Evaluation of probability of failure of static equipment in pressurized mud systems on an offshore drilling installation

Abstract

Degradation of pressurized topside static mud equipment is a very complex process. Inspection is a helpful tool to monitor degradation and helps reduce the number of critical failures. If it is left undetected and unchecked, it can lead to leakage resulting in accidents. To set up an effective inspection program, the concepts of Risk-Based Inspection (RBI) can be utilized. RBI helps to develop an optimum inspection program by evaluating the probability of failure (PoF) and consequence of failure (CoF), and combining them to estimate risk. The parts that have risk higher than the acceptable limit are then prioritized for inspection. This thesis studies the probability of failure in static equipment in pressurized mud systems on an offshore drilling installation, due to different degradation mechanisms and its influencing factors.

Det Norske Veritas (DNV), suggests a number of models to estimate internal and external degradation. By analyzing inspection data from the industry, it is observed that the models can at times be inconvenient to use when the degradation process is complex. For example, it is difficult to develop a simple, yet reliable, model that can accurately predict rate of degradation in situations where corrosion and erosion are simultaneously taking place. In static equipment in high pressurized mud systems, inspection have shown that the main reason for internal degradation is high amounts of solids in the fluid, high velocity, presence of seawater, and corrosive chemicals. DNV-RP-G101 does not present any model for this situation. For external corrosion, the current models presented in DNV-RP-G101 can be used to analyze inspection data.

In an old installation, the accurate quantitative records are often not available due to a number of reasons, like difficulty in measuring, old data management system, unsystematic inspections or lost records. Thus, it often becomes difficult to develop any qualitative model. On the other hand, the inspection and maintenance engineers have extensive experience that may be utilized for developing effective subjective models. In this thesis, based on some simplified parameters, a methodology for evaluating probability of failure is established. The methodology is divided into external and internal degradation, where the subjective judgments are more evident for internal degradation. Simplified flowcharts and tables are developed to easily evaluate probability of failure.