More container ship operators are deciding to run their ships at reduced speed to cut bunker costs and atmospheric emissions. While slow steaming poses challenges, there is a growing awareness that operating vessels at the high speeds for which they were originally designed is not essential

Five years ago, if container ship operators were asked if they needed
to sail their large deepsea vessels at 25 knots, they would feel the
question hardly merited an answer. It was an accepted fact of life that
these “floating warehouses” needed to move over the oceans at high
speed and to a tight schedule in order to make their just-in-time
deliveries and then to be off quickly on the return voyage to make way
for the next vessel in the rotation. Shops and industrial premises
needed to have their stocks continuously topped up and speedy container
ships were integral to customer satisfaction and healthy balance sheets
across the supply chain.

To make this finely tuned system work, naval architects and engine
builders were required to extend the ship technology envelope. As
container ships increased in size to enable economies of scale to be
realised, hull lines became a compromise between optimising
cargo-carrying capacity and minimising resistance to passage through
the sea. Equally important to naval architects was the requirement to
provide a hull which, considering the open deck construction of the
ship, was strong and robust enough to cope with the high loads
encountered while proceeding at high speed in open seas.

Engine builders played their part by crafting the largest marine diesel
propulsion units ever constructed, able to deliver 70,000 kW of main
engine power. As ship size increased towards that threshold beyond
which twin diesels appeared the only viable option for maintaining the
high ship speeds required, engine manufacturers coaxed even more form
their single engine designs. Because the disadvantages of twin diesels,
in terms of vibration loads and fuel costs, were deemed to outweigh the
advantages, the pressure was on to further improve the performance of
the existing engines.

Today, the situation is markedly different. Following the credit crunch
of September 2008, world trade went into decline and consumer demand
has fallen away. As a result, approximately 10% of the global container
fleet is currently idle, at precisely the time that the new generation
of large, powerful container ships, which were ordered three to four
years ago, are being delivered.

With pressure off the need for just-in-time deliveries, the container
shipping industry has had time to think about what would have seemed
inconceivable five years ago, i.e. slowing ships down. The benefits of
curtailing speed – reductions in both fuel consumption and ship
atmospheric emissions – were always there; it was just that they were
outweighed by the importance of speedy cargo deliveries. Today, the
drive to tighten the regulatory regime governing greenhouse gas
emissions and the high cost of fuel at a time of reduced revenues have
greatly increased the importance of these two factors.

As a result of several recent studies shipowners have discovered that
they are able to run their two-stroke diesel engines at rates down to
20% of the full engine load, and possibly as low as 10% under certain
circumstances, whereas previously it was thought that 40-60% was the
minimum acceptable level. This makes it possible to sail ships at
50-60% of full speed, termed slow steaming and super slow steaming at
the lowest loads, and to achieve reductions of up to 30% in fuel costs
and carbon dioxide (CO2) emissions.

The operation of slow speed diesels below 50% engine load poses certain
challenges, not least an increased risk of engine fouling and excessive
component temperatures. However, engine manufacturers have been able to
respond through the provision of upgrade kits for their existing diesel
propulsion units. By effectively cutting out a turbocharger when the
engine is operating at low load, these kits enable vessels to slow
steam without the need for any notable modifications to the machinery.
In addition, the ability to run the ship at the full design service
speed, if desired, is not compromised.

Several container shipping lines have implemented slow steaming over
the course of the past year in response to the drop in world trade. In
addition, some shipowners in a position to do so amended slightly the
designs of the new ships that had already been ordered prior to
commencement of their construction. The changes, aimed at reducing
bunker costs and reducing emissions, have included larger propellers,
reconfigured bow shapes and enlarging the take-home power of exhaust
boilers.

Over the past 20 years commercial pressures have driven up the average
speed of a container ship from 17.5 to 21.6 knots while for the ultra
large container ships (ULCSs) of 12-13,000 TEU capacity ordered in
recent years a service speed of 25 knots has been the common
specification. Now, all of a sudden, shipowners are waking up to the
fact that fuel costs rise exponentially with ship speed.

The growing realisation that speeds as low as 12-14 knots may be more
desirable means that the next generation of container ships will have a
different design specification to the ULCSs now being delivered.
Unfortunately, in view of the current fleet oversupply and the number
of large ships still to be delivered, it will be some time before the
new energy-efficient container ships are ordered.

In the meantime current market analyses are showing the extent to which
the implementation of slow steaming by a number of shipping lines is
not only absorbing excess tonnage but also achieving cost savings. With
bunker costs now above USD 400 per tonne, it is estimated that by
slowing ships on the Far East – North Europe route down slightly, from
an eight-week to a nine-week rotation, cost savings of up to 6% can be
achieved. These savings increase if ships are slowed down further and
also if bunker costs rise.

Slow steaming is not a universal panacea and there are challenges posed
by issues such as time-sensitive cargoes, the performance of existing
engines operating at low loads over extended periods, maintenance
programme repercussions, increased auxiliary engine power requirements,
lube oil consumption and other downside aspects of the increased time
at sea.

The full benefits of slow steaming will only accrue with the
construction of the next generation of container ships, complete with
their newly designed propellers and engines. Super slow steaming is
unlikely to prove to be the optimum answer and the ideal solution will
probably turn out to be a compromise. At the very least, the events of
the past 12 months have shown that there is no need to drive container
ships along at speeds that would break the limits in most residential
areas.

Source: BIMCO Features

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