A reliability evaluation methodology, referred to as Sustainable Capacity, has been developed to quantify reliability expectations and provide a basis for the explicit consideration of reliability in pipeline design. The methodology, which has been adapted from one widely used in the electric power industry, is based on the representation of the load and capacity processes, which define reliability, with Markov models. These models generate time-series of simultaneous load and capacity profiles which are evaluated to determine reliability expectations. A cost-benefit methodology which recognizes the variation in the value attributed to pipeline reliability by individual customers is then used to assess the economic efficiency of any proposed facility addition. Further to its application as a design tool, the Sustainable Capacity methodology can also provide support in other areas such as operations and maintenance planning.


NOVA Gas Transmission Ltd. (NGTL) owns and operates a major gas transmission system located in the province of Alberta as shown in Figure 1. As of December 3 1, 1993, the NGTL system consisted of approximately 11,700 miles of pipe ranging in size from NPS 2 to 48 with total installed compression power of over 1,100,000 HP located at 45 compressor stations. The natural gas that is transported is collected at 838 meter stations and delivered to 157 delivery stations including 3 major border delivery points for subsequent shipment to ex-Alberta markets. The primary export markets which are served by gas transported by NGTL includes: California and the Pacific Northwest through the Pacific Gas Transmission System; Ontario and the Eastern Seaboard through the TransCanada Pipeline system, and; the Midwest through the Northern Border Pipeline. During 1993, a total of 3.8 tcf, or an average of about 10.33 bcf/d, was transported through the NGTL system with maximum daily receipts of 11.33 bcf/d. The throughput levels achieved in 1993 represent a 37% increase over the last 5 years which is generally reflected in the delivery history at EmpressNcNeill, NGTL's eastern border delivery statons, as shown in Figure 2. In addition to the growth in annual deliveries, another trend that has become evident is the increased year-round utilization of the system as indicated by the load factors in Figure 2. Annual average utilization of the delivery capacity at Empress/McNeil1 has steadily increased from about 80% in 1988 to a level of 92% in the first half of 1994. This increasing utilization, a trend which is forecast to continue, is generally attributed to changes in the natural gas market and more specifically the increased use of gas for electric generation and the use of storage by local utilities. The trend towards both increasing peak day loads as well as higher year-round load factors has resulted in increased stress on the NGTL system operations and has served to test the existing design methodologies. The effects on the NGTL system of the sustained loads was first experienced in the summer of 1992.

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