1. Field of the Invention
The present invention relates to electrical heating of subsea pipelines. More particularly the invention relates to corrosion protection while electrical heating with a pipe-inside-pipe configuration.
2. Description of Related Art
Offshore hydrocarbon recovery operations are increasingly moving into deeper water and more remote locations. Often satellite wells are completed at the sea floor and are tied to remote platforms or other facilities through extended subsea pipelines. Some of these pipelines extend through water that is thousands of feet deep and where temperatures of the water near the sea floor are in the range of 40xc2x0 F. The hydrocarbon fluids, usually produced along with some water, reach the sea floor at much higher temperatures, characteristic of depths thousands of feet below the sea floor. When the hydrocarbon fluids and any water present begin to cool, phenomena occur that may significantly affect flow of the fluids through the pipelines. Some crude oils become very viscous or deposit paraffin when the temperature of the oil drops, making the oil practically not flowable. Hydrocarbon gas under pressure combines with water at reduced temperatures to form a solid material, called a xe2x80x9chydrate.xe2x80x9d Hydrates can plug pipelines and the plugs are very difficult to remove. In deep water, conventional methods of depressuring the flow line to remove a hydrate plug may not be effective. Higher pressures in the line and uneven sea floor topography require excessive time and may create more operational problems and be costly in terms of lost production.
The problem of lower temperatures in subsea pipelines has been addressed by a variety of heating methods, including electrical heating. Heating by a variety of electrical methods is well known in the industry. Most of the proposals for electrical heating of pipelines have related to pipelines on land, but in recent years industry has investigated a variety of methods for electrical heating of subsea pipelines. (xe2x80x9cDirect Impedance Heating of Deepwater Flowlines,xe2x80x9d OTC 11037, May, 1999). One electrical heating method is the pipe-in-pipe-method. In one configuration of this method, a pipe-in-pipe subsea pipeline is provided by which a flow line for transporting well fluids is the inner pipe and it is surrounded concentrically by and electrically insulated from an electrically conductive outer pipe until the two pipes are electrically connected at bulkhead at the distal or remote end of a heated segment. Voltage is applied between the inner and outer pipes at the proximate or electrical input end and electrical current flows along the exterior surface of the inner pipe and along the interior surface of the outer pipe. This pipe-in-pipe method of heating is disclosed, for example, in U.S. Pat. No. 6,142,707, which is commonly assigned and hereby incorporated by reference herein. Other variations of the general pipe-in-pipe method exist.
In pipe-in-pipe electric heating configurations, an annulus design that electrically isolates the inner and outer pipe and provides thermal insulation and load sharing is desirable. Electrical isolation between the inner and outer pipe is needed so that the pipes will not short out. Thermal insulation is advantageous because it minimizes heat loss from the inner pipe and reduces the amount of electrical current necessary to achieve the desired temperature in the inner pipe. Load sharing between the pipes helps limit the stress on the outer pipe during laying.
It has been found that leakage currents enter and leave a subsea pipe-in-pipe pipeline when electrical current flows along the pipeline. The currents enter or leave the outer pipe of the pipeline only near where the pipeline enters the water and near the remote end of the heated segment. It has also been found that accelerated corrosion of metal occurs when electrical current flows from the metal into seawater, even when 60 Hz alternating current is used, if the current density per unit area is excessive. This corrosion can deplete the pipe thickness and possibly result in pipe rupture or a decrease in overall life of the pipeline. A method for protecting the pipe-in-pipe flowline from the corrosive effects of the leakage current and a method for selecting the size of the apparatus are needed.
Towards providing these and other advantages, the present invention in one embodiment provides a method for mitigating corrosion during electrical current flow in a pipe-in-pipe subsea pipeline by applying a protective layer over the outer pipe in and near the splash zone and in the vicinity of the bulkhead and attaching a sacrificial current discharge electrode over the protective layer. The discharge electrode is electrically connected to the outer pipe below the splash zone and near the bulkhead. In another embodiment, a bare area of the outer pipe, i.e., an area without any coating over the metal, is provided to serve as a discharge electrode. The area of the discharge electrode or the bare pipe is selected to decrease current density from seawater to the area of the heated segment where leakage current leaves or enters the pipe. This current density is preferably decreased to a value that provides corrosion rates not greatly affected by the current flow through the surface of the sacrificial electrode or bare pipe. The required area of the sacrificial electrode or bare pipe may be estimated from calculations of current density at different values of electric field along the pipeline and calculations or measurements of electric field near the ends of the heated segment of the pipeline, along with results of measurements of corrosion rates versus current density. Measurements of electric field along the pipeline are preferably made for the pipeline to be used.