Road Crossing
1.1. Cover
In this design of pipeline at road crossings we have adopted a depth of cover of 1.2 meters minimum from the top of the pipe to the travelled surface of the road, in accordance with API RP 1102.
1.2. Load
1.2.1. General
The pipeline at road crossing will be subjected to both internal loads from pressurization and external loads from earth force (dead load) and highway traffic (live load). An impact factor should be applied to the live load. The tandem axles were use in this design for the live load from highway traffic.
Pipe stress calculation was design to assure that sufficient pipe strength is provided at the road crossing to anticipated design loads. In this design calculation, the AASHTO HS20-44 truck used as a design load with a safety factor of 1.25.
Total Effective Stress of the pipeline at road crossing shall be less than or equal to the Design factor, F times the pipe specified minimum yield strength.
Pipe stress calculation was performed for pressurized conditions at internal operating pressure equals to maximum allowable operating pressure (MAOP).
The pipe horizontal deflections (ovality) due to combined effect of the external loads shall not exceed 3% of pipe outside diameter.
1.2.2. External Load
1.2.2.1. Earth Load
The earth load is the force resulting from the weight of the overlying soil that is conveyed to the top of pipe.
1.2.2.2. Live Load
It is assumed that the pipeline is subjected to t
he loads from two trucks travelling in adjacent lanes, such that there are two sets of tandem or single axles in line with each other.
The crossing is assumed to be oriented at 90 degrees with respect to the highway and is an embankment-type crossing.
1.2.2.3. Internal Load
The internal load is produced by internal pressure, p, in pounds per square inch. The maximum allowable operating pressure, MAOP, or maximum operating pressure, MOP, was used in the design.
1.3. Stresses
The detailed information of stresses to be used in the design approaches can be described as below.
1.3.1. Stress Due to External Loads
External loading on the pipeline will produce both circumferential and longitudinal stresses. It is assumed that all external loads are conveyed vertically across a 90 degree arc centred on the pipe crown and resisted by a vertical reaction distributed across a 90 degree arc centred on the pipe invert.
1.3.1.1. Stress Due to Earth Load
The circumferential stress at the pipeline invert caused by earth load, SHe (psi), is determined as follows:
Where:
KHe = stiffness factor for circumferential stress from earth load.
Be = burial factor for earth load.
Ee = excavation factor for earth load.
g = soil unit weight, in psi.
D = pipe outside diameter, in inches.
1.3.1.2. Stress Due to Live Load
Stress due to live load can be classified in two items:
Surface Live Loads
The live external highway load, w, is due to the wheel load, Pt, applied at the surface of the roadway. For design, only the load from one of the wheel sets needs to be considered. In this design calculation, the maximum wheel load from a truck’s tandem axle set was used.
The applied design surface pressure, w (psi) is determined as follows:
P = Design tandem wheel load, Pt (lbs)
Ap = The contact area over which the wheel load is applied;
Ap is taken as 144 in².
Impact Factor It is recommended that the live load be increased by an impact factor, Fi, which is a function of the depth of burial, H, of the pipeline at crossing.
Highway Cyclic Stresses
The cyclic circumferential stress due to highway vehicular load, DSHh (psi) can be determined as follows:
Where:
KHh = highway stiffness factor for cyclic circumferential stress.
GHh = highway geometry factor for cyclic circumferential stress.
R = highway pavement type factor.
L = highway axle configuration factor.
Fi = impact factor.
w = applied design surface pressure, in psi.
The cyclic longitudinal stress due to highway vehicular load, DSLh (psi) can be determined as follows:
Where:
KLh = highway stiffness factor for cyclic longitudinal stress.
GLh = highway geometry factor for cyclic longitudinal stress.
R = highway pavement type factor.
L = highway axle configuration factor.
Fi = impact factor.
w = applied design surface pressure, in psi.
1.3.2. Stress Du to Internal Load
The circumferential stress due to internal pressure, SHi (psi) can be determined as follows:
Where :
p = internal pressure, taken as MAOP or MOP, in psi.
D = pipe outside diameter, in inches.
tw = wall thickness, in inches.
1.4. Limit of Calculated Stresses
1.4.1. Check for Allowable Stress
The circumferential stress due to internal pressurization, as calculated using Barlow formula, SHi (Barlow) in psi must be less than the factored specified minimum yield strength. This check for natural gas is given by the following:
Where :
p = internal pressure, taken as MAOP or MOP, in psi.
D = pipe outside diameter, in inches.
tw = wall thickness, in inches.
F = design factor = 0.6, for road/river crossing and bend
E = longitudinal joint factor.
SMYS = specified minimum yield strength, in psi.
1.4.2. Check for Principal Stresses
The principal stresses S1, S2, and S3 are used to calculate Seff. The principal stresses are calculated from the following:
Where:
S1 = maximum circumferential stress.
DSH = DSHh, in psi, for highways.
Where:
S2 = maximum longitudinal stress.
DSL = DSLh, in psi, for highways.
Es = Young’s modulus of steel, in psi.
aT = coefficient of thermal expansion of steel, per °F.
T1 = temperature at time of installation, in °F.
T2 = maximum or minimum operating temperature, in °F.
ns = Poisson’s ratio of steel.
Where:
S3 = maximum radial stress.
1.4.3. Total Efective Stresses
The total effective stress, Seff (psi) can be determined as follows:
Based on API RP-1102 Steel Pipelines Crossing Railroads and Highways, the total effective stress limit can be determined as follows:
1.4.4. Check for Fatigue
The check for fatigue is accomplished by comparing a stress component normal to a weld in the pipeline against an allowable value of this stress, referred to as a fatigue endurance limit.
1.4.4.1. Girth Weld
The cyclic stress that must be checked for potential fatigue in a girth weld located beneath a highway crossing is the longitudinal stress due to live load. The design check is accomplished by assuring that the live load cyclic longitudinal stress is less than the factored fatigue endurance limit.
The fatigue endurance limit of girth welds, SFG, is taken as 12,000 psi for all steel grades and weld types.
The general form of the design check against girth weld fatigue is given by the following:
Where :
DSL = DSLh, in psi, for highways.
SFG = fatigue endurance limit of girth weld = 12,000 psi.
F = design factor = 0.6, for road/river crossing and bend
1.4.4.2. Longitudinal Weld
The cyclic stress that must be checked for potential fatigue in a longitudinal weld located beneath a highway crossing is the circumferential stress due to live load.
The check may be accomplished by assuring that the live load cyclic circumferential stress is less than the factored fatigue endurance limit. The fatigue endurance limit of longitudinal welds, SFL, is dependent on the type of weld and the minimum ultimate tensile strength.
The general form of the design check against longitudinal weld fatigue is as follows:
Where:
DSH = DSHh, in psi, for highways.
SFL = fatigue endurance limit of longitudinal weld, in psi.
F = design factor = 0.6 for road/river crossing and bend
