CROSS-REFERENCE TO RELATED APPLICATIONS
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISK
BACKGROUND OF THE INVENTION
The field of this invention is that of preventing hydrate blockages in subsea pipelines, subsea riser pipes, and subsea equipment installations.
Hydrates are a porous solid which is formed primarily of water with a mixture of gases. It is effectively similar to crushed ice which is stuck together. There is a tendency for hydrates to form in pipelines departing from a subsea gas well, especially during well flow startup.
The temperature of the seawater at depths will often approach 34° F., with the temperature in a non-blowing pipe being the same. When a subsea valve is opened, the gas expansion can cause substantial additional cooling. In these cold and high pressure conditions, hydrates of the gas and water quickly form.
Frequently when the hydrate forms, it forms a blockage. The blockage will typically be somewhat porous. At that time, a high pressure will exist on the upstream side of the blockage and a lower pressure will exist on the downstream side. This means that additional gas will move thru the hydrate and expand and therefore cool as it does. This means that not only can the expansion of this gas keep the formed hydrate cool, but can literally continue to grow additional hydrate blockage.
Attempts have been made to enter the accessible end of the pipeline with a somewhat flexible string of steel coiled tubing to get to the blockage and wash it out. This is an expensive operation due to the cost of the equipment and the time delay in arranging for and deploying the equipment. As the blockage most often occurs at the opposite end from the accessible end, the blockage can be 5 or more miles away. Removal by the use of coiled tubing is further complicated if there are bends in the pipeline, making it more difficult to impossible.
Another method of prevention of the formation of hydrates is to place expensive chemicals at the likely location of the formation of the hydrates. Chemicals such as methanol will reduce the temperature of the liquid/gas combination required for the formation of the hydrates, hopefully below the temperature which occurs during the well startup operations. In addition to the chemicals being expensive, the delivery of the chemicals to the remote location of the likely hydrate formation requires expensive capital equipment.
BRIEF SUMMARY OF THE INVENTION
The object of this invention is to provide a method for the prevention of the formation of hydrates in subsea pipes when subsea valves are opened.
A second object of the present invention is to provide a method for the removal of hydrates and/or wax blockages which have formed in subsea pipes.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
FIG. 1 is a partial section of a subsea system comprising a subsea Christmas tree, a vertical riser pipe, a mid-ocean buoy, and a flexible flowline to a surface vessel.
FIG. 2 shows an expanded section of FIG. 1 illustrating the location of the valve to be opened which will provide the gas throttling to reduce temperature to the hydrate formation range.
FIG. 3 shows the FIG. 1 with equipment to implement the present invention installed.
FIG. 4 shows an expanded section of FIG. 3 illustrating the addition of the heating section to prevent and/or remediate the hydrates.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a subsea installation 1, landed on the seafloor 3, a valve 5 to be opened, a riser pipe 7, a gooseneck 9, a flexible pipe 12, a surface vessel 14, at the ocean's surface 16, and a buoyant tank 18 to support the riser pipe 7.
Referring now to FIG. 2, valve 5 has been closed for an extended period of time sufficient for all equipment and liquids to have been cooled to the temperature of the sea water, presumably 34° F. Valve 5 was opened and the cooled high pressure gas throttled across the opening valve into the lower pressure area in the riser pipe 7 above the valve. The blast of gas (probably methane) with a water content literally begins to freeze into a hydrate ice on the internal walls of the riser pipe 7 to the point of complete blockage as indicates as 20. At this time the operation has been required to wait until the hydrate melted of its own accord, in temperatures as low as 34° F.
Referring again to FIG. 1, access to the hydrate for mechanical removal is limited because the bending radius of the gooseneck 9 makes it difficult to pass a work string. In the best case scenario, the scheduling and deployment of a work string can take weeks. This means weeks of lost production. Once the hydrate is removed, the valve 5 must be opened again, potentially causing a repeat hydrate.
Referring now to FIG. 3, the system looks similar to FIG. 1, except an outer pipe 30 is placed around the riser pipe 7 and an ROV 32 with a belly skid 34 is positioned next to the outer pipe 30.
Referring now to FIG. 4, outer pipe 30 is shown around riser pipe 7, with a substantial volume of water 40 in the annular area 42 between. Belly skid 34 is designed to use the power in the ROV umbilical 44 (typically 150 hp of electrical power) to heat seawater and pump it into the annular area between the outer pipe 30 and the riser pipe 7. This is accomplished by a seawater intake 46, a circulation pump 48, an electric heater 50, and an attachment interface 52.
Prior to the opening of valve 5 and throttling gas into the riser pipe 7, the ROV 32 and belly skid 34 heats seawater to 150° F. and circulates it into the annular area 42, This fills the annular area 42 with 150° F. seawater rather than the 34° F. seawater which would have been in it otherwise. When the valve 5 is opened a hydrate is not formed as it enters an area as hot as you want it to prevent gas and water vapor mixture from going down to a temperature low enough to form hydrates.
Other methods are available for providing a heat source to prevent the formation of hydrates, such as providing electric heaters within the annular area 42. Prior to the opening of the valve 5, electricity could simply be sent to the electric heaters from the surface or from the ROV to heat the seawater 40 in the annular area 42.
The present method of preheating a volume of seawater to prevent the formation of hydrates is shown in these drawings as a part of a vertical riser pipe 7 going to the surface. The method could be equally well used along a horizontal flowline associated with a subsea installation, or the subsea installation itself. In the case of some subsea installations, there is enough piping to allow formation of the hydrates directly on the subsea system.
Hydrates are formed and are remediated at a combination of temperatures and pressures rather than having a single simple temperature or pressure to design equipment in accordance with. For this reason the temperature best to increase the contained water to and the volume of contained water will vary from installation to installation.
The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.