Section 1 Introduction

1.1 Background

1.1.1 This guidance note has been produced to provide guidance on the Classification of tugs with Lloyd’s Register, including information on tug types, operational restrictions, notations and design calculations. In addition, for tugs with a (freeboard) length L_LL ( as defined in the International Convention on Load Lines (ICLL)) of not more than 100 m, this note goes beyond the limited scope of Class Rules to provide guidance on best practice and general statutory requirements.

1.2 Scope

1.2.1 The content of this guidance note relating to Classification aspects is relevant to all tugs eligible for Classification to which one or more of the Type Notations specified in Ch 2, 1.1 Type and Service Restriction Notations intended to be are assigned.

1.2.2 The content of this guidance note relating to statutory aspects is relevant to tugs having a (freeboard) length L_LL, as defined in the International Convention on Load Lines (ICLL), of not more than 100 m.

1.2.3 Classification of tugs requires that the vessel complies with the Rules and Regulations of Lloyds Register. The relevant requirements are made up of the general requirements for all vessels in the Rules and Regulations for the Classification of Ships (The Rules and Regulations for the Classification of Ships, July 2022) and any relevant specific requirements.

For all tugs the Classification requirements may include;

The above list of applicable requirements is not exhaustive and may vary from vessel to vessel based on the operational profile, the tug type, equipment, fit out etc.

1.2.4 All vessels are subject to the requirements set out by their relevant Flag Administration(s). Typically, such statutory requirements go beyond the scope of class. This Guidance note sets out some general guidelines in these areas however in all cases the requirements of the relevant flag Administration(s) are to be applied.

1.2.5 There should be no instances where this guidance conflicts with the requirements of the Flag Administration, however if an instance is identified for a vessel that is to be Classed the conflict should be highlighted to Lloyd’s Register at the earliest opportunity to ensure that the conflict is rectified as soon as possible to allow Classing and Flagging of the vessel.

1.2.6 The Rules and these guidance notes assume that no escort operations will be conducted at speeds greater than 10 kn, however for vessels designed to conduct escort at speeds greater than this application of the Rules and these guidance will be specially considered.

1.2.7 The sizing of the prime mover and the propulsion chain to develop sufficient thrust for the vessel to perform her predicted duties is beyond the scope of Classification.

1.3 Definitions

1.3.1 Design bollard pull

The design bollard pull T_BP, in kN, is the maximum sustained towline force a tug is capable of generating at zero forward speed, to be initially specified by the designer and to be verified by a full scale test, generally referred to as bollard pull test.

Where T_BP is not available, the following default values may be used as an estimate for a preliminary design review:

T_BP

0,204NP_S for conventional tugs with propellers fitted with nozzles;

T_BP

0,176NP_S for tractor tugs and ASD tugs with steerable propellers fitted with nozzles

where:

N : number of propellers;

P_S : maximum continuous power per propeller shaft, in kW.

1.3.2 Escort forces and speed

The steady towline force during escorting, F_t in kN, is the towline force associated with the (quasi-static) equilibrium in indirect towing mode, excluding short time-duration dynamic effects, for a given loading condition and escort speed V, see Figure 1.1.1 Typical escort configuration. The steady towline force is applied by the tug on the stern of the escorted ship.

Figure 1.1.1 Typical escort configuration

Additionally, the steady towline force F_t can be decomposed into a steering force F_s and a braking force F_b :

The steering force F_s, in kN, is the transverse component of the steady towline force F_t with respect to the escorted ship;
The braking force F_b, in kN, is the longitudinal component of the steady towline force F_t with respect to the escorted ship.

For the purpose of this guidance note the following rated values of the above defined escort forces are defined as:

The rated steady towline force F_t,R, in kN, is the highest anticipated steady towline force F_t, as obtained from the evaluation of the escort forces for a particular loading condition and escort speed, taking into account the applicable stability and strength criteria in this guidance note;
The rated steering force F_s,R, in kN, is the highest anticipated steering force F_s, as obtained from the evaluation of the escort forces for a particular loading condition and escort speed, taking into account the applicable stability and strength criteria in this guidance note;
The rated maximum braking force F_b,R, in kN, is the highest anticipated braking force F_b, as obtained from the evaluation of the escort forces for a particular loading condition and escort speed, taking into account the applicable stability and strength criteria in this guidance note.

And the associated maximums are,

The design maximum steady towline force F_t,MAX, in kN, is the highest rated steady towline force F_t,R over the applicable range of loading conditions and escort speeds;
The design maximum steering force F_s,MAX, in kN, is the highest rated steering force F_s,R over the applicable range of loading conditions and escort speeds;
The design maximum braking force F_b,MAX, in kN, is the highest rated braking force F_b,R over the applicable range of loading conditions and escort speeds.
The maximum escort speed V_MAX, in kn, is the highest escort speed V for which the escort tug is considered to perform escort operations.

For the purpose of this guidance note the following relevant angles are defined as:

The towline angle α, in deg, is the angle between the towline and the centreline of the escorted ship and;
The drift angle β, in deg, is the angle between the centreline of the tug and the centreline of the escorted ship (also referred to as yaw angle).

1.3.3 Reference towline force

The reference (quasi-static) towline force T, in kN, is considered to represent:

the design bollard pull T_BP for type notations tug, see Ch 1, 1.3 Definitions 1.3.1;
the maximum steady towline force F_t,MAX for type notation escort tug, see Ch 1, 1.3 Definitions 1.3.2.

1.3.4 Design load

The design load (DL), in kN, is the force taken into consideration for the strength assessment and testing of the towing equipment and the associated supporting structures, and for the purposes of design appraisal it is taken as not less than:

DAF x T

where

DAF: dynamic amplification factor

The dynamic amplification factor takes into consideration dynamic effects. Reference values for the dynamic amplification factor are given in:

For type notation tug: Ch 2, 2.2 Towing arrangements for Tugs 2.2.3
For type notation escort tug: Ch 2, 2.3 Towing arrangements for Escort Tugs 2.3.5.

1.3.5 Winch brake holding load

The winch brake holding load (BHL), in kN, is the maximum towline force the towing winch can withstand without slipping of the (activated) brake, considering the towline at the first inner layer.

The BHL is a reference value for strength assessment and testing of towing winches and associated towing fittings (e.g. fairlead, staple, gob-eye) as well as their supporting structures.

1.3.6 Towline breaking strength

The towline breaking strength, in kN, is the tension required to cause failure of the towline (parting of the towline).

1.4 General Guidance

1.4.1 All bollard pull tests should be performed in accordance with a recognised Standard, such as the ‘Lloyd’s Register Bollard Pull certification procedures guidance information’, and witnessed by a Lloyd’s Register Surveyor.

1.4.2 For tugs capable of towing over the stern (ahead towing) as well as over the bow (astern towing), the bollard pull test should be performed for both scenarios.

1.4.3 If the measured bollard pull for any vessel is higher than the design bollard pull (T_BP) by 1 per cent or more then aspects of the design appraisal of the vessel may need to be redone reflecting this new bollard pull. The extent of reappraisal is at the discretion of Lloyds Register.

1.4.4 Angles α and β and maximum escort speed V_MAX (see Figure 1.1.1 Typical escort configuration) should be defined by the designer prior to commencement of design appraisal.

1.4.5 The matrix of rated steady towline forces F_t,R, steering forces F_s,R and braking forces F_b,R should be specified by the designer for design appraisal and are latterly verified by Lloyd’s Register on the basis of the results of:

full scale trials, or
model testing, or
a computer simulation program accepted by Lloyd’s Register.

1.4.6 All full scale trials conducted to verify the above matrix of forces, should be performed in accordance with a procedure agreed with Lloyd’s Register prior to commencement of the trials. Further guidance on such trials is contained in Ch 3, 2.3 Intact stability 2.3.6.

1.4.7 All Model testing, where applicable, should be performed in accordance with a procedure agreed with Lloyd’s Register before commencement of the tests. The testing should comply with the relevant aspects of Ch 3, 2.3 Intact stability 2.3.6.

1.4.8 Special attention should be paid to scale effects when processing any model scale measurement results to create predictions at full scale.

1.4.9 Computer simulation programs for predicting escort performance should comply with the relevant aspects of Ch 3, 2.3 Intact stability 2.3.5.

1.4.10 Lloyd’s Register will accept escort performance predictions from computer simulation programs in lieu of full scale trials where the predictions are carried out in accordance with Lloyd’s Register’s ShipRight Procedure titled Guidelines for CFD Escort Tug Performance as detailed in Pt 4, Ch 3, 9.4 Computational Fluid Dynamics Predicted Performance of the Rules and Regulations for the Classification of Ships, July 2022

1.4.11 In order to maintain the Classification of any tug, the vessel will be subject to an ongoing periodical survey regime to ensure that the vessel and the equipment relevant for Classification remain in a worthy condition. Details of the through life survey requirements can be found in Part 1 of the Rules and Regulations for the Classification of Ships, July 2022.

1.4.12 For high powered escort tugs (with a free running speed of more than 15 kn) Lloyd’s Register will specially consider the application of the Rules and these Guidance notes to the vessel assuming an escort speed of 12 kn.

1.4.13 Propulsion engines and propulsion train should develop sufficient thrust for manoeuvring the tug quickly for any drift angle, and in the case of loss of propulsion, the heeling moment due to the remaining forces should lead to a safe equilibrium position of the tug with reduced heeling angle.

1.5 Escorting dynamics

1.5.1 For the purpose of this guidance note, escorting is considered to include active (emergency) steering, braking and otherwise controlling of the escorted ship by the tug operating in indirect towing mode, whereby the ahead speed of the escorted ship is within a typical speed range of 6 to 10 kn.

1.5.2 In indirect towing mode the towline force is the resultant of the (quasi-static) equilibrium condition reached between the forces and moments arising from the hydrodynamic lift and drag forces acting on the hull and appendices of the tug advancing through the water at a drift angle relative to the water flow, the thrust vector and the towline force (In direct towing mode the thrust is directly applied to generate the towline force, hydrodynamic lift and drag forces play no significant role).

Escort tugs may work in different indirect towing modes, depending on the required action towards the escorted ship (e.g. steering, braking). The main indirect towing modes relevant for escort tugs are schematically shown in Figure 1.1.2 Schematic overview of indirect towing modes (escort tug). Where reference is made to ‘indirect steering’ the objective is to maximise the steering force in indirect towing mode. Where reference is made to ‘indirect braking’ the objective is to maximise the braking force in indirect towing mode.

In (basic) indirect mode the towline force is generated primarily by the hydrodynamic forces acting on the hull and skeg, with the thrust used solely to maintain the desired drift angle (also referred to as yaw angle).

In powered indirect mode (indirect steering) the transverse component of thrust is used to maintain the desired drift angle, while a significant longitudinal component of thrust is applied in forward direction of the tug.

Compared to the (basic) indirect mode, the tug is operating more sideways of the escorted ship with a relatively large towline angle, generating a higher steering force.

In combination mode (indirect braking) the same principle as for the indirect steering mode is applied, except that the longitudinal component of thrust is applied in aftward rather than forward direction.

Compared to the (basic) indirect mode, the tug is operating more behind the escorted ship with a relatively small towline angle, generating a higher braking force.

For indirect towing modes it is generally recognised that it is beneficial to design the tug to generate high (indirect) towline forces with minimal propulsion thrust, while respecting the limits imposed by stability and strength considerations (towing equipment, general hull structure).