Section 2 Operations and arrangements
Clasification Society 2024 - Version 9.40
Clasifications Register Guidance Information - Guidance Note for the Classification, Safe Design, Construction and Operation of Tugs, August 2018 - Chapter 1 Background information - Section 2 Operations and arrangements

Section 2 Operations and arrangements

Parent topic: Chapter 1 Background information

2.1 Typical tug operational profiles

2.1.1 This sub-Section provides an overview of some of the typical operational profiles for which a tug might be designed and equipped.

2.1.2 Harbour tugs

Harbour tugs are considered to be tugs specially equipped to assist ships and/or floating offshore units while entering or leaving port and during berthing and unberthing operations. Harbour tugs are considered to navigate in calm stretches of water (sheltered waters) and commonly have a Lloyd’s Register service restriction notation. Usually harbour tugs work from a fixed port; the crew is familiar with the operating area and shore side facilities for maintenance, repairs, spare parts, etc. should be readily available. Also, in case of emergency, shore side assistance should be readily available.

2.1.3 Seagoing tugs

Seagoing tugs are considered to be tugs specially equipped to assist ships and/or floating offshore units at sea, but may also be involved in harbour towage operations. Seagoing tugs can either operate without any restriction (deep sea towage, in any sea area and any period of the year), within short distance from shore (for example: coastal towage), or at a specified location (for example offshore terminal tugs). The Lloyd’s Register service restriction notation should be tailored to reflect the operational profile of the vessel. Seagoing tugs may include but are not limited to:
  • Salvage tugs: These tugs are considered as seagoing tugs having specific equipment for salvage operations and due to the nature of their operations, are to be able to operate under all conditions. Consequently, even if the crew may be familiar with the operating area, Lloyd’s Register considers that shore side facilities and emergency assistance are not readily available for these vessels.
  • Coastal and offshore terminal tugs: These tugs are considered to operate within a short distance from shore and the crew is considered to be generally familiar with the operating area and readily available shore side facilities. Lloyd’s Register also considers that in case of emergency shore side assistance is readily available for this type of tug provided it is restricted to operating no more than four hours at operational speed from a place of safe sheltered anchorage. Again such an operational restriction should be recorded in the Lloyd’s Register service restriction notation.
  • Deep sea tugs: These tugs operate without any restriction and may perform duties in any seaway. For deep sea towage Lloyd’s Register considers the crew to not necessarily be familiar with the operating area and that shore side facilities will not generally be readily available in both normal operations and in emergency situations.

2.1.4 Escort tugs

An escort tug is a tug intended for escort operations. An escort operation is an operation in which the tug closely follows the assisted ship providing control by steering and braking as necessary using forces generated by a combination of propulsive and hydrodynamic forces developed by the tug, acting on a towline to the attended ship. Lloyd’s Register considers escorting to include active (emergency) steering, braking or otherwise controlling of the assisted ship while approaching a port or terminal, or while navigating in confined waters. Typically escorting involves indirect towage at speeds in excess of 6 kn. Lloyd’s Register considers escort tugs to operate in open sea areas and/or in sheltered (confined) waters (e.g. in ports) any limitations on the area of operation should be documented by means of the Lloyd’s Register service restriction notation.

If escort tugs operate from a fixed station, the crew is generally familiar with the operating area and shore side facilities (for maintenance, repairs, spare parts, etc.) are readily available. It is considered that in case of emergency shore side assistance is readily available provided that the escort tug does not proceed in the course of the voyage more than four hours at operational speed from a place of safe sheltered anchorage.

2.2 Typical tug arrangements

2.2.1 This Section provides a general and indicative description of some typical tug design arrangements. This list is not exhaustive and it is recognised that other arrangements exist in addition to the ones specified here. This guidance note is applicable to the specific design arrangements mentioned below, but could also be applied to other design arrangements, as far as deemed reasonable and practicable.

2.2.2 Conventional tugs

Conventional tugs have single or multiple shaftline arrangement. The propeller(s) can be of fixed pitch or controllable pitch type normally fitted with Kort nozzle(s). Steering is by means of rudder(s) or steerable nozzle(s). The towing point is normally located slightly aft of the centre of lateral resistance for towing over the stern with a towing hook and/or towing winch.

Conventional tugs may also be equipped to perform pushing operations.

2.2.3 Tractor tugs

Tractor tugs are fitted with omnidirectional thrusters (typically two steerable propellers or Voith–Schneider type cycloidal propulsion units), which are located forward of the towing point (usually not more than 30 per cent of the length from the forward end). A skeg or vertical fin is fitted aft. Towing is performed over the stern with a towing winch and/or towing hook.

Tractor tugs may also be equipped to perform pushing operations.

2.2.4 Azimuth stern drive tugs

Azimuth stern drive (ASD) tugs are fitted with multiple steerable propellers located near the aft end. Typically, ASD tugs can perform towing operations over the bow with the forward towing winch (towing operation similar to a tractor tug) and over the stern with a towing hook and/or second towing winch (towing operation similar to a conventional tug). In both cases the towing point is located forward of the thrusters. ASD tugs may be fitted with a skeg, the size and location depending on the intended function.

ASD tugs may also be equipped to perform pushing operations.

2.2.5 Other design arrangements

Other tug design arrangements, having towing and propulsion configurations different from the design arrangements described above, may be specially considered by Lloyd’s Register in accordance with the Rules and the concepts presented within these guidelines.

Specific reference is made to tugs which are equipped with multiple omnidirectional thrusters distributed along the length. Such tugs have the capability to generate relatively high transverse thrust compared to tugs with omnidirectional thrusters installed at a single point along the length of the vessel. Care should be taken when considering the guidance provided by this note with respect to the self-tripping heeling arms of the stability criteria, as these statutory requirements assume that every thruster with the same longitudinal location is to be considered as belonging to the same group of thrusters.

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