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

Offshore Rig Ventilation design

In offshore rig ventilation design, attention is to be given to the vents inlet and outlet locations and airflow in order to minimize the possibility of cross contamination. Ventilation inlets are to be located in non-hazardous areas. Ventilation for hazardous areas is to be completely separate from that for non-hazardous areas.

Enclosed hazardous spaces are to be provided with ventilation so as to maintain them at a lower pressure than less hazardous zones. The arrangement of ventilation inlet and outlet openings in the space is to be such that the entire space is efficiently ventilated, giving special consideration to location of equipment which may release gas and to spaces where gas may accumulate. Enclosed hazardous spaces containing open active mud tanks are to be ventilated with high capacity mechanical venting systems capable of changing the air every two minutes.

The outlet air from Zone 1 and Zone 2 spaces is to be led in separate ducts to outdoor locations which in the absence of the considered outlet are of the same or lesser hazard than the ventilated space. The internal spaces of such ducts are the same Zone as the inlet space. Ventilation ducts for hazardous areas are to be at under pressure in relation to less hazardous areas and at overpressure in relation to more hazardous areas, when passing through such areas, and are to be rigidly constructed to avoid air leaks.

Ventilation inlets and outlets for non-hazardous spaces are to be located in non-hazardous areas. Where passing through hazardous areas, ducts are to have overpressure in relation to the hazardous area.



Courtesy of ISO standard

ISO_8861

Floaters model test - verify of design

Model testing of marine floaters
Small scale model testing is normally required in the process of designing new semi-sub, spars, ships and other marine structures. The ocean basin laboratory is made for testing of offshore structures and the scaling laws and the modelling principles are the same as those developed and used in traditional ship model testing in towing tanks. Model test could be done from large gravity-base concrete platforms to semisubmersibles, spar buoys and floating hoses, in model scales ranging from 1/40 to 1/150 in water depths from 30 m to 1200 m, and these contributed to a comprehensive and unique expertise and knowledge about design of small scale models, instrumentation, and data collection.
Focus of the model tests is to provide information about aspect of the behaviour that are difficult or impossible to obtain from theoretical analysis. These are normally related to extreme wave conditions, steep or breaking waves, interaction phenomena between waves and currents, or other nonlinear hydrodynamic interaction phenomena.

The size of the test basin allows modelling of a complete anchor system down to about 1000 m depth. For structures on greater depth, a technique of truncating the anchor system has been developed. In these cases, results from the model tests are used to provide input for a numerical simulation model that is subsequently used to obtain results for the full depth system. This approach is built on an accumulated experience of modelling truncated systems, and advanced parameter estimation techniques are used for extracting nonlinear hydrodynamic force coefficients from the model test results.


Floater Model Test

Basic Pump design

The inlet and outlet of a pump are two locations where pressure is of special interest. The difference in pressure head (the term pressure head refers to the energy associated with pressure divided by the weight of fluid displaced) between these two points is known as the Total Head. A system equation will be developed based on fundamental principles from which the Total Head of the pump can be calculated, as well as the pressure head anywhere within the system. These principles can be applied to very complex systems.

Centrifugal pumps are by far the most common type of pump used in industrial processes. This type of pump is the focus of the book. The challenge in pump sizing lies in determining the Total Head of the system, not the particular pump model, or the materials required for the application. The pump manufacturers are generally more than willing to help with specific recommendations. Information on models, materials, seals, etc., is available from pump manufacturer catalogs.

The Total Head of the pump provides the energy necessary to overcome the friction loss due to the movement of the fluid through pipes and equipment. It also provides the energy to compensate for the difference in height, velocity and pressure between the inlet and the outlet.

Pump Design

Your Business Fitness check

Knowledge of the financial "fitness" of your business is necessary for you to decide what you can and should do. Alot of financial health and performance of your business can be obtained by analysing your financial statements through financial ratios, terms like ROI, ROE, EBDITA,etc.. Comparing these ratios against past performance and similar businesses could give you some relevant indications of the strengths and weaknesses of your business.


SWOT analysis ( very common in Business studies and one of the framework for business analysis )
You should undertake a SWOT - strengths, weaknesses, opportunities and threats - analysis to determine the state of your business, its capacity to recover now the downturn is over, and what additional capacity may be needed.

When analysing opportunities and threats, you should:
· Research changes in customer taste
· Research how the tough times have affected your suppliers and competitors.
· Determine what changes may need to be made, including adding capacity to your business, shifting capacity or disposing of excess capacity.

Review your business plan and rewrite where appropriate
You should review your current business plan to ensure it reflects the capacity of your business and the the ability to grow that capacity. You should consider where you want to take your business and understand any uncertainties that remain. Important areas to focus the plan on include:

· How to expand capacity
· How much such expansion is likely to cost
· How the business is going to pay for such expansion.

Focus on innovation and efficiency :
When a business is in recovery mode, it is likely to become more innovative while keeping a strong focus on efficiency. Areas innovation should be limited, while staff should be empowered to search for new opportunities. To ensure limited resources are focused on the most promising innovations, pre-action reviews of projects should be undertaken.

Take advantage of opportunities :
Businesses that now find themselves in a strong financial position should consider opportunities to expand. Opportunities may be favourable, as some asset prices remain depressed.

Review and revise your marketing plan :
You should consider reviewing and revising your marketing plan. Such a plan should reflect the likelihood of limited resources continuing to be a constraint on marketing activity. It is, therefore, important that your marketing plan be focused on helping improve the cash position of your business, its profitability and promoting any new - or revived - products and services.

A marketing plan for a business seeking to recover should:
· Focus on sales that have high margins and bring in cash quickly
· Reward staff for sales of higher-margin products and when payment is received
· Avoid discounting, unless it can achieve a better gross profit margin through increased sales
· Measure the success of each promotional activity or campaign
· Encourage customers to pay at the point of purchase or pay early.

Remain focused on improving cash position
Focus on improving your cash position by improving working capital management - such as by reducing stock levels, increasing the percentage of cash sales and reducing the time you give debtors to pay. Such moves add to your cash reserves, which can be an important source finance.

Focus on improving profitability

Amid the recovery, discounts and other incentives introduced to improve your cash position during the tough times should be removed, so you can focus on improving profitability. Increased profitability builds retained earnings, creating another internal source of finance.

Funding recovery efforts

When considering borrowing from a bank, a business should:
· Determine what the funds are to be used for and the time they are required
· Be realistic about the amount of funds that can be afforded
· Determine the level of security that can be offered
· Start early.

Addressing weaknesses in your business
The actions a business may have taken to get through the slowdown can have negative consequences that may need to be addressed amid the recovery. Activities that may need to be undertaken to address the consequences of actions taken during tough times include:

· Increase stock
· Pay down high-priced external debt
· Broaden the focus of sales
· Revisit staffing arrangements
· Beware of a false recovery.