A pipeline is subjected to three primary loading conditions during installation by HDD: tension, bending, and external pressure. A thorough design process requires examination of the stresses that result from each individual loading condition as well as an examination of the combined stresses that result from the interaction of these loads.
Hence, the stresses imposed on steel pipe during both the HDD installation process and subsequent operation should be carefully evaluated.
Bending stress resulting from a rigid steel pipe
being forced to conform to the drilled radius of curvature
E =modulus of elasticity for steel (psi)
D = outside diameter of the pipe (inch)
R = Shortest Radius of Curvature (ft)
Steel pipe will fail by buckling or collapse when under the influence of hoop stress. Itβs important to calculate the wall thickness required to prevent collapse of round steel pipe under the influence of hoop stress.
P = Operating Pressure (psig)
H = Depth of the Pipe at Horizontal Section (ft)
Hw = Groundwater Table below Datum (ft)
Thermal stress resulting from changes in pipe temperature from the point in time at which the pipe is restrained by the surrounding soil to typical operating conditions.
ππ = πΈπΌ (π1 β π2)
πΈ β Youngβ²s Modulus of Elasticity πΈ = 29 x 106
πΌ β Coefficient of Thermal Expansion for steel Ξ± = 6.5 x 106 [in/inβ]
T1 = Installation Temperature (Β°F)
T2 = Operating Temperature (Β°F)
ππ = -ππ + πβ * π+ ππ
fl = Total Longitudinal Compressive Stress
π = Poissonβ²s Ratio = 0.3
ππ£π = 0.45ππππ
SMYS β Specified minimum Yield Stress for Steel Pipe (psi)