Phillip J. Murdock
Lone Star Gas Co.
Dallas
Major reconfiguration of a West Texas pipeline hub will allow the operator significant flexibility in transporting natural gas either to U.S. West Coast markets or to similar pipeline hubs in central and East Texas for movement to Midwestern or eastern U.S. markets.
In July 1988, Lone Star Gas Co., Dallas, declared itself an "open access" transporter under U.S. Federal Energy Regulatory Commission (FERC) Orders 436 and 500.
At that time, the company began to consider various alternatives for best utilizing its pipeline system, considering both operations and economics, to provide transportation services and expand sales opportunities.
Lone Star's pipeline system is configured to transport natural gas between three major pipeline hubs in Texas: Carthage (Panola County), Katy (Waller County), and Waha (Pecos County).
Lone Star had previously installed facilities necessary to receive or deliver natural gas for transport at the Carthage and Katy hubs. Installation of facilities at Waha to deliver and receive natural gas for transport would enable Lone Star to transport gas to or from the three major pipeline hubs in Texas.
HUB CONCEPT
Fig. 1 details the hub concept and Lone Star's major transmission pipelines.
Analyses of each hub area considered several parameters:
- Number of other pipelines in each hub area and their proximities to Lone Star's pipeline system
- Physical characteristics and operating parameters of other pipelines including diameter, maximum allowable operating pressure (MAOP), maximum and minimum operating pressures, and, if known, percentage of pipeline capacity being utilized
- Lone Star's ability to deliver through its pipeline system volumes of natural gas in sufficient quantities for viable transportation and exchange opportunities
- Existing interconnects at each hub between Lone Star and various other pipelines
- The potential market for transportation and exchange services to the other pipeline companies at each hub.
At two of the three existing Texas pipeline hubs, Carthage and Katy, Lone Star has pipeline facilities in place necessary to receive or deliver natural gas for transport. In some cases, the only capital expenditures necessary to achieve required interconnects would be costs associated with the installation of measurement facilities between Lone Star's system and other pipeline systems.
Additionally, both hubs are in the vicinity of natural-gas processing plants with residue-gas headers to which Lone Star and other pipeline systems are connected. Such configurations may allow for transport gas to be allocated to Lone Star at residue-gas headers with minimal capital expenditures.
Lone Star's presence at the Carthage and Katy hubs provides access to major pipeline systems serving markets on the Gulf Coast, on the East Coast, and in the upper Midwest.
These locations do not, however, provide a means readily to access those pipeline systems serving growing markets in California and on the West Coast. An estimated two thirds of the projected increase in gas consumption between now and 2010 will occur in California and the Atlantic regions.
Additionally, recent development of coal-seam gas in the San Juan basin will, in all likelihood, create demand for transportation services to move gas from western hubs, such as Waha, to eastern hubs, such as Katy and Carthage, in order to interconnect with major transmission pipelines serving the East Coast and the upper Midwest.
WAHA HUB
To be in a position to take advantage of these new transportation and marketing opportunities, Lone Star Gas Co., in partnership with American Central Gas Companies Inc., decided in early 1989 to develop a pipeline hub at Waha.
Three major factors contributing to this decision were the following:
- The presence of numerous interstate and intrastate pipelines within a small geographic area around Waha, including systems operated by El Paso Natural Gas, Valero, Oasis, Intratex, Red River, American Pipeline, and Northern Natural (Fig. 2a)
- Lone Star Gas Co.'s existing 36-in. pipeline, maximum allowable operating pressure (MAOP) = 960 psig, available for transporting gas to and from Waha
- Lone Star Gas Co.'s existing pipeline system being located to transport gas between Waha, Katy, Carthage, and points in between.
Initial analysis of the Waha hub concept indicated that several design requirements had to be met in order to make the project viable.
Perhaps the most important of these criteria was that the system have sufficient operating flexibility to deliver or receive gas from one or more pipelines on a daily basis as dictated by operating parameters and transportation orders.
Such operating flexibility would allow gas to be transported to or from individual pipelines within the hub area, a practice which has come to be known as "wagon wheeling."
Other pipelines in the Waha area normally operate in the following pressure ranges: minimum pressure, 450-850 psig; normal pressure, 700-1,000 psig; and maximum pressure, 890-1,050 psig.
Because of the wide variance in operating pressures, both within given pressure ranges and between minimum and maximum, it was determined that a single pipeline connecting various pipelines and Lone Star's 36-in. pipeline would not provide the required operating flexibility simultaneously to deliver gas to or from selected pipelines without the operating pressures of those selected pipelines having to be raised or lowered.
For example, if deliveries were to be made to Pipeline A operating at 900 psig and deliveries are called for from Pipeline B operating at 750 psig, a single interconnecting pipeline would require that the operating pressure of Pipeline B be raised or the operating pressure of Pipeline A be lowered in order to accomplish simultaneous receipt and delivery.
Such an operating scenario is not feasible considering the potential number of pipeline interconnects involved and the operating difficulties associated with raising and lowering pipeline operating pressures for what could be relatively short periods.
RANGE OF PRESSURES
To solve this operating dilemma and provide necessary operational flexibility, the concept of two interconnecting pipelines, as opposed to a single pipeline, was developed whereby one pipeline, operating at 600-900 psig, would be used as a low-pressure header and the second pipeline, operating at 950-1050 psig, would be utilized as a high-pressure header.
Such an arrangement would allow gas to be delivered to or from pipelines with different operating pressures without necessarily raising or lowering operating pressures. By a connection of selected pipelines to both the high-pressure header and the low-pressure header, a wide range of operating pressures can be accommodated when gas is being transported from pipeline to pipeline.
The high-pressure/low-pressure header concept also provides an ideal piping arrangement for installation of compression with the low-pressure header being used for suction and the high-pressure header being used for discharge.
The project (Fig. 2b) became known as the "Waha header" project.
With the header concept decided upon, the physical layout of the proposed header system could be determined.
A review of the location of existing pipelines in the Waha area, as well as of the operating parameters of those pipelines, indicated that the logical terminus points of the system should be El Paso Natural Gas Co.'s 36-in. Waha-Ehrenberg pipeline at the west end and Lone Star's 36-in. pipeline at the east end, a distance of approximately 2.94 miles.
Because El Paso normally maintains the highest operating pressures in the Waha area, the proposed interconnecting pipeline between these two points was designated as the high-pressure header. Interconnecting the El Paso and Lone Star pipelines with the high-pressure header was seen to provide a number of advantages, including the following:
- Under the majority of operating conditions, El Paso's higher operating pressures would allow gas to be delivered into the header and, subsequently, be transported to other pipelines connected to the header without compression.
- Under most operating scenarios, gas could be delivered without compression into Lone Star's 36-in. pipeline for transport via Lone Star's pipeline system.
- Interconnecting with El Paso's 36-in. pipeline would provide transportation opportunities to California and West Coast markets and for transporting San Juan basin coal-seam gas to eastern hubs.
The high-pressure header at Waha was designed to transport a volume of 350 MMcfd between El Paso's 36-in. pipeline and Lone Star's 36-in. pipeline, assuming an operating pressure of 1,050 psig on El Paso's pipeline.
COMPRESSION
Because compression would be required for delivery into El Paso, a maximum pressure drop of 10 psig was used in sizing the high-pressure header. The parameters, Q = 350 MMcfd, P1 = 1,060 psig, P2 = 1,050 psig, L = 2.94 miles, indicated a required nominal pipe diameter of 24 in.
The MAOP of El Paso's system at the point of tie-in is 1,123 psig. Therefore, the high-pressure header design parameters were established at 24-in. diameter with a minimum design pressure of 1,123 psig.
The second interconnecting pipeline at Waha, the low-pressure header, was to provide a means whereby gas from those pipelines operating at pressures lower than El Paso's normal operating pressure of 1,000 psig can be delivered into the system without raising operating pressures to match El Paso's.
The low-pressure header extends from Lone Star's 36-in. pipeline on the east to an American Pipeline 20-in. pipeline on the west, a distance of approximately 1.05 miles. The low-pressure header was designed to transport a volume of 200 MMcfd from American Pipeline's line to Lone Star's 36-in. pipeline with a pressure drop of 10 psig and pressure parameters of 650 psig and 640 psig.
The parameters Q = 200 MMcfd, P1 = 650 psig, P2 = 640 psig, and L = 1.05 miles indicated a required nominal pipe diameter of 20 in. Under certain operating scenarios, for example, when deliveries are being made simultaneously to a number of pipelines, the operating pressure of the low-pressure header will float with the operating pressure of the 24-in., high-pressure header. Therefore, the minimum design pressure of the low-pressure header was also established at 1,123 psig.
Design parameters established for the low-pressure and high-pressure headers at Waha, therefore, were as shown in Table 1.
To provide full advantage of the operating flexibility provided by a dual-header system, compression is required between the low-pressure header and the high-pressure header. Compression will allow gas to be "wagon wheeled" between pipelines connected to the Waha header enabling gas to be delivered from a pipeline operating at a low pressure to a pipeline operating at a higher pressure.
Initial deliveries at Waha are anticipated to be provided via El Paso's 36-in. pipeline which is a source of high pressure. Therefore, compression facilities for volumes greater than 50 MMcfd are not expected to be required in the initial phases of operation.
Compression will be installed in stages, with skid-mounted packages, in order to meet changing compression requirements during the life of the project. Compression totaling 1,150 hp has been installed for 50 MMcfd at a suction pressure of 750 psig and a discharge pressure of 1,100 psig.
Details of the compression facility are shown in Fig. 3.
MEASURING STATION
Gas delivered between the Waha header and interconnect pipelines must be measured as custody transfer occurs flowing both into and out of the system. With the volume of gas to be measured (more than 350 MMcfd) at relative high pressures (up to 1,100 psig), orifice meters were the measurement method of choice.
Other measurement methods, such as turbine meters, are certainly alternatives to orifice measurement. Lone Star, however, has traditionally installed orifice meters for large-volume, high pressure measurement applications.
At any given time, gas can flow into or out of the system; therefore, measurement facilities have dual-direction flow measurement capabilities.
At the terminus points of the high-pressure header, custody-transfer measurement is required for up to 350 MMcfd into and out of both El Paso's pipeline system and Lone Star's pipeline system. At each of these two locations, measurement is accomplished through three 12-in., dual-direction orifice meter tubes with Daniel Senior orifice fittings (Fig. 4).
By installation of 0-150 in. water-column Rosemount differential pressure transducers, a volume of 350 MMcfd can be measured using three 12-in. meter tubes as opposed to four 12-in. meter tubes if more traditional 0-100 in. water column differential-pressure instruments were installed.
Volume and pressure control into and out of the header is accomplished through parallel 8-in. Fisher V-100 control valves equipped with bidirectional seals (Fig. 5). Volume control is the primary control parameter with pressure being an overriding parameter should the header pressure approach the MAOP of an interconnected pipeline.
Overpressure protection is provided by a high-pressure shutdown system which closes gas-operated block valves should the pressure-override control function fail to operate. Electrical and instrumentation within 25 ft of aboveground facilities were designed and installed in accordance with the National Electrical Code for Class 1. Division 2, Group D locations.
The design philosophy and control scheme incorporated in the measuring stations (Fig. 6) at El Paso and Lone Star will also be incorporated in those measurement facilities installed between the header and other interconnected pipelines. Each of these stations will be designed for a volume of 100 MMcfd with parallel 8-in. dual-direction orifice meter tubes and a single 4-in. Fisher V-100 control valve with bidirectional seals.
At each interconnect point, provisions are being made for installing a second, parallel 100-MMcfd station which would allow for an ultimate volume of 200 MMcfd to be delivered into or out of the header at each interconnect point.
One unique aspect of the 100-MMcfd measuring stations (Fig. 7) is that the stations will be skid-mounted and fabricated and tested in Lone Star's welding and fabrication shop in Dallas.
The skid-mounted stations, consisting of a control valve skid. two header-block valve skids. arid two 8-in. orifice meter tubes, will be shipped to the job site. There, the skids are set on concrete slabs, individual skids and components bolted together, interconnecting piping constructed, and tie-ins made at the header and interconnected pipeline.
Advantages of skid-mounted stations include a reduction in fabrication and construction costs and better quality Control utilizing shop vs. field fabrication. Stations can be stocked for immediate shipment. In addition, uniformity of facilities can be maintained throughout the system.
INSTRUMENTATION, CONTROLS
Transporting gas in today's market requires that real time operating parameters, such as pressure, volume, gas-quality parameters, valve position, etc., be known in order to allow an operator to respond to changing operating conditions and, perhaps more importantly, to maintain a balance between gas delivered into and out of the system.
The decision was made in the early stages of the project to incorporate real-time measurement capabilities into the project, to provide for remote operation of all facilities, and to provide a communication link between Waha and Lone Star's gas-control center in Dallas.
Lone Star has been installing electronic flow measurement (EFM) devices since 1983 and has developed confidence in the accuracy, operation, and dependability of such devices.
If fact, EFM devices have been installed on Lone Star's system without chart backup since 1987.
The EFM devices previously installed were, for the most part, solar powered with limited or no control of communication capabilities. Therefore, if remote control of, or communications with, a facility were required, a separate remote terminal unit (RTU) was installed.
This practice resulted in a number of stations having redundant measurement systems with one system being used for custody-transfer measurement and the second system used for the supervisory control and data acquisition (scada) system.
Given recent technological advances in EFM/RTU devices, a cost-effective option now exists for combining EFM/RTU functions into a single device. At the Waha header, a Bristol 3330 RTU is being installed at each measuring station to accomplish both custody-transfer measurement and scada functions.
Installation of the Bristol 3330's allows for the remote monitoring and control of numerous station functions from Lone Star's gas-control center in Dallas. The Bristol 3330's are being installed as standalone units without chart backup or other redundant measurement systems except check measurement installed by the other parties.
Additionally, Daniel gas chromatographs are being installed at each measuring station to provide real time heating value (BTU), specific gravity, and component analysis.
Communications, for real time system monitoring and control, between Waha and Lone Star's gas-control center in Dallas are provided via a C-band satellite network.
A satellite dish installed at Waha transmits and receives signals to and from a satellite dish in Dallas via a double-hop transmission system: from Waha to satellite and from satellite to a ground facility for the first hop, then from the ground station back to satellite and satellite to Dallas for the second hop.
In the Waha area, information is transmitted and received from individual station RTUs and sent to the satellite dish via a radio network. In Dallas, information is transferred via hardwire from the satellite dish to a mainframe computer.
Similar communication systems are in place or planned between pipeline hubs at Katy and Carthage and the gas-control center in Dallas.
CONSTRUCTION
Construction of the Waha header and related facilities began in July 1989 with a projected start-up date of December 1989. Six months is more than ample time to complete a project involving 2.94 miles of 24-in. pipeline, 1.05 miles of 20-in. pipeline, and two measuring stations, one at Lone Star and one at El Paso.
When construction was authorized in July 1989, however, right-of-way (ROW) had not been acquired, design and construction drawings had not been completed, and materials, including pipe, valves, fittings, flanges, etc. had not been ordered.
The 6-month time frame allotted for construction was in actuality, therefore, the time frame allotted in which to acquire ROW, complete design, prepare construction drawings, and order and receive materials, as well as complete construction.
Preparation of specifications, construction bidding, and construction contracts were not required as Lone Star maintains and utilizes in-house construction forces capable of accomplishing such projects.
ROW work for the project was straightforward, since Lone Star Gas and El Paso Natural Gas already owned property at the terminus points of the high-pressure 24-in. header as well as the sites for the El Paso and Lone Star measuring stations.
The low-pressure, 20-in. header parallels the high-pressure, 24-in. header for 1.05 miles. A 75-ft wide easement was obtained in which both the 20-in. and 24-in. pipelines were laid. The remaining 1.89 miles of 24 in. was laid in a 50-ft wide easement.
At potential pipeline interconnects, 200 x 200-ft lots were obtained for the installation of tap valves and measurement facilities.
Main line ROW and lots are adjacent to an existing county road; therefore, accessibility during construction was facilitated.
Material for the project was ordered concurrently with acquisition of ROW and final preparation of construction drawings.
For the high-pressure, 24-in. header, 24-in. OD, 0.375-in. W.T., API 5L-X52 pipe was available from a supplier's stock.
Because it was installed in a Class 1 location (design factor = 0.72), use of this pipe established a design pressure for the 24-in. header at 1,170 psig which was higher than the required design pressure of 1,123 psig.
A portion of the 24-in. header, in the vicinity of a plant site, was in a Class 2 location (design factor 0.60) in which case 24-in. OD, 0.500-in. W.T., API 5LX52 pipe was required to maintain a design pressure of 1,170 psig.
The low-pressure, 20-in. header was constructed in a Class 1 location with 20-in. OD, 0.312-in. W.T., API 5LX56 pipe with a design pressure established at 1,170 psig to match the design pressure of the high-pressure header.
All main line pipe was yard coated with a thin-film epoxy coating with heat-shrink sleeves used for field-joint coatings.
Cathodic protection for underground facilities will be provided by an impressed-current groundbed system to be designed and installed within the required time frame specified in DOT 49 CFR 192.455.
Construction of the Waha header and related facilities was straightforward and proceeded smoothly, considering materials were being delivered concurrently with construction activities.
A 250-ft bore was required to install the 24-in. pipeline a minimum of 3 ft beneath the lowest pipeline in a corridor containing 6, 8, 16, 20, 24, and 30-in. high-pressure, natural-gas pipelines.
As an interesting side light, the San Francisco Bay area earthquake in October 1989 delayed the shipment of valves by 4 weeks from the valve manufacturer in Oakland, Calif.
Facilities were constructed or fabricated without valves, however.
Valves were installed upon receipt in early December 1989.
Pipeline and measurement facilities were completed and tested by mid-December, and initial gas deliveries from EPNG to Lone Star via the 24-in. header were made in late-December.
Facilities currently installed and operational at Waha include the 24-in., high-pressure header from El Paso's 36-in. Waha-Ehrenberg pipeline to Lone Star's 36-in. Line X pipeline; the 20-in., low-pressure header extending from American Pipeline's 20-in. pipeline to Lone Star's 36-in. Line X pipeline; a 1,150-hp compressor; and measurement facilities at El Paso and Lone Star for delivery or receipt of approximately 250 MMcfd at each point.
Work is under way to connect additional facilities to the header.
With these facilities in service, the Waha header will be positioned effectively and competitively to serve as a major gas-transfer facility in the Waha area. Through provision of these services, the Waha header will greatly enhance the capability readily and effectively to move gas supplies from coast to coast in response to ever changing markets.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.