1. Differences in tug design and assisting methods (1.1)
The way tugs assist ships in ports and port approaches around the world is not uniform. Methods grew out of local conditions, particular situations and long-standing customs and traditions. Those differences in practice are mirrored in the requirements placed on the tugs themselves, and so in the wide range of tug types that has developed.
Source: HENSEN, Henk. Tug Use in Port: A Practical Guide. 4th ed. Rotterdam: STC Publishing, 2021. Chapter 1 — Tug Design Factors.
Edital: Anexo 2-B, Área II (Arte Naval / Shiphandling), item 9 (Hensen) → Chapter 1 — Tug design factors · Anexo 2-A, Área II, item 26.1 (Fatores de projeto de rebocadores portuários), com itens 4 (Emprego de rebocadores na manobrabilidade) e 31 (Métodos de utilização de rebocadores no Brasil).
Harbour tugs have developed rapidly in recent years. New types offer high manoeuvrability and much greater engine power; modern steering devices, new towing appliances and new towline materials have been fitted. These advances change both the methods of assistance and the number of tugs used. One specific driver was the Exxon Valdez disaster, after which the requirement to escort tankers in certain port approaches produced purpose-built escort tugs and tugs with escort capabilities.
Two trends push the same way. Modern ships manoeuvre better, and modern tugs tow better — so fewer tugs are needed per ship. Economic pressure reinforces this: captains and pilots are often pushed to use the minimum number of tugs. The result is that each individual tug now plays a more crucial role, which demands a high level of operational safety, reliability and suitability for the job.
Answering the question "which type of tug, or which working method, is best for a certain port?" takes extensive research and knowledge: a profound understanding of the different tug types, their capabilities and limitations, and a good insight into the local situation. The operational requirements a harbour or terminal tug must meet for ship assistance are determined mainly by the following broad factors:
- The kind of port and approaches, foreseeable future developments and the existing geographical environmental conditions.
- The type of ships calling at the port.
- The services required in and around the port and, where relevant, at offshore locations — jetties, LNG terminals, SPMs, F(P)SOs, FLNGs or oil rigs.
2. Categories of port and their approaches (1.2.1)
The first factor is the port itself. Ports can in general be divided into four categories, each creating its own requirements for tugs and tug assistance.
| Category | Defining feature | Consequence for tugs |
|---|---|---|
| a. Conventional ports | The majority of ports, often with a long history. Ships berth in harbour basins, docks or river berths and frequently pass locks and/or bridges. Connection to the sea may be short (Rotterdam, Le Havre, Hong Kong) or long and complicated (Antwerp, Calcutta, New Orleans). | Tugs must suit assistance both inside the harbour and outside it — on the river or at sea. Ports often do not grow as fast as the ships calling, so manoeuvres get harder and demand more engine power, manoeuvrability and the right dimensions. |
| b. Ports with mainly terminals | Container and similar terminals often have plenty of room to manoeuvre, so manoeuvres can be more standardised. | Very well suited to the push-pull method; tugs are specifically designed for this type of operation. |
| c. Ports with mainly piers and jetties | Jetties may lie in the open sea or in protected waters. The key difference from a normal harbour is the method of mooring: at a jetty, mooring is usually on dolphins or a finger/T-pier. | That layout lets tugs operate on both sides of the assisted vessel, whereas in harbour basins and at quays assistance is, in most cases, restricted to one side. |
| d. Mooring facilities at remote locations | Offshore jetties and LNG terminals, where ships are often made fast under wind and waves. Tugs handling SPMs and certain F(P)SOs and FLNGs face specific demands. | Special requirements: good stability, sturdy fendering, render-recovery winches and high engine power; for SPM/F(P)SO/FLNG work, specific engine power, manoeuvrability and towing winches according to ship size and conditions. |
Ports under development
Ports keep developing — new berths, new basins, new ports altogether. It is desirable that tugboat companies and pilots take part in the design studies at an early stage: they can advise from real shiphandling experience and order new tugs suited to the new situation. Regular consultation between port authorities, designers, tug companies and pilots favourably affects the accessibility of ports. In container ports especially, the demand for large land area to stack containers may conflict with the minimum manoeuvring area ships and tugs need, calling for specific tug type, power, towing equipment and assisting method.
Port approaches
Approaches are under the influence of the open sea — wide or narrow, sandy or rocky, winding or straight. Tugs used there should be capable of working in more open-sea conditions with waves and/or swell. After the Exxon Valdez disaster there is a growing tendency to require an escort for oil and gas tankers in port approaches, and escort tugs must comply with very specific requirements.
3. Environmental conditions (1.2.2)
Geographical environmental conditions matter greatly to a tugboat company. Most older ports sit in river estuaries, subject to tides and seasonal effects; fairways and rivers change constantly. Differences in water depth, bridge passages and lock entries may force the adoption of time windows, so accessibility can be complicated. The requirements a tug must meet may therefore change continuously along the route — from the entrance or approach, up to the berth and the final mooring. Some ports solve this by using different tug types for the various parts of the route.
Ports close to the sea may be hit by waves and swell, adding requirements; the same applies to tugs working at offshore locations or in colder areas where ice may be met. Limited water depth where harbour tugs operate gives rise to special requirements on a tug's maximum draft.
4. The ships concerned · Services required (1.2.3–1.2.4)
4.1 The ships concerned (1.2.3)
Another important factor is the type of ship to be assisted — large LNG or LPG tankers, FPSOs, FLNGs, bulk carriers, oil tankers, container ships, car carriers, ro-ro ships and so on. Different ships impose specific requirements on the tug: engine power, towing equipment, fendering, manoeuvrability and superstructure. Where only tankers moor alongside an oil jetty, the tug and power required are easy to determine; a large variety of ship types and sizes means the tug requirements vary just as much.
4.2 Services required in and around the harbour (1.2.4)
Beyond assisting seagoing vessels, harbour tugs are often used for other duties, each demanding a specific type, size, manoeuvrability, equipment and towing method:
- Towing offshore material, such as oil rigs and crane barges.
- Towing inland barges, elevators and floating cranes.
- Assisting push-tow barges.
- Fire-fighting and pollution-control duties.
The same is true of tugs that must operate at SPMs, F(P)SOs, FLNGs, LNG terminals or oil rigs, and of tugs that assist at dockyards.
5. Assisting method in use (1.2.5)
The method of assistance a port uses depends on a set of conditions, and the type of tug used depends largely on the assisting method — the tug must, as far as possible, meet the requirements of that method.
- Port, jetty, terminal layout and/or offshore installation.
- Types of ship.
- Environmental conditions.
- Navigational complexity of river, channels and port approach.
- Whether bridges and locks have to be passed.
- Whether escorting is needed.
- Often, simply tradition.
The method may also depend on the availability of mooring boats. When none are available, the ship must be brought very close to — or even alongside — the berth to pass the mooring lines, so tugs must be able to push at the ship's side. In a number of ports tugs are used to brake and control the ship's speed, for example when approaching a lock or passing a bridge, and must be capable of doing so.
6. Available experience · Safety requirements (1.2.6–1.2.7)
6.1 Available experience (1.2.6)
Pilots and tug captains are accustomed to the assisting methods and tug types used in their port. They have built up experience with these tugs and their crews, know the advantages and shortcomings, and can therefore anticipate. Changing to a new system or new tug type may introduce difficulties, takes time and must be weighed carefully: training and instruction are needed, especially when the new tug and the way it operates differ totally from the existing system. A well-planned changeover is essential, and all this must be taken into account before introducing a new tug type or assisting method.
6.2 Safety requirements (1.2.7)
Tugs sometimes have to work in hazardous areas — for instance when handling LNG carriers or working at LNG terminals — and must then meet certain safety requirements. Tug assistance always includes risks for the tug and her crew; these are minimised by good training and by a well-designed, well-equipped tug, and the type of tug also influences the level of safety.
7. Summary of the factors (1.2.8)
No two ports are the same. Many factors influence the choice of tug type — local customs, environmental conditions, water depth, the type of ships using the port and escort requirements — and they can differ enormously from port to port. Requirements for tugs working at offshore installations may be an additional factor. Crucially, these factors influence not only the type of tug needed but also the assisting method used, and that method has, in turn, a strong relation back to the type of tug.
| Factor group | Examples given in the book |
|---|---|
| Passage / Berth | Sea/approach, river, channel, water depth, locks/bridges, jetties & terminals (open sea or protected waters), harbour basins, river berths, mooring buoys, mooring boats. |
| Environmental conditions | Swell, waves, wind, current, ice, fog, hazardous areas. |
| Types of ship | General cargo, container vessels, car carriers, ro-ro, tankers/VLCCs, gas tankers, bulk carriers, ferries, passenger/cruise vessels, navy vessels (e.g. submarines, aircraft carriers). |
| Other services required | Offshore installations, SPMs, FPSO/FLNGs, barges, floating cranes, dockyards, escorting. |
| Assisting methods | Towing on a line, push-pull, alongside towing, escorting. |
| Existing tugs | Conventional single/twin/triple screw, tractor tug, VS (Voith) tractor tugs, azimuth, ASD tugs, reverse-tractor tugs, Rotor tugs, Z-tech tugs, tugs with one propulsion unit forward and one aft. |
| Available experience | Tug-type experience; assisting-method experience. |
| Safety of tugs | Tug type, training, port/State regulation, classification regulations, environmental conditions, hazardous-area regulation. |
| Financial aspects | Budget, tug price, operating costs. |
8. Types of tug · Assisting methods · Conclusion (1.3–1.5)
8.1 Types of tug (1.3)
The factors above have produced different tug types worldwide. At present the following are mainly used:
- Single screw tugs.
- Twin screw tugs.
- Tractor type tugs.
- Tugs with azimuth propellers aft.
- Rotor tugs.
- Tugs with one propulsion unit forward and one aft.
Single and twin screw tugs are generally well known; many are fitted with nozzles to increase towing power, with fixed or variable pitch propellers. To improve manoeuvrability a bow thruster — on single screw tugs often a 360° steerable and retractable bow thruster — may be installed, which also increases towing force. Tractor tugs have their propellers under the forebody, either Voith propellers or 360° steerable azimuth propellers. Tugs with azimuth propellers aft resemble twin screw tugs but, thanks to the 360° steerable thrusters, are far more manoeuvrable. Rotortugs have two azimuth units forward and one aft and are very manoeuvrable. Various tug types also exist with one propulsion unit forward and one aft. The different tug types are detailed in Chapter 2.
8.2 Assisting methods (1.4)
Depending on local experience and circumstances, the following assisting methods — or combinations of them — are mainly used:
- Tugs towing on a line.
- Tugs operating at the ship's side.
Tugs working at the ship's side can be made fast with one, two or three lines. In the push-pull method the tug is usually made fast with one line. In alongside towing the tug is securely lashed alongside the ship with a minimum of three lines. The different assisting methods are discussed in Chapter 3.
8.3 Conclusion (1.5)
No port is the same with respect to tug requirements — layouts differ, as do the ships frequenting the port, the environmental conditions and local traditions, and consequently the tug types and assisting methods. A simple answer to "which type of tug and/or which towing method is most suitable for the port?" cannot easily be given; too many factors play a role. It takes reliable research, weighing all advantages and disadvantages. Most important is not just what forces must be considered, but how, when and under what conditions they act — ship's speed, confined areas, environmental conditions and underkeel clearance. The outcome may be a tractor type with azimuth or Voith propulsion, or even a conventional tug; escorting tankers brings additional requirements.