Introduction
In some cases, underwater pipelines were damaged by lightning. These accidents were concentrated in areas close to lightning lands. Single lands are more susceptible to damage to underwater pipelines with lightning strikes. The bridge bridges spread the current density from a lightning strike with greater efficiency than an earthquake alone.
Careful consideration should be given to placing a single globe, sensitive to lightning strikes, in the immediate vicinity of the pipeline. This document is a brief study of the placement of earth beds in the immediate vicinity of high pressure underground gas pipelines.
Soil resistance plays a major role in the susceptibility of a subcontracted pipeline object that is delayed by lighting.
The purpose of this document is to establish guidelines for installing individual grounding points in close proximity to high pressure gas pipelines. Soil resistance is the most influential factor in the impact of lightning strikes on subsurface assets. Localized soils can vary greatly; the models expressed in this document will only be as accurate as the measurements made.
The circuit for shutting down a lightning pipeline through an underwater conductor consists, first, for striking an above-ground protective device. From there, the current is diverted to earth or subsurface wire. Current flows from the conductor through ground zone 1. This zone consists of the radial resistance of the soil between the nearest point to the pipe and an earthquake. The resistance of this zone is called R (to_pipe). The theory suggests that we have a radial flow. Since the grounding resistance is often not uniform, this assumption may have some limitation in conditions of high dispersion of soil resistance.
As soon as the current reaches the bypass zone, it will take 2 paths, one will be on the pipe, and then on the “correct ground” another path will pass through the bypass zone and on the “correct ground”. The resistance from the pipe to the proper ground is called R (Pipe), and the resistance of the soil in the bypass zone is called R (bypass) or resistance to bypass. In surveys, a pipe is usually measured to an appropriate grounding resistance. This value will depend on the number of coating defects on the pipe and the specific resistance of the soil surrounding the coating defects.
Assuming the delivery time for critical design
The feed current from a lightning strike varies greatly. According to a number of research organizations, lightning strikes can deliver on average up to 30,000 amperes per hit (ref. 1). It is known that close simultaneous multiple impacts of points reach more than 100,000 amperes. For calculations, it is commonly assumed that one lightning strike will have a feed current of 30,000 amperes.
Uniformity of soil resistance.
Localized soil resistance varies greatly. For all calculations based on this, the modeling presented in this document uses average soil resistance, in some cases this may not correspond to the true state of the soil. In order to achieve greater accuracy, it is necessary to provide multiple tests for resistance to mass, and great care should be taken if large areas are expected to have medium soil resistance. Here is the range of typical soil resistance.
Tidal marshes: 0.2 to 1 ohm. M, artificial clay: from 2 to 100 Ohm.m, sandy clay: from 100 to 150 Ohm.m, gravel: from 5 to 700 Ohm, concrete from 300 to 500 Ohm.m
The size of the defect coating for impact (bare metal to the ground)
Pipelines are usually covered with an insulating coating to facilitate the cathodic protection system. A large coating defect will reduce the likelihood of damage to the lightning tube, since the current density will decrease back with an open metal on the tube. Separation in the coating, which allows large volumes of current to flow over large areas of> 60 mm2 for soils> 20 Ohms in resistivity, most likely leads to current densities that are not so large as to cause significant damage to most pipes exceeding 150 mm in diameter . Smaller coating defects are more susceptible to damage. On some coatings, the impact itself will quickly spread damage to the coating, which will have the effect of reducing the current density on the pipe. For the majority of blows, the effect of a metal pipe was observed, the diameter of which exceeds the diameter of 12 mm. Metal loss is usually conical. Using a safety factor of 4, the diameter of the coating defect of 3 mm can be used as a nominal value when calculating safe distances to the earth's fractions. Most gas pipes have many coating defects distributed along its length, because most likely lightning damage will occur at the point of construction closest to the light stroke. Defects along the length of the pipe offer current dispersion paths for lightning strikes.
Critical geometry of metal loss for pipeline impact
Observation of historical data on high-pressure gas pipelines leads to the fact that the impact geometry tends to be conical, although metal losses close to cylindrical failures are observed. Since the rejection of the loss of a metal metal surface is based on the minimum assumed destruction of the coating, the worst condition is that the piece of metal is the diameter of the defect and has a volume height equivalent to the thickness of the pipe. To calculate the metal loss on impact, we use the following formula:
ML = (x x Hole diameter x Hole diameter x Wall thickness x Metal density x 1/4)
Where: ML - pipe material lost to failure in kg
Hole diameter = 0.003 m
Wall thickness is in m
The density of the metal is 7200 kg / m3
Energy loss of metal.
For a standard pipe material, we have a melt constant that is used to determine the amount of energy to melt a certain amount of steel. This melt constant depends on the type of steel and is the amount of energy needed to deliver within 1 second with a 1 gram change from solid to liquid. The melt constant is denoted by Qm, and for standard carbon steel pipe pipes it is 1.584 kW / day. Egypt 1,584 KJ / kg.
To calculate the energy required to obtain sufficient metal loss for pipe failure, we use the following formula:
PEL = ML x Qm / (Impact Time)
Where:
PEL = Min. Energy to change the phase of the metal on the cylindrical path of the defect (KW)
ML = Metal Liquefied (g)
Qm = Chang phase phase constant (1584 kJ / kg)
Scrum time (seconds)
The thermodynamic effects are complex, and the model equation above gives only a very simplified analysis of the mechanisms involved.
Shock or temporary time
The strike time is based on an average illumination of 0.00003 seconds or 30 microseconds. This time has been researched by a number of weather research organizations around the world, including the US Bureau of Meteorology. Suppliers of potentially damaged current radiating assets should state the maximum damage period.
Actual power applied to pipe during impact
To find out whether a blow will lead to a defect in the pipe, it is necessary to determine the actual power applied to the defect of the pipe during the impact and compare it with the minimum power of metal loss. To achieve this, the following scheme must be resolved for unknowns.
Determination of the resistance of the earth zone 1
The resistance of ground zone 1 is determined by the first detection of soil resistance. The next step is to determine the distance from the earth cola to the pipe. These numbers are then inserted into the formula:
Rto_pipe = ρ / ((2 x x xr))
This formula is based on calculations of the basic soil resistance.
Determination of Bypass Resistance
When calculating the resistance of the soil in the bypass zone, we include the volume of the pipeline and we believe that the resistivity of the pipe will be the same as the resistivity of the earth. The change in accuracy will decrease as the subsurface conductor moves further away from the pipe, and the variation has a conservative effect on the output of the structure.
R_Bypass = R_pipeOD - R_topipe
R_pipeOD = ρ / (2 x x x (r + pipeOD))
Determination of the applied current on the pipe
R total = Rtopipe + 1 / ((1 / Rpipe) + (1 / Rbypass))
Determine the voltage drop across the pipe
V Drop Through Pipe = Rtotal x I total - (Rtopipe * Itotal)
= I total x (Rtotal - Rtopipe)
Pipe current calculation
Pipe Current = V Cross Pipe / Rpipe
= I total x (Rtotal - Rtopipe) / Rpipe
Tubular Applied Energy (PAE)
Pipe Applied energy = (pipe current x pipe voltage) x Impact time
(Rtotal - Rtopipe)) x (I) x (I) total x (Rtotal - Rtopipe)
Determining the occurrence of a failure
If the pipe feed energy is greater than PEL (the minimum energy to melt a defect in a pipe), then the pipe most likely does not work when the lighting is triggered, and the grounding point should be removed farther from the pipe.
