26.6.3 Detailed Pipeline Design and Engineering

The two largest tasks associated with cross-country pipeline design are selecting the route and choosing the pipe diameter, wall thickness, and materials strength.

Route Selection. Engineers start with a straight line between the specified beginning and ending points and then begin looking for geographic, topographic, ecologic, social, and political obstacles. These obstacles include water bodies, population centers, severe topographic features (e.g., mountains or canyons), commercial or industrial development, and sensitive environmental and cultural locations. Generally, the route is selected based on limiting the total pipeline length, offset by constructability and permitting.

Cross-country pipeline construction is most often contracted on the price per foot of pipe installed, with adders for river, stream, road, and other crossings, as well as nontypical construction. Accordingly, after several preliminary routes are identified, the cost of the project is evaluated by estimating the different types of installations required and the length and cost of each. For example, constructing across farm pasture or fields, constructing up a steep slope, crossing waterways or highways, and constructing in congested suburban or urban areas are increasingly more costly on a per-foot basis. Classifying and identifying the degree of construction difficulty, as measured by estimated cost, allows the engineer to compare different route alternatives on a total-installed-cost basis and to select the most cost-effective route for further analysis.

With several routes preliminarily selected, the engineer works with land specialists, identifying and classifying sections of the pipeline along the route as

- Rural undeveloped (agricultural, grazing lands, crop lands, etc.)

- Residential developed

- Residential undeveloped

- Industrial

-Parks or recreational

- Reservoirs and water storage

- Forested lands

Together, the engineer and land-acquisition specialist develop the project implications associated with each of these land classes; difficulty and cost of acquiring easements, public sentiment, constructability, and often other factors are considered.

Advantageous features for pipeline routing include

• Existing utility corridors

• Existing roads that may accept pipeline installation

• Access for transport of workers and materials for construction and maintenance personnel for later operation

• Level or near-level terrain

Features to avoid or mitigate include

• Major freeways

• Residential areas

• Parks and open spaces

• Earthquake fault lines

• Areas subject to flooding

• Other unstable geologic areas

• Highly rugged terrain and side slopes

• Highly environmental sensitive areas

With all these factors considered, final route selection is made on the basis of project economics, construction availability, and environmental and permitting analysis.

Permitting. Permits are authorization by a governmental agency to cross or otherwise impact country, state, or locally managed resources or utilities. Securing permits generally is separate and apart (but closely connected to) securing rights-of-ways (ROW), way leaves, easements, and private access rights required for pipeline installation.

In many cases, there is no single permitting process. Each country, state, or local government has one (or more) permitting processes. Major projects usually require extensive studies to identify their environmental, cultural, social, and economic impact. In the United States, for example, when projects will cross federal lands or otherwise require federal action for implementation (such as a FERC permit), they require extensive environmental review as provided by the Environmental Protection Act of 1970. The review process may require one of two levels of environmental analysis: environmental impact statement or environmental analysis. Other counties have similar processes. Countries that do not have their own process may follow a World Bank process and use World Bank guidelines.

Another common permitting requirement for pipeline construction and installation in the United States is the Clean Water Act, Section 404 permit, administered by the U.S. Army Corps of Engineers. This permit is required any time a water body of the United States is crossed, and this includes most water bodies, rivers, and streams, including some that are intermittent. Additionally, if sensitive wildlife or aquatic environments are impacted, the U.S. Fish and Wildlife Service may be involved based on the prospective impact of the project.

Other permit requirements may include archeological sites, usually administered by a state historical administration, stream and river crossing state permits (Department of Environmental Quality), railroad crossing permits, permits for crossing other utilities, highway and road crossing permits, and local building or access permits. A pipeline is a very long facility that crosses many miles, and the associated permitting is usually complex and involved.

Sizing the Pipe. Selecting the optimal line diameter and wall thickness begins with the flow-rate design basis and fluid properties. Receipts into the line, deliveries out of it, and any other factors that define the quantity and characteristics of fluids are all considered as the design engineer, using hydraulic software, models line performance to determine the appropriate pipe size in combination with compressor (or pump) unit sizing and station location.

The single most important variable in pipeline sizing, friction loss owing to fluid movement, is directly related in an exponential fashion to the diameter of the pipe. (Recalling that the velocity component in the Darcy-Weishbach formula is squared explains the exponential relationship.)

Selecting the optimal pipe size requires evaluating:

- Pipeline hydraulics to determine total system pressure requirements

- Maximum allowable operating pressure (MAOP) to ensure safe operating conditions

- Pump or compressor station sizing to determine horsepower requirements

- Station locations and number to determine total energy requirements

These operations are all performed through computer modeling, but conceptually (and what was done prior to the advent of computers), the system resistance curve from Figure 26.7 is being overlaid with the pump curve from Figure 26.31 to establish the operating point for the pipeline (Figure 26.40). The system resistance curve changes as pipe diameter and fluid properties change. Compressor or pump curves are matched to the resulting systems resistance curves to establish the best range of operating points to meet the established design parameters.

FIGURE 26.40 System resistance curves for 16- and 20-inch pipelines overlaid with a centrifugal pump curve. (Source: Oil and Gas Pipeline Fundamentals Course, Courtesy Pipeline Knowledge & Development.)

The MAOP line shown in Figure 26.40 is established through the use of Barlow’s hoop stress formula:

MAOP = (2t * SMYS)SF/D

where t = wall thickness

SMYS = specified minimum yield strength of the pipe parent metal

D = outside diameter of the pipe

SF = safety factor established through standards or codes

Based on various combinations of line sizes and properties, as well as energy requirements, cost estimates demonstrating the trade offs between line pipe diameter, wall thickness and steel strength, and number of pumping or compressor stations required to achieve the design basis flow rate are prepared (Table 26.2).

TABLE 26.2 Unit Cost Comparison Pipe Size versus Station Trade-Off

12-inch 16-inch 20-inch

Total pressure required 6,000 psi 2,000 psi 700 psi

Number of stations required 5 2 1

Pressure per station (balanced system) 1,200 psi 1,000 psi 700 psi

Pipeline investment $12,000,000 $16,000,000 $22,400,000 (materials and construction)

Station investment $5,550,000 $2,100,000 $1,000,000 (materials and construction)

Total investment $17,550,000 $18,100,000 $23,400,000

Annual operating costs

-Salaries, wages, station operation, $686,000 $353,000 $244,000 power, maintenance, etc.

Depreciation (annual cost of the investment)

- Pipeline @3% $360,000 $480,000 $672,000

- Stations @4% $220,000 $84,000 $40,000

- Property taxes @1% $175,000 $181,000 $234,000

Total annual costs (present value) $1,441,000 $1,098,000 $1,190,000

Annual cost per unit of capacity $0.079 $0.060 $0.065

Source: Adapted from “Hydraulics for Pipeline Engineers,” unpublished. Courtesy of Vanderpool Pipeline Engineers Inc.

The capital cost of line pipe is compared with the operating costs and capital costs of the stations. As line-pipe size increases, the cost of the pipeline material and construction can increase significantly. As line-pipe size increases, however, the friction losses decrease greatly, as do the requirements for compressor or pump stations. The economical line size, where costs are minimized considering both construction costs and operating costs, is chosen as the solution for continued engineering design. As shown by Table 26.2, pipeline diameter is critically important in the flow-resistance calculations. It also has a large impact on the flow-rate capacity.

Wall Thickness and Material Strength. From the MAOP calculations, it is clear that pipe wall thickness and steel strength are critical elements in the structural capability of the pipe to hold pressure. The pipe behaves as a thin-walled pressure vessel, with the pressure inside the pipe trying to push the pipe outward in every direction, thereby stressing the steel. The strength of the material resists the tensile force applied by the pressure, and the thickness of the material provides the structure to resist the applied pressure. Table 26.3 shows the MAOP range for a 20-inch-diameter, 0.375-inch-wall thickness for various steel grades. Table 26.4 shows the MAOP range for a 20-inchdiameter X-60 pipe for various standard wall thicknesses.

TABLE 26.3 Comparison of Grade and

MAOP

Grade MAOP MAOP (psi) (kpa)

X-42 1,134 7,819

X-52 1,404 9,680

X-56 1,512 10,425

X-60 1,620 11,170

X-70 1,890 13,031

Source: Adapted from Oil and Gas Pipeline Fundamentals Course. Courtesy Pipeline Knowledge & Development.

TABLE 26.4 Comparison of Wall Thickness and MAOP

Wall Thickness MAOP MAOP (in) (psi) (kpa)

0.218 942 6,495

0.250 1080 7,446

0.375 1620 11,170

0.500 2160 14,893

Source: Adopted from Oil and Gas Pipeline Fundamentals Course.

Courtesy Pipeline Knowledge & Development.

Pipe generally is priced by the ton, so it seems the best solution is always to buy the higheststrength (and thinnest walled) pipe. But higher-strength pipes generally are less ductile and require different welding process than lower-strength pipes. Additionally, if the diameter-to-wall-thickness ratio (D/t) is too large (􀀞 80 is a general rule of thumb), the pipe may not have the structural integrity to maintain its roundness during handling and many not adequately support the weight of backfill materials. Two other useful rules of thumb: Always use a wall thickness greater than 0.250 inch (6.35 mm) for buried pipe and 0.375 inch (9.53 mm) for above-ground pipe to mitigate the risk of outside force damage.

Optimization: Balancing the Factors. Oil and gas pipeline design and engineering require balancing many critical factors. Pipelines are expensive and permanent. Making wise choices regarding the proper line sizing requires the best possible balancing of foreseeable economics and creating options for an unpredictable future.

Generally, if there is any possibility of expansion requirements in the future beyond current foreseeable needs, it is wise to install or construct the largest possible diameter pipeline that can be justified. Because of the physics of friction loss, flow-rate capacity is gained exponentially with incremental increases in pipe diameter. This has the impact of increasing flow rate exponentially while increasing construction costs only linearly, so an incremental dollar spent on larger pipe diameter will have a greater than unity effect on flow-rate capacity. This best positions the pipeline for expansion years into the future because the reasonably expected life of a steel pipeline system that is maintained property can be 50 to 100 years.

Looping and Future Expansions. When the flow-rate capacity of an existing pipeline becomes a limiting factor in the business of the operating company, the capacity can be expanded in several ways. One is looping, or adding a second pipeline parallel to the first. From an engineering perspective, this is analogous to an electrical circuit problem, when looping the conductor allows the same current flow at half the resistance, or twice the flow at the same total resistance. Natural gas is a largely homogeneous mixture, so looping is a common expansion solution. However, looping of batched liquids lines is seldom undertaken because loops allow increased mixing at the interface between the various grades of refined products or types of crude oil. The first expansion of liquid lines is commonly accomplished through adding chemical drag-reducing agents (DRAs), which reduce the friction loss per mile by reducing turbulence within the fluid.

Oil Pump Station Engineering and Layout. Oil pipelines require significant amounts of energy to overcome friction loss and push oil up and over elevation changes. Pump stations, consisting of one or more pump units, provide this energy. For cross-country pipelines, the pumping units are usually multistage centrifugal pumps driven by electric motors. Where a stable and substantial electrical grid is not available, the pumps may be driven by natural gas turbines or even by engines fueled with diesel, crude oil, or natural gas.

The first pump station on the pipeline is referred to as the origination station, and subsequent stations are called booster stations. Origination pump stations normally are part of a larger receipt station, including meter stations that measure the oil before it enters the pump units. Some stations are designed to either originate or boost depending on the need (Figure 26.41).

FIGURE 26.41 Oil pump station. One grade of oil is received into one of the tanks from local crude oil gathering, whereas another grade of crude oil originating upstream is boosted by this station. When the local gathering tank is full, flow from upstream is diverted into the other tank, and this station originates the local oil into the pipeline. (Source: Oil and Gas Pipeline Fundamentals Course. Courtesy Pipeline Knowledge & Development.)

FIGURE 26.42 Schematic of a pump station. Either pump can be used individually, or both pumps can be used in series. (Source: Oil and Gas Pipeline Fundamentals Course. Courtesy Pipeline Knowledge & Development.)

Almost all liquid pump stations employ centrifugal pumps arranged in series; that is, the discharge of one connects to the suction of the next. Piping is arranged so that either or both pumps can be used (Figure 26.42). In the case of series operations, the flow rates through each pump are the same, and pressures are additive.

The system rate depends on the discharge pressure from the pump station, which is controlled with either a control valve or a variable-speed controller. Pump station control loops monitor the overall station as well as individual unit, suction, and discharge pressures. Programmable logic controllers (PLCs) compare these pressures to the setpoints programmed into the PLC for each pressure and make adjustment to the control valve or variable-speed controller as required, thereby maintaining station pressures within the setpoints.

Practical pipe wall thickness economics dictate that pump stations typically are designed to discharge between 1,400 and 2,500 psi (9,653 and 17,240 KPa), and typical pump station spacing ranges between 30 and 100 miles. When pipelines are forecast to gain increased volume over their life, the originating and then every other station may be constructed initially. Midpoint stations are added later between the original stations as pipeline volumes increase. As a rule of thumb, adding midpoint station increases capacity by the square root of 2 or about 40 percent.

Pump station mechanical designers consider flow and pressure requirements as they select the optimal pumps and drivers. Then they lay out station piping for efficiency and ease of maintenance, all the while working with the other engineers on the team, including instrumentation, controls, and electrical power engineers, to achieve station and pipeline control.

Natural Gas Compressor Station Engineering and Layout. Analogous to liquid pipeline pump stations, natural gas compressor stations are installed on gas pipelines to boost pressures. But the compressible nature of natural gas means that in addition to overcoming friction loss, compressor stations also compress the gas, increasing its density so that it can be moved more efficiently. Natural gas at 1,200 psi (8,273 kPa) and 60􀁮F, for example, is 100 times more dense than at standard temperature and pressure. Compression requires power and generates heat, both of which are considered as engineers select equipment and layout stations.

The compressible nature of natural gas introduces the compression ratio, a concept not found in pump selection or pump station design. Compression ratio is the ratio of inlet to outlet pressure. A compressor with 100 psi (689 kPa) inlet and 300 psi (2,068 kPa) outlet pressures has a compression ratio of 3, for example. Higher compression ratios mean higher heat generation and higher forces on compressor parts. Compression ratios of between 1.5 and 2 are common, but origination stations generally have higher compression ratios because they compress gas from gathering up to transmission pressures. Compressors are arranged either in parallel or series depending primarily on the compression ratio needed.

Positive-displacement compressors driven by gas engines for years have been the compression strategy of choice. Centrifugal compressors, driven by gas turbines or electric motors, however, are gaining popularity owing to their generally lower initial costs.

Use of electric motors have the added attraction that the emissions they generate are released at power plants and not at the station site. Of course, using electric motors rather than gas engines means total emissions are increased owing to electrical generation and transmission inefficiencies.