ALBERTA EXPANSION-1 ANGTS PREBUILD EXPANSION SOLVES PRESSURE CONSTRAINTS

Stanley R. Kitt
Foothills Pipe Lines Ltd.
Calgary

Foothills Pipe Lines Ltd., Calgary, has employed a unique solution to pipeline-capacity delivery problems around its Empress, Alta., extraction plants.

The problems arose as Foothills began planning for its expansion to export more Canadian gas into the U.S. The location is just north of Foothills' Monchy, Sask., delivery point to the U.S. at Northern Border Pipeline.

This capacity constraint resulted from the requirement for gas stripping by low-pressure Empress extraction plants severely limiting the operating pressure level of the high-pressure pipeline there.

The solution entailed construction of facilities for de-compressing then recompressing gas on the pipeline adjacent the Empress extraction plants.

Such facilities would enable the pipeline to operate at high pressure, while allowing the low-pressure extraction plants to maintain gas stripping.

The segment of line at that point forms part of the 396-mile Eastern Leg prebuild section of the future Alaska Natural Gas Transportation System (Angts) pipeline project (Fig. 1). This first of two articles on the project describes the pre-expansion system operation, the possible expansion options, and critical engineering and operating factors.

Hydraulic transient analyses performed to evaluate the operational stability of the candidate designs justified the decompression-recompression facilities. The conclusion of this series will review those analyses and describe the facilities which recently began operating.

The existing Foothills pipeline, although designed for a maximum operating pressure of 1,260 psig, was operationally integrated with the lower pressure NOVA Corp. of Alberta pipeline system in Alberta (Fig. 2).

Both pipelines delivered gas volumes to existing extraction plants at Empress at a fixed inlet pressure of approximately 610 psig.

The Foothills system, thus constrained in pressure at the Alberta-Saskatchewan border, was nevertheless required to transport gas the remaining distance to Monchy for delivery to Northern Border at maximum 1,260 psig. In the expansion case, this pressure was to be further increased to 1,435 psig.

Several expansion system designs were examined, including the unusual scenario which employed decompression-recompression facilities at the Empress extraction plants.

Based on extensive engineering analyses, including offline pipeline transient simulations to evaluate potential operational problems, the company concluded that the best way to expand the Foot-hills system would be to raise the operating pressure in the Foothills pipeline at Empress to nominal 1,000 psig and then to construct the decompression-recompression facilities to maintain gas-stripping capabilities.

In late June 1989, Foothills received approval from the Canadian National Energy Board (NEB) to construct the decompression-recompres-sion facilities. Construction was completed and the facilities placed into service in November 1990.

The new facilities expand the gas from the nominal 1,000 psig pipeline pressure to the nominal 600 psig extraction-plant operating pressure then recompress the extraction-plant residue gas to original pipeline pressure.

With the gas directed through turboexpanders coupled to brake compressors, a substantial portion of the expansion energy is captured and used in gas recompression. Supplemental compression required to return gas to the original pipeline pressure is provided by electric compressors.

ANGTS PROJECT

Foothills is the Canadian sponsor of Angts, the pipeline project selected in both Canada and the U.S. as the most effective and environmentally sound means of transporting Alaskan natural-gas volumes to U.S. markets.

(See OGJ, May 20, p. 19 for a full discussion of recent developments in Canadian natural gas exports to the U.S.)

In July 1977, upon completion of extensive hearings of alternative systems, the NEB recommended Angts. And, in April 1978 when the Canadian Parliament enacted the Northern Pipeline Act, the NEB granted certificates of public convenience and necessity for the construction and operation of the Canadian segment of Angts to Foot-hills' subsidiaries.

In September 1977, U.S. President Jimmy Carter, in his Decision and Report to Congress on the Alaska Natural Gas Transportation Sys-tem, likewise selected Angts as the pipeline system for transporting Prudhoe Bay volumes to U.S. markets.

In late 1977, the U.S. Congress ratified the president's Decision and the Agreement Between Canada and the U.S. on Principles Applicable to a Northern Natural Gas Pipeline.

The Angts project, when completed, will comprise approximately 4,780 miles of pipeline, divided into four segments (Fig. 1):

The Alaskan segment-746 miles from Prudhoe Bay to the Alaska-Yukon border and sponsored by Alaskan Northwest Natural Gas Transportation Co.
The Canadian segment-2,034 miles of pipeline from the Alaska-Yukon border to western and eastern export points at Kings-gate, B.C., and Monchy, Sask., respectively, sponsored by Foothills.
The U.S. Western Leg-869 miles from Kingsgate to the San Francisco area sponsored by Pacific Gas Transmission Co. and Pacific Gas and Electric Co. (PGT/PG&E).
The U.S. Eastern Leg-1,130 miles from Monchy to the Chicago area sponsored by Northern Border Pipeline Co.

Provision is also made in the Angts project for connecting Canadian gas supplies in the Mackenzie Delta region of the Canadian Arctic via the Dempster lateral.

More recently, Foothills has proposed the Mackenzie Valley Pipeline as a potential alternate for connecting Mackenzie Delta volumes should delivery of Mackenzie Delta volumes precede those of Prudhoe Bay.

The Canadian NEB and the U.S. president in 1977 raised the potential benefits of prebuilding the project's southern segments.

In 1978, the prebuild project was initiated when contracts were signed for the sale of new Canadian exports to California and U.S. Midwestern, southern, and eastern markets. By July 1980, necessary approvals to proceed with prebuilding portions of the Western and Eastern Legs were received, and facilities design and construction commenced.

The Western Leg portion of the Angts prebuild was placed into service on Oct. 1, 1981, while the Eastern Leg prebuild commenced service on Sept. 1, 1982.

The Angts prebuild, as constructed in the early 1980s and as its exists today, consists of approximately 1,51 1 miles of pipeline in four segments:132-mile Canadian Western Leg, 161-mile U.S. Western Leg, 396-mile Canadian Eastern Leg, and 822-mile U.S. Eastern Leg.

The Western Leg prebuild extending from Caroline, Alta., to Stanfield, Ore., consists of approximately 293 miles of Angts pipeline segments which are integrated for operational purposes with existing NOVA and Alberta Natural Gas Co. Ltd. (ANG) facilities in Canada and with PGT facilities in the U.S.

The Eastern Leg prebuild likewise commences at Caroline, Alta., but extends as one continuous pipeline about 1,218 miles to Ventura, Iowa. The Angts prebuild in its two legs constitutes approximately one third of the ultimate Angts project length.

PRE-EXPANSION

The existing Foothills Eastern Leg prebuild pipeline originates at a point near Caroline in Alberta and extends approximately 396 miles southeasterly across the provinces of Alberta and Saskatchewan to a point of interconnection with Northern Border on the Canada-U.S. border near Monchy, Sask.

From ownership as well as operational perspectives, the pipeline is divided into two segments: the Foothills Pipe Lines (Alta.) Ltd. segment operated by NOVA (as agent) and the Foothills Pipe Lines (Sask.) Ltd. segment operated by TransCanada PipeLines Ltd. (as agent).

As configured before expansion, the Foothills (Alta.) system consisted of about 235 miles of 42-in. pipeline with maximum allowable operating pressure (MAOP) of 1,260 psig, and one compressor station housing two compressor units at Station 367 near the town of Jenner (Fig. 2).

The Foothills (Alta.) pipeline parallels the existing NOVA eastern mainline system along its entire length from Caroline to the Empress extraction plants and the points of interconnection with TransCanada and Foothills (Sask.) at the Alberta-Saskatchewan border.

The pre-expansion Foothills (Alta.) and the NOVA pipeline systems were integrated for operational purposes with links between the two pipelines at NOVA's Schrader Creek, Didsbury, Beiseker, Hussar, and Princess compressor stations, as well as at Sandy Point immediately upstream of the Empress extraction plants.

In pre-expansion operation, gas was injected into the Foothills pipeline by NOVA's Schrader Creek compressor station near Caroline at pressures of 950-1,200 psig, depending upon the operating conditions in the upstream NOVA pipeline and the operating status of the Schrader Creek compressor unit.

Thereafter, the flow in the Foothills pipeline varied depending upon the strategy of operations of NOVA gas control on any particular day. It was gas control which determined which of the intercon-necting links were to be opened or closed to satisfy operational requirements.

The two pipelines, acting as one entity operationally, delivered gas to the Empress area at a nominal pressure of 610 psig for measurement and distribution to the extraction plants before removal from the province.

The NOVA eastern system in the Caroline-to-Empress segment consists of a mainline and several loop lines with MAOP significantly lower than that of the Foothills Eastern Leg prebuild.

In the segment from Caroline (Schrader Creek) to NOVA's Beiseker station, the pipeline's MAOP is 935 psig, while in the downstream sections the MAOP's range is 896-826 psig. The maximum operating pressure of the Foothills (Alta.) pipeline on the other hand is 1,260 psig.

Thus the operational integration of the Foothills and NOVA systems, coupled with the low-pressure delivery requirements at Empress, resulted in the Foothills system being seriously constrained hydraulically.

The Foothills (Sask.) segment of the Eastern Leg prebuild consists of 161 miles of 42 in. with MAOP of 1,260 psig, four single-unit, gas-turbine-driven compressor stations, and a custody mea-surement station at Monchy.

Three of the compressor stations were installed during initial prebuild operations, while the fourth (Station 393) was constructed only recently and placed into service primarily as a security station.

The last compressor station, Station 394, is at Monchy and is responsible for compressing gas to satisfy pressure obligations for delivery into Northern Border.

In its pre-expansion operation, the Foothills (Sask.) segment received processed gas from the immediate downstream side of the Em-press extraction plants at a nominal pressure of 580 psig.

The gas was then compressed by the three operating stations for delivery to Northern Border at Monchy at a pressure of 1,260 psig maximum.

Thus, the operation required compression not only to overcome pressure losses associated with pipeline flow, but also to maintain a fixed static pressure differential of 680 psig. With system expan-sion for accommodating increased Canadian exports through Monchy, the maximum delivery pressure at Monchy was to be increased to 1,435 psig, which is the MAOP of the downstream Northern Border pipeline.

Accordingly, if the low-pressure receipt conditions at Empress were to be retained for the expansion, the operational problems associated with the large pressure differential between Empress and Monchy would be exacerbated by such expansion.

EMPRESS EXTRACTION PLANTS

Before removal from Alberta, gas exported via the Empress-McNeill export points is generally processed by one or more of four extraction plants located in this area. The four plants and respec-tive design capacities are shown in Fig. 3.

In the pre-expansion operation, the NOVA eastern mainline pipeline system, which comprises four largediameter pipelines in the Princess-to-Empress section, delivered gas to the NOVA Empress meter station for measurement before distribution in accordance with an established allocation formula to the four plants.

The Foothills (Alta.) pipeline, which parallels the NOVA system to the Empress region, delivered gas to the Empress meter station via the Sandy Point crossover at a pressure equivalent to the arrival pressure in the NOVA pipelines. The combined NOVA-Foothills gas streams were measured by the Empress meter station, then distributed to the four extraction plants.

After processing, the gas destined for export via the Foothills pipeline and Monchy was returned via the McNeill crossover and meter station to Foothills, while the remainder was delivered to TransCanada. The NOVAFoothills pipeline systems delivered gas to the Empress meter station at nominal 610 psig.

With the maximum allowable design pressure of the Empress extraction plants at 650 psig, pressure-regulating facilities at the Empress meter-station site limited the pipeline pressure on the downstream side of the meter station to this level.

Operating pressures, however, could sink to the minimum allowable 550 psig from time to time.

The pre-expansion regime in the Empress area constituted a complex interconnection of pipeline and extraction plant facilities whose operations directly affected one another. The Foothills (Alta.) and NOVA pipelines were directly interconnected upstream of the extraction plants at the Sandy Point crossover.

Downstream of the extraction plants, the NOVA pipeline interconnected with the TransCanada pipeline as well as the Foothills pipeline. The TransCanada pipeline itself has a large compressor station located in the immediate proximity of the extraction plants, approximately 2 miles downstream near the town of Burstall, Sask.

Similarly, Foothills has a compressor station located on its pipeline approximately 14 miles downstream of the extraction plants. Finally, the four extraction plants themselves straddle the NOVA pipeline.

This complex network of pipelines, compressor stations, and extraction plants posed some potentially serious operational difficulties because compressor upsets on either the downstream TransCanada or Foothills systems would directly and immediately affect the operation of all interconnecting facilities.

The interdependence of operations, therefore, was an important consideration in the selection of a system design for Foothills pipeline expansion.

EXPANSION PLAN

The pre-expansion Foothills Eastern Leg facilities and op-erations configuration entailing low-pressure, integrated operation (with NOVA) in Alberta was implemented at the system start-up in fall 1982.

Foothills contractually received gas volumes from NOVA at two locations and transported the net receipts (less fuel) to Empress for redelivery to NOVA via the Sandy Point crossover at a pressure of about 610 psig. The Foothills system in Alberta thus configured was at maximum capacity.

Downstream of the Empress extraction plants, Foothills received gas destined for the Monchy export point at nominal 580 psig. At Monchy, Foothills was obligated to deliver gas to Northern Border at pressures up to 1,260 psig.

The capacity of the existing Foothills (Sask.) system under such established receipt-delivery conditions was approximately 1.1 billion cfd (bcfd). Increased gas-transportation requirements in Alberta and Saskatchewan for NOVA and Monchy exporters, respectively, necessitat-ed expansion of the Foothills Eastern Leg prebuild.

The level of increased transportation requirements in the immediate term was uncertain, but there was an expectation that exports at Monchy could reach as high as 1,800 MMcfd. In any event, with the existing system at capacity, it was essential that how expansion should proceed be established first.

Accordingly, Foothills, in cooperation with NOVA, proceeded to evaluate expansion alternatives for two levels of Monchy exports, an initial volume of 1,350 MMcfd, and an ultimate volume of 1,800 MMcfd. For purposes here, however, discussion will generally be limited to the 1,350 MMcfd case.

In broad terms, only two expansion alternatives were considered: low-pressure, integrated and high-pressure, segregated options.

The former entailed expansion through retention of the original operating regime, wherein the Foothills-NOVA systems in Alberta remained integrated operationally while delivering gas to the Empress area at low pressure (61 0 psi g).

The latter involved expansion through segregating the Foothills pipeline from the NOVA pipeline in Alberta, operating the Foothills pipeline in Alberta at high pressure, and delivering gas to the Em-press area at relatively high pressure (1,000 psig).

Both expansion options had several suboptions possible; these were evaluated separately in the design process. All system alternatives had to accommodate the following important design crite-ria and considerations:

The maximum delivery pressure at Monchy was to be increased from 1,260 psig to 1,435 psig, the latter being the MAOP of the Northern Border pipeline.
Pipeline flow loss under the outage of a single compressor station was generally to be limited to 10% of contract.
Design was to be performed under average July ambient conditions.
Existing compressor-station discharge piping maximum temperature constraints could not be violated.
Series coupling of centrifugal compressor units was to be limited to single-stage compressors only.
Operations stability and reliability were to be maintained at a high level.

LOW-PRESSURE CONFIGURATION

The low-pressure, integrated option for system expansion involved the following strategies:

Continuing the integrated mode of operation of the Foothills and NOVA pipelines in Alberta
Expanding the combined and integrated FoothillsNOVA systems in Alberta to accommodate the total Empress (TCPL)-McNeill increased volume requirements
Increasing the capacity of the Foothills system in Alberta by increasing the receipt pressure from NOVA at Schrader Creek
Continuing to deliver gas to the Empress extraction plants at nominal 610 psig in both pipelines
Expanding the Foothills pipeline downstream of the Empress plants to Monchy by adding compression as appropriate to accommodate the increased Monchy flow and pressure requirements.

The virtue of the low-pressure, integrated mode of expansion was its retention of the original regime in both the operation of the pipeline in Alberta and the extraction plants at Empress.

The integrated manner of pipeline operation with low-pressure delivery to Empress was preferred from the considerations of pipeline reliability and operational flexibility. From the perspective of the extraction plants, the low-pressure delivery was preferred because it permitted the increased gas volumes to be processed without the ad-dition of any new plant facilities, existing plant capacities being adequate to process the anticipated increased volumes.

On the other hand, the low-pressure delivery at Empress would severely constrain the capability of the downstream Foothills pipeline, especially with the increased pressure requirement at Monchy (maximum delivery pressure to be increased to 1,435 psig from the current 1,260 psig).

In addition, the high pressure differential (of 860 psig) that would prevail in such an operation would pose serious operational stability and reliability concerns. An inadequate design could even result in an Eastern Leg prebuild pipeline shutdown under certain situations because of compressor unit restart difficulties as a result of high station pressure differential conditions.

In selection of the system design for the low-pressure, integrated option, several expansion alternatives for the Empress-to-Monchy segment of pipeline were evaluated, including the following for the 1,350-MMcfd flow case:

Addition of 30,000-hp (ISO) units at existing Compressor Stations 391 and 392 in Saskatchewan
Construction of a new, single-unit, 35,000-hp (ISO) compressor station at Empress (Station 369), with con-sideration for a spare
Construction of a new, twin-unit, 70,000-hp (ISO) compressor station at Empress, arranged in series, with Station 391 in Saskatchewan relegated to a standing spare
Construction of a new, multiple-unit, 36,000-hp (ISO) compressor station at Empress, arranged in parallel, with consideration for a spare.

The case alternatives for the initial 1,350-MMcfd flow case were selected to be compatible with ultimate 1,800-MMcfd case require-ments. The steady-state hydraulic, cost, and cost-of-service analyses yielded the conclusion that the optimum configuration for the lowpressure, integrated option included the construction of the large unit (35,000 hp) compressor station at Empress (Station 369).

For the initial 1,350-MMcfd nominal flow, based on steady-state hydraulic analysis, a single 35,000-hp compressor unit was found to be adequate, although there were considerable concerns for the operational stability of the system.

For the ultimate 1,800 MMcfd flow, this station would consist of three 35,000-hp units arranged in parallel.

There was, accordingly, more comfort with this ultimate configuration.

The low-pressure, integrated system configuration for the 1,350-MMcfd flow case is illustrated in Fig. 4 and includes a single 35,000-hp compressor station at Empress (Station 369).

The normal system operation would include leaving Station 393 on standby, with the resultant steady-state hydraulic profile being as illustrated in Fig. 5.

It remained for the transient analysis to determine the need for a second standing spare at Station 369 to preserve operational stability.

HIGH-PRESSURE CONFIGURATION

The high-pressure, segregated mode of system expansion involves the following strategies:

Effectively segregating the Foothills pipeline in Alberta from the NOVA system, excepting that Foothills would continue to receive gas from NOVA's Schrader Creek compressor station near Caroline with receipt pressure increased to approximately 1,200 psig
Operating the Foothills pipeline in both Alberta and Saskatchewan at or near its maximum design pressure of 1,260 psig, consistent with the original design of the Angts Eastern Leg in Canada.
Delivering gas to the Empress area at high pressure (approximately 1,000 psig) in the Foothills pipeline, while continuing to deliver gas to Empress at low pressure in the NOVA pipeline
Constructing decompression-recompression facilities at Empress to maintain stripping of the gas stream in the Foothills pipeline
Constructing compression facilities on the Foothills pipeline in Alberta in accordance with the flow requirements, while preserving one standing spare at Station 367 in order to maintain constant delivery pressure at Empress to the decompression-recompression facility (No new compression facilities are required for the 1,350-MMcfd expansion case.)
Constructing compression facilities on the Foothills (Sask.) segment in accordance with flow requirements.

The function of the decompression-recompression facilities is to facilitate gas stripping of the high-pressure pipeline gas stream by the low-pressure extraction plants by initially decompressing or expanding the high-pressure pipeline gas to the normal operating pressure of the extraction plants. Then the facilities recompress the residue gas to original pipeline pressure.

The process of decompression entails the routing of the high-pressure gas through turboexpanders and brake compressors to recover power for use in gas recompression. The process of recompression then involves supplementing the partial recompression provided by the expanders-brake compressors with electric-motor-driven centrifugal compressors.

In addition, the decompression-recompression facilities process design includes the temperature conditioning of both the inlet gas prior to its expansion, as well as the outlet gas prior to its return to the pipeline. As a general rule, the decompression-recompression facilities' design would render the facilities neutral to pipeline operations.

The decompression-recompression facilities' process design is more fully addressed in the conclusion to this series. From the per-spective of the pipeline, the high-pressure, segregated expansion includes the following benefits:

Higher hydraulic efficiency operation of the Foothills pipeline in both Alberta and Saskatchewan as a result of the generally higher operating pressure
Debottlenecking of the Foothills pipeline downstream of the extraction plants by virtue of the significantly higher arrival pressure at Empress
Incremental compression facilities compatible with ultimate Angts requirements
Significantly improved operations stability of the downstream Foothills (Sask.) segment by virtue of the substantially reduced static pressure differential between Empress and Monchy (reduction of approximately 400 psig)
Improved pipeline deliverability since the decompression-recompression facilities will be neutral to pipeline operations
Reduced pipeline-sparing requirements because any decompression-recompression sparing requirement would be justified on a basis equivalent to that of the extraction plants.

Fig. 6 illustrates the facilities configuration for the high-pressure, segregated expansion case based on 1990-91 Empress (TCPL) requirements and 1,350-MMcfd Monchy requirements.

Because of high receipt pressure from NOVA's Schrader Creek compression station, no new compression facilities are required on the Foothills pipeline in Alberta. Likewise, because of the high pipeline pressure at Empress, no new compression facilities are required in Saskatchewan.

The only incremental facilities necessary are the decompression-recompression facilities with their associated interconnecting pipelines. It was determined that for several reasons the decompres-sion-recompression facilities ought be located adjacent to the Empress 11 extraction plant.

The normal operation of the Foothills pipeline includes the operation of one compressor unit at Station 367 and three of the four compressor stations in Saskatchewan.

The steady-state hydraulic profile of the high-pressure, segregated operation is illustrated in Fig. 5, where its superiority over the low-pressure, integrated option is immediately apparent.

The operational superiority of the high-pressure alternative under transient conditions will be discussed in the conclusion to this series.

ACKNOWLEDGMENTS

The author thanks Sam Sampath, Sampath Consulting Ltd., Calgary, and, Stanley P. Chan, engineering specialist in system planning of Foothills Pipe Lines Ltd. for their valuable technical assistance in the preparation of this article. Additionally, the author thanks the numerous other individuals within the engineering and corporate communications departments of Foothills Pipe Lines who contributed time and talents in making this article possible.

Likewise, the engineering staff of Amoco Canada Petroleum Co. Ltd. and Delta Projects Inc. are acknowledged for their responsibil-ities in engineering design and construction management of the decompression-recompression facilities project.