AN Ch. 9 — Escort Tugs

1. The background to escorting (9.1)

Source: HENSEN, Henk. Tug Use in Port: A Practical Guide. 4th ed. Rotterdam: STC Publishing, 2021. Chapter 9 — Escort tugs.

Edital: Anexo 2-B, Área II (Arte Naval / Shiphandling), item 9 (Hensen) → Chapter 9 — Escort tugs · Anexo 2-A, Área II, item 26 (Utilização de rebocadores portuários — emprego em modo de escolta) com itens 4 (Emprego de rebocadores na manobrabilidade) e 31 (Métodos de utilização de rebocadores empregados no Brasil).

Escorting by tugs is nothing new, and this chapter follows on from all the earlier ones rather than standing apart. Ports along rivers, canals or behind locks have long sent tugs to accompany large ships from the river entrance up to the berth. The service is also used for big tows such as offshore rigs, for ships with limited manoeuvrability after engine or rudder trouble, and in bad weather. Those older services, however, stay inside port areas and adjacent rivers. The escorting this chapter is really about is something narrower: the escorting of tankers and gas carriers in port approaches.

Specific attention to tanker escorting began in the USA around 1975, when the State of Washington mandated Foss Maritime to escort tankers over 40,000 dwt to limit oil spills in Puget Sound; laden tankers in Prince William Sound, Alaska, have been escorted since 1977.

Tanker, oil-tanker and gas-tanker escorting then spread, especially across Europe. In Norway it became mandatory in the Grenland area in 1979 after the gas tanker Humboldt incident, with a purpose-built tractor tug tethered to the ship from 1981; after the bulk carrier Mercantile Marcia sank in 1989 it was extended to tankers over 30,000 dwt at Mongstad and Sture. Sweden (Gothenburg, 1990), Finland (early 1990s) and Spain (La Coruña, after the Aegean Sea, 1992) followed; in the UK escorting is decided port by port and began on the Solent in 1991.

Underway in inland and restricted waters — where escort tugs would operate — allisions/collisions and groundings together account for 17 % of the tanker accidents that caused spills over 700 t. Whether escort tugs could have prevented a given accident depends on its location, the nature of the failure, whether human error on board or ashore was involved, and the environmental conditions at the time.

2. Studies on escort requirements (9.2)

Whether escorting suits a particular port or approach has to be settled by a thorough study, not assumed. Such a study reviews the present situation, runs a risk assessment, checks whether escorting would actually reduce the risks of a passage, and then defines escort-tug requirements, escorting procedures and training.

2.1 Reviewing the present situation

The review weighs many factors together: the port and its approach (restrictions, bends, distances, depths, tides), the environmental conditions along the whole passage, the bottom and channel sides (rocky or sandy, steep or flat), anchorages, traffic separation schemes, aids to navigation, vessel traffic services, the shipping traffic itself (number, size, draft, speed, cargo), arrival/departure policy, pilotage, accident statistics, the environmental impact of an incident, and the tugs available. It may already lead to better procedures or services. Only when no further improvement is possible, or it is not enough, is a formal risk assessment carried out — establishing the probability and severity of an incident and the areas of concern.

2.2 Accident scenarios and response time

To find out whether escort tugs could reduce the risk, a set of accident scenarios is built for the areas of concern, covering engine and rudder failures and a powered ship steering a dangerous course. Each scenario must include the response time — the gap between a failure happening and the tugs becoming effective — based on realistic assumptions, because that time is critical to limiting how far a tanker advances and transfers after a failure.

3. Developments in escorting (9.3)

3.1 Oil tankers

Escorting has long focused on tankers, to cut the risk of grounding and the oil spill that may follow. Much has changed: in 1992 MARPOL was amended to require double hulls on tankers of 5,000 dwt and more ordered after 6 July 1993, with single-hull tankers phased out by 2010. Partly because of this, tanker spills have fallen sharply even as oil and gas transport grew — 2019 and 2020 saw the lowest spill numbers in 50 years, with about 1,000 t lost in each, the lowest annual figures in five decades.

3.2 LNG carriers

Whether an LPG grounding spills more than an LNG one should be investigated separately: LPG (mainly propane and butane) is carried in various tank types. In April 2018 a collision on the Singapore Roads between the tanker Astro Saturn and the LPG carrier Crystal Sunrise leaked an estimated 1,796 t of butane, posing no danger to other shipping.

3.3 Blockage of the approach channel

There is also an economic reason to escort. If a navigational error or an engine/rudder failure leaves a large ship blocking the approach channel, clearing it can take hours or days — so ships are escorted to keep the channel open, as at Port Hedland and the ports of Dampier and Weipa (Australia) and for large container vessels bound for Southampton. Escorting bulk carriers raises the demands on deck fittings: about 35 % of Capesize vessels at Port Hedland have bitts and leads not usable by the port's tugs, so indirect towage can put over 120 t in the towline. Since 1 February 2021 the Pilbara Ports Authority therefore requires all Capesize vessels (≥ 120,000 dwt) to have a set of bitts and Panama lead/roller fairleads aft, near the centreline, rated to at least 120 t SWL.

4. Escorting objectives and tug placement (9.4)

The objectives of escorting are to ensure a safe passage through the approach channel — applying steering and braking forces to a disabled vessel and keeping it afloat, or at least limiting the impact of a collision or grounding — and to reduce the risk of pollution from groundings or collisions caused by technical or human failures on board a tanker.

Two broad methods are used: escorting by a number of normal harbour tugs, and escorting by specifically designed escort tugs. In either case the tugs accompany the ship either with towline(s) secured or free-sailing at close quarters, ready to make fast if a failure occurs. Harbour-tug escorting is generally done inside port areas, over short distances and at low speed; purpose-built escort tugs work in port approaches, over longer distances and at higher speed.

5. Escorting by normal harbour tugs (9.5)

In some ports just one harbour tug, of any type, regularly escorts tankers; in others the number depends on ship size and the tugs available. The escort distance is usually only a few miles, sometimes longer through rivers and channels, at about five to six knots — though unsecured tugs, or longer distances, can push speeds up to nine knots. Escorting can be done with tugs at the ship's side (possibly including a rudder tug), with tugs towing on a line, or a mix; the choice depends on local practice and the tugs available.

5.1 Secured or not — the trade-off

5.2 Effective and ineffective positions (Fig. 9.5)

In the example, a loaded tanker underway has an engine failure and veers to starboard, which the rudder cannot stop. As explained in section 4.3.5, the forward tug (no. 1) is not in a position to counter the sheer effectively — it may even make things worse — while the after tug (no. 2) and the rudder tug (no. 3) are much more effective. A rudder tug is as effective in steering as a side tug secured with lines, but with the advantage that it can apply force to port and starboard. Side tugs and forward tugs applying braking also create a turning moment, another reason tugs are needed on both sides; a rudder tug brakes without a high turning moment. Tugs at the side with a bow line can apply braking forces as well as steering forces.

5.3 Towing on a line, and escort speed

When tugs tow on a line (Fig. 9.5B), securing near the bow means ship's speed should not exceed about six knots. A stern tug with omni-directional propulsion, or a carrousel or combi-tug with an after towing point, can apply braking and steering forces both ways; a forward tug towing on a line, like a side tug, increases ship's speed when steering and is poor at opposing a sheer compared with a stern tug, though it does pull the ship away from danger. Towing on a line allows somewhat higher speeds than conventional side tugs — about four to five knots with a suitable conventional forward tug and an omni-directional after tug. A conventional tug used aft instead limits speed to a maximum of three to four knots, since it can brake and steer to both sides only at very low speed, and a conventional forward tug cannot brake at all.

Training and escort planning matter for harbour-tug escorting too, though they depend on the local situation — they are discussed under purpose-built escorting (9.6.4).

6. Escorting by purpose-built tugs — methods (9.6.1)

Two methods are used: escorting by one escort tug (Fig. 9.9), the most common for years, and escorting by two escort tugs — dual escorting, also called team towing or tandem escort towing (Figs. 9.10–9.11). The tandem method, both tugs tethered, began in the Port of Long Beach, California, using modern VS or ASD/reverse-tractor tugs, so that relatively small escort tugs can handle heavy ships. Escort speeds with team towing are fairly low — about six knots, up to perhaps eight depending on tug design, crew training and sea conditions.

7. The indirect mode and the geometry of forces (9.6.2)

An escort tug is, properly, a tug designed to escort ships over long distances at relatively high speeds; all are of the omni-directional type, ASD/reverse-tractor or tractor (most escort tractor tugs use VS propulsion). They are secured (tethered) to the ship's stern or kept unsecured (untethered) but ready. Tethered to the stern, they can apply high steering and/or braking forces if a failure occurs, and at high speed the steering force is generated in the so-called indirect method.

7.1 Direct, indirect and powered-indirect (Fig. 9.8)

Figure 9.8 shows the direct and indirect towing methods, with the terms once used by propulsion maker Aquamaster: the indirect arrest mode is recommended to initiate a turn, and the combination arrest mode to oppose a turn at low and higher speeds. In direct mode the achievable steering force falls as speed rises; above the normal harbour speed of about 5–6 knots, higher steering forces come from the indirect mode (Fig. 9.12).

High steering forces result — higher than in direct mode. In the 5–7 knot range, US measurements show line pulls of 75–125 % of the tug's bollard pull: a 4,400 hp tug of about 52 t bollard pull reached roughly 62 t steering force at 5 knots (≈ 120 %), falling to about 75 % at 7 knots. Crucially, the powered indirect mode delivers steering force much faster than the direct (dynamic arrest) mode, in which the whole tug body must be dragged across the flow — slow, especially at 5–7 knots. Escort speeds, though, can reach 10 or even 12 knots, which is why the indirect mode is developed here.

7.2 Centre of pressure and towing point (Figs. 9.13–9.14)

For tug performance, the size and especially the points of application of the transverse forces matter most. Fig. 9.13 shows a tractor tug in indirect mode: the propeller thrust holds the transverse and longitudinal forces — from the hydrodynamic force on hull and skeg and from the towline — in balance. The two locations that govern performance are the centre of pressure (COP) and the towing point (T), described by four levers:

• Lever x vs lever y (horizontal). The larger x is compared with y, the less sideways thrust is needed to balance the hydrodynamic force at the COP — so the higher the towline force. Good design pushes the COP as far forward as possible.
• Lever a (vertical, T above COP). The larger the vertical distance between towing point and COP, the larger the list (heel) for a given towline force.
• Lever b (vertical, T above propulsion point P). The larger the vertical distance between propulsion point and towing point, the more the sideways thrust reduces the list.

8. Braking and steering forces, speed and stability (9.6.2)

Escort tugs must deliver steering and/or braking forces in an emergency. Steering forces are considered the more important — as long as there is room ahead and the bends are not too sharp, a ship can be steered clear of danger. But whether steering will be enough depends on many factors; environmental conditions may drift a ship into danger as soon as its speed falls after an engine failure, whatever steering is given. When the area or distance ahead is very restricted, braking is needed, and the most effective way to take way off — given room — is to initiate a turn, which slows a tanker or gas carrier quickly and cuts head reach.

Rudder forces are largest at about a 25° rudder angle, typical of most tankers' rudders. Required steering and stopping forces rise as underkeel clearance falls (Chapter 6), and after a failure beamy full-bodied ships develop the fastest rates of turn — so insight into the ship's behaviour is part of setting escort requirements. The maximum forces a local escort tug should apply must come from a study of representative failure scenarios for the ships and area, validated by practical tests; a tethered tug reacts faster and so needs relatively lower steering forces than a passive (untethered) one, which suffers a much larger time delay.

Maximum forces also depend on sea conditions. The SAFETUG project (Chapter 4), testing an ASD and a Voith tug, found the tug's own escort towing capability in waves (up to 2.5 m at 6 s period, up to 4 m at 10 s) was in most cases close to its calm-water value — but it was the transit towards the ship that produced the highest, critical transverse accelerations at the tug master's position, and all assist work in waves demands operation-specific tug-master training. The largest purpose-built escort tugs seem limited to about a 4 m wave height, and even then need a towing winch with a load-reducing system.

8.1 Required free-sailing speed

Maximum escort speed is usually around 10 knots but can be up to 12, so the tug's free-sailing speed must be higher — present escort tugs reach 12.5 to 15 knots. An escort tug needs this over-speed to overtake the escorted ship in time, to overcome the ship's propeller slipstream when approaching the stern to pass or connect the towline, to keep reserve power for strong interaction effects at high speed, and because in bad weather a tug's speed may fall faster than a large ship's, and any deviation from a straight course cuts the tug's speed in line with the ship.

8.2 Stability — heeling is the limit

9. Design, towlines, fail-safe, communication and planning (9.6.2)

9.1 Design developments and the additional towing point

An escort tug must be seaworthy and use its best capabilities. ASD-tugs run bow first, free-sailing or tethered — the fastest and safest direction for the deck crew, especially in high waves and in indirect mode. Tractor tugs run bow first when free, but tethered they run stern first, with the lower after deck leading and a lower maximum speed; at speed and in waves, water comes over the after deck easily. So some VS escort tugs (the Bess and Boss, Fig. 9.20; also the Ajax) were redesigned with higher sheer and bulwarks at the skeg end, a more pointed hull, and the wheelhouse turned 180° for a good view in the operating direction. Skeg developments continue for both VS and ASD tugs: modern VS-tugs use high-lift (hydrofoil-shaped) skegs, and the Voith Turbo Fin (VTF), with a rotating tube at the skeg end, is claimed to add about 18 % steering force.

9.2 Deck equipment and towlines

Towlines are generally HMPE fibre (Spectra/Dyneema) with pennants of the same material, sometimes nylon stretchers; lengths are usually 100–150 m, sometimes 60–80 m at the tug master's discretion. HMPE lines are light and easy to handle — important when the ship's crew may have to lift the line aboard by hand with no power — and they float, so they do not easily foul propellers; but their stretch is very low, so short lines easily give high peak loads. A pennant, often of the same or larger breaking strength, is cow-hitched or spliced eye-to-eye and extends the main towline's life (wear is largest where the line is taken aboard the ship). All towing equipment must be highly reliable and sized for the highest expected loads, and escort winches should have high brake-holding power, fast deployment and retrieval, a high pull and a load-reducing system to avoid excessive towline loads, especially in waves. Some class societies set detailed escort-winch requirements (e.g. BV: the brake must act directly on the drum and work if primary power fails; winches for wave areas need an active pay-out / haul-in system reacting at 110 % of rated towline force; escort in such areas may not rely on the drum brake).

Deck equipment on the escorted ship must suit the high towline loads and the type of towline; complaints about a lack of suitable strong points and fairleads are common. Where they are too weak, escort speed may have to be reduced, or dual escorting used (smaller tugs or limited towline force), or — for tankers not meeting OCIMF recommendations — a second tug kept standing by. OCIMF recommends using the SOLAS emergency towing arrangement for escort/pull-back too, without impeding the towing arrangement.

9.3 Connecting, fail-safe and communication

10. Escort tugs in use, training and a summary of requirements (9.6.3–9.6.5)

10.1 Escort tugs in use

The tugs used are ASD/reverse-tractor and VS tugs, with bollard pull between roughly 40 and 140 t. A sharp definition is hard: an escort tug is properly one built specifically to escort ships — tankers and gas carriers — at relatively high speed, but harbour tugs with escort capabilities are often used for escorting too, so the two cannot be cleanly separated. Even so, purpose-built escort tugs keep being built — the new tugs for Norway and for Prince William Sound, the icebreaking Ocean Tundra (Fig. 9.27) and the icebreaking Yuribey for the Russian Arctic port of Sabetta.

10.2 Training and pilotage

10.3 Summary of escort-tug requirements (9.6.5)

Hensen lists what a good escort tug needs:

• Optimal manoeuvrability and high free-sailing speed; high working reliability.
• Good sea-keeping both free-sailing and in the escort operating direction; sufficiently high freeboard; good static and dynamic stability.
• A safe working deck for handling towlines in rough seas at high speed.
• Ability to apply high steering and/or braking forces over the whole escort speed range, and to assist in different ways.
• Bollard pull enough to keep the escorted ship under control once speed has reduced to zero, allowing for wind and current.
• A safe, effective towing-point location for heeling angle, achievable towline force and engine-failure behaviour; an extra towing point if needed.
• Deck equipment that easily withstands the high towline forces; towlines with a high safety factor, light and strong synthetic fibres with positive buoyancy, passable by hand if the ship has no power at its stations.
• Good fendering, preferably all round; a reliable radio system; a well-equipped wheelhouse with all-round visibility, towline-tension control and displays, emergency abort, and window heating.
• Weather-deck openings giving access below fitted with watertight doors, kept closed during escort operations.

Fire-fighting and pollution-control duties, and operations in cold climates, add their own specific requirements (as with Dux, Pax and Audax).

11. Escort regulations and concluding remarks (9.7–9.8)

11.1 United States — OPA 90 and 33 CFR 168

US federal, state and local regulations apply in several compulsory escort areas, federal rules overriding the rest; "escort vessel" can mean any suitably fendered and outfitted tug. OPA 90 requires the Secretary of Transportation to define areas where single-hull tankers over 5,000 gross tonnage must have two escort vessels — Prince William Sound and Puget Sound — with rule-making delegated to the US Coast Guard (33 CFR 168, "Escort Requirements for Certain Tankers"). The 2010 Coast Guard Authorization Act extended this to double-hull tankers over 5,000 t in those waters, also requiring at least two towing vessels.

11.2 Europe and the Turkish Straits

In Europe escort rules are state or local terminal regulations agreed between port authority, pilots and tug owners, with no fixed bollard-pull rule; most ports use one purpose-built tug, but two if the ship does not meet OCIMF fittings recommendations. In Norway, the Sture Terminal compels escort for all crude tankers and LPG vessels over 20,000 grt (one tug to 180,000 dwt, two above); Mongstad for oil tankers over 20,000 grt (two above 320,000 dwt); and dedicated stretches (Fugløya–Porsgrunn, Melkøya). In the UK, Southampton escorts inbound/outbound oil tankers over 60,000 dwt (Esso Fawley) and certain large container ships, and Sullom Voe requires two tugs — one secured at the stern in drag mode — with tankers at a moderate speed not over 8 knots. In the Turkish Straits escorting is not compulsory but strongly recommended: LNG carriers by tugs of at least 60 t each totalling 150 t bollard pull, LPG carriers over 200 m LOA by tugs of at least 90 t.

11.3 Concluding remarks (9.8)