Marine Grounding and Potential Equalization of Tank Ships

SAFE AND MONITORED DISSIPATION OF ELECTRICAL COMPENSATING
CURRENTS BETWEEN SHIP AND PIER/JETTY

In this article you will learn how and why electrical equalizing currents arise between ship and pier.

We explain the difference between grounding in electrostatic sense and marine grounding, describe how marine grounding can be implemented safely and how it complements the concept of isolation between ship and pier.

Finally, we show the marine grounding system SEK-3 as best practice recommendation.

More detailed information about marine grounding as an additional safety barrier during (un-)loading of tank ships can be found in our whitepaper.

What are the dangers of electrical equalizing currents between ship and pier?

The danger of compensating currents between ships and pier arises from electrochemical processes. The ship, the metal construction of the loading pier and the water as electrolytic connection between these two form a galvanic cell:
Ship Grounding galvanic effect battery effect potential equalization SEK-3 Marine Grounding System

galvanic cell

Galvanic cell from the arrangement ship – water – pier

The ship and the pier can each be viewed as electrodes of the galvanic cell.

An electrical potential difference of up to 1 volt exists between the electrodes of such a cell. Any conductive connection between the two electrodes leads to an undefined electrical current. Which can be up to several amperes. A dangerous consequence of this current flow is the uncontrolled formation of sparks. Sparks occur in particular when the electrical connections are opened, e.g. at the connection points of loading arms or loading hoses for liquids or gases, but also at gangways, hawsers or other connections.

In hazardous areas, these sparks can be an effective source of ignition, igniting flammable liquids or gases.

How does marine grounding help to prevent these dangers?

When loading and unloading tank ships, an explosive atmosphere can arise. Therefore, marine loading piers/jetties are classified as hazardous areas of Zone 1.
Fire and explosion protection measures must be taken to prevent the explosive atmosphere from igniting. This means that all potentially effective sources of ignition must be excluded.

In order to counteract the risk of compensating currents, various safety guidelines for tanker shipping and tank terminals stipulate that tankers should be electrically isolated from the loading ports using insulating measures.

Typically, insulating flanges are used at the filling interfaces between ship and shore.

Despite appropriate insulation, the risk of accidental or faulty conductive connections and the resulting current flows remains. Examples are conductive loading hoses that touch the ship’s hull or the pier construction after the isolation point, gangways, hawsers, as well as faulty or bridged isolation points.

2.2 Marine Grounding

Ex-Zone Area

Designation of potentially explosive areas

Taking this into consideration, isolation between ship and loading terminal cannot be sufficient as the only safety measure.
 
It is therefore recommended to additionally use a controlled, highly conductive equipotential bonding connection. This short-circuits the galvanic cell, reduces the potential difference and dissipates parts of the ignitable energy safely via the equalizing connection. In practice, the potential equalization connection described is referred to as marine grounding. Marine grounding represents an effective risk-reducing measure at loading terminals and complements the flange insulation of the loading arms and loading hoses as an additional explosion protection measure.

What is the difference between marine grounding and electrostatic grounding?

Electrostatic grounding is used where electrostatic charges can lead to dangerous sparks.
 
This is also the case when loading flammable liquids and gases on tanker. Therefore, measures to protect against electrostatic charging must also be taken here: all conductive equipment and objects must be bonded and connected to earth. On land, this applies to the product transfer lines, system parts, etc. and is implemented via bonding and grounding measures and connection to earth rods of the pier. On the ship, all conductive equipment must be connected to the ship’s hull. Electrostatic charges are then dissipated via the water. 
 
Electrostatic charges result in very high voltages, in the range of several kilovolts,  with relatively low currents at the same time.
 
They can be safely dissipated via a relatively high leaking resistance (up to 106 Ω). With the galvanic effect, however, electrostatic discharge connections are not effective. On the contrary, due to their design for very low currents and the possibility that sparks may occur at the connection points, such connections can themselves become an ignition source. Therefore, grounding clamps and cables as well as grounding control devices for road tank trucks, rail tank wagons, IBC etc. must not be used for marine grounding.

What requirements must marine grounding meet?

Conventional grounding control devices for electrostatic charges, but also so-called simple potential equalization lines in regards of unmonitored cables, with or without Ex switches, are not suitable for safe marine grounding.
 
Safe marine grounding must provide at least the following properties and functions:
  • Hazardous area approval for Zone 1 (ATEX)
  • Extremely high-conductive potential equalization line
  • Capability of the potential equalization connection and internal circuits to carry several amperes while maintaining the explosion protection
  • The equalizing connection must be open when attaching the grounding clamp to the ship
  • The equalizing connection must open automatically when loosening the grounding clamp on the ship so that the breakaway sparks do not occur in the potentially explosive area
  • The electrical load of the equalizing connection must be monitored
  • If the equalizing connection is overloaded, e.g. by active sources, it must be opened safely
  • There should be control signals to indicate dangerous situations to supervising control system

What are the best practice recommendations for marine grounding?

The explosion-proof marine grounding system SEK-3 was developed to meet the special requirements of safe potential equalization of tank ships. It is used for the controlled and safe creation and monitoring of the equipotential bonding between tankers and shore-based pier facilities during the loading of flammable gases and flammable liquids, whose vapors can form an explosive atmosphere.

Marine Grounding System SEK-3

The marine grounding system is permanently installed on the pier.

It has a special cable that is up to 30 m long and a special marine grounding clamp. The grounding clamp has to be attached to a suitable point on the ship’s hull after the ship has docked and before the loading equipment is connected. The potential difference between the ship and the pier is reduced by the monitored connection. At the same time, part of the energy is safely dissipated via the marine grounding system. As long as the conditions ensure safe loading, the signal LEDs of the marine grounding system show the status “green” and the control outputs switch to “release”.

In the event of loss of contact of the marine grounding clamp, or if the electrical conditions between ship and shore exceeds safety specifications, SEK-3 immediately disconnects internally and the spark energy is kept away from the ship’s connection point. As long as the conditions do not ensure safe loading, the signal LEDs of the ship’s grounding system show the status “red”.

Before and during the contacting of the grounding clamp, the internal equipotential bonding connection within the control unit is interrupted.

This ensures that the electrically conductive connection between ship and shore is only released after mechanically sound and electrically tested contacting. In the event of a fault or hazard, the object voltage monitoring and other monitoring functions signal the faulty state by means of a clear “flashing red” status display and also activate an external alarm signal.
The use of marine grounding increases the level of safety on ship loading/unloading terminals and mitigates the risk of fire or explosion caused by galvanic compensating currents.

 

Are you unsure whether marine grounding is necessary in your application?

Or would you like to learn more exact details about the best practice? Feel free to reach out to our team of experts.
 

We look forward to helping you.