Abstract:
A pile driving transition piece. The transition piece avoids the shock loading in the stabbing points of pile driving transition pieces that otherwise would be induced when the pile driving hammer strikes the transition piece. Avoiding the shock loading prevents the characteristic fatigue cracks from forming in the stabbing point and the consequent failure of the stabbing points from rapid growth of the fatigue cracks.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is generally related to pile driving and more particularly to the design of pile driving transition pieces. 
     2. General Background 
     Pile driving hammers, particularly hydraulic hammers designed to drive pipe piles in the offshore environment, are of two types. One type has an external sleeve that enables the hammer to be cantilevered from the top of the pile that is being driven, sometimes known as a free riding hammer. The other type has a constant diameter that is equal to the diameter of the pile that is being driven, sometimes known as a slimline hammer. The free riding and slimline hammers are represented in FIGS. 1 and 3, respectively. The slimline hammer cannot cantilever from the pile top and must be supported by guides. The external guide and stabbing bell of the free riding hammer cannot clear support guides, so the pile must cantilever the free riding hammer above any obstructions. 
     A free riding hammer can be used to drive a battered pile. A couple at the level of the pile driving head and the bottom of the external sleeve of the hammer develops the necessary cantilever moment. The arrows in FIG. 1 represent the couple. Frequently, in pile driving operations a pile is driven to partial penetration with a smaller hammer and driven to final penetration with a larger hammer. Often, the external sleeve of the smaller hammer will not fit over the pile, which fits the larger hammer. To solve this problem a pile driving transition piece is stabbed into the pile top and the smaller hammer is stabbed over the smaller top end of the transition piece, as shown in FIG.  2 . The hammer cantilevers from the transition piece, developing the couple C 1  represented by the arrows acting on the top end in FIG.  2 . The transition piece cantilevers from the pile top, developing the couple C 2  represented by the arrows acting on the transition piece stabbing point  23 . A greater pile batter will develop a greater couple acting on the stabbing point. Making the stabbing point longer reduces the magnitude of the couple. 
     Historically, transition piece stabbing points have had a fatigue problem, which gets worse as the stabbing points are made longer. A fatigue crack  24  forms a few inches below the driving shoulder and runs circumferentially around the stabbing point, causing the stabbing point to break off and fall into the interior of the pile. If the pile is battered enough, the transition piece and hammer will fall off the top of the pile once the stabbing point breaks off. The typical location for the fatigue crack  24  is indicated in FIG.  2 . 
     A slimline hammer, as seen in FIG. 3, can only fit one diameter of pile. To use a slimline hammer on a pile with a diameter greater than the hammer diameter presents a support problem because the support guides must be sized for the larger diameter pile. Then the guides are too large to support the smaller diameter hammer. 
     SUMMARY OF THE INVENTION 
     The invention addresses the above needs of preventing fatigue cracks and allowing a slimline hammer to be used on more than one pile diameter. What is provided is a shock avoiding pile driving transition piece. The transition piece separates the function of delivering the driving energy of the hammer to the pile top from the function of cantilevering the hammer and transition piece from the top of the pile. A shock isolation unit provides the only axial connection between the transition cantilever element and the driving element. The connection is made with an elastomeric material. The material is resilient, so that when a blow is struck on top of the driving element, the driving energy passes through the driving element to the pile top without a significant portion of the energy being diverted into the cantilever element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a further understanding of the nature and objects of the present invention reference should be made to the following description, taken in conjunction with the accompanying drawing in which like parts are given like reference numerals, and wherein: 
     FIG. 1 illustrates a prior art free riding hammer and transition piece. 
     FIG. 2 illustrates the prior art where a transition piece is used to accommodate a hammer smaller than the pile, and a typical fatigue crack. 
     FIG. 3 illustrates a prior art slimline hammer driving a battered pile. 
     FIG. 3A is a view taken along lines  3 A— 3 A in FIG.  3 . 
     FIG. 3B is a detail view of the interface between the slimline hammer and the pile top taken along lines  3 B— 3 B in FIG.  3 . 
     FIG. 4 illustrates the configuration of the invention for use with a free riding hammer. 
     FIG. 5 illustrates an alternate embodiment of the invention in FIG.  4 . 
     FIG. 6 illustrates the configuration of the invention for use with a slimline hammer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a prior art arrangement for a free riding hammer  10  resting on a pile  12 . Among other components, a free riding hammer  10  is generally formed from a hammer anvil  14  and an external sleeve  16 . The sleeve  16  has an internal diameter that is slightly larger than the outer diameter of the pile  12 . A stabbing bell  18  is provided on the lower end of the sleeve  16  to provide for easier positioning of the sleeve over the pile. The upper end of the pile  12  is provided with an increased wall thickness driving head  20 , which the hammer anvil  14  strikes. As indicated above, a free riding hammer can be used to drive a battered pile (a pile that is at an angle from the vertical). A couple C 1 , acting on the sleeve  16  at the level of the driving head  20  and the level at the bottom of the sleeve  16 , develops the necessary cantilever moment. 
     FIG. 2 illustrates a prior art arrangement for a free riding hammer  10  where the size of the free riding hammer  10  and pile.  12  do not match. It should be noted that the free riding hammer  10  is not shown in this drawing. To accommodate the size difference, a transition piece  22  is stabbed into the top of the pile  12  and the free riding hammer  10  is stabbed over the smaller top end of the transition piece  22 . As described above, the upper end of the pile  12  is provided with an increased wall thickness driving head  20  that the hammer anvil  14  strikes. A fatigue crack  24  that is characteristic of this arrangement is indicated in the location that the crack normally develops, i.e., shortly below the driving shoulder of the transition piece. 
     The couple C 1  cantilevers the free riding hammer  10  from the transition piece  22 . The couple C 2  cantilevers the transition piece  22  from the pile  12 . 
     FIGS. 3,  3 A, and  3 B illustrate a prior art arrangement for a battered pile  12  driven by a slimline hammer  26  with pile  12  and hammer  26  guided by supports  28  extending out from the leg  30  of a jacket. It can be seen in FIG. 3B that the hammer  26  has an anvil  32  that stabs into the upper end of the pile  12 . 
     It is seen in FIG. 4 that the invention is generally indicated by numeral  32 . The shock avoiding pile driving transition piece  32  is generally comprised of a driving element  34 , a cantilever element  36 , and a shock isolation unit  38  between the driving and cantilever elements. This arrangement is for use with a free riding hammer, not shown. A free riding hammer is received over the top of the transition piece  32  during pile driving operations. 
     The cantilever element  36  is received in the upper end of the pile  12  and extends above the upper end of the pile  12 . The shock isolation unit  38  provides the only axial connection between the driving element  34  and the cantilever element  36 . The shock isolation unit  38  is formed from an elastomeric material, i.e. rubber vulcanized to the steel cylinders, or a material such as urethane bonded to the steel cylinders. The shock isolation unit material is resilient, so that when a blow is struck on top of the driving element, the driving energy passes through the driving element to the pile top without a significant portion of the energy being diverted into the cantilever element. In the prior art, the cantilever element is welded to the driving element, which results in a significant portion of the driving energy being diverted into the cantilever element, thus causing the running fatigue crack  24 . 
     It is seen in FIG. 4 that Couple C 1  cantilevers the free riding hammer  10  (not shown) from the transition piece  32 . Couple C 2  cantilevers the driving element  34  from the cantilever element  36 . Couple C 3  cantilevers the cantilever element  36  from the pile top. The couples are developed between close fitting cylindrical surfaces on the opposed elements, which permit relative axial motion of the elements when the hammer strikes a blow. 
     A fail-safe stop  40  may also be provided on the interior diameter of the driving element  34 . The fail-safe stop  40  is formed from complimentary shoulders  42  and  44  on the driving element  34  and the cantilever element  36 . The stop  40  prevents the cantilever element  36  from falling if the shock isolation unit  38  should fail completely during pile driving operations. 
     FIG. 5 illustrates an alternate embodiment of the invention of FIG.  4 . In this embodiment, the shock isolation unit  38  is located on the top of the cantilever element  36 . In this position, the shock isolation element  38  also serves to develop the upper force of Couple C 2 . This embodiment eliminates the required close tolerances of the cylindrical surfaces in this region. The fail-safe stop  40  is moved lower to accommodate the location of the shock isolation unit  38 . 
     FIG. 6 illustrates the shock avoiding pile driving transition piece  32  in an arrangement for use with a slimline hammer  46 . In this arrangement, a first cantilever element  36   a  is received in the upper end of the pile  12  as in the arrangement of FIG. 4. A second cantilever element  36   b  extends above the pile  12  and is substantially the same diameter as the pile  12 . The driving element  34  is positioned between the first and second cantilever elements and contacts the upper end of the pile for transferring driving force thereto. Two shock isolation units  38  are provided on either side of the driving element  34  to resiliently connect the driving element to the cantilever elements. The slimline hammer  46  is received in the second cantilever element  36   b  for contacting the driving element during pile driving operations. As indicated in the arrangement of FIG. 4, a fail-safe stop  40  may also be provided. Stop  40  is formed from shoulders  42  and  44  on the driving and cantilever elements, respectively. Couple C 1  cantilevers the slimline hammer  46  from the second cantilever element  36   b . Couple C 2  cantilevers the second cantilever element  36   b  from the driving element  34 . Couple C 3  cantilevers the driving element  34  from the first cantilever element  36   a . Couple C 4  cantilevers the first cantilever element  36   a  from the pile  12 . 
     Because many varying and differing embodiments may be made within the scope of the inventive concept herein taught and because many modifications may be made in the embodiment herein detailed in accordance with the descriptive requirement of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.