ART of Tugnology

Paper To:

Whom it may concern

From:

M.W. Jansen M. sc.

CC: Date:

November 30, 2012

Re:

The ART of tugnology: Safe escorting duties put in perspective

Introduction How much power and steering force do we really need to perform safe escorting duties? How much power does a tugboat deliver and what are my alternative design options. Do I want to use dedicated escort tugs or will multi-purpose tugboats suffice for the type of operation? Table 1, Steering forces required for oppose manoeuvre on 15° rudder angle

Type of escorted ship

Steering force at 10 knots speed (kN)

Steering force at 10 knots speed (mt)

Handymax bulk carrier

393

40

Panamax bulk carrier

589

60

Suezmax tanker

863

88

VLCC tanker

1,138

116

30,000 m³ gas carrier

314

32

60,000 m³ gas carrier

422

43

Designers have a natural tendency to talk about their own designs a lot. They typically display a number of figures to indicate their designs performance. The downside is comparisons of available tugboat concepts are found only very rarely. This paper puts escort performance of tugboats in a broader context. This paper reflects upon escort tugboat requirements and presumes a level of basic knowledge obtained from our “ART of tugnology: the ultimate balance between safe ship assist and safe escorting duties” paper. Escort performance criteria In-line with LRS class rules we use the following benchmarks to measure a tugboats escort performance: •

Braking forces: reduce speed and bring an assisted vessel under control (retard manoeuvre);

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• •

•

Steering forces: assist a ship’s turn or oppose turning direction in case of rudder failure; Reaction time: how long does it take before a tugboat applies an effective force to an assisted vessel. Classification societies generally use a time (in seconds) criteria. I prefer using the towline angle at which the previous mentioned forces are generated. This eliminates any discussion on outstanding towline length; Vessel speed: how does a tugboat perform the above functions at different vessel speeds. Generally speaking sea-going vessels will reduce speed in confined waters.

All the above mentioned criteria can be visualized using a polar diagram plotting a tugboats towline force and towline angle characteristics. Polar diagrams are difficult to interpret correct, but contain a wealth of information for experts in the field of tugnology. Figure 1, Polar diagram

Steering force (kN)

90°

90°

75° Braking force (kN)

75°

60°

45°

60°

45°

30°

30° 15°

0°

15°

Figure 1 illustrates possible locations of an escort tug while tethered to an assisted vessel. A polar diagram lists the towline forces generated at these respective towline angles. The previously mentioned performance criteria translate into a polar diagram as follows: • • •

•

Maximum braking forces: maximum y-axis value; Maximum steering forces: maximum x-axis value; Reaction time: towline angle θ at which maximum steering – and braking forces are generated. Independent from outstanding towline length, this offers a simple parameter to indicate reaction speed between a request for steering force and the actual application of such forces. IACS recommends limiting maximum towline angles to 60° for assessing maximum steering forces; Vessel speed: each speed really requires its own polar diagram. This is the one remaining variable not represented in a polar diagram (though you can plot different speeds in one diagram);

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Theoretical intermezzo Conventional escort towing relies on hydrodynamic forces acting on the hull of an escort tug when its brought under an oblique angle. Rotortugs combine this principle with thruster direct forces and wake interaction commonly used during transverse arrest operations. This combination enables the Rotortug® to generate higher towline forces at lower speeds and towline angles. Figure 2, Conventional escort tug

Figure 3, Escort Rotortug®

This working principle for a Rotortug® operating over the bow translates back in the polar diagrams of available tugboat systems. Figure 4 provides a polar diagram comparison between two multi-purpose workhorse tugboats. Figure 4, Polar diagram multi-purpose ASD and – Rotortugs at 10 knots speed

Steering force (kN)

90°

514 662

90°

200 kN 400 kN 600 kN 800 kN

75°

75° Braking force (kN)

1.000 kN 1.200 kN 60°

60°

45°

45° 30°

30° 15°

15° 0°

•

ASD 80mt BP, 32m RT 80mt BP, 32m

Both tugboats from figure 4 illustrate typical high-end harbor – or multi-purpose tugboats also used for the occasional escort duty;

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•

• •

Due to the combined principles of direct thrust, hydrodynamic forces and wake interaction, the Rotortug® is able to generate significantly higher steering forces at lower towline angles = reaction time ↓ and safety ↑ at equal towline lengths Max steering forces are delivered at 40° for a Rotortug® versus 90° for a popular ASD-type. IACS reccomends limiting the max feasible towline angle to 60°; Very Large Gas Carriers and Panamax bulk carrier at 10 knots (589kN steering force required) can safely be opposed by 80mt 32m Rotortug , though larger vessels require tandem operation or more powerful escort tugs ;

Escort performance With many dedicated escort tugs relying on hydrodynamic forces to generate steering forces. Increasing vessel size and including large hull appendages traditionally limited an escort tugs service profile to escort duties only. From lengths of 37m onward a tug is just to big to effectively engage in ship-handling operations. Figure 5, Comparison dedicated escort Voith-Schneider tractor tug and multi-purpose Rotortug®

Steering force (kN)

200 kN 400 kN 600 kN 800 kN 1.000 kN 1.200 kN 75° 1.400 kN 1.600 kN 1.800 kN 60°

831

1.128

75°

60°

45°

45° 30°

30° 15°

15° 0°

• • •

90°

Braking force (kN)

90°

VSP 95mt BP, 45m ART 88mt BP, 35m

Figure 5 illustrates a multi-purpose Rotor®Tug design from Robert Allan plotted against a dedicated Voith-Schneider tractor escort tug; The tractor tug is some 10m longer in length and more powerful, yet the Rotortugs combination of direct thruster power, hydrodynamic forces and wake creates higher towline forces between 10°-40° towline angles compared to the dedicated escort tug; A tugboat should not only be able to perform an oppose manoeuvre in case of rudder failure, but also return the assisted vessel to a safer and more controlled state at slower speeds;

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• • •

At slower speeds the dedicated escort tugs steering performance will strongly decline due to the reduced hydrodynamic forces; The Rotortug® from figure 5 was developed to mitigate navigational risks in the Port Hedland approach channel; Operating in tandem, these Rotortugs successfully opposed Cape-size bulk carriers with a rudder failure at 20° rudder angle at 10 knots during navigational risk simulations conducted at Force (Denmark).

Figure 5 clearly illustrates that a Rotortug® can be a very versatile tugboat concept for projects requiring both powerfull and versatile tugboats. Yet in combination with table 1 it also illustrates the need for ever more powerful escort tugs safe-guarding vulnerable ecological systems (tandem operations not always being a feasible solution). Figure 6 displays a polar diagram of the most powerfull Rotortug® designed by Robert Allan Ltd. This particular design is a 110 mt BP, 37m length Rotortug® fitted with a retractable skeg. The ART110-37 design combined superior steering forces with excellent maneuvering characteristics during shiphandling operations. Figure 6, Polar diagrams dedicated escort Voith-Schneider tractor tug and multi-purpose Rotortug®

Steering force (kN)

200 kN 400 kN 600 kN 800 kN 1.000 kN 1.200 kN 75° 1.400 kN 1.600 kN 1.800 kN 60°

831

1.128 1.393

Braking force (kN)

90°

75°

60°

45°

45° 30°

30° 15°

15°

VSP 95mt BP, 45m ART 110mt BP, 37m

0°

• •

90°

The multi-purpose Rotortug® can actually successfully oppose a VLCC at 10 knots speed, whereas the dedicated escort VSP doesn’t meet the listed criteria for VLCC’s from table 1; Furthermore the multi-purpose Rotortug® can still feasibly operate in a terminal or harbor environment for ship-handling duties offering increased flexibility;

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•

With some 20% lower capital investment a Rotortug® can offer a cost-effective navigational risk mitigation tool for protected areas and remote terminals a like;

Conclusion None of the information listed in this paper was in any way new information. In fact some of the test data used is over 10 years old. References for this data is available on your request as a reader, but also to verify our statements and continue the discussion on this topic on www.rotortug.com. • • • • • • •

The escort tug ability to generate steering and braking forces within shortest possible time while tethered and changing position from port to starboard is of utmost importance. As escort operations only occur within fairly confined waters and speeds up to 10 knots, the maximum feasible towline angle for escort manoeuvres is 60°; The smaller the towline angle at which effective forces are generated, the faster a tethered escort tug can respond to emergency situations; Maximum steering capability is required at speeds of 6-10 knots, while at speeds above 10 knots braking assistance will be predominant over steering. Due to the direct thruster forces generated by the triangular thruster configuration a Rotortug® is, in the speed range of 4-8 knots, able to deliver advanced steering in both direct and indirect arrest mode compared to other tug concepts. In addition to this advanced steering capability, a Rotortug® escorting in Direct/Combi-Mode only sustains a heeling angle of 7° enabling the safest possible operation at all times. A Rotortug® is a versatile, multi-purpose escort tug with advanced safety for crew during emergency escort operations and superior manoeuvrability during ship-handling operations (not being hindered by large hull appendages commonly found on escort tugs).

References • • • • •

Escorting Ships with Tractor Tugs, Capt. Gregory Brooks & Capt. Wallace Slough Escort Tug Design Alternatives And A Comparison Of Their Hydrodynamic Performance Robert G. Allan, (FL), President, Robert Allan Ltd. & David Molyneux, (M), National Research Council of Canada ITS2000 papers: The Rotortug®, Ton Kooren, owner, Kotug International; Frans Quadvlieg, Principal Consultant Manoeuvring, MARIN, Wageningen; Arie Aalbers, Professor University of Technology Delft Report No. 2506 / 2509 - Hydrodynamic Model Tests - Escort Tug ART / RRT 85-35 ROTORTUG Tug Use In Port, Captain Henk Hensen, the Nautical Institute, 1997

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Appendix 1 - Rotortug® designs ART 80-32, 80mt BP, 32m

ART85-35, 88mt BP, 35m

ART110-37m, 100mt BP, 37m