Warren R. True
Pipeline/Gas Processing Editor
Earlier this month, Union Pacific Resources Co., Ft. Worth, began operating its sour-gas Wahsatch gathering system in southwestern Wyoming and northeastern Utah (Fig. 1).
The project currently ties in 36.7 miles of 4, 6, and 10-in. pipeline among six wells delivering design capacity of 65 MMcfd at 1,600 psig. Two more wells adding 3.1 miles of 6 in. line will come on stream later in the project's life to maintain the design capacity (Table 1).
In addition to the pipelines, the project consists of a separation and delivery facility at the inlet of Amoco Production Co.'s Whitney Canyon gas plant and an existing flare site on the Fawcett & Sons lateral (Fig. 2).
The project was budgeted for $44 million but has exceeded that, by how much Union Pacific declines to say. Additionally, its targeted start up date was November 1993.
But it is not a simple project.
Design and construction have employed numerous corrosion protection techniques to guard against accidental release of the wet gas that carries as much as 15% H2S.
OPENING PLUGGED WELLS
The purpose of the project is to produce reserves from the Phosphoria and Weber formations of the Yellow Creek Deep and Cave Creek fields of Wyoming and Utah, to transport the sour gas and liquids to the Whitney Canyon plant for separation of liquids and gas, and to deliver the raw gas and condensates to the Whitney Canyon plant for H2S removal and processed gas to interstate pipelines and gas markets.
Marketable products include pipeline gas, NGLs, condensates, and sulfur. Produced water from the Yellow Creek Deep and Cave Creek fields is being disposed of by Amoco into an already approved, existing salt-water disposal well adjacent the Whitney Canyon plant inlet.
Union Pacific says the eight wells included in the Wahsatch gathering system permit were drilled, production tested, and shut in a decade ago.
No commercial production occurred because there was no nearby processing facility and gas reserves were insufficient to justify construction of a separate processing plant.
At that time, Amoco considered a gathering system that would have been nearly identical to what Union Pacific has constructed, but only recently has declining production in other fields opened up long-term processing capacity in Whitney Canyon.
GATHERING SYSTEM
The Wahsatch gathering system consists of 24.8 miles of seamless 10.75-in. OD x 0.438-in. W.T. X-52 line pipe. Additionally, the line consists of 11.4 miles of seamless 6.625 in. OD x 0.280-in. W. T. and a 0.5-mile extension of seamless 6.625-in. x 0.280-in. W.T.
Later in the project, a 3.1-mile extension of seamless 6.625-in. OD x 0.280-in. W.T. is planned for future production along the 6.625-in. Kindler-Reese lateral and Champlin No. 375C-1 well.
The gathering system currently collects sour gas from six wells which Union Pacific purchased in 1991 for an undisclosed amount from Amoco Production: the Harry Moon No. 1, Bradbury No. 1, Urroz No. B-1, Urroz No. 1, Celsius No. 4-36, and the Fawcett & Sons No. 1 (Fig, 2).
An accompanying box shows the gas composition flowing on the gathering system as well as that for the fuel-gas line running with it.
At peak capacity, as stated, the pipeline delivers 65 MMcfd at 1,600 psig; maximum allowable operating pressure (MAOP) is 1,760 psig. Although early delivery-facility pressures have ranged from 850 to 1,760 psig, pressures later in the anticipated 20 year life of the system will decline to 300 psig.
The sour-gas lines are laid in 8-ft ditches to depths of at least 6 ft at the top of the pipe, well deeper than usual even for sour-gas lines (Fig. 3).
At a depth of at least 4 ft and directly over the sour-gas line, Union Pacific has laid a 3 and 4 in. fuel-gas line for most of the project's length. This line supplies heater fuel gas and control gas to system components. The sweet gas for this line is delivered in the line by Amoco at the Whitney Canyon plant inlet.
Parallel with the fuel-gas line was laid the fiber optic cable that carries signals to the system's valves and emergency shutdown (ESD) system components.
Union Pacific believes this configuration provides a margin of safety against third-party damage to the main line.
Also built with the pipeline were 17 heaters, 6 located at wellsites, 8 in the main line, and 3 at the delivery facility. These heaters are part of the flow-control strategy to inhibit hydrate formation.
Fifty-eight ESD valves have also been installed, 30 at wellsites, 12 at line heaters, 7 at main line valves, 5 at the Wahsatch flare site, 3 at the Urroz junction, and 1 at the Wahsatch delivery site.
The fuel-gas line includes 33.1 miles of newly installed pipeline: 24.8 miles of 4 in. and 8.3 miles of 3 in. line.
Another portion of the fuel-gas line, about 6.7 miles of 6 in., was already in place between the Wahsatch flare site and the Harry Moon well as part of the Wahsatch flare test system.
Along this portion of the route, the sour gas line has been laid in a completely separate ditch.
The fuel-gas line, as stated, supplies fuel to equipment located along the sour gas line at both heater and ESD valve sites and also at the well sites. It Operates at 250 psig at a rate of 2 MMcfd and is equipped with manual and automatic block valves.
The 4-in. line carries sweet fuel gas from the inlet to the Whitney Canyon plant south 24.8 miles to the Urroz junction. There, it ties into an existing 6-in. line.
During July 1992, this 6-in. line was hydrotested for 24 hr at 3,300 psig to ensure its integrity. This 6-in. line travels 6.7 miles to the Harry Moon wellsite and southwest to the Wahsatch flare site.
At the Wahsatch flare site, this existing line ties into a new 3-in. line which travels nearly 5 miles south to the Fawcett & Sons wellsite.
The newly installed portion of the sweet-gas line is laid in the same ditch, as stated, and approximately 2 ft above the sour gas line so that if unauthorized trenching or digging occurs, the sweet-gas line would be encountered before the sour-gas line would be exposed or damaged.
A safety warning ribbon has been laid in the trench 2 ft above the sweet-gas line for the same purpose.
At special crossings, the two lines usually lie side by side in the trench or casing to provide separate independent support for each of the lines.
A separate ditch has been made for the sour-gas line where it runs parallel to the 6-in. existing line.
FLARE, DELIVERY SITES
The Wahsatch flare is on the Fawcett & Sons lateral 3.5 miles southwest of the Urroz No. 1 wellsite in Summit County, Utah.
The flare site already existed and was used during original well-production testing 10 years ago. During well recompletion, a temporary 90-ft flare stack was used for initial production testing of the Harry Moon, Bradbury, Urroz B, and Urroz No. 1 wells. The flare site was used in September 1992 for production testing of the Urroz No. I well.
During system operation, the flare site will be used in an emergency shut down to allow depressurization of the gathering line.
Currently at this site are a new flare stack, 95 ft high, a 30,000-gal knockout drum, and an in-line heater. The heater is designed for a duty of 1.0 MMBTU/hr and a design temperature of 170 F.
The 132 in. ID, 44-ft flare knock-out drum is used for knocking out liquids from the sour gas as it is blown down through the flare in an emergency.
Liquids accumulated during flare-line pigging will be collected in the flare knockout drums and hauled from the flare site and redelivered to the Whitney Canyon plant inlet area.
This site also includes three ESD valves for the sour-gas system plus two ESDs on the heater and one for the fuel-gas system.
The Wahsatch delivery facility sits adjacent Amoco's Whitney Canyon gas-plant inlet and about 16 miles northeast of Evanston, Wyo. This location includes a flare, 2,600-bbl slug catcher, tankage, separators, pumps, heaters, and a small control room.
Three buildings at the delivery facility include a pump building; a building housing the sour-water stripping vessel, the inlet-gas separator, and the inlet-liquid separator; and a control building housing telemetry equipment, warehouse space, and a remote field office.
A main line pig receiver precedes a 42-in. OD, 1,700-ft slug catcher capable of holding 2,600 bbl. Its purpose is to intercept the large amounts of liquids that arrive during the regular pipeline pigging operations.
Once pigging is complete, the fluid from the slug catcher is pumped through the condensate heater and then on to the liquids separator. The condensate heater requires a duty of 3.0 MMBTU/hr.
The liquids from the slug catcher are processed in the 72 in. OD x 30-ft liquids separator that removes remaining sour gas from sour water in the liquids.
At this point the condensate moves to the nearby plant for further processing, and the sour water is treated in a sour-water stripper (16 in. OD x 21 ft high). The water is then either stored in one of the two available 1,000-bbl storage tanks or pumped directly to Amoco for disposal, as previously mentioned.
Gas coming off the slug catcher is treated in the inlet-gas separator (60 in. OD x 20 ft tall). From this point, the gas is heated and sent to Amoco for further processing. The gas heater requires a duty of 1.5 MMBTU/hr.
The building housing these components is equipped with H2S sensors, hydrocarbon-gas detection equipment, and warning lights.
In an emergency, the gas flows through the flare knock-out drum (12 in. OD x 35 ft long) to remove any liquids before flaring.
The 30 in. x 130 ft tall flare has a blow down rate of 175 MMcfd.
CORROSION PROTECTION, SAFETY
The entire Wahsatch gathering system is a Class 1 location as defined in ANSI B31.8 and U.S. Department of Transportation (DOT) 49 Code of Federal Regulations (CFR) Part 192.
DOT requires an area of 220 yd on either side of the centerline of the pipeline be surveyed for buildings for human occupancy. Class I locations are intended to reflect such areas as waste lands, deserts, rugged mountains, grazing land, farmland, and sparsely populated areas that have 10 or fewer buildings for human occupancy along any 1-mile section.
Because of the toxicity of the sour gas, Union Pacific has designed the system to exceed Class I requirements for valve spacing, burial depth, design pressure, corrosion allowance, and pipeline monitoring.
The design safety factor of the pipeline has been improved beyond current standards for wall thickness, external coatings, and overall system operation.
Additionally, pipeline depth and valve locations are also affected by the class location. A Class 1 location requires pipeline valves to be located within 10 miles of each other.
Union Pacific has designed 25 automatic ESD valve locations into the gathering system. The maximum valve distance is 5.3 miles, with a minimum valve distance of 300 ft.
PIPES, PROTECTION
The burial depth required by DOT in Class 1 service is 30 in. to the top of the pipe. Minimum depth of burial for the fuel gas line for the Wahsatch system is 30 in. to the top of the pipe, as stated earlier.
As an additional safety precaution, Union Pacific required a minimum burial depth of 72 in. to the top of the sour-gas pipe. This is to ensure the integrity of the system and avoid unintentional pipeline damage.
Table 2 compares DOT and Union Pacific standards for design pipewall thickness.
The seamless 4, 6, and 10-in. X-52 pipe used met the requirements of the latest edition of API 5L, "Specification for Line Pipe." It has a maximum carbon equivalency of 0.40. Table 3 shows the required chemical composition of the pipe steel.
Steel pipe was manufactured by Mannesmann S.A. in Belo Horizonte, Brazil, specifically for the Wahsatch pipeline.
A set of Charpy impact tests was performed at -20 F. on one length of pipe from every 100 and with a minimum average impact energy of 30 ft-lb with no single value less than 25 ft-lb.
Pipe wall thickness tolerances are +15%, -12.5%. And Rockwell B hardness is between 74.0 minimum to 85.0 maximum and does not exceed a 22 HRC ASTM STD E 18.
Maximum hoop stress, calculated according to DOT CFR 49 Part 192, is 3,050 psig for the entire sour-gas system.
Union Pacific's system design includes proven control technology and practices for an H2S environment.
Internally the pipeline would be protected by use of corrosion-inhibition fluids, increased pipe wall thickness, specially manufactured, H2S-resistant, low carbon X-52, seamless steel pipe and fittings, and periodic pigging of the pipeline to prevent fluid buildup.
Regular injection of corrosion-inhibition fluids will reduce internal corrosion. How much corrosion-inhibitor is injected and how often are determined once the pipeline is fully operational and based upon observations and measurements of the system.
Union Pacific included a 0.0625-in. internal corrosion allowance in the pipeline design. An additional wall thickness of 0.0625 in. was included for external corrosion at all cased crossings.
CORROSION MONITORING; HEATERS
Corrosion monitoring spools have been installed upstream of the wellsite heaters and line heaters on the pipeline and immediately upstream of the Whitney Canyon delivery facility.
The spools are located on the bypass into the heaters so that the main pipeline can be periodically pigged.
Coupons are located in the spool upstream of the chemical injection points.
In addition to the coupons, access fittings have been installed in the spool to allow for other types of internal monitoring devices such as electric resistance probes, hydrogen probes, and flush probes.
A conventional coupon retainer assembly has been used that allows coupons to be inserted and retrieved under pressure so that the line does not have to be shut-in and loss of sour gas is held to a minimum.
Because hydrate formation can plug the line and stop flow, heaters are necessary to keep the pipeline at a temperature above hydrate formation. Operating temperatures range from 70 to 170 F. with normal temperatures at 140 F. or cooler. Gas temperature is maintained at greater than 90 F.
Seventeen heaters along the gathering system are fueled by gas from the tailgate of the Whitney Canyon plant. Total heater load for the system is approximately 42 MMBTU/hr.
The heaters are located at all wellhead sites, accounting for six heaters. The designed duty requirements for these heaters is 3.3 MMBTU/hr, with the maximum designed heating temperature for all heaters at 170 F.
The pipeline coatings can withstand temperatures to 180 F.
There are also 8 in.-line heaters along the pipeline whose duty ranges from 1.0 to 1.3 MMBTU/hr.
Three heaters are located at the Wahsatch delivery facility: one to heat the sour-gas condensate requiring 3.0 MMBTU/hr, oi.e to heat the sour gas requiring 1.5 MMBTU/hr, and one to provide space heating within the process and metering building requiring 1 MMBTU/hr.
Heater units plus a small building to protect equipment are skid-mounted.
CROSSINGS; CONCRETE COATINGS
Casing has been installed at three points along the gathering system.
One of the cased crossings is the Union Pacific railroad. The others are highway crossings at Interstate 80 and Utah State Highway 16.
The cased highway crossings have been installed to comply with Utah Department of Transportation requirements.
The carrier pipe will be concrete coated and placed inside the 30-in. casing. Concrete coating of the carrier pipe will cathodically insulate the two lines and prevent the development of an electrical short.
A cathodic protection test station has been installed at both ends of the cased crossing.
The 10.750 in. sour-gas pipe has been concrete coated for a length of 80 ft for installation at the Chapman Canal crossing and 4,880 ft at the Woodruff Narrows crossing.
A minimum concrete coating thickness of 4 in. has been applied to the exterior of the insulated 10-in. line. This coating is necessary to prevent the pipeline from floating in submerged areas.
A concrete coating was chosen instead of weights to reduce crushing the insulation underneath. The weight is evenly distributed instead of being localized at one point.
The 4 and 6-in. sour gas pipelines have also been concrete coated through all water crossings. The 4-in. line required 1-in. concrete coating; the 6-in. line, 2 in.
PIGGING; EXTERNAL COATINGS
At the Fawcett & Sons well site and at the Harry Moon well site are located 6-in. pig launchers; at the Urroz junction location, 6-in. pig receivers and a 10-in. pig launcher. A 10-in. pig receiver is located at the Wahsatch delivery facility.
In November and December 1993, Pipetronix Inc., Houston, ran an ultrasonic inspection pig through the 6 in. and 10-in. main lines to verify pipeline geometry and provide a base line for future runs for comparison of corrosion rates.
Inspection pigging was unnecessary for the fuel-gas line because the gas is sweet and dry.
After this initial survey, the inspection pig will be run once a year the first few years of operation to assess the corrosion-control program for the line. Future frequencies of smart-pig runs depend upon the amount of corrosion observed.
All records from the inspection-pigging process are being retained for the life of the pipeline.
By routinely pigging the sour-gas line, Union Pacific can also greatly reduce liquid build-up, particularly in low lying areas of the pipeline.
The sour-gas pipeline has been externally coated by two systems (Fig. 4):
- A fusion-bonded epoxy coating has been covered by Pritec 1040, a polyethylene wrap over a proprietary butyl mastic. The polyethylene gas applied to a thickness of 40 mils, the butyl mastic 10 mils, both supplied by Energy Coatings Inc., Houston, and applied in Fontana, Calif., and Harvey, La.
Over these, a polyurethane-foam insulation has been applied. The insulation is necessary to maintain the gas stream's temperature to prevent hydrates forming.
- A final layer of high-density polyurethane has been applied over the insulation to protect it. On the 10-in. sour-gas main line, 2 in. of high-density polyurethane will be applied.
When necessary, as stated earlier, concrete coating has been applied over the insulation for installation at cased crossings or at water crossings.
In October 1993, the sour gas and fuel-gas pipelines were hydrostatically tested to ensure their integrity before start-up. The minimum test pressure for the sour-gas line was 180% of the MAOP of 1,760 psig.
VALVES, FITTINGS
All the system's ball valves were manufactured for sour gas service to meet requirements in API Specifications 6D and 6A, as well as NACE MR-01-75.
All components of the valves resist sulfide stress cracking and are designed to operate between -50 F. and 180 F.
The valve bodies were forged and manufactured from AISI 4130, 8630, or 2 1/2 Cr-1 Mo steel. The valve balls were supplied by American Energy Services Co., Houston, and made from solid Incoloy 825 or carbon-steel weld overlaid with a nickel, chromium alloy.
Union Pacific required the valve supplier to meet the following specifications:
- Impact tests were performed at or less than - 50 F. and exhibited a minimum average value of 15 ft-lb in the transverse direction or 20 ft-lb in the longitudinal direction.
No test was acceptable if a single value is less than 12 ft lb in the transverse direction or 15 ft-lb in the longitudinal direction.
- The tensile properties were tested with a minimum of one specimen for each heat-treatment lot. All metallic components of the valves were also function tested to ensure they could be opened and closed against differential hydrostatic pressure.
- All valve parts and components were capable of complete traceability. If any part of the valve was not traceable, that component was rejected.
There are 58 ESD valves at 25 locations along the sour-gas system; these include the line heaters, wellheads, the Urroz No. I junction, main line valve locations, the Wahsatch flare site, and the Wahsatch delivery facility.
Five ESD valves are located at each of the six well sites. Each of the line heaters has two ESD valves placed to allow maintenance on the heater.
The purpose behind the valve design is to limit how much sour gas could be released into the atmosphere in an emergency. An ESD valve will close if abnormal pressures appear in the gas line, if for example pressures exceeded preset high or low pressure conditions at any individual valve location.
Sour gas line ESD valves are designed to "fail" in the closed position.
The basic design is also used on the sweet-gas line, but these valves activate in lost pressure. The sweet-gas valve is designed to "fail" close.
VALVE SPACING; FIBER OPTICS
According to DOT 49 CFR Part 192, as stated earlier, valve spacing within a Class 1 location shall not exceed 10 miles from any point along the pipeline. The spacing between any two valves along the Wahsatch system varies from a maximum of 5.3 miles to a minimum of 300 ft.
Union Pacific says its control instrumentation will constantly monitor the pipeline with air monitors located at all aboveground installations to detect H2S gas.
Alarms are activated if H,)S concentrations reach 10 ppm. At 40 ppm H,S, the ESD valves automatically activate to their designed "fail safe" closed positions.
Automation of the system is controlled by its supervisory control and data acquisition (scada) system which monitors operating parameters and output from monitors.
The scada system is in communication with each site through a remote telemetry unit (RTU) and a programmable logic controller (PLC). The PLCs monitor local site conditions and report to the master station which is in communication with the scada system.
AU fittings manufactured for sour-gas service meet the requirements of MSS SP-75 "Specification for High Test Wrought Butt Welding Fittings" and NACE MR 01-75. Their chemical composition and carbon equivalency are similar to those used in the main line pipe.
Union Pacific required that the primary fitting suppliers distributor National Oilwell for manufacturer Texas Forge & Fitting, both of Houston-meet the following specifications:
- The hardness of the fittings was to be measured with requirements of Rockwell hardness B 72.0 minimum and HRB 85.0 maximum.
- Charpy impact tests were to have been performed at - 50 F. with a minimum average impact energy of 25 ft-lb. No single value from the tests was to be less than 20 ft-lb with a minimum shear area of 85%.
- 100% visual and non-destructive examinations were to have been performed on the fittings.
The fiber optic line installed in the pipeline trench links all sensors, ESD valves, and system control components with the Wahsatch control room in Evanston.
The fiber optic tine, laid at the side of the trench at the same depth as the fuel gas tine, has been installed within a protective conduit consisting of a layer of polyethylene, a steel jacket, and an outer layer of polyethylene.
MONITORING; EMERGENCY SHUT DOWN
A scada system monitors the operating parameters of the pipeline and associated facilities and regulates critical equipment. This includes the ESD valves and the H2S sensors.
The scada system consists of a master terminal station and redundant backup station in Union Pacific's office in Evanston where they are monitored 24 hr/day.
The system uses PLCs to determine field conditions. These monitor conditions at designated locations and report these conditions to the master terminal through a continuous scanning process.
PLCs are stationed at all wellsites, heaters, ESD valves, and flare sites.
In an upset of pipeline conditions at a specific location, the PLC can close valves and sound alarms without relying on feedback from the master station before performing preprogrammed functions.
In the event of lost communication between the PLC and the master station, the available redundant communication paths would be used. If power were temporarily lost or all communications or the computer system failed, the ESD valves would shut down part or all the pipeline.
To detect leaks along the pipeline, the scada system monitors pressures, field equipment, and process control. if a probable leak were detected, alarms would sound and valves surrounding the leak would close.
Any immediate upstream wells affected by the valve closures would be shut-in.
A major component and safety feature of the automatic shut down system is the number of ESD valves included in the system. The 58 valves allow Union Pacific the flexibility to isolate a small portion of the line or to shut-in the entire system, depending upon the severity of the problem.
The valves will begin the shut-in process under a variety of conditions:
- High, low pipeline pressure. A shut down process may be initiated by abnormal pressures experienced in the pipeline or at a wellsite. Unusual conditions would be determined by preset high or low pressures at the valve.
If unusual operating conditions were present, the automatic shut down process would begin.
Having all of the emergency valves designed to fail close further increases the safety factor. In an emergency shut down, the flares at the Wahsatch delivery facility and at the Wahsatch hue site provide a means to depressurize the pipeline.
If pressures deviate 10% from the normal operating pressure range, the line will shut down.
- Fusible Links. Another safety feature incorporated in the pipeline design is the use of fusible links at the well-sites.
In fire or exposure to extreme heat, these links will melt and create a pressure loss in the control line that will cause the downhole valve to shut in the well.
This same configuration has been installed at heater locations as well.
- H2S sensors. Valve closure will be activated if H2S gas is detected at 40 ppm along the pipeline, at any well or heater, the flare, or the delivery site. The gas would be detected by the H2S sensors.
Every wellsite has a total of seven sensors in place, one at each corner, one at the wellhead, one at the heater vent, and one in the heater building.
Every heater location has sensors in all comers, one at the heater vent, and one in the building. All casing vents have one sensor. Several sensors are located at the Wahsatch delivery facility, and a sensor is located at each main line ESD valve.
If H2S gas is detected at concentrations of 10 ppm, the sensors trigger alarms at the wellsite and in the control room. At H2S concentrations of 40 ppm, the valves begin the automatic shut-in process which cannot be overridden from any other location.
- Lower explosive limit sensors. Lower explosive limit sensors are tied into the scada system at heater buildings and delivery process buildings. Their purpose is to detect combustible gas, and they are used by scada to provide for emergency shut down.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
