ABSTRACT

Two techniques are used for reliability analysis of a blowout preventer and a hydraulic control system. Failure Modes and Effects Analysis (FMEA) examines each part and the consequences of its malfunctions. Fault Tree Analysis (!7A) traces undesired events to their causes. Reliability calculations and data sources are addressed.

INTRODUCTION

In the wake of recent disasters in oil and gas exploration and production, as well as the petrochemical industry, 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 to engineers in the offshore oil and gas business.

Blowout preventers (BOPS) 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 is dependant upon proper functionality of the well control equipment. Hence, 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, of BOPS and controls now working on North Sea platform rigs, has served as the basis for this report.

SYSTEMS ANALYZED

Systems which provide high-prioriiy safety protection are particularly good subjects for this type of analysis. One example is a Blowout Preventer, a hydraulic/mechanical system, which requires special techniques in evaluation of the reliability data. Analysis of a hydraulically operated control system shows the techniques used in a more complex system.

Ram Type BOPS

Ram blowout preventers are used to contain pressure below the wellhead while drilling a well. Typically, two ram assemblies are driven radially inward by diametrically opposed hydraulic cylinders. See Figure 1. These rams are designed to accommodate various sizes of pipe in the well bore, sealing on the pipe and against each other to shut in the well.

Alternative ram configurations include pipe rams designed to seal on a specific pipe size, variable rams designed to seal on a range of pipe sizes, blind rams designed to seal on an open bore, and shear rams designed to sever pipe in the hole and seal well bore pressure.

Once the well bore has been shut off, abnormal pressure can be neutralized by circulating drilling fluid through the drill pipe and/or side outlets on the BOPS for dissipating pressure in the choke manifold.

When the well bore pressure has been neutralized, the rams are hydraulically retracted to clear the well bore and enable a return to normal drilling operations.

Hydraulic Controls

BOP stacks on drilling rigs are usually controlled hydraulically. Petroleum or water-based hydraulic control fluid is mixed and stored in a dedicated reservoir. Electric motor driven and air driven multi-stage 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 way of control valves manifolded on the control system power unit.

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