Abstract:
The method of taking a remotely operated vehicle to the ocean floor to land on and move along a subsea pipeline above or below the seafloor and repeatedly circulate seawater which has been heated electrically, mechanically, or chemically across the outer surface of the pipeline to melt hydrates or paraffins which have formed on the inside of the pipeline.

Description:
BACKGROUND OF THE INVENTION 
   The field of this invention is that of removing blockages in remote subsea pipeline, typically from subsea gas well installations. 
   Hydrates are a porous solid which is formed primarily of water with a mixture of gases. It is effectively similar to ice. There is a tendency for hydrates to form in the pipelines departing from a subsea gas well, especially on well startup. 
   The temperature of seawater at depths will often approach 32° F., with the temperature in non-flowing pipelines being the same. When a subsea pipeline valve is opened, the gas expansion can cause substantial additional cooling. In these cold and high pressure conditions, hydrates of the gas and water can quickly form. 
   Frequently when the hydrate forms, it forms a blockage, the blockage will be somewhat porous. At that time, a high pressure will exist on the upstream side and a lower pressure will exist on the downstream side of the blockaged. 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. 
   It is difficult to tell where the hydrates are actually located in deepwater pipelines, especially when the pipelines are buried. 
   Hydrates formed like this can last for weeks or months, with a substantial loss of gas flow and therefore revenue to the owner of the pipelines and subsea wells. 
   Paraffins can form blockages in pipelines by building up on the inner diameter of the cold pipelines as relatively warm oil circulates out of an oil well and cools as it flows down a subsea pipeline. As the layer of paraffin builds up on the subsea pipeline inner diameter, the inner diameter of the paraffin becomes smaller and smaller. Ultimately a pigging device intended to clean the paraffin will cause the paraffin to separate from the inner wall of the pipeline and become a plug. In some cases the paraffin will release from the subsea pipeline inner diameter without a pig and cause a blockage. In either case, if the pressure in the pipeline is enough to move the plug along the pipeline, it will continue to collect additional paraffin as it moves until the length of the blockage cannot be moved by the available pressure. 
   Some attempts have been made to enter the end of the pipeline with a somewhat flexible string of coiled tubing to get to the blockage and wash it out. This is an expensive operation, and in some cases the blockage can be 10 to 20 miles away. 
   Removal by use of coiled tubings can be further complicated if the pipeline has bends in it, making passage of the coiled tubing difficult if not impossible. 
   SUMMARY OF THE INVENTION 
   The object of this invention is to provide a system which will approach a subsea pipeline and remove hydrate and/or paraffin blockages from within the pipeline. 
   A second object of the present invention is to provide a method of removing the hydrate or paraffin without requiring that the pressure integrity of the pipeline be compromised. 
   A third object of the present invention is to provide for recirculation of seawater to allow the heat not absorbed into the pipeline to increase the inlet temperature to the seawater heating means—thereby increasing the outlet temperature of the seawater. 
   Another object of the present invention is to provide a means for applying heat to the outer surface of a subsea pipeline without uncovering the pipeline to minimize the disturbance to the pipeline. 
   Another object of the present invention is to provide means for applying heat to the outer surface of a subsea pipeline without uncovering the pipeline so that the insulating effects of the covering will retain the heat in the pipeline. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a section thru the pipeline and the thermal operating module showing the circulation thru the circulation chamber. 
       FIG. 2  is a larger section of the view of  FIG. 1  including a trench dug to uncover the pipeline and the remotely operated vehicle (ROV). 
       FIG. 3  is a similar section thru the system, but showing wheels which might engage the exterior of the pipeline to drive the thermal operating module along the pipeline. 
       FIG. 4  is a partial section of the system taken axially along the pipeline showing a spatial relationship between the driving wheels and a heating chamber utilizing electric resistance heating of the seawater used for the heating. 
       FIG. 5  is a similar figure to  FIG. 4 , but showing the heat being generated by a pressure drop across an orifice rather than electric resistance heating. 
       FIG. 6 . shows an alternate embodiment of this invention with the circulation chamber being disposed a distance away from the main portion of the system to allow the circulation chamber to follow the pipeline under the mudline without uncovering the pipeline. 
       FIG. 7 . shows the rollers of the alternate embodiment and their method of engaging the pipeline. 
       FIG. 8  shows a side view of the thermal operating module of the alternate embodiment with the circulation chamber spaced a distance away from the upper section by a pair of legs. 
       FIG. 9  is a section thru one of the legs of  FIG. 8 . 
       FIG. 10  is a view of the alternate embodiment engaging the subsea pipeline below the mudline. 
       FIG. 11  is a section thru the alternate embodiment showing the heated seawater flowing down the rear leg, into an axial tube, and across the upper surface of the pipeline. 
       FIG. 12  is a section thru the alternate embodiment showing the heated seawater flowing across the upper surface of the pipeline, into an axial tube, and up the front leg. 
       FIG. 13  is a partial section thru the alternate embodiment showing the full circulation of the heated water down the rear leg, along the circulation chamber and back up the front leg. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , a subsea pipeline  10  has a blockage indicated at  12 . The subsea pipeline  10  is setting on the seafloor  14  and is covered by seawater  16  which may be as cold as 32° F. Arrows  18  indicate the flowing of heated seawater across the upper portion  20  of the subsea pipeline  10  for the purpose of disassociating the pipeline blockage  12 . The disassociation may be melting a hydrate, melting a paraffin blockage, or softening a paraffin blockage to the point that it will flow. 
   Two tubes  22  and  24  run parallel to the subsea pipeline and have a plurality of holes  26  and  28  which direct heated seawater into the circulation chamber  30  or out of the circulation chamber  30  respectively. Line  32  feeds heated seawater into tube  22  and return line  34  draws the seawater out of tube  24 . The return line  34  leads to pump  36  and then to heating element  38  and back to tube  32 . The heated seawater which is introduced into the combustion chamber by tube  22  is somewhat cooled by flowing across the upper portion  20  of subsea pipeline  10  before it enters tube  24  to return thru the pump and back into the heating element  38 . As the returning seawater is only partially cooled, the inlet to the heating element  38  is higher, so the output seawater from the heating element will be progressively higher each circulation until a temperature is reached in which the heat losses thru the insulation will equal the heat input and so a form of steady state will be achieved. 
   Resilient flap type seals  40  are placed around the perimeter of the circulation chamber  30  to restrict the mixture of the seawater  42  within the circulation chamber with the seawater  44  outside the circulation chamber. 
   Most conventional ROVs have a minimum of 100 horsepower of electricity which can almost all be converted into heat thru a resistance heater, so it can be readily seen that if the same seawater is circulated with only minimal leakage, it can be quickly brought to a high temperature. 
   Referring now to  FIG. 2 , the apparatus of  FIG. 1  is shown below an ROV  50  and operating in an area of the subsea pipeline  10  which has been uncovered in a ditch form  52 . By uncovering only the top portion of the pipeline, the lower portion of the seafloor is left intact to support the pipeline. The thermal operating module is generally referred to as  55 . 
   Referring now to  FIG. 3 , an alternate section through the thermal operating module  55  showing rollers  60  and  61  mounted on motors  62  and  63  and pivoted about pin  64 . Cylinders  66  and  67  will move to push the rollers  60  and  61  against the pipeline  10  and the motors  62  and  63  will turn the rollers  60  and  61  to drive the thermal operating module along the pipeline. 
   Referring now to  FIG. 4 , rollers  60 ,  61 ,  70 , and  71  are shown positioned to move the thermal operating module  55  along the pipeline  10 . The speed along the pipeline would be calculated to allow the hydrates and/or paraffin in the pipeline to melt during the heating cycle. Umbilical  74  typically provides 100 to 150 horsepower of electricity to operate the ROV, but in the case of the thermal operating module will use a majority of this electrical horsepower to generate heat. 
   Referring now to  FIG. 5 , an alternate method of converting energy into heat is illustrated. Rather than providing electrical resistance heating, the energy is directed toward the pump  80  which is a high pressure pump (i.e. 10,000 p.s.i.) rather than a circulating pump like  36  (i.e. 15 p.s.i.). The high pressure output of pump  80  is directed across a pressure reducing means such as an orifice  90  to drop the pressure to a low pressure (i.e. 15 p.s.i.). In dropping the pressure in this “inefficient” manner, the horsepower required to operated the pump is lost into heat, which is our goal. In some situations it may be convenient to run a hose or pipe from the surface to simply provide high pressure fluid for heat generation at the subsea location in a similar manner. Using pressure to transport energy to a subsea location is practical, whereas attempting to directly pump high temperature fluid to a subsea location will result in substantial thermal energy losses. 
   Alternate methods of generating heat at the subsea location adjacent to the pipeline such as chemical reactions can also be used to provide the heat necessary for the task of pipeline blockage remediation. 
   Referring now to  FIG. 6 , an embodiment is illustrated which can eliminate the requirement for uncovering and then recovering the subsea pipelines. Both uncovering and recovering subsea pipelines are expensive tasks. It is inherently true if the sea bottom is soft enough for the pipeline to be buried into it, it is relatively soft.  FIG. 6  illustrates the circulation chamber  30  being streamlined and mounted on streamlined legs such that they will simply plow through the soft seafloor bottom. Leg  100  extends down from the heat generation section  101  and supports the circulation chamber  30  and a pair of rollers  102  and  104 . Operation of the rollers will be discussed in the next figure. Drive shafts  106  and  108  go up to motors  110  and  112  (not seen) which are mounted on cylinders  114  and  116  (not seen). 
   Referring now to  FIG. 7 , roller  104  is supported on axle 120  which pivots about pivot  122  and extends to universal joint  124  and then upward by shaft  108  to motor  110  (not shown). When shaft  108  is pushed down by cylinder  114  (not shown), the roller 104  pivots away from the central opening  126 . When the cylinder  114  pulls up, the roller  104  is moved toward the central cavity  126  and toward subsea pipeline  10  when subsea pipeline  10  is in the central cavity  126 . 
   Referring now to  FIG. 8 , a side view of the alternate embodiment is shown illustrating that two stream lined legs  130  and  132  connect the circulation chamber  30  to the heat generation section  101 . 
   Referring now to  FIG. 9 , a cross section of leg  130  of  FIG. 8  is shown illustrating the streamlined shape and the location of drive shafts  106  and  108  and the return line  34 . 
   Referring now to  FIG. 10 , the circulation chamber  30  along with rollers  102  and  104  are shown engaging the pipeline  10  below the seafloor. Cylinders  114  and  116  are shown retracted to the position in which the rollers  102  and  104  are engaging the pipeline  10 . 
   Referring now to  FIG. 11 , the rear leg  132  is shown with the heated seawater going down the tube  32  and across the circulation chamber  30  along arrow  140 . 
   Referring now to  FIG. 12 , the front leg  130  is shown with the somewhat cooled seawater returning back to the pump for recirculation along arrow  141 . 
   Referring now to  FIG. 13 , a partial section of the alternate embodiment is shown with the full circulation path. The components as discussed in  FIG. 6  are shown within the front leg  130 , and matching components are shown in the rear leg  132 . Both the front leg  130  and the rear leg  132  would have similar streamlined profiles as illustrated in  FIG. 9 . 
   The foregoing disclosure and description of this invention are illustrative and explanatory thereof, and various changes in the size, shape, and materials as well as the details of the illustrated construction may be made without departing from the spirit of the invention.