Reliability Analysis Of Thistle "A" Platform
Authors: Bosler William H. , Dolan Thomas J. 
Published: December 2007

A detailed analysis of equipment and systems has confirmed that the Thistle "A" platform has both the reliability and the flexibility to achieve the results anticipated in the huge platform's design. The reliability assessment utilized an advanced technique which recognizes the long-term, continuous operational characteristics of offshore production facilities as well as the relationships between equipment failure rates and operating time. The technique used and the results obtained are the subjects of this paper.

Introduction

The Thistle "A" oil production platform, operated by BNOC (Development) Ltd.—a subsidiary of British National Oil Corporation—started drilling producing wells in Thistle Field of the North Sea in late 1977. Located about 270 miles northwest of Scotland, the platform is constructed in 530 feet of water and is platform is constructed in 530 feet of water and is designed for production of 200,000 barrels of oil per day. Most of the production from Thistle "A" eventually be transported by pipeline to Sullom Voe in the Shetland Islands for storage and subsequent shipment to refining centers. Pending completion of the pipeline link, however, production is transported via pipeline link, however, production is transported via tanker from a mooring system located near the platform.

About 11 months prior to the start of oil production, BNOC (Development) Ltd., retained Bonner and production, BNOC (Development) Ltd., retained Bonner and Moore Associates, Inc., a Houston, Texas, consulting firm, and the Systems Reliability Service of the UKAEA Safety and Reliability Directorate for assistance in measuring the platform's reliability. The reliability assessment was considered necessary for several reasons.

First, BNOC wanted an accurate, independent audit of the platform's design. A reliability study— conducted at the proper level of detail—can identify opportunities for increasing reliability prior to start-up. Several of the component systems of the platform were of particular concern as potential limiting factors.

Secondly, BNOC wanted an accurate reliability assessment in order to predict production levels for use in economic and financial forecasts.

Finally, BNOC wanted to use the results and data from the reliability study in still another analysis to assess safety and hazards from process upsets.

This paper describes the technology and the methodology employed in measuring the reliability of the Thistle "A" platform.

SUMMARY OF RELIABILITY STUDY RESULTS

The reliability of production facilities aboard the Thistle "A" platform has been computed at 96.4 percent of design capacity. This means that day-today operation of the platform at nominal design capacity (200,000 barrels per stream day) throughout a year would result in the actual average production of 192,800 barrels of oil per calendar day. Stated another way, if the platform is to produce an average of 200,000 barrels of oil platform is to produce an average of 200,000 barrels of oil per calendar day during a year of operation, then it per calendar day during a year of operation, then it must operate above nominal design capacity at about 208,000 barrels per stream day.

The relationship between stream-day operating capacity and actual calendar-day production is shown by the curve in Figure 1. The linear relationship which would result at 100 percent reliability is represented by the dashed line.

Stream day operating capacity and calendar-day production do not match because equipment downtime or production do not match because equipment downtime or reduced operating capacity will be experienced during platform operation as equipment fails and requires platform operation as equipment fails and requires repair or as inspection and maintenance work are performed to assure that high reliability of equipment performed to assure that high reliability of equipment continues.

As the curve of Figure 1 shows, reliability decreases as stream-day capacity increases—resulting in a declining ratio between stream day and calendar day production rates. At the maximum production rate production rates. At the maximum production rate (267,700 barrels per day) a calendar day production of about 256,460 barrels would result.

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