Electrical heating of subsea flowlines represents an effective method to prevent wax and hydrate formations, especially at long transportation distances and in deep waters with a low seabed temperature.
Such systems are available to be used in conjunction with bundles, pipe-in-pipe and wet thermal insulation systems, enabling an environmental friendly fluid temperature control without use of chemicals and flaring for pipeline depressurizing.
Enhanced production availability is achieved as no time is lost by unnecessary de-pressurizing, pigging, heating medium circulation or removal of hydrate and wax blockages.
The seabed temperature at 100 m water depth and lower may range from 7 °C to -1.5° C, causing a rapid cooling of hot well streams being transported in subsea flowlines, see Fig. 1.
Vulnerable crudes and multiphase compositions will at such super cooling deposit wax and asphalts, and the gas - water phase may freeze solid hydrate particles that can permanently block the pipeline.
A conventional approach to secure a safe subsea transportation has been to use thermal insulated flowlines in combination with the following measures:
Continuous injection of chemicals to reduce the hydrate freezing point and the rate of wax deposition.
Depressurization of the flowline to enable a further reduction of the hydrate freezing-point.
Use of twin parallel flowlines to achieve a more effective depressurizating of potential hydrate plugs, and to perform pig cleaning operations. Further to circulate hot waterloil in order to melt out wax and to pre-warm the flowlines after long lasting shut downs.
It is obvious that these measures do have physical, economical and environmental limitations especially in deeper waters and over very long transportation distances:
The pressure head in deep water pipelines may give an insufficient pressure relieve at blow downs.
Blow down and pressurisation of pipelines will involve pressure drop expansions and a related Joule-Thomson supercooling of multiphase fluids, that by itself can cause severe wax deposits and hydrate plugs.
Heat loss in long twin pipelines that shall be preheated in a serial configuration, will restrict adequate heating above 10-15 km length.
Continuous injection of large amounts of hydrate and wax inhibitors will dissolve in the produced water that may have a restricted release to sea.
Long twin flowline installations will generate high investment and operational costs.
Thermal insulated and electrically heated flowlines represent an alternative prevention method for wax and hydrates that will not be restricted with the same limitations.
A market review of different designs and functional principles conclude that electrical heating technology is available for flexible risers and flowlines, pipelines, pipe-in-pipe and bundle systems for all practical lengths in subsea transportation. Some of these have been in operation with a satisfactory performance over a period of more than 20 years.
