The Art of Dredging

Dredging and shipping

H2S    (hydrogen sulphide)


In 2005 Gerardus Mercator was employed at the Ulsan Breakwater Project, South Korea.


We dredged clay, silt, mud, big boulder rocks and all kinds of rubbish, and all had a jolly good time at it.













During the first dredge trips we noticed a nasty smell on deck.

The smell was H2S, a toxic gas, present in the dredged mud.


Through shore staff we got hold of an electronic H2S -meter.

Only then we found out that levels on deck were very high.


The gas was flowing over the sides of the hopper,

and often concentrations of 15 ppm and more were measured.


We dredged for appx. 45 minutes every trip.


On a windless day the H2S gas cloud completely covered the ship, like fog.

We could not see the aftship at times.

On such days we registered peak values of 100 ppm, a lethal dose.



I heard another story of a trailer dredger,

starting up a dredging job some place in Africa,

unknowing of the high content of H2S in the soil.

The story goes that all crew present in the wheelhouse

and on deck fainted, because of the extreme high

levels of gas during the first minutes of dredging.

The chief mate managed to stop the dredging process.

 If not; this dredger would have become a ghost ship,

probably killing everyone onboard with H2S.


Immediate risks


H2S is a toxic gas, it is inflammable and heavier than air.


If measurements show the release of H2S (hydrogen sulphide) during dredging, the following actions must be taken:


All outer doors, openings, hatches, etc… should be kept closed during dredging.

All crew should be locked inside.  

Contact with H2S must be avoided at all costs, check regularly all inner spaces of gas is entering one way or the other.


Air-conditioning which, in normal circumstances, takes air from outside, should be switched to 100% recycling.

No outer air must be allowed in airconditioned spaces.


The main symptom arising with high concentrations are irritation followed by respiratory problems.

Be aware that symptoms only arise 2 hours after exposure.


At low concentrations (10-15ppm), the smell is pretty nasty,

but at higher concentrations, the human nose becomes numb,

an does not register the high dose of gas.


H2S is heavier than air. Toxic gas could stay in the hopper, or in pipes or pumps.

Before working (e.g. in the hopper) it is essential to ensure no more gas is left.


H2S (in higher concentrations) is inflammable and could cause explosion.

Therefore: no open fire, no sparks, no smoking when dealing with H2S during and after dredging.



Delayed risks

The presence of hydrogen sulphide during dredging contains many safety risks.

But little is known about the later effects of H2S, especially on the electric installation onboard.

Copper will be severely corroded by hydrogen sulphide.

This includes copper in electric switchboards, cabling, etc…

This effect appears almost immediately.

The corrosion in the electric switchboards -caused by H2S- may lead to so-called “hotspots”, and become as many fire hazards.

These hotspots can be detected with infra-red thermal imagery equipment.

Just get electric power on the switchboard, and check for any infrared hotspots.


The effect of H2S on the formation of “hotspots” in electric switchboards is still underestimated.


It is imperative that –while or after dredging H2S-contaminated soil-  all electric equipment onboard –especially switchboards- are checked.



 Actual infrared thermal imagery of a switchboard, indicating "hotspots"; potential sources of electrical fire.





















If no thermal-image investigation is done, the risk of electric fire may go undetected.



Marc Van de Velde


Subscribe to our newsletter:

Write us: (at)

Manu's scripts

- a sailor's fifth column


Oops! This site has expired.

If you are the site owner, please renew your premium subscription or contact support.