process to more effectively implement inspection resources. The process identifies which platforms expose Amoco to the greatest risk and which platforms are most likely to benefit the most from a more focused inspection effort. The process is based upon key characteristics of each platform (year designed, number of legs, framing scheme, manning level, etc.) as well as results from previous inspections (date of last inspection, amount of inspection, flooded members, excessive marine growth, anode status, etc.). Using this information, the overall "risk" of the platform is determined using a rule-based scoring estimation of the likelihood and consequence of failure. The platforms are then ranked from highest to lowest risk, with the highest risk platforms receiving priority for inspections. The use of RBUI for inspection planning is demonstrated using a fleet of 29 platforms.

Introduction
Underwater inspections, along with structural assessment and data management, are an integral part of structural integrity management of an offshore platform. A diagram of the structural integrity management (SIM) process (from ISO TC67/SC7/WG3/P8 Section 18 Draft D1) is shown in Figure 1. As may be noted in this figure, there are four elements in SIM: Data, Evaluation, Inspection Strategy, and Inspection Program. These elements are joined into a continuous SIM cycle through consideration of the underwater inspection data obtained from the Inspection Program.
ISO Section 18 recommends that an operator use operational experience, inspection data, engineering expertise, analysis, and experimental data to develop and implement an Inspection Strategy. The Inspection Strategy describes which platforms should be inspected, why, how, and at what time interval. From the Inspection Strategy, an Inspection Program is developed which includes specific scopes of work, specifications, and contracting strategy, etc. During the Inspection Strategy stage, an operator may decide to use a platform selection (or categorization) system to prioritize platforms for inspection. ISO does not specify methodologies for doing so, but provides guidance on factors which are important for developing a defensible Inspection Strategy.
Amoco has long had an age based categorization system for platforms which corresponds to significant changes in the API RP2A2. Amoco platforms have been identified as 1st Generation (1963-1970), 2nd Generation (1971–1974) and 3rd Generation (1975 to present). These generations correspond closely to Pre-API RP2A, development of RP2A 1st edition, and development of a RP2A 9th edition respectively. The Generation ranking does not capture consequence but indirectly captures specific platform characteristics (e.g. number of legs, bracing configuration) as analysis and design practice evolved over time.
In developing an inspection strategy for a fleet of platforms, one approach may be to rank platforms according to relative "risk" posed to the operator by each platform. By "risk" we mean consideration of the likelihood of a platform failure (due to environmental overload) and the consequence of such a failure. Consideration of likelihood allows the use of pecific structural characteristics while consequence allows the use of safety, business interruption, and environmental impact costs.