Feature 1 | TUGS
Figure 6: Heeling arm comparison of the ASD vs. EDDY tug
to conventional tugs of the same performance, lower wetted surface, less draught, improved hydrodynamics, course stability (ahead and astern), reduced motions and low wake. Tere are numerous interrelated aspects which are key to the concept’s success, but which are too lengthy to explain here. Te impact of a reduced heeling arm (simplified, being the vertical distance between the towing point and Vertical Center of Buoyancy) is just one example (see Figure 6). Tugs with transverse thrusters have,
by necessity, a different hull shape and different weight distribution than EDDY tugs. Thrusters are the single heaviest components onboard a tug (for example, a thruster weighs 35tonnes against 8tonnes for a main engine). Mounting both thrusters on the same end requires a lot of volume locally to keep the tug afloat. Transverse thrusters also need to be
sufficiently separated. Both factors result in a wide and voluminous stern, which (amongst others) liſts the stern up in a seaway, simultaneously pushing the bow down (Figure 7). To compensate for this, the bow of ASD
tugs needs to be high, resulting in a high towing-staple location. Te heeling arm of ASD tugs is typically 40% larger than that of EDDY tugs. Tis has a dramatic impact
Figure 7: The stern of an ASD vs. EDDY-X
on dynamic towage stability, requiring increased beam. And so the inefficiency- snowball starts to roll: a wider tug is heavier, which increases fuel consumption, which requires larger fuel tanks, which makes the hull bigger, heavier, etc (see Figure 8). Te key philosophy of EDDY tugs is
exactly the opposite: melt the snowball by simplification, and optimisation of natural balance, natural strength and natural flow. Despite the low bow of EDDY tugs, it stays dry and safe, also in bad weather, while ship-assist and escort performance is superior. The same principles apply to the second generation EDDY-X tugs, which are further refined for cost-effective decarbonisation potential. When comparing the EDDY-X with the
EDDY 30, the same low displacement is achieved, but with the capability to carry 1,600kWh batteries without compromising endurance and speed. Its improved energy efficiency and ship-assist performance allow for near-zero-emission operations. Once it is realised that it is rather
difficult to achieve 70%+ emission reduction, one will understand why energy efficiency becomes so important. Figure 9 schematises the energy balance of tugs with azimuthal
thrusters at
various stages of electrification. Tugs with Voith-Schneider propulsion (even those with in-line VSPs) are not
taken into
consideration because VSPs are around 20% less efficient than azimuthal thrusters, while these units are also significantly heavier and larger. Tis requires bigger and heavier tugs which consume more
Alternative fuels Zero-emission energy sources are key to achieving the GHG emission targets. Te problem however is that zero-emission fuels are not commercially available yet.
gre profile e of the 30 tug (green) vs. an ASD of similar performance (orange)
fuel, thereby reducing energy efficiency and increasing emissions. The first column of Figure 9 shows
that EDDY tugs have a head start when it comes to electrification: their roughly 30% intrinsically lower energy need (before batteries are added) has a knock-on effect when sizing the energy storage system (ESS). The middle column indicates that an EDDY-X with an 800kWh ESS can meet the IMO 2030 GHG emission targets. The last column demonstrates that an EDDY-X with a 1,600kWh ESS can perform a typical ship-assist operation virtually emission-free, while conventional tugs with the same ESS capacity achieve only roughly a 50% reduction, thus not meeting GHG emission targets. Unfortunately, a 1,600kWh ESS already
requires a volume equivalent to two 20’ containers (including converters and other related equipment), which is considerable for a tug. Adding more batteries results in bigger and heavier tugs and the inefficiency- snowball starts rolling again. Since batteries are expensive, heavy and consume considerable space, the fewer the better. Besides these serious drawbacks,
batteries play a key role in reducing emissions. They also reduce fuel and maintenance costs and noise, and improve responsiveness, performance and safety.
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Ship & Boat International November/December 2018
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