PRIORITY STATEMENT

The present application hereby claims priority under 35 U.S.C. § 119 to European patent application number EP18176118.0 filed Jun. 5, 2018, the entire contents of which are hereby incorporated herein by reference.

FIELD

Embodiments of the invention generally relate to a subsea direct electrical heating power supply system and a method of operating such a system. Embodiments of the invention further generally relate to a subsea direct electrical heating power supply system.

BACKGROUND

Hydrocarbon flow is a key concern in subsea hydrocarbon production. Traditionally chemicals have been injected to prevent hydrate forming. More recently pipeline heating systems and in particular direct electric heating (DEH) systems have been developed in order to avoid the risks and cost associated with transporting chemicals to the wellhead and injecting them into the process fluid.

Early DEH systems such as disclosed in European patent application EP 2 324 550 utilize a topside power supply for providing electrical power to the pipeline section to be heated. This approach works well for short and medium step-outs but has disadvantages at longer step-outs, for example because of losses that occur over the length of the supply lines.

In more recent systems such as disclosed in European patent application EP 2 764 599 power is therefore provided to the sea floor using a subsea power cable adapted to be coupled to a three phase electric power source for providing three phase electric power to a subsea location and two or more subsea direct electric heating (DEH) modules, each module being provided for heating a different pipeline section of the subsea pipeline. The subsea DEH modules are adapted to be installed subsea at different subsea locations, for example in proximity to the pipeline section to be heated by the respective subsea DEH module. Each such DEH module may comprise a compensation unit adapted to compensate for reactive power that may lead to a reduced power factor. The compensation unit can be adapted to compensate for this reduced power factor and may comprise a capacitance, the value of which can be controlled, for example, in accordance with a control signal. The compensation unit may be adapted to allow an on-load control of the capacitance value. The capacitance may be implemented by way of a capacitor bank and may comprise two or more capacitors being connected in parallel, and switches for connecting or disconnecting the capacitors, the switches being controllable in accordance with the control signal for adjusting the value of the capacitance.

The system disclosed in EP 2 764 599, see FIGS. 2 and 3, uses a common supply cable 41 to power each DEH module, and transformers 13 are arranged in each DEH module 10 to downconvert the high transport voltage to a voltage compatible with the single-phase loads 21, 22, 23. Adapting the AC frequency is only possible for the entire system 100. Consequently the frequency cannot be adapted individually for the different DEH modules 10.

SUMMARY

At least one embodiment of the present invention provides an improved pipeline direct electrical heating power supply system.

In accordance with a first embodiment of the present invention there is provided a subsea direct electrical heating power supply system for providing electrical power for heating a subsea pipeline section. The subsea direct electrical heating power supply system comprises input device(s) adapted to couple the direct electrical heating power supply system to a power supply.

Thus, at least one embodiment of the inventive subsea direct electrical heating power supply system in its most generic form does not comprise a power source as such but provides an adaptation between a power source and a subsea pipeline section.

The subsea direct electrical heating power supply system of at least one embodiment further comprises a subsea variable speed drive for receiving electrical power from the input device(s) and for providing an AC output, the subsea variable speed drive comprising a plurality of series-connected power cells, each power cell comprising an inverter and a bypass device to selectively bypass the power cell. A surface-use variable speed drive comprising a plurality of series-connected power cells, each power cell comprising an inverter and a bypass device to selectively bypass the power cell, is for example known from granted U.S. Pat. No. 9,876,347 (the entire contents of which are hereby incorporated herein by reference), particularlyFIG. 1and accompanying description.

The subsea direct electrical heating power supply system of at least one embodiment of the present invention further comprises an adjustable subsea capacitor connected to the AC output of the subsea variable speed drive and output device(s) adapted to couple the direct electrical heating power supply system to a subsea pipeline section. The subsea direct electrical heating power supply system further comprises a controller adapted to adjust the capacitance of the adjustable subsea capacitor such that if the system output voltage is reduced as a result of bypassing one or more power cells the current output by the direct electrical heating power supply system is increased.

In accordance with another embodiment of the present invention, there is provided a subsea direct electrical heating power supply system for providing electrical power for heating a subsea pipeline section, comprising:

at least one input device, adapted to couple the direct electrical heating power supply system to a power supply;

a subsea variable speed drive to receive electrical power from the at least one input device and to provide an AC output, the subsea variable speed drive including a plurality of series-connected power cells, a power cell of the plurality of series-connected power cells, including an inverter and a bypass device to selectively bypass the power cell;

an adjustable subsea capacitor connected to the AC output of the subsea variable speed drive;

an output device, adapted to couple the subsea direct electrical heating power supply system to a subsea pipeline section; and

a controller, adapted to adjust capacitance of the adjustable subsea capacitor to, upon a system output voltage being reduced as a result of bypassing one or more power cells of the plurality of series-connected power cells, increase a current output by the subsea direct electrical heating power supply system.

In accordance with another embodiment of the present invention there is provided a direct electrical heating system comprising a power supply, a subsea pipeline section and a subsea direct electrical heating power supply system according to the present invention's first embodiment.

In accordance with yet another embodiment of the present invention, there is provided a method of operating a subsea direct electrical heating power supply system comprising a controller, a subsea variable speed drive comprising a plurality of series-connected power cells, each power cell comprising an inverter and a bypass device to selectively bypass the power cell, and an adjustable subsea capacitor connected to an AC output of the subsea variable speed drive. The method comprises, through operation of the controller, monitoring the condition of the power cells and upon detecting a faulted power cell, activating this power cell's bypass device. The method further comprises increasing the output current by adjusting the adjustable subsea capacitor and adjusting the output frequency so as to at least partially compensate for the power factor change caused by the adjusting of the adjustable subsea capacitor.

In accordance with yet another embodiment of the present invention, there is provided a method of operating a subsea direct electrical heating power supply system including a controller, a subsea variable speed drive including a plurality of series-connected power cells wherein each power cell of the plurality of series-connected power cells includes an inverter and a bypass device to selectively bypass the power cell, and an adjustable subsea capacitor connected to an AC output of the subsea variable speed drive, the method comprising, through operation of the controller:

monitoring a condition of the plurality of series-connected power cells;

activating, upon detecting a fault in a power cell of the plurality of series-connected power cells, a bypass device of the power cell;

increasing output current by adjusting the adjustable subsea capacitor; and

adjusting an output frequency, to at least partially compensate for a power factor change caused by the adjusting of the adjustable subsea capacitor.

