MIL and MIRO Diagrams for Risk-Based Positioning of Drilling Rigs With Dynamic Positioning System

Abstract

Currently, most offshore oil exploration operations are performed by Dynamically Positioned (DP) units; in Brazil, the use of these units is traditional in drilling operations (DP drilling rigs). The advent of DP systems, which allow the vessel to maintain a certain position without the need for anchor lines, has brought great flexibility to the oil field; however, usually in drilling areas, there are a large number of operations being performed simultaneously and subject to weather conditions and a failure of the DP system may cause these vessels to drift, which may occasionally result in a collision with other field equipment or vessels, causing material, personal and environmental damage. In this context, it is necessary to analyze what would be the best positioning points for these units according to the configuration of potential obstacles present in the area (such as risers, anchor systems and floating production systems), the characteristics of the DP unit and the expected environmental conditions. It is vital to know the risk of collision associated with the positioning of these units. The risk of collision will depend mainly on the meteo-oceanographic variables of the operating region, the hydrodynamic characteristics of the unit, the DP system reliability and its repair time, and the distribution of obstacles in the area. The objective of the ongoing research is the development of a methodology to define the risk associated with the positioning of the DP units, through a statistical method and a validated drift mathematical model under the influence of environmental agents. The proposed methodology allows us to demonstrate compliance with a widely accepted RAC (Risk Acceptance Criterion). The developed methodology proposes the use of two instruments: Location Iso-Probability Maps (MIL) and Operational Iso-Risk Maps (MIRO), to synthesize the information to the decision making about the operation of the DP units at a specific location, considering the overall collision risk (at MIRO) and the probability of the rig being at a specific location (at MIL).