Sunday, September 25, 2011

Electrical Installation Concept on a MODU rig

One of the earliest tasks for the electrical engineer who is designing a power system is to estimate the normal operating rig power plant load. He is also interested in knowing how much additional margin he should include in the final design. There are no ‘hard and fast’ rules for estimating loads, and various basic questions need to be answered at the beginning of a project, for example,

• Is the rig power a new plant?
• How long will the offshore rig power system to exist e.g. 10, 20, 30 years?
• Is the old rig power being extended?
• Does the owner have a particular philosophy regarding the ‘sparing’ of equipment?
• Are there any operational or maintenance difficulties to be considered?
• Is the power factor important with regard to importing power from an external source?
• If a generator suddenly shuts down, will this cause a major interruption to the rig operation?
• Are there any problems with high fault levels?
The electrical engineer will need to roughly draft a key single-line diagram and a set of subsidiary single-line diagrams. The key single-line diagram should show the sources of power e.g. generators, utility intakes, the main switchboard and the interconnections to the subsidiary or secondary switchboards. It should also show important equipment such as power transformers, busbars, busbar section circuit breakers, incoming and interconnecting circuit breakers, large items of equipment such as high voltage induction motors, series reactors for fault current limitation, and connections to old or existing equipment if these are relevant and the main earthing arrangements. The key single-line diagram should show at least, the various voltage levels, system frequency, power or volt-ampere capacity of main items such as generators, motors and transformers, switchboard fault current levels, the vector group for each power transformer and the identification names and unique ‘tag’ numbers of the main equipment.
Vital loads are normally fed from a switchboard that has one or more dedicated generators and one or more incoming feeders from an upstream switchboard. The generators provide power during the emergency when the main source of power fails. Hence these generators are usually called ‘emergency’ generators and are driven by diesel engines. They are designed to automatically start, run-up and be closed onto the switchboard whenever a loss of voltage at the busbars of the switchboard is detected.

Vital AC loads, example below

Emergency lighting
Emergency generator auxiliaries
Helicopter pad lighting
Control room supplies
Vital LV pumps

Essential AC loads, example below
Diesel fuel transfer pumps 
Diesel fire pump auxiliaries
Main pump auxiliaries
Main compressor auxiliaries
Main generator auxiliaries
Electric fire pumps

Living quarters
Air compressor
General service water pumps
Fresh water pumps
Equipment room HVAC supplies
Life boat davits
Anti-condensation heaters in panels and switchboards
Security lighting supplies
Control room supplies
UPS supplies
Radio supplies
Computer supplies
Battery chargers for engine starting systems
Instrumentation supplies

Vital DC Loads, example below 
Public address system
Plant alarm systems
System shutdown system
Telemetry systems
Emergency radio supplies
Fire and gas detection system
Navigation aids

Hence each switchboard will usually have an amount of all three of these categories. Call these C for continuous duty, I for intermittent duty and S for the standby duty. Let the total amount of each at a particular switchboard j be Cjsum, Ijsum and Sjsum. Each of these totals will consist of the active power and the corresponding reactive power.

In order to estimate the total consumption for the particular switchboard it is necessary to assign a diversity factor to each total amount. Let these factors be Dcj for Csumj , Dij for Isumj and Dsj for Ssumj . Offshore rig companies that use this approach have different values for their diversity factors, largely based upon experience gained over many years of designing plants. Different types of plants
may warrant different diversity factors.

Electrical Installation Concept on MODU_ Choong
The info  presented in the slides are samples only and may not represent the total correctness of what is being installed subjected to the specifications of the contract scope.    
Comparison of US and IEC Nomenclature, eg. below
While there are many similarities and even direct interchangeabilities between U.S. and IEC recognized standards, specific applications must be considered.
Motors may be acceptable under all standards but not necessarily certified under all standards.
The IEC "flame-proof" motor is essentially the same as the U.S. "explosion-proof" motor. Each design withstands an internal explosion of a (specified) gas or vapor and prevents ignition of the specified gas or vapor that may surround the motor. However, construction standards are not identical. The U.S. standard is generally more stringent and acceptability can be based on approval of local authorities.
The U.S. totally enclosed "purged and pressurized," or "inert gas filled," motors are manufactured to similar standards as those of IEC pressurized motors. Each operates by first purging the motor enclosure of any flammable vapor and then preventing entry of the surrounding (potentially explosive
or corrosive) atmosphere into the motor enclosure by maintaining a positive gas pressure within the enclosure.
IEC Type 'e' (Increased Safety) motors are nonsparking motors with additional features that provide further protection against the possibilities of excess temperature and/or occurrence of arcs or sparks.
NEMA and IEEE standards and testing are more comprehensive than the IEC standards. In general, motors designed to NEMA/IEEE standards should be suitable for application under IEC standards from a rating, performance, and testing viewpoint.


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