EXPERIENCES AND CHALLENGES IN SIMULATING SHIP TO SHIP TRANSFER, STS
Knut Bakken (Ship Manoeuvring Simulator Centre AS (SMSC), Norway)
Kjell Martinussen (Norwegian Marine Technology Research Institute AS (MARINTEK), Norway)
Abstract: The training course for lightering operations at SMSC is described and some of the experience obtained is presented. Some results from model tests performed at MARINTEK to demonstrate the phenomena occurring during operation of ships close one another are also presented.
Ship to ship transfer, lightering, is an operation where two ships moor together in open waters. The larger vessel is usually the Ship To Be Lightened (STBL). The ship in ballast is the Service Ship (SS). The approach and mooring operations are carried out in several ways. The most common way when professional lightering masters are involved is where the STBL is under way at low speed.
Lightering has been practiced for almost 30 years off the U.S. coast. This is a cost effective method of transporting liquid cargo when the vessels used for the overseas transportation are too large to call at the loading or discharging port. Lightering is becoming more and more common around the world. In the U.S. today 25% of the imported crude oil is lightered. The safety record is excellent. In the period of 1993-1997 less than 0.003% of the lightered oil volume was spilled. However, no record is made of other damage like broken fenders, hull damage, broken moorings etc.
2. SIMULATOR TRAINING AT SMSC
2.1 Training course
SMSC has developed a training course for lightering operations. So far approximately 300 ship officers and lightering masters have taken the course. One important reason for the high demand for the course is that according to experienced lightering masters the physic al phenomena experienced during lightering operations are well represented by the numerical simulation model. The simulation model and the course material have been improved over several years in co-operation with the lightering masters.
The training is normally performed on a single simulator bridge. This is more cost effective with regard to training. The training can also be performed on two simulator bridges in co-operation. This method is important for teamwork training of officers on the two ships undertaking the operation. Any deficiencies in communication are exposed as are also any action by one ship that is seen as potentially dangerous by the other ship.
The need for simulator training has been clearly demonstrated in the case of these demanding operations involving safe and efficient handling of large ships operating at close range and where interaction between the two ships have to be taken into account. The understanding of the behaviour and control of two ships at close range is usually very limited among ship officers with little or no previous experience of such operations. This experience involves relating the effects of own actions to the other vessel, including relative motion, relative direction and relative speed and time. In addition the effects of the hydrodynamic interaction between the ships at close range need to be understood. In some cases an inexperienced ship officer may actually refuse to make the correct action at the given stage of the operation. Such behaviour may lead to too much responsibility having to be shouldered by the captain of the other vessel.
The emphasis of the course is on normal approach and departure procedures. The approach procedure has been standardized. A considerable effort is applied to the description of and training on the practical effects of the hydrodynamic interaction between the vessels and the way of handling these for optimal safety. The departure of the loaded SS is also a demanding operation. The STBL accelerates from dead in the water. Trainees often find it difficult to safely carry out the procedure where the SS first is matching the speed to the accelerating STBL and then gets clear by angling the bow outwards from the STBL. In many cases situations develop that in real life will result in structural damage to the afterbody of the ships. Many trainees have concluded that the departure is more demanding than the approach. A distinct improvement in performance is observed during the course with regard to approach and departure of the SS.
Emergency manoeuvring with malfunctioning engine or rudder is also part of the training programme. At the start of the course many of the trainees do not know the procedures that may save a critical situation.
The training course at SMSC has been developed with the following objectives:
・Simulation models that realistically express the effects on ship handling of the hydrodynamic phenomena occurring between the ships in dependence of the ship speed and relative position.
・Simulation of the physical properties of the fenders including recording of fender loads. Realistic visual presentation of fenders.
・Incorporate existing operational procedures in the trade including checklists in the course.
・Perform exercises with training objectives approved by experienced lightering masters and other experienced personnel.
・Description of the actions involved in handling the SS safely and efficiently from any approach location till moored alongside the STBL.
2.2 Effects of hydrodynamic phenomena
The hydrodynamic interaction forces acting are dependent on motions and velocities of the ships as well as their relative position. The model for simulation of the interaction hydrodynamics is based on methods derived from experiments. The results from the experiments involving static force measurements have been implemented in the general hydrodynamic hull force model in such a way that the change in forces with relative position is accounted for, and the dynamics of the manoeuvring vessel is integrated during the complete approach or departure. In order to obtain the final realistic reactions of the vessels to the interaction forces the hydrodynamic model has been adapted on the basis of advice from experienced lightering masters. The pressure distribution along the hulls of the two vessels is quite complex and dependent on factors like ship speed, relative position and direction, and use of propellers and rudders. The interaction force is proportional to square of the velocity and inversely proportional to more than the square of the distance between the ships. The result of this is that the control force from the rudder has to be increased with decreasing distance, i,e. the rudder angle is increased. If the bows or the sterns of the vessels are in line it may be extremely challenging for the SS to get alongside in a controlled manner and to avoid touching the STBL. The influence of the relative position is such that the control problem is different when the SS is somewhat ahead, abeam, or somewhat astern of the midship of the STBL. In the last stage of the approach, the angles between the two vessels are usually kept between 3°to 5°, increasing the sway force and the yaw moment on the SS. An attempt at illustrating the flow and indicating the inherent control problems is made in Fig. 1 and 2.
There is a chaotic flow around the vessels: bow waves, drag on the afterbody and underpressure along the sides.
The sway force and yaw moment is increased on the SS in the last stage of the approach.
In order to provide the trainees with reasonable understanding of the effects of the pressure distribution and the resulting forces on the ships without confusing them with too much detail, a practical but somewhat simplistic description is provided: When making headway, the vessels are driven trough the water. The bow moves water away creating a bow wave and an overpressure. At the stern the water has to flow into the space left by the vessel creating an underpressure around the afterbody. When the ship are at close range the duct effect created between the ships by their sides leads to high water velocity and the underpressure extends also to this area. Often surprisingly little attention is paid among mariners to the effects of the pressure distribution along two vessels operating at close range. A measure of the forces involved is given by a simple indication of the amount of water passing the ship, taking breadth x draft x speed, or for a VLCC: 50m x 20m x 2.5 m/s = 2500 m3/s.
The fenders are described in the guidelines. There may be four large fenders in use and in addition smaller fenders forward and at the spring aft. Part of a controlled approach is to avoid overloading the fenders. The fender load is monitored by the instructor and is a parameter in the evaluation of the trainee. After being exposed to the force from the approaching SS the fender (normally the forward one) create some recoil effect that must be controlled to avoid the possibility of the ships touching astern.
The fenders may be rigged on either vessel and at the simulator it should be possible to observe them from the operator's position on the bridge. The trainee is taught the essentials of kinetic energy, learning that the displacement of the vessel and the sway and yaw velocities are the parameters determining the fender load.
2.4 Deciding the speeds during approach
The interaction effects between the ships should be kept as small as possible. Hydrodynamic forces increase by the square of the velocity and consequently the ship speed should be kept to a minimum with the following limitations:
・The engine of the STBL must be able to maintain the speed over a long period of time.
・The STBL must maintain steering speed.
・The SS must be able to steer and in order to control longitudinal position relative to the STBL, must be able to reduce to smaller speed than the STBL without stopping the engine. Stopping the engine may lead to loss of control.
2.5 Communication and co-operation
Optimum communication and co-operation are important aspects of the training course. In an accident in 1995 850 barrels of oil were spilled. The cause was inadequate communication and procedural errors. The course uses the STS guidelines/check lists and other procedures from the manual of OCIMF .
The STBL is ordered to keep course and speed. If the SS deviate from agreed procedures the master of the STBL may be confused, resulting in potentially dangerous actions. Training in common situation awareness is emphasised throughout the course by using a proper communication style. Results from research in the aviation business prove that people who communicate their intents and plans and update the members of the team, are less involved in accidents than those who do not.
A thorough understanding of shiphandling techniques is emphasised in the course. Efficient handling at low speed by combined use of rudder and propeller is given particular importance. The rate of turn technique and the way a difference in relative heading creates relative transverse speed is important knowledge. The approach from abeam should be carried out with small relative angle. This knowledge improves ship handling also in earlier stages of the approach.
Relation between angles and transverse speed. Increases with about 10% for every 6°.
2.7 Navigation by eye
Lightering operations are carried out by navigation by eye. This means that the visual presentation of the situation on the simulator must be very realistic. It is of utmost importance to be able to observe relative motions and the change in relative directions. The quality of texture and the presentation of depth in the visual system is important when judging distances during training.
During departure training it is important to observe the afterbody of the vessels and being able to judge how the distance between the vessels in this area may decrease when the angle between the foreships is increasing. If there seems to be a risk of collision remedial action must be taken very quickly. The visual system of the simulator must be very realistic to allow correct decisions in this respect during training sessions.