"An arrangement for heating a hydrocarbon transport line"

The present invention relates in general to oil and gas transport pipe lines, and more particularly to heating of such transport lines, in order to counteract clogging of the pipe interior.

When a subsea process plant that includes piping for transporting hydrocarbons, is in normal operation, the temperature of the well flow, and possible MEG additives (monoethylene glycol) in the flow, will have the effect that formation of hydrates is avoided in the pipes and valves of the system. Such hydrates, formed from hydrocarbons and water, may, if the temperature becomes too low, form a solid phase that will settle and constitute a hindrance to continued flow.

Particularly when the subsea system comes to a shut-down, the risk of having hydrate plugs in sections of the hydrocarbon pipelines will increase, and it may actually be impossible to restart the process. It is therefore important to have an efficient means for preventing the formation of such hydrate plugs, and if plugs have already been formed, they must be removed in an efficient manner.

The solution most commonly used, is to increase the level of MEG when a shutdown occurs, and to provide a pressure relief in the system. In order to remove hydrate plugs that have already formed, parts of the equipment may have to be removed, to be brought to the surface and defrosted in a sea vessel. Such operations are of course both costly and time-consuming.

Another previously known method for preventing and removing hydrate plugs, is by heating the pipelines, in particular by induction heating. International publication WO 90/05266 discloses such a system. The pipe is thermally insulated, and heat is generated by eddy currents in the pipe wall, induced from conductors extending along the pipe outside the thermal insulation layer.

European patent application EP 0036322 A1 discloses a similar system based on induction heating, for shorter hydrocarbon transport lines, less than 1 km. Further, Brazilian publication Pl 0101546-OA discloses a pipe induction heating system primarily for local heating, by means of a compact apparatus that can be transported to and used at a spot where a hydrate plug has already formed.

The present invention concerns the above described induction heating technique for preventing and removing hydrate plugs from short pipeline lengths and associated equipment like e.g. valves, and concerns more particularly an efficient manner of supplying power for such induction heating.

Consequently, in accordance with the present invention there is provided a method such as defined precisely in the appended claim 1 , and an arrangement such as defined precisely in the appended claim 7.

The method and arrangement of the invention are based on using a power source that is free to deliver power to the induction wiring on the hydrocarbon transport line due to a shut-down. It is just when a shut-down occurs, that the need for pipe heating arises, as explained earlier, so using "free" power for this purpose is quite advantageous.

In the following, the invention shall be illuminated further by presenting a detailed explanation of favourable embodiments. It is at the same time referred to Fig. 1 , that shows a schematic drawing of a preferred system layout for an embodiment of the present invention.

In the upper right corner of Fig. 1 , a hydrocarbon transport line section 6 appears, equipped with induction wiring 7 along its length. As shown, three cables 7 are used, short-circuited at one end of the pipe section to be heated. It is to be noted that the pipe section 6 may also be equipped with valves and other details that may also benefit from the heating.

The induction cables 7 may be installed simply by seizing with unmagnetic strips, for instance plastic strips. The pipe 6 may be thermally insulated or not, and the induction cables 7 are favourably arranged under the possible thermal insulation layer.

The gist of the present invention lies however in the power supply chain leading up to those induction cables. In the embodiment shown in the drawing, a general indication of a power supply 1 is found to the far left. The box 1 may represent a generator system or simply a line feed from a power grid. Reference numeral 2 indicates a so-called Variable Speed Drive (VSD) that is normally used for feeding power to one or more subsea motors 4.

As indicated by selector switch 3 between the VSD 2 and the motor 4, it is possible to re-route power to the induction cables 7 instead. Particularly when power to motors and other equipment is shut down, there is an increased need for providing extra heat to the transport line so as to avoid hydrate ice plugs. The drawing thus shows a situation where the selector switch 3 has been switched over to route power to the induction heating. The switch 3 is preferably operated automatically by a control signal on a separate line (not shown) on the basis of a control parameter in connection with system operation, so that switching takes place immediately or a predetermined time period after a shutdown. Or, the switch 3 may be operated "manually", for instance by an ROV.

In the embodiment shown in the drawing, an optional transformer 5 is included to provide a suitable voltage for the induction wiring 7.

The drawing shows a situation in which power is taken from the normal supply chain to a motor 4, but power may equally well be taken from a power chain leading to another heating system, for instance a system for heating an antisurge loop for a subsea gas compressor.

As stated above, the transformer 5 is optional, and so is the VSD 2. VSD 2 provides a frequency that may be higher than the normal grid frequency of 50 or 60 Hz, actually for motor operation, but such increased frequencies are favourable also for the induction heating system. A higher frequency will give a better heating effect, and the frequency adjustability can also be used to choose between maintenance heating, for preventing forming of plugs, or a high power setting in order to defrost plugs that have already been formed.

However, normal grid frequency may be used also. , , ^• ,. ^{• • '} ; ^{• '}

In the above explanation, reference has all the time been made to a subsea system. But the invention comprises also systems in other environments, for instance arctic conditions, where extra heating of pipelines may be necessary to avoid hydrate plugs.