Features: - Tanker Operator

TECHNOLOGY - FOULINGS
An idle ship is the
devil’s playground
The economic slow-down has resulted record breaking amount of ships at anchor
and a speed-up of organisms attaching themselves to their hulls.
Alarge percent of anchored ships
are in warm Asian waters with
known high level of ‘fouling
pressure’ activity from the
standpoint of slime, and marine growth.
Performance losses, just due to light growth
an inactivation of the coating system or
inability to ‘foul release’ can result in an
Aframax losing six tonnes per day ($2,700 per
day).
The concern from a scientific and industry
standpoint is that the accumulation of fouling
will cause an inactivation of the anti-fouling
properties for biocide hull coatings. For foul
release coatings, the possibility of slime and
marine growth accumulation is exacerbated by
their non-biocide nature.
The local diving contractors in proximity to
these anchored vessels will no doubt have
some good business in the future when these
ships return to service (assuming Port States
allow underwater cleanings) but what are the
consequences of not cleaning hulls and
propellers in a timely manner, ie before
sailing again?
According to Daniel Kane, Propulsion
Dynamics vice president; “From the
standpoint of ship technical performance,
there are robust formulas to calculate the
ship’s added resistance while sailing, however,
no formulas to work with in order to
Increase of resistance
Source: Propulsion Dynamics.
determine the increase in hull resistance of
idling ships.
“Factors such as age of ship, time since last
docking, type of coating, underwater surface
area, and duration at anchor will all result in
various degrees of fouling and resistance. Then
additional factors while underway (speed and
loading conditions) are factors that will make it
impossible to determine which ships need
husbandry and which ships will recover their
fuel efficiency prior to anchoring. In other
words, which ships with anti-foulants will have
deactivated antifoulant and which ships with
foul release will be unable to wash away the
fouling is yet to be seen.
“The astute shipowner will at minimum
polish the propeller and at the same time order
an in water hull inspection prior to departure,
other shipowners will simply sail and observe
the speed and fuel losses in order to schedule
husbandry (if needed) at the next port of call,
but the latter may mean wasted bunkers.
“Of the hundreds of ships in our
programme, we do have several dozen that are
at anchor until further notice. We look
forward to observing the changes in resistance
of these ships when they lift anchor and
eventually do sail. Our CASPER® service
will benefit customers by working closely
with them to assess fouling effect on fuel
efficiency and mitigate fuel losses in the most
60%
55%
50%
45%
40%
35%
30%
25%
20%
15%
10%
5%
0%
2910 2930 2950 2970 2990 3010 3030 3050 3070 3090 3110 3130 3150 3170 3190
Days for development of added resistance
timely and appropriate manner,” he concluded.
For ships in long layup periods, docking
may be needed, according to other experts.
Below is an example of the dramatic
increase in hull and propeller resistance for a
ship that was at anchor for only four weeks
off the coast of West Africa. The x-axis
represents the time in days (2,940 days since
delivery). The last set of performance data
was received on ‘Day 3,130.25’ the ship then
anchored until we received the next set of
performance data on ‘Day 3,155’.
The resistance of the ship increased 30%
which is normal for an older tanker to 52%,
which for this ship represented a 0.9 knot loss
in speed at 85% MCR. The resistance then
climbed steadily indicating that the antifoulant
had been inactivated by the marine
growth. These cases are not common because
today’s hull coating systems are doing a
terrific job, but with thousands of ships at
anchor now, these cases will be more
common.
According to Daniel Kane: “Let’s look at
some numbers again, assuming the following
ship types are at anchor for only a few months
and then sail at design speed and draft. The
Aframax tanker at six tonnes per day ($2,700
per day) was just due to light growth an
inactivation of the coating system, or inability
to ‘foul release’. A conservative pay back
time for hull and propeller cleanings varies
from a few months for smaller ships and as
little as two weeks for larger ships!
These figures shed light on why hull and
propeller performance monitoring are key
areas of the Ship Energy Efficiency
Management Plan (SEEMP) drafted by IMO
in order to increase awareness of fuel
conservation measures. This means that CO2
reductions, when a part of reduction in fuel
consumption is achieved at a negative cost per
tonne of CO2 avoided.
In the future, more and more focus will be
on the biological risk of hull fouling, and
integration of reducing biorisk whilst
simultaneously improving fuel efficiency, “
Kane said.
TO
November/December 2009 TANKEROperator 55