2.11 Emergency escape

　

 If one of the vessels loses control of heading there will be a risk of collision. After the normal procedures are carried out satisfactorily on the simulator some exercises concentrate on mechanical malfunctions, for instance that the rudder of one vessel is locked in a hard over position. The first objective is to keep the vessels as far as possible on parallel course. The one with the problem reduces the effect of the rudder by stopping the engine. The result of panicking and going full astern with a fixed pitch propeller has been demonstrated. The effect of the reversing propeller increases the rate of turn, and if the turn is in the wrong direction the result will be a collision. If proper action is taken the vessels can increase distance without touching each other. Training in handling critical situations improves situation awareness. Feedback from participants is taken as evidence that collisions have been avoided due to this type of training.

　

　

2.12 Wind, waves and swell

　

 The limiting weather conditions for lightering operations is a very important subject were there is different opinions in the trade. However, there are 3 main cases that should be considered:

　

 Beam sea may cause problems. The closer the wavelength is to the sum of the beams of the vessels, the worse the problem may be. Bridge-wings have been damaged due to roll motion. In addition the tremendous forces caused by heave and pitch may easily break the moorings. Good information concerning the performance of the rudder in following seas seems not to be available.

　

 Other important subjects are the development of waves and wave forces in the space between interacting ships, and the effect on the motions of ships at anchor of environmental conditions and approaching ships.

　

　

3.1 Recourses for development

　

 In addition to its own staff SMSC has access to the recources of MARINTEK and NTNU (The Norwegian University of Science and Technology) for development of theoretical and experimental methods that can be further developed into numerical models for simulation training. The Marine Technology Department of NTNU is among the biggest in the world, while MARINTEK is a research organization that is well established in all aspects of marine technology. SMSC, MARINTEK and NTNU are all located in the city of Trondheim which makes for easy communication and co-operation.

　

3.2 Development of training in lightering operations

　

 Even if the simulator training in lightering operations at SMSC is considered satisfactory in most respects there is room for improvements. The limitations in the present day simulation model is related to the possibilities of demonstrating the consequences of not following the normal approach or departure procedures, particularly regarding the effect of unusual relative positions between SS and STBL.

　

 A co-operation has therefore been started between SMSC and MARINTEK with regard to lightering operations. The first definition of a development program has been formulated recently. The objectives are to improve simulator training, also by including onboard PC simulators, increase the understanding of the dominating flow phenomena by means of flow calculations and visual presentation, preferably video presentation, of the actual flow and the resulting pressures and forces, and the development of an active operating guidance system for lightering operations.

　

 Before the formulation of the development program a lightering operations workshop was arranged by MARINTEK. It was attended by many of the largest operators in the lightering business. It was stated that even if lightering can be considered a safe operation with regard to oil spill there is considerable room for improvement concerning damage to ships, fenders and moorings. The question of safe weather conditions for the operations should have an answer with scientific backing. Reverse lightening (transfer of oil from the smaller ship to the larger ship) looks like becoming more important in the future. The formulation of the development program is now proceeding on the basis of the experience from the workshop.

　

3.3 Initial model tests with two ship models

　

 As a prelude to the development program some model tests with two ship models side by side were undertaken by MARINTEK. Existing models from stock were used so the model scale is arbitrary and the smaller model was more representative of a general cargo ship than an SS, while the larger model may be representative of a relatively small STBL. If a model scale of 1:73.5 is chosen the following ship dimensions are obtained:

 STBL: Length 276,4 m, breadth 45,9 m, draft 17,1 m, displacement 179700 m³

 SS: Length 167,6 m, breadth 28,5 m, draft 9,1 m, displacement 29550 m³.

　

 The tests were carried out in calm water and in regular waves from ahead in the Towing Tank of MARINTEK (Length x Breadth x Depth = 260 m x 10.5 m x 5.6/10 m). The STBL was fixed to the towing carriage and running on constant heading with constant speed. The SS was free-running, speed and heading being manually controlled from a steering position close to both models. The STBL was equipped with a fixed rudder but no propeller. The SS was equipped with working propeller and rudder. The SS model always had the STBL on the starboard side. This is the opposite of normal lightering practice and was dictated by the arrangement for tests on the towing carriage. This has no practical effect because of port and starboard symmetry, i.e. were the results indicate starboard rudder, read port rudder, In future experiments where perhaps the detailed effects of propeller direction of rotation will be studied a true to life model set-up will be used. In each test the SS was steered to a specified longitudinal position and transverse distance relative to the STBL. When the specified position and distance were obtained the objective was to keep the SS at this location for the remainder of the test.

　

Results: Some results from the tests in calm water will be presented here. The following parameters were varied in these tests:

 Three test speeds corresponding to 5 knots, 10 knots and 15 knots.

 Four transverse distances from ship centreline to ship centreline: 1.0 x BSTBL, 1.5 x BSTBL, 2.0 x BSTBL and 3.0 x BSTBL. BSTBL = breadth of STBL. This corresponds to ship side to ship side distances of approximately 9 m, 32 m, 55 m and 100 m respectively.

 Five longitudinal positions defined by the distance of the midship section of the SS forward of the aft perpendicular of the STBL: 0 x LSTBL, 0.25 x LSTBL, 0.5 x LSTBL, 0.75 x LSTBL, 1.0 x LSTBL. LSTBL = length of STBL. For distances 1.0 x BSTBL and 1.5 x BSTBL the longitudinal position 1.125 x LSTBL was also tested. This corresponds approximately to the midship section of the SS being the following distances forward of the aft perpendicular of the STBL: 0 m, 70 m, 140 m, 210 m, 275 m, 310 m.

　

 Fig. 10 shows the mean rudder angle used when the SS is sailing alondside the STBL at a transverse distance of 1.0 x BSTBL, corresponding to 9 m between ship sides. The rudder angle is given as a function of the longitudinal position of the midsection of the SS relative to the aft perpendicular of the STBL expressed in terms of the length of the STBL. Fig. 11 shows similar results for transverse distance 1.5 x BSTBL, corresponding to 32 m between ship sides. Positive and negative rudder angle is rudder to port and to starboard, respectively. One objective of the model tests was to explore extremes, so the tests were run at speeds corresponding to 10 and 15 knots in addition to the normal speed of 5 knots. The results for 5 and 10 knots are shown here.

　

　

　

　

　

 It is seen that large rudder angles are necessary to control the SS at these transverse distances, particularly when the SS is aft relative to the STBL. The frequency of rudder motions also indicated the high demand on the ship handler when the SS is in the aft positions relative to the STBL. In these positions the frequency of change of rudder angle was high and the magnitude of the changes were large. It is also seen that the mean rudder angle is strongly dependent on the longitudinal position of the SS :

　

 It can be observed that the normal speed of 5 knots is not necessarily the speed that results in the smallest rudders angles, i.e. presenting lesser control problems. The mean rudder angle required is in many cases significantly smaller at 10 knots. These results are reflecting the minimum steering speed of the SS, indicating that a speed somewhat more than 5 knots may be favourable for this particular ship. Rudder size and propeller rudder arrangement strongly influence this speed. However, the consequences of losing control increase with the square of the speed, which must also be considered. The risk of loosing control will also increase due to the increased need of quick reactions at higher speeds.

　

 Fig. 12 and 13 show an attempt at rating the control problems that were observed during the tests. The horizontal axes in the figures are the same as in Fig. 10 and 11 , while the vertical axes present control problems on a scale from 0 to 6 according to the following rating:

　

 The following conclusions can be drawn, from the observations:

　

 Distance 1.0 x BSTBL: Control problems occur at aft (0 x LSTBL and 0.25 x LSTBL) and forward (0.75 x LSTBL and 1.0 x LSTBL) positions. The control problems at forward positions are more pronounced at the higher speed.

 Distance 1.5 x BSTBL: Control problems occur at aft position (0 x LSTBL). The control problems are more pronounced at the lower speed.

　

 The simulator course is appreciated among professional lightering masters as a valuable training tool. However, there is room for improvement to demonstrate the consequences of and to provide realistic training when the normal approach and departure procedures are not being adhered to. This is particularly the case when the SS is side by side with the STBL, close to it, and either relatively far astern or far ahead relative to the STBL.

　

 It is hoped that any restrictions in the simulation model will be removed as a result of a development program to be started in co-operation with MARINTEK and, hopefully, with participation from the industry. The program will employ experiments and theoretical calculation in order to arrive at a very detailed model of the interaction effects including visual demonstration of the flow phenomena occurring.

　

[2] Oil Spill Risk from Tank Vessel Lightering, 1998

[3] Ship to Ship transfer Guide (Gas), OCIMF, 1997