Sunday, September 26, 2010

Rig electric propulsion system

Electrical installations are present in any ship, from powering of communication and navigation equipment, alarm and monitoring system, running of motors for pumps, fans or winches, to high power installation for electric propulsion.

Electric propulsion is an emerging area where various competence areas meet. Successful solutions for vessels with electric propulsion are found in environments where naval architects, hydrodynamic and propulsion engineers, and electrical engineering expertise cooperate under constructional, operational, and economical considerations. Optimized design and compromises can only be achieved with a common concept language and mutual understanding of the different subjects.

Electric propulsion with gas turbine or diesel engine driven power generation is used in hundreds of ships of various types and in a large variety of configurations. Installed electric propulsion power in merchant marine vessels was in 2002 in the range of 6-7 GW (Gigawatt), in addition to a substantial installation in both submarine and surface war ship applications.

By introduction of azimuthing thrusters and podded thrust units, propulsion configurations for transit, maneuvering and station keeping have in several types of vessels merged in order to utilize installed thrust units optimally for transit, maneuvering and dynamically positioning (dynamic positioning - DP).

At present, electric propulsion is applied mainly in following type of ships: Cruise vessels, ferries, DP drilling vessels, thruster assisted moored floating production facilities, shuttle tankers, cable layers, pipe layers, icebreakers and other ice going vessels, supply vessels, and war ships. There is also a significant on-going research and evaluation of using electric propulsion in new vessel designs for existing and new application areas.

The following characteristics summarize the main advantages of electric propulsion in these types of vessels:

- Improved life cycle cost by reduced fuel consumption and maintenance, especially where there is a large variation in load demand. E.g. for many DP vessels a typically operational profile is equally divided between transit and station keeping/maneuvering operations.
- Reduced vulnerability to single failure in the system and possibility to optimize loading of prime movers diesel engine or gas turbine).
- Light high/medium speed diesel engines.
- Less space consuming and more flexible utilization of the on-board space increase the payload of the vessel
- Flexibility in location of thruster devices because the thruster is supplied with electric power through cables, and can be located very independent on the location of the prime mover.
- Improved maneuverability by utilizing azimuthing thrusters or podded propulsion.
- Less propulsion noise and vibrations since rotating shaft lines are shorter, prime movers are running on fixed
speed, and using pulling type propellers gives less cavitation due to more uniform water flow.

These advantages should be weighted up against the present penalties, such as:
- Increased investment costs. However, this is continuously subject for revisions, as the cost tends to decrease with increasing number of units manufactured.
- Additional components (electrical equipment – generators, transformers, drives and motors/machines)between prime mover and propeller increase the transmission losses at full load.
- For newcomers a higher number and new type of equipment requires different operation, manning, and
maintenance strategy.

High availability of power, propulsion and thruster installations, as well as safety and automation systems, are the key factors in obtaining maximum operation time for the vessel. The safety and automation system required to monitor, protect, and control the power plant, propulsion and thruster system, becomes of increasing importance for a reliable and optimum use of the installation.

The advantages of diesel-electric propulsion can be summarized as follows:
- Lower fuel consumption and emissions due to the possibility to optimize the loading of diesel engines / gensets. The gensets in operation can run on high loads with high engine efficiency.
This applies especially to vessels which have a large variation in power demand, for example for an offshore supply vessel, which divides its time between transit and station-keeping (DP) operation.
- Better hydrodynamic efficiency of the propeller. Usually Diesel-electric propulsion plants operate a FP-propeller via a variable speed drive. As the propeller operates always on design pitch, in low speed sailing its efficiency is increased when running at lower revolution compared to a constant speed driven CP-propeller. This also contributes to a lower fuel consumption and less emission for a Diesel-electric propulsion plant.
- High reliability, due to multiple engine redundancy. Even if an engine / genset malfunctions, there will be sufficient power to operate the vessel safely. Reduced vulnerability to single point of failure providing the basis to fulfill high redundancy requirements.
- Reduced life cycle cost, resulting from lower operational and maintenance costs.
- Improved manoeuvrabilty and station-keeping ability, by deploying special propulsors such as azimuth thrusters or pods. Precise control of the electrical propulsion motors controlled by frequency converters enables accurate positioning accuracies.
- Increased payload, as diesel-electric propulsion plants take less space compared to a diesel mechanical
plant. Especially engine rooms can be designed shorter.
- More flexibility in location of diesel engine / gensets and propulsors. The propulsors are supplied with electric power through cables. They do not need to be adjacent to the diesel engines / gensets.
- Lower propulsion noise and reduced vibrations. For example a slow speed E-motor allows to avoid the gearbox and propulsors like pods keep most of the structure bore noise outside of the hull.
- Efficient performance and high motor torques, as the electrical system can provide maximum torque also at low speeds, which gives advantages for example in icy conditions.
Electric Propulsion Basics

The propulsion system of a DP vessel is sized to provide stationkeeping forces for the most severe operating scenario specified by the owner or operator of the vessel. During most of its operating time, the DP vessel operates in environmental conditions which are far less severe than the ones used as the design basis for the power and propulsion system. As a result, during the majority of its operating time, the DP vessel operates the propulsion system at partial load. The power system is typically equipped with a multiple installation of Diesel-generator sets; the number of generators on-line is selected (mostly automatically by the power management system) according to the power demand of the vessel. As a result, the engine generators operate at relatively high loads, at conditions of optimum fuel efficiency.

The propulsion system consists of a multiple installation of thrusters. During most of its operational time, only a part of the installed thrust capacity is required. The operator has two basic choices:

· Operating all thrusters at the required low load or
· Operating a few thrusters at high load

The environmental elements, such as wind, current, and wave drifts generate forces and moments on the vessel. The thrusters have to generate counter forces and moments to create a force and moment equilibrium. The thrust allocation logic of the DP controller calculates the magnitude and direction required for each thruster to establish a counter. The closed-loop DP control system for an FP propeller thruster faces a problem in that the ideal control would be to control the force generated by each thruster and to use a measurement of the force as the feedback. It is, however, not feasible to directly command force (thrust) or to measure thrust and use it as a feedback signal.

The thruster with CP propeller operating at constant speed is limited to the pitch angle as the control value; the drive motor power is used in addition in many cases. In the case of FP propeller thruster driven by an VSD, the only control value is the thruster rpm. Many DP systems use the rpm also as the feedback value. Optional values which can be used as feedback signal are motor torque (current, Amps) or motor power (kW).

Class 0 was a sort of “never mind” operations where nothing could go seriously wrong. Any vessel that could operate in DP mode at all, could not avoid meeting Class 3 requirements. That class disappeared with the introduction of the IMO Guidelines.

Class 1 operations are those where loss of position may cause some pollution and minor economical damage, but excluding severe harm to people.

Class 2 operation are those where loss of position may cause severe pollution, large economical damage, and accidents to people.

Class 3 operations are those where major damages may occur, severe pollution, and fatal accidents.

With the IMO Guidelines, the term equipment class was introduced, which is an inversion of the concept. A vessel is now equipped according to a chosen set of class requirements, and that will allow the vessel to undertake the corresponding consequence class of operations.

The assessment of the hazard level of the operation still has to be done by involved parties, which include owner, operator, and national authorities.

The major difference between Class 1 and Class 2 is that Class 1 vessels are allowed to fail completely, i.e. lose both position and heading. The Class 2 vessel is not expected to do that. The maximum failure that can be defined is assumed to be feasible, it will happen, and it is not to be ignored with reference to optimistic statistics. Once this failure occurs, the vessel shall still be able to maintain both position and heading, at least initially. For how long this ability shall remain, is governed by the time required to secure the operation.

For Class 2 all failures of a technical nature are relevant, but certain types of equipment of a passive nature are trusted to stay out of harms way. For Class 3 vessels, all of the Class 2 requirements are adopted, and then is added failures that are brought about by fire and flooding events. This latter requirement results in need of physical separations that are not necessary for Class 2.

For both Class 2 and 3, single acts of maloperation are defined as relevant failure modes.

Hence, the difference between Class 2 and 3 is the failure mode definition, which briefly said consists of the need for physical separation of redundant components/systems in case of Class 3.
The typical redundant DP vessel, e.g. most drill ships, are based on two almost identical half systems for power generation and thruster configuration, which are controlled by a dual control system. When done properly, each half system shall carry on after full failure of the other half. Both halves will normally continue undisturbed after failure of one of the control systems.

This solution is acceptable for Class 2, and the vessel will be quantified by the smaller of the half systems, if they are unequal. Strictly speaking, systems are never equal. The thruster configuration will consist of units that will have variable efficiency, depending on external circumstances. In simple terms, if there are two equal bow thrusters, the forward one will be most valuable in situations where yawing moment is critical. Therefore, the most valuable system is not a static choice. This selection is taken care of by the “consequence analysis”, which will be explained later, if time permits.

For such a vessel to comply with Class 3, there must be physical separation of the two half systems, both with regard to fire and flooding hazards. There is common agreement that this would require no less than two engine rooms, with fire separation by A-60 protection. Less obvious is that there should also be watertight separation of engine rooms below waterline, and thruster rooms. The excuse for not having that is often reference to bottom and side tanks that will protect against collision damage. That is not adequate, there are ample cases of flooding caused by inboard water sources.

DP Class Type

No comments: