Memo Pilot Transfer

December 11, 2017 | Author: PietGebruiker | Category: Tugboat, Ships, Watercraft, Water Transport, Shipping
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Paper To:

To whom it may concern

From:

Marinus Jansen Msc.

Date:

August 17, 2012

Re:

Pilot transfers with tugs

Introduction Tugboats perform a multitude of services in ports all over the world. Apart from their primary shiphandling and escorting duties, tugboats can perform various auxiliary services like fire-fighting and pilot transfers. To enable a safe pilot transfer operation the tugboat should be able to come safely alongside, provide a stable platform for the pilot transit and get in position for its primary service (either being shiphandling - or escort duties). Sailing away from a sea-going vessel at 8-10 knots with negative wave fields around the ship can be a challenging operation for any tugboat master. A great number of tugboat accidents have been contributed by the inability of conventional tugboats to steer away from an assisted vessel and tugboat masters losing control over their vessel. It is our firm belief that a master should always be in control of his vessel. Therefore a tugboat master should be able to operate in a safe manner and have the ability to steer away at all times. In addition a tugboat used for pilot transfers should be able to maintain a stable position alongside a vessel when delivering pilots. Some tugboat types are more suited for this purpose then others as this paper will explain.

Basic hydrodynamics There are two types of ship resistance being friction – (water flowing alongside the hull) and wave resistance (by alternating pressure fields from moving a body (the ship) through the water. This latter resistance creates the wave system around a ship as displayed in figure 1 and - 2. Figure 1 , Wave system around a ship – side view

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Figure 2 , Wave system around a ship – top view

These waves are the result of high/low pressure field generated by the ship moving through a body of water and its surface boundary. In either case this pressure will pull a tugboat into a vessel, or push it away. Neither case is favorable when maintaining a safe- and stable position near a ship. For purposes of the below examples we assume a negative wake field when steering away and positive wake pressure field pushing a tugboat away from the assisted vessel when transferring the pilot. A tugboat master has no control of this wave system and it generally depends on the assisted vessel length (on the waterline) and speed while boarding. It is entirely possible that a tugboat is pulled towards the vessel near its front and pushed away towards its stern and/or vice versa.

Ship handling Generally three types of propulsion configurations for tugboats are accepted in today’s market environment. See also appendix 1 for more information on these types and their respective propulsion configurations In order to safely perform pilot transfer operations we’re looking for two key criteria in a tugboat 1) it should be able to steer away from the vessel at all times; and 2) a tugboat should be able to maintain its position to enable safe boarding by the pilot at high speed (10 knots). Figure 3 illustrates the respective tugboat types when steering away from a vessel. Figure 3 presumes a negative pressure pulling the tugboat to the assisted vessel. This effect being the worst case scenario when steering away from an assisted vessel. Figure 3, Steering clear with a tugboat



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Tractor – and Rotor®tug type tugboats can steer away in a safe manner;



Although these types are pulled towards the assisted vessel, the tugboat master can safely steer away using (one of) the forward thrusters. The thruster unit pulls the tugboat out of the negative wake pressure field;



ASD tugs have a serious problem, the tugboats front is pulled towards the assisted vessel by the negative wave pressure field and there is no propulsion unit available to pull the tugboat clear;



ASD tugboats risk collision on their aft side due to the tugboats midship turning point;

Pilot transfer Main key tugboat criteria for pilot delivery is maintaining a stable position alongside an assisted vessel in order for the pilot to safely board. As previously indicated the wake pressure field alongside a vessel can vary from pulling nearby bodies next to it, to pushing them away. In order for the tugboat master to have full control of his vessel the tugboat should be able to exert forces towards its stern and bow. This same criteria applies for example when operating at a LNG carrier’s side bitt . See also figure 4 for the respective tugboat types. Figure 4, Pilot transfer with tugboats



Tractor and ASD type tugboats experience difficulties maintaining their position alongside assisted vessel at10 knots;



Wave system (and pressure effects) increase with speed (fewer, but higher waves);



Rotor®tugs are able to maintain position alongside and enable safe pilot transfer due to thrusters fore and aft at 10 knots;



Rotor®tugs can safely establish a towing connection on LNG carrier’s side bitts;



Large area hull appendages and skegs commonly found on ASD and tractor escort tugs increase susceptibility to the listed effect.

Conclusion 

Tractor - and Rotor®tugs can safely steer clear from assisted vessels when operating in close proximity to same (for example also when operating near the bow of an assisted vessel);



Both tractor- and ASD-tugboats have great difficulty in maintaining a position alongside an assisted vessel at higher speeds;



Rotor®tugs can safely maintain a position alongside assisted vessels enabling safe boarding of pilots at high speed;



To be in control of their vessel a tugboat should always have the ability to operate in a safe and controlled manner;



Rotor®tugs offer a tugboat master full control of his vessel during pilot transfers.

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Appendix 1 – Tug boat comparison Figure 5, Azimuth stern driven tug

Figure 6, Tractor tug boat

Figure 7, 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

Towline control at narrow passages Positioning in current without force on townline Escort capabilities Propulsion or winch brake down while towing

Behaveour in swell during assistance

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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

good, less when speed decreases

good, less when speed decreases, risk of capsizing at high speed (10 knts)

assisting strongly restricted

assisting strongly restricted

risk of propeller ventilation

no propeller ventilation

good, thruster configuration makes dynamic position possible good, also when speed decreases still 66 pct BP, good maoeuvrability or via other winch good, no propeller ventilation. Extra: ability for wave damping with fwd thruster units during pushing

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