Design Considerations of DP- Systems for Offshore Windpark Installation Vessels

Design Considerations of DP- Systems for Offshore Windpark Installation Vessels Stefan Krüger 1 and Hendrik Vorhölter 2 ABSTRACT The political decision in Germany to escape from nuclear power generation has lead to a severe pressure on the German offshore wind industry. A large number of wind turbines shall be installed in a short time. As these offshore wind parks have to be erected quite far away from the German coastline, the wind turbine installation vessels have to operate in a potentially harsh environment. Prior to the jacking process, the vessel has to keep exactly its position, where wind forces, current forces and wave forces act on the vessel. During the concept design phase of such vessels, these forces have to be determined quickly and with a sufficient accuracy to serve as input basis for the DP- system. Such DP- systems consist of several maneuvering components which are alternatively used for propulsion purposes. Typically, lateral thrusters are combined with azimuthing units to achieve the highest DP- flexibility. However, this often leads to the situation where the DP- system has much more degrees of freedom than possible equations which makes it difficult to develop a control algorithm for the system. Moreover, there is a strong interaction between the DP- components as such and between the individual components and the hull. This can result in forbidden zones for each DP- component, which makes the development of the control system even more challenging. Therefore an alternative method for the design of the DP- system and the hull form was used during the concept design of the Type 187 Offshore Wind Farm Transport and Installation Vessel of Sietas Shipyard, which was on order for a Dutch client. For this DP system, experiences from the crabbing requirement of twin screw ferries were used to design a DP system which has less degrees of freedom, but a higher efficiency, as the single DP components can more efficiently used. At the same time, the DP task was introduced into the hull form design from the very beginning of the project. Further, due to its simplicity, the DP system is quite cost effective and easy to maintain. The paper will show that the use of appropriate computational methods during the first phase of the ship design process will lead to efficient and simple solutions. Further it will be shown that such methods need to be used especially by the responsible building yard to ensure that the final product will meet the desired purpose in the desired quality. The paper is original and the subject for the first time presented in public. KEY WORDS Dynamic Positioning; Crabbing; Wind farm Installation Vessel;Slow Speed Manoeuvring INTRODUCTION Dynamic positioning has become a major design issue during the last decade for some ship types. One reason is the booming offshore segment, where during operation it is required that the ship can maintain its position even in harsh weather conditions. As the weather conditions under which the ship can still be operated are typically part of the building contract, the design and demonstration of a properly working dynamic positioning (DP) system has become an important issue of the ship design process. Moreover, there are some applications where the DP- system has become the main design driver for the layout of the whole machinery plant, as the total power demand required for DP- purposes can be larger than the power demand required for the propulsion. Consequently, the DP- system influences the complete ship design from the very beginning, and as the initial design is the most relevant design phase from cost point of view, reliable computational methods are required which allow the proper layout of all major DP- components. These DP- systems include a variety of quite complex components, which have interactions and which all have to be integrated into a functional system. The larger the forces are that act on the ship, the more complex does the system become, which is simply a consequence of the fact that more individual components need to be installed which have to operate together with other components. This does especially hold for a very new ship type: The wind farm installation vessels. These vessels are highly specialized and very 1 Hamburg University of Technology, Institute of Ship Design and Ship Safety 2 J.J. Sietas KG Schiffswerft GmbH & Co., Hamburg

expensive, and down times which occur when the ship can not operate are expensive, too. The present designs of wind farm installation vessels serve two purposes: The wind turbines are transferred from the shore to the Windpark, the vessel jacks up and the turbine is installed. The bottle neck in the procedure is the situation prior to jacking: When the vessel can not keep its position, it can not jack up and the installation of the turbine is delayed. So the reliability of DP- system has a large influence on the efficiency of the installation process, and therefore on the number of turbines that can be installed per vessel and year. The decision of the German government to close down the German nuclear power plants while not installing fossile power plants at the same time has put an enormous pressure on the German renewable energy industry. Wind farming is presently regarded as the number one option, and many wind parks are in the planning process. For other political reasons, the planned wind parks are quite far away from the German coastline. This poses the following design problems on wind farm installation vessels: - The service speed must be quite high to allow for short transit times. - The number of wind turbines carried during transit must be quite large. - The operation able water depths are quite large, and harsh weather situations become more probable. For the design of the DP- system, this can be translated into the following demands: - The large water depth requires very long jacking legs, which results in high wind forces when the legs are raised and in high current forces when the legs are lowered. - The high number of wind turbines carried on deck results in large aerodynamic forces for the DP- system. - The relatively high service speed requires a hull form which is directionally stable during transit, which on the other hand results in lager current forces during the DP- operation. As a consequence, DP- systems of this type of vessels have reached a complexity which has become hard to handle: It has become a typical layout of such systems that a couple of lateral thrusters is installed in the forward part of the ship, and a some azimuthing thrusters are installed in the aft part of the ship, the latter are used for propulsion purposes during transit, but their efficiency during transit is very low as they have been designed for bollard pull conditions. A typical arrangement of such a system is shown in Fig. 1. Additionally, the system may be supported by so called retractable thrusters which Figure 1: Typical components of a DP- system for a wind farm installation vessel and forbidden zone definition for multiple azimuthing thrusters. are also of azimuthing type and which are retracted when not in use, e. g. during transit. The large number of different components results in the situation that the system is extremely difficult to control. Ruth (2009) and Sörensen have shown for the simple case of four azimuthing thrusters which are each located at the corner of a platform that situations can occur where the control algorithm fails, as there are multiple solutions of the problem possible. Moreover, interactions between the azimuthing thrusters and hull exist (e.g. Palm (2010)) which results in so called forbidden zones where the thrusters can not be operated, see also Fig. 1. This makes the development of a control algorithm even more complex and results in the situation that the system losses due to improper control of the components can be that large that the DP- capability of the system drops significantly. This is in line with the observations reported from experienced masters of such kind of vessels: In most cases, the DP- task prior to jacking was performed by manual control of the manoeuvring devices, not by

the automation. The installation of additional components does not solve the problem, as it makes the control algorithm even more complex. And from the designer s point of view the system is inefficient, as quite a lot of money is invested into many components which are not efficiently used. And this poses additional problems to the shipyard, as the yard has the system responsibility for a lot of complex components which have to ensure the operability of the vessel from contractual point of view. However, in this context it should be remembered that the so- called DP- problem is in fact not a really new issue. Each RoRo, RoPax or Cruise vessel design is confronted with the same type of problem, when the vessel has to berth at slow speeds in narrow spaces against harsh weather conditions. For large cruise ships, the situation have become even more comparable, as these ships nowadays approach destinations which are environmentally sensible and the ships are not allowed to drop the anchor. Therefore, they also have to maintain their position using their propulsion and manoeuvring system, also against unfavourable weather conditions. The designers of these ships have come to efficient technical solutions to solve nearly the same type of problem. Therefore, when Sietas Shipyard had to design their wind farm installation vessel Type 187, the DP- system was designed based on those principles that have since long time been used by the ferry industry. The following sections deal with the design concept of that DP- system and shows how modern numerical design methods can already be used to support the design. But it will also be shown that at present, severe problems exist from the practical design point of view that require intense research activities in the future. PRINCIPLES OF DP OR CRABBING Basic Problem y Wind, Current and Seastate N F E E AP T PS y S F P CL F S (backing Propeller) x T SB F B F R x S x B Figure 2: Principle of the crabbing manoevre for ferries, with the starboard propeller in backing mode. The design of any DP- (or slow speed manoeuvring) system can be split into two tasks: - The static analysis which is based on force and moments equilibrum delivers the required nominal forces of the individual components. - The dynamic analysis which is based on time dependent external forces determines the ability of the system to alter the required control forces in sufficient time. It is obvious that the second task can not be performed if the first task was not done properly. Further, the dynamic functionality depends on the ability of the system to alter the manoeuvring forces with sufficient speed, provided a clear functional dependency exists between the alteration of these forces and the DP- problem. If that can be guaranteed, the dynamic functionality of the system only depends on the possible maximum time derivatives of the maneuvering forces of the individual DP- components and, consequently, on the ability of the prime movers to cope with the time derivatives of the required power demand. It will later be shown that these dynamic requirements can easily be fulfilled if the correct combination of prime movers and manoeuvring devices is used. Therefore, we will in the first step restrict ourselves to a static analysis of the problem, and we will make use of manoeuvring components which are used during conventional twin screw ship design, as shown in Fig. 2. The system consists of two conventional shaft lines with spade rudders located in the propeller slipstream, located in a distance y S perpendicular to the ship s centre line. Further, a set of lateral bow and stern thrusters located at a distance x B and x S from the aft perpendicular of the ship is installed. Using these devices, we have to

fulfill three simple elementary equations, namely two force equilibrii and a yaw moment equilibrium. We denote the time averaged sum of all external forces by wind, current and seastate by F XE and F Y E and the resulting external yawing moment by N E. If we assume for the first step that the lateral stern thrusters are not required for the DP- problem, we obtain: F Y = 0 = F B F Y E (1) Where F B denotes the nominal force of the bow thruster(s). This nominal force may be significantly decreased in case of forward speed and/or current effects. This effect will be quantitatively treated below. It becomes obvious from this equation that the cross force generation of the thrusters must compensate the external cross force, which results in a yawing moment dominated by the thrusters, because their centre of effort is much more forward compared to the centre of effort of the external forces. This additional yawing moment resulting from the thruster operation must in our configuration be compensated by the net propeller shoulder moment, which is generated by one propeller acting forwards and one propeller acting backwards. In this context, net propeller thrust means the nominal propeller thrust decreased by the thrust deduction of the propeller(s), where the backing propeller acts against the hull. At the same time, the force equilibrium in X direction reads: F X = 0 = T PS T SB F XE (2) where the index SB denotes starboard side, PS port side. T denotes the net thrust of the propeller, which is the thrust minus the thrust deduction at that thrust loading, which needs to be computed individually for both propellers. The equilibrium for the yawing moment reads: N = 0 = N E F B x B + T PS y S + T SB y S. (3) As a result, we obtain three simple equations which do easily allow the computation of the two thrusts and the lateral thruster force. The remaining unknowns are simply the thrust deduction factors for the two propellers and, eventually, the reduction factor of the lateral thrusters when operating at nonzero forward inflow speed. Now this extremely simplified system is not able to hold the ship against large external forces, and it is quite inefficient as the full propeller shoulder moment atcs against the bow thrusters. If we introduce now the net thrust of the stern thrusters in the system, we have a fourth degree of freedom, but only three equations. Consequently, the problem can be formulated as a nonlinear optimization problem with one free variable and three dependent ones, where a target function can be introduced that helps to find those solutions which are most interesting from technical point of view. Several target functions are possible, if for example one of the propeller thrusts is implemented as free variable, for example: - Maximize thruster output - Maximize shoulder moment - Minimize energy consumption Applications have shown that there is no significant difference in the results when either the first or the second option are used, whereas the selection of the third option results in a minimized net thrusts of the propellers, as the yaw moment balance of the thrusters is optimally balanced against the external yawing moment, because at given total cross force, the latter can be generated by a distribution between bow and stern thruster(s) in such a way the the resulting yawing moment is minimized. Further, detailed computations for different designs we have carried out have shown that the limiting factor of this concept is the thrust which can be delivered by the backing propeller. Any design option to increase the latter increases the DP- capability of the system remarkably. One efficient option to do so is to enable the backing propeller to efficiently absorb 100%MCR of the available power of its propulsion train. Crabbing with rudders As additional devices which have not been used until now we have two (spade) rudders located in the propeller slipstream. These are typically used as passive control devices during normal operation. From DP- or crabbing point of view, rudders are quite efficient devices as they can deliver large cross forces without notable energy consumption. Rudders are also interesting devices from redundancy point of view: For some DP- notations, redundancy requirements exist that require full DP capability even if one ore more components fail. This redundancy requirement is extremely cost inefficient, as the installation of backup units is required which are only intended to operate in case of a failure. In this context, the rudders can for example be used in case of a lateral thruster failure. Or the rudders can simply be used as additional devices when the DP- capability shall be increased. If we denote the rudder forces by F R,X and F R,Y, our basic equations - including stern

thruster(s) - read when the yawing moment of the rudders with respect to AP can be neglected: F X = 0 = (T PS F R,X,PS ) T SB F XE (4) F Y = 0 = F B + F S F Y E + F R,Y,PS (5) In both equations it is assumed that relevant rudder forces only exist for the rudder which is located in the slipstream of the forward acting propeller (PS propeller in our example). The equation for the yawing moment reads: N = 0 = N E F B x B F S x S + (T PS F R,X,PS )y S + T SB y S (6) The equations shows that the beneficial effects of the rudder(s) result from two different effects: The rudder increases the possible propeller shoulder moment, as the forward net thrust is decreased when at the same time the sum of both propeller thrusts needs to match the external longitudinal force. At given thrust limit of the backing propeller, this allows for larger output of the propeller acting forward. Further, the cross force of the rudder(s) supports the lateral thrusters. As the rudder itself decreases the net thrust of the propeller (besides the thrust deduction), the larger propeller thrust possible allows for larger rudder cross forces, additionally. The additional rudder forces can easily be introduced into the equation system when the rudder angle during DP is set to a fixed value, because in this case, no additional variable is introduced into the problem due to the fact that at a given rudder angle, the rudder force only depends on the propeller variables. The required thrust of the propellers and of the thrusters can be expressed by the individual rpm and pitch setting values, which easily allows to compute the required power demand as a non linear optimization problem with one free variable and three dependent ones. It also becomes obvious that the system layout as described above can easily be controlled for time dependent force fluctuations, because the system design is straight forward: - In case pure additional cross force is required, the output of the lateral thrusters needs to be increased. - In case pure additional longitudinal force is required, the pitch (or rpm) setting of both propellers is increased simultaneously. - In case pure yawing moment is required, one propeller pitch or rpm setting is increased, the setting of the other propeller is decreased accordingly. It becomes clear that controllable pitch propellers are most favourable for all DP- units because the propeller pitch can much faster be altered (typical hydraulic servo units deliver approx. 0.6-0.8 deg blade angle/s) than its revolutions. This is due to the fact that whenever the rpm is altered, rotational masses need to be accelerated. Interaction Forces In the equations above, all forces have been written independently from each other. This is in fact not correct as there are some interaction effects which have a significant influence on the problem. The strongest and therefore most important interactions are: - propeller- rudder interaction - propeller- hull interaction - hull- thruster interaction at forward speed The propeller- rudder interaction can be formulated as a one directional one, which means that the upstream induction of velocities from the rudder to the propeller can be disregarded. This is true if the distance between rudder and propeller is larger than 0.25 times propeller diameter according to numerical investigations by Abels (2005). If the rudder design is fixed, then the rudder forces at zero speed for a given rudder angle do solely depend on the propeller pitch setting, if a CPP is used and constant rpm mode is selected for the DP- mode. The rudder forces required for DP purposes are computed by a direct panel method (Soeding (1998)), where the lift is modelled by wake panels at the trailing edge of the rudder blade, the top and the sole. The propeller slipstream is modelled by a lifting line method adapted for CPPs in off design conditions (Haack (2006), Krüger (1998)). Prior to the computations, the circulation distribution of the propeller is computed for the given rpm and pitch setting under bollard pull conditions. For this purpose we have selected a lifting line method as it is well known that this method is very robust with respect to the characteristics of the free vortex system. Further, the analytical formulation of the free vortex induced velocities allows to compute the slipstream speeds at all positions without numerical problems. Fig. 3 shows the computation of the pressure distributon of a full spade rudder with Costa Bulb and twisted leading edge for bollard pull condition. The right picture shows the pressure side, the left picture the suction side.

If such kind of computations are performed prior to the DP- calculation for a set of rudder angles and propeller rpm or pitch settings, the rudder forces F R,X and F R,Y are available and can be interpolated during the DP- calculation as a function of the propeller pitch and rpm settings. From such kind of computations we can further develop a set of design rules for spade rudders that increase the DP- capability at low investment costs. These rules are: - The rudder should be located as far as practically possible away from the propeller, because the slipstream speeds increase downstream. Due to the fact that the pitch of the free vortex system is quite small (the inflow speed is zero), we have computed as a rule of thumb 0.5D as efficient distance, measured approximately from the propeller generator line to the 1/4 rudder chord line. - An important design feature of the rudder is to increase the maximum lift at high rudder angles. This can be achieved by thick profiles having large nose radii and a twisted leading edge. - The steering gear should be able to reach larger rudder angles than 35 Degree, which is the standard. Figure 3: Rudder force computation in the propeller slipstream with our panel method for bollard pull condition, 100% MCR engine output. Left: suction side, right: pressure side. A similar approach is used for the interaction forces between hull and propellers. For the DP- Problem, only the portion T (1 t) of the propeller thrust is available, where t is the well known thrust deduction fraction. For the DP- situation the problem occurs that t is hard to define, as the propeller operates under bollard pull condition and the resistance R T, which is also required to define t is zero. Further, two different situations have to be distinguished: The forward acting propeller produces a suction force on the hull, whereas the backing propeller creates an inflow to the hull. The latter causes problems with the Bernoulli equation to compute the pressure on the hull from the computed velocities, because it has to be distinguished between those parts of the hull which are affected by the propeller slipstream and those which are not. Haack (2006) has developed a panel method for the computation of the propeller hull interaction forces based on the principles of our rudder panel method. The lift is generated by wake panels which are connected to the panel grid of the hull at the ship s centre line. The propeller is again modelled by a lifting line approach, and the velocities are computed from Biot- Savart s law by direct integration. Fig. 4 shows an example of the result of such kind of interaction force computations for a single screw vessel under bollard pull condition. The left picture shows the panel grid, the propeller and the wake panels, the right picture the computed pressure distribution along the hull for a backing propeller at 100% MCR. Such kind of computations need to be carried out for a set of propeller conditions, and the interaction forces can be made dimensions less with the individual propeller thrust. If a CPP is used, the correct combination of pitch settings and rpm settings should be used, which can be taken from the individual combinator curve. The interaction between the lateral thrusters and the hull is much more difficult to compute. In contrast to the propeller hull interaction, which is dominated by potential flow effects, the thruster hull interaction is mainly due to viscous effects. Therefore, viscous computations are required to solve this type of problem. Unfortunately, these computations are extremely time consuming which makes their application not possible during the initial design stage. Therefore, we make use of measurements carried out by Brix (1993), but we have to admit that this is at present the weak point of our concept. Brix has investigated the nominal thrust and yaw moment loss of bow and stern thrusters acting in forward or backward flow, see Fig. 5. For the DP- analysis, the flow component parallel to the ship s centre line is computed, an then the reduction factors for the tunnel thrusters are interpolated from Fig. 5. It is of course obvious that these reduction factors must depend on the individual hull form and the tunnel arrangement, and therefore the present approach can only be a rough guess. However,

Figure 4: Example of propeller hull interaction for a single screw vessel bollard pull condition, 100% MCR engine output, backing propeller. Left: Panel grid, right: Pressure distribution. better material is not available to us, and it is intended to perform a set of RANS- computations for selected hull forms and tunnel thruster arrangements in the future. Figure 5: Reduction factors for tunnel thrusters in longitudinal flow according to measurements by Brix (1993). Now, all information is available to perform the DP- analysis if the external loads are known. EXTERNAL LOADS The external loads are wind loads, current loads and loads from the sea state. The determination of the external loads is not really a problem for conventional vessels, with respect to the accuracy that is required during the initial design phase for the system layout. Wind loads are available from wind tunnel tests, and current forces - if there are any - can easily computed from our slender body manoeuvring model Soeding (1984) if the correct hydrodynamic cross force coefficients are used. But these are well known for typical hull forms from the evaluation of full scale trials. And external loads from the sea state are rarely applied for conventional vessels, and if necessary, they can be computed from well known strip theories with sufficient accuracy. For the new ship type of wind farm installation vessels, practically all design loads are unknown, because no data of reference vessels are available. Further, no computational methods for these forces exist which can be applied during the initial design stage. This holds for example for wind loads, which are the dominating external loads with the jacking legs in raised position. Fig. 6 shows the results obtained for a wind park installation vessel according to different computational methods obtained from different parties. The left graph shows that nearly all methods including model tests come to similar results with respect to the cross force, but the yawing moment results differ significantly. This uncertainty is a major problem the designer has to face, and consequently, he has to rely on model tests. However, wind farm installation vessels have a windage area which is composed of many ragged slender cuboids, where the wind yawing moment contribution of each cuboid is dominated by the product of the force and the lever arm with respect to AP (in fact, the yawing moment contribution of each cuboid around its own axis is in fact small). This might indicate that it might be possible to develop simple design formulae for rough yawing moment estimations which come to more reliable results as shown above if the

contributions of each cuboid are superimposed. Nevertheless, at present, wind tunnel tests are definitively needed. With Figure 6: Wind forces of a wind farm installation vessels according to different methods. The read and green line shows different computational methods, the markers model test results. Left: Cross Force, Right: Yawing Moment. It becomes clear that further development is required. respect to current or sea state forces, computational methods exist which are based on the assumptions of slender ships, but in fact these vessels are not of slender type and it is questionable in how far the violation of this basic assumption has a quantitative effect on the results. However, the application of these findings to the Sietas Type 187 design shows what can be achieved under these limitations during practical design work. DP-SYSTEM DESIGN FOR SIETAS TYPE 187 WIND FARM INSTALLATION VESSEL The Sietas Type 187 is a jack-up-vessel for the transport and installation of offshore wind turbines. The vessel is intended to operate in the North Sea. In order to reduce down times for the installation process, two goals have been set for the design of the vessel concerning the layout of the propulsion system: - Fast and economic transit from the shore basis to the wind farms, e.g. high service speed with minimised power consumption - Sufficient DP-capability to ensure the jacking process even in harsh weather conditions The basis for the design was a detailed survey of the wind farm installation vessels on the market and several design projects on the Sietas Yard since 2009. Therefore, Sietas was able to lay out a the principal design of the Type 187 vessel in a very short time when the customer contacted the yard in November 2010. In order to compete with other projects especially in Middle and Far East, special consideration has been taken into the use of standard components for the propulsion system. Thus, it was possible to reduce the purchase costs for the yard and the maintenance costs for the customer at the same time. The purpose of the vessel is to provide transport space for the wind turbines and a stable platform for the crane operations during the installation. The first task requires a large deck space and the second a large breadth and short length as the jack-up legs have to be positioned as far as possible out most. Thus, the length to breadth ratio is only about 3.5, which further increases the hydrodynamic problems for this type of ship. The main characteristics of the vessel are shown in Tab. 1: Length over all 139.40m Breadth moulded 38.00m Draught (design) 5.70m Speed (at design draught) Deadweight (at design draught) Crane lift main host Maximum water depth for jacking 12.00kn 6500t 900t at 30m outreach 45m Table 1: Main characteristics of the Sietas Type 187 vessel

For the economic transit, significant effort was placed into the hull form design and analysis. Extensive CFD analyses have been conducted both with potential and viscous flow methods without and with consideration of the free surface. The aim was not only to reduce the wave resistance but to increase propulsion efficiency by providing a good wake field and low thrust deduction. The final model tested showed that all of these goals could be achieved. Fig.7 shows two snapshots from the CFD optimisation: Figure 7: Optimisation of wave resistance and propeller inflow with CFD methods The propulsion arrangement consists of two controllable pitch propellers mounted on conventional shaft lines. In the slipstream of each propeller one high lift flap rudder is positioned. The propellers are used for the generation of axial forces during DP-operations according to the above mentioned concept. For this purpose the reduction of the thrust deduction is of advantage as the bollard pull is increased both in push and in pull mode. The arrangement of the propellers can be seen in the side view in Fig. 8. The problem in the design phase was to determine the necessary design loads for the layout Figure 8: Side view of the Sietas Type 187 vessel of the DP-system. In the first step data of comparison from various vessels have been used. Here again Sietas was facing the problem that the public data basis for this new type of ship is very rare. In the later design process, model tests have been performed in the wind tunnel to determine the wind and current loads (see Fig. 9). As the above mentioned concept was used during the design work, it was decided to perform complementary model tests only to determine the single load components and not to perform complete DP tests. This was also done in order to control the known and unknown deficiencies of model tests. For the wave forces model tests have been conducted in the model basin in different sea states. The result of the model tests was that the assumption for the necessary power for the DP operation could be verified. The final propulsion system consists of 2 CPPs mounted on conventional shaft lines and two bow and two stern thrusters. All propulsion units are operated in constant speed mode which allows a fast alteration of the thrust over the whole power range. Each tunnel thruster has 2500kW input power whereas the main propulsors are driven by 5000kW each. In order to reduce purchase and maintenance costs eight electric drives of the same type are used, for the main propellers two of them act via a collective gear on the shaft line. Due to the successful hull form optimisation with reduced power demand for the transit was it possible to optimise the propellers for the bollard pull condition. Special consideration has been taken

Figure 9: Windtunnel models for determination of wind and current loads into the arrangement of the tunnel thrusters. The aim was to maximise the thruster performance during DP operation and to minimize the tunnel interference at design speed. A difficult task was to arrange the tunnels with 2.8m in diameter into the hull form, which had to be designed accordingly. With the help of CFD analysis the task could be solved. The simplicity of the propulsion configuration for the DP-system has not only advantages for the DP-control system but also for the electric power supply. As every thruster performs only a single task it was possible to work with only two different power lines. Thus, the effort for the installation of the auxiliary systems for the power generation and distribution could be reduced compared to other jack-up designs for the offshore wind market. The lack of data from other projects is a serious obstacle for the design development in this new market field. Therefore further development at the tools for the ship design is required. But the above presented concept for the layout of propulsion systems for DP-vessels helped Sietas Shipyard to design a simple and smart solution for the transportation and installation of offshore wind turbines. Hydrodynamic problems concerning the forbidden zones for azimuth thrusters for the DP-problem and the design speed could be avoided. Thus a more efficient product could be developed. CONCLUSIONS The DP- problem of a wind farm installation vessel is characterized by the fact that due to the many possible degrees of freedom and the interactions of all components, it is very difficult to design such a system in a straight forward way. Our analysis has shown that it is possible to simplify such systems when experiences from conventional twin screw ship design are used, which leads then to the well known crabbing problem of ferries with conventional CPP- propulsion. The use of first principle based design methods can assist the design of such systems by computing interaction forces. This could be demonstrated by the design of the DP- system of the SIETAS Type 187 wind farm installation vessel which is at present under construction. The main problem arises for the design load determination for these ships, because the data or methods from conventional ships can not be transferred to this type of ship. Further research is necessary if the layout of such systems shall in the future be possible without extensive model testing, which is important especially for the initial design phase. REFERENCES ABELS, W., 2005. Zuverlässige Prognose propellererregter Druckschwankungen auf die Aussenhaut mittels Korrelation

direkter Berechnung. PhD Thesis, Hamburg University of Technology. BRIX, J. (ED.), 1993. Manoeuvring technical manual. Seehafen Verlag Hamburg, 24. HAACK, T., 2006. Simulation des Manövrierverhaltens von Schiffen unter besonderer Berücksichtigung des Verhaltens der Antriebsanlage. PhD Thesis, Hamburg University of Technology. KRÜGER, S., 1998. A panel method for predicting ship- propeller interaction in potential flow. Ship Technology Research 45, 134 140. PALM, M., JÜRGENS, D., BENDL, D., 2010. Comparison of thruster axis tilting versus nozzle tilting on the propeller hull interactions for a drillship at dp- conditions. Proc. Dynamic Positioning Conf. Houston. RUTH, E., SÖRENSEN, A. J., 2009. Effect of thruster configutation on dp- capability. Proc. IMDC Trondheim. SOEDING, H., 1984. Bewertung der Manövriereigenschaften im Entwurfsstadium. Proc. STG 78, 179 204. SOEDING, H., 1998. Limits of of potential theory in rudder flow prediction. Ship Technology Research 45.