In-depth Issue 17 - Rolls-Royce

www.rolls-royce.com
09
TABLE 1:
All are based on a tug powered by two 1705 kW engines driving azimuth thrusters, with a
typical annual operating profile of 300 days per year with 4,200 operating hours.
Operating Mode
TIME%
Standby (coupled & uncoupled) 38%
Transit (sailing from & towards a job) 33%
Assist (connected to ship) 29%
1. The Rolls-Royce
LNG propulsion
system for a
harbour tug.
2. The number of
LNG bunkering
facilities is
growing and
refuelling is a
simple operation
that can take
under an hour.
3. Gas propulsion is
also an option for
pusher tugs.
TABLE 2:
Illustrates the potential saving in fuel and lube oil consumption.
Fuel use comparison MGO* LNG**
Average fuel consumption g/kWh 194 157
Annual fuel use tonnes per year 868 702
Lube oil consumption g/kWh 0.8 0.4
Annual lube oil use tonnes per year 3.3 1.6
Test
Cycle Type
E3 (Variable speed propulsion)
* MGO Specific Heat 43.0 MJ/kg. Density 858 kg/m³
** LNG Specific Heat 49.4 MJ/kg. Density 430 kg/m³
TABLE 3:
Shows the projected reduction in emissions. The Bergen gas engine has a low methane slip of
3.10g/kWh at full load, which has been taken into account when calculating the equivalent CO 2
reduction of 23 per cent.
Emissions comparison MGO LNG
NOx emissions g/kWh 9.6 1,1
NOx tonnes per year 41 5
SOx emissions g/kWh 0.2 0
SOx tonnes per year 0.9 0
CO 2
tonnes per year 2,716 2,202*
Environmental Ship Index ESI 7.27 90.66
*Includes an averaged 3.7 g/kWh methane slip equivalent CO 2
mechanical drive or as gensets and can accept rapid changes in load. They
can therefore be used in a variety of propulsion solutions, depending on the
operating profile of the tug.
A study of ship assist tug operations shows that a tug has a varied working
profile. Long-term operational studies demonstrate that harbour tugs will
typically only spend 29 per cent connected to a ship, actually carrying out its
assistance duties, 33 per cent of total operation time in transit, sailing from
and towards a job, while the remaining 38 per cent is spent on standby and
loitering.
Throughout a day or week, a tug’s power requirements can vary
tremendously. Diesel engines normally have a much higher level of specific
emissions and fuel consumption at low loads, compared with the most
efficient operation at high loads in terms of emissions and fuel burnt per
unit of power output. A characteristic that has resulted in the recent move
to hybrid tugs by some operators. It is here that Rolls-Royce lean burn gas
engines score.
Rolls-Royce has evaluated a number of LNG-fuelled propulsion systems for
harbour tugs and the first are now under contract (see page 2). The system
selected produces over 65 tonnes bollard pull and uses a stern drive azimuth
thruster layout based on two US 35 CP thrusters with controllable pitch
propellers. They are directly driven by two Bergen C26:33L6PG gas engines,
each delivering 1,705kW and running at 1,000rpm. Aquapilot controls and
ACON automation are part of the package.
A vertical C-type gas tank of
78m3 capacity and two separate
cold boxes are installed under the
foredeck and provide sufficient
capacity for 150 hours running at
50 per cent load. Refuelling is a
simple operation and is required
once a week from either a
shore-based tank, road tanker or
transport storage unit. Fuel transfer
time is just 45 minutes, provided a
100m3/hr 3
system is used.
Although the initial capital costs
are higher for gas propulsion due to
the cost of the fuel system, this can
be offset within a relatively short
time by significantly lower annual
operating costs. You also have a very
environmentally friendly vessel that
satisfies forthcoming legislation,
with the emissions reduction
advantage there from day one.
Ship construction data is used to
calculate the vessel environmental
ship index (ESI). This is a score
based on the vessels environmental
credentials. It is expected that in
the future, this, or a similar scoring
system will be used to tax vessels
entering port. The score does give a
good indication of how much more
environmentally friendly LNG as a
fuel is, compared to MGO.
These analyses are based on
long-term experience with tug
propulsion. Rolls-Royce supplies
engines, thrusters and winches
for tugs all over the world and the
US series azimuth thrusters are
particularly popular due to their
good performance and robustness.
US 205 FP thrusters already propel
environmentally friendly tugs and
have been selected for the second
of the Foss hybrid tugs, following
the successful operation of the first
of the type, Carolyn Dorothy, at Long
Beach and Los Angeles over the past
two years. [AR]