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A North Sea platform power system
Electric Power Systems Research, 17 (1989) 159 - 169
159
A North Sea Platform Power System* G. L. MICHAEL
Marathon Oil Company, P.O. B o x 3128, Houston, T X 77253 (U.S.A.) (Received May 30, 1989)
ABSTRACT
Over the last twenty-five years, North Sea platform electrical generation designs have grown from relatively small, simple systems to large sophisticated fully integrated units. Brae 'B', installed in the North Sea during the summer of 1987, is one o f the largest platforms in the world. The electrical system design, with 72 MW o f main power generation, includes a complete diversity o f electrical engineering applications. Following an introductory description of the platform facilities, this paper identifies the more important electrical system design objectives and describes the methods and equipment used to meet them.
INTRODUCTION
Brae 'B' is the second platform to be installed and operated b y Marathon Oil U.K. Ltd. in the U.K. sector of the North Sea. The Brae Field is located in Block 16/7, approximately 155 miles northeast of Aberdeen, Scotland. Extensive exploratory drilling has identified at least five reservoirs referred to as South, North, Central, East and West Brae. Brae 'B' will exploit the North Brae reservoir and follows closely behind Brae 'A' which has been producing from the South Brae reservoir since July 1983. Whereas South Brae is a more conventional black-oil reservoir, North Brae contains a highly retrograde condensate. This means that North Brae reservoir fluid exists above its dew point at the reservoir pressure of a b o u t 6850 lb in -2 and temperature of 242 °F. Sample fluid *Presented at the Southern Electric Industry Application Symposium, New Orleans, LA, U.S.A., Nov. 15 - 16, 1988. 0378-7796/89/$3.50
analysis and testing identified that the normal depletion method of production would result in substantial amounts of liquid condensing in the reservoir as the pressure declined, and thereby render them unrecoverable. By recycling lean gas after removing condensate from the rich production stream, the pressure in the reservoir can be maintained above the bubble point for a longer period, thereby maximizing recoverable reserves. In this respect, Brae 'B' will have the distinction of being the first such gas cycling project to be operated in the North Sea. The development program calls for a gas cycling period lasting up to nine years, during which liquid condensate will be recovered from a continually leaning produced gas stream. This will be followed b y a blow-down period during which the resulting lean gas production will be exported from the platform. The process facilities required to support such a program are enormous by offshore standards. This is due to the need to a c c o m m o d a t e gas-liquid separation facilities, a substantial gas processing plant, and large gas injection compressors, all in one facility. Project economics also require a rapid drilling program to achieve early plateau production, necessitating two drilling rigs. This also contributes significantly to the size of the platform. Process design is based on a peak average liquid production capacity of 7 5 0 0 0 bl/day from a wet gas production rate of 400 × 106fta/day, increased b y 10% for contingency. Main elements of the process include an inlet gas scrubber, a three-stage separation train, condensate metering and export, LP and MP separator compression, gas drying, natural gas liquid recovery and four parallel trains of gas injection compressors. The NGL recovery section incorporates a substantial gas chilling plant operat© Elsevier Sequoia/Printed in The Netherlands
160
ing at --30 ~F. Along with utility and life support equipment the topside facilities' operating weight approaches 42 000 metric tons. The topside is supported in 326 feet of water by an eight leg, piled steel jacket which alone weighs 18 900 metric tons. A modular installation concept was used on Brae 'B', with main equipment modules ranging in weight from 2000 to 3000 metric tons. Figure I indicates the platform's modular configuration. T h e two module support frames (MSFs) contain most of the platform's utility equipment in a plan area approximately 190 ft wide by 260 ft long. Wellheads,
well control equipment and production and injection manifolds are located in modules 01 and 02. Modules 03, 04 and 05 contain the bulk of the production process equipment. Modules 06 and 16 house two each of the four gas injection compression trains. Modules 11, 12, 13, 14, 21, 22, 31 and 32 contain all of the equipment which supports drilling. Module 18 houses the main power generation sets and high voltage switchgear while module 08 includes the platform's central control room, safe area HVAC supply equipment and other utilities. Modules 07 and 17 provide self-sufficient living accommoda-
01 WELLHEAD (WEST) 0 2 WELLHEAD (EAST) 0 3 SEPARATION 0 4 L P / M P COMP. & OIL EXPORT 0 5 NGL • REFRIGERATION OS INJECTION COMPRESSORS 1S INJECTION COMPRESSORS O? ACCOMMODATION 8ERVICE8 17 ACCOMMODATION 15 CONSTRUCTION 27 HELIDECK 0 8 CCR & UTILITIES 18 POWER GENERATION 2 8 EXHAUST STACK 11 SKIDDING MOD. (WEST) 12 SKIDDING MOD. (EAST) 13 DRILLING (WEST) 14 DRILLING (EAST) 21 SUBSTRUCTURE (WEST) 2 2 SUBSTRUCTURE (EAST)
27
31 32 38 57 58
07
104
03
Fig. 1. Brae 'B' module configuration.
DERRICK (WEST) DERRICK (EAST) WORKSHOPS MOD. SUPPORT (WEST) MOD. SUPPORT (EAST)
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tion for 240 personnel and support the helideck. Module 15 is a unique innovation on Brae 'B', providing additional accommodation, offices and hook-up power generation to support the platform hook-up and commissioning (HUC) program. This module was designed to be removed later should future reservoir or field development require additional process equipment. The foregoing should provide some insight to the platform facilities and indicate the size and complexity of the installation. The electrical power generation and distribution systems necessary to support such a facility are in themselves quite n o t e w o r t h y and present considerable challenge to the design effort.
DESIGN OBJECTIVES
There are several fundamental engineering considerations which must be satisfied by the power system for an integrated platform design. The more essential requirements to be met are reliability, flexibility, operability, safety, weight and space conservation, hookup and commissioning efficiency, and cost effectiveness. The power system for an oil drilling and production platform must be completely self-contained and have a very high level of reliability. This is essential owing to the remoteness of the installation, the necessity of providing life support services to hundreds of personnel and the total dependence of the production process for electrical power. The system must also be flexible to cope with wide variations in power demand. Under adverse conditions w h e n equipment failure occurs it must provide alternative means of sustaining the operation with minimal and preferrably no disruption. The power system is one of m a n y services necessary for oil production. With this in mind, and the need to concentrate resources primarily on production facilities, the power system should be capable of operation with little supervision. When intervention is required controls need to be readily accessible and easy to understand and use. Safety of operation is vitally important owing to the very nature of the facility and operation. Hazardous area classification must be carefully assessed and properly accom-
modated. Failure modes, whether process or power system oriented, must be reviewed to ensure that safe conditions ensue automaticaUy. In this respect power system control interfaces are an important aspect of the platform's emergency shutdown system. Adverse process upsets may require power isolation to a given area of the platform at all utilization levels and in extreme cases shutdown of power generation itself. Apart from the safety and operations oriented aspects, consideration of equipment weight and size is particularly important and somewhat unique to a platform installation. There is a direct relationship between the size and weight of a platform and its associated capital cost. The topside's size and weight also dictate the jacket design requirem e n t and its associated cost. The objective of each engineering discipline is to minimize weight and space usage while still achieving an efficiently operable and maintainable facility. Hook-up and commissioning efficiency is a further design consideration which was an objective throughout the Brae 'B' project. It is considered that this particular aspect sets Brae 'B' apart from other previous platforms of its magnitude. Apart from being costly in itself, the HUC time stands between the large capital outlay for the facility and the payback from production. The current status of the Brae 'B' hook-up, which is now in progress and well ahead of schedule, demonstrates the success of the thought and planning devoted to this subject. The design of the electrical power system reflects this objective throughout. Consideration of equipment cost is an engineering requirement for any major project. Platforms are no exception, particularly at today's oil prices. While power system reliability and safety are essential to the project's long-term economic success, a careful balance must be maintained for system design and equipment selection. Stringent cost control procedures were enforced equally across all design engineering disciplines on the Brae 'B' project. SCOPE
There are essentially three permanent power systems associated with an offshore
162 platform: main p o w e r system, emergency p o w e r system, and drilling p o w e r system. Provision must also be made for supply of power during the HUC phase before these permanent facilities are complete and operational. Various equipment configurations are possible depending on the level of importance placed on reliability as opposed to cost. During the Brae 'B' early design phase in 1983 a conservative design was adopted. As with many other platforms, dedicated power generation is provided for each power system. Further, numerous interconnections and backfeeds are incorporated between the associated distribution systems to enhance reliability for security of supply. During normal operating conditions the main power generation system is the primary p o w e r source for all permanent platform electriced power systems. Should equipment or generation failure render primary feeds unserviceable, the dedicated emergency and drilling generation facilities automatically restore power to their associated distribution systems.
DESIGN PHILOSOPHIES During the conceptual design phase of the earlier Brae 'A' platform, Marathon adopted an all-electric philosophy for equipment drives powered by a central generating plant. Studies conducted at that time indicated that this scheme would give the best operational availability and the lowest cost, since no drives exceeded 6 MW of power. Other benefits related to equipment layout, space conservation and maintenance aspects were also identified. Although this same philoso p h y was generally favored for Brae 'B', the installed power requirement for gas injection alone exceeded 95 MW. Electric drives for this d u t y were not practical, nor cost effective. Also the total platform power requirements would have been prohibitive owing to switchgear fault-level limitations. An o p t i m u m solution resulted in gas turbine drives on each of the four injection compressors with central electric supply for all other platform loads. Selection of the same gas turbine driver for both the gas compressors and power generating sets enhanced the attractiveness of this scheme.
A further design philosophy carried over from Brae 'A' design provided that the generation facilities should be able to carry the total platform load with one generator offline for maintenance or repair. Because of the desire to contain the generation system in one module and the associated layout restrictions, a clear preference was established for three generating sets instead of four. The goal was to design an integrated generation module. This resulted from the emphasis placed on simplifying hook-up and commissioning. The lifting capacities of installation crane ships had only recently improved to the point of making a single lift generation module possible. This goal presented a considerable challenge owing to the numerous HUC related objectives identified. The module needed to contain all three power generation sets, complete with their associated control panels, auxiliary m o t o r control centres (MCCs), and primary switchgear. Installation of the module complete with the gas turbines' inlet air systems further complicated the problem. Installation analysis had shown that this would save a significant a m o u n t of offshore work. However, with inlet systems in place the module's center of gravity was near the cruciform limits imposed by the installation crane ship capabilities. Stringent weight control measures had to be enforced throughout the design and fabrication periods to insure that the finished module's center of gravity was acceptable. A thorough onshore testing program was performed, including the actual running of the generating sets. All equipment was commissioned and load tested onshore. Both load share and load shed equipment were exercised. The electrical system control panel was used to synchronize and switch main generators and loads. The power module was successfully designed, built, tested and lifted into position as planned, thereby saving considerable offshore manhours. Preference for a 60 Hz system frequency, rather than the U.K. standard 50 Hz, was also decided during the Brae 'A' design. A study commissioned at the time indicated that at 8 0 M W fault currents would determine generating voltage. At 11 kV fault currents exceeded the capability of standard 11 kV
163
equipment. At 13.8 kV, the fault currents, although marginally lower, were just within the capability of 13.8 kV equipment. The study also identified potential weight and space savings on 4.16 kV and 13.8 kV m o t o r frame size selections. Thus the American standards of 13.8 kV and 60 Hz were selected. Brae 'B' utilization voltages and frequency were selected for the same reasons, as well as to be compatible with those of the existing Brae 'A' platform. Lighting and small power loads are supplied at 254 V. Application load ranges were set as follows:
Peak condensate recovery is established from day one and declines thereafter. Gas production, which is limited initially b y well availability, peaks in year three and then continues at plateau throughout the cycling period. The drilling program is intensive during the first t w o and a half years and then decreases significantly. Combining the associated loads gives a peak platform load of 41.6 MW in year three. Thereafter, platform load will decline steadily.
13.8 kV for motors above 1500 kW; 4.16 kV for motors b e t w e e n 160 kW and 1500 kW; 440 V for motors below 160 kW, welding supplies and heaters.
With a peak load near 42 MW established, 24 MW generating sets were ultimately selected to match the rating of the Rolls-Royce RB-211 gas turbine driver. Three main generators, each rated 30 MVA, 13.8 kV, 60 Hz, supply the main electrical power to the platform. The RB-211 engines are dual-fuelled for m a x i m u m security. Several fuel sources are available with automatic changeover controls to insure continuous supply even during process upsets. Normally fuel gas is derived from the platform production process. An alternative gas supply from the Brae 'A' platform is also available. There is also a fully independent diesel fuel system in reserve as a final backup.
PLATFORM
MAIN POWER GENERATION
LOAD
An electrical load study was carried o u t to establish a load profile and to determine power generation requirements. The electrical operating load profile is a function of the drilling and production programs. Figure 2 depicts the overall platform load profile determined.
50
- - - 4 1 . 6 MW 40-J
7 7
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GAS EXPORT BEGINS
DRILLING PROGRAM COMPLETE 20. PEAK GAS PRODUCTION PEAK CONDENSATE PRODUCTION
10.
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I 3
4
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t
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Fig. 2. Platform load profile.
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8
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164 MAIN POWER DISTRIBUTION
13.8 k V system Figure 3 depicts a simplified single-line diagram of the main distribution system. It serves to indicate the typical configuration used to achieve the level of security required at the main utilization levels. The 13.8 kV system consists of a main switchboard and separate MCC, both of which are located in the lower level of module 18 directly under the generators. It was necessary to split the two switchboards owing to module layout restrictions. The switchboard and MCC utilize withdrawable vacuum circuit-breakers t h r o u g h o u t for generator incomers, bus sections, transformer feeders and m o t o r controllers. Bus section circuit-breakers split the main switchboard into three sections and the MCC into two. Each section of the MCC is fed from an outer section of the main switchboard. The 13.8 kV switchboards are normally operated with all bus section and interconnecting feeder breakers closed. Should either of the bus section circuit-breakers on the main switchboard open, the MCC can be used to tie the outer sections of the main switchboard together. Computer modelling was used to investigate system load flows and to study pro-
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spective short-circuit levels and decrement characteristics. These studies are particularly critical to a large platform power system design owing to the generating capacities involved, the close proximity of equipment, and the high percentage of induction motor load. The results of the short-circuit study showed a symmetrical fault current of 32.4 kA RMS at fault inception for a worst case three-phase 13.8 kV busbar fault. The associated asymmetric peak-making d u t y was calculated to be 76.7 kA peak. Further transient studies were required to determine the system fault decrement characteristics and to prove the suitability of the 13.8 kV switchgear purchased. The AC and DC components of fault current were plotted for both peak load and low load cases. For the peak load case a current zero was not shown to occur until the seventh cycle in the fully offset phase following a three-phase short-circuit. At the point of breaker opening AC and DC current components were computed at 14.3 kA RMS and 21.1 kA respectively. The low load case study showed the delay to the first current zero to be extended by one cycle. Review of the 13.8 kV switchgear certification test data and breaker minim u m operating times verified the suitability of the equipment. Test data indicated that the circuit-breakers have a symmetrical breaking
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Fig. 3. Simplified single-line diagram.
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165 capacity of 31.5 kA RMS with as much as 131% a s y m m e t r y and a peak-making capacity of 96 kA. Fault currents at other voltage levels were shown to be well within equipm e n t ratings.
4.16 k V system Referring again to Fig. 3 it can be seen that all 4.16 kV motors are served by a single two-section switchboard and MCC. Each side of the board is supplied by a 12.5 MVA, 13.8 kV/4.16 kV transformer. Both transformers and the switchboard are located in the lower level of module 18. Transformer sizing is such that either can carry the full 4.16 kV operating load. Incoming and bus section circuit-breakers are of the non-withdrawable vacuum type. Motor starters are withdrawable fuse-protected vacuum contactors. Circuit-breakers have a symmetrical breaking capacity of 31.5 kA RMS and a peak-making capacity of 78.7 kA. Supply transformer star point ground resistors limit ground fault currents to 200 A. Typically, main and standby process equipm e n t drives are connected to opposite sides of the switchboard for m a x i m u m security. While switchgear equipment ratings would allow b o t h incomers and the bus section circuit-breaker to be closed on the 4.16 kV board, the bus section is normally operated open. In the event of a fault on either incomer, an auto-transfer scheme isolates the faulty incomer and closes the bus section breaker. Thus, supply is automatically reestablished to the affected switchboard side ready for remote starting of the interrupted motors. The auto-transfer scheme is selective in responding only to incomer faults so as not to expose the switchboard side served by a healthy incomer. When a fault has been rectified a manually initiated auto-selection scheme permits a no-break return to the normal configuration. This scheme also allows removal of a feeder or transformer from service w i t h o u t any interruption of supply. 440 V system The 440 V system configuration is similar in concept to that of the 4.16 kV system. There are, however, numerous MCCs located in each of the major modules. There are also four sets of 2.5 MVA transformers located in modules 04, 07, 08 and 57 to establish
distributed 440 V feeder switchboards. Each two-section switchboard in turn feeds a group of two-section module MCCs. The distributed system concept minimized cable size and weight as well as hook-up manhours since fewer and smaller offshore cables were required. Provision of MCCs in every major module also allowed the m a x i m u m degree of construction completion and equipm e n t commissioning to be achieved onshore. All 440 V switchboards are three-phase, four-wire, and transformer star points are solidly grounded. Primary switchboard busbar systems are rated for continuous operation at 3500 A and are certified for a short-circuit d u t y of 70 kA for 1 second. Incoming circuitbreakers and bus section breakers are certified for 100 kA RMS break and 200 kA peak make. All outgoing breakers are certified for 70 kA RMS break and 154 kA peak make. At the 440 V user level, main and standby process equipment drives are provided extensively. Auto-transfer and auto-selection schemes are provided on bus section and feeder breakers as described for the 4.16 kV system.
DRILLING POWER SYSTEM Platform drilling facilities are traditionally an entity unto themselves. On Brae 'B' an a t t e m p t was made to provide commonality between the drilling and production facilities' power systems where possible. The two drilling rigs each have separate power systems for independent operation and to facilitate early drilling capability after module installation independent of other platform systems. Four diesel engine driven generators, each rated at 850 kW, supply power to each rig at 600 V AC. Single 2.5 MVA transformers in turn supply 440 V users. An interconnection between each drill rig's 440 V system may be activated in case of equipment failure. This provides substantial flexibility to the critical drilling function. Typically, DC motors are used to provide variable speed drive to the drilling rig's drawworks, rotary table and m u d pumps. Variable DC m o t o r supplies are developed by SCR equipment supplied at 600 V AC. The cost associated with supply of clean fuel to the diesel engine drilling generators
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prohibits their continuous running. Therefore, for normal operation following commissioning of the platform main generation, each drilling rig power system will be supplied by a three-winding transformer fed from the 13.8 kV switchboard. Figure 3 illustrates this supply connection from main power generation to one of the drilling rig systems.
former fed from the 13.8 kV switchboard. Again all bus section and feeder circuitbreakers have auto-transfer control for added security. Should main generation fail the emergency switchboard is automatically isolated and the diesel generator automatically starts and closes onto the board. The diesel generator has a continuous rating of 1250 kW. Engine auxiliaries and room ventilation fans are energized automatically after the engine starts. A diesel fuel storage tank provides for 24 hours of running at rated load. The mandatory and essential supplies, including those to explosion-proof emergency light fittings, are re-energized automatically upon restoration of power to the emergency switchboard. Many of the associated loads are in turn made further secure by use of alternative forms of dedicated battery-backed supplies. For example, a 150 kVA rotary uninterruptable power supply (UPS) and 115 V distribution system supply power to the platform fire and gas detection system, emergency shutdown system and the main process control system. Similarly, a 40 kVA rotary UPS system supplies telecommunications equipment and the platform public address (PA) system. Other equipment such as emergency lighting, helideck lighting and
EMERGENCY SYSTEM
The emergency power system is a key element of any platform installation serving various mandatory and essential loads necessary for safety and life support. A diesel engine driven generator and 440 V threephase switchboard form the heart of the emergency supplies on Brae 'B'. The generator and switchboard are located in the lower living quarters module, which is considered to be one of the safest and least vulnerable areas of the platform. The two-section emergency switchboard is normally fed by two full-size feeders from the main 440 V distribution switchboard in module 07 (Fig. 4). This configuration is similar to that of other 440 V supplies except that the module 07 switchboard has three sections, each supplied by a 2.5 MVA trans-
M.I O N
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FROLM DRI LING
Fig. 4. Emergency and accommodation switchboards.
167 navigational aids incorporate integral battery/ inverter supplies. The result is that mandatory and essential loads have two alternative selfenergizing sources of power in addition to main generation supplies. Apart from providing power to mandatory and essential loads under emergency conditions the emergency power system fulfills two further functions on Brae 'B'. These functions are referred to as 'black start' and 'five~lay habitation'. 'Black start' facilities provide for orderly and safe start-up of the platform following a complete shutdown in the unlikely event of a total loss of power. Should gas leakage accompany such an incident, the ventilation system is needed to purge the affected area prior to re-energizing equipment. On Brae 'B' the emergency switchboard can supply platform ventilation equipment as well as main generator auxiliaries to accomplish the black start functions. A further design criterion was established on Brae 'B' to facilitate habitation of the platform on a short-term basis without main power generation. The electrical system design provides for the ability to backfeed the living quarters from the emergency generator switchboard under manual supervision. To achieve this it was necessary to incorporate special electrical protection features for the associated feeders and breakers.
HOOK-UP AND COMMISSIONING POWER
As mentioned previously, HUC provisions were considered throughout the design. The objective was to incorporate as much flexibility in the permanent system configuration as possible to minimize wasted time and materials associated with temporary equipment. The need to establish HUC momentum on the platform quickly after installation is also paramount. Owing to the time required to fully commission any sizeable power generation plant offshore, small temporary diesel engine driven generators were used on Brae 'B'. These were tied directly into the permanent 440 V MCCs in selected modules to provide lighting and small power for tools and welding machines. These supplies were installed onshore and commissioned along with other permanent
equipment. Within hours of landing each module on the platform offshore, lighting and small power were available. This contributed significantly to safe access and utility during the very early days following module installation. As the HUC program progresses electrical load builds up rapidly. Progress may be impeded if adequate supplies are not made available efficently. On Brae 'B' provisions were incorporated to make extensive use of the permanent diesel engine driven generators on the platform. The 1250 kW emergency generator provided the early means of powering the living quarters. Interconnections between the two living quarters modules and the helideck were rapidly installed. Two fully commissioned 1000 kW generators were incorporated within the previously mentioned module 15, the construction module. Feeders from this facility to the living quarters module 07 and other selected modules quickly established the availability of additional power. The small temporary generators could then be phased out. Drilling generators were the next to be commissioned. A permanent feeder to the living quarters module further enhanced flexibility. The main generation supplies to the drilling rigs via the three-winding transformers are also designed to be operated in a backfeed mode. With permanent distribution system feeders installed this allows the drilling generators to supply power virtually anywhere on the platform. Not only do the various interconnections provide a diversity of power during the HUC program but also considerable flexibility to the long-term platform operation.
PROTECTION Needless to say, the complexity of the overall system demands a high degree of protection. With its several sources of power generation, interconnections and backfeeds, manual operation would be extremely challenging and risky. Numerous protective interlocks throughout the system prevent unsafe system configurations. Synchronizing facilities and protective devices are incorporated where necessary to insure safe paralleling of generation where allowed. Protective devices
168 are provided throughout the electrical system to insure that faults are isolated quickly. The majority of protective relays are state-of-theart electronic devices. High resistance grounding of the main generators limits ground fault currents to very low levels on the 13.8 kV system. Wherever possible, protection relay settings are selected such that the device nearest a fault operates first to achieve discrimination and thereby minimize system disturbance. LOAD SHARING AND LOAD SHEDDING SYSTEM Load sharing and load shedding functions are provided b y a single microprocessor-based system. The load sharing program balances the platform electrical load between the main generators and insures minimal variation of the system voltage and frequency. The load shedding function is activated if the main p o w e r system becomes overloaded b y addition of load or b y the loss of a generator. Preprogrammed m o t o r loads at the 13.8 kV and 4.16 kV level are then shed to restore the balance b e t w e e n generation capacity and load demand, thereby avoiding a main generation shutdown. CONTROL FACILITIES A system electrical control panel (ECP} is provided in the generator control room in module 18. The panel's mosaic mimic displays the status of the main generators and the distribution of power throughout the system. Centralized generator controls are provided in addition to circuit-breaker controls for b o t h 13.8 kV and 4.16 kV switchboards. Each generating set has its own unit control panel for individual sequencing and detailed status display. A remote selection switch on each unit control panel allows control from the ECP. Load sharing and load shedding functions are selectable at the ECP as well as voltage and frequency raise/lower controls. System status indications are also repeated to the platform central control r o o m (CCR). System graphic representations can be displayed there on the distributed control system video interface screens. The three main generating sets can be started from the CCR under auto-synchronizing sequence control. Other electrical system controls are inten-
tionally excluded in favor of concentrating CCR emphasis on process monitoring and control. The emergency generator control panel (EGCP) is another important control location of the electrical system. Voltage and frequency raise/lower controls for the main generators are duplicated on the EGCP in module 07. These controls are used in conjunction with manual synchronizing facilities to parallel a main generator with the emergency switchboard during a black start. In practice, a start would normally be initiated from the ECP and control would then be temporarily transferred to the EGCP for the paralleling operation. After paralleling the emergency generator would be taken off line.
SHUTDOWN SYSTEM INTERFACE Another important control function, which is critical to the safety of platform operation, is provided by the platform shutdown system. The shutdown system provides for the safe and orderly shutdown of equipment on detection of fire, gas and abnormal process operating conditions. An important function of the system is to isolate electrical power to motors and other equipment in a given area of the platform in accordance with predetermined hazard analyses. This is accomplished by hardwire interfaces between the distributed shutdown system and the p o w e r distribution system throughout the platform. Trip signals are connected to m o t o r and feeder control circuits at all voltage levels. The fact that 440 V MCCs, the shutdown system and the fire and gas system are all distributed systems greatly simplified the hardwire connection requirements. This also allowed for the majority of these interfaces to be commissioned and tested onshore. The overall combination is extensive but well coordinated and very effective. CONCLUSION A North Sea platform presents a unique challenge to the electrical engineering discipline. Within relatively small confines tremendous amounts of power are generated, distributed, and utilized. By its very nature an offshore platform demands an extremely
169
reliable electrical service. Its power system must be flexible and safe while requiring a minimum of attention. The Brae 'B' power system meets these requirements impressively by employing sophisticated state-of-the-art equipment. Innovative interconnections and backfeeds between power sources have helped contribute to the success of a rapid hook-up program. Attention paid to safe and proper application of equipment will no doubt insure its future operational success. ACKNOWLEDGEMENTS
The author wishes to express appreciation
to Marathon Oil Company and the Brae Field Participants, namely, Britoil PLC; Bow Valley Exploration {U.K.) Ltd., a wholly owned subsidiary of Bow Valley Industries Ltd.; Kerr-McGee Oil (U.K.) PLC, a wholly owned subsidiary of Kerr-McGee Corporation; Dyas Oil U.K.; LL & E (U.K.} Inc., a subsidiary of The Louisiana Land and Exploration Company; Sovereign Oil and Gas PLC; and Norsk Hydro Oil and Gas Ltd., for encouragement to publish this paper. Many thanks are also due to Mr. Brian Le Masurier of Marathon's London office for gathering much of the technical reference material.
159
A North Sea Platform Power System* G. L. MICHAEL
Marathon Oil Company, P.O. B o x 3128, Houston, T X 77253 (U.S.A.) (Received May 30, 1989)
ABSTRACT
Over the last twenty-five years, North Sea platform electrical generation designs have grown from relatively small, simple systems to large sophisticated fully integrated units. Brae 'B', installed in the North Sea during the summer of 1987, is one o f the largest platforms in the world. The electrical system design, with 72 MW o f main power generation, includes a complete diversity o f electrical engineering applications. Following an introductory description of the platform facilities, this paper identifies the more important electrical system design objectives and describes the methods and equipment used to meet them.
INTRODUCTION
Brae 'B' is the second platform to be installed and operated b y Marathon Oil U.K. Ltd. in the U.K. sector of the North Sea. The Brae Field is located in Block 16/7, approximately 155 miles northeast of Aberdeen, Scotland. Extensive exploratory drilling has identified at least five reservoirs referred to as South, North, Central, East and West Brae. Brae 'B' will exploit the North Brae reservoir and follows closely behind Brae 'A' which has been producing from the South Brae reservoir since July 1983. Whereas South Brae is a more conventional black-oil reservoir, North Brae contains a highly retrograde condensate. This means that North Brae reservoir fluid exists above its dew point at the reservoir pressure of a b o u t 6850 lb in -2 and temperature of 242 °F. Sample fluid *Presented at the Southern Electric Industry Application Symposium, New Orleans, LA, U.S.A., Nov. 15 - 16, 1988. 0378-7796/89/$3.50
analysis and testing identified that the normal depletion method of production would result in substantial amounts of liquid condensing in the reservoir as the pressure declined, and thereby render them unrecoverable. By recycling lean gas after removing condensate from the rich production stream, the pressure in the reservoir can be maintained above the bubble point for a longer period, thereby maximizing recoverable reserves. In this respect, Brae 'B' will have the distinction of being the first such gas cycling project to be operated in the North Sea. The development program calls for a gas cycling period lasting up to nine years, during which liquid condensate will be recovered from a continually leaning produced gas stream. This will be followed b y a blow-down period during which the resulting lean gas production will be exported from the platform. The process facilities required to support such a program are enormous by offshore standards. This is due to the need to a c c o m m o d a t e gas-liquid separation facilities, a substantial gas processing plant, and large gas injection compressors, all in one facility. Project economics also require a rapid drilling program to achieve early plateau production, necessitating two drilling rigs. This also contributes significantly to the size of the platform. Process design is based on a peak average liquid production capacity of 7 5 0 0 0 bl/day from a wet gas production rate of 400 × 106fta/day, increased b y 10% for contingency. Main elements of the process include an inlet gas scrubber, a three-stage separation train, condensate metering and export, LP and MP separator compression, gas drying, natural gas liquid recovery and four parallel trains of gas injection compressors. The NGL recovery section incorporates a substantial gas chilling plant operat© Elsevier Sequoia/Printed in The Netherlands
160
ing at --30 ~F. Along with utility and life support equipment the topside facilities' operating weight approaches 42 000 metric tons. The topside is supported in 326 feet of water by an eight leg, piled steel jacket which alone weighs 18 900 metric tons. A modular installation concept was used on Brae 'B', with main equipment modules ranging in weight from 2000 to 3000 metric tons. Figure I indicates the platform's modular configuration. T h e two module support frames (MSFs) contain most of the platform's utility equipment in a plan area approximately 190 ft wide by 260 ft long. Wellheads,
well control equipment and production and injection manifolds are located in modules 01 and 02. Modules 03, 04 and 05 contain the bulk of the production process equipment. Modules 06 and 16 house two each of the four gas injection compression trains. Modules 11, 12, 13, 14, 21, 22, 31 and 32 contain all of the equipment which supports drilling. Module 18 houses the main power generation sets and high voltage switchgear while module 08 includes the platform's central control room, safe area HVAC supply equipment and other utilities. Modules 07 and 17 provide self-sufficient living accommoda-
01 WELLHEAD (WEST) 0 2 WELLHEAD (EAST) 0 3 SEPARATION 0 4 L P / M P COMP. & OIL EXPORT 0 5 NGL • REFRIGERATION OS INJECTION COMPRESSORS 1S INJECTION COMPRESSORS O? ACCOMMODATION 8ERVICE8 17 ACCOMMODATION 15 CONSTRUCTION 27 HELIDECK 0 8 CCR & UTILITIES 18 POWER GENERATION 2 8 EXHAUST STACK 11 SKIDDING MOD. (WEST) 12 SKIDDING MOD. (EAST) 13 DRILLING (WEST) 14 DRILLING (EAST) 21 SUBSTRUCTURE (WEST) 2 2 SUBSTRUCTURE (EAST)
27
31 32 38 57 58
07
104
03
Fig. 1. Brae 'B' module configuration.
DERRICK (WEST) DERRICK (EAST) WORKSHOPS MOD. SUPPORT (WEST) MOD. SUPPORT (EAST)
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tion for 240 personnel and support the helideck. Module 15 is a unique innovation on Brae 'B', providing additional accommodation, offices and hook-up power generation to support the platform hook-up and commissioning (HUC) program. This module was designed to be removed later should future reservoir or field development require additional process equipment. The foregoing should provide some insight to the platform facilities and indicate the size and complexity of the installation. The electrical power generation and distribution systems necessary to support such a facility are in themselves quite n o t e w o r t h y and present considerable challenge to the design effort.
DESIGN OBJECTIVES
There are several fundamental engineering considerations which must be satisfied by the power system for an integrated platform design. The more essential requirements to be met are reliability, flexibility, operability, safety, weight and space conservation, hookup and commissioning efficiency, and cost effectiveness. The power system for an oil drilling and production platform must be completely self-contained and have a very high level of reliability. This is essential owing to the remoteness of the installation, the necessity of providing life support services to hundreds of personnel and the total dependence of the production process for electrical power. The system must also be flexible to cope with wide variations in power demand. Under adverse conditions w h e n equipment failure occurs it must provide alternative means of sustaining the operation with minimal and preferrably no disruption. The power system is one of m a n y services necessary for oil production. With this in mind, and the need to concentrate resources primarily on production facilities, the power system should be capable of operation with little supervision. When intervention is required controls need to be readily accessible and easy to understand and use. Safety of operation is vitally important owing to the very nature of the facility and operation. Hazardous area classification must be carefully assessed and properly accom-
modated. Failure modes, whether process or power system oriented, must be reviewed to ensure that safe conditions ensue automaticaUy. In this respect power system control interfaces are an important aspect of the platform's emergency shutdown system. Adverse process upsets may require power isolation to a given area of the platform at all utilization levels and in extreme cases shutdown of power generation itself. Apart from the safety and operations oriented aspects, consideration of equipment weight and size is particularly important and somewhat unique to a platform installation. There is a direct relationship between the size and weight of a platform and its associated capital cost. The topside's size and weight also dictate the jacket design requirem e n t and its associated cost. The objective of each engineering discipline is to minimize weight and space usage while still achieving an efficiently operable and maintainable facility. Hook-up and commissioning efficiency is a further design consideration which was an objective throughout the Brae 'B' project. It is considered that this particular aspect sets Brae 'B' apart from other previous platforms of its magnitude. Apart from being costly in itself, the HUC time stands between the large capital outlay for the facility and the payback from production. The current status of the Brae 'B' hook-up, which is now in progress and well ahead of schedule, demonstrates the success of the thought and planning devoted to this subject. The design of the electrical power system reflects this objective throughout. Consideration of equipment cost is an engineering requirement for any major project. Platforms are no exception, particularly at today's oil prices. While power system reliability and safety are essential to the project's long-term economic success, a careful balance must be maintained for system design and equipment selection. Stringent cost control procedures were enforced equally across all design engineering disciplines on the Brae 'B' project. SCOPE
There are essentially three permanent power systems associated with an offshore
162 platform: main p o w e r system, emergency p o w e r system, and drilling p o w e r system. Provision must also be made for supply of power during the HUC phase before these permanent facilities are complete and operational. Various equipment configurations are possible depending on the level of importance placed on reliability as opposed to cost. During the Brae 'B' early design phase in 1983 a conservative design was adopted. As with many other platforms, dedicated power generation is provided for each power system. Further, numerous interconnections and backfeeds are incorporated between the associated distribution systems to enhance reliability for security of supply. During normal operating conditions the main power generation system is the primary p o w e r source for all permanent platform electriced power systems. Should equipment or generation failure render primary feeds unserviceable, the dedicated emergency and drilling generation facilities automatically restore power to their associated distribution systems.
DESIGN PHILOSOPHIES During the conceptual design phase of the earlier Brae 'A' platform, Marathon adopted an all-electric philosophy for equipment drives powered by a central generating plant. Studies conducted at that time indicated that this scheme would give the best operational availability and the lowest cost, since no drives exceeded 6 MW of power. Other benefits related to equipment layout, space conservation and maintenance aspects were also identified. Although this same philoso p h y was generally favored for Brae 'B', the installed power requirement for gas injection alone exceeded 95 MW. Electric drives for this d u t y were not practical, nor cost effective. Also the total platform power requirements would have been prohibitive owing to switchgear fault-level limitations. An o p t i m u m solution resulted in gas turbine drives on each of the four injection compressors with central electric supply for all other platform loads. Selection of the same gas turbine driver for both the gas compressors and power generating sets enhanced the attractiveness of this scheme.
A further design philosophy carried over from Brae 'A' design provided that the generation facilities should be able to carry the total platform load with one generator offline for maintenance or repair. Because of the desire to contain the generation system in one module and the associated layout restrictions, a clear preference was established for three generating sets instead of four. The goal was to design an integrated generation module. This resulted from the emphasis placed on simplifying hook-up and commissioning. The lifting capacities of installation crane ships had only recently improved to the point of making a single lift generation module possible. This goal presented a considerable challenge owing to the numerous HUC related objectives identified. The module needed to contain all three power generation sets, complete with their associated control panels, auxiliary m o t o r control centres (MCCs), and primary switchgear. Installation of the module complete with the gas turbines' inlet air systems further complicated the problem. Installation analysis had shown that this would save a significant a m o u n t of offshore work. However, with inlet systems in place the module's center of gravity was near the cruciform limits imposed by the installation crane ship capabilities. Stringent weight control measures had to be enforced throughout the design and fabrication periods to insure that the finished module's center of gravity was acceptable. A thorough onshore testing program was performed, including the actual running of the generating sets. All equipment was commissioned and load tested onshore. Both load share and load shed equipment were exercised. The electrical system control panel was used to synchronize and switch main generators and loads. The power module was successfully designed, built, tested and lifted into position as planned, thereby saving considerable offshore manhours. Preference for a 60 Hz system frequency, rather than the U.K. standard 50 Hz, was also decided during the Brae 'A' design. A study commissioned at the time indicated that at 8 0 M W fault currents would determine generating voltage. At 11 kV fault currents exceeded the capability of standard 11 kV
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equipment. At 13.8 kV, the fault currents, although marginally lower, were just within the capability of 13.8 kV equipment. The study also identified potential weight and space savings on 4.16 kV and 13.8 kV m o t o r frame size selections. Thus the American standards of 13.8 kV and 60 Hz were selected. Brae 'B' utilization voltages and frequency were selected for the same reasons, as well as to be compatible with those of the existing Brae 'A' platform. Lighting and small power loads are supplied at 254 V. Application load ranges were set as follows:
Peak condensate recovery is established from day one and declines thereafter. Gas production, which is limited initially b y well availability, peaks in year three and then continues at plateau throughout the cycling period. The drilling program is intensive during the first t w o and a half years and then decreases significantly. Combining the associated loads gives a peak platform load of 41.6 MW in year three. Thereafter, platform load will decline steadily.
13.8 kV for motors above 1500 kW; 4.16 kV for motors b e t w e e n 160 kW and 1500 kW; 440 V for motors below 160 kW, welding supplies and heaters.
With a peak load near 42 MW established, 24 MW generating sets were ultimately selected to match the rating of the Rolls-Royce RB-211 gas turbine driver. Three main generators, each rated 30 MVA, 13.8 kV, 60 Hz, supply the main electrical power to the platform. The RB-211 engines are dual-fuelled for m a x i m u m security. Several fuel sources are available with automatic changeover controls to insure continuous supply even during process upsets. Normally fuel gas is derived from the platform production process. An alternative gas supply from the Brae 'A' platform is also available. There is also a fully independent diesel fuel system in reserve as a final backup.
PLATFORM
MAIN POWER GENERATION
LOAD
An electrical load study was carried o u t to establish a load profile and to determine power generation requirements. The electrical operating load profile is a function of the drilling and production programs. Figure 2 depicts the overall platform load profile determined.
50
- - - 4 1 . 6 MW 40-J
7 7
I
I
30,
GAS EXPORT BEGINS
DRILLING PROGRAM COMPLETE 20. PEAK GAS PRODUCTION PEAK CONDENSATE PRODUCTION
10.
0
I 1
I 2
I 3
4
I
5
t
6
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YEARS
Fig. 2. Platform load profile.
t
8
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9
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10
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11
I
12
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13
:
14
164 MAIN POWER DISTRIBUTION
13.8 k V system Figure 3 depicts a simplified single-line diagram of the main distribution system. It serves to indicate the typical configuration used to achieve the level of security required at the main utilization levels. The 13.8 kV system consists of a main switchboard and separate MCC, both of which are located in the lower level of module 18 directly under the generators. It was necessary to split the two switchboards owing to module layout restrictions. The switchboard and MCC utilize withdrawable vacuum circuit-breakers t h r o u g h o u t for generator incomers, bus sections, transformer feeders and m o t o r controllers. Bus section circuit-breakers split the main switchboard into three sections and the MCC into two. Each section of the MCC is fed from an outer section of the main switchboard. The 13.8 kV switchboards are normally operated with all bus section and interconnecting feeder breakers closed. Should either of the bus section circuit-breakers on the main switchboard open, the MCC can be used to tie the outer sections of the main switchboard together. Computer modelling was used to investigate system load flows and to study pro-
30 MVA
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spective short-circuit levels and decrement characteristics. These studies are particularly critical to a large platform power system design owing to the generating capacities involved, the close proximity of equipment, and the high percentage of induction motor load. The results of the short-circuit study showed a symmetrical fault current of 32.4 kA RMS at fault inception for a worst case three-phase 13.8 kV busbar fault. The associated asymmetric peak-making d u t y was calculated to be 76.7 kA peak. Further transient studies were required to determine the system fault decrement characteristics and to prove the suitability of the 13.8 kV switchgear purchased. The AC and DC components of fault current were plotted for both peak load and low load cases. For the peak load case a current zero was not shown to occur until the seventh cycle in the fully offset phase following a three-phase short-circuit. At the point of breaker opening AC and DC current components were computed at 14.3 kA RMS and 21.1 kA respectively. The low load case study showed the delay to the first current zero to be extended by one cycle. Review of the 13.8 kV switchgear certification test data and breaker minim u m operating times verified the suitability of the equipment. Test data indicated that the circuit-breakers have a symmetrical breaking
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165 capacity of 31.5 kA RMS with as much as 131% a s y m m e t r y and a peak-making capacity of 96 kA. Fault currents at other voltage levels were shown to be well within equipm e n t ratings.
4.16 k V system Referring again to Fig. 3 it can be seen that all 4.16 kV motors are served by a single two-section switchboard and MCC. Each side of the board is supplied by a 12.5 MVA, 13.8 kV/4.16 kV transformer. Both transformers and the switchboard are located in the lower level of module 18. Transformer sizing is such that either can carry the full 4.16 kV operating load. Incoming and bus section circuit-breakers are of the non-withdrawable vacuum type. Motor starters are withdrawable fuse-protected vacuum contactors. Circuit-breakers have a symmetrical breaking capacity of 31.5 kA RMS and a peak-making capacity of 78.7 kA. Supply transformer star point ground resistors limit ground fault currents to 200 A. Typically, main and standby process equipm e n t drives are connected to opposite sides of the switchboard for m a x i m u m security. While switchgear equipment ratings would allow b o t h incomers and the bus section circuit-breaker to be closed on the 4.16 kV board, the bus section is normally operated open. In the event of a fault on either incomer, an auto-transfer scheme isolates the faulty incomer and closes the bus section breaker. Thus, supply is automatically reestablished to the affected switchboard side ready for remote starting of the interrupted motors. The auto-transfer scheme is selective in responding only to incomer faults so as not to expose the switchboard side served by a healthy incomer. When a fault has been rectified a manually initiated auto-selection scheme permits a no-break return to the normal configuration. This scheme also allows removal of a feeder or transformer from service w i t h o u t any interruption of supply. 440 V system The 440 V system configuration is similar in concept to that of the 4.16 kV system. There are, however, numerous MCCs located in each of the major modules. There are also four sets of 2.5 MVA transformers located in modules 04, 07, 08 and 57 to establish
distributed 440 V feeder switchboards. Each two-section switchboard in turn feeds a group of two-section module MCCs. The distributed system concept minimized cable size and weight as well as hook-up manhours since fewer and smaller offshore cables were required. Provision of MCCs in every major module also allowed the m a x i m u m degree of construction completion and equipm e n t commissioning to be achieved onshore. All 440 V switchboards are three-phase, four-wire, and transformer star points are solidly grounded. Primary switchboard busbar systems are rated for continuous operation at 3500 A and are certified for a short-circuit d u t y of 70 kA for 1 second. Incoming circuitbreakers and bus section breakers are certified for 100 kA RMS break and 200 kA peak make. All outgoing breakers are certified for 70 kA RMS break and 154 kA peak make. At the 440 V user level, main and standby process equipment drives are provided extensively. Auto-transfer and auto-selection schemes are provided on bus section and feeder breakers as described for the 4.16 kV system.
DRILLING POWER SYSTEM Platform drilling facilities are traditionally an entity unto themselves. On Brae 'B' an a t t e m p t was made to provide commonality between the drilling and production facilities' power systems where possible. The two drilling rigs each have separate power systems for independent operation and to facilitate early drilling capability after module installation independent of other platform systems. Four diesel engine driven generators, each rated at 850 kW, supply power to each rig at 600 V AC. Single 2.5 MVA transformers in turn supply 440 V users. An interconnection between each drill rig's 440 V system may be activated in case of equipment failure. This provides substantial flexibility to the critical drilling function. Typically, DC motors are used to provide variable speed drive to the drilling rig's drawworks, rotary table and m u d pumps. Variable DC m o t o r supplies are developed by SCR equipment supplied at 600 V AC. The cost associated with supply of clean fuel to the diesel engine drilling generators
166
prohibits their continuous running. Therefore, for normal operation following commissioning of the platform main generation, each drilling rig power system will be supplied by a three-winding transformer fed from the 13.8 kV switchboard. Figure 3 illustrates this supply connection from main power generation to one of the drilling rig systems.
former fed from the 13.8 kV switchboard. Again all bus section and feeder circuitbreakers have auto-transfer control for added security. Should main generation fail the emergency switchboard is automatically isolated and the diesel generator automatically starts and closes onto the board. The diesel generator has a continuous rating of 1250 kW. Engine auxiliaries and room ventilation fans are energized automatically after the engine starts. A diesel fuel storage tank provides for 24 hours of running at rated load. The mandatory and essential supplies, including those to explosion-proof emergency light fittings, are re-energized automatically upon restoration of power to the emergency switchboard. Many of the associated loads are in turn made further secure by use of alternative forms of dedicated battery-backed supplies. For example, a 150 kVA rotary uninterruptable power supply (UPS) and 115 V distribution system supply power to the platform fire and gas detection system, emergency shutdown system and the main process control system. Similarly, a 40 kVA rotary UPS system supplies telecommunications equipment and the platform public address (PA) system. Other equipment such as emergency lighting, helideck lighting and
EMERGENCY SYSTEM
The emergency power system is a key element of any platform installation serving various mandatory and essential loads necessary for safety and life support. A diesel engine driven generator and 440 V threephase switchboard form the heart of the emergency supplies on Brae 'B'. The generator and switchboard are located in the lower living quarters module, which is considered to be one of the safest and least vulnerable areas of the platform. The two-section emergency switchboard is normally fed by two full-size feeders from the main 440 V distribution switchboard in module 07 (Fig. 4). This configuration is similar to that of other 440 V supplies except that the module 07 switchboard has three sections, each supplied by a 2.5 MVA trans-
M.I O N
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FROLM DRI LING
Fig. 4. Emergency and accommodation switchboards.
167 navigational aids incorporate integral battery/ inverter supplies. The result is that mandatory and essential loads have two alternative selfenergizing sources of power in addition to main generation supplies. Apart from providing power to mandatory and essential loads under emergency conditions the emergency power system fulfills two further functions on Brae 'B'. These functions are referred to as 'black start' and 'five~lay habitation'. 'Black start' facilities provide for orderly and safe start-up of the platform following a complete shutdown in the unlikely event of a total loss of power. Should gas leakage accompany such an incident, the ventilation system is needed to purge the affected area prior to re-energizing equipment. On Brae 'B' the emergency switchboard can supply platform ventilation equipment as well as main generator auxiliaries to accomplish the black start functions. A further design criterion was established on Brae 'B' to facilitate habitation of the platform on a short-term basis without main power generation. The electrical system design provides for the ability to backfeed the living quarters from the emergency generator switchboard under manual supervision. To achieve this it was necessary to incorporate special electrical protection features for the associated feeders and breakers.
HOOK-UP AND COMMISSIONING POWER
As mentioned previously, HUC provisions were considered throughout the design. The objective was to incorporate as much flexibility in the permanent system configuration as possible to minimize wasted time and materials associated with temporary equipment. The need to establish HUC momentum on the platform quickly after installation is also paramount. Owing to the time required to fully commission any sizeable power generation plant offshore, small temporary diesel engine driven generators were used on Brae 'B'. These were tied directly into the permanent 440 V MCCs in selected modules to provide lighting and small power for tools and welding machines. These supplies were installed onshore and commissioned along with other permanent
equipment. Within hours of landing each module on the platform offshore, lighting and small power were available. This contributed significantly to safe access and utility during the very early days following module installation. As the HUC program progresses electrical load builds up rapidly. Progress may be impeded if adequate supplies are not made available efficently. On Brae 'B' provisions were incorporated to make extensive use of the permanent diesel engine driven generators on the platform. The 1250 kW emergency generator provided the early means of powering the living quarters. Interconnections between the two living quarters modules and the helideck were rapidly installed. Two fully commissioned 1000 kW generators were incorporated within the previously mentioned module 15, the construction module. Feeders from this facility to the living quarters module 07 and other selected modules quickly established the availability of additional power. The small temporary generators could then be phased out. Drilling generators were the next to be commissioned. A permanent feeder to the living quarters module further enhanced flexibility. The main generation supplies to the drilling rigs via the three-winding transformers are also designed to be operated in a backfeed mode. With permanent distribution system feeders installed this allows the drilling generators to supply power virtually anywhere on the platform. Not only do the various interconnections provide a diversity of power during the HUC program but also considerable flexibility to the long-term platform operation.
PROTECTION Needless to say, the complexity of the overall system demands a high degree of protection. With its several sources of power generation, interconnections and backfeeds, manual operation would be extremely challenging and risky. Numerous protective interlocks throughout the system prevent unsafe system configurations. Synchronizing facilities and protective devices are incorporated where necessary to insure safe paralleling of generation where allowed. Protective devices
168 are provided throughout the electrical system to insure that faults are isolated quickly. The majority of protective relays are state-of-theart electronic devices. High resistance grounding of the main generators limits ground fault currents to very low levels on the 13.8 kV system. Wherever possible, protection relay settings are selected such that the device nearest a fault operates first to achieve discrimination and thereby minimize system disturbance. LOAD SHARING AND LOAD SHEDDING SYSTEM Load sharing and load shedding functions are provided b y a single microprocessor-based system. The load sharing program balances the platform electrical load between the main generators and insures minimal variation of the system voltage and frequency. The load shedding function is activated if the main p o w e r system becomes overloaded b y addition of load or b y the loss of a generator. Preprogrammed m o t o r loads at the 13.8 kV and 4.16 kV level are then shed to restore the balance b e t w e e n generation capacity and load demand, thereby avoiding a main generation shutdown. CONTROL FACILITIES A system electrical control panel (ECP} is provided in the generator control room in module 18. The panel's mosaic mimic displays the status of the main generators and the distribution of power throughout the system. Centralized generator controls are provided in addition to circuit-breaker controls for b o t h 13.8 kV and 4.16 kV switchboards. Each generating set has its own unit control panel for individual sequencing and detailed status display. A remote selection switch on each unit control panel allows control from the ECP. Load sharing and load shedding functions are selectable at the ECP as well as voltage and frequency raise/lower controls. System status indications are also repeated to the platform central control r o o m (CCR). System graphic representations can be displayed there on the distributed control system video interface screens. The three main generating sets can be started from the CCR under auto-synchronizing sequence control. Other electrical system controls are inten-
tionally excluded in favor of concentrating CCR emphasis on process monitoring and control. The emergency generator control panel (EGCP) is another important control location of the electrical system. Voltage and frequency raise/lower controls for the main generators are duplicated on the EGCP in module 07. These controls are used in conjunction with manual synchronizing facilities to parallel a main generator with the emergency switchboard during a black start. In practice, a start would normally be initiated from the ECP and control would then be temporarily transferred to the EGCP for the paralleling operation. After paralleling the emergency generator would be taken off line.
SHUTDOWN SYSTEM INTERFACE Another important control function, which is critical to the safety of platform operation, is provided by the platform shutdown system. The shutdown system provides for the safe and orderly shutdown of equipment on detection of fire, gas and abnormal process operating conditions. An important function of the system is to isolate electrical power to motors and other equipment in a given area of the platform in accordance with predetermined hazard analyses. This is accomplished by hardwire interfaces between the distributed shutdown system and the p o w e r distribution system throughout the platform. Trip signals are connected to m o t o r and feeder control circuits at all voltage levels. The fact that 440 V MCCs, the shutdown system and the fire and gas system are all distributed systems greatly simplified the hardwire connection requirements. This also allowed for the majority of these interfaces to be commissioned and tested onshore. The overall combination is extensive but well coordinated and very effective. CONCLUSION A North Sea platform presents a unique challenge to the electrical engineering discipline. Within relatively small confines tremendous amounts of power are generated, distributed, and utilized. By its very nature an offshore platform demands an extremely
169
reliable electrical service. Its power system must be flexible and safe while requiring a minimum of attention. The Brae 'B' power system meets these requirements impressively by employing sophisticated state-of-the-art equipment. Innovative interconnections and backfeeds between power sources have helped contribute to the success of a rapid hook-up program. Attention paid to safe and proper application of equipment will no doubt insure its future operational success. ACKNOWLEDGEMENTS
The author wishes to express appreciation
to Marathon Oil Company and the Brae Field Participants, namely, Britoil PLC; Bow Valley Exploration {U.K.) Ltd., a wholly owned subsidiary of Bow Valley Industries Ltd.; Kerr-McGee Oil (U.K.) PLC, a wholly owned subsidiary of Kerr-McGee Corporation; Dyas Oil U.K.; LL & E (U.K.} Inc., a subsidiary of The Louisiana Land and Exploration Company; Sovereign Oil and Gas PLC; and Norsk Hydro Oil and Gas Ltd., for encouragement to publish this paper. Many thanks are also due to Mr. Brian Le Masurier of Marathon's London office for gathering much of the technical reference material.