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Paper To:

To whom it may concern

From:

Marinus Jansen M.Sc.

Date:

September 13, 2012

Re:

Ship-handling in port

Introduction A seagoing vessel needs tug assistance when its manoeuvrability gets restricted. This control is decreased or lost when the vessel reduces its speed as it approaches its load or discharge harbour. Figure 1, Tugboat requirement

Figure 1 illustrates a tugboats primary service which is to control sea-going vessels in confined waters. Subject to the speed of a sea-going vessel two fundamentally different services can be distinguished. This paper focuses on ship-handling and harbour ship assistance in port (restricted waters) including making fast, ship turning and berthing/unberthing.

Basic ship-handling Basic ship-handling involves three basic maneuvers: 1) making fast 2) turning a ship in the harbor basin and 3) berthing a ship to its designated berth. Being in control, requires an understanding of how assisted vessels are controlled by the assisting tugboats. Push-pull operations are generally accepted and known worldwide. Alternative style operations are available such as the Rotterdam developed Full pull system (no pushing). Ship-handling operations remain very much subject to personal preference of the pilots in charge and tugboat masters.

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Making fast Tugboats make fast at about anywhere between 10 knots for escort duties and 6-8 knots for your general harbor approach. Now imagine making fast in the turbulent wake of a big sea-going vessel sailing at 8 knots and what effect this has on for example tugboats with big lateral underwater areas. A Rotor®Tug enables a tug master to make fast in between 5-7 minutes. Conventional tugboats with big lateral surface areas generally make fast in excess of 10 minutes. This doesn’t look like much, but the sea-going vessel moved some 750 meter ahead in the meantime. Ship turning Generally a ship calling at a port will enter the port under escort of tugboats before being turned in a turning basin and towed to its designated berth. Turning a ship before berthing generally enables simple un-berthing and sailing from a port after loading or discharging. Figure 2, Ship turning using push-pull

The assisted vessel is turned by the turning couple (Bollard pull x Lever arm L) enacted by the assisting tugboats about its pivot point P;

Lever arm L ↑ means increased turning moment = bollard pull effectiveness ↑;

Figure 3, Ship turning using full-pull

Lever arm L ↑ 30% due to towing in the line instead of pushing on the designated tug area;

Increased turning moment reduces turnaround time ↓ 30% and is an effective way to increase number of ship movements in fixed timeframes;

A combination of figure 2 and figure 3 ship turning is also feasible dependent on: → local conditions (wind/current/under keel clearance/ jetty type or quay side structure) → vessel size

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→ available tugboat power → tugboat type •

For example only Tractor – and Rotor®Tugs can safely operate as front tug in the configuration of figure 3.

Berthing Berthing a ship means bringing the ship alongside its berth such that the line-handling crew and ship’s mooring system can safely connect the assisted vessel to its designated berth. In order to achieve this the tugboats bring the assisted vessel alongside and keep her stationary until a safe shore connection is established. Berthing with an ASD- or tractor tug are such similar operations that figure 4 displays both. Control of the assisted vessel’s transverse speed is critical when berthing. Figure 4, Ship berthing with Push-Pull method using conventional tugs

Repositioning required between pulling and pushing mode with two tugs → reaction time ↑ when changing vessel direction. Generally acceptable to Beaufort 2 wind speeds;

Additional tugs can be added pushing in the side while simultaneously pulling on front and stern → reaction time ↓ with > 2 tugs;

Berthing with >2 tugs is a slow but controlled maneuver

But pushing in waves risks damage to assisted vessel and tugboat;

Thrust deduction from propeller wash when pulling to berth (up to 60%!) dependent on towline length. Limited under keel clearance of the seagoing vessel enhances this effect;

Figure 5, Ship berthing with Full Pull method using Rotor®Tugs

Limited repositioning when changing vessel direction from/to berth → reaction time ↓;

Additional tugs can be added pushing in the side while simultaneously pulling on front and stern on pilots command if circumstances require same;

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No thrust deduction from propeller wash in case of Rotor®Tug propulsion configuration;

Rotor®Tugs can deliver ±60% bollard pull transverse force on the assisted vessel at zero risk to damage to assisted vessel and tugboat;

Bollard pull There are various graphs available to determine required (transverse) bollard pull dependent on the local circumstances (wind, current and waves) near a berth or quay-side and assisted vessel type and size. Berth conditions, (tidal) currents and waves are more or less fixed and predetermined by port infrastructure, geography and main commodities. The remaining primary drivers are thus wind speed and - direction, especially important considering the large wind area of container vessels and Very Large Gas Carriers. Figure 6, Required bollard pull per wind speed for a VLGC 25

Wind speed (m/s)

20

15

10

5

0 0

200 300 Required Bollard Pull (mt)

100

400

500

Figure 6 displays required bollard pull for a typical Very Large Gas Carrier with full beam winds;

Combined with histograms for wind conditions (figure 7) in a harbor or berth it is straight forward to determine required bollard pull and thus the # of tugs required per % of time;

Figure 7, Wind speed histogram (W-Australia) 20% 18% 16% 14% 12% 10% 8% 6% 4% 2% 0% 0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Wind speed (m/s)

Figure 8 displays two histograms on how many tugs are needed, by % of time to achieve this required bollard pull. Mean average current – and wave forces are presumed for this example;

The bollard pull in the example of figure 8 is delivered by 80t BP tugboats using push-pull methods (figure 3) with conventional tugboats and full-pull method (figure 5) with Rotor®Tugs;

Navigational computer simulations are generally available to verify towing methods and tugboat requirements by situation. Computer simulations also enable cross-training between pilots and tugboat masters if available near a port of operations increasing skill-levels and mitigating navigational risk.

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Figure 8, Required # tugs per % of time 100% 90% 80% 70% 60% 50%

Conventionl tugs

40%

Rotor®Tugs

30% 20% 10% 0% 2

3

# Tugs

4

5

Three (3) Rotor®Tugs can deliver the same effective Bollard Pull compared to four (4) conventional tugboats. Even in case of equipment failure (propulsion unit breakdown), Rotor®Tugs can provide safe ship-handling operations 98% of all times (86% for conventional tugboats);

Cost savings amount to capital investments (18%), maintenance – (20%) and crewing cost (25%) respectively in the listed example. Reviewing towing methods and available tugboat types for ship-handling operations can offer substantial benefits;

Conclusion •

Control: tugboats are an effective (navigational) risk management tool. Tugboats primary role in ship-handling operations is to improve maneuverability of big sea-going ships in or near port areas and when berthing a ship;

Safety: tugboats execute their primary role by moving sea-going ships as fast as possible through high-traffic areas with associated navigational risks. Reducing making fast- and turnaround times offer an effective way to mitigate navigational risks;

Reliability: detailed analysis of local circumstances and port lay-outs can clearly describe the safe working limits in a port. This analysis and data can also be used to display safety level of different towing methods and tug fleet profiles in case of equipment failures.

Performance: three (3) 80 ton BP Rotor®Tugs, can deliver the same effective bollard pull compared to four (4) 80 ton BP conventional tugboats 99% of the time. More and high-power tugs do not necessarily improve safety levels;

Redundancy: in case one propulsion units onboard the Rotor®Tugs fails, the fleet can still deliver the required bollard pull 98% of the time. Conventional twin-drive tugboats are ineffective operating with one thruster-unit only, and can only perform in 86% of all working conditions in case of a propulsion unit failure;

Cost: Detailed analysis of local circumstances, available towing methods and tugboat types can enable significant cost savings; maintenance (↓20%), crewing (↓25%) and capital investment (↓ 18%) reductions at higher safety levels;

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Appendix 1 – Tug boat types Figure 9, Azimuth stern driven tug

Figure 10, Tractor tug boat

Figure 11, Rotor tug

Table 1, Comparison between an ASD -, Tractor - and Rotor tug boat AZIMUTH STERN DRIVE TUG

TRACTOR TUG (f.i. VSP)

ROTOR TUG

Draught > 75 TBP

less than Tractortug more than Rotortug

more than ASD and more than Rotortug

less than ASD and less than Tractortug

Draught < 75 TBP

less than Tractortug and Rotortug

more than ASD and more than Rotortug

more than ASD and less than Tractortug

Safe towing points

1 safe towing point at the bow

1 safe towing point at the stern

2 safe towing points at stern as well as bow

Towing over stern

risk of capsizing by girting

safe

safe

Towing over bow

safe

not possible

safe

good, safe over the bow

good, safe over the stern

good, safe over bow and stern

good, safe over the bow

good, safe: if waves not too high

good, safe over bow

good, safe over the stern

good, safe over stern

high, approx 80% of max pull

Push/Pull Safe connecting to stern of speeding ship in waves and current Safe connecting to bow of speeding ship in waves and current BP sideways (pushing with the side) Side stepping Towline control at mooring in confined or restricted areas

unsafe, due to less control with waves on aft deck limited BP

limited BP

approx 3-4 knts

approx 3-4 knots

high, approx 6-7 knts

only in line with tow line

only in line with tow line

good, high pulling in any direction (rotoring)

due to one end propeller configuration

due to one end propeller configuration

due to unique triangle thruster configuration

goes outside path width,

goes outside path width,

stays within path width

need to reposition see

need to reposition see

no need to reposition see

not possible

not possible

Towline control at narrow passages good, thruster configuration makes dynamic

Positioning in current without force

position possible

on townline good, less when speed decreases, Escort capabilities

good, less when speed decreases

good, also when speed decreases risk of capsizing at high speed (10 knts) still 66 pct BP, good maoeuvrability

Propulsion or winch brake down assisting strongly restricted

assisting strongly restricted

risk of propeller ventilation

no propeller ventilation

or via other winch

while towing

Behaveour in swell during assistance

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good, no propeller ventilation. Extra: ability for wave damping with fwd thruster units during pushing

Shiphandling  

Shiphandling with Rotortugs

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