Risk Based Assessment (RBA) for inspection activities within geothermal energy plants has become commonplace in New Zealand to control the risk of premature or unexpected failure and for maintenance planning. Geothermal energy companies often administer their own pressure vessel inspection requirements following recognized standards, with audit and certification provided by accredited inspectors.
The RBA methodology is based on the ability to predict the corrosion chemistry in key parts of the plant so that material-environment combinations can be modeled. This allows the prediction of damage mechanisms and the most likely location in the plant for that damage to occur. The intent of the RBA process is to provide operators with a targeted approach to predict areas of potential damage, and thus the ability to focus their time and resources more efficiently. The RBA provides a basis for an Overall Inspection and Test Plan which describes the "what and when to inspect" and provides a living inspection document that identifies "where and how" to inspect.
A natural transition occurs from the initial RBA to inspection based assessments through knowledge gained by physical internal inspections, which lead to adjustment of the RBA. Management of change is an integral part of this process, as new equipment or change in equipment or process conditions initiates a review of the RBA process. Such RBA and inspection activities are part of an operations philosophy that aims to achieve 96% availability in a geothermal plant with no forced outages.
This paper outlines the processes used for the RBA of geothermal plants and the methodology to capitalize on the knowledge gained from risk assessments and downstream inspection activities.