Design Considerations of DP- Systems for Offshore Windpark ...
Design Considerations of DP- Systems for Offshore Windpark ...
Design Considerations of DP- Systems for Offshore Windpark ...
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<strong>Design</strong> <strong>Considerations</strong> <strong>of</strong> <strong>DP</strong>- <strong>Systems</strong> <strong>for</strong> <strong>Offshore</strong> <strong>Windpark</strong><br />
Installation Vessels<br />
Stefan Krüger 1 and Hendrik Vorhölter 2<br />
ABSTRACT<br />
The political decision in Germany to escape from nuclear power generation has lead to a severe pressure<br />
on the German <strong>of</strong>fshore wind industry. A large number <strong>of</strong> wind turbines shall be installed in a short<br />
time. As these <strong>of</strong>fshore wind parks have to be erected quite far away from the German coastline, the<br />
wind turbine installation vessels have to operate in a potentially harsh environment. Prior to the jacking<br />
process, the vessel has to keep exactly its position, where wind <strong>for</strong>ces, current <strong>for</strong>ces and wave <strong>for</strong>ces act<br />
on the vessel. During the concept design phase <strong>of</strong> such vessels, these <strong>for</strong>ces have to be determined quickly<br />
and with a sufficient accuracy to serve as input basis <strong>for</strong> the <strong>DP</strong>- system. Such <strong>DP</strong>- systems consist<br />
<strong>of</strong> several maneuvering components which are alternatively used <strong>for</strong> propulsion purposes. Typically,<br />
lateral thrusters are combined with azimuthing units to achieve the highest <strong>DP</strong>- flexibility. However,<br />
this <strong>of</strong>ten leads to the situation where the <strong>DP</strong>- system has much more degrees <strong>of</strong> freedom than possible<br />
equations which makes it difficult to develop a control algorithm <strong>for</strong> the system. Moreover, there is a<br />
strong interaction between the <strong>DP</strong>- components as such and between the individual components and the<br />
hull. This can result in <strong>for</strong>bidden zones <strong>for</strong> each <strong>DP</strong>- component, which makes the development <strong>of</strong> the<br />
control system even more challenging. There<strong>for</strong>e an alternative method <strong>for</strong> the design <strong>of</strong> the <strong>DP</strong>- system<br />
and the hull <strong>for</strong>m was used during the concept design <strong>of</strong> the Type 187 <strong>Offshore</strong> Wind Farm Transport<br />
and Installation Vessel <strong>of</strong> Sietas Shipyard, which was on order <strong>for</strong> a Dutch client. For this <strong>DP</strong> system,<br />
experiences from the crabbing requirement <strong>of</strong> twin screw ferries were used to design a <strong>DP</strong> system which<br />
has less degrees <strong>of</strong> freedom, but a higher efficiency, as the single <strong>DP</strong> components can more efficiently<br />
used. At the same time, the <strong>DP</strong> task was introduced into the hull <strong>for</strong>m design from the very beginning<br />
<strong>of</strong> the project. Further, due to its simplicity, the <strong>DP</strong> system is quite cost effective and easy to maintain.<br />
The paper will show that the use <strong>of</strong> appropriate computational methods during the first phase <strong>of</strong> the ship<br />
design process will lead to efficient and simple solutions. Further it will be shown that such methods<br />
need to be used especially by the responsible building yard to ensure that the final product will meet the<br />
desired purpose in the desired quality. The paper is original and the subject <strong>for</strong> the first time presented<br />
in public.<br />
KEY WORDS<br />
Dynamic Positioning; Crabbing; Wind farm Installation Vessel;Slow Speed Manoeuvring<br />
INTRODUCTION<br />
Dynamic positioning has become a major design issue during the last decade <strong>for</strong> some ship types. One reason is the<br />
booming <strong>of</strong>fshore segment, where during operation it is required that the ship can maintain its position even in harsh<br />
weather conditions. As the weather conditions under which the ship can still be operated are typically part <strong>of</strong> the building<br />
contract, the design and demonstration <strong>of</strong> a properly working dynamic positioning (<strong>DP</strong>) system has become an important<br />
issue <strong>of</strong> the ship design process. Moreover, there are some applications where the <strong>DP</strong>- system has become the main design<br />
driver <strong>for</strong> the layout <strong>of</strong> the whole machinery plant, as the total power demand required <strong>for</strong> <strong>DP</strong>- purposes can be larger than<br />
the power demand required <strong>for</strong> the propulsion. Consequently, the <strong>DP</strong>- system influences the complete ship design from the<br />
very beginning, and as the initial design is the most relevant design phase from cost point <strong>of</strong> view, reliable computational<br />
methods are required which allow the proper layout <strong>of</strong> all major <strong>DP</strong>- components. These <strong>DP</strong>- systems include a variety<br />
<strong>of</strong> quite complex components, which have interactions and which all have to be integrated into a functional system. The<br />
larger the <strong>for</strong>ces are that act on the ship, the more complex does the system become, which is simply a consequence <strong>of</strong> the<br />
fact that more individual components need to be installed which have to operate together with other components. This does<br />
especially hold <strong>for</strong> a very new ship type: The wind farm installation vessels. These vessels are highly specialized and very<br />
1 Hamburg University <strong>of</strong> Technology, Institute <strong>of</strong> Ship <strong>Design</strong> and Ship Safety<br />
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 <strong>of</strong> wind<br />
farm installation vessels serve two purposes: The wind turbines are transferred from the shore to the <strong>Windpark</strong>, the vessel<br />
jacks up and the turbine is installed. The bottle neck in the procedure is the situation prior to jacking: When the vessel can<br />
not keep its position, it can not jack up and the installation <strong>of</strong> the turbine is delayed. So the reliability <strong>of</strong> <strong>DP</strong>- system has<br />
a large influence on the efficiency <strong>of</strong> the installation process, and there<strong>for</strong>e on the number <strong>of</strong> turbines that can be installed<br />
per vessel and year.<br />
The decision <strong>of</strong> the German government to close down the German nuclear power plants while not installing fossile power<br />
plants at the same time has put an enormous pressure on the German renewable energy industry. Wind farming is presently<br />
regarded as the number one option, and many wind parks are in the planning process. For other political reasons, the<br />
planned wind parks are quite far away from the German coastline. This poses the following design problems on wind farm<br />
installation vessels:<br />
- The service speed must be quite high to allow <strong>for</strong> short transit times.<br />
- The number <strong>of</strong> wind turbines carried during transit must be quite large.<br />
- The operation able water depths are quite large, and harsh weather situations become more probable.<br />
For the design <strong>of</strong> the <strong>DP</strong>- system, this can be translated into the following demands:<br />
- The large water depth requires very long jacking legs, which results in high wind <strong>for</strong>ces when the legs are raised and<br />
in high current <strong>for</strong>ces when the legs are lowered.<br />
- The high number <strong>of</strong> wind turbines carried on deck results in large aerodynamic <strong>for</strong>ces <strong>for</strong> the <strong>DP</strong>- system.<br />
- The relatively high service speed requires a hull <strong>for</strong>m which is directionally stable during transit, which on the other<br />
hand results in lager current <strong>for</strong>ces during the <strong>DP</strong>- operation.<br />
As a consequence, <strong>DP</strong>- systems <strong>of</strong> this type <strong>of</strong> vessels have reached a complexity which has become hard to handle: It has<br />
become a typical layout <strong>of</strong> such systems that a couple <strong>of</strong> lateral thrusters is installed in the <strong>for</strong>ward part <strong>of</strong> the ship, and a<br />
some azimuthing thrusters are installed in the aft part <strong>of</strong> the ship, the latter are used <strong>for</strong> propulsion purposes during transit,<br />
but their efficiency during transit is very low as they have been designed <strong>for</strong> bollard pull conditions. A typical arrangement<br />
<strong>of</strong> such a system is shown in Fig. 1. Additionally, the system may be supported by so called retractable thrusters which<br />
Figure 1: Typical components <strong>of</strong> a <strong>DP</strong>- system <strong>for</strong> a wind farm installation vessel and <strong>for</strong>bidden zone definition <strong>for</strong><br />
multiple azimuthing thrusters.<br />
are also <strong>of</strong> azimuthing type and which are retracted when not in use, e. g. during transit. The large number <strong>of</strong> different<br />
components results in the situation that the system is extremely difficult to control. Ruth (2009) and Sörensen have shown<br />
<strong>for</strong> the simple case <strong>of</strong> four azimuthing thrusters which are each located at the corner <strong>of</strong> a plat<strong>for</strong>m that situations can occur<br />
where the control algorithm fails, as there are multiple solutions <strong>of</strong> the problem possible. Moreover, interactions between<br />
the azimuthing thrusters and hull exist (e.g. Palm (2010)) which results in so called <strong>for</strong>bidden zones where the thrusters<br />
can not be operated, see also Fig. 1. This makes the development <strong>of</strong> a control algorithm even more complex and results<br />
in the situation that the system losses due to improper control <strong>of</strong> the components can be that large that the <strong>DP</strong>- capability<br />
<strong>of</strong> the system drops significantly. This is in line with the observations reported from experienced masters <strong>of</strong> such kind <strong>of</strong><br />
vessels: In most cases, the <strong>DP</strong>- task prior to jacking was per<strong>for</strong>med by manual control <strong>of</strong> the manoeuvring devices, not by
the automation. The installation <strong>of</strong> additional components does not solve the problem, as it makes the control algorithm<br />
even more complex. And from the designer’s point <strong>of</strong> view the system is inefficient, as quite a lot <strong>of</strong> money is invested into<br />
many components which are not efficiently used. And this poses additional problems to the shipyard, as the yard has the<br />
system responsibility <strong>for</strong> a lot <strong>of</strong> complex components which have to ensure the operability <strong>of</strong> the vessel from contractual<br />
point <strong>of</strong> view.<br />
However, in this context it should be remembered that the so- called <strong>DP</strong>- problem is in fact not a really new issue. Each<br />
RoRo, RoPax or Cruise vessel design is confronted with the same type <strong>of</strong> problem, when the vessel has to berth at slow<br />
speeds in narrow spaces against harsh weather conditions. For large cruise ships, the situation have become even more comparable,<br />
as these ships nowadays approach destinations which are environmentally sensible and the ships are not allowed to<br />
drop the anchor. There<strong>for</strong>e, they also have to maintain their position using their propulsion and manoeuvring system, also<br />
against unfavourable weather conditions. The designers <strong>of</strong> these ships have come to efficient technical solutions to solve<br />
nearly the same type <strong>of</strong> problem. There<strong>for</strong>e, when Sietas Shipyard had to design their wind farm installation vessel Type<br />
187, the <strong>DP</strong>- system was designed based on those principles that have since long time been used by the ferry industry. The<br />
following sections deal with the design concept <strong>of</strong> that <strong>DP</strong>- system and shows how modern numerical design methods can<br />
already be used to support the design. But it will also be shown that at present, severe problems exist from the practical<br />
design point <strong>of</strong> view that require intense research activities in the future.<br />
PRINCIPLES OF <strong>DP</strong> OR CRABBING<br />
Basic Problem<br />
y<br />
AP<br />
CL<br />
T<br />
SB<br />
F R<br />
x S<br />
Wind, Current and Seastate<br />
N<br />
F<br />
E<br />
E<br />
T PS y S<br />
F S<br />
(backing Propeller)<br />
x B<br />
F B<br />
F P<br />
x<br />
Figure 2: Principle <strong>of</strong> the crabbing manoevre <strong>for</strong> ferries, with the starboard propeller in backing mode.<br />
The design <strong>of</strong> any <strong>DP</strong>- (or slow speed manoeuvring) system can be split into two tasks:<br />
- The static analysis which is based on <strong>for</strong>ce and moments equilibrum delivers the required nominal <strong>for</strong>ces <strong>of</strong> the<br />
individual components.<br />
- The dynamic analysis which is based on time dependent external <strong>for</strong>ces determines the ability <strong>of</strong> the system to alter<br />
the required control <strong>for</strong>ces in sufficient time.<br />
It is obvious that the second task can not be per<strong>for</strong>med if the first task was not done properly. Further, the dynamic<br />
functionality depends on the ability <strong>of</strong> the system to alter the manoeuvring <strong>for</strong>ces with sufficient speed, provided a clear<br />
functional dependency exists between the alteration <strong>of</strong> these <strong>for</strong>ces and the <strong>DP</strong>- problem. If that can be guaranteed, the<br />
dynamic functionality <strong>of</strong> the system only depends on the possible maximum time derivatives <strong>of</strong> the maneuvering <strong>for</strong>ces<br />
<strong>of</strong> the individual <strong>DP</strong>- components and, consequently, on the ability <strong>of</strong> the prime movers to cope with the time derivatives<br />
<strong>of</strong> the required power demand. It will later be shown that these dynamic requirements can easily be fulfilled if the correct<br />
combination <strong>of</strong> prime movers and manoeuvring devices is used. There<strong>for</strong>e, we will in the first step restrict ourselves to a<br />
static analysis <strong>of</strong> the problem, and we will make use <strong>of</strong> manoeuvring components which are used during conventional twin<br />
screw ship design, as shown in Fig. 2. The system consists <strong>of</strong> two conventional shaft lines with spade rudders located in the<br />
propeller slipstream, located in a distance y S perpendicular to the ship’s centre line. Further, a set <strong>of</strong> lateral bow and stern<br />
thrusters located at a distance x B and x S from the aft perpendicular <strong>of</strong> the ship is installed. Using these devices, we have to
fulfill three simple elementary equations, namely two <strong>for</strong>ce equilibrii and a yaw moment equilibrium. We denote the time<br />
averaged sum <strong>of</strong> all external <strong>for</strong>ces by wind, current and seastate by F XE and F Y E and the resulting external yawing moment<br />
by N E . If we assume <strong>for</strong> the first step that the lateral stern thrusters are not required <strong>for</strong> the <strong>DP</strong>- problem, we obtain:<br />
∑F Y = 0 = F B − F Y E (1)<br />
Where F B denotes the nominal <strong>for</strong>ce <strong>of</strong> the bow thruster(s). This nominal <strong>for</strong>ce may be significantly decreased in case<br />
<strong>of</strong> <strong>for</strong>ward speed and/or current effects. This effect will be quantitatively treated below. It becomes obvious from this<br />
equation that the cross <strong>for</strong>ce generation <strong>of</strong> the thrusters must compensate the external cross <strong>for</strong>ce, which results in a yawing<br />
moment dominated by the thrusters, because their centre <strong>of</strong> ef<strong>for</strong>t is much more <strong>for</strong>ward compared to the centre <strong>of</strong> ef<strong>for</strong>t<br />
<strong>of</strong> the external <strong>for</strong>ces. This additional yawing moment resulting from the thruster operation must in our configuration be<br />
compensated by the net propeller shoulder moment, which is generated by one propeller acting <strong>for</strong>wards and one propeller<br />
acting backwards. In this context, net propeller thrust means the nominal propeller thrust decreased by the thrust deduction<br />
<strong>of</strong> the propeller(s), where the backing propeller acts against the hull. At the same time, the <strong>for</strong>ce equilibrium in X−direction<br />
reads:<br />
∑F X = 0 = T PS − T SB − F XE (2)<br />
where the index SB denotes starboard side, PS port side. T denotes the net thrust <strong>of</strong> the propeller, which is the thrust minus<br />
the thrust deduction at that thrust loading, which needs to be computed individually <strong>for</strong> both propellers. The equilibrium<br />
<strong>for</strong> the yawing moment reads:<br />
∑N = 0 = N E − F B x B + T PS y S + T SB y S . (3)<br />
As a result, we obtain three simple equations which do easily allow the computation <strong>of</strong> the two thrusts and the lateral thruster<br />
<strong>for</strong>ce. The remaining unknowns are simply the thrust deduction factors <strong>for</strong> the two propellers and, eventually, the reduction<br />
factor <strong>of</strong> the lateral thrusters when operating at nonzero <strong>for</strong>ward inflow speed. Now this extremely simplified system is<br />
not able to hold the ship against large external <strong>for</strong>ces, and it is quite inefficient as the full propeller shoulder moment atcs<br />
against the bow thrusters. If we introduce now the net thrust <strong>of</strong> the stern thrusters in the system, we have a fourth degree<br />
<strong>of</strong> freedom, but only three equations. Consequently, the problem can be <strong>for</strong>mulated as a nonlinear optimization problem<br />
with one free variable and three dependent ones, where a target function can be introduced that helps to find those solutions<br />
which are most interesting from technical point <strong>of</strong> view. Several target functions are possible, if <strong>for</strong> example one <strong>of</strong> the<br />
propeller thrusts is implemented as free variable, <strong>for</strong> example:<br />
- Maximize thruster output<br />
- Maximize shoulder moment<br />
- Minimize energy consumption<br />
Applications have shown that there is no significant difference in the results when either the first or the second option are<br />
used, whereas the selection <strong>of</strong> the third option results in a minimized net thrusts <strong>of</strong> the propellers, as the yaw moment<br />
balance <strong>of</strong> the thrusters is optimally balanced against the external yawing moment, because at given total cross <strong>for</strong>ce, the<br />
latter can be generated by a distribution between bow and stern thruster(s) in such a way the the resulting yawing moment is<br />
minimized. Further, detailed computations <strong>for</strong> different designs we have carried out have shown that the limiting factor <strong>of</strong><br />
this concept is the thrust which can be delivered by the backing propeller. Any design option to increase the latter increases<br />
the <strong>DP</strong>- capability <strong>of</strong> the system remarkably. One efficient option to do so is to enable the backing propeller to efficiently<br />
absorb 100%MCR <strong>of</strong> the available power <strong>of</strong> its propulsion train.<br />
Crabbing with rudders<br />
As additional devices which have not been used until now we have two (spade) rudders located in the propeller slipstream.<br />
These are typically used as passive control devices during normal operation. From <strong>DP</strong>- or crabbing point <strong>of</strong> view, rudders<br />
are quite efficient devices as they can deliver large cross <strong>for</strong>ces without notable energy consumption. Rudders are also<br />
interesting devices from redundancy point <strong>of</strong> view: For some <strong>DP</strong>- notations, redundancy requirements exist that require<br />
full <strong>DP</strong> capability even if one ore more components fail. This redundancy requirement is extremely cost inefficient, as the<br />
installation <strong>of</strong> backup units is required which are only intended to operate in case <strong>of</strong> a failure. In this context, the rudders<br />
can <strong>for</strong> example be used in case <strong>of</strong> a lateral thruster failure. Or the rudders can simply be used as additional devices when<br />
the <strong>DP</strong>- capability shall be increased. If we denote the rudder <strong>for</strong>ces by F R,X and F R,Y , our basic equations - including stern
thruster(s) - read when the yawing moment <strong>of</strong> the rudders with respect to AP can be neglected:<br />
∑F X = 0 = (T PS − F R,X,PS ) − T SB − F XE (4)<br />
∑F Y = 0 = F B + F S − F Y E + F R,Y,PS (5)<br />
In both equations it is assumed that relevant rudder <strong>for</strong>ces only exist <strong>for</strong> the rudder which is located in the slipstream <strong>of</strong> the<br />
<strong>for</strong>ward acting propeller (PS propeller in our example). The equation <strong>for</strong> the yawing moment reads:<br />
∑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)<br />
The equations shows that the beneficial effects <strong>of</strong> the rudder(s) result from two different effects: The rudder increases the<br />
possible propeller shoulder moment, as the <strong>for</strong>ward net thrust is decreased when at the same time the sum <strong>of</strong> both propeller<br />
thrusts needs to match the external longitudinal <strong>for</strong>ce. At given thrust limit <strong>of</strong> the backing propeller, this allows <strong>for</strong> larger<br />
output <strong>of</strong> the propeller acting <strong>for</strong>ward. Further, the cross <strong>for</strong>ce <strong>of</strong> the rudder(s) supports the lateral thrusters. As the rudder<br />
itself decreases the net thrust <strong>of</strong> the propeller (besides the thrust deduction), the larger propeller thrust possible allows <strong>for</strong><br />
larger rudder cross <strong>for</strong>ces, additionally. The additional rudder <strong>for</strong>ces can easily be introduced into the equation system when<br />
the rudder angle during <strong>DP</strong> is set to a fixed value, because in this case, no additional variable is introduced into the problem<br />
due to the fact that at a given rudder angle, the rudder <strong>for</strong>ce only depends on the propeller variables. The required thrust <strong>of</strong><br />
the propellers and <strong>of</strong> the thrusters can be expressed by the individual rpm and pitch setting values, which easily allows to<br />
compute the required power demand as a non linear optimization problem with one free variable and three dependent ones.<br />
It also becomes obvious that the system layout as described above can easily be controlled <strong>for</strong> time dependent <strong>for</strong>ce fluctuations,<br />
because the system design is straight <strong>for</strong>ward:<br />
- In case pure additional cross <strong>for</strong>ce is required, the output <strong>of</strong> the lateral thrusters needs to be increased.<br />
- In case pure additional longitudinal <strong>for</strong>ce is required, the pitch (or rpm) setting <strong>of</strong> both propellers is increased simultaneously.<br />
- In case pure yawing moment is required, one propeller pitch or rpm setting is increased, the setting <strong>of</strong> the other<br />
propeller is decreased accordingly.<br />
It becomes clear that controllable pitch propellers are most favourable <strong>for</strong> all <strong>DP</strong>- units because the propeller pitch can<br />
much faster be altered (typical hydraulic servo units deliver approx. 0.6-0.8 deg blade angle/s) than its revolutions. This is<br />
due to the fact that whenever the rpm is altered, rotational masses need to be accelerated.<br />
Interaction Forces<br />
In the equations above, all <strong>for</strong>ces have been written independently from each other. This is in fact not correct as there are<br />
some interaction effects which have a significant influence on the problem. The strongest and there<strong>for</strong>e most important<br />
interactions are:<br />
- propeller- rudder interaction<br />
- propeller- hull interaction<br />
- hull- thruster interaction at <strong>for</strong>ward speed<br />
The propeller- rudder interaction can be <strong>for</strong>mulated as a one directional one, which means that the upstream induction <strong>of</strong><br />
velocities from the rudder to the propeller can be disregarded. This is true if the distance between rudder and propeller is<br />
larger than 0.25 times propeller diameter according to numerical investigations by Abels (2005). If the rudder design is<br />
fixed, then the rudder <strong>for</strong>ces at zero speed <strong>for</strong> a given rudder angle do solely depend on the propeller pitch setting, if a CPP<br />
is used and constant rpm mode is selected <strong>for</strong> the <strong>DP</strong>- mode. The rudder <strong>for</strong>ces required <strong>for</strong> <strong>DP</strong> purposes are computed by<br />
a direct panel method (Soeding (1998)), where the lift is modelled by wake panels at the trailing edge <strong>of</strong> the rudder blade,<br />
the top and the sole. The propeller slipstream is modelled by a lifting line method adapted <strong>for</strong> CPPs in <strong>of</strong>f design conditions<br />
(Haack (2006), Krüger (1998)). Prior to the computations, the circulation distribution <strong>of</strong> the propeller is computed <strong>for</strong><br />
the given rpm and pitch setting under bollard pull conditions. For this purpose we have selected a lifting line method as<br />
it is well known that this method is very robust with respect to the characteristics <strong>of</strong> the free vortex system. Further, the<br />
analytical <strong>for</strong>mulation <strong>of</strong> the free vortex induced velocities allows to compute the slipstream speeds at all positions without<br />
numerical problems. Fig. 3 shows the computation <strong>of</strong> the pressure distributon <strong>of</strong> a full spade rudder with Costa Bulb and<br />
twisted leading edge <strong>for</strong> bollard pull condition. The right picture shows the pressure side, the left picture the suction side.
If such kind <strong>of</strong> computations are per<strong>for</strong>med prior to the <strong>DP</strong>- calculation <strong>for</strong> a set <strong>of</strong> rudder angles and propeller rpm or pitch<br />
settings, the rudder <strong>for</strong>ces F R,X and F R,Y are available and can be interpolated during the <strong>DP</strong>- calculation as a function <strong>of</strong><br />
the propeller pitch and rpm settings. From such kind <strong>of</strong> computations we can further develop a set <strong>of</strong> design rules <strong>for</strong> spade<br />
rudders that increase the <strong>DP</strong>- capability at low investment costs. These rules are:<br />
- The rudder should be located as far as practically possible away from the propeller, because the slipstream speeds<br />
increase downstream. Due to the fact that the pitch <strong>of</strong> the free vortex system is quite small (the inflow speed is zero),<br />
we have computed as a rule <strong>of</strong> thumb 0.5D as efficient distance, measured approximately from the propeller generator<br />
line to the 1/4 rudder chord line.<br />
- An important design feature <strong>of</strong> the rudder is to increase the maximum lift at high rudder angles. This can be achieved<br />
by thick pr<strong>of</strong>iles having large nose radii and a twisted leading edge.<br />
- The steering gear should be able to reach larger rudder angles than 35 Degree, which is the standard.<br />
Figure 3: Rudder <strong>for</strong>ce computation in the propeller slipstream with our panel method <strong>for</strong> bollard pull condition,<br />
100% MCR engine output. Left: suction side, right: pressure side.<br />
A similar approach is used <strong>for</strong> the interaction <strong>for</strong>ces between hull and propellers. For the <strong>DP</strong>- Problem, only the portion<br />
T (1 − t) <strong>of</strong> the propeller thrust is available, where t is the well known thrust deduction fraction. For the <strong>DP</strong>- situation<br />
the problem occurs that t is hard to define, as the propeller operates under bollard pull condition and the resistance R T ,<br />
which is also required to define t is zero. Further, two different situations have to be distinguished: The <strong>for</strong>ward acting<br />
propeller produces a suction <strong>for</strong>ce on the hull, whereas the backing propeller creates an inflow to the hull. The latter causes<br />
problems with the Bernoulli equation to compute the pressure on the hull from the computed velocities, because it has to be<br />
distinguished between those parts <strong>of</strong> the hull which are affected by the propeller slipstream and those which are not. Haack<br />
(2006) has developed a panel method <strong>for</strong> the computation <strong>of</strong> the propeller hull interaction <strong>for</strong>ces based on the principles<br />
<strong>of</strong> our rudder panel method. The lift is generated by wake panels which are connected to the panel grid <strong>of</strong> the hull at the<br />
ship’s centre line. The propeller is again modelled by a lifting line approach, and the velocities are computed from Biot-<br />
Savart’s law by direct integration. Fig. 4 shows an example <strong>of</strong> the result <strong>of</strong> such kind <strong>of</strong> interaction <strong>for</strong>ce computations<br />
<strong>for</strong> a single screw vessel under bollard pull condition. The left picture shows the panel grid, the propeller and the wake<br />
panels, the right picture the computed pressure distribution along the hull <strong>for</strong> a backing propeller at 100% MCR. Such kind<br />
<strong>of</strong> computations need to be carried out <strong>for</strong> a set <strong>of</strong> propeller conditions, and the interaction <strong>for</strong>ces can be made dimensions<br />
less with the individual propeller thrust. If a CPP is used, the correct combination <strong>of</strong> pitch settings and rpm settings should<br />
be used, which can be taken from the individual combinator curve.<br />
The interaction between the lateral thrusters and the hull is much more difficult to compute. In contrast to the propeller<br />
hull interaction, which is dominated by potential flow effects, the thruster hull interaction is mainly due to viscous effects.<br />
There<strong>for</strong>e, viscous computations are required to solve this type <strong>of</strong> problem. Un<strong>for</strong>tunately, these computations are extremely<br />
time consuming which makes their application not possible during the initial design stage. There<strong>for</strong>e, we make use<br />
<strong>of</strong> measurements carried out by Brix (1993), but we have to admit that this is at present the weak point <strong>of</strong> our concept. Brix<br />
has investigated the nominal thrust and yaw moment loss <strong>of</strong> bow and stern thrusters acting in <strong>for</strong>ward or backward flow, see<br />
Fig. 5. For the <strong>DP</strong>- analysis, the flow component parallel to the ship’s centre line is computed, an then the reduction factors<br />
<strong>for</strong> the tunnel thrusters are interpolated from Fig. 5. It is <strong>of</strong> course obvious that these reduction factors must depend on the<br />
individual hull <strong>for</strong>m and the tunnel arrangement, and there<strong>for</strong>e the present approach can only be a rough guess. However,
Figure 4: Example <strong>of</strong> propeller hull interaction <strong>for</strong> a single screw vessel bollard pull condition, 100% MCR engine<br />
output, backing propeller. Left: Panel grid, right: Pressure distribution.<br />
better material is not available to us, and it is intended to per<strong>for</strong>m a set <strong>of</strong> RANS- computations <strong>for</strong> selected hull <strong>for</strong>ms and<br />
tunnel thruster arrangements in the future.<br />
Figure 5: Reduction factors <strong>for</strong> tunnel thrusters in longitudinal flow according to measurements by Brix (1993)<br />
.<br />
Now, all in<strong>for</strong>mation is available to per<strong>for</strong>m the <strong>DP</strong>- analysis if the external loads are known.<br />
EXTERNAL LOADS<br />
The external loads are wind loads, current loads and loads from the sea state. The determination <strong>of</strong> the external loads is not<br />
really a problem <strong>for</strong> conventional vessels, with respect to the accuracy that is required during the initial design phase <strong>for</strong> the<br />
system layout. Wind loads are available from wind tunnel tests, and current <strong>for</strong>ces - if there are any - can easily computed<br />
from our slender body manoeuvring model Soeding (1984) if the correct hydrodynamic cross <strong>for</strong>ce coefficients are used.<br />
But these are well known <strong>for</strong> typical hull <strong>for</strong>ms from the evaluation <strong>of</strong> full scale trials. And external loads from the sea<br />
state are rarely applied <strong>for</strong> conventional vessels, and if necessary, they can be computed from well known strip theories<br />
with sufficient accuracy.<br />
For the new ship type <strong>of</strong> wind farm installation vessels, practically all design loads are unknown, because no data <strong>of</strong><br />
reference vessels are available. Further, no computational methods <strong>for</strong> these <strong>for</strong>ces exist which can be applied during the<br />
initial design stage. This holds <strong>for</strong> example <strong>for</strong> wind loads, which are the dominating external loads with the jacking legs<br />
in raised position. Fig. 6 shows the results obtained <strong>for</strong> a wind park installation vessel according to different computational<br />
methods obtained from different parties. The left graph shows that nearly all methods including model tests come to similar<br />
results with respect to the cross <strong>for</strong>ce, but the yawing moment results differ significantly. This uncertainty is a major<br />
problem the designer has to face, and consequently, he has to rely on model tests. However, wind farm installation vessels<br />
have a windage area which is composed <strong>of</strong> many ragged slender cuboids, where the wind yawing moment contribution <strong>of</strong><br />
each cuboid is dominated by the product <strong>of</strong> the <strong>for</strong>ce and the lever arm with respect to AP (in fact, the yawing moment<br />
contribution <strong>of</strong> each cuboid around its own axis is in fact small). This might indicate that it might be possible to develop<br />
simple design <strong>for</strong>mulae <strong>for</strong> rough yawing moment estimations which come to more reliable results as shown above if the
contributions <strong>of</strong> each cuboid are superimposed. Nevertheless, at present, wind tunnel tests are definitively needed. With<br />
Figure 6: Wind <strong>for</strong>ces <strong>of</strong> a wind farm installation vessels according to different methods. The read and green line<br />
shows different computational methods, the markers model test results. Left: Cross Force, Right: Yawing Moment.<br />
It becomes clear that further development is required.<br />
respect to current or sea state <strong>for</strong>ces, computational methods exist which are based on the assumptions <strong>of</strong> slender ships, but<br />
in fact these vessels are not <strong>of</strong> slender type and it is questionable in how far the violation <strong>of</strong> this basic assumption has a<br />
quantitative effect on the results. However, the application <strong>of</strong> these findings to the Sietas Type 187 design shows what can<br />
be achieved under these limitations during practical design work.<br />
<strong>DP</strong>-SYSTEM DESIGN FOR SIETAS TYPE 187 WIND FARM INSTALLATION VESSEL<br />
The Sietas Type 187 is a jack-up-vessel <strong>for</strong> the transport and installation <strong>of</strong> <strong>of</strong>fshore wind turbines. The vessel is intended<br />
to operate in the North Sea. In order to reduce down times <strong>for</strong> the installation process, two goals have been set <strong>for</strong> the<br />
design <strong>of</strong> the vessel concerning the layout <strong>of</strong> the propulsion system:<br />
- Fast and economic transit from the shore basis to the wind farms, e.g. high service speed with minimised power<br />
consumption<br />
- Sufficient <strong>DP</strong>-capability to ensure the jacking process even in harsh weather conditions<br />
The basis <strong>for</strong> the design was a detailed survey <strong>of</strong> the wind farm installation vessels on the market and several design projects<br />
on the Sietas Yard since 2009. There<strong>for</strong>e, Sietas was able to lay out a the principal design <strong>of</strong> the Type 187 vessel in a very<br />
short time when the customer contacted the yard in November 2010. In order to compete with other projects especially in<br />
Middle and Far East, special consideration has been taken into the use <strong>of</strong> standard components <strong>for</strong> the propulsion system.<br />
Thus, it was possible to reduce the purchase costs <strong>for</strong> the yard and the maintenance costs <strong>for</strong> the customer at the same time.<br />
The purpose <strong>of</strong> the vessel is to provide transport space <strong>for</strong> the wind turbines and a stable plat<strong>for</strong>m <strong>for</strong> the crane operations<br />
during the installation. The first task requires a large deck space and the second a large breadth and short length as the<br />
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<br />
further increases the hydrodynamic problems <strong>for</strong> this type <strong>of</strong> ship. The main characteristics <strong>of</strong> the vessel are shown in Tab.<br />
1:<br />
Length over all 139.40m<br />
Breadth moulded 38.00m<br />
Draught (design) 5.70m<br />
Speed (at design draught)<br />
Deadweight (at design draught)<br />
Crane lift main host<br />
Maximum water depth <strong>for</strong> jacking<br />
12.00kn<br />
6500t<br />
900t at 30m outreach<br />
45m<br />
Table 1: Main characteristics <strong>of</strong> the Sietas Type 187 vessel
For the economic transit, significant ef<strong>for</strong>t was placed into the hull <strong>for</strong>m design and analysis. Extensive CFD analyses have<br />
been conducted both with potential and viscous flow methods without and with consideration <strong>of</strong> the free surface. The aim<br />
was not only to reduce the wave resistance but to increase propulsion efficiency by providing a good wake field and low<br />
thrust deduction. The final model tested showed that all <strong>of</strong> these goals could be achieved. Fig.7 shows two snapshots from<br />
the CFD optimisation:<br />
Figure 7: Optimisation <strong>of</strong> wave resistance and propeller inflow with CFD methods<br />
The propulsion arrangement consists <strong>of</strong> two controllable pitch propellers mounted on conventional shaft lines. In the<br />
slipstream <strong>of</strong> each propeller one high lift flap rudder is positioned. The propellers are used <strong>for</strong> the generation <strong>of</strong> axial <strong>for</strong>ces<br />
during <strong>DP</strong>-operations according to the above mentioned concept. For this purpose the reduction <strong>of</strong> the thrust deduction is<br />
<strong>of</strong> advantage as the bollard pull is increased both in push and in pull mode. The arrangement <strong>of</strong> the propellers can be seen<br />
in the side view in Fig. 8. The problem in the design phase was to determine the necessary design loads <strong>for</strong> the layout<br />
Figure 8: Side view <strong>of</strong> the Sietas Type 187 vessel<br />
<strong>of</strong> the <strong>DP</strong>-system. In the first step data <strong>of</strong> comparison from various vessels have been used. Here again Sietas was facing<br />
the problem that the public data basis <strong>for</strong> this new type <strong>of</strong> ship is very rare. In the later design process, model tests have<br />
been per<strong>for</strong>med in the wind tunnel to determine the wind and current loads (see Fig. 9). As the above mentioned concept<br />
was used during the design work, it was decided to per<strong>for</strong>m complementary model tests only to determine the single<br />
load components and not to per<strong>for</strong>m complete <strong>DP</strong> tests. This was also done in order to control the known and unknown<br />
deficiencies <strong>of</strong> model tests. For the wave <strong>for</strong>ces model tests have been conducted in the model basin in different sea states.<br />
The result <strong>of</strong> the model tests was that the assumption <strong>for</strong> the necessary power <strong>for</strong> the <strong>DP</strong> operation could be verified. The<br />
final propulsion system consists <strong>of</strong> 2 CPPs mounted on conventional shaft lines and two bow and two stern thrusters. All<br />
propulsion units are operated in constant speed mode which allows a fast alteration <strong>of</strong> the thrust over the whole power<br />
range. Each tunnel thruster has 2500kW input power whereas the main propulsors are driven by 5000kW each. In order<br />
to reduce purchase and maintenance costs eight electric drives <strong>of</strong> the same type are used, <strong>for</strong> the main propellers two <strong>of</strong><br />
them act via a collective gear on the shaft line. Due to the successful hull <strong>for</strong>m optimisation with reduced power demand<br />
<strong>for</strong> the transit was it possible to optimise the propellers <strong>for</strong> the bollard pull condition. Special consideration has been taken
Figure 9: Windtunnel models <strong>for</strong> determination <strong>of</strong> wind and current loads<br />
into the arrangement <strong>of</strong> the tunnel thrusters. The aim was to maximise the thruster per<strong>for</strong>mance during <strong>DP</strong> operation and<br />
to minimize the tunnel interference at design speed. A difficult task was to arrange the tunnels with 2.8m in diameter<br />
into the hull <strong>for</strong>m, which had to be designed accordingly. With the help <strong>of</strong> CFD analysis the task could be solved. The<br />
simplicity <strong>of</strong> the propulsion configuration <strong>for</strong> the <strong>DP</strong>-system has not only advantages <strong>for</strong> the <strong>DP</strong>-control system but also<br />
<strong>for</strong> the electric power supply. As every thruster per<strong>for</strong>ms only a single task it was possible to work with only two different<br />
power lines. Thus, the ef<strong>for</strong>t <strong>for</strong> the installation <strong>of</strong> the auxiliary systems <strong>for</strong> the power generation and distribution could be<br />
reduced compared to other jack-up designs <strong>for</strong> the <strong>of</strong>fshore wind market. The lack <strong>of</strong> data from other projects is a serious<br />
obstacle <strong>for</strong> the design development in this new market field. There<strong>for</strong>e further development at the tools <strong>for</strong> the ship design<br />
is required. But the above presented concept <strong>for</strong> the layout <strong>of</strong> propulsion systems <strong>for</strong> <strong>DP</strong>-vessels helped Sietas Shipyard<br />
to design a simple and smart solution <strong>for</strong> the transportation and installation <strong>of</strong> <strong>of</strong>fshore wind turbines. Hydrodynamic<br />
problems concerning the <strong>for</strong>bidden zones <strong>for</strong> azimuth thrusters <strong>for</strong> the <strong>DP</strong>-problem and the design speed could be avoided.<br />
Thus a more efficient product could be developed.<br />
CONCLUSIONS<br />
The <strong>DP</strong>- problem <strong>of</strong> a wind farm installation vessel is characterized by the fact that due to the many possible degrees <strong>of</strong><br />
freedom and the interactions <strong>of</strong> all components, it is very difficult to design such a system in a straight <strong>for</strong>ward way. Our<br />
analysis has shown that it is possible to simplify such systems when experiences from conventional twin screw ship design<br />
are used, which leads then to the well known crabbing problem <strong>of</strong> ferries with conventional CPP- propulsion. The use <strong>of</strong><br />
first principle based design methods can assist the design <strong>of</strong> such systems by computing interaction <strong>for</strong>ces. This could be<br />
demonstrated by the design <strong>of</strong> the <strong>DP</strong>- system <strong>of</strong> the SIETAS Type 187 wind farm installation vessel which is at present<br />
under construction. The main problem arises <strong>for</strong> the design load determination <strong>for</strong> these ships, because the data or methods<br />
from conventional ships can not be transferred to this type <strong>of</strong> ship. Further research is necessary if the layout <strong>of</strong> such<br />
systems shall in the future be possible without extensive model testing, which is important especially <strong>for</strong> the initial design<br />
phase.<br />
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direkter Berechnung. PhD Thesis, Hamburg University <strong>of</strong> Technology.<br />
BRIX, J. (ED.), 1993. Manoeuvring technical manual. Seehafen Verlag Hamburg, 24.<br />
HAACK, T., 2006. Simulation des Manövrierverhaltens von Schiffen unter besonderer Berücksichtigung des Verhaltens der<br />
Antriebsanlage. PhD Thesis, Hamburg University <strong>of</strong> Technology.<br />
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