5.2.1 Thruster Failure Modes
Some DP rules and guidelines have specific requirements that thrusters should fail safe and not develop uncontrolled thrust or change direction with thrust applied as the result of a single failure. Fail safe conditions are generally considered to be:
fail as set;
fail to zero thrust;
motor stop;
uncontrolled change in thrust direction may be accepted provided thrust goes to zero.
Fail as set may not be a good choice in some situations.
5.2.2 Thruster Types
There are several different types of thruster:
propeller with high lift rudder;
gill jet;
Voith Schneider;
tunnel thruster;
azimuth thruster (several forms).
Propellers can be of fixed pitch or variable (controllable) pitch.
Propeller: This is a component part of many thrusters as well as the most common form of propulsion. The propeller can be of fixed pitch or variable pitch. The pitch of a propeller is the theoretical distance moved through the water for one revolution, but due to slippage this is never achieved. One way to visualise this is to consider the axial distance moved when a wood screw is turned through one revolution in a piece of wood. Propeller efficiency is an important consideration as if the propeller is not correctly matched to the vessel then it will never perform to expectations. This is not only an important consideration for operational costs but also for the environmental emissions.
The high lift rudder can be connected to a DP control system and may be accepted as contributing to athwart-ship‟s thrust in certain conditions.
Gill jet: This system is not as common now as it was in the past. The thruster consists of an axial flow pump with vertical shaft axis delivering water downwards across a grill with angled plates (gills) in the bottom of the vessel to direct the thrust in the required direction.
The gill plate is circular and can be rotated through 360° to provide a multidirectional thrust capability.
Voith Schneider unit: This type of unit is capable of thrust delivery in any direction.
When used as a means of propulsion it does not need a rudder. The blades are attached at right angles to the rotor casing and rotate around a vertical axis. Each blade performs an oscillatory motion around its own axis. This is superimposed on the uniform rotary action of the entire unit. When the unit is fitted in the hull, only the blades protrude from the hull, as shown in Figure 89.
ROTOR
BLADES
Figure 89 – Voith Schneider propeller
Tunnel thruster: The tunnel thruster requires a tubular water passage running athwart-ships with the unit placed at its centre. This allows for the thrust to be directed in either port or starboard directions by reversing propeller pitch or direction of rotation. The limitations of this type of thruster are the length of tunnel in which it is situated and the distance it is located from the bow or stern. The longer the tunnel the greater the possibility of cavitation at high loads as the water flow may become restricted. The further from the bow or stern the less the turning moment created about the vessel‟s centre of rotation.
Tunnel thrusters located at the stern may also be susceptible to aeration of the water caused by the main drive propellers. There are no protrusions from the hull when tunnel thrusters are used.
Rim drive thrusters are a relatively recent innovation which essentially remove the need for a gearbox as the rotating element is the propeller which sits inside the stator of the motor.
This has the advantage of reducing the central body of the propeller, thus aiding the flow of water.
MOTOR
GEARBOX
TUNNEL PROPELLER
Figure 90 – Tunnel thruster
Azimuth thrusters: The azimuth thruster is mechanically similar to the tunnel thrusters, it has the advantage however of being able to direct the thrust in any direction as opposed to port and starboard only. It also operates in open water which has fewer problems in relation to the dynamics of flow as compared to the tunnel thruster.
MOTOR
GEARBOX
NOZZLE PROPELLER AZIMUTHING
GEAR
Figure 91 – Azimuthing thruster
Propulsion thruster: This is used in the same manner as the conventional shafted propeller system, with steering being achieved by rotating the thruster rather than operating a rudder. It also forms part of the station keeping system when operating in DP. On some types of vessel all the thrusters are of this type and are designed to be removed without the need to put the vessel in drydock. Class will apply elements of steering gear rules to designated propulsion thrusters.
Retractable azimuth thruster: This is similar to the propulsion thruster but it can be withdrawn into the hull of the vessel in order that it does not create extra drag while the vessel is in transit. For short transit distances, the thrusters may be left deployed and under power. For long transit distances, the increase in speed achieved by their use is not justified normally due to the high additional operating cost incurred.
Combined retractable thruster and tunnel thruster: Theoretically, this provides the advantages of both types but as with any multifunctional system it is a compromise which may be suitable for some situations but not for all. As the hull has less material the possibility of structural deformation as a result of the forces developed increases, therefore the hull has to be considerably strengthened to compensate for this loss of strength. The increase in weight created may be detrimental to the vessel or the thruster power may need to be decreased to accommodate it adequately. It also has to be drawn further into the vessel thus increasing the use of internal space.
Contra rotating azimuth thruster: Thrusters with contra-rotating propellers offer higher efficiencies of between 10-15% because the „aft‟ propeller regains some of the energy losses in the stream as well as rotational losses. Contra-rotating propeller thrusters also have low noise and vibration, and for the same power have propeller diameters 20% smaller than single screw units, giving a shallower draught. The unit requires a variable speed drive as there is no option for a CP propeller. There are also more complex sealing and thrust containment systems to consider.
Podded azimuth thruster: The podded drive provides the means to deliver greater power than previously possible with a geared azimuth thruster by eliminating the gear train in the thrusters, as shown in Figure 92. The shaft of the thruster is also the motor rotor with the pod casing being the stator. The seal arrangement becomes much more critical as there is now the possibility of electrical failures occurring as a result of seawater ingress. A bilge arrangement is provided to remove any leakage into the pod itself and assess the rate of leakage. Power ratings are of the order of 2MW to 25MW.
MOTOR
PROPELLER
SHAFT POD
BRAKE
SLIP RING
Figure 92 – Podded thruster
Azipull thrusters: This appears to be a back to front azimuth thruster but there are advantages in this design. A pulling propeller (CPP or FPP) is mounted ahead of the leg, which is a streamlined unit incorporating the gear house and a lower fin. The leg/housing/fin combination recovers swirl energy from the propeller slipstream which would normally be wasted, converting it into additional forward thrust. At the same time the underwater unit has more steering effect than a conventional azimuth thruster, improving the steerability of many hull forms.
Portable thruster: At least one manufacturer now offers hydraulically driven swing down thrusters which are largely independent in terms of power and control and can be added to a dumb barge to provide a DP capability with relatively little effort compared to a conventional unit.
5.2.3 Choice of Thruster
In any DP Class 2 or DP Class 3 new building or conversion project, the choice of which thrusters to use is often made at an early stage in the basic design process due to the long manufacturing lead times for such units. In addition to lead time, there are many other factors to consider when choosing a thruster for a particular application such as:
thruster type – tunnel or azimuthing;
thrust capability;
physical size – headroom under deck head etc.;
fixed or retractable;
variable speed, variable pitch or combinatory;
electrical drive, direct diesel drive;
reliability;
maintainability;
availability of service engineers.
One of the most important points to consider is:
How will the choice of thruster influence the development of the redundancy concept?
The choice of thruster type will significantly influence the redundancy concept and it is important to ensure that the redundancy concept incorporates the necessary features to support that particular choice.
5.2.4 Thruster Power
DP Class 2 and DP Class 3 vessels should have thrusters installed in number and power rating such that they can maintain position and heading following the worst case failure.
The surviving thrusters must be able to generate the required surge, sway and yaw forces.
The surviving generators must be capable of supplying the power required by the thrusters.
The vessel‟s post-failure DP capability is determined by these factors. Stern thrusters may be sized for transit speed and may operate at a fraction of their rating on DP.
5.2.5 Physical Constraints
Fixed pitch thrusters driven by variable speed AC drives are very popular but it is not always appreciated how much space and weight can be taken up by the converter and its related support equipment, such as drive cabinets, phase shifting transformers, de-ionized cooling water units, UPSs, pre-charge units, etc. There will also be a need to provide connections to FW cooling systems, HVAC and electrical supplies for all these units.
Although the variable pitch propeller may have some perceived disadvantages in terms of increased maintenance requirements and lower reliability due to its mechanical complexity, it can be packaged into a very compact arrangement at low and medium power levels and requires very little in the way of ancillary equipment and support services. This might be an important consideration in small and medium sized DP vessels.
5.2.6 Low Load Performance and Related Issues
The advantages of the fixed pitch, variable speed thruster are its mechanical simplicity and low power consumption at low propeller speed. Many DP vessels spend only a fraction of their working life operating in conditions close to their maximum post-failure capability and therefore thrust demand levels can be very low much of the time. The result is that the vessel has to operate with a few lightly loaded generators online which can be an uncomfortable condition both in terms of power plant stability and running conditions for diesel engines, which need to be well loaded to prevent carbon build-up reducing performance.
Vessels with variable pitch thrusters can depend on a guaranteed base load from each thruster of around 20% but this is not the case with variable speed drives. The solution for vessels employing variable speed drives is to use the thrusters in bias mode (fixed azimuth with opposing thrust vectors to create the desired resultant force) and apply significant amounts of force bias to increase the load on the generators by having the thrusters work against each other. This method works well and has advantages of improved station keeping stability in benign environmental conditions and reduced wear and tear on thruster steering gear. However, the need to manage this force bias correctly following a power plant failure was not fully understood in some early applications. In particular, it was not always properly controlled by the power management system, particularly if the PMS was a standalone unit not supplied by the DP control system provider. Two issues associated with early implementations were the need to shed the bias load before initiating overall thrust reduction as a means of blackout prevention and also the need to shed bias in such a way that the desired thrust vector is maintained, otherwise a drive off will result.
If the sum of the base load provided by the variable pitch thrusters and the hotel load is larger than the rating of the largest generator on the vessel, the power plant is relatively immune to failure to excess fuel generator faults. In this type of failure one faulty generator takes the entire load and others trip on reverse power leading to cascade failure and blackout. With fixed-pitch, variable speed propellers there may be times when the total system load falls within the rating of one generator leaving the system vulnerable to this type of failure.
5.2.7 Effect of Propeller Law and Power Factor on Post-Failure Capability
Because the relationship between propeller thrust and power is a not a straight line, as shown in Figure 88, a vessel which is holding position with all thrusters available may need significantly more power to hold station in the same conditions following a failure that leads to loss of some thrusters. Thruster tripping was (and still is) a popular last resort load-shedding feature on vessels with variable pitch thrusters. The poor low load power factor of large asynchronous motors means that more generators have to be online even at relatively low load, thus tripping thrusters reduces the total current demand even if the power demand increases. On vessels with fixed pitch thrusters, which tend to have a high power factor throughout their operating range, the advantage lies in keeping as many thrusters running as
possible following a failure, as it is more efficient to divide the available power between them than to have a few thrusters working hard. In reality the advantage may be quite small and each case needs to be considered on its merits, taking into account the power savings associated with thruster auxiliaries which can also be tripped.
5.2.8 Regenerated Power
Other issues associated with the use of large power electronic variable speed drives are the need to manage power regenerated by braking action. This is not usually an issue for DP but more for transit and vessel manoeuvring when much higher levels of power can be returned to the power plant. Some types of drives are not capable of regenerating power. With this type of drive, care must be taken when using speed control not to reduce the speed command set point at a rate faster than the propeller will naturally decelerate, otherwise the inverter part of the drive will attempt to return power. Because power cannot be transmitted beyond the drive to the power plant, the drive will only succeed in storing the energy within itself to the point where it will be tripped by its own over voltage protection.
Where drives are designed to return power to the power generation system or to dynamic breaking resisters, care must be taken to manage the return of this power in such a way that generators are not tripped on reverse power or braking resistors overloaded.
This problem is often eliminated when variable speed drives are designed for true torque control rather than speed control.
5.2.9 Effect of Harmonics
Power system harmonics have already been discussed in relation to phase shifting transformers in 4.1.10. All variable speed drives produce harmonics of one form or another.
Generally, the more sophisticated the drive, the smaller the levels of harmonic distortion produced. However, this is another area where the choice of thruster type can influence the redundancy concept. If it is necessary to add harmonic filters to deal with the effects of harmonics then it is necessary to consider the effect of these on DP redundancy.
It is notoriously difficult to specify a passive harmonic filter that will be effective in all power plant configurations. The additional capacitance these filters add to the system may also affect the power factor to the point where there are restrictions on the number of generators that can be run with certain combinations of filters. If harmonic filters are to be part of a DP redundancy concept then a very careful study of their failure effects needs to be carried out to ensure they do not create undesirable operational restrictions.
More recently there is a trend to use variable speed drives with so called active front ends.
These are generally advertised as a solution to the problems of harmonic distortion associated with older six- and twelve-pulse drives. These modern drives make use of individual filters at the thruster rather than attempting to correct the entire power system.
However, even with these modern devices there have been concerns about system resonance. It is also necessary to consider whether failure of the filter will lead to equipment malfunction elsewhere in the plant; studies backed up by suitable testing should be carried out to establish this.
5.2.10 Starting Transients and Inrush Current
Before solid state power electronic frequency converters of large power rating were available, DC drives were the most popular method of obtaining speed control of motor loads. Since this time, power electronics have advanced to the point where frequency converters of very high power rating are available. Two device types dominate the market, the gate turn off thyristor (GTO) and the insulated gate bipolar transistor (IGBT). The advantages of fixed pitch propellers using variable speed drives are higher efficiency under all operating conditions (typically in excess of 90% at full load), mechanical simplicity, improved control and negligible starting transients. The reduction of starting transients is a significant advantage as many motor failures can be linked to excessive heating and the large electromagnetic forces generated by starting currents. Many motor manufacturers impose limits on the number of starts per hour to control the thermal effects associated with direct
on line starting of large motors. It should be noted that some classification societies specify that there should be no restriction on thruster starting intervals.
When variable speed drives were first introduced, one of their advantages was the ability to soft start large motors. Prior to the application of power electronic technology, this was achieved through the use of reduced voltage starting techniques such as star-delta and Korndorffer starters, both of which have a significant degree of mechanical complexity.
It is fair to say that variable speed drives do remove the large starting current transient of thruster motors when the thruster is started. This effect is achieved by ramping the speed order up from zero to the desired speed. What is not so obvious is that there is still a large inrush current associated with connecting the drive‟s phase shifting or isolation transformer.
The problem has therefore been shifted from normal starting and stopping of the thruster to blackout recovery when it is arguably more important to be able to start thrusters with as few generators as possible. Because of the transformer inrush current it is still possible to
The problem has therefore been shifted from normal starting and stopping of the thruster to blackout recovery when it is arguably more important to be able to start thrusters with as few generators as possible. Because of the transformer inrush current it is still possible to