Pipeline design consideration and standards

Pipeline design consideration and standards

Wall thickness calculations - comparisons

Additional comparison of Standard B31.3 to both B31.4 and B31.8 indicates the following:

*ANSI/ASME Standard B31.3 is more conservative than either Standard B31.4 or B31.8, especially relative to API 5L, X-grade pipe and electric-resistance-welded (ERW) seam pipe.

*ANSI/ASME Standard B31.8 does not allow increases for transient conditions.

*The ANSI/ASME Standard B31.3 specification break occurs at the fence, whereas B31.8’s occurs at the “first flange” upstream/downstream of the pipeline.

Using ANSI/ASME Standard B31.3 criteria for oil- and gas-facility piping will assure a very conservative design. However, the cost associated with the Standard B31.3 piping design may be substantial compared to the other codes and may not be necessary, especially for onshore facilities.

Velocity considerations

In choosing a line diameter, consideration also has to be given to maximum and minimum velocities. The line should be sized such that the maximum velocity of the fluid does not cause erosion, excess noise, or water hammer. The line should be sized such that the minimum velocity of the fluid prevents surging and keeps the line swept clear of entrained solids and liquids.

API RP14E provides typical surge factors that should be considered in designing production piping systems. These are reproduced in the following table (Table 9.16).

Table 9.16 – TYPICAL SURGE FACTORS FOR TWO-PHASE-FLOW PIPELINES (Courtesy of API)

Service Factor, %

Facility handling primary production from its own platform 20

Facility handling primary production form another platform remote

Well in less than 150 ft of water 30

Facility handling primary production from another platform or remote

Well in greater than 150 ft of water 40

Facility handling gas lifted production from its own platform 40

Facility handling gas lifted production from another platform or

Remote well 50

For additional information, see API RP14E

Liquid line sizing

The liquid velocity can be expressed as

V=0.012 QL/d (Eq.1)

Where:

QL = Fluid-flow rate, B/D, and

d = pipe ID, in

In piping systems where solids might be present or where water could settle out and create corrosion zones in low spots, a minimum velocity of 3 ft/sec is normally used. A maximum velocity of 15 ft/sec is often used to minimize the possibility of erosion by solids and water hammer caused by quickly closing a valve.

Gas line sizing

The pressure drop in gas lines is typically low in gas-producing facilities because the piping segment lengths are short. The pressure drop has a more significant impact upon longer segments such as gas-gathering pipelines, transmission pipelines, or relief or vent piping.

The velocity in gas lines should be less than 60 to 80 ft/sec to minimize noise and allow for corrosion inhibition. A lower velocity of 50 ft/sec should be used in the presence of known corrosives such as CO2. The minimum gas velocity should be between 10 and 15 ft/sec, which minimizes liquid fallout.

Gas velocity is expressed in Eq.2 as:

Vg=60*(Qg TZ)/(d^2 P) (Eq.2)

where:

Vg = gas velocity, ft/sec

Qg = gas-flow rate, MMscf/D

T = gas flowing temperature, °R

P = flowing pressure, psia

Z = compressibility factor, dimensionless

d = pipe ID, in

Multiphase line sizing

The minimum fluid velocity in multiphase systems must be relatively high to keep the liquids moving and prevent or minimize slugging. The recommended minimum velocity is 10 to 15 ft/sec. The maximum recommended velocity is 60 ft/sec to inhibit noise and 50 ft/sec for CO2 corrosion inhibition.

In two-phase flow, it is possible that liquid droplets in the flow stream will impact on the wall of the pipe causing erosion of the products of corrosion. This is called erosion/corrosion. Erosion of the pipe wall itself could occur if solid particles, particularly sand, are entrained in the flow stream. The following guidelines from API RP14E should be used to protect against erosion/corrosion.

Calculate the erosional velocity of the mixture with Eq. 3

Ve=C / (ρM)^(1⁄2) (Eq.3)

Where:

C = Empirical constant

ρM = the average density of the mixture at flowing conditions.

It can be calculated from

M=[(12409)(SG)P+(2.7)RSP] / [(198.7)P+ZRT] (Eq.4)

Where:

SG = Specific gravity of the liquid (relative to water),

P = Operating pressure, psia

R = gas/liquid ratio, ft3/bbl

T = operating temperature, ºR

Z = gas compressibility factor

S = Specific gravity of the gas relative to AIR, at standard conditions.

Industry experience to date indicates that for solids-free fluids, values of C = 100 for continuous service and C = 125 for intermittent service are conservative. For solids-free fluids where corrosion is not anticipated or when corrosion is controlled by inhibition or by employing corrosion-resistant alloys, values of C = 150 to 200 may be used for continuous service; values up to 250 have been used successfully for intermittent service. If solids production is anticipated, fluid velocities should be significantly reduced. Different values of C may be used where specific application studies have shown them to be appropriate.

Where solids and/or corrosive contaminants are present or where C values higher than 100 for continuous service are used, periodic surveys to assess pipe wall thickness should be considered. The design of any piping system where solids are anticipated should consider the installation of sand probes, cushion flow tees, and a minimum of 3 ft of straight piping downstream of choke outlets.

Once a design velocity is chosen, to determine the pipe size, Eq. 5 can be used.

d=[((11.9+ ZTR/16.7P)*QL)/1000V]^(1⁄2) (Eq.5)

Where:

D = pipe ID, in

Z = gas compressibility factor

R = gas/liquid ratio, ft3/bbl

T = operating temperature, ºR

Z = gas compressibility factor

V = maximum allowable velocity, ft/sec

QL = liquid-flow rate, B/D

Valve, fitting, and flange pressure ratings

Pipe fittings, valves, and flanges are designed and manufactured in accordance several industry standards including API, ASTM, ANSI/ASME and Manufacturer’s Standardization Soc. (MSS) (large-diameter pipeline fittings/flanges). The piping components are designed and manufactured to the industry standards to:

Ensure the consistency of the material properties and specifications
Set uniform dimensional standards and tolerances; specify methods of production and quality control
Specify service ratings and allowable pressure and temperature ratings for fittings manufactured to the standards
Provide interchangeability between fittings and valves manufactured to the standards

Piping materials manufactured to these standards can be traced to the source foundry and the material composition verified. Material traceability is another important feature of standardization. Each fitting, valve, and flange can be certified as to the material, specification, and grade.

Pressure ratings

ANSI Standard B16.5, Steel Pipe Flanges and Flanged Fittings, has seven pressure classes: ANSI 150, 300, 400, 600, 900, 1500, and 2500. Table 11 illustrates the maximum, nonshock working pressures for Material Group 1.1, which is the working group for most oil and gas piping and pipeline applications.

API Spec. 6A prescribes seven pressure classes: 2000, 3000, 5000, 10000, 15000, 20000, and 30000. API 2000, 3000, and 5000 lbf have the same dimensions as ANSI 600, ANSI 900, and ANSI 1,500, respectively. When the API flange is bolted to an ANSI flange, the connection must be rated for the ANSI pressure rating. Table 12 shows the temperature and pressure ratings for API-specification fittings.

API flanges are required for extreme high pressures and are typically used for wellheads. ANSI flanges are less costly and more available than the API flanges and are used in the production facility. Typically, API flanges are used in the flowline near the wellhead, but ANSI flanges are used downstream. Manifolds and production headers may be API or ANSI, depending upon the operating pressures.