Abstract:
A method producing a relative vacuum in a subsea pipeline to assist in the disassociation of a hydrate comprising providing a tool assembly comprising a sealing cup to engage the bore of the pipeline, a vacuum pump, and slips to engage the bore of the pipeline, attaching the tool assembly to a coiled tubing string and inserting the tool assembly into an access point in the pipeline, pumping into the annular area between the bore of the pipeline and the outer diameter of the coiled tubing string to move the tool assembly to a distal location within the pipeline at a lower elevation than the access point, pumping into the coiled tubing string to set the slips and to power the vacuum pump to pull the relative vacuum within the pipeline, pumping into the annular area to vent the relative vacuum into the coiled tubing string, and pumping through the coiled tubing string into the area in front of the tool assembly to assist in the recovery of the tool assembly from the pipeline.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to the method of providing drawing a relative vacuum locally on a hydrate formation in a pipeline to disassociate the hydrate. 
       BACKGROUND OF THE INVENTION 
       [0002]    At certain temperatures and pressures in a pipeline, hydrates will form. Hydrates are a combination of hydrocarbon gases and water which resembles crushed ice and will completely block the flow in a pipeline. As hydrate formation is facilitated by higher pressure and lower temperature, subsea pipelines are particularly susceptible to hydrates. The ocean in deepwater is characteristically about 34 degrees F., and if there was not the gas volume inherent with significant pressure the pipeline would not exist. 
         [0003]    When hydrates form, the typical solution has been to reduce the pressure as much as practical at the end of the pipeline and wait until they melt. This process can take several months with associated loss of revenue. A second method is to locally heat the area with a subsea heating module as is shown in U.S. Pat. No. 6,939,082. The application of this method has been restricted as the concern with hydrates has caused operators to apply insulation to the pipelines and this dampens the effectiveness of trying to get heat to the hydrate. 
         [0004]    The problem is so expensive that the industry has not only gone to the expense of insulating pipelines, but literally installing double wall pipelines for insulation characteristics. If you imagine a double wall pipeline with gas flowing through the inner pipeline, the larger outer pipeline is likely more expensive than the inner one and then you have the problem of how you assemble one pipeline inside another. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The objective of this invention is to provide a method of providing a relative vacuum in a subsea pipeline between a pig and a hydrate blockage to cause the hydrate blockage to disassociate. 
         [0006]    A second objective of this method is to provide a high differential pressure across the pig without putting the running coiled tubing string in tensile stress. 
         [0007]    A third objective of this method is to allow the pig to grip the internal bore of the pipeline without damaging the bore of the pipeline. 
         [0008]    Another objective of this method is to use power fluid flow down the coiled tubing string to power a pump/motor combination to cause the relative vacuum. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a view of an offshore platform and pipeline showing a hydrate formed in the pipeline and a prior art method of remediation. 
           [0010]      FIG. 2  is graphical representation of hydrate formation criteria in a pipeline. 
           [0011]      FIG. 3  is a view of an offshore platform and pipeline similar to  FIG. 1  showing a hydrate formed in the pipeline and a prior art method of remediation including using a coiled tubing string to mechanically pull a local vacuum. 
           [0012]      FIG. 4  is a view of an offshore platform and pipeline similar to  FIG. 1  and  FIG. 3  showing a hydrate formed in the pipeline and a method of the present invention remediating the hydrate. 
           [0013]      FIG. 5  is a half section of the pipeline pig of the present method indicating the flow paths as the pipeline pig is being run into the pipeline. 
           [0014]      FIG. 6  is a half section of the pipeline pig of the present method indicating the flow paths as the pipeline pig is located proximate the hydrate and is remediating the hydrate. 
           [0015]      FIG. 7  is a half section of the pipeline pig of the present method indicating the flow paths as the flow to the pipeline pig is reversed to reset the tool for recovery. 
           [0016]      FIG. 8  is a half section of the pipeline pig of the present method indicating the flow paths as the pipeline pig is being recovered from the pipeline. 
           [0017]      FIG. 9  is an enlargement of a portion of  FIG. 6  for clarity. 
           [0018]      FIG. 10  is a quarter section graphic of a set of conventional slips except without the conventional sharp teeth to discuss the purpose of the sharp teeth. 
           [0019]      FIG. 11  is a quarter section graphic of a set of conventional slips except with the conventional sharp teeth to discuss the purpose of the sharp teeth. 
           [0020]      FIG. 12 . is a quarter section of slips without sharp teeth which will provide failsafe support in a pipeline. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Referring now to  FIG. 1 , a platform  10  is shown with a pipeline  12  terminating above the deck  14 , going down a side  16  of the platform, and away from the platform along the ocean floor  18 . This would be a reasonable representation of a pipeline which is receiving gas or oil from the pipeline or is sending gas or oil from the platform to another location, e.g. to the shore. In this case a hydrate formation  20  is shown near the end  22  of the portion of the pipeline which is shown. The ocean  24  is shown with surface  26 . 
         [0022]    Referring now to  FIG. 2 , a graph is shown which shows a “sweet spot” range of temperature and pressure where hydrates form. There is a hydrate zone  30 , a hydrate risk zone  32 , a hydrate free zone  34 , a formation curve  36 , and a dissociation curve  38 . As can be seen, one can escape from the hydrate zone  30  by increasing the temperature or decreasing the pressure. In the referenced U.S. Pat. No. 6,939,082, the objective was to escape the hydrate zone  30  by heating the pipeline. This requires going to the subsea site of the pipeline and locating the hydrate from outside the pipeline. The alternative which is addressed in the present invention is to reduce the pressure. 
         [0023]    Referring again to  FIG. 1 , if appropriate valving  40  at the end of pipeline  12  is opened and the pressure within the pipeline is vented, the pressure proximate the hydrate formation  20  can be reduced. If that causes the hydrate formation  20  to disassociate, the problem is solved. If vacuum equipment (not shown) is attached to the valving  40  and a vacuum is drawn at that location, the pressure will be reduced by only 14.7 p.s.i., which is not likely to make a difference. 
         [0024]    Referring now to  FIG. 3 , an alternate proposal for reducing the pressure proximate the hydrate formation  20  is illustrated. A pipeline pig  50  with sealing cup  52  is run into the pipeline pulling coiled tubing string  54  as it moves forward. As it moves forward, it is pushed forward by flow into the annular area  56  outside the coiled tubing  54  and inside the pipeline  12 . Any gas or liquid in front of pipeline pig  50  is forced by up the bore of coiled tubing string  54  and to platform  10  for disposal. When the pipeline pig  50  is nearing the hydrate formation  20 , valve  58  on the coiled tubing is closed and the coiled tubing string  54  is pulled in tension. This tension will effectively pull a relative vacuum between the hydrate blockage  20  and the pipeline pig  50 , which if sufficient will cause the hydrate blockage  20  to disassociate or melt. The vacuum is called relative as it is relative to the pressure in the pipeline. If the pipeline pressure is 1000 p.s.i. and the pressure is reduced to 600 p.s.i. to remediate the hydrate, it is a relative vacuum of 400 p.s.i., but it still has a 400 p.s.i. pressure. For this method to be effective it is important to get the pipeline pig  50  as close to the hydrate blockage  20  as practical to reduce the volume of gas to be expanded to lower the pressure. Unfortunately this means several factors are working against the effectiveness of the method. Some of these factors are (1) the weight of the coiled tubing in the pipeline riser section  60  of the pipeline  12 , (2) the drag around the bends  62  in the pipeline which are literally requiring enough force to bend the coiled tubing as it goes around the bend, (3) the simple weight friction  64  of the coiled tubing string  54  along the pipeline  12 , and (4) the sealing friction of the sealing cup  52  in the bore of the pipeline  12 . All of these factors are working against the effectiveness of the system, when the strength of the coiled tubing may not be sufficient to pull and adequate relative vacuum in the first place. 
         [0025]    Referring now to  FIG. 4 , a figure is shown similar to  FIG. 3 , except the pipeline pig  50  of  FIG. 3  is replaced with combo pig  70  of the present invention. Combo pig  70  comprises sealing cup  72 , motor  74 , pump  76 , and slips  78 , as will be described in subsequent figures. Combo pig  70  is designed to be moved into the pipeline  12  as pipeline pig  50  was, however, it is not important that it is moved near to the hydrate blockage  20 . It needs to be moved as closely as practical to the same elevation as the hydrate blockage  20  so that the relative vacuum pulled in front of combo pig  70  will be the relative vacuum which the hydrate blockage  20  sees. In many cases, it means the pig can be run to the bottom of the pipeline riser section  60  and not even be required to navigate the bends  62 . At this point slips  78  are set on the internal diameter of the pipeline and the motor  74  is run to drive the pump  76  to displace fluids and gases in front of combo pig  70  up the bore of coiled tubing string  54  to pull a relative vacuum on the hydrate. As the force of the differential pressure across the sealing cup  72  is withstood by the slips  78 , the coiled tubing string  54  is not loaded or stretched. If the relative vacuum is not sufficient to remediate the hydrate, the pump motor combination simply continues to run until it is. As the hydrate begins to disassociate or melt and releases gases and liquids to functionally reduce the extent of the relative vacuum, the pump/motor combination continues to run to remove the released gases and liquids. 
         [0026]    Referring now to  FIG. 5 , combo pig  70  is shown in pipeline  12  with coiled tubing string  54  connected to combo pig  70  with connection  100  and with sealing cup  102  engaging the internal bore  104  of pipeline  12 . Arrow  106  illustrates the direction of flow in the annular area  56  which engages sealing cup  102  and moves the combo pig  70  and coiled tubing string  54  towards the hydrate blockage  20 . Fluids and gases between combo pig  70  and hydrate blockage  20  return thru combo pig  70  and up the internal bore of the coiled tubing string  54  as indicated by arrows  108 - 120 . This includes passing through a check valve  122 . 
         [0027]    Referring now to  FIG. 6 , when combo pig  70  is as far into the pipeline as desired, flow is reversed and pumped into the coiled tubing string  54  to combo pig  70 . As flow will not go through check valve  122  in the reverse direction, sleeve  130  is moved downwardly on  FIG. 6 . This movement of sleeve  130  releases pivoting dogs  132  which in turn releases ring  134  which is attached to slip segments  136 . Spring  138  pushes slip segments  136  upwardly on  FIG. 6 , and slip segments  136  ride on tapers  140  on top sub  142 , with low friction bearings  144  there between. The purpose of low friction bearings  144  will be discussed in  FIG. 9 . The flow along the coiled tubing string  74  takes the path indicated by arrows  150 - 162  to power a motor  164 . Exhaust from motor  164  flows back to the annular area  56  as indicated by arrows  166  and  168 . Motor  164  powers pump  170  by shaft  172 . Pump  170  draws fluids and gases from the area  174  between the combo pig  70  and the hydrate blockage  20  as indicated by arrows  176  and  178 . Flow from pump  170  returns to the annular area  56  as indicated by arrows  180  and  168 . By this method flow from the coiled tubing string  54  powers motor  164  to drive pump  170  to pull a relative vacuum in area  170  or effectively on the hydrate blockage  20 . The longer the pump and motor combination run, the lower the pressure in the relative vacuum becomes. 
         [0028]    Referring now to  FIG. 7 , flow into the annular area  56  as indicated by arrows  190  and  192  shifts valve  194  downwardly in the figure. 
         [0029]    Referring now to  FIG. 8 , arrows  202 - 230  indicate the newly opened flow path which allows flow from the coiled tubing string  54  to flow to the front of the combo pig  70  to repressure the area which was subjected to a partial vacuum and then to provide fluid or gas volume in front of the combo pig  70  as it is being retrieved so it will not tend to cause another partial vacuum. 
         [0030]    Referring now to  FIG. 9 , an enlarged portion of  FIG. 6  is shown. As can be seen, sleeve  130  has been shifted downwardly but valve  194  has not been shifted downwardly at this time. Enlarged portion  240  of sleeve  130  has been moved downwardly from enlarged portion  242  of slotted collet portion  244  of valve  194 . This means that when enough pressure force is imparted to valve  194  (as discussed in  FIG. 7 ) enlarged portion  242  will move from behind shoulder  244  and allow valve  194  to move downwardly. 
         [0031]    Slip segments  136  are shown engaged with internal bore  104  of pipeline  12 , but have a smooth engagement surface rather than the sharp teeth as are characteristic of normal slips. The reason this is possible is due to the low friction bearings  144 , as will be described in  FIGS. 10-12 . This is extremely important as the extent to which normal sharp toothed slips will cut into the pipeline internal bore is unacceptable in this service. 
         [0032]    Referring now to  FIG. 10 , a quarter section graphic of a slip assembly without sharp teeth is shown. Pipe  250  is shown around centerline  252 . Slip insert  254  is touching the outside diameter  256  of pipe  250  and is contacting slip bowl  258  on the opposite side. The contact surface  260  between slip insert  254  and slip bowl  258  is tapered at approximately eight degrees as is conventional in the art. The coefficient of friction  262  at contact surface  260  and the coefficient of friction  264  at contact surface  266  between the slip insert  254  and the outer diameter  256  of pipe  250  are likely to be the same. When pipe  250  is loaded downwardly by the force indicated as  268 , slippage will occur at either at contact surface  260  or at contact surface  266 . As the coefficient of friction is the same for both surfaces and the eight degree angle of surface  260  gives an additional resisting force, the slippage will occur at surface  260 . This is shown graphically with  270  being the normal (perpendicular) force to the surface,  272  being the horizontal component of the force, and  274  being the vertical component. There is no comparable vertical component associated with the normal force to the contact surface  266  as the force is simply horizontal. This means the pipe  250  will simply slip at contact surface  266  and fall rather than the slip insert  250  sliding down the taper and wedging more tightly to grip the pipe. 
         [0033]    Referring now to  FIG. 11 , the same general geometry as was in  FIG. 10  is repeated, however, sharp teeth  280  are introduced at interface  282 . It is a general industry practice to equate the effect of sharp teeth to be a coefficient of friction of 0.5, whereas the typical coefficient of friction at a surface like  284  to be 0.1. As the sliding friction force is a product of the normal force times the coefficient of friction, the high coefficient of friction at interface  282  will more than offset the vertical component  274  as seen in  FIG. 10 , so the slip insert  286  will slide down the taper  288  and more tightly grip the pipe for failsafe support. 
         [0034]    Referring now to  FIG. 12 , instead of gripping on the outside diameter of a pipe, the need is to grip on the inside diameter  300  of a pipeline  302 . It is not acceptable to use sharp teeth on the slip insert  304  as the sharp teeth put potentially damaging teeth marks on the inside diameter  300 . A tapered surface  306  exists on the inner body  308 . The method used herein is for the coefficient of friction at interface  310  to be lower than the 0.1 coefficient of friction at interface  312 . The method for accomplishing this is to incorporate needle roller bearings  314  into the interface  310  which will have a coefficient of friction approximately 0.01 or very close to zero. Other bearings such as Teflon bearings can be used, however, the simple rolling friction of the needle roller bearings is very predictable. Using this method the problem of non-marking failsafe gripping on the inside of a pipeline is resolved. The needle roller bearings provide another feature, the elimination of hysteresis. Hysteresis in this application means that friction works against you to tighten something such as setting the slips, but when set the friction in the opposite direction can lock it in place. In simpler terms, it will get stuck. Getting stuck remotely inside a pipeline is a very bad and expensive condition. As the needle roller bearings roll into position, they simply roll out also and therefore do not get stuck. 
         [0035]    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.