Effects of Geomagnetic Disturbances on Pipelines

摘要

Time-varying geomagnetic field affects extended conducting systems at or near the earth's surface causing induced or telluric currents. A changing pipe-to-soil potential (PSP) can be dangerous for pipelines. Electric field variations caused by geomagnetic disturbances can disturb operation of the system of cathode protection and even cause an intensified corrosion at the places of electrodes protecting joints. The systems operating in high latitudes are mostly vulnerable to impacts of induced fields systems because these are the regions where strong ionospheric currents are developing during geomagnetic storms. A corrosion pit caused by telluric currents was found in the pipeline in Quebec, Canada, after 5 years of its operation [1]. The chain of physical phenomena causing significant telluric currents is the following: a disturbance appearing on the Sun is spreading through the interplanetary media (solar wind disturbances) causing magnetospheric disturbances and changes the interconnection between the magnetosphere and the ionosphere at that the induced ionospheric currents generate variations of the magnetic field (a magnetic storm) and geomagnetically induced (telluric) currents (GIC) on the Earth surface and conducting networks. Figure 1 can be taken as an example of such a chain of events [1]. A solar flare and associated coronal mass ejection occurred on 4 April 2000. This disturbance was recorded at ACE satellite on 6 April at 16:03 as large changes in magnetic field and solar wind speed. Changes in magnetosphere-ionosphere interaction leads to creation of a large eastward electrojet started at 18:00, as seen from geomagnetic recordings in Ottawa magnetic observatory. Recordings of the ground electric field in Ontario, GIC in Ontario power system and telluric currents in Martime pipeline had a maximum around midnight on 6 April. In the first order variations of ionospheric currents are the reason of inductive fields at the ground. Ionospheric currents (or jets) are flowing at approximately 100 km altitude and during a strong magnetic storm can rich the magnitude up to 10~6 A. Scale dimensions are of the order (1-3) • 10~3 km in E-W direction and up to 100 km along meridian. Frequency of jets (and consequently GIC) variations ranges from 10~(-4) (12 hours diurnal variations) to a few Hz (pulsations). The magnitude of time derivative of horizontal magnetic component (dB/dt) can be used for estimation of inductive electric field [2], dB/dt>200 nT/min corresponds to strong enough disturbance with Et ~ 1 V/km [3].