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Development of a calculation program for escort forces of stern drive tug boats Ir. Frans Quadvlieg, Maritime Research Institute Netherlands Dipl.-Ing. Stefan Kaul, SCHOTTEL GmbH & Co. KG SYNOPSIS: The escort class notation of several classification societies requires the test of the dynamic towing forces at escort speeds of 8 or 10 knots. These tests should be performed with the completed tug according to a special procedure. Accurate prediction methods are available mostly for tractor type tug boats. As more and more stern drive tugs (ASD-tugs) are built, aiming also for an escort class notation, SCHOTTEL and the Dutch Model Test Establishment MARIN have developed a computer prediction program for the escort forces of these tugs. The program is based on executed escort tests and systematic model tests with different configurations of the hull, skeg, arrangement of thrusters and towing positions. The program allows optimising the above configurations and it helps to predict the escort rating number of ASD-tugs in a very early stage of the design. INTRODUCTION Tools to predict the escort capabilities of tugs are not new. Earlier references are available in the various ITS conferences, and presented by Damen Shipyards [1], The Glosten Associates [2], and others, e.g. [3] [4] (this list is not complete). As a leading firm in manufacturing of rudder propellers, SCHOTTEL has already a huge experience in tugs, with dedicated engineering capabilities, see [5]. Providing on-the-fly predictions of bollard pulls and speed-power relations are for example of prime importance in a very early stage of the discussions between SCHOTTEL and their clients. Nowadays, it is also necessary to make predictions for the escort capabilities in the very early stage of the design and therefore the current project was initiated. Compared to other projects in the past, this project contains some new elements: 1) To generate a tool that can be used for making

predictions in a quick fashion, meaning that with a minimum amount of input, a reasonable estimate should be given. In addition, advice can be given on the improvement of the escort capabilities. The typical handling time of a calculations should be 15 minutes or below. The escort capabilities are expressed in terms of steering force and braking force at a certain speed (for example 8 or 10 knots).

2) The project gives engineering information on the operational conditions of the propulsors during escorting conditions. When the tug is operating in indirect towing mode, the propulsor and engines are doing a tough job in off-design conditions. As nowadays systems are designed more and more towards the edge of the possibilities, it is essential to know details of the behaviour of the propulsors in escorting conditions.

ESCORTING OPERATIONS Escorting of ships is a substantial task of tugs. The demands for this operation are excellent manoeuvrability, high thrust and large steering and breaking forces as well. Figure 1 shows an escort tug boat (Kotug's Rotor Tug RT Magic) in operation equipped with three Z-Drives (Rudder propellers). This is a full scale test to determine the Escort capabilities.

Figure 1: Full scale test for determination of the Escort Notation (Courtesy Kooren Shipbuilding and Trading) Generally tugs with propulsion and steering drives that are able to provide max. thrust around 360° are used for this kind of operation. These tug boats are equipped with Voith-Schneider vertical axis rotors (VSP) or azimuthing rudder propellers with ducted propellers as fixed pitch version (FP) or controllable pitch version (CP). The two most used types are Tractor-Tugs and ASD-Tugs (Azimuth Stern Drive). The distinctive mark is the installation position of the propulsors at the bow or respectively at the stern. In addition, the typical hull forms used for tractor tugs and stern-drive tugs is significantly different. Both hull form types have seen significant optimisations during the last years (see [6]). New tug designs are also being made such as the Rotor Tug [7]

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and the SDM (Ship Docking Module). Which type of tugs is used depends on the requirements of the operators. Not only is the thrust of the propulsors used to provide maximal towing forces. The lift and drag forces generated by the hull of the tug are other very important parts. The intention is to turn the tug in a position of highest resistance behind the ship by using the steerable thrusters. The different modes are described as:

• Indirect towing (indirect arrest) • Direct towing (Combined arrest) • Reversed arrest • Transverse arrest

Which mode is suitable depends on different parameters: the intended manoeuvre, speed of reaction, behaviour of the hull (resistance, agility), available thrust in combination with the characteristic of the engine and the characteristic of the propellers at oblique and backwards inflow etc. The objective of escorting is to provide maximum braking and steering forces at usual escorting speeds of 8 or 10 knots. Additionally moving from the force equilibrium at the port side to the force equilibrium at the starboard side has to be as quick as possible. This is the analogy of the rudder function concerning steering force, steering angle and rudder speed as well. The escorting capability of a tug is expressed as the Escort Rating Number derived from a full scale procedure (see [8] and Figure 3). It is not yet clear whether it is allowed to get this notation based on a model test or theoretical calculations.

Figure 2: Typical escort configuration

Figure 3: Example from the DNV-Rules regarding Escort Notation [8] For tug boat operators, it is of great importance to consider all aspects at the project stage to reach the optimum tug boat design and thruster arrangement for getting maximal towing forces. A suitable choice of the position of the tow rope, the size and form of the skeg, hull form itself, location, size and arrangement of the thrusters are all important aspects to obtain optimum forces while escorting. On the other hand a compromise between yaw stability and agility of the tug has to be found. For optimisations it is important to be able to predict the achievable escort forces and to have a tool to study the influence of different tug boat configurations. This is the aim of the new software tool with focus on ASD-Tugs. The goals of the tool are:

• Prediction of steering and braking forces at escort operations at the project stage

• Giving recommendations to the clients on the configuration: thruster power, thruster position, tow rope arrangement and hull design, especially with respect to the skeg

• Calculation of the static escort forces according to the escort rating number

• Easy-to-use software tool Apart from the software, it was decided to devote ample attention to the generation of knowledge on the so-called hull-to-thruster interaction in escorting

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conditions. The following aspects had to be investigated and included in the software:

• Interaction between thrusters and hull especially while escorting [w, t, βR = f(VS, β)]

• Thruster-thruster interaction due to direct or indirect influence of the slip stream

• Interaction of thruster/propeller characteristic and engine characteristic at oblique inflow

SOFTWARE TOOL The results of the model tests and theoretical considerations (to be discussed further on in this paper) had to be implemented in a calculation model. This calculation model should be able to generate results in an automated way: the used should not spend too much time tuning the calculations or performing manoeuvres manually. After having performed a basic amount of input, a polar diagram should be generated. Obviously, it should work under Windows, easy to use on laptops in planes and with clients. The User Interface is written in Java, while the calculations modules are written in FORTRAN.

General description Obviously the program should be able to run with a minimum amount of input. At the project stage for the propulsors manufacturer, only limited information is available, but a reasonable prediction should be made. In that respect, it is chosen to make predictions valid for a certain family of tugs. Obviously, as this tool is to be used by SCHOTTEL employees, the SCHOTTEL rudder propeller types are input. The other input parameters are restricted to the following:

• Hull type • Main dimensions • Metacentic height • Skeg type • Rudder propeller position, power, diameter

and type • Position and location of tow line

It is realised that the escorting capabilities can be enhanced much by appendages and features such as skegs, bilge keels and dynamic fairleads. It is not the target to predict the individual contributions of these special features. Often, these features are becoming important at a stage where special and extremely dedicated escort tugs have to be designed. At that stage, more elaborate studies are required to quantify these aspects.

Visual aids As also non-expert (read non-naval architects) users will use the program, some robustness has to be present. Therefore, visuals are added that clearly indicate what the ship looks like when the user has performed all its input (see Figure 4).

Figure 4: Input Mask with visualisation of the arrangement

Output In general, in the project stage, the output will be the most important. Polar diagrams are created by the software (see Figure 5) and the users can compare different versions of the same tug. In such a way, the impact of several changes such as power changes, tow line attachment points or GM variations on the design can be visualised very quickly.

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Figure 5: Output as Polar Diagram of the maximum towing forces, expressed in steering and braking forces

CALCULATION MODEL

General system set-up In the objective of the tool development, it was clear that a good insight in the performance of the omni-directional thrusters was important. Apart from this, it was MARIN’s experience that in other research projects the weakest point was the hull-to-propeller interaction. Realising this importance, this was treated very thoroughly. This is the major difference with other research studies carried out in the past. To describe the complete forces on the whole tug, the following aspects are distinguished:

• Hull forces (including skeg), • Hull to propeller interaction (meaning the

quantification of the inflow velocity and angle towards the thruster with the tug in escorting condition).

• Propeller forces under influence of the above. • Thruster-to-thruster interaction causing

efficiency decrease of the ‘leeward’ thruster • Thruster-to-hull interaction: when the thruster

wash blows against the hull, thrust degradation occurs, resulting in efficiency loss.

• Hull stability • Tow line forces

The purpose of including all these aspects is obvious: it allows for easy recalculation of the tugs with for example different propulsion configurations and different geometrical configurations.

Engine and propeller characteristics Figure 6 shows the change of the torque coefficient as function of the advance ratio and of the angle of attack respectively of the steering angle. These curves are derived from systematic test series, which have been carried out for different types of nozzles and different

pitch settings. The changes of the nozzle induction and the local inflow speed lead to a dramatic change of the characteristic curves, which has a significant influence on the interaction with the engine. It can be noticed, that the KQ-value at J = 0 can be exceeded at oblique or backwards flow. Therefore it has to be taken into account that the achievable thrust depends on the actual available torque of the engine and on the adjusted pitch in case of a CP-propeller. This has to be considered during model tests as well. The thrust and torque behaviour of thrusters as found during the present project depends on the type of nozzle, the pitch of the propeller and the geometry of the housing. Earlier series of similar contents are mentioned by Oosterveld [9] and Minsaas [10]. These test series are amongst others used in the present tool. The big advantage is that these systematic tests ([9] and [10]) have been carried out over a large range of angles of attacks and a large range of propeller loadings.

Figure 6: Torque coefficient KQ as function angle of attack and advance ratio

Systematic Captive Tests The thruster-hull interaction and the thruster-thruster interaction can only be analysed by systematic model tests. The tug model used during the project was a non-existing tug design, from MARIN’s stock. The tug is not intended to be an optimum escort tug but can however be a representative tug that one could buy commercial off-the-shelf. Every designer would distinguish aspects that would improve lift on the hull and hence the escort capabilities. It was however the intention to work with an average tug design, which is the reason why a MARIN stock model was selected. During the tests, the tug model was equipped with models of SCHOTTEL Rudder propellers (Scale 13, Figure 7).

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Figure 7: Model of a Tug with twin SRP 1515 Using this model a systematic test series has been carried out. The systematic set-up was such that each of the components of the calculation model could be identified after analysis of the test series. Propeller loading, ship speed, heel angles, drift angles and steering angles were varied. The idea was that each test series gives answers to a specific interaction factor. In addition to that, a number of tests were performed in a typical escorting condition. These tests serve as independent case to validate that the formulas describing the escort forces are correct. Table 1: Overview of test series

Test-series 1 Towing Forces 2 Hull Forces 3 Steering Forces 4 Drift Tests 5 Escort Indirect Mode 6 Escort Direct Mode 7 Escort Direct Arrest Mode 8 Escort Transverse Arrest Mode

Overall about 480 tests have been carried out. The forces and torques (powers) have been measured for the conditions mentioned above. The data have been transformed into useful non-dimensional coefficients with parameters to quantify the influencing effects. Figure 8 gives an impression of the captive tests at

MARIN. The test results have been used as basis for the algorithm and the software.

Figure 8: Captive tests As an example of some test results, the influence of the skeg area is given. Figure 9 shows that the transverse forces can be increased considerably by increasing the skeg area and optimal positioning of the skeg. The lift force could be more or less increased by 100 % at a representative drift angle of 30 degrees.

Figure 9: Lift Force as function of drift angle for two different skegs

Hull to propeller interaction An important aspect in the calculation model is the definition and the quantification of hull-to-propeller interaction. Hull-to-propeller interaction is an important phenomenon for any ship, but for tugs it plays a particularly pronounced role due to the relatively large

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drift angles occurring during the operations and the flow around the hull and skeg. In this paper, hull-to-propeller interaction is defined as the influence of the hull on the orientation and velocity of the flow at the location of each propeller. The change of the orientation of the flow compared to the undisturbed flow is related to the so-called flow straightening, while the change of the velocity of the flow compared to the undisturbed flow is expressed by the wake fraction. In escorting conditions, one can distinguish a ‘weather side’ thruster and a ‘leeward side’ thruster (see Figure 10). From propeller open water tests, the drift tests and the steering tests, the wake fraction for each thruster as a function of the drift angle can be quantified.

Figure 10: Leeward and weather side propellers The influence of the hull on the ‘weather side’ thruster is relatively small and therefore this propeller encounters a smooth transition in the flow properties as function of the drift angle: the direction and magnitude are differing from the ‘open water’ inflow, but not to a large extent. The ‘leeward side’ propeller however operates in a completely distorted flow, influenced by skeg, hull and weather side thruster. Viscous flow calculations (often called Computational Fluid Dynamics or CFD) allow naval architects to study in detail the flow directions and magnitudes around the hull at the location of the thrusters. These calculations are feasible nowadays and can supply the naval architects with useful information. Therefore, the flow around the ship was further studied using a CFD calculation for a steady drift angle of 20° with MARIN's viscous flow solver PARNASSOS. More information about PARNASSOS can be found in Hoekstra [11] [12], while other examples of calculations for ships sailing at steady drift are presented in e.g. Toxopeus [13] and Van Oers and Toxopeus [14]. An impression of the calculated flow around the hull is shown in Figure 11. In the CFD calculations, only the bare hull with the large skeg was modelled. The influence of the thrusters or other appendages on the flow was not considered. At first, the validity of these calculations is briefly discussed. Secondly observations are made and lessons-learnt are discussed.

Figure 11: Impression of the flow around the hull when sailing at 20° drift angle. Earlier work by Toxopeus; see e.g. reference [13], showed good agreement between calculations and flow field measurements for other hull forms. From the viscous flow calculations performed for the current study, the integral forces on the hull can be derived and these can be compared to the measurements. In the following table, a comparison between the measurements and the calculations is given, together with the percentual deviation from the measurements: Table 2: Comparison between experiments and calculations, large skeg configuration, β=20° Experiment Calculation Error Y' -0.35 -0.31 -9.2% N' -0.10 -0.09 -8.8% N'/Y' 0.29 0.29 0.4%

This comparison shows quite an acceptable agreement and combined with earlier experience with PARNASSOS, it is expected that the results can be used with confidence for the purpose of the present project. In Figure 12 the axial velocity contours at the transverse plane through the thrusters are presented. Additionally, the propeller discs are indicated. From this figure, it is seen that the port side propeller (leeward side) is located in the wake of the vortex shed from the (upstream) centre skeg. This means that the hull-to-propeller interaction for the leeward side propeller will be largely dominated by the location and magnitude of the longitudinal vortex from the skeg. The starboard (weather side) propeller, however, is more or less positioned in undisturbed flow. Some flow retardation (wake) is found at the top of the propeller disc caused by the longitudinal vortex generated at the bilge.

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Figure 12: Axial velocity contours at transverse plane through thruster locations, β=20°. The objective of these CFD efforts was to derive the direction and magnitude of the flow through the discs at the propeller locations. In the following table, some quantitative results regarding the average flow properties in the discs at the thruster locations are presented: Table 3: Averages of flow velocity and orientation derived from viscous-flow calculation, β=20° Item PS propeller

(lee side) SB propeller

(weather side) axial velocity u/V 0.8 0.9 wake fraction w 0.2 0.1 horizontal angle 8° to SB 22° to PS flow straightening coefficient CFS -0.4 1.1

vertical angle 3° downward 4° upward with the flow straightening coefficient CFS defined as the local flow angle at the location of the propeller (βp) divided by the drift angle of the ship (βp):

β=

βp

FSC

These results show that at the weather side, the flow is attracted towards the centreline, see also Figure 11. Also at the leeward side, an inward flow is experienced. However, this is mainly caused by the vortex shed from the centreline skeg. The clock-wise rotation of this vortex generates a flow to starboard at the thruster position. Knowing the velocities and orientation of the flow is essential for deriving a reliable mathematical model for the calculation tool. When the propeller works in a low-speed region, much more thrust can be generated than when it works in a high-speed region. In this case, about 30% more thrust is obtained. This is important to take into account in the calculations of the escorting capability. Furthermore, if a CP propeller is used, it could be advantageous to apply different pitch settings for the leeward and weather side propellers in escorting mode in order to improve the breaking or

steering capability of the tug. Finally, the side forces generated by steering actions are strongly dependent on the effective inflow angle of the thrusters. From the combinations of the model tests with the theoretical calculations such effects can be quantified better.

Fitting of the calculation model For the prediction tool, the individual components are described in different formulas. The formulas have to fulfil three requirements:

• A good fit of the measured data • Representative for the class of tugs • Robust formulations yielding reliable and

stable results. Secondly, it should be easy to implement new and updated information on other and newer propulsors when they become available.

Demonstration of importance of hull-to-propeller interaction To demonstrate the importance of the hull-to-propeller interaction, the polar diagram of escort forces is calculated with and without the hull-to-thruster interaction. These results are illustrated in Figure 13.

Figure 13: Predictions with and without hull-to-thruster interaction

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Figure 13 shows two lines: One of the lines indicates the braking and steering forces obtained while ‘forgetting’ the hull-to-propeller interaction, while the bold line indicates the hull-to-propeller interaction, properly taken into account. It is demonstrated that there is a distinct impact on steering forces and braking forces.

APPLICATION Using the tool, it is now very easy to demonstrate to clients the influence of certain choices made in the very early design stage, see Figure 14 and Figure 15. The installed power that can be used during escorting or the position of the tow line point are important input values.

Figure 14: Influence of different SCHOTTEL Rudder Propeller types Figure 14 demonstrates the impact of mounting three different types of the SCHOTTEL rudder propellers on the performance of the tug with. In this figure, the SRP1212, SRP1515 or SRP2020 on the same tug are compared. Obviously, the maximum steering forces are influenced: in this case increasing from 550 kN up to 675 kN.

Figure 15: Influence of the tow line attachment point Figure 15 demonstrates another aspect: the influence on the tow line attachment point in longitudinal direction. In these calculations, the tow line fairlead is virtually moved from the foremost point of the vessel in several steps aftwards, up to 3.5 meter aft of the foremost point. By placing the tow line attachment point more aft, the maximum steering forces are increasing in this case from 592 kN up to 669 kN.

CONCLUSIONS Based on a systematic series of model tests towards hull to thruster interaction, knowledge is developed on the performance of thrusters under the hull of a tug. This knowledge is implemented in an escort force prediction tool. The software is able to predict the escort forces and to compare different tug configurations to each other. SCHOTTEL is confident that the tools will support yards, designers and operators of ASD-Tugs extensively with focus on the special requirements of each customer to get the optimal tug arrangement and thruster equipment.

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ACKNOWLEDGEMENTS This paper is devoted to Mr. Uwe Gragen of SCHOTTEL, who passed away during the course of this project. He was the visionary seeing how the result of such a project could and should be used.

REFERENCES [1] Sas, F.M.; Timmers, R.A. and Gallin, C.;

Simulation of the effective pull forces produced by tugs. RINA international conference on escort tugs – Defining the technology, October 1993.

[2] Hutchison, Bruce L., Gray, David L. and Jagannathan, Sridhar, New Insights into Voith Schneider Tractor Tug Capability, Marine Technology 30(4):233-242, 1993.

[3] Brandner, P.A.; Performance and effectiveness of omni-directional stern drive tugs. PhD thesis, Tasmania, November 1995.

[4] Smith, J.R. and Birmingham, R.; Creating the Virtual Tug, ITS 2002, 17th International Tug & Salvage Convention, Bilbao, May 13-17, 2002.

[5] Jensen, G.; Rudder propellers for tugs – developing the state of the art. ITS2002 17th International Tugs & Salvage Convention, Bilbao, Spain, May 13-17 2002.

[6] Allen, R.; The evolution of escort tug technology: Fulfilling a promise. SNAME transactions, Vol. 108, pp. 99-122, 2000.

[7] Kooren, A. et al, Rotor Tugnology. ITS 2000, Jersey.

[8] DNV-Rules [DNV Vol. 3 PT5 Chpt. 7 Sec.16] regarding Escort Notation.

[9] Oosterveld, M.W.C. and van Oortmerssen, G.; Thruster systems for improving the manoeuvrability and position keeping capability of floating objects. OTC paper 1625, May 1972Oosterveld and van Oortmersen. OTC paper, 1978.

[10] Minsaas, K.J. and Lehn, E.; Hydrodynamical characteristics of rotatable thrusters. NSFI report R-69.78, January 1978.

[11] Hoekstra, M. and Eça, L. PARNASSOS: an efficient method for ship stern flow calculation. 3rd Osaka Colloquium on advanced CFD applications to ship flow and hull form design, pages 331–357, May 1998.

[12] Hoekstra, M. Numerical Simulation of Ship Stern Flows with a Space-Marching Navier-Stokes Method. PhD thesis, Delft University of Technology, Faculty of Mechanical Engineering and Marine Technology, October 1999.

[13] Toxopeus, S.L. Verification and validation of calculations of the viscous flow around KVLCC2M in oblique motion. 5th Osaka Colloquium on Advanced CFD Applications to Ship Flow and Hull Form Design, March 2005.

[14] Oers, B.J. van and Toxopeus, S.L. On the relation between flow behaviour and the lateral force distribution acting on a ship in oblique motion. 10th International Cooperation on Marine Engineering Systems ICMES, March 2006.