Introduction
On a number of occasions sub surface pipelines have been damaged by lightning. These incidents have been concentrated on areas close to lightning earth stakes. Single earth stakes are more susceptible to damaging sub surface pipelines with lightning strikes. Earthing grids spread the current density from a lightning strike with greater effectiveness than a single earth stake.
The placement of a single earth stake that is susceptible to lightning strikes should be carefully considered when within close vicinity of a gas pipeline. This document is a brief study of the placement of earth stakes in close proximity to subsurface high pressure gas pipelines.
Soil resistivity plays a major factor in the susceptibility of a sub service pipeline asset being struck by lightning.
The objective of this document is to establish guidelines for the installation of single earthing points in close proximity to high pressure gas pipelines. Soil resistivity is the most influential factor on the effects of a lightning strike on sub surface assets. Localised soils have the potential to vary greatly; the models expressed in this document will only be as accurate as the measurement taken.
A lightning pipe strike circuit via a sub-surface conductor consists firstly of the striking of an above ground protective device. From there, the current is diverted to the earth stake or sub-surface conductor. The current then flows from the conductor through ground zone 1. This zone consists of the radial soil resistance between the nearest point to the pipe and the earth stake. The resistance of this zone is called R(to_pipe). The theory assumes we have radial current flow. Since ground resistance is often not uniform this assumption may present some limitation in conditions of high soil resistance variance.
Once the current reaches the bypass zone it will take 2 paths, one will be onto the pipe and then to “proper earth” the other path will be through the bypass zone and onto “proper earth”. The resistance from the pipe to proper earth is called R(Pipe) and the resistance of the soil in bypass zone is called the R(bypass) or the bypass resistance. The pipe to proper earth resistance is usually measured during surveys. This value will depend on the number of coating defects on the pipe and the resistivity of the soil surrounding the coating defects.
Assuming Delivery Current for Critical Design
Delivery current from a lightning strike varies greatly. According to a number of research organisations lightning strikes on average can delivery up to 30,000 Amps in a single strike (Ref 1). Close proximity simultaneous multiple point strikes have been known to reach over 100,000 Amps. For design calculations it is generally assumed that a single lightning strike will a delivery current of 30,000 amps.
Uniformity of ground resistance.
Localised Ground resistance varies greatly. For all calculation based on this an average soil resistivity is used in the modelling represented in this document, in some case this may not represent the true condition of the soil. To achieve greater accuracy multiple earth resistivity tests should be undertaken and great care should be undertaken when average soil resistivity are assumed over large areas. Here are a range of typical soil resistivity’s
Tidal swamps:0.2 to 1 Ohm.m., Compacted Clay: 2 to 100 ohm.m, Sandy Clay: 100 to 150 ohm.m, Gravel: 5 to 700 Ohm.m, concrete 300 to 500 ohm.m
Coating Defect Size for a strike (Exposed metal to earth)
Gas pipelines are usually coated with an insulative coating to facilitate the cathodic protection system. A large coating defect will result in a lower chance of lightning damage to a pipe because the current density will reduce inversely with exposed metal on the pipe. Splits in the coating that allow large amounts of current to flow over large areas > 60 mm2 for soils > 20 ohm.m in resistivity will most likely will result in current densities not great enough to do significant damage on most pipes that are greater than 150 mm in diameter. Smaller coating defects are more prone to damage. On some coatings the strike itself will rapidly propagate the coating damage this will have the effect of reducing the current density onto the pipe. For most strikes it has been observed pipe metal exposure greater than 12 mm diameter has resulted. The metal loss is usually in a conical shape. Using a safety factor of 4 a coating defect diameter of 3 mm can be used as a nominal value when calculating safe distances to earth stakes. Most gas pipes have many coating defects distributed over its length it is therefore most likely that lightning damage will occur at the conduction point closest to the lightning strike. The defects over the length of the pipe offer current dissipation paths for the lightning strike.
Critical Metal Loss Geometry For a Strike on a Pipeline
Observing historic data on high pressure gas pipeline lightning strikes the geometry of the strikes tends to be conical, although close to cylindrical metal loss failures have been observed. Since a cylindrical metal loss failure is based on the minimum assumed coating failure then the worst case condition is a piece of metal being the diameter of the defect and having the volumetric height equivalent to the pipe thickness. To calculate the metal loss in a strike we use the following formula:
ML=(π x Hole Diameter x Hole Diameter x Wall thickness x Metal density x 1/4)
Where: ML is the pipe material lost for a failure in kg
Hole Diameter = 0.003 m
Wall thickness is in m
Metal Density is 7200 kg/m3
Metal Loss Energy.
For standard pipe material we have a melt constant that is used to determine the amount of energy to melt a given amount of steel. This melt constant is dependent on the steel type and is the amount of power required to be delivered in 1 second to phase change 1 gram from solid to liquid. The melt constant is denoted Qm and for standard carbon steel pipes cane be assumed as 1.584 KW.S/g. or 1,584 KJ/kg.
To calculate the Energy required to cause enough metal loss for a pipe failure we use the following formula:
PEL=ML x Qm/(Strike Time)
Where:
PEL = Min Energy to phase change metal on cylindrical path of defect (KW)
ML = Metal Liquefied (grams)
Qm = Metal phase Chang constant ( 1584 kj/kg)
Strike Time (Seconds)
The thermodynamic influences are complex and the model equation above offers only a very simplistic analysis of the mechanisms involved.
Strike or Fault Current Time
The strike time is based on an average lighting strike which is 0.00003 seconds or 30 micro seconds. This time has been researched by a number of weather research organisations throughout the world including the US bureau of meteorology. Suppliers of potentially fault current emitting assets should specify the maximum fault current period
Actual power applied to pipe during a strike
To find out if a strike will cause a defect in a pipe then the actual power applied to a pipe defect during a strike needs to be determined and compared with the minimum metal loss power. To achieve this the circuit below needs to be solved for the unknowns.
Ground Zone 1 resistance determination
Ground zone 1 resistance is determined by first finding the resistance of the soil. The next step is to determine the distance from the earth stake to the pipe. These numbers are then inserted in the formula:
Rto_pipe = ρ /((2 x π x r))
This formula is derived from the basic earth resistance calculations
Bypass Resistance Determination
When calculating the resistance of the soil in the bypass zone we includes the volume of the pipeline and assume the pipe’s resistivity to be the same as the earth resistivity. The variation in accuracy will reduce as the sub surface conductor is moved farther from the pipe and the variation has conservative influence on the designs output.
R_Bypass =R_pipeOD – R_topipe
R_pipeOD=ρ/(2 x π x (r + pipeOD))
Pipe Applied Current Determination
R total=Rtopipe+ 1/((1/Rpipe)+(1/Rbypass))
Determine the Voltage Drop Across Pipe
V Drop Across Pipe = Rtotal x I total – (Rtopipe * Itotal)
= I total x (Rtotal – Rtopipe)
Pipe Current Calculation
Pipe Current = V Drop across Pipe /Rpipe
= I total x (Rtotal – Rtopipe) /Rpipe
Pipe Applied Energy (PAE)
Pipe Applied Energy = ( Pipe Current x Pipe Voltage ) x Strike Time
PAE= (( I total x (Rtotal – Rtopipe) /Rpipe) x( I total x (Rtotal – Rtopipe))) x Strike Time
Determining if failure will occur
If pipe applied energy is greater than PEL (Min energy to melt defect in pipe) then the pipe will most likely fail in a lighting strike and the earth point should be moved further away from the pipe.
