System Safety Analysis of Well-Control Equipment
- J.H. Fowler (On-Line Resources) | J.R. Roche (Hydril Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 1994
- Document Type
- Journal Paper
- 193 - 198
- 1994. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 1.7 Pressure Management, 6.1.5 Human Resources, Competence and Training, 1.6.5 Drilling Time Analysis, 1.6 Drilling Operations, 4.1.5 Processing Equipment, 1.7.5 Well Control, 1.10 Drilling Equipment, 5.1.2 Faults and Fracture Characterisation
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Two techniques are used for reliability analysis of a blowout preventer (BOP) and a hydraulic control system. Failure modes and effects analysis (FMEA) examines each part and the consequences of its malfunction. Fault tree analysis (FTA) traces undesired events to their causes. Reliability calculations and data sources are addressed.
In the wake of recent disasters in the oil and gas E&P and petrochemical industries, the importance of system safety analysis is becoming recognized. Reliability assessment techniques, which were developed in the nuclear-power-generation and defense industries, are potentially valuable tools for engineers in the offshore oil and gas business. BOP's and their control systems used on offshore rigs are typically made up of several subsystems. Hydraulic, pneumatic, and electronic modules are interfaced to provide functional control and monitoring of the mechanical BOP's and valves.
Safety of personnel, the environment, and the rig equipment depends on proper functioning of the well-control equipment. Therefore, a high priority is placed on maintaining the readiness and reliability of the well-control equipment. Formal reliability studies are now becoming contractual requirements when new well-control systems are designed and manufactured. A compilation of such analyses, BOP's, and controls now being used on North Sea platform rigs serves as the basis for this report.
Human error was not considered in this analysis because it is out of the control of the system designer and is influenced by such things as the training level and number of people present. This does not mean that a system cannot be designed with human factors in mind (which it is), but that this is purely a hardware analysis. For this analysis, all personnel intervention and support systems, such as air and electrical supplies, are considered to be 100% reliable.
Human error reduces equipment reliability in ways that cannot be predicted accurately by equipment suppliers. Reliable equipment supplied by a quality manufacturer is strongly dependent on a well-trained user workforce in the field for maintaining and operating the equipment to achieve the expected reliability.
Systems that provide high-priority safety protection are particularly good subjects for this type of analysis. One example is a BOP, a hydraulic/mechanical system that requires special techniques to evaluate the reliability data. At a minimum, ram-type BOP's must open, close, contain pressure, and seal tightly; each function requires the operation of different elements. Analysis of a hydraulically operated control system shows these evaluation techniques used on a more-complex system. Hydraulic BOP control systems include electrical, electronic, and mechanical components; hydraulic systems; and pneumatic systems with manual overrides. All these systems interact in a complex way.
Electric motor- and air-driven multistage pumps are used to pressurize the hydraulic fluid while transferring it to banks of accumulators. This stored power fluid is selectively routed through pressure regulators to the appropriate BOP or valve-operating cylinder(s) by means of control valves manifolded on the control system power unit. Fig. 1 shows a typical ram BOP and simplified control system for a surface installation.
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