SOME CONSIDERATIONS ON THE ARRANGEMENT OF THE THRUSTERS SET FOR A DYNAMIC POSITIONING SYSTEM

Lázaro Moratelli Jr.Hélio Mitio Morishita

Dept. of Naval Arch. & Ocean Eng. Escola Politécnica

University of São Paulo

Abstract

Some aspects of design of a dynamic positioning system (DPS) for ships especially those related to offloading operation are considered. The design of a DPS involves a complex integration of a large number of different components in which performance and reliability are two important issues to be taken into account. In order to achieve the desired ship dynamics a careful study of the performance of the thrusters and their interaction with the hull need to be carried out. Therefore, among several aspects, the integrated analysis for the thruster and their power, taking into account both dynamic performance and reliability, is a major issue for the design of the DPS. However, that analysis is not straightforward because the overall performance of a thruster depends on the arrangement of the thrusters set.

The focus of this paper is to discuss some aspects for specification of the thruster for a given arrangements defined from reliability consideration. In order to do it, an overview of the hydrodynamic performance of different propeller and thrusters installed in vessels with DPS is made. Thruster generators devices as fixed and controllable pitch propellers, steerable and ducted propellers are considered. Interactions among thrusters and between them and the hull are commented.

1 – Introduction

An FPSO-based offshore oil exploiting system is very common in Brazil and the oil is transferred to the shore through a shuttle tanker (ST). During the offloading operation the

heading and position of the ST have to be within recommended limits in order to avoid an offloading interruption or an undesired incident. For that reason the ST has thrusters to compensate the forces and moments of the environment.

The DPS shuttle tankers demand special requirements in order to guarantee a safe offloading operation. Their design involves a complex integration of a large number of components, such as thrusters and their prime movers, electric devices, controllers, sensors, power generation set and all items which have any function in dynamic positioning (Morgan (1978) and Bray (1998)). Furthermore, reliability issues are important to guarantee the offloading operation in rough sea conditions. (Moratelli et al (2008)). However, the calculation of reliability is not simple since it is affected by the arrangement of the system. For instance, in the case of the thruster set, it is necessary to consider their position along the hull, since thruster-hull, thruster-thruster and thruster-environment interactions need to be taken into account. Those arrangements influence the dynamic performance of the vessel.

In this paper some requirements of the DPS for floating vessels are commented. In special operational modes and regulatory agencies requirements are pointed out. The operational modes have a considerable effect in the propulsion efficiency. Hence the efficiency of the thrusters is discussed, especially when they are installed at the shuttle tanker with DPS. Thruster devices as fixed and controllable pitch propellers, steerable and ducted propellers have their hydrodynamic performance and reliability issues analyzed.

Eventually, the choice of the thrusters set and its power specification is commented taking into account all aspects presented.

2 – Requirements of the DPS for ST

DPS requirements depend on the kind of the floating vessel and its operation. The classification and regulatory agencies specify requirements to DPS for ST (Lloyd´s (1999), DNV (2004), ABS (2007) and IMO (1994)). In addition, IMCA (1999) proposes guidelines based on those agencies in order to design and to operate the ST. The regulatory agencies define classes based on the level of the redundancy of the components that are related to the reliability of the system. In case of the thrusters, the rules indicate the number of the propellers in redundancy for each class. There are basically three classes: first class, there is no necessary thruster redundancy; second class, the redundancy is required; third class, more than redundancy, the thrusters may be isolated from each other in the burnt fire subdivision or flooded watertight compartment. Hence, the classes of the ship define some initial arrangement of the thrusters set.

In terms of operation, the ST has both DP and cruise speed modes (Dang et al, 2004). In DP mode, the longitudinal and transversal speeds are so low that they can be considered null. In this case, the thrusters operate in somewhat like the bollard pull point. Also, the thrusters used in DP mode work in dynamical operation. It means that the thrusters have to provide thrust for both directions to support the oscillatory forces and moments. In cruise speed mode, the thrusters which provide thrust to advance the ship have both operational modes, such as the main propeller. It implies special requirements for those propellers and propulsion efficiency for both modes needs be analyzed.

3 – Propulsion efficiency and coefficients

In general, the propeller is characterized by three coefficients: thrust coefficient at the equation (1), torque coefficient in equation (2) and efficiency at the equation (3), as showed by Lewis (1998).

(1)

(2)

(3)

(4)

(5)

Where,

- Thrust coefficient;- Thrust delivered by the

propeller in [N];- Water density;- Rotation of the propeller in

[rps];- Diameter of the propeller in

[m];- Torque coefficient;- Torque of the propeller in

[N.m];- Propeller efficiency in open

water;- Advance coefficient;- Operational speed of the ship

[m/s];- Speed at the propeller [m/s];

- Wake fraction coefficient.

However, as discussed in section 2, the DP mode is somewhat like bollard pull operation. When the speed is null, the advance coefficient is also null. The figure 1 shows the classic curves of the open propellers from B-Thrust Series, as presented by Dang et al (2004). The figure 1 shows the advance coefficient is null, the efficiency is null also, but in fact the thrust is not. Hence, there is some efficiency involved and is not based on the equation 3.

Figure 1: Open characteristics of a typical open propeller (B-Thrust Series with and

), extracted from Dang et al (2004).

For the DP operation mode (bollard pull), there are others coefficients that are used to indicate the efficiency. In general, these coefficients are called merit coefficients. These merits do not reflect the concept of efficiency, but they are comparative figure among the propellers at bollard pull operation. Different merit coefficients are found at Beveridge (1972), Dang et al (2004), Morgan (1978), Norrby et al (1980), Schneiders et al (1975), Taniguchi et al (1966). In this paper, it is used the merit coefficient defined by Taniguchi et al (1966), as showed by equation (6).

(6)

Where,

- Merit coefficient;

Other factors used to compare different thrusters are the slopes of the curves of the thrust and the torque coefficients, as showed by Dang et al (2004). Modifying a few the constants of the equations, they yield at the equation (7) and at the equation (8).

(7)

(8)

Where,

- Slope of the thrust coefficient;- Slope of the torque coefficient.

These slopes determine the sensibility of the thrust and torque to the flow, current or to the rotation of the propeller. When the curve or

are flat in some region of the curve, it means that the coefficient is insensible to variation of the flow, current or rotation of the propeller. This is a characteristic useful in order to choice thrusters which are less affected by flows or that have the torque insensible to perturbations.

The last coefficient presented in this paper is the thrust load coefficient. This coefficient is used to study the difference among the thrusters in open waters, but eliminating the influence of the rotation and diameter of the propeller, as showed the equation (9). The thrust load coefficient shows the thrust load that propeller is supporting for the advance coefficient point.

(9)

Where,

- Thrust load coefficient;

4 – Thrusters characteristics

In order to guide the DP designer to choose the thrusters for ST with DPS, there are commented some hydrodynamic and reliability characteristics about the most common thrusters installed in shuttle tankers.

4.1 – Fixed Picth propellers

Fixed Picth propellers are the most common thrusters for merchant ships. Lewis (1988) comments some aspects of these propellers and gives the B-Thrust Series. This systematic series of the propeller presents and coefficients and the efficiency of the propeller

for a lot of values of the number of the blades, ratio and ratio.

Where,

- Expanded area of the propeller;- Disk area of the propeller- Pitch of the propeller.

The reliability of this kind of propeller is higher than other thrusters used in DP mode. Modarres et al (1999) shows that the more components the system has, the less reliability it will have. Further, the conventional fixed propellers are the thruster which have the least number of components. Other advantage is that for merchant ships with high draft, it is possible to install propellers with high diameters. Propeller with high diameter is interesting because it increases the efficiency in open water. However, in DP mode, the merit coefficient of these propellers is lower than other kinds, as showed at figure 2.

Figure 2: Comparison of bollard pull efficiency among different type of propellers, extracted from Dang et al (2004).

Also, the thrusters used in DP mode have to be reversible easily to support the oscillatory forces and moments of the rough seas. In general, the conventional propellers are used with a diesel engine because the match between the propeller and engine rotations are easily achieved. But the diesel machine can not reverse the shaft speed easily. In spite of the fact of the fixed propellers have high efficiency in open water, they are not ideal for the DPS because their merit coefficient at low speed is also low and they are not able to reverse the rotation quickly.

4.2 – Controllable pitch propellers (CPP)

In order to use the diesel engine matched with high diameter propellers, it was installed the controllable pitch propellers. These propellers are able to turn their blades and hence to modify the pitch angle. The thrust is modified also when the blades turn. It is possible to create a negative thrust turning the blades until the pitch angle is negative. So the CPP are widely used in shuttle tanker with DPS because these propellers attend to both operational modes.

Other advantage is that in order to optimize the efficiency at low shaft speeds, the pitch angle can be changed. It requires a study to guarantee the match between the diesel machine and the CPP for the all pitch ratio.

However, some disadvantages are found in the CPP. The mechanisms of turning of the blades lay some space at the centre of the propeller and affect the efficiency of the propeller, as commented by Rupp (1948). These mechanisms also lay some space at the engine room and the arrangement needs be reviewed. Reliability issues and maintenance are prejudice also because of the increased of the component.

4.3 – Ducted thrusters

The ducted thrusters are the most common thrusters to generate thrust in the transversal direction. A lot of works have studied the design of these lateral thrusters, such as Beveridge (1972), English (1963), Morgan (1978), Norrby et al (1980), Ridley (1969), Schneiders et al (1975), Stuntz (1964) and Taniguchi (1966).

These works presents a lot of characteristics of the ducted thrusters that makes difference on their performances. Beveridge (1972) proposes a procedure to select the bow thruster taking account the specified turning rate of the ship and thrust operation at ahead speed for control ship in restricted waterways. The author discusses also a cavitation criterion for the thruster selection. Ridley (1969) studies the effect of the entrance configuration of the duct at the hull resistance. Stuntz et al (1964) studied the bow thruster-endings in order to maximize the thrust and to decrease the hull resistance.

English (1963) carried trials out with lateral units installed on models in order to evaluate the thrust loss at low speeds of the main stream. The author makes an extensive analysis of the hydrodynamic of the ducted propeller. Schneiders et al (1975) discusses about the thruster merits and the design points. He proposed a design procedure taking account the mechanical pitch restrictions, operation range and cavitation and noise criterions.

Taniguchi et al (1966) carried trials out to analyze different aspects of the lateral thrusters and there are commented here the major results of part of these trials. The propellers tested have their characteristics summarized at the table 1.

Table 1: Propellers characteristics, extracted from Taniguchi (1966).

D (mm)P/D 0,750d/D 0,300

Ae/Ad 0,5216 0,300 0,600 0,3375Blade contourBlade section Elliptic

Z 3Rake, Skew 0

Elliptic Kaplan-typeAerofoil (symmetrical)4

200,00ajustável : 0 ~1.3

0,4000,450

Where,

- Boss (hub) diameter;- Number of blades.

In that work, there are analyzed the following measurements of the size and shape of the duct:

a) Effect of bottom immersion;b) Effect of the duct length;c) Effect of the radius of the roundness of

the ducted-end corner (R);d) Effect of the tip clearance;e) Effect of the duct shape;f) Effect of the guide vanes;g) Effect of grids;h) Effect of duct-end-wall inclination.

In relation to the propellers, the analysis is made to evaluate:

a) Blade contour;b) Fixed pitch;c) Blade area ratio;d) Blade number;e) Boss ratio.

In relation at the bottom immersion, for length of the bottom twice the diameter of the propeller or more, there is no influence in the

merit coefficient, as showed at the figure 3. It happens because the flow created around the beginning and the end of the duct is no more modified for the bottom boundary .

Figure 3: Bottom immersion series, extracted from Taniguchi et al (1966).

In relation at the length of the duct, the better merit coefficient of the thrusters is around once or twice the diameter of the propellers. For long ducts, the frictional losses are significant, as showed at the figure 4, and the efficiency decreases.

Figure 4: Duct length series, extracted from Taniguchi et al (1966).

When the radius of the roundness of the ducted-end corner is between mm and

, there is no significant influence at the merit coefficient. However, the tip clearance until 6 mm has the more negative impact at the same merit. Values higher than 6 mm, the merit seems to stay constant, but with low values.

The important results about the wall inclination are presented. The increase of the inclination tends to reduce the merit coefficient. It is showed at the figure 5. In the case of the ship, thruster located close of the bulb or the stern, it suffers this effect of the inclination for both horizontal and vertical planes. So it is awaiting high influences at the merit coefficient.

Figure 5: Duct wall inclination, extracted from Taniguchi et al (1966).

In relation to the dimensions of the propellers, it is interesting to comment three of them. The increased of the area ratio has a negative effect at the merit coefficient, as showed at the figure 6.

Figure 6: Area ratio series, extracted from Taniguchi et al (1966).

The number of the blades propeller modify the merit coefficient as showed at the figure 7. For low pitch ratio, propellers with three blades has high merit coefficient. Propellers with four blades has high merit coefficient for high pitch ratio.

Figure 7: Comparison of blade numbers, extracted from Taniguchi et al (1966).

The eventual result commented here is the influence of the boss diameter. The more the

ratio, the less the merit coefficient, as showed at the figure 8. The boss propeller causes a kind of “shadow” at the flow generated by the propeller. This behavior increases the difference between the thrust of the propeller for the two directions of the flow in the duct.

Therefore, not only the dimension of the propeller, but also the shape of the duct and its position at the ship has an effect at the efficiency of the ducted thruster.

Figure 8: Boss ratio series, extracted from Taniguchi et al (1966).

4.4 – Steerable thrusters

In general, the use of the steerable thrusters has increased in DP operation because these thrusters are able to generate thrust for any direction desired. So, these propellers improve the maneuverability of the ships. In terms of the reliability, the thrusters give redundancy at both longitudinal and transversal direction. However, the mechanisms are more complex because of the gearing systems used to turn the thruster. So, there is necessary a good analysis of the maintenance to avoid the fail of the steerable components.

The main and auxiliary propulsions have used these steerable thrusters. At auxiliary propulsion, an advantage is the possibility of retracting the propeller inside of the hull, as

showed at the figure 9. At main propulsion, an advantage is the utilization of a strut that improves the hydrodynamic characteristics of the propellers, as showed at the Funeno (2004). It is usual to call these thrusters as pooded thrusters.

Figure 9: Hydraulically retractable Ro-thurster in the operational (left) and retracted (right) positions, extracted from Norrby et al (1980).

Certain kind of the thrusters has nozzles at the propellers. These nozzles have the function to increase the thrust or to increase the static pressure. Some works about these nozzles can be found at Dang et al (2004), Lewis (1988), Oosterveld (1973). Dang et al (2004) presents the improvement of the merit coefficient at the steerable thrusters with nozzles, as showed at the figure 2. However, as showed at the figure 10, the slope of the thrust coefficient curve is high for low values of the advance coefficient. It means that the thrust has sensibility in relation to the flow and the shaft speed. Other disadvantage is the drag of the nozzle in high advance coefficient.

However, at the same figure, the torque coefficient curve shows an interesting flat behavior for low values of the advance coefficient. Hence, the dynamic of the shaft and flow speed do not much influence in the power installed. It is a grate advantage in terms of the DP operation because it helps to specify and to manage the power of the motors.

The possibility of working under high load in cruise mode is other advantage of the steerable thrusters with nozzles. The figure 11

shows that for high values of the thrust load coefficient, the efficiency of the thruster can be significantly higher than other kind of the propellers.

However, there is necessary to study the conditions of the operation to choose the steerable thruster as well as possible. Some thrusters without nozzle has high thrust coefficient and, at the same time, high sensibility because of the slope of the thrust coefficient curves. Funeno (2004) presents the coefficients of the propeller alone and the steerable thrusters like a pooded propeller, as showed at the figure 12. The figure 13 presents the shape of the pooded studied. The same thrust characteristic has the counter-rotating steerable thruster discussed by Dang et al (2004). The figure 14 shows the coefficients of this kind of the propeller and the figure 15 shows a lateral view of the thrust.

So, the steerable thruster used for both bollard pull and operational speed conditions has some contradictory requirements. For cruise speed, the higher the thrust coefficient is, stronger is the propeller. But, the slope of the thrust can increase and the thruster begins to have some influence of the shaft and flow speeds. A possible solution is the installation of a nozzle. However, the nozzle causes drag forces for operational speed condition.

Figure 10: Open water characteristics of a typical nozzle propeller 19ª nozzle (with Ka 4-70,

), extracted from Dang et al (2004).

Figure 11: Comparison of the open water efficiency, extracted from Dang et al (2004).

Figure 12: Open-water characteristics of propeller alone and pooded propulsion, extracted from Funeno (2004).

Figure 13: Grids on surface of pod and strut with full geometry of a propeller, extracted from Funeno (2004).

Figure 14: Open water characteristics of typical counter-rotating propellers, extracted from Dang et al (2004).

Figure 15: Thruster with Z-drive and CRP installation, extracted from Dang et al (2004).

Other factors can affect the choice of the propeller. Dang et al (2004) comments that the configuration of the motor (“L” or “Z” types) has influence for the choice of the motor and the space laid. Moreover, the author discusses that the configuration of the propeller (“push” or “pull arrangement) has influence at the efficiency of the propeller.

Therefore, the choice of the steerable thruster needs to be based on the characteristics of the operation and the operational modes of the shuttle tanker as well.

5 – Hydrodynamic Interactions

The flotation systems with DPS have some interactions which should be previously estimated in order to specify the thruster. It discusses in this section the interactions that a shuttle tanker has in offloading condition. Basically three interactions are commented: thruster-thruster, thruster-hull and thruster-environment.

However, there are two others interactions that are important. The first one is the “shadow” effect. When the shuttle tanker is operating in front of a platform like a FPSO, there is no incident current at the ST because the presence of the FPSO causes a “shadow” of current, as studied by Fucato et al (2004). Other interaction is operation close to a wall. Taniguchi et al (1966) studies this operation as well.

5.1 – Thruster-thruster interaction

Interactions can occur among the thrusters that are close each other because their flows can cross. An example of this interaction is studied by Ekstrom et al (2002). This work discusses the interaction between two steerable thrusters as function of the direction of the thrusters. The figure 16 shows the interaction for the presented configuration for some values of the current inflow. It is possible to see that current can improve or reduce the thrust. The author also comments the same results for others thruster angles. Dang et al (2004) also presents some similar results.

Figure 16: Left thruster force, left thruster at 135° to current inflow right thruster rotated through 360°, extracted from Ekstrom et al (2002).

Moreover, the interaction can occur in the following situations:

a) Two or more duct thruster which are close each other;

b) One steerable and one or more duct thrusters which are at the stern or at the bow;

c) One steerable thruster at the stern and the main propeller (fixed pitch, CPP or steerable too).

In order to avoid some interactions, it is necessary to locate the thrusters as far as possible from each other or to study their interactions to specify thrust requirements for the ship design.

5.2 – Thruster-Hull interaction

Van Dijk et al (2004) classify the thruster-hull interactions in following three categories of losses:

a) Frictional losses;b) Induced pressures losses;c) Forces due to blockage.

In addition to these losses, in section 4.3 it discusses that the location of the thrust at the ship also influence its efficiency. Other effect commented by the Van Dijk et al (2004) is the Coanda effect. This effect is the tendency of the jet to follow the shape of the hull. It can be reduce the thrust in effective direction.

A common interaction with steerable thrusters is the frictional losses. Depending on the direction of the thruster, it can face more hull area and so needs to accelerate the water from a long hull area. So, it can decrease the thrust at some direction. Norrby et al (1980) shows this effect in a model test of a Ro-

thruster, as showed at the figure 17. The author shows the percentage thrust reduction as function of the thruster angle.

Figure 17: Model results of a Ro-thruster/hull interaction study, extracted from Norrby et al (1980).

Therefore, the effective thrust at the ship has some influence of the interactions with the hull that may be taking account in the ship design in order to avoid some effects at the dynamic performance.

5.3 – Thruster-environment

Van Dijk et al (2004) discuss the thruster-current and the thruster-wave interactions. The thruster-current interaction can be from two factors: direct effect due to the inflow and the modification of the thruster-hull interaction due to changes of the pressure fields.

At the figure 18, Van Dijk et al (2004) presents the thrust coefficient reduction as function of the advance coefficient. Here, the speed is the relative speed between the current and the thruster. It is possible to notice that depending on the thruster angle, the current can be beneficial at the total thrust. However, at some angles, the thrust coefficient reduces for high values of the advance coefficient.

The thruster-waves interactions have two kind of the degradations: first one is caused by the oscillating flow at the ship due to incident waves; the second one is caused by the occurrence of the ventilation at the duct thrusters.

Figure 18: Efficiency curves for a thruster, extracted from Van Dijk et al (2004).

For submerged thrusters, such as steerable ones, there are no significant losses for wave’s effect. However, for duct thrusters in ships, it can be serious. The major degradation can occur when the motion of the ship is high and the duct has contact with air (ventilation). Van Dijk et al (2004) presents a study of this ventilation at the duct thrusters. At the figure 19, the thrusts of the duct propeller without and with ventilation are compared. It is possible to see that the thrust is strongly affected.

Figure 19: Simulated bow thruster performance in a time domain DP simulation, the same phase of positioning is shown without and with ventilation due to waves, extracted from Van Dijk et al (2004).

Therefore, the thruster-environment interact-tions need to be studied in order to quantify the level of the thrust losses due to current and waves. In practice, it is not desired that these interactions modify the dynamic performance as well.

6 – Considerations about the design of the thruster set

For guaranteeing that the ST maintains the safe position from the reference, it is necessary to study the available forces of the ship in front of the environment conditions. IMCA (2000) specifies a procedure to make the capability plots in order to evaluate these forces. For that, it is necessary to specify correctly the thrusters and their power. The IMCA´s guideline also specifies the ratio between power and thrust for some kind of the thrusters. IMCA´s guideline shows the environment conditions that have to be used in this capability plots. The dynamic thrust factor is added for taking account the dynamic effects in DP mode. However, it is carefulness to study the local rough sea and the particular requirements from the local operation mode in order to draw the plots. Factors taking account the interactions are also important to add these effects in those plots.

After studing the environment conditions and their forces, it is required the thrusters set specification. Harrington (1992) presents some kind of main engine for ships, including some comments about thruster devices to DPS. The characteristics of the main and auxiliary propulsion are very important to determine the power management in both conditions of the operation for ST with DPS.

For optimizing the plant, some new ships have the diesel-electric propulsion. This kind of propulsion consists of the power generators which attend the entire demand of energy on board. The main propeller receives torque from a prime electric motor as well as the prime motor of others thrusters. Power electronics are development to provide energy for electric motors in AC or DC. For use the alternated current, the reversion of the motor is made by frequency inverters. The direct current motors has power supplied by rectifiers. Others power demands at the ship are supplied by transformers in desired voltage. Various factors can be influence the choice of the prime electric motor, such as power, cost, consumption, maintenance, size. However, in terms of DP design, dynamic performance and the reliability issues are the major factors.

In relation on reliability issues, the ABS (2007), DNV (2004) and Lloyd´s (1999) require an extensive utilization of the FMEA (Failure Modes and Effect Analysis) at the entire phases of the design. However, besides the

FMEA, it is recommended to analyze the subsystem and their components in terms of their individual reliability. Maintenance is a factor that is important to avoid the fails of the subsystems. The choice of the prime movers, thrusters and location of the thrusters are important to reliability characteristics also.

Therefore, both reliability issues and the dynamic performance are important aspects to study in order to maintain the shuttle tanker operating in safe mode.

7 – Conclusions

The demand for new DPS vessels in Brazil is increasing and this work intends to help DP designers to take in consideration some hydrodynamic aspects of different thrusters during design of a DPS especially for shuttle tanker. The design of a DPS involves a complex integration of a large number of different components, such as thrusters and their prime motors, electric devices, controllers, sensors, power generation devices and all items which have any function in dynamic positioning. Among several aspects, the integrated analysis for the thruster and their power, taking into account both dynamic performance and reliability, is a major issue for the design of the shuttle tanker with DPS. However, that analysis is not straightforward because the overall performance of a thruster depends on the arrangement of the thrusters set.

For studying the choice of the arrangement of the thrusters set, an overview of the hydrodynamic characteristics of the most common thrusters is made. The conventional propellers are not ideal for the DPS because its efficiency at low speed is also low and they are not able to reverse the rotation easily because of the diesel engine which is frequently installed. In order to use the diesel engine matched with the high diameter propellers, it is installed the controllable pitch propellers.

In spite of the large diameter of the propeller, the pitch angle can be turned in order to optimize the efficiency at low shaft speeds. But this requires a study to guarantee the match between the diesel machine and the CPP for the all pitch ratio. At the duct thrusters, not only the dimension of the propeller, but also the shape of the duct and its position at the ship has an effect at the efficiency of the ducted thruster. In relation to the steerable thrusters,

the choice of the kind of the thruster needs to be based on the characteristics of the operation points of the shuttle tanker as well.

Moreover, the interactions among thrusters and between them and the hull and the environment have also some effect on the dynamic performance of the ship. It is important to study all interactions in order to attend the dynamic performance and reliability issues. In order to avoid some interactions, it is necessary to locate the thrusters as far as possible from each other or to study their interactions to specify thrust requirements for the ship design. It is also carefulness to study the stronger local environment and the particular requirements from the local operation in order to add some factors containing these studies.

Therefore, both reliability issues and the dynamic performance are important to maintain the shuttle tanker operating in safe mode and both characterisctis affect the choice of the arrangement of the thrusters set.

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