Emergency Electrical Power

The emergency load schedule (including critical loads such as thrusters), will determine the size of the emergency generation capacity. The load schedule for an FPSO will be considerably larger and different from a tanker. The designer, who may be more familiar with ships than with offshore production platforms, may require additional guidance from the owner and the production facilities designer to ensure full emergency power coverage.

On loss of main power, it is essential that the production facilities can be shut down safely and that the rest of the FPSO, with all its essential and emergency control and management systems and life support systems (including TR) can continue to function normally and safely.

In the event of a major emergency, the battery back up systems must ensure that minimum control room functions are maintained along with emergency lighting, radio and communications facilities, navigation aids etc. Battery systems design must take account of access for inspection and maintenance, heat removal and ventilation of the battery room.

Consideration has to be given to starting up the entire FPSO from what is commonly known as “black start conditions”. The preparation of a black start philosophy at an early stage in the electrical design will highlight the basic power supply needs to establish and maintain life support and emergency systems.

Larger systems requiring power from emergency generators will include auxiliary seawater, instrument air, potable water, HVAC, diesel fuel etc in sufficient quantities to get at least one main generator on line. It should also be possible to operate at least one pedestal crane, especially if it is an electro-hydraulic unit, to permit essential fuel and supplies to be transferred from supply boats. As design progresses, it is prudent to check that the actual loads on the emergency and critical load schedule do not exceed the capacity of the emergency generator. Eventually it may be necessary to carry out a full connected load test of the generators to ensure that they are capable of supplying the total connected power for the required period of time.

The size of the emergency generator will in general be dictated by the size of the FPSO, ranging from 500 to 800 KW for smaller production units to over 1MW for an FPSO with a throughput of over 100,00bpd. There are well-established guidelines for the design of the emergency generator and its support systems including fuel supply, starting facilities, inlet air and exhausts, the enclosure and its protection and ventilation. (For example, reference can be made to Lloyds Register, (ref 22), Part 6, chapter 2, section 3). There are also guidelines for the design and protection of the emergency switch room and the distribution system.

It is usual to have two 100% rated emergency generators. It is possible to justify the need for a single emergency generator on the basis of the risk of losing main power and emergency power at the same time and for an extended period being low. Making the case for a single or a second emergency generator will also have to take into account the potential loss of production should the FPSO be unable to return to normal operation within a realistic time interval.

The location of the emergency generator(s) and the emergency switch rooms should take account of their accessibility relative to the main control room or switch rooms should manual intervention be needed quickly in an emergency situation.

3.7.11 HVAC

As stated earlier in Part 1, the development of an HVAC Design Philosophy or Basis of Design document will highlight the fundamental requirements for an offshore oil and gas installation operating as an FPSO. Furthermore, the document should clarify the differences between an FPSO and a tanker to ensure that designers, due to a lack of familiarity or understanding, do not initially design and specify marine type systems. Although the Classification Societies now generally recognise that floating production installations have particular requirements, their guidance on HVAC is still contained in their rules for the classification of ships or mobile offshore drilling units. An alternative source of guidance can be found in the Department of Energy Fourth Edition Guidance Notes (ref 4), section 47.

Although a large proportion of the HVAC design is for the Living Quarters, the overall design should reflect the requirements of the FPSO as a unit. Should a specialist HVAC design contractor be engaged, the contractor should demonstrate a proven track record in the design of offshore systems. Experience of hotel systems alone is not sufficient. The main outputs from the HVAC systems design will be the flow diagrams, the schematics and the D&ID’s (ducting and instrumentation diagrams). Good quality documents will greatly assist those commissioning and operating the systems and can prove beneficial if and when modifications are needed at a later date.

In the specification and selection of HVAC equipment and in the development of layouts, three main requirements have to be addressed:

Suitability of equipment for offshore use (robustness, materials of construction)

The range of equipment and components is considerable e.g., fans (supply, extract and recirculation), air handling units, dampers (fire and gas, pressure control, shut-off, volume control), silencers, heater units, louvres, mixing boxes, humidifiers, ductwork, bellows units etc etc. The specifications have to ensure that all items (not just the high value items) are suitable for duty offshore with its high saline atmosphere, high relative humidity levels and the risk of condensation. Incorrect materials can lead to premature corrosion and the inconvenience and cost of replacement. Special attention should be given to battery rooms and laboratories

Accessibility for operation and testing

Once the system(s) have been commissioned and satisfactorily balanced, there will still be a need to perform regular tests and checks on certain components, principally those connected with the Fire and Gas system. These include the fire and gas dampers and the smoke and gas detectors in the inlet and exhaust ducts. These must be readily accessible as they are essential parts of the safety system.

Maintainability

During layout development of the FPSO, HVAC maintenance needs can often be overlooked with equipment and ducting being shoehorned in after the main piping runs and cable rack routings have been determined.

Where there are large spaces to be ventilated, the size of fans, filters, and air handling units will be commensurate. Adequate provision should be made for access to these items and for removal of components. In-duct fans units are a good example of items where handling difficulties can arise due to inadequate provision.