ELECTRICAL POWER TRANSMISSION SYSTEM

FIELD OF INVENTION

The present invention relates to an electrical power transmission system and more particularly, an electrical power transmission system which utilizes High Voltage Direct Current (HVDC) technology for transmitting power to underwater systems and equipment such as a modular seabed processing system.

BACKGROUND OF THE INVENTION High Voltage Direct Current (HVDC) is an electrical power transmission technology whereby electrical power is converted from Alternating Current (AC) to Direct Current (DC) at the transmitting end and inverted from DC to AC power at the receiving end. The HVDC technology is a solution to transmit AC electrical power over long distance (more than 100 kilometers) in which directly transmitting AC power over long distance is impracticable due to excessive power loss. Moreover, HVDC technology allows power transmission between unsynchronized AC distribution systems. Thus, HVDC increases system stability by preventing cascading failures from propagating from one part to another of a wider power transmission grid, whilst still allowing power to be imported or exported in the event of smaller failures.

Since the first commercial installation of HVDC in Sweden on 1954, a huge amount of HVDC transmission systems have been installed around the world. In Itaipu, Brazil, HVDC was chosen to supply a total rated power of 6300 MW from the Itaipu hydropower plant to the AC distribution network in Sao Paulo which is the industrial centre of Brazil. The Itaipu HVDC transmission consists of two bipolar DC transmission lines, transmitting DC power at 500 kV over 800 km to Sao Paulo, where it is converted back to AC to suit the Brazilian power network. Another implementation of HVDC system is the Leyte-Luzon Project in Philippines whereby power is transmitted from a geothermal power plant on the island of Leyte to the southern part of the main island of Luzon in order to feed the existing AC distribution network within the region of Manila. In the oil and gas

industry, HVDC is used to supply power from a mainland power source or offshore platform to other offshore platforms.

As for subsea systems, the existing conventional subsea systems are powered and controlled by electro-hydraulic systems. These conventional subsea systems have step-out distance limitations, while, pressure losses and response time increase proportionally to the length of the umbilical which connect the subsea systems to a main system or an offshore platform. Hence, there has been a shift towards subsea systems that are electrically powered and controlled. Numerous different approaches have been implemented to provide all electrical subsea systems; however, all of these approaches have technical limitations.

In one approach, AC power transmission is used to supply power to the subsea system, though, these AC voltages transmission are constrained by the availability of suitably rated underwater connectors. Thus, these effectively restrict the step-out distance for subsea AC power transmission to more or less 80 km.

In another approach, DC power transmission is used by solutions such as Cameron DC™ which is suitable for low power applications. Furthermore, power buoys are used to self-generate power locally to supply power to subsea systems. These power buoys include diesel powered generators and diesel storage tanks in addition to the control, communications and chemical injection equipments that are integrated to the power buoys. However, there is a need for a regular maintenance on all of the equipment integrated in the power buoys and thus, leading to the need for accessing both the power buoys and the equipments within and around the buoys for maintenance. Accessing to the buoy and maintenance activities depend on the weather, as this can only be undertaken during calm weather.

Gas turbines have also been introduced as an alternative to the diesel powered generators in the power buoys. The gas turbines can be fueled via a small conduit from a

seabed system, thereby avoiding the need for a storage tank in the power buoys and this also eliminates the need for refueling. Though, gas turbines require more maintenance than diesel generators.

Alternatively, the inversion of the DC power transmitted over long distances could be done by locating the DC to AC inverter and transformer at subsea level. The problem arises as the large size of the equipments are required to be housed in a 1 bar environment. In addition, the inversion of the DC power to AC power causes the equipments to release heat and thus, a cooling means is also required to prevent the equipments from overheating. As the electrical equipments are located underwater, a large numbers of wet- mateable connections are required which will adversely affect the reliability of the electrical power transmission system.

Hence, a need still exists in the art of transmitting electrical power over long distance to underwater systems such as modular seabed processing system. Additionally, a need still exists to provide an electrical power transmission which requires less frequent maintenance.

SUMMARY OF INVENTION The present invention discloses an electrical power transmission system which utilizes HVDC technology for transmitting power to at least one underwater system. The system comprises a DC power generation system, wherein the DC power generation system is connected to an external power source for converting AC power to DC power; a conduit means to transmit DC power to a DC to AC inverter positioned above water; and a transformer, wherein the transformer is connected to the DC to AC inverter. Moreover, the system is characterized in that the DC to AC inverter and the transformer are connected to a local control station; and wherein the DC to AC inverter, the transformer and the local control station are arranged in a remote power conversion facility; and wherein at least one umbilical cable is connected to transmit AC power from the remote power conversion facility to the at least one underwater system; and wherein the local control station is

configured for monitoring and controlling the DC power transmitted from the DC power generation system, monitoring and controlling the at least one underwater system through the at least one umbilical cable, transmitting and/or receiving data from a master control station, and transmitting and/or receiving data from the at least one underwater system.

In another aspect of the present invention, an electrical power transmission system which utilizes HVDC technology for transmitting power to at least one modular seabed processing system is provided. The system includes a DC power generation system, wherein the DC power generation system is connected to an external power source for converting AC power to DC power; a conduit means to transmit DC power to a DC to AC inverter positioned above water; and a transformer, wherein the transformer is connected to the DC to AC inverter. Moreover, the system is characterized in that the DC to AC inverter and the transformer are connected to a local control station; and wherein the DC to AC inverter, the transformer and the local control station are arranged in a remote power conversion facility; and wherein at least one umbilical cable is connected to transmit AC power from the remote power conversion facility to the at least one modular seabed processing system; and wherein the local control station is configured for monitoring and controlling the DC power transmitted from the DC power generation system, monitoring and controlling the at least one modular seabed processing system through the at least one umbilical cable, transmitting and/or receiving data from a master control station, and transmitting and/or receiving data from the at least one modular seabed processing system.

A method of transmitting electrical power to at least one underwater system by utilizing the electrical power transmission system in accordance to the present invention is also provided. The method comprises the steps of generating DC power by means of a DC power generation system; transmitting DC power to a remote power conversion facility; receiving the DC power transmitted from the DC power generation system and converting the DC power to AC power; and transmitting the AC power to the at least one underwater system through at least one umbilical cable.

A method of transmitting electrical power to at least one modular seabed processing system by utilizing the electrical power transmission system in accordance to the present invention is also provided. The method comprises the steps of generating DC power by means of a DC power generation system; transmitting the DC power to a remote power conversion facility; receiving the DC power transmitted from the DC power generation system and converting the DC power to AC power; and transmitting the AC power to the at least one modular seabed processing system through at least one umbilical cable.

Other features are inherent in the method and apparatus disclosed or will become apparent to those skilled in the art from the following detailed description of embodiments and its accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of an electrical power transmission system in accordance with the present invention.

FIG. 2 is a block diagram of an electrical power transmission system in accordance with the present invention.

FIG. 3 is a block diagram of an electrical power transmission system suitable for controlling an electro-mechanical underwater system in accordance with the present invention.

FIG. 4 is a diagram of an electrical power transmission system suitable for a modular seabed processing system.

FIG. 5 is a schematic diagram of a modular seabed processing system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well known functions or constructions are not described in detail since they would obscure the description with unnecessary detail.

Reference is made initially to FIG. 1 which illustrates an electrical power transmission system (100) arranged in accordance with certain illustrative embodiments of the present invention to implement the principles thereof. The electrical power transmission system (100) is installed to provide electrical power to an underwater system (130) such as a modular seabed processing system. The electrical power is supplied from an offshore platform to the underwater system (130) over distances of more than 100 km. The electrical power transmission system (100) includes a host station (110), a remote power conversion facility (120) and an Integrated Services Umbilical or ISU (150). The host station (110) transmits the DC power to the remote power conversion facility (120) through a submarine power cable (140). The cable (140) is designed with the cable core optimized to reduce power loss during DC power transmission, while the cable insulation is able to withstand the underwater environment. The conversion from AC to DC power (provided by the DC power generation system (112)) and inversion from DC to AC power (provided by the DC to AC inverter (123)), within the system (100) utilizes either with Natural Commutated Converters, Capacitor Commutated Converters, Voltage Source Converters or any other means for the purpose of converting from AC to DC power and inverting from DC to AC power.

FIG. 2 shows a block diagram of the electrical power transmission system (100) in accordance with the present invention. The host station (110) which is located at an offshore platform includes a Master Control Station or MCS (111) and a DC power generation system (112). The MCS (111) is a monitoring and control station whereby it acts as a means of communicating with a Local Control Station or LCS (121) which is

located on the remote power conversion facility (120). The communication between the MCS (111) and LCS (121) is through a communication link which may include satellite communication, line-of-sight, high frequency (HF) radio systems or any other communication link technologies. The MCS (111) is connected to the DC power generation system (112) to obtain the required DC power for transmission along the DC power line to the remote power conversion facility (120). In the event that a fault is detected at the remote power conversion facility (120), the LCS (121) transmits the fault information to the MCS (111). Thereon, the MCS (111) will take the necessary action based on the information received; this action may include ceasing power transmission to the remote power conversion facility (120) by shutting down or halting the DC power generation system (112). The MCS (111) may also be connected to another monitoring system (not shown in the accompany drawings) such as a mobile computer in order to remotely monitor and control the power transmission to the underwater system (130).

The DC power generation system (112) of the host station (110) is where DC power is generated either through a power generator or by conversion of AC power from another AC power source. Moreover, the DC power generation system (112) comprises AC filters (not shown) and protection devices or systems (not shown) such as circuit breakers, surge suppressors and/or arc fault detectors. The resulting DC power from the DC power generation system (112) is rich in harmonics and thus, DC filters are installed in order to limit the amount of harmonics to an acceptable level. From the DC power generation system (112), the DC power is transmitted to the remote power conversion facility (120) through the submarine power cable (140).

The remote power conversion facility (120) includes the Local Control Station or

LCS (121), transformer (122), the DC to AC inverter (123) and a chemical storage system (124). Preferably, the remote power conversion facility (120) is a remote buoy, though, depending on the depth of the water level and prevailing weather, the remote buoy may also be replaced with a minimum facilities platform or a lightweight structure or vessel. The LCS (121) gathers the data gathered from the transformer (122), from the DC to AC

inverter (123), from the chemical storage system (124) and from the underwater system (130) through the Integrated Services Umbilical or ISU (150). Thereon, the LCS (121) diverts the data to the MCS (111) via the communications link. This is for monitoring the condition and performance of the equipments in the remote power conversion facility (120) and checking the condition of the power received from the host station (110). Moreover, the LCS (121) can be remotely controlled from the MCS (111) in order to configure the transformer (122) or halt the power transmission to the underwater system (130).

The DC power is transmitted from the DC power generation system (112) to the DC to AC inverter (123) in the remote power conversion facility (120). The DC to AC inverter (123) is designed to convert the DC power from the host station (110) to the desired AC power for the underwater system (130). Such inverters (123) which may be utilized in the system include thyristor valves, voltage source converters or any other devices which is capable of converting a DC power to an AC power. The inverter (123) also includes electrical filters (not shown) to discard any unwanted interferences in the converted AC power line. For example, AC filters are installed in the inverter as a shunt element to limit the amount of harmonics to the level required. The transformer (122) integrated in the remote power conversion facility (120) is used to enable the optimal voltage transformation. Moreover, the transformer (122) is monitored through the LCS (121) and is connected to the underwater system (130) through the ISU (150).

The chemical storage system (124) consists of the equipment necessary to store and deliver the required chemicals to the underwater system (130) in the necessary quantities. The chemicals are delivered to the underwater system (130) through the ISU (150). The ISU (150) is connected to the underwater system (130) to supply the necessary AC power and chemicals. The ISU (150) also includes control lines which are connected to the LCS (121). The control lines are used to carry information between the underwater system (130) and the MCS (111) via the LCS (121) and the communication link to the MCS (111).

FIG. 3 is a block diagram of an electrical power transmission system (200) for an underwater system (130) that includes motor devices in accordance with the present invention. As used herein and in the appended claims, the term 'motor' is to be construed broadly to any electro-mechanical device or system which includes valve, pump or actuator. The speed of an induction motor can be controlled by modulating the frequency of the AC power being delivered to it. This can be done by using a variable frequency drive. In the system as shown in FIG. 3, a variable frequency drive (125) is connected to the transformer (122). The variable frequency drive (125) is used to provide AC power to the induction motor whereby the variable frequency drive (125) can control and vary the speed of the motor. For example, supplying AC electrical power at 50Hz to the motor through the variable frequency drive (125) allows the motor to rotate at 3000 rpm, while supplying AC electric power at 200Hz allows the motor to rotate at the speed of 12000 rpm. Thus, the rotation speed of the motor is dependent on the frequency of the AC power transmitted to the motor.

Referring to FIG. 4 of the accompanying drawing, there is shown an electrical power transmission system in accordance with the present invention configured for transmitting electrical power to a modular seabed processing system (300). The modular seabed processing system (300) is connected by underwater flow lines (301) to wells (302) which remove fluid mixture comprising water and oil/gas from reservoirs beneath the seabed. The modular seabed processing system (300) comprises a foundation system to which a docking unit or manifold (303) is engaged.

Referring to FIG. 5, there is shown a schematic diagram of a system module (310) integrated in the modular seabed processing system (300). The system module (310) comprises a controller unit (311), valves (312), temperature sensors (313), pressure sensors

(314) and a booster pump (315). The controller unit (311) collects information from pressure sensors (314) and temperature sensors (313) and thereon, transmits the information to the LCS (121) that is connected to the MCS (111) through the communication link. The controller unit (311) can also be controlled either from the MCS

(111) and/or LCS (121) for reprogramming or shutting down the modular seabed processing system (300) which is otherwise autonomous during normal operation. The controller unit (311) is also connected to a plurality of valves (312) to control the commingling of fluid flowing from the wells through the docking unit or manifold (303) into the system-module (310) of the modular seabed processing system (300).

The booster pump (315) is configured to pump the fluids from the wells (302) to interconnecting pipes or to other system modules. The booster pump (315) is powered by high voltage power supply wherein it is connected to the ISU (150) and the frequency of the AC power is adjusted accordingly to control the booster pump. The AC power is adjusted through the variable frequency drive (125) as shown in FIG. 3. Hence, for the modular seabed processing system (300) to function, AC power is required to power-up the controller unit (311), booster pump (315) and other electrical devices integrated in the modular seabed processing system (300). Chemicals can also be injected to the modular seabed processing system (300) from the chemical storage system (124) of the remote power conversion facility (120) as part of a normal operation or as a result of a planned or unplanned shut down to prevent unwanted chemical reactions such as hydrate formation, wax deposition and corrosion.

Although described in the preferred embodiments that the power source from the offshore platform is connected to a remote power conversion facility (120) and thereon to the underwater system (130), the electrical power from the host station (110) may also be transmitted to a plurality of remote power conversion facilities (120) and/or from a remote power conversion facility (120) to a plurality of underwater systems (130). The electrical power transmissions to a plurality of remote power conversion facilities (120) and underwater systems (130) are dependant on the rated power that could be supplied by the power source. The previously described host station (110) may also be located at an onshore location rather than on an offshore platform. Advantageously, the electrical power transmission system requires less maintenance time and can be remotely monitor.

While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrated and describe all possible forms of the invention. Rather, the words used in the specifications are words of description rather than limitation and various changes may be made without departing from the spirit and scope of the invention.