Abstract:
A rig skidding method involving raising a rig on jacks, positioning skid tracks comprising low friction plastic underneath the rig, lowering the rig, and skidding the rig along the skid tracks to a new site for rig operations. At the new site, the process is reversed to leave the rig on the ground at the new location.

Description:
TECHNICAL FIELD 
     Rig skidding apparatus and method. 
     BACKGROUND 
     It is often desirable to move drilling rigs short distances between wells within a drill site. Disassembling the rig and reassembling it at the new location is time and labor intensive, and increases the rate of wear of some rig components. To avoid disassembly and reassembly, one solution has been to skid the rig structure across steel framed rig matting. However, the force required to overcome the metal-to-metal sliding friction between rig and track was problematic. Other solutions have included sliding the rig on rollers and using rig walkers. 
     SUMMARY 
     It is desired to achieve a means of moving a rig structure to a well site without requiring extensive disassembly of the rig structure that is labor and time efficient. 
     In one embodiment, there is disclosed a method of skidding a rig structure, comprising raising the rig structure; after raising the rig structure, placing a skid track under the rig structure, wherein the skid track comprises a low friction plastic sheet; lowering the rig structure onto the skid track; and sliding the rig structure along the skid track. The low friction plastic sheet may be supported by a base and the skid track may comprise a cooperating sheet, for example a continuous flexible metal sheet, placed between the plastic sheet and the rig structure. In one embodiment, there is disclosed a method of skidding a rig structure comprising placing beams in a position to support the rig structure; using jacks to raise the beams; raising the rig structure with the jacks; placing a skid track under the rig structure; lowering the rig structure onto the skid track; removing the jacks and the beams; and sliding the rig structure along the skid track, wherein the skid track comprises a low friction plastic sheet. 
     In various embodiments, there may be included any of the following features: upon sliding the rig structure to a desired location, placing beams in a position to support the rig structure; using jacks to raise the one or more beams, raising the rig structure with the jacks, removing the skid track from under the rig structure, lowering the rig structure and removing the jacks; sliding the rig structure along the skid tracks may occur by towing; the low friction plastic sheet may be a lubricant filled plastic sheet; the low friction plastic sheet may be self-lubricating; and the low friction plastic sheet may be ultra-high-molecular-weight polyethylene. 
     These and other aspects of the device and method are set out in the claims, which are incorporated here by reference. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Embodiments will now be described with reference to the figures, in which like reference characters denote like elements, by way of example, and in which: 
         FIG. 1  is a perspective view of a portion of a rig structure resting on skid tracks. 
         FIG. 2  is a front view of a portion of a rig structure raised on beams by jacks with skid tracks comprising a sliding layer and base placed underneath the structure. 
         FIG. 3  is a side view of a portion of a rig structure raised on beams by jacks with skid tracks placed underneath the structure. 
         FIG. 4  is a perspective view of a portion of a rig structure lowered onto skid tracks with a cooperating layer in this instance comprising a metal sheet. 
         FIG. 5  is a side view of a portion of a rig structure lowered onto skid tracks. 
         FIG. 6  is a perspective of a representative portion of a rig structure being towed on skid tracks, with beams and jacks removed. 
         FIG. 7  is a side view of a representative portion of a rig structure being towed on skid tracks, with beams and jacks removed. 
         FIGS. 8A, 8B and 8C  are respectively a perspective exploded view, a top view and an exploded section of an exemplary skid track. 
         FIG. 9  shows a rig structure resting on a ground surface. 
     
    
    
     DETAILED DESCRIPTION 
     Immaterial modifications may be made to the embodiments described here without departing from what is covered by the claims. 
     At the beginning and end of moving a rig, the rig rests on the ground as shown in  FIG. 9 . Removal and later replacement of catwalks and related peripheral equipment will also be carried out. The rig is represented by rig structure  10 , which comprises conventional elements of a rig. The rig structure  10  comprises a ground contacting portion on which the mast, power sources, pumps, drill pipe, etc., are carried. The ground contacting portion may comprise I-beams. When preparing to move the rig structure  10 , a set of jacks  12  are arranged to support the rig structure  10  while the skid tracks  14  are positioned underneath it, for example, under the I-beams if the rig structure  10  is supported on I-beams. The jacks  12  may, for example, be hydraulic jacks. In an embodiment beams  16  are placed through portions of the rig structure  10 , resting upon the jacks  12 . The jacks  12  are activated, raising the beams  16  and with them the rig structure  10  (see  FIGS. 2 and 3 ). The beams  16  may in some embodiments be fabricated aluminum beams. The rig structure  10  only needs to be raised sufficiently to place the skid tracks  14  underneath the rig structure  10 . Only a few inches of clearance may be required. 
     Once the rig structure  10  has been raised off the ground, skid tracks  14  are slid underneath those portions of the rig structure which would contact the ground surface  19  when lowered. The ground surface  19  may be any surface on which the rig rests including a manufactured or prepared surface such as conventional rig matting. The skid tracks  14  include a sliding layer  18  of continuous or sectioned low friction plastic sheet (see  FIGS. 8A, 8B and 8C  for construction of an exemplary skid track  14 ). In an embodiment, this plastic sheet is a sheet or sheets of ultra-high-molecular-weight polyethylene (UHMW-PE) such as TIVAR® DrySlide UHMW-PE produced by Quadrant Engineering Plastic Products. The sliding layer  18  may comprise a porous polymer having pores that include a lubricant. The particular plastic used for the sliding layer  18  should be selected for appropriate strength, corrosion resistance, and abrasion resistance. The lubricant may be a dry film lubricant or solid, such as PTFE, molybdenum disulfide, or graphite, selected for compatibility with the plastic. 
     As shown in  FIGS. 8A, 8B and 8C , in some embodiments, the layer  18  is supported by a base  20 , for example a layer of plywood sheets. The base  20  may be formed of overlapping plywood sheets, for example two layers of overlapping 8 foot long ¾ inch×24 inch sheets screwed together at 6 inch intervals. A cooperating layer  22  of stainless steel sheet may be placed between the rig structure and the sliding layer  18 . The cooperating layer  22  may be a continuous flexible metal sheet. Continuous in this context means extending the full distance along the ground engaging part of the rig structure. Flexible in this context means sufficiently flexible that the continuous sheet can be placed under the rig structure when the rig structure is raised off the ground by jacks. In an embodiment, the layer  22  may be a continuous sheet of 16-gauge  304  annealed stainless steel, 12 inches wide. The layer  22  may run the length of the rig structure or more. The layer  22  may be secured to the front of the rig structure  10  by any suitable means, so that the layer  22  moves with the rig structure  10  when it is towed. For example, if the rig structure  10  has a tubular or other element connecting I-beams, the layer  22  may be bent around the tubular element to hold the layer  22  stationary in relation to the rig structure  10 . It may be sufficient in some cases that the friction between the layer  22  and the ground contacting portions of the rig structure  10  is sufficient to cause the layer  22  to move with the rig structure  10  when it is moved. 
     In an embodiment, the sliding layer  18  may be ½ inch thick. The sliding layer  18  may be the same width as or narrower than the base  20 . The cooperating layer  22  may be as wide as the ground contacting parts of the rig structure. The layer  22  may be narrower than the layer  18 . The layer  18  may be provided in sections 24 inches wide. The layer  18  may be screwed onto the base  20 . A dry film coefficient of friction of 0.08 between the layers  18  and  22  has been found adequate, for example, as occurs between TIVAR® DrySlide UHMW-PE used as the sliding layer  18  and stainless steel used as the cooperating layer  22  but other levels of friction may be adequate depending on the towing ability of a vehicle used to pull the rig structure and the integrity of the rig structure  10  at the tow points. With the skid tracks  14  in position, the rig structure  10  may then be lowered onto the skid tracks  14  with the rig structure  10  contacting the cooperating layer  22  when present or the sliding layer  18  (see  FIGS. 4 and 5 ). 
     Once the weight of the rig structure  10  is off of the jacks  12 , the beams  16  and jacks  12  may be removed (as shown in  FIGS. 4 and 5 ). The rig structure  10  is then connected to be pulled or pushed along the skid tracks  14  ( FIGS. 6 and 7 ). In a preferred embodiment the rig structure  10  is towed by one or more vehicles  24 . Additional vehicles (not shown), positioned alongside or behind the rig structure can also be used to stabilize the rig structure on the skid tracks. 
     When towed, the rig structure slides on the sliding layer  18 , usually with the sliding interface being between the plastic sheet forming the sliding layer  18  and the cooperating layer  22  formed of stainless steel. The plastic sheet such as sheets of UHMW-PE are self-lubricating, thereby significantly reducing the sliding friction despite the substantial weight of the rig structure. In an embodiment, the cooperating layer  22  moves in relation to the sliding layer  18 , while the rig structure remains stationary with respect to the cooperating layer  22 . In this instance, the important coefficient of friction for sliding purposes is the coefficient of friction between the sliding layer  18  and the cooperating layer  22 . Depending on the rig structure base, the cooperating layer  22  could be omitted in some circumstances, but it is recommended to use a cooperating layer  22  in most instances. Instead of a metal cooperating layer  22 , other smooth and strong cooperating layers may be used. 
     Once the rig structure  10  has arrived at the desired location, the rig structure  10  may be raised again upon the jacks  12  as shown in  FIGS. 2 and 3 , and the skid tracks  14  removed. The rig structure  10  can then be lowered, the jacks  12  removed, leaving the rig structure on the ground surface  19  again as shown in  FIG. 9  after which rig operations may resume at this new location. 
     In an experimental design, a base layer  20  was made of high density hardwood (oak) plywood painted to reduce moisture and oil absorption was too brittle and would fracture when a raise pad was encountered. Therefore, it was found that a fracture resistant base layer  20  was needed, for example, ¾ inch cabinet grade fir plywood was found to have the required flexibility and strength. Other materials could also be used in place of the fir plywood having substantially the same or better fracture resistance. In the experimental design, a sliding layer  18  made of Jaytrex Virgin Natural White UHMW did not work well in higher temperatures (+25 C). It seemed to get a little “sticky.” The Dry Slide UHMW by Quadrant Plastics was firmer at higher temperatures exhibiting less friction. Therefore, it is preferred to use a sliding layer  18  that is temperature resistant, namely that retains its sliding properties above 25 C, for example, up to 40 C. For the cooperating layer, it has been found that lighter stainless steel (18 gauge) did not disperse the weight at the edges of the load, reducing the effective surface area between the cooperating layer  22  and sliding layer  18 . 16 gauge stainless steel has alleviated this issue. Therefore, it is preferred to use a cooperating layer  22  that is sufficiently flexible to be slid under the lifted drilling structure when it is lifted a very short distance, yet being bending resistant under the load of the drilling structure to disperse the weight of the drilling structure at the edges of the load and maximize the effective surface are of contact between the cooperating layer  22  and sliding layer  18 . 
     In the claims, the word “comprising” is used in its inclusive sense and does not exclude other elements being present. The indefinite articles “a” and “an” before a claim feature do not exclude more than one of the feature being present. Each one of the individual features described here may be used in one or more embodiments and is not, by virtue only of being described here, to be construed as essential to all embodiments as defined by the claims.