DK201500424A1 - Energy generation and storage system for drilling rigs - Google Patents

Abstract

The present invention relates in power plan systems such as for drilling rigs with or without dynamic positioning.

Description

ENERGY GENERATION AND STORAGE SYSTEM FOR DRILLING RIGS

The present invention relates in power plan systems such as for drilling rigs with or without dynamic positioning.

BACKGROUND OF THE INVENTION

The present inventions relate to various improvements of energy generation and power supply systems of drilling rigs and similar microgrid applciations. In the field of oil well drilling, significant amount of power is required during the drilling activity. The power requirements as used on a drilling rig serve to supply the drawworks, multiple electric thrusters, mud pumps, top drives, rotary tables, dynamic braking systems and other peripheral electrical loads. Equipment utilized in oil well drilling activities often comprises oversized power systems to accommodate the "peak" power requirements of all loads coupled to the energy generation and storage system of the drilling rig. Historically, the number of typically active engines/generators is more than the number required by the load of the application due to redundancy and necessary peak power demand during certain phases of the operation as discussed below.

Drilling rings are normally provided with dynamic positioning systems comprising electrically driven propellers, i.e. electric thrusters, powered by the energy generation and storage system of the drilling rig. The task of the dynamic positioning systems is to keep a desired position of the drilling rig in the sea during oil and gas drilling operations, station keeping, anchoring, port maneuvering etc. For certain types of high risk operations where increased risk of oil pollution, loss of life, collisions etc. exists, it is vital that a desired position of the drilling rig can be maintained. Integrity and failure safe operation of energy generation and storage system which supplies the power to the multiple thrusters of the drilling rig are therefore of particular importance. The drilling rigs are classified in different classes such as DP2 (dynamic positioning 2) or DP3 (dynamic positioning 3). High risk operations such as drilling operations or the approach of other vessels may for example require a certain mode of operation for a particular vessel class. To ensure that a malfunction of a component does not lead to a complete blackout of the energy generation and storage system of the drilling rig, the energy generation system is often split into several power system sections, for example between 2 and 4 separate sections, in such high risk mode of operation. Each power system section is often located in a separate engine room which is isolated from the other engine room or rooms with fire proof and water tight walls. During such high risk operations, the power system sections are electrically isolated by opening connections provided by electric cables termed bus tie breakers. One or more engines/generators are active in each of the power system sections to supply electric power to connected loads such as the electric thrusters. Operating such energy generation and storage systems with interconnected power system sections, i.e. with connected bus tie breakers, is generally not possible since a fault, such as a short circuit or generator failure, will generally result in a total blackout of the energy generation and storage system. The blackout may result in a risk of loss of the position of the drilling rig.

One advantage of the present energy generation and storage system is that the impact of a single point failure in a power system section is minimized. The power system section comprises an AC bus subsection divided into first and second segments which are electrically connected through a pair of series connected bus tie breakers housed in physically separate switchboard compartments to selectively connect and disconnect the first and second segments of the AC bus subsection. If one of the first and second switchboard compartments is flooded, or otherwise damaged and faulty, by a single point failure the other switchboard compartment together with its associated segment of the AC bus subsection, AC generator and electric thruster may be undamaged. Hence, the single point failure leads merely to the loss of a single AC generator and a single thruster drive (and its associated thruster).

The previously discussed peak power demands of the drilling rig energy generation and storage system exist during the so-called "tripping" of the pipe or drill stem. During normal operations, there is a base load of lighting, pumps, agitators, mixers, air compressors, etc. on the energy generation and storage system of the drilling rig. This base load can make up typical loads of 400-600 kilowatts. Mud pumps, top drives and rotary tables contribute another fairly consistent KW demand. This demand will vary based on the particular well, depth of drilling, and material being drilled. During oil well drilling activities, the most intermittent load is the drawworks. This intermittent load is directed toward the peak power demand during the raising or lowering of the drill pipe upwardly and downwardly in the well. This peak demand can have loads 2-3 times larger than the base loads of the other demands on the drilling rig. When drilling and at times when the downhole tool has to be inspected or changed, it is required to pull the entire drill pipe from the hole. This distance can be 10,000 feet or more. The drill pipe must be taken apart and stacked as it is being removed. After repair or replacement, the reverse procedure must take place so as to reinsert all the components back to the desired depth. During the tripping in or out of the hole, the driller (operator) demands extreme power consumption in very short power bursts as the driller raises (or lowers) the string of drill pipe. Since there is a limitation on the height of the drilling mast, the operator must lift the sections in increments and unscrew the different sections. These sections are stacked one at a time. This process is repeated during the reinsertion of the drill pipe back into the hole. This process is referred to as "making a trip". The intermittent high power demand occurs when this load (300,000 pounds or more) is applied to the electric motor or motors lifting the pipe sections over and over again. The load is variable since the weight of the drill stem becomes less and less as pipe sections are removed. The base load requirements for the drilling rig are approximately 600-800 KW. The peak demand can be 1.5 MW and as high as 2.0 MW. Because of these power requirements, the emissions of the engines/generators for a typical drilling rig are quite high. To deliver such short power bursts without overloading the active engines/generators or requiring an excessive number of simultaneously active engines/generators it is advantageous to deliver power or energy for these short power bursts from an energy storage assembly via a common DC bus of the energy generation and storage system. The energy storage assembly may for example comprise a flywheel energy storage device which possesses several advantageous properties for drilling rig energy generation and storage systems such as high energy density, long lifetime, rapid energy charging and large maximum power output.

SUMMARY OF THE INVENTION A first aspect of the invention relates to a power generation and distribution system for a drilling rig, comprising two or more electrically interconnectable power system sections, wherein each power system section comprises: an AC bus subsection electrically connectable to a common AC bus of the power generation and distribution system through first and second bus tie breakers connected to respective ends of the AC bus subsection to selectively disconnect and connect the AC bus subsection to the common AC bus, a first ac generator electrically connected to a first segment of the AC bus subsection through a first generator circuit breaker, a first thruster drive electrically connected to the first segment of the AC bus subsection through a first thruster circuit breaker, a second ac generator connected to a second segment of the AC bus subsection through a second generator circuit breaker, a second thruster drive electrically connected to the second segment of the AC bus section through a second thruster circuit breaker, a third bus tie breaker and a fourth bus tie breaker connected in series between the first and second segments of the AC bus subsection to selectively connect and disconnect the first and second segments of the AC bus subsection, wherein the third bus tie breaker is housed in a first switchboard compartment and the fourth bus tie breaker is housed in a second and physically separate switchboard compartment.

Power generation and distribution system in accordance with the present invention may be utilized on various types of drilling rigs such as drillship, semi-submersible rig, jack-up rig, barge or landrig etc.

The first and second switchboard compartments are preferably physically isolated from each other with by fire proof and water tight walls of the compartments. There are several noticeable advantages by the use of series connected third and fourth bus tie breakers housed in separate switchboard compartments to separate the first and second segments of the AC bus subsection instead of using a single physical compartment for housing a single bus tie breaker as used in prior art drilling rig power distribution systems. If one of the first and second switchboard compartments is flooded, or otherwise damaged and left in-operational, by a single point failure the other switchboard compartment together with its associated segment of the AC bus subsection and AC generator may be left undamaged. Hence, in the present power generation and distribution system the single point failure leads merely to the loss of a single AC generator and a single thruster drive (and its associated thruster) connected thereto instead of the loss of both the first and second AC generators and the first and second thruster drives (and their associated thrusters) as in prior art drilling rig power distribution systems. The loss of a single thruster of the drilling rig instead of two thrusters by a single point failure in the present power system section leads to numerous advantages such as a marked decrease of the minimum power requirement to each of the typically four to eight individual thrusters of the drilling rig as discussed in additional detail below in connection with the appended drawings. A second aspect of the invention relates to a power generation and distribution system for a drilling rig, comprising two or more electrically interconnectable power system sections. Each of the power system sections comprises: a first AC bus subsection electrically connectable to a common AC bus of the power generation and distribution system through first and second bus tie breakers connected to respective ends of the first AC bus subsection to selectively disconnect and connect the first AC bus subsection to the common AC bus, a first ac generator electrically connected to the first AC bus subsection through a first generator circuit breaker, an AC bus electrical load, such as a first thruster driver, electrically connected to the first AC bus subsection through a first load circuit breaker, a first three phase power transformer electrically connected between the first AC bus subsection and a first DC bus subsection through a first three phase AC-DC power converter, wherein the first three phase power transformer comprises a plurality of primary side transformer windings and a plurality of secondary side transformer windings wound around a common magnetic core to convert three primary side voltage phases of the first AC bus subsection into three corresponding secondary side voltage phases, one or more DC bus electrical loads such as a drawworks motor, a mud pump motor, a cement pump motor, a rotary table motor etc., electrically connected to the first DC bus subsection, a first DC-AC power converter coupled from the first DC bus subsection to an auxiliary transformer winding of the first three phase power transformer to energize the auxiliary transformer winding from the first DC bus subsection, wherein the auxiliary transformer winding is wound around the common magnetic core to energize the plurality of primary side transformer windings and supply power to the first AC bus subsection.

The auxiliary transformer winding of the first three phase power transformer allows the first DC bus subsection to energize the first AC bus subsection, via the first DC-AC power converter, and hence supply power to the AC bus electrical load or loads such as a first thruster driver and associated thruster. This feature has several advantages. The first AC bus subsection may be powered through the auxiliary transformer winding during temporary generator failure such that power supply to the AC bus electrical load or loads remains intact. This is particularly helpful in an embodiment where the power system section comprises a first energy storage assembly connected to the first DC bus subsection to selectively supply power to the first DC bus subsection and absorb power from the first DC bus subsection in accordance with a supply control signal. The first energy storage assembly may comprise a flywheel energy storage device and an appropriate power converter operating in accordance with the supply control signal. In the latter embodiments, energy stored in the first energy storage assembly may be directed through the first DC-AC power converter and auxiliary transformer winding to provide the power to the first AC bus subsection during the temporary generator failure. The energy storage capacity of the flywheel energy storage device may be sufficiently large, such as more than 500 MJ, to power large AC bus electrical loads for a prolonged period of time. In one embodiment of the invention, the flywheel energy storage device is configured to power a 5 MW thruster of the drilling rig for at least 5 minutes during a failure of the first ac generator that would have left the the first AC bus subsection powerless without the alternative power supply through the auxiliary transformer winding of the first three phase power transformer. BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the invention will be described in more detail in connection with the appended drawings, in which: FIG. 1) is a schematic diagram of a drilling rig energy generation and storage system in accordance with a preferred embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 below is a simplified schematic diagram of a first power system section 100 of a power generation and distribution system for a drilling rig in accordance with a preferred embodiment of the present invention. The power generation and distribution system may comprise two or more electrically interconnectable power system sections in total where each of these power system sections may be substantially identical to the illustrated first power system section 100. A second power system section and third power system section may for example be connected to right side and left side, respectively, of the first power system section 100 via respective AC bus connections and DC bus connections as discussed in additional detail below.

The first power system section 100 comprises a first AC bus tie breaker 102a which electrically connects a first segment 104 of a first AC bus subsection of the first power system section 100 to a corresponding AC bus subsection of the second power system section (not shown) through tie breaker 162b of the second power system section. A first DC bus tie breaker 160 is configured to electrically connect a first segment 114 of the DC bus subsection to a corresponding DC bus subsection of the second power system section. Likewise, a second AC bus tie breaker 122a electrically connects a second segment 124 of the AC bus subsection (AC BUS) of the first power system section 100 to a corresponding AC bus subsection of the third power system section (not shown) through tie breaker 142a of the third power system section. A second DC bus tie breaker 140 electrically connects a second segment 134 of the DC bus subsection (DC BUS) to a corresponding DC bus subsection of the third power system section. The skilled person will understand that the first and second segments 104,124 of the first AC bus subsection and the corresponding bus segments of the additional AC bus subsections of the second and third power system sections may form part of a common AC bus of the power generation and distribution system. Likewise, the first and second segments 114, 134 of the first DC bus subsection and the corresponding bus segments of the additional DC bus subsections of the second and third power system sections may form part of a common DC bus of the power generation and distribution system. In one exemplary embodiment of the present power generation and distribution system the respective AC bus subsections of the two or more power system sections are electrically connected in a so-called ring configuration such that the common AC bus possesses a ring configuration. The respective DC bus subsections of the two or more power system sections are preferably also electrically connected in a ring configuration such that the common DC bus possesses a ring configuration. The ring configuration has the advantage that even if a single AC bus subsection such as the illustrated first AC bus subsection 104, 124 is disconnected from the common AC bus by the second and third AC bus tie breakers 102a, 122a arranged at respective ends of the of the first AC bus subsection all the remaining AC bus subsection(s) may remain powered. The second and third AC bus tie breakers 102a, 122a are operable to selectively disconnect and connect the first AC bus subsection to/from the common AC bus and may be controlled by a suitable central power management/supervision controller (not shown). The first AC bus subsection may be disconnected from the common AC bus for various reasons for example in the event of an AC generator GEN 1 failure or a short circuit on a load or component connected to the first AC bus subsection. The first power system section 100 additionally comprises the first AC generator GEN 1 which is electrically connected to the first segment 104 of the AC bus subsection through a first generator circuit breaker 113. The first AC generator GEN 1 may comprise a synchronous three phase generator which at least energizes the first segment of the AC bus subsection 104 during operation of the first power system section 100 unless the circuit breaker 113 is active to disconnect the first AC generator. The first AC generator GEN may be configured to generate an AC voltage level between 5 kV and 15 kV such as around 11 kV (for each phase of three voltage phases of the AC bus) on the first segment of the AC bus subsection 104. A first thruster drive 115 is electrically connected to the first segment of the AC bus subsection 104 through a first thruster circuit breaker 112. The first thruster drive 115 may comprise a variable frequency drive (VFD) or AC-AC power converter to drive an electric motor (not shown) of the first thruster Th1 of the drilling rig. The VFD unit is configured for converting a frequency of the supplied AC voltage on the first segment 104 of the AC bus subsection to an appropriate frequency for operating the first thruster drive 115. The first thruster drive 115 supplies AC power to rotate a propeller of the first thruster Th1. The skilled person will understand that further loads such as a low voltage power distribution bus (not shown) may be connected to and powered by the first segment 104 of the AC bus subsection. The low voltage power distribution bus may comprise a low voltage AC bus or a low voltage DC bus with an voltage level around 690 V and be used to power various peripheral power consumers of the drilling rig such as electrical lighting systems, kitchen appliances etc.

The first power system section 100 additionally comprises a second AC generator GEN 2 which is electrically connected to the second segment of the AC bus subsection 124 through a second generator circuit breaker 133. The second AC generator GEN 2 may be substantially identical to the first AC generator GEN 1 and operational to at least energizes the second segment of the AC bus subsection 124 during operation of the first power system section 100 unless the circuit breaker 133 is active to disconnect the second AC generator. A second thruster drive 135 is electrically connected to the second segment of the AC bus subsection 104 through a second thruster circuit breaker 132. The skilled person will understand that the second thruster drive 135 and a second thruster Th6 connected thereto may be substantially identical to the first thruster drive 115 and first thruster Th1 discussed above.

The first and second segments 104,124 of the AC bus subsection are electrically connected through series connected third and fourth bus tie breakers 102b, 122b, respectively. The third and fourth bus tie breakers 102b, 122b are accordingly operational to selectively connect and disconnect the first and second segments 104,124 of the AC bus subsection. The third bus tie breaker 102b is arranged or housed inside a first switchboard compartment 106 while the fourth bus tie breaker 122a is arranged or housed inside a second and physically separate switchboard compartment 126. The first and second switchboard compartments 106,126 are preferably physically isolated from each other with by fire proof and water tight walls of the compartments. There are several noticeable advantages by the use of two series connected bus tie breakers 102b, 122b housed in two separate switchboard compartments 106,126 to separate the first and second segments 104,124 of the AC bus subsection, instead of a single physical compartment housing a single bus tie breaker as used in prior art drilling rig power distribution systems. In case of a single point failure where one of the first and second switchboard compartments 106, 126 is flooded or otherwise damaged and left non-functional, the other switchboard compartment may be undamaged such that the AC generator and thruster connected to the still functional segment of the AC bus subsection remain operative. Hence, the single point failure only leads to the loss of only a single AC generator and a single thruster connected thereto in the present power generation and distribution system instead of a loss of both the first and second AC generators and the first and second thrusters associated therewith as in prior art drilling rig power distribution systems. The first and second AC generators GEN 1, GEN 2 may be housed in a shared generator room 117 as schematically indicated on the drawing.

To appreciate the advantages of the separate switchboard compartments one can consider a typical drilling rig power generation and distribution system that may comprise 6 thrusters connected to, and powered by the common AC bus of the drilling rig power generation and distribution system. In addition, the common AC bus of such a typical drilling rig power generation and distribution system is powered by six AC generators each possessing a maximum power generation capacity of 9 MW. The minimum requirement to comply with DP3 operation mode is the presence of 26 MW of thruster power in response to a single point failure such as flooding of a switchboard compartment. Hence, a flooding of the switchboard compartment in the above-discussed prior art drilling rig power distribution systems leads to the loss of two thrusters such that the minimum power requirement of each of the six thrusters is 6.5 MW. In contrast, the same single point failure in the power generation and distribution system in accordance with the present invention only leads to the loss of the single thruster associated with the single failing switchboard compartment such that the minimum power requirement of each of the six thrusters is reduced to 5.2 MW. This marked decrease of thruster power requirement leads to smaller dimensions, considerably reduced costs and better power efficiency of each to the six thrusters.

The skilled person will understand that even if the shared generator room 117, where the first and second AC generators GEN 1, GEN 2 are situated, there will be enough generator capacity to power the residual five thrusters because the remaining functional AC generators connected to the common AC bus, after the disconnection of the first and second AC generators GEN 1, GEN 2 (by activating the first and second generator circuit breakers 113,133), still possess a power generation capacity of 36 MW.

In some embodiments the power system comprises two or more power system sections according to any of the claims, such as three or more, such as 4 or more. Typically a drilling rig will have 6 generators so that with three power system sections each generator will have an allocated segment of a power system section. In some embodiments each set of the first and second generators from each power system section are housed shared generator room which is typically fire and/or water tight, so that with a rig having 6 generators these will be placed in three generator rooms. However, further safety and/or freedom of designing capacity when considering a single-point failure may be obtained by placing each generator (also referred to as genset) in a separate room so that the first and second generators housed are separated from each other in a separate generator room which is typically fire and/or water tight.

One embodiment of the first power system section 100 of the present power generation and distribution system comprises a DC bus subsection that is powered from the first and second segments 104,124 of the first AC bus subsection via first and second first three phase power transformers 101,121 as illustrated on FIG. 1. This embodiment of the invention comprises the first three phase power transformer 101 electrically connected between the first segment 104 of the AC bus subsection and a first segment 114 of the DC bus subsection through a first three phase AC-DC power converter 103. The skilled person will understand that one or more DC bus electrical loads M1, M2 such as a drawworks motor, a mud pump motor, a cement pump motor, a rotary table motor etc. may electrically connected to the first segment 114 of the DC bus subsection for example via respective DC circuit breakers (not shown). The skilled person will understand that the one or more DC bus electrical loads M1, M2 may be connected to the DC bus subsection via respective unidirectional DC-AC power converters 105,105a. Each of the DC-AC power converters 105, 105a may comprise a variable frequency and/or variable output voltage type of converter. Likewise, a second three phase power transformer 121 is electrically connected between the second segment 124 of the AC bus subsection and a second segment 134 of the DC bus subsection through a second three phase AC-DC power converter 123. The skilled person will understand that one or more further DC bus electrical loads 107 (M1, M2...) such as a drawworks motor, a mud pump motor, a cement pump motor, a rotary table motor etc. may be electrically connected to the second segment 134 of the DC bus subsection for example via respective DC circuit breakers (not shown). A DC segment circuit breaker 110 is configured to selectively electrically connect and disconnect the first and second segments 114,134 of the DC bus subsection. The DC segment circuit breaker 110 may be controlled by the previously discussed central power management or supervision controller and the same applies for the previously discussed first and second DC bus tie breaker 140,160 arranged at respective ends of the DC bus subsection. This feature allows the central power management/supervision controller to electrically isolate or insulate each of the first and second segments 114, 134 of the DC bus subsection from each other and/or electrically isolate the entire DC bus subsection from the common DC bus of the power generation and distribution system if or when needed.

The first segment 114 of the DC bus subsection comprises a first energy storage assembly 109, 120 electrically connected to the first segment 114 of the DC bus subsection to selectively supply power to the first segment 114 of the DC bus subsection and absorb power from the first segment 114 of the DC bus subsection in accordance with a supply control signal (not shown). The first energy storage assembly preferably comprises a flywheel energy storage device 120 and may comprise other types of energy storage device such as rechargeable batteries. The flywheel energy storage device 120 is electrically connected to the first segment 114 of the DC bus subsection through a bi-bidirectional AC/DC power converter 109 allowing the assembly to either supply power to the DC bus subsection to for example feed power to the DC bus electrical loads M1, M2 etc. or absorbing power from the DC bus subsection and storing absorbed power in the flywheel 120 as kinetic energy via an integral ac motor/generator coupled to a rotating wheel member or members of the flywheel 120.

The presence of the first energy storage assembly with the bi-directional bidirectional power transfer capability to the DC bus subsection entails numerous advantages. The first energy storage assembly is capable of nearly instantaneously supply large currents, Upike, onto the common DC bus subsection and thereby effectively suppress voltage spikes on the DC bus caused by the previously discussed intermittent peak power loads drawn by one or more of the DC bus electrical loads, in particular the drawworks motor. The first energy storage assembly 109,120 may be charged from power on the DC bus subsection 114 generated by the ac generator GEN 1 and supplied via the AC bus subsection, the first three phase power transformer 101 and the first three phase AC-DC power converter 103. The first energy storage assembly 109,120 may additionally or alternatively be charged by regenerative energy captured from braking energy of the one or more of the DC bus electrical loads M1, M2 etc. such that the power production requirements on the ac generator GEN 1 decreases and its fuel consumption lowered. The first energy storage assembly 109,120 is also important to various safety issues with well control and circulation of drilling mud and drawworks control because of the ability of the storage assembly to supply back-up power to the DC bus subsection 114 for a prolonged period of time if the ordinary power supply from the ac generator GEN 1 (through the three phase high voltage transformer 101) for any reason fails. The first energy storage assembly 109,120 preferably comprises a flywheel device 120 as mentioned above which adds further advantageous properties to the energy storage assembly for example a high energy density, long lifetime, rapid charging and large maximum power output. The large maximum power output of the flywheel based energy storage system makes it very effective in suppressing the previously discussed voltage spikes on the DC bus subsection. The flywheel energy storage device 120 may possess a peak power delivery capability larger than 2.0 MW, i.e. 3000 A at 720 V DC onto the common DC bus. The flywheel energy storage device 120 may possess an energy storage capacity of more than 500 MJ such as more than 1200 MJ and may be designed to meet a particular driving time of the thrusters Th1 and Th6.

Finally, the first energy storage assembly 109,120 is highly useful for temporarily powering the first segment 104 of the AC bus subsection through a failure of ac generator GEN 1 in connection with a second aspect of the invention. This second aspect of the invention concerns a novel design of the three phase high voltage transformer 101 as described in further detail below.

As described above, the first three phase power transformer 101 is electrically connected between the first segment 104 of the AC bus subsection and the first segment 114 of the DC bus subsection through the first three phase AC-DC power converter 103. Hence, during normal operation of the first power system section 100 the DC bus subsection 114 is powered by the first segment 104 of the common AC bus which in turn is powered by the first ac generator GEN 1, or one or more of the residual AC generators GEN 2-6 depending how many of these that are active at any particular moment of operation. The first three phase power transformer 101 may comprise a plurality of primary side transformer windings and a plurality of secondary side transformer windings (not shown) to convert three individual voltage phases of the AC bus subsection applied to three primary side windings into three corresponding voltage phases on three secondary side windings. The plurality of primary side and secondary side transformer windings are preferably wound around a common magnetic core. The common magnetic core may comprise a single laminated magnetic core. The AC voltage level on the three secondary side windings is lower than the AC voltage level on the three primary side windings. The three secondary side windings of the transformer 101 are connected to respective inputs of the first three phase AC-DC power converter 103. However, the first three phase power transformer 101 comprises an auxiliary transformer winding 101a in addition to the plurality, e.g. six, primary side and secondary side transformer windings which could be considered main transformer windings. This auxiliary transformer winding 101a is preferably wound around the common magnetic core such that ac voltage and current applied to the auxiliary winding 101a is coupled to each of the three individual primary side transformer windings on the primary side of the transformer 101. The three individual primary side transformer windings are thereby energizing the individual voltage phases of the first segment 104 of the AC bus subsection. The auxiliary transformer winding 101a is supplied with ac voltage and current from the first segment 114 of the DC bus subsection through a second DC-AC power converter 108 coupled between the first segment 114 of the DC bus subsection and the the auxiliary transformer winding 101a. The flow of this ac voltage and current through the auxiliary winding 101a is schematically indicated by current arrow laUx. The previously discussed central power management/supervision controller may be configured to control when the second DC-AC power converter 108 is activated such that the auxiliary winding 101a is energized. The central power management controller may for example monitor an AC voltage level on the first segment 104 of the AC bus subsection and activate the second DC-AC power converter 108 if the ac voltage level falls below a certain voltage threshold or other suitable criterion. An ac voltage level below this voltage threshold may indicate that GEN 1 is disconnected or failing such that the AC bus subsection is left without power supply. In response to this condition, the central power management controller may decide to activate the second DC-AC power converter 108 and energize the first segment 104 of the AC bus subsection via the auxiliary winding 101a such that the appropriate or nominal ac voltage level is reestablished. Under these circumstances, the first segment 114 of the DC bus subsection may be energized by energy stored in the flywheel energy storage device 120 through power converter 109. The capability of the first three phase power transformer 101 to power the first segment 104 of the common AC bus through the auxiliary winding 101a during generator GEN 1 failure has several noticeable advantages. One advantage is that the first three phase AC-DC power converter 103 may be a unidirectional power converter without the ability to transmit “reverse” power from the DC bus subsection 114 to the AC bus subsection 104. This reverse transfer of power may instead be handled by the auxiliary transformer winding 101a and the second DC-AC power converter 108. The size and costs of a unidirectional version of the three phase AC-DC power converter 103 are markedly lower than the costs of a bi-directional counterpart of the same. While the second DC-AC power converter 108 is an additional component, the power rating of the latter converter can often be much smaller than the power rating of the three phase AC-DC power converter 103, because markedly less power is often needed in the reverse direction. Another noticeable advantage of the ability of the first power system section 100 to transmit “reverse” power from the DC bus subsection 114 to the AC bus subsection 104 is the ability to increase average loading of each of the active ac generators GEN 1 -GEN 6 and thereby achieve a reduction of the number of simultaneously active ac generators. The reduction of the number of simultaneously active ac generators is achieved because of the energy back-up from the flywheel energy storage device 120 in conjunction with the ability to selectively transfer this energy when required to the AC bus subsection 104 via the second DC-AC power converter 108 and auxiliary transformer winding 101 a. The energy back-up reduces the required peak power demand from the active ac generators GEN 1 - GEN 6 of the present drilling rig power generation and distribution system because energy for intermittent power peaks or spikes on the AC bus subsection 104 is supplied by the energy back-up instead of the active ac generators. Hence, the present drilling rig power generation and distribution system may operate with a smaller number of active ac generators each running with a higher average loading than conventional power systems for drilling rigs. The higher average loading of the active ac generators increases the efficiency of each active ac generator and reduces its power consumption and pollution.

In addition, the energy back-up from the flywheel energy storage device 120 may be used to power the previously discussed low voltage power distribution bus connected to the AC bus subsection 104 during tripping or failure of the first ac generator GEN 1. In this manner the power supply to the various peripheral power consumers of the drilling rig such as electrical lighting systems, kitchen appliances etc. remains undisturbed of the generator failure.

More importantly, the flywheel energy storage device 120 may in one embodiment of the invention possess sufficient energy to drive one or both of the thrusters Th1 and Th6 by energizing the first segment 104 AC bus subsection and, optionally, the second segment 114 of the AC bus subsection for predetermined amount of time during an emergency state of the power system for example a so-called Emergency Shut Down (ESD) of the power system. The powering of the thrusters Th1 and Th6, and possibly other thrusters, is critical under numerous drilling rig operations that require the ability to dynamically maintain a certain position of the drilling rig in the sea. The second energy storage assembly 129,140 comprising a second flywheel energy storage device 140 is electrically connected to the second segment 134 of the DC bus subsection and may be operative to energize the second segment 124 of AC bus subsection through an auxiliary transformer winding 121a of the second three phase power transformer 121 in a corresponding manner to the first three phase power transformer 101. In this manner, the first and second flywheel energy storage devices 120,140 may cooperate to powering of the first and second thrusters Th1 and Th6 by energizing the first and second segments 104,124 of AC bus subsection.

Claims (7)

1. A power generation and distribution system for a drilling rig, comprising two or more electrically interconnectable power system sections, wherein each power system section comprises: a first AC bus subsection electrically connectable to a common AC bus of the power generation and distribution system through first and second bus tie breakers connected to respective ends of the first AC bus subsection to selectively disconnect and connect the first AC bus subsection to the common AC bus, a first ac generator electrically connected to the first AC bus subsection through a first generator circuit breaker, an AC bus electrical load, such as a first thruster driver, electrically connected to the first AC bus subsection through a first load circuit breaker, a first three phase power transformer electrically connected between the first AC bus subsection and a first DC bus subsection through a first three phase AC-DC power converter, wherein the first three phase power transformer comprises a plurality of primary side transformer windings and a plurality of secondary side transformer windings wound around a common magnetic core to convert three primary side voltage phases of the first AC bus subsection into three corresponding secondary side voltage phases, one or more DC bus electrical loads such as a drawworks motor, a mud pump motor, a cement pump motor, a rotary table motor etc., electrically connected to the first DC bus subsection, a first DC-AC power converter coupled from the first DC bus subsection to an auxiliary transformer winding of the first three phase power transformer to energize the auxiliary transformer winding from the first DC bus subsection, wherein the auxiliary transformer winding is wound around the common magnetic core to energize the plurality of primary side transformer windings and supply power to the first AC bus subsection.

2. A power generation and distribution system according to claim 19, further comprising a voltage controller configured to: monitoring an AC voltage level on the first AC bus subsection, selectively activate and deactivate the first DC-AC power converter based on the AC voltage level such that the auxiliary transformer winding is energized by the the DC-AC power converter if the AC voltage level meets a predetermined voltage criteria.

3. A power generation and distribution system according to claim 19 or 20, wherein the first three phase AC-DC power converter is a unidirectional power converter configured for transmitting power from the first three phase power transformer to the the first DC bus subsection.

4. A power generation and distribution system according to any of claims 19-21, further comprising: a first energy storage assembly connected to the first DC bus subsection to selectively supply power to the first DC bus subsection and absorb power from the first DC bus subsection in accordance with a supply control signal.

5. A power generation and distribution system according to claim 22, wherein the first energy storage assembly comprises one or more energy storage devices selected from a group of [a rechargeable battery, a capacitor, a flywheel].

6. A power generation and distribution system according to claim 23, comprising a flywheel energy storage device having an energy storage capacity of more than 500 MJ, preferably more than 1200 MJ (driving the first and second thrusters (each 4-6 MW) to 50 % maximum power for at least 5 minutes).

7. A power generation and distribution system according to any of claims 19-24, wherein respective AC bus subsections of the two or more power system sections are electrically connected to the common AC bus in a ring configuration; and wherein respective DC bus subsections of the two or more power system sections are electrically connected to the common DC bus in a ring configuration.