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Overshot Fishing Assembly Design In Oil & Gas

Overshot Fishing Assembly design
Overshot

Overview             

When you are designing an overshot fishing assembly, you will need to select the:

  • overshot size
  • connection on the top sub
  • grapple type
  • grapple size
  • pack-off type
  • grapple controtype
  • overshot guide type
  • overshot extension size.

Selecting the Overshot Fishing Assembly Size

The OD of the fish and the ID of the hole determine which size overshot you should use.  Under normal conditions, you should run the largest overshot that will go in the hole, allowing the best possible chance of catching the fish.

Make sure you establish the dimensions of the fish and find out about any hole ID restrictions in order to select the appropriate overshot size.  A good rule of thumb is to use an overshot about 1/8″ less than the casing ID and about ½” less than the open hole size.

Table 1 provides the OD and maximum catch of the overshot recommended for each size of API casing.

Casing SizeCasing WeightRecommended Overshot ODRecommended Overshot Maximum Catch
4 ½”9.50–11.6#3 ¾”3 1/16″
4 ½”13.50#3 ¾”3 1/16″
4 ½”15.10#3 3/8″2 ½”
5″11.50–15#4 ½”3 1/8″
5″18#3 7/8″3 1/8″
5″29.3–24.2#3 ¾”3 1/16″
5 ½”13–17#4 11/16″3 21/32″
5 ½”13–17#4 11/16″3 1/8″
5 ½”20#4 9/16″3 21/32″
5 ½”23–26#4 3/8″3 3/8″
6 5/8″325 1/8″4 ¼”
7″17–23–26#5 ¾”4 ¾”
7″17–23–26#5 7/8″5″
7″29–32#5 ¾”4 ¾”
7″35#5 ¾”4 ¾”
7″38#5 9/16″4 ¼”
7 5/8″20–33–37#6 3/8″5 ¼”
7 5/8″39#5 7/8″5″
8 5/8″24–40#7 3/8″6 ¼”
8 5/8″44–49#7 1/8″6″
9 5/8″29.3–36#8 1/8″7″
9 5/8″40–43.5#8 1/8″7″
9 5/8″47#8 1/8″7″
9 5/8″53.5#8 1/8″7″
Table 1: Recommended overshot sizes

Selecting the Grapple Type

The fish size and type will determine the type of grapple that you should use.

IF the fish…THEN use a…
diameter is near the maximum catch of the overshot,spiral grapple, spiral grapple control, and type “A” packer.
is a tubing collar,type “D” collar stop pack-off.
diameter is ½” less than the maximum catch size,basket grapple and a mill control packer.

Grapple Catch Size

Grapples are manufactured with small tolerances in catch sizes.  For example:

  • The tolerance for a catch size of spiral/basket type grapples is 3/32″ undersize to 1/32″ oversize.  This only applies to FS type overshots.
  • Basket grapple tolerances for slim hole overshots are 1/16″ undersize with no oversize tolerance.
  • Tolerances for slim hole spiragrapples are 1/16″ oversize and undersize.

Selecting The Pack-off Type

Select the appropriate pack-off according to the criteria below:

IF you are using a…THEN select a…
spiral grapple,type A packer.
basket grapple,control packer (plain or with mill teeth).

Selecting The Overshot Fishing Assembly Grapple Control

There are three types of grapple controls:

  • Spiral grapple control
  • Mill control packer
  • plain control packer.

When using a spiral grapple, use a spiral grapple control.

Use a mill control packer for basket grapples if the top of the fish is burred. As you rotate, the mill control packer will enable you to dress off any bur on the fish.  However, a mill control packer will not dress a badly flared fish.

A plain control packer is best if you want to offer the fish the least resistance while entering the grapple.  For example, you might use a plain control packer in a wire line-stripping job.

Selecting The Overshot Fishing Assembly Guide Type

You should be aware that the fish may be:

  • off-centered in the hole
  • much smaller than the hole
  • in a washed out area of the hole.

In these situations, an overshot guide may help the overshot to catch the fish.

Use Table 7-7 to select the guide necessary for the fishing operation.

Guide TypeUse it when…
cut-lippedthe top of the fish is likely to be centered in the hole.  The cut lip guide pulls the fish into the center of the overshot, enabling the fish to enter into the overshot.
oversizethe hole size is so much larger than the fish size that the overshot could pass along side the fish.  The oversize guide may be used to better utilize inventory by keeping a smaller selection of overshots.  However, be careful not to sacrifice strength by using an oversize guide.
wall hookthe fish cannot be pulled into the overshot with a standard cut lip guide.  The wall hook guide is used with a bent joint of pipe or a hydraulic knuckle joint to sweep a washed out section of the hole.
fluted millthe top of the fish is badly flared or jagged.
Table 2: Overshot guide types

Selecting The Overshot Extension Size Design

The overshot extension should be the same OD as the overshot itself. Ensure that the ID of the extension will accommodate the fish.

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