Nayak — Marine Pilotage · Ch. 18: Emergency Manoeuvring Options

1. Overview — Emergency Manoeuvring Options

Source: NAYAK, Capt. Santosha K. Theory and Practices of Marine Pilotage. Chapter 18 — Emergency Manoeuvring Options.

Syllabus: Anexo 2-B, Área II (Arte Naval), item 8 (NAYAK), Cap. 18 — opções de manobra de emergência. Cross-list Área III, item 6.

Emergencies do occur in life, and we should be prepared and trained to face and handle them when they strike without notice. In ship handling, the emergency manoeuvring options are the last resorts — the handler turns to them only after all other options have been consumed, when action must be taken to prevent an actual emergency from happening.

1.1 Emergency scenarios while manoeuvring in port limits

The emergency situations that may arise while manoeuvring in port limits include grounding, running onto hazards, collision with other vessels or with shore objects and jetty cranes, anchor dragging, engine failure, steering failure, a tug becoming ineffective, excessive bad weather, man overboard and the sudden onset of poor visibility. Well-defined action plans are kept ready for emergencies out at sea, but when an emergency strikes while manoeuvring in restricted waters the challenges increase manyfold.

Managing an emergency in port needs a different approach than at sea. The nature of the emergency is similar, but its consequences affect not only the vessel — they reach the nearby vessels and craft, the port itself and the local and international communities. Because each port, anchorage, channel or canal has its own rules, topography and challenges, there is no single set procedure; pilots, ship handlers, navigators and masters must follow the basic principles of handling emergencies in proper coordination with the relevant shore authorities.

The scenarios that may arise while the vessel is within port limits include, but are not limited to, the following.

Although emergencies may arise at any time, the pilot or ship handler must stay mentally ready to handle the vessel efficiently and move her from the emergency scenario to a safe position. The handler must understand and appreciate the emergency procedures and the vessel's emergency manoeuvring characteristics, and must grasp the gravity of each distress scenario.

2. The Vessel's Manoeuvring Characteristics

The vessel's pilot card, together with the manoeuvring diagram and the manoeuvring booklet, gives the pilot the information needed for immediate reference. These particulars are needed during ordinary manoeuvring, but the emergency characteristics are compiled together with the general ones.

2.1 Main engine characteristics

An example of the main-engine information given on the pilot card, with the speeds in loaded and ballast condition, is shown below.

The corresponding astern RPMs are the same as the ahead RPMs, but the astern speeds differ because the actual power of the astern engine is only about 30 % of the ahead power. The remaining engine particulars given on the card are as follows.

2.2 Steering characteristics

An example of the steering particulars given on the pilot card is shown below. The rudder is a semi-balanced spade rudder.

2.3 Anchors

The example anchor information given on the pilot card is: anchor type Stockless AC14; two anchors, port and starboard bow, with no stern anchor; the port cable has 10 shackles and the starboard cable 11 shackles; and the maximum rate of heaving the anchor is of the order of 1'58"–2'04" per shackle.

2.4 Blind sector at the bow

The blind sector at the bow is another feature that determines the emergency actions and how effectively they can be executed. When the vessel clears an obstruction from the bow by swinging or stopping, the view from the bridge may look scarier than the actual scene because of the blind sector. When an object falls under the blind sector the handler cannot judge the exact distance at which the bow clears the obstruction; a lookout on the fo'c'sle may help, but only if his reporting is correct.

In confined waters and within harbour limits the restrictions are high, and objects fall under the blind sector of the bow while turning or stopping — the clearing distance of buoys in the turning circle, the jetty while turning in front of the berth, the distance from another vessel when berthing alongside, and so on. The example below gives the blind-sector distances for a vessel of LOA 170 m with the bridge 27.4 m above the keel.

3. Stopping Ability — Track Reach and Side Reach

The stopping ability of a vessel is measured by the track reach and the time to be dead in the water, realized in a crash stop or crash astern manoeuvre.

In a crash-stop scenario the engine can be reversed after about 3 minutes, while still running ahead at about 30 % rpm; a slower vessel, or one in ballast, takes less time to reach that point. A single-screw ship cants her bow briskly to starboard under astern power and brings her stern into the wind once sternway is built up. Most vessels travel about 5 to 12 times their own length before coming to rest from full ahead — depending on displacement, trim, speed and type of machinery — and take from 4 to 10 minutes to do so.

3.1 Track reach and the stopping-distance formula

An example of the stopping distances of a handymax bulk carrier is shown in the figure. The track reach is longer in the loaded condition than in ballast, and longer when the astern movement is given from a higher initial engine order.

The stopping distance on a straight track can be estimated from the following relation, where S is the stopping distance in ship lengths and A, B and C are the coefficients defined below.

A depends only on the type of ship and the shape of its hull; it is inherent in the hull form and cannot be changed unless resistance is significantly increased.

B is inversely related to the astern power expressed as a percentage of the ahead power. In diesel machinery the astern power is usually about 85 % of the ahead power, whereas with steam-turbine machinery it is about 40 %; B can only be reduced by building more astern power into the engine, which is unrealistic for a steam turbine.

C is half the distance, in ship lengths, travelled by the ship while the engine is reversed and full astern thrust develops. C is larger for smaller ships, and larger still if more than one minute is taken to reverse the engines from the astern order to full astern thrust.

Worked example. For a VLCC with steam-turbine machinery, 300 m long, at 15 knots, taking one minute to develop full-astern thrust, the coefficients are A = 16, B = 1.5, C = 0.8. The stopping distance is S = 16 log(1 + 1.5) + 0.8 = 15.5 ship lengths, which exceeds the 15-ship-length stopping criterion.

3.2 Side reach

The behaviour of a ship during a stopping manoeuvre is extremely complicated. The longest stopping distance is obtained when the ship runs in a straight line along the original course after the astern order. In reality she veers off to port or starboard and runs along a curved track, giving a shorter track reach because of the increased hull drag, but also producing a lateral deviation — the side reach. Side reach is very sensitive to initial conditions and wind disturbances.

The two tables below give the track reach, head reach and side reach (in cables) with the corresponding time, for the case where the engines are stopped and for the case where full astern is given, from each initial engine order in loaded and ballast condition.

The track-reach deceleration factor is about 0.10 miles per minute in all cases, except from full-ahead-sea where it varies from 0.14 to 0.16 miles per minute.

3.3 How the crash stop or astern is executed

When the engine is running ahead and a crash astern is ordered, the sequence of events is as follows.

a. The bridge orders crash astern. In engine-room control mode the traditional way to signal it was to bring the telegraph from ahead to full astern, then again to full ahead and again to full astern; the engine room then implements the astern order. In bridge control mode, common in modern automated ships, the bridge telegraph is directly connected to the engine controls (a remote-controlled telegraph device) and no engine-room personnel are involved — the officer simply puts the telegraph to full astern and the engine complies. A provision links both telegraphs so manual operation is possible if the automation fails.

b. The main-engine fuel supply is immediately put to zero.

c. The engine may keep turning ahead from the momentum of the vessel even with the fuel cut off, so the bridge tachometer may still show ahead rpm.

d. The engine waits for the ahead rpm to fall to the reversing level prescribed by the manufacturer, which is around 25 % to 30 % of MCR rpm.

e. As the rpm reaches the reversing level, starting air is admitted in the astern direction — known as braking air. Admitting braking air at too high an rpm can cause very high stresses and break the crankshaft.

f. Repeated kicks of braking air bring the engine rpm down to zero.

g. Once the rpm is at zero, starting air is admitted in the astern direction.

h. As the rpm reaches the start level astern, fuel is injected.

i. The engine starts turning on fuel in the astern direction.

j. Vibration may set up from heavy wake disturbance, so the astern rpm has to be increased gradually. The same sequence takes place automatically on a bridge-controlled vessel.

3.4 Telegraph failure — emergency operation of the engine from the engine room

If the telegraph from the wheelhouse fails, engine commands can be executed manually from the local position near the main engine. The telegraph no longer works, so the duty officer must inform the duty engineer at the local station to execute the commands; the ship handler must then allow for the delay in execution and the chance of a manual mistake. The main engine's direction and rotation are normally controlled remotely from the bridge or the ECR through the telegraph and fuel-lever control, but a local manoeuvring control system is provided for use if the remote system fails from both remote stations, or if the engine governor becomes faulty. The local control stand is normally near the fuel-pump platform of the main engine.

a. Changeover from remote to local position. First change control from the wheelhouse to the ECR, with both telegraphs in the stop position; bring the fuel levers of the wheelhouse and ECR to stop; turn the changeover switch in the ECR to Local; at the local station, change the fuel-pump control shaft from local to manual; and operate the fuel rack through a cone-clutch or mechanical-lever arrangement acting as the manual control. The changeover can be done with the engine stopped or running, but is better done stopped if the situation permits.

b. Operation from the local stand. As the wheelhouse order comes by the local telephone, respond to it and give fuel and air through the local control levers. If the engine fails to start, give extra fuel and air, since manual control needs more push on the linkage. Once the engine starts, follow the telegraph and maintain the speed from the local fuel lever.

4. Reducing Speed on Engine Failure

When the engine fails without notice the vessel quickly loses control, and as the speed drops the turning ability also falls seriously. A not-under-control vessel with sufficient speed can create unimaginable disasters, so the handler's immediate objective is to bring her to a safe location and control the speed — often reducing it substantially while the engine remains failed.

4.1 Rudder cycling

If sea-room is available, the handler may use rudder cycling to reduce speed. Rudder cycling uses the drag of the rudder and of the vessel to break her momentum; it suits open seas but may also be used at suitable locations in restricted waters.

4.2 Back pull by tugs

If tugs are already made fast, they may be used for back pulling to reduce speed; when the engine fails the tugs should be advised immediately. Sometimes a tug is made fast right astern through the centre lead solely to reduce speed — such tugs are called braking tugs. Tugs made fast on the sides for lateral movement at slow speed may also back-pull from their positions while resting on the ship's side. Usually the after tug is used, but both bow and stern tugs may be deployed.

5. Emergency Methods of Killing the Speed to Avoid a Disaster

When every available method has been deployed and it is apparent that the speed cannot be reduced, with an imminent threat of grounding or collision, the emergency methods of speed reduction may be used to save life or property. These methods may cause the vessel some structural or machinery damage, but the bigger objective is to prevent a disaster of loss of life or greater property damage; there may be legal implications afterwards, but the best action is to avoid the bigger incident.

5.1 Dredging anchors underfoot

When all previous options are exhausted, the navigator may resort to dredging the anchor to reduce speed. A critical decision is which anchor to use. As soon as the anchor is dropped, the stern begins swinging to the opposite side, the anchor acting as a pivot, so there must be no obstruction where the stern will turn — to avoid a bow collision there should not be a stern collision. The bow on the chosen side must be clear of boats, with sufficient depth; it is preferable to drop the weather-side anchor if sufficient stern-room is available, otherwise the lee side may be considered.

It is prudent to keep both anchors on standby for emergency use when entering restricted waters. Usually the depth in harbour limits is less than one shackle of cable, i.e. 27 metres, so once ready the anchor is let go up to one shackle on the water and held back with the brake tight and the gear disengaged, so that the anchor dredges on the seabed. There will be tremendous force on the cable depending on the vessel's speed and momentum: anchors and cables are designed to hold the static load and prevent yawing, but here they are subjected to a dynamic load excessive enough to break them. Once one anchor is let go for dredging, the vessel must be ready to let go the second if the first gives way. Before starting, note the speed, head-room, stern-room and weather effects on the vessel.

5.2 Smelling ground at bow or quarter (side touching)

When the vessel is at high speed and dredging the anchor is not viable, allowing her bow or stern to smell the ground laterally may be considered to break the speed. This is a very precise and critical exercise: the vessel touches ground laterally at the bow or stern and returns to safe water with the help of the bank cushion, the marginal touch reducing the speed almost instantaneously. Helm and engine — or tugs if available — carry out a controlled swing towards a shallow patch; just before touching, opposite helm is made ready with higher engine rpm to move the bow or stern away from the patch, brushing the vessel away from the ground and dropping her speed dramatically. The ground must be of a non-damaging nature — soft mud, sand and the like.

5.3 Beaching to avoid collision

Since cadetship we read that grounding is better than a collision. If all resources are consumed and all the critical time is lost, the last resort is to cause a beaching or grounding. When this is required, it is better to ground the vessel from the side or from the bow, as this causes less damage and is easier to re-float.

6. Turning Ability and Crash Astern vs Full-Ahead-Hard-Over

Opposed to directional stability, the ship must also turn satisfactorily when a rudder order is given; she must change heading within a specified minimum time (T) with minimum heading overshoot after the order. Both responses depend on the rudder's dimensions, the rudder angle and the ship's speed.

6.1 The turning circle — advance, transfer and tactical diameter

The turning circle is the path traced by the pivot point when the vessel turns with a fixed rudder angle and a particular engine rpm. Its shape changes with the rudder angle (10°, 20°, 30°) and with the engine rpm; the shortest turning circle results when the engine runs full ahead and the helm is put hard over to either side. As the vessel traverses the circle she moves both ahead and laterally.

The advance, transfer and tactical diameter depend on the displacement (loaded or ballast), the depth of water, the rudder angle and the running rpm; the distances and the times taken are recorded in the manoeuvring booklet. The example below is a loaded bulk carrier in shallow water, engine at full sea speed, wheel hard to starboard.

6.2 Crash astern vs full ahead and hard-over wheel

Having seen the stopping manoeuvres in terms of head reach and side reach, and the advance and transfer of a turning circle at full ahead with hard-over wheel, the two manoeuvres can be compared to judge which better avoids a collision — with other vessels in the open sea, or with infrastructure and moving vessels inside harbour limits.

Comparing the two, it is clearly advisable to turn with full ahead and hard-over wheel if the vessel has a lateral tolerance of about 4–5 cables to avoid a collision: it gains less longitudinal distance and takes less time. This is generally preferable when ample sea-room is available for turning, but it may not always be possible, since in harbour limits the lateral sea-room is usually restricted.