Swedish coastal Tanker Thuntank 5, grounding on the east coast of Sweden on Dec 21, 1986
2.1 The tanker was approaching the port of Gavle on the Swedish east coast
with a full load of fuel oil.
The weather was bad with snow storm and a wind gusting up to 25 m/s.
In darkness and poor visibility due to snow and freezing spray,
an navigational misinterpretation led to the vessel grounding
on a shoal besides the marked channel.
2.2 The vessel was built with five pairs of wing tanks and four center tanks.
cargo tanks 1P, 1S, 2C and 5C were penetrated in the bottom at the impact or later,
before the cargo could be salvaged.
The vessel was loaded with 5024 metric tons of cargo
and fully loaded to the mark.
The cargo was heavy fuel oil with a density of 0.93 and a pour point of +12C.
Due to the high density of the cargo,
not all the cargo tanks were volumetrically full.
After a few days of heavy weather when no actions could be taken,
the vessel was lightened, pulled off the ground and towed to a repair yard.
From the initial amount of cargo, the amount transferred to the lightening vessel
and the amount recovered at the repair yard,
it was concluded that about 150 tons had escaped to the sea.
2.3 Due to the wave height reported to be 4 to 5 metres,
a considerable surging action took place in the damaged tanks,
extracting more cargo from these tanks than otherwise would be the case.
The high viscosity of the oil when being cooled down by the sea water
created some dampening of the surging effect.
Figure 1 [not included] illustrates the loading condition of the tanks
which were damaged.
The quantity lost equivalents as effective low water level of 1.4 metres
below the main level
which seems likely in consideration of the wave height and the dampening effect
of the solidyfing oil.
2.3 The amount leaking was considerably reduced by the fact
that only one of the damaged tanks was full.
The outflow was also reduced by the fact that the master
immediately after the accident decided not to attempt to take any soundings
in the tank in order not to admit air into the tanks
and thereby increase the outflow.
The heavy weather would also have made sounding the tanks as extremely dangerous task.
2.4 The maximum underpressure that would have been needed in the tanks
in order to retain the oil despite the heavy surging from the wave action
would have been 2.05 metres of water column.
Operation with fully closed deck openings and P/V valves with an opening
pressure of 2 metres wg would have been fully feasible in this case.
In case of no heavy sea, the cargo would have been contained by a maximum underpressure of 0.7 metres wg.
The decision in the case to leave the tanks closed with an estimated underpressure
in the ullage space of about 0.5 metres wg on the on the other hand
have saved some additional 60 tons from escaping into the sea
during the prevailing weather conditions.
2.5 The oil that escaped could not be collected due to the heavy weather
and no cleanup work was possible before the approaching winter
due to the freezing of the sea.
The oil was therefore spreading along a large area of the eastern coastline.
The cleanup work went on for the major part of the spring and summer season of 1987
and became expensive due to the circumstances.
The total cost for shoreline and on-site cleanup was about 30 million Swedish crowns
(equivalent to about 5 million USD).
A considerable portion of the oil has however sunk and is still laying on the seabed,
likely to require further cleanup activities.
2.6 It seems for a study of this accident that the use of a vaccum system
to retain oil in damaged tanks would have worked very efficiently.
The tanker is typical of a large fleet of coastal tankers
operating in the coastal and sensitive waters worldwide.
|Draught at tank inc 0.5 m rise of bow
|Press of cargo, sg = 0.93, P/V vlvs closed at 0.5 m wg
|Press of sea incl 1.4 m wave effect
|Diffential [at tank bottom]
|Outflow of oil, m3