A report by the Germany Federal Bureau of Maritime Casualty Investigation (BSU) of the grounding of 42,000 dwt chemical tanker PAZIFIK in Indonesia in July 2018, published in January 2020, gives interesting insights into the importance of crew understanding the quality of official electronic navigation chart (ENC) data accuracy.
The root cause of the accident could be described as the
vessel hitting a rock. The rock was shown on the ECDIS display with a note
“underwater rock (always underwater / submerged 1 MAR 2017)”.
From this, they assumed that the rock was not a hazard,
since surrounding water was a comfortable 100m depth.
The crew also thought the vessel was at a safe distance from
the underwater rock, since the electronic chart display had a “cross track distance”
of 182m either side of the vessel, and the rock was much further away than
182m, according to the ECDIS display. (The cross track distance is a system on
ECDIS displays where vessels are given a safe corridor shown by red and green
lines, rather than a specific course, taking uncertainty into account).
But in reality, the rock was only 9m below the water
surface, and located 400m away from where it was stated to be on the ENC. The
ENC’s stated accuracy was +/- 500m. The rock’s location was also shown
accurately on a small-scale paper chart mapped in a 1904 Dutch survey, and
warnings were published in “Sailing Directions” available onboard. BSU heard
from local sources that several other ships have ran aground on the same rock.
The route chosen was recommended by the vessel’s passage
planning software. The vessel’s master was familiar with a route through the
Lombok Strait, which would have added 200 nautical miles to the voyage.
He decided to take the route recommended by the software to
save the 200nm, the Selat Snape strait between Komodo and Banta.
The vessel was loaded with 18,000 tonnes of ammonia –
although no cargo escaped because only the forepeak / ballast water tanks were
damaged.
It was able to refloat 5 days later after transferring cargo
and ballast water to other tanks, and could proceed to a shipyard in Singapore
under its own power, supported by a tug. The repair included renewing 50m of
the double bottom.
The company has decided that the vessel will avoid the Selat
Sape passage from now on.
Navigation background
In its original plan (which was changed due to fishing
vessels), the vessel had planned to pass the rock at a distance of 0.7 nautical
miles (1300m).
Its ENC was classified as “Zone of Confidence Category C”,
which means a position accuracy of +/- 500m horizontally, and “full area search
not achieved”.
But the ECDIS was set to a cross track distance of 0.1nm
(180m) on each side.
This fits company procedures, where it recommends to keep a
“cross track distance setting” of 2 x the vessel’s beam in confined waters, or
just 64.4m, and this passage is considered “confined waters” in the procedural specifications,
so the 0.1m (180m) cross track was considered within limits.
There could have been an alarm in the ECDIS that the cross
track was set to 180m, while the chart had an accuracy of 500m.
The crew could have brought up data about the chart accuracy
on the ECDIS display, including both horizontal and vertical accuracy, such as for
submerged rocks. But it was quite hard to understand how to use it, BSU says.
If the vessel had been navigating with paper charts, the
crew would probably have been more considerate of possible inaccuracies in the
chart, and looked up all the “Sailing Directions” if the vessel was going to an
area the master was not familiar with.
Or concerns about using paper charts in an unknown area may
have led the crew to take on a pilot, who may have had his own accurate soundings
map, or had better local knowledge, BSU said.
Sailing directions
The relevant section of Sailing Directions for the strait
between Komodo and Banta states “The passage E of Pulau Banta is navigable but is
seldom used, other than by ferries and other local craft, as tidal streams are strong
and fewer anchorages are available.”
The Sailing Direction for the island of Tokohgilibanta
states, “a drying rock, 1 mile farther NNW, is small and dangerous; the breakers
on it being indistinguishable from the normal overalls and sea conditions in
the area.”
(Confusingly, on the ENC, the name of the island changes
from Tokohgilibanta to Nisabedi when the viewer zooms in). This description
reflects the location of the rock where the vessel ran aground.
A digital version of these sailing directions would have
been available onboard, but without any reference to the ENC, which would be required
for the computer to connect them.
BSU says that the ECDIS could be described as “not fully
engineered” - since it displaces sources of information such as paper sailing directions,
without being a consistent replacement for them.
“There are significant differences between traditional
voyage planning using paper charts and digital voyage planning using ENCs.
Planning a voyage using paper charts often entails referring
to sailing directions, the list of lights and pilot charts with proposed routes
plotted.”
“Besides drawing on their experience, officers of the
navigational watch therefore refer to sources of data other than the
navigational chart. Paper charts and sailing directions have developed over
centuries and became more accurate in many areas.”
“Most of the world’s sea areas are looked upon as being
inaccurately surveyed, while paper charts only provide an indication of the
data of a survey.”
“The accident is therefore attributable to the ECDIS and
settings specified,” BSU says.
More details
The vessel ran aground on a shoal between the islands of
Komodo and Banta, Indonesia.
It was using a Transas ECDIS, with Passage Manager software
from ChartCo, with ENCs supplied by ChartCo using data from the Indonesian
Hydrographic Office. It was using voyage planning software “BonVoyage System” (BVS)
from StormGeo.
The ChartCo software proposed a route via Selat Sape,
passing between Banta and Komodo, going between the tiny islands of Nisabedi
and Lubuhtare, which have only 1.5nm between them. The master and officer
decided not to take this route, but instead take a route between the islands of
Nisabedi and Banta, which have 2.5nm between them.
BSU looked at the Indonesian and UK Hydrographic paper
charts of the region. TheUKHO charts (1:500,000) show a “rock awash” symbol,
meaning a rock submerged at high tide or temporarily.
The Indonesian smaller scale chart (1:200,000) shows a rock
symbol without specifying whether it is sometimes submerged, while the larger
scale Indonesian chart (1:50,000) shows a shallow area with water depth of 9m.
This chart was drawn from Dutch surveys carried out in 1904.
The ENC shows the shoal 2 cable lengths (400m) from the
scene of the accident, with a note saying, “underwater rock (always underwater/
submerged 1 MAR 2017)”. The general water depth around the rock is about 100m.
The reason for the discrepancy between the ENC and paper
chart is not clear.
The ENC has a “Zone of Confidence Category C” (CATZOC) which
means “a position accuracy of +/- 500m horizontally, and “full area search not
achieved”. This data could be fed into the ECDIS to illustrate the range of
“cross track distance” needed.
IHO has a Data Quality Working Group looking at options for
improve user awareness and presentation of quality data.
Full report may be read at
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