Development of a Cost-Benefit Model for Inspection of Offshore Jacket Structures in Mexico

Planning of inspections is an important task for optimal maintenance of a jacket platform system, and the current practice has been based mostly on judgment and experience. Given the limited resources available for maintenance, there is a growing interest of PEMEX in the optimization of inspection strategies. In order to make easier and more practical the determination of the inspection strategies, in this work, it is assumed that inspections are performed at a constant time interval. A strategy is formulated so that the elements with larger failure probability have priority over other components in the inspection schedule. In this paper, the areas most prone to be damaged are identified, the frequency of damage as a function of its depth in a platform is studied, the levels of damage on joints and elements are identified to define the levels of repair, and the importance of each element is selected according to its capacity to contribute on the base shear resistance of the platform. In addition, a fatigue analysis of the platform is performed to identify the joints most prone to be cracked by fatigue, and to estimate the evolution of damage, where limits in measures for crack magnitude are found and a uniform probability distribution for crack magnitudes on joints are proposed. Several inspection intervals are proposed, their corresponding life-cycle costs are estimated and the optimal schedule is chosen so that the expected life-cycle cost is minimized. The expected costs associated with the no failure event include the costs of inspection and repair whereas the expected failure costs involve the potential consequences of damaged equipment, cost associated with the pollution by crude oil, injuries, cost of avoiding the loss of life, deferred production and indirect loss. In this paper, indirect losses are estimated through an assessment based on the concepts of the Leontief’s input-output model. In addition, relationships between each item costs and the global damage index are proposed. As before mentioned, the optimal inspection schedule corresponds to the minimum life-cycle cost.Copyright © 2003 by ASME