1. Overview — Special Operations
Source: NAYAK, Capt. Santosha K. Theory and Practices of Marine Pilotage. Chapter 19 — Special Operations.
Syllabus: Anexo 2-B, Área II (Arte Naval), item 8 (NAYAK), Cap. 19 — operações especiais (STS, reboque de barcaças). Cross-list Área III, item 6.
Pilotage includes many special operations in special circumstances besides the relatively simpler berthing and un-berthing procedures. Though the basic principles of pilotage and ship handling are similar, some operations pose a higher challenge to the pilot. Such operations may include — but are not limited to — towing, cold moves, trans-shipment, STS operations, handling laden Capesize bulk carriers or VLCCs, and handling large, unusually dimensioned un-propelled objects.
When such operations are carried out regularly, the pilot's experience makes them relatively easy. But at times a pilot has to carry out a special operation only once in a while, out of some compulsion, and then he does not have much experience in handling it. This chapter treats three of them: STS operations, SBM operations and customary towing operations.
2. STS Operations — Pre-Approach, Vessel Combination and Fendering
An STS (ship-to-ship) transfer generally involves trans-shipment between two ships and is sometimes called lightering. The large vessel is the SBL (Ship to be Lightered) and the small one the SS (Service Ship). They are positioned alongside each other — either stationary or underway — to commence cargo transfer. The operation is usually carried out for huge oil tankers in the open sea, when the ship does not berth in a port or jetty, especially because of draught restrictions or port berthing charges. The motivation is a lack of deep-water ports and economic aspects. Sometimes the transfer is needed between damaged ships after an accident, to save the cargo and mitigate emission to the environment. Accidents involving oil tankers represent approximately 10% of the world's marine accidents, and STS operations are expected to increase significantly in frequency and expand into new geographical areas in the coming years.
2.1 Pre-approach planning and preparations
STS operations are carried out in different scenarios — at anchor, at berth, or while underway. When STS is planned, the mother vessel (SBL) has to be prepared in many aspects and assessed as to whether she is ready for double banking in all respects. After careful examination of the case, the necessary risk assessment, and being equipped with all required arrangements, the vessel decides on double banking. The vessel must be provided with big Yokohama fenders to create sufficient space between the two hulls.
The pre-approach check covers the following points.
| Check | What to verify |
|---|---|
| Combination of vessels | Small + big, same size, or big + small — assess how the freeboard and manoeuvring differ for the pair |
| Freeboard difference | Check the freeboard difference between SBL and SS, which drives fender height and lead of ropes |
| Fenders | Condition of the fenders must be in good order; big Yokohama fenders create sufficient space between the hulls |
| Ropes | Prepare good ropes through the designated leads, otherwise chafing occurs |
| Crew | Crew must be trained and ready to carry out the mooring operations |
3. STS Approach Manoeuvre
The most common incident during STS operations is a collision between the two ships while manoeuvring alongside each other or while sailing. Such a collision typically occurs for one of these reasons: an incorrect approach angle between the manoeuvring ships; approaching at excessive speed; or the failure of one or both ships to appreciate the weather conditions.
3.1 Approach while at anchor or berthed
While the Service Ship approaches the SBL that is berthed alongside or lying at anchor, the following precautions apply.
| Precaution | Reason |
|---|---|
| Do not approach at an angle at any cost | Any angle may cause metal-to-metal contact between the ships |
| Make the best use of the wind | Let the wind help set the SS gently onto the fenders |
| Slowly fall off the fenders / touch parallel to the fenders | Parallel contact spreads the load; an angle points a corner at the other hull |
| Touch the fenders with no forward or aft speed | Fore-and-aft speed at contact may break the lines |
| Use tugs judiciously, if used | Injudicious tug use can drive the vessels together or off station |
3.2 Approach while underway
The standard approach manoeuvre in STS lightering has the SBL maintain a constant heading at minimum controllable speed (5 knots or less) or drifting with wind and current. The Service Ship approaches and berths normally on the lee side of the constant-heading ship. The standard manoeuvre is divided into two phases.
The initial phase is basically a collision-avoidance manoeuvre from the current position to the final position, in order to obtain the required safe distance between the two ships. That safe distance is the Distance at Closest Point of Approach (DCPA), appropriate to the conditions. During this phase the SS must approach the SBL on a parallel course and adjust its velocity to equal that of the SBL. The second phase — operation of the ships alongside — takes place after the safe distance has been maintained: the manoeuvring ship positions itself relative to the constant-heading ship, and makes contact by reducing the distance until the fenders touch. Both ships are then on parallel courses at similar velocity, with their manifolds in line, so the force of berthing bears simultaneously on all fenders.
In open waters the standard approach begins at a distance of 0.5 NM from the destination point and finishes at a DCPA of approximately 50–100 m off; the mooring lines start about 20–30 m away from each ship. Normally the manoeuvre is made with the wind and sea ahead, though this depends on local conditions and on the knowledge and experience of the SS navigator. Throughout the operation, visibility must be good enough for safe manoeuvring, taking safe-navigation and collision-avoidance requirements into account. This standard manoeuvre applies when the ships are under power in a normal STS transfer; the procedure may vary for an emergency with oil spill, an inshore operation, a limited geographical scope, particular dynamical and kinematical ship properties, and the weather and traffic density.
4. Emergency STS Transfer with Oil Spill — the Three Phases
A major activity during any STS transfer is the safe approach to the SBL, which moves on a constant heading at slow speed or drifts. The approach is a sequence of navigation manoeuvres in a specific environment with environmental and operational constraints. In an emergency STS transfer, additional aspects appear: the ship and cargo condition (trans-shipment from the undamaged side), time limits, water-area constraints, and the avoidance of a moving oil spill or other rescue units. These extra constraints may call for a specific type of manoeuvre depending on the scenario. The pilot's objective is to define the approach as a problem of safe trajectory planning, taking into account weather, traffic density and the stop-and-speed control performance of the vessels.
Manoeuvres are made from leeward and to the starboard side of the constant-heading ship. Service Ships dedicated to STS transfer are mostly equipped with an aft main propeller, usually with a rudder and a bow tunnel thruster. The main propeller produces the surge force needed for transit; the rudder produces the yaw moment for steering control; the tunnel thruster produces a sway force and is only effective at low speed. Azimuth thrusters can produce two force components — surge and sway — in the horizontal plane. To control the SS during the approach, the manoeuvre is divided into three parts in order to reach the final distance from the SBL, a parallel course and equal speed.
| Phase | Speed regime | Controls used / objective |
|---|---|---|
| Trajectory tracking (planning) | Moderate or high (> 2 m/s) | Minimise the tracking error; rudder for course control and main propeller for speed control (ROT and rudder-angle control) are the only effective means |
| Stopping manoeuvre | Reducing along the trajectory | Reduce the ship's velocity; judge stopping ability by emergency or normal stop; stop the engine and keep course with rudder amidships, or use engine astern |
| Dynamic positioning | Low (< 2 m/s) | Course and position control only; steering mostly by azimuth, bow and/or stern thrusters, since rudder efficiency drops at low speed; allows longitudinal, transverse, rotation-about-axis or side movement at an angle |
5. STS Mooring, Cargo Transfer, Unmooring and Departure
5.1 Mooring and cargo transfer
After the approach manoeuvre, the ships lie alongside at the required safety distance (DCPA), both keeping constant heading and constant speed or drifting at about zero. In this condition the berthing — by tunnel thruster — and the mooring — by lines — can start. During cargo transfer, careful attention must be given to the prevailing circumstances. Checking the mooring lines is of utmost importance, particularly in these conditions.
| Watch this during cargo transfer | Why it matters |
|---|---|
| Inclement weather condition | Wind and swell load the lines and set up motion between the hulls |
| Change of tide | Alters relative levels and line tension |
| Freeboard of the vessels changing | As cargo transfers, freeboards diverge and lines slacken or come taut |
| Changes in drafts and trims | Change the lead and tension of the lines |
| Changes in rolling period due to change in GM | Altered rolling can surge the pair on the fenders |
5.2 Unmooring and departure manoeuvre
The sequence of unmooring must be decided carefully after examining the existing situation. Once both vessels are ready in all respects to sail the Service Ship away from the SBL, the unmooring follows. Cast off the head and stern lines and hold on to the springs forward and aft. If tugs are available they should avoid pushing the vessel onto the Yokohama fenders. Once the head and stern lines are on board, cast off the springs from the inner vessel. When all lines are clear, get the vessel pulled by both tugs parallel to the fenders; once sufficiently clear, she can be manoeuvred away from the fenders.
6. Single Buoy Mooring (SBM) Operation
A Single Point / Single Buoy Mooring (SBM/SPM) is an arrangement whereby VLCC tankers moor to a single buoy for loading or discharging to or from storage where no dedicated cargo facility exists; the cargo is transferred through subsea manifold structures. The SBM is usually anchored to the seabed with multiple anchors spread evenly, so as to keep it stable in all sea and swell conditions. The buoy is held in position in two ways: CALM (Catenary Anchor Leg Mooring) and SALM (Single Anchor Leg Mooring). An SBM has five main parts: the buoy body; the anchors and cables running down to the seabed; the fittings for mooring; the product-transfer system carrying the product from the buoy to the subsea manifold; and the fenders that prevent damage should the vessel hit the buoy. The mooring fittings are arranged so that the buoy moves freely within defined limits.
6.1 Preparations prior to arrival and mooring
The vessel also keeps ready to receive the hose-connection gear and crew when they board while approaching the SPM, with the crane ready to lift the hose and gear; the lifting and handling equipment on the buoy allows the connection of hoses and tools.
6.2 Manoeuvring, mooring and cargo
The pilot boards from a tug boat at the pilot station, usually a few miles from the SBM. Normally two tugs guide the vessel in the berthing process. After berthing and mooring are complete, one tug is released while the other remains standby near the vessel for pull-back when required; the pilot usually stays on board to help maintain position with the one tug. Approaching the pickup lines, the pilot or berthing master may move to the focs'le for better conning, in communication with the bridge — a better forward view than from the bridge. As the vessel approaches the pickup lines, speed is reduced to less than half a knot or so; the pickup lines pull the chafing chains on board, and the slack is picked up slowly while the lines are kept a little slack. At some SBM terminals there are no tugs but two line boats: one clears the floating hoses of the tanker's approach and the other pulls the ropes to the vessel's bow.
After mooring, the hose-connection procedure follows: hose gear and crew board while approaching the SPM, and the crane lifts the hose for connection. Normally a loading master or pilot stays on board once berthed, to coordinate between ship and terminal, assess the weather and decide to stop loading in severe weather. Engines remain standby on short notice, to prevent any unwanted movement of the vessel towards the SBM; an astern engine is required if the vessel is riding up to the buoy because of weather from astern.
6.3 Un-berthing and departure from SBM
A careful assessment of the existing weather must be made before considering to un-berth. If the vessel is heading to the weather — tide, wind or current — the lines remain taut; it is then necessary to give a kick on the engine to move the vessel marginally ahead, which slacks the lines, helps the chafe chains release from the stoppers, and facilitates the Smitt bracket to be unshackled. Once the lines are cast off, the vessel may give an astern engine order or be back-pulled by the astern tug to fall astern, clearing the pickup ropes from the focs'le. This takes some time; once all the ropes are clear she may leave the berth.
7. Customary Towing — Preparations and Bollard Pull
Towage is 'a service rendered by one vessel to aid the propulsion or to expedite the movement of another vessel'. It can be part of a salvage or wreck-removal operation following a casualty, or occur when a ship is in distress to avoid a casualty. In the vast majority of cases, however, towage is a routine operation, particularly within the confines of a port — this is customary towage. An agent of the ship, or the charterer, usually requests the tug; once engaged, the tug may take its orders from any pilot on board the towed ship. The responsibility for engaging tug assistance rests with the ship's master, who may be found negligent for not engaging a tug where the circumstances warrant it and an accident occurs. The rights and responsibilities of tug and towed ship are generally dealt with in the applicable towage contract.
7.1 Planning and preparation
Planning before a tow commences might include: assessing the size and type of vessels or barges to be towed and any limitations of the tow; confirming that the tug is suitable in size, manning, sea-keeping, horsepower (HP) and bollard pull (BP); verifying that the tow wire and equipment suit the planned tow; planning the route and passage — safe transit times, narrows, bridges (with a list of maximum and minimum heights and tide height for each arch), areas of high traffic, tight river bends, reduced depths, tidal limitations and currents; a weather forecast with an outlook of at least 48 hours; connection and disconnection arrangements with sufficient fuel, water and spares; and emergency contingency plans.
7.2 Bollard pull formula for a towed barge
To calculate what the bollard pull of the tug should be for a towed barge of displacement Δ and the stated dimensions, the following formula has been used as a guide.
| Symbol | Meaning |
|---|---|
| Δ | Full displacement of the towed vessel (tonnes) |
| V | Tow speed (knots) |
| B | Breadth of the towed vessel (metres) |
| D | Depth of the exposed transverse section of the towed vessel, including deck cargo, measured above the waterline (metres) |
| K | Factor reflecting potential weather and sea conditions: 1.0 to 3.0 for exposed coastal tows; 0.75 to 2.0 for sheltered coastal tows |
8. Rough Weather and Length of Towing Line
8.1 Precautions for rough weather
Rough weather for a small tug or workboat is not restricted to strong winds. Many claims arise where the tug-and-tow unit contacts a third-party vessel, berth or fixed floating object through misjudging the prevailing weather when manoeuvring. Adverse conditions can be caused by the action of wind against tide; tidal bores, rip tides or strong currents; the interaction of strong river currents with prevailing currents or winds at the mouths of large rivers; sudden changes in current from increased rain; turbulence, undertows or wash reflected off river or channel banks; wash from passing craft; and geographical or seasonal issues such as the freshet or seasonal ice flows. In extreme cases, water over the bow of the tow can impact on barge stability, with extra strain on towing and mooring lines and potential damage to barges towed alongside or in tandem.
8.2 Length of the towing line
The less water under the keel, the more power the tug must apply — increasing the wash effect. A longer towline reduces or avoids the wash effect; a short towline in a confined area produces a significant wash. Tractor tugs pulling over the stern and ASD tugs pulling over the bow reduce the wash, since the propellers are further from the towed unit's hull.
Where sea room is restricted, the master must consider shortening the tow wire for better control of the barge; the length is at the master's discretion. Shortening should preferably be done in deep water, weather permitting, and well before entering congested waters — shortening in deep water reduces the wear that dragging on the seabed would cause. However, if the weather is severe, there may be no choice but to defer it to as late as possible.
9. Establishing the Tow Connection — Multiple Barges and Pushing Ahead
There are no strict rules for making fast the tow — each barge differs in size, shape, draught, weather, current strength, light and location. Prior planning makes the operation safer, and a briefing between the tug master and his crew is vital. Before arrival at the connecting location, effective communications should be established between tug and towed unit if manned; ideally a risk assessment is in place, and tug speed is adjusted for a safe rendezvous and connection. The equipment and configuration are dictated by the nature of the voyage, and must be discussed, made ready and inspected before arriving at the tow.
9.1 Towing multiple barges
If the tow consists of a number of barges with different loads, sizes and shapes, the barges should preferably be arranged by similar size and design, with similar-sized barges as the lead. If possible, loaded barges should be placed first with empty barges astern.
9.2 Pushing ahead
Tugs regularly have to push barges ahead even when not specifically designed to do so.
10. Exchange of Information
It is generally assumed that tug operations are routine for ships' crews and that mooring parties will handle them efficiently, so master–pilot exchanges do not usually address the issue. However, to ensure effective harbour towage, the relevant information must be exchanged between master and pilot beforehand, so the mooring parties can be called to their stations in time, fully briefed. A lack of skill may delay securing a tug, putting time pressure on the crew and increasing the risk of personal injury or of the vessel sailing in unsafe conditions — for instance in dense traffic — before the tug is ready. The tug information can be exchanged during the voyage under the pilot's advice, rather than at pilot boarding when other navigational priorities exist. At the start of the operation everyone should be at their mooring stations in good time, with heaving lines ready; if the operation is at night, the crew should have ample time to wake and prepare.
10.1 Information from pilot to master
Because of the different types and sizes of tugs and harbour manoeuvres, the master should find out from the pilot, to pass to his crew: whether ship's lines or tug lines are used; the method of getting the rope on board (most commonly, a thin heaving line from the vessel picks up a larger messenger rope from the tug, led to the winch warping drum to heave the tow wire on board; when using ships' ropes, lower a ship's rope or send it over with a heaving line); the position for passing over the heaving line and which fairlead to use; the maximum speed for securing the tug, so the bridge team can monitor; the bollard pull of the tug(s); and the VHF channels for working with the tugs.
10.2 Information from master to pilot
The master should provide to the pilot: the SWL of the mooring and towing equipment; which fairleads are suitable for securing the tugs — if they are off-centre and only one tug is used, this must be specifically brought to the pilot's attention; and the pushing-point strength, if known — and if no pushing points are marked but the ship has a reinforced belt all around, it is important to convey that fact to the tug master.
11. During the Towing Operation
11.1 Position of tug and interaction
The position of the tug is always important, especially when assisting a barge or vessel. The safe position of the tug relative to the assisted unit depends on many factors: the size and pivot point of the unit; the number of tugs assisting; the speed of the unit being assisted; the depth of water; the amount of manoeuvrable room; and the currents and winds.
11.2 Communications
The commands used by the officer in charge should be clear and well understood by the deckhands, using standard terminologies subject to the ship's working language. The officer in charge must ensure the mooring party knows which bollard(s) will be used for the tug(s); how the messenger line will be led to the warping drum; how the tow wire will be stopped off so the strain comes off the messenger line and the soft eye is put over the bollard; and the releasing procedure. The officer in charge must always be in visual contact with the tug during securing, to exchange hand signals — usually better than a walkie-talkie in windy conditions. Where high bulwarks hide the deck from the tug crew, one person must be stationed to signal visually. The crew should signal to the tug when the tow wire is secured and the tug can safely apply power, and confirm the wire's status to the master (secured, in the water, propeller cleared).
11.3 Safe working practices
Handle a tow wire only with leather (or equivalent) gloves — never cotton — and avoid loose clothing. A snapback zone exists when a mooring line is under tension; crews should stand back from it and it is good to mark these areas permanently on deck. Beware of hands and fingers — sudden jerks can injure. Use only a suitably weighted heaving line: a monkey's fist should carry no additional weight, but a heaving line should not be thrown without one; keep a second heaving line ready. Never throw a thick messenger line instead of a heaving line — its weight may injure the tug crew.
12. Dangers of Towing and Emergency Release
| Danger | What happens |
|---|---|
| Girting / girding / tripping (GGT) | The most prevalent reason for tugs to capsize; can cause fatalities and happens very quickly at either end of the tow. Particularly relevant to conventional single-screw tugs; tractor and ASD tugs are less likely to girt. Towing from near amidships on a conventional tug is inherently unstable — the load on the tow rope can heel the tug to a large, dangerous angle. Understanding the quick release of the tow wire is essential. |
| Interaction | Dangerous where a larger vessel or barge moves at speed close to a smaller vessel such as a tug; the effect increases in confined and shallow waters and when the tug is overtaken by a larger, faster vessel in a narrow waterway. Suction and pressure forces around the hull grow with speed, and water flow under the tug's hull decreases effective stability and can capsize her. |
| Effect of wind | Not appreciating the wind can cause collisions, groundings, parted towlines, injury and girting. Wind changes headings, increases speeds and drifts a towed craft; manoeuvring becomes difficult if the wind rises or shifts suddenly. Know the forecast and local conditions. |
| Effect of current | Less important in open waters than in confined waters, where it can be significant. River tugs work where currents are strong and change over short distances; the current at the starboard bank acts on the port quarter, and as the vessel turns the bow enters weaker current, giving a turning moment to port. The effect can be sudden and should not be underestimated. |
12.1 Emergency release systems for the towline
Tugs working on a towing winch have a "let go" system: the ship's crew does nothing to disconnect the wire — the tug master sets his winch drum free and lets the wire run out until it breaks from its securing bolt, while he manoeuvres to safety. This, however, leaves the ship's crew trailing up to 140 m of steel wire, which must be recovered from the water before the tug can approach again with its spare wire. Once the messenger line is entirely on board or on the warping drum, winding the remaining wire is difficult, probably impossible; a stopper must be used and a second messenger line tied to the towing wire further down, then winding resumed — possibly repeated many times. It will probably be necessary to reduce speed if conditions allow. This is a dangerous operation and great care must be taken.