2.8 Human limitations
The human mind is limited with regard to number of subjects that can be treated simultaneously, and with regard to the duration of concentrating on the subjects. This fact points to the importance of pre-planning and rational procedures. This consideration was important when the approach operation was divided into three steps in the training programme.
2.9 The steps of the approach procedure (Fig. 4)
Step 1, bring the SS from any site to a location 1/2 nautical mile or more astern of the STBL.
Step 2, bring the SS abeam of the STBL at a distance of about 150-200 metres.
Step 3, bring the SS alongside the STBL with a small angle 3°-5°.
Exercises might start from any location
Fig. 5 Timing relative motion
Performance of the approach procedure: The first exercise starts with performing leg 2 and 3, Fig. 4. The SS is on parallel course and 1 nautical mile astern of the STBL. During training the time needed for taking the SS abeam of the STBL is typically reduced from ll/2 hours or more to 30 minutes. In order to make it easier to assess speed and relative motion, the diagram in Fig. 5 was designed. (1): the STBL is steaming with an agreed speed, normally approximately 5 knots. The SS is 1 nautical mile astern of the STBL, the speed relative to the STBL is 4 knots. (2): the SS may be abeam in less than half an hour. (3): when the relative distance is 5.5 cables the relative speed of the SS is reduced to 3 knots. The reduction in speed is easily carried out within 4 minutes, remembering that the drop is actually from 9 to 8 knots true speed. (4) : When the distance to go is 2 cables the relative speed is reduced to 2 knots, and so on until the speed matches that of the STBL and the transverse distance is approximately 1 cable.
After some training the approach is carried out efficiently. The safety is improved because the trainees are more focused on timing when executing the necessary manoeuvres. The accident in 1995 where 850 barrels of crude oil was spilled may be taken as a warning against making shortcuts in the approach procedure. In that case the vessels were more or less on opposite courses, starboard to starboard. The STBL was ordered to proceed at approximately 5 knots and RPM was ordered accordingly. The SS turned starboard believing that the STBL had obtained the speed ordered. The intention was to be astern the STBL at the completion of the turn. Unfortunately the STBL had achieved only 2 knots. The situation was now out of control, the engine on the SS was stopped and collision was unavoidable. It can be demonstrated that if the SS turns away to port it will be possible to perform the operation safely and requiring only 6 additional minutes compared to the original intention. In other words, the collision occurred in in order to save 6 minutes!
First leg of the approach: The performance is improved by training for increased competence in taking the SS from any site to a planned relative location and heading with appropriate margins and without spending time unduly. The person in charge should waste as little of his energy as possible during this phase The fatigue and relaxation after being intensely concentrated for a relatively short period of time may lead to incidents resulting from human error. Every master that has handled critical situations that takes some minutes to solve, has experienced the relaxation when everything is back to normal.
(1) The SS turns to starboard (2) The collision is a fact.
The demonstration of and training in efficient execution of the first leg of the approach involves the SS proceeding from different locations and with different headings relative to the STBL, figure 4. The objective is to be about 1/2 nautical mile astern of the STBL and on parallel course at a transverse distance of about 1 cable. A total of about 10 exercises are carried out during the course. When starting l nautical mile from the location where the second leg starts, the time used to be abeam of the STBL after training is about 1/2 hour.
Second leg of the approach: When on parallel course the observation of any deviation in the track of the two vessels becomes easier. Such deviation may be the result of compass error, different angle of drift due to wind or layers of current acting differently on the vessels or other causes. However, the main task is to control the relative speed and to time the approach to be abeam at a transverse distance of about 1 cable.
Before starting the next phase it must be checked that the speed of the two vessels matches.
Third leg of the approach: Finally the SS continues from abeam of the STBL. At a safe distance of about 150 metres it is checked that the headway of the two vessels matches and that the longitudinal position of the SS is as desired and not too far ahead or astern. The relative position is important for the final approach. If the SS is too far aft the suction towards the STBL is stronger than in the optimum position and may become uncontrollable. By ensuring that the SS has the bow sufficiently forward advantage may be taken of the cushion provided by the bow wave of the STBL, making the situation more predictable and easier to handle. If the SS is too far ahead we may also be encountering a critical situation because the afterbody of the SS is pushed away by the bow wave and overpressure along the forebody of the STBL. The end result may be that the SS turns across the bow of the STBL. Training in finding the optimum longitudinal position during approach is therefore of great importance. When the SS starts the last part of the approach the relative angle should be kept to a minimum, often as little as 3°-5°. It should be noted that 6°at 5 knots produces a transverse speed of 0.5 knot, or about 0.25 metres per second, in addition comes the interaction effects, see Fig. 3. The transverse distance of 150 metres are covered in approximately 10 to 15 minutes. More and more rudder has to be applied as the distance decreases. The engine should not be stopped before the mooring has been completed. The moorings forward should be made fast before the SS is swung parallel to the STBL. Otherwise there is a risk that the foreships will separate and the last part of the approach has to be made again.
2.10 The departure procedure
The departure is done as the STBL starts to move ahead, towing the SS by the moorings. The speed must be controlled very carefully or the vessels may be sucked together. When casting off it may be difficult for the SS to match the speed of the STBL when they start their engine.
The location of the aft fender is quite far ahead due to the spring lines of the vessels. When the bow on the SS is allowed to swing out there is a risk of contact steel against steel behind the aft fender. Due to the underpressure in the area of the afterbodies the development may take place so quickly that it is not possible to control. To provide realism the visual system of the simulator must provide the view far abaft the beam. Sometimes it is difficult to swing out the bow of the SS, and the trainees are taught how the situation may be improved by moving the SS forward until some ouward force from the bow wave of the STBL is acting.
Fig. 7. There may be a risk of contact steel against steel.