Targeted Subsea Inspection of Offshore Structures Based on Risk and Criticality
Authors: Descamps B. , Woolley K.J. , Baker M.J. 
Published: October 2008

Abstract
Despite recent technical advances in underwater technology, the inspection of steel jacket structures is still a very costly activity, However it is recognised in the industry that inspection costs could be further reduced by combining technical advances with a much more rigorous planning philosophy. This paper presents the preliminary results of a two-year project carried out jointly by Quest Consulting Ltd. and the University of Aberdeen, Scotland. The aim has been to develop practical methods and tools for targeting the underwater inspection of offshore structures in areas of greatest criticality on a sound scientific basis. The work has involved an evaluation of traditional methods of inspection planning and the development of a practical risk-based strategy capable of minimizing the inspection costs of offshore structures while maintaining effective safeguards against structural deterioration, The benefits of this new inspection planning strategy are discussed hereafter.

Introduction
Underwater inspection, repair and maintenance (IRM) of offshore structures consumes a major proportion of the overall maintenance budget and consequently any reduction in this area is expected to have significant impact on the overall costs. Regular inspection of offshore jackets is carried out as part of the efforts to guarantee structural integrity and to meet statutory requirements for certification. Actual inspection planning in the UK North Sea is still very much a qualitative process mainly based on engineering judgement and experience. Technical advances have been achieved in recent years, particularly in ROV (Remotely Operated Vehicle) capability, which have led to a general reduction in highly expensive diver time. However there is a common appreciation of the need for scientific inspection planning which would reduce further the total cost of inspection activity whilst at the same time maintaining or improving the effectiveness of the work carried out.
The underwater inspection of steel offshore installations is concerned with the examination of specific areas and components which are normally hidden from view, in search of major or minor defects, flaws or other conditions which could affect the integrity of the structure or the functioning of the whole installation. Jacket structures will have some components whose integrity is critical to the reliability of the structure, while others will not have a great effect on the system reliability. Jacket structures are therefore ideal candidates for targeted risk-based inspection planning strategies where both the probability and consequences of failure of each component are taken into account.
A new inspection planning strategy, which uses the concept of risk to assess inspection priorities, has already been outlined in a previous publication. This strategy has now been developed further into an applicable scientifically sound inspection planning methodology for jacket structures. This methodology comprises two distinct steps. A semi-probabilistic ranking tree approach is first used in order to make an initial assessment of the areas of greatest criticality. The results of this method are then used in a probabilistic approach to produce a cost effective inspection programme.