Section 9 Escort operation, performance numeral and trials

9.1.1 An escort tug is a tug intended for escort operation. Escort operation is an operation in which the tug closely follows the assisted ship providing control by steering and braking, as necessary.

9.1.2 Escort tugs are to be capable of utilising methods of towing through which steering and braking forces are generated by a combination of propulsive and hydrodynamic forces developed by the tug, acting on the towline to the attended ship, see example in Figure 3.9.1 Forces generated during a typical escort mode.

9.1.3 The intact stability of the tug during escort operation is to comply with a Standard recognised by the National Administration with whom the ship is registered and/or by the National Administration within whose territorial jurisdiction the tug is intended to operate, as applicable. Attention is drawn to the inherent problems relating to the quick release of the towline and the sudden loss of propulsion power during the escort operation in addition to the maximum steering and braking forces.

9.2.1 The specified breaking strength of the towline is to be at least 2,5 x maximum design towline force.

9.2.2 The towing winch is to include a system of continuous load monitoring, with a bridge readout display and an overload prevention system, which is to be operational during escort duties. The overload prevention system is to be designed with the capability to pay out the towline in a controlled manner when the load reaches the maximum design towline force, and is to be capable of alerting the Master and crew.

Figure 3.9.1 Forces generated during a typical escort mode

9.3.1 Escort tugs which carry out full scale performance trials in accordance with the requirements of this Section will be eligible to have the escort performance numeral EPN (F,B,V,C) appended to the escort tug notations, see Pt 4, Ch 3, 1.2 Class notations 1.2.1 and Pt 4, Ch 3, 1.2 Class notations 1.2.2 and Pt 4, Ch 4, 1.2 Class notations 1.2.2,

where

9.3.2 The performance numeral may be determined with speed V equal to either 8 knots or 10 knots. If both sets of numerals are determined at the trials then the class notation will include them all.

9.3.3 A trials plan, which includes the estimated forces, is to be submitted and approved prior to trials being undertaken.

9.3.4 The trials of the escort tug are to be performed using a ship capable of maintaining almost constant heading and speed when subjected to the steering and braking forces from the escort tug.

9.3.5 The following trials are to be carried out in calm weather conditions and in the presence of a Clasifications Register Surveyor:

• Steering and braking force capability test, see Pt 4, Ch 3, 9.3 Performance numeral and trials 9.3.6.
• Bollard pull test, see Pt 4, Ch 3, 9.3 Performance numeral and trials 9.3.8.

A record of the results is to be kept on board the escort tug.

9.3.6 Prior to commencing a trial, the following data are to be recorded:

• Wind speed and direction.
• Sea state.
• Current speed and direction.
• Water depth.
• The main particulars and the loading condition of the assisted ship.
• Loading condition of the escort tug.

9.3.7  Steering and braking force capability test is a test by which the steering force, F _s, and braking force, F _b, are determined when utilising the method, shown in Figure 3.9.1 Forces generated during a typical escort mode, of towing at a range of towline angles, α, from 0 to 90 degrees and for a range of operating speeds up to and including the maximum escort speed. The following parameters are to be continuously recorded during the test:

• Position, speed and heading of the assisted ship and the escort tug.
• Towline force, F _t.
• Angle of towline, α.
• Heel angle of the escort tug.
• Direction of thrust and power absorbed by all propellers and thrusters of the tug.
• Rudder angles of the tug.

9.3.8 The length of the towline is to represent a typical operating condition and is to be recorded prior to and at the completion of the test. The steering and braking forces for a given speed and angle can be calculated by using the average values of the recorded towline force.

9.3.9  Bollard pull test is to be carried out in accordance with LR's Bollard Pull Certification Procedures Guidance Information.

9.4.1 Where the performance of the escort tug during escort operation in indirect towing mode has been predicted and reported using computational fluid dynamics in accordance with the requirements of this Section and in lieu of full-scale performance trials, the vessel will be eligible to have EPN (CFD: F,B,V) appended to the escort tug notation, see Pt 4, Ch 3, 1.2 Class notations 1.2.1, Pt 4, Ch 3, 1.2 Class notations 1.2.2 and Pt 4, Ch 4, 1.2 Class notations 1.2.2.

9.4.2 The computational fluid dynamics investigation is to include escort performance prediction with an escort speed equal to 8 knots and/or 10 knots.

9.4.3 A computational fluid dynamics report is to be submitted to LR for consideration. The performance prediction simulating the trials detailed in Pt 4, Ch 3, 9.3 Performance numeral and trials is to be carried out in accordance with LR’s ShipRight Procedure titled Guidelines for CFD Escort Tug Performance.

9.4.4 The length of the towline in the computational fluid dynamics simulation is to represent a typical operating condition.

9.4.5 The report or a summary of the results obtained therein is to be kept on board the vessel and made available prior to the commencement of any full-scale performance trials.