The Releasing Overshot fishing tools (check our guide to successful fishing operations) are used to externally engage and retrieve all sizes of tubing, drill pipe, and casing. Here we will discuss its components including basket grapple, spiral grapple & controls. We shall also explain in detail its operations.
Overshot: First choice tool because it does not reduce I.D. and has a better pack-off than a spear.Weatherford Fishing Manual
The overshot is designed to assure positive external engagement over a large area of the fish and is ruggedly built to withstand severe drilling jarring devices actions and pulling strains without damage or distortion to either tool or fish.
Overshot Fishing Tool Components
Most overshot tools consist of a bowl, top sub, guide, and grapple or slip, control, and pack-off. The overshot bowl is turned with a taper on a helical spiral internally and then the grapple, which is turned with an identical spiral and taper, is fitted to it. Let’s have a look at the most famous Bowen Overshot as an example to check its components.
The Bowl of the Overshot is prepared with a helically tapered spiral section on its internal side. This section is equipped also with Spiral Grapple or Basket Grapple which acts as a gripping member.
So if apply an upward pull to the fish, an expansion strain is spread and becomes over a long length of the fish. This action will cause no distortion to either the fish or the overshot tool.
The Spiral Grapple
A Spiral Grapple is formed as a left-hand helix with a tapered end so that it can work with the helically tapered section in the overshot Bowl. Its interior is manufactured in a way that helps in the engagement with the fish.
The Basket Grapple
A Basket Grapple is an expansible cylinder equipped with an end that can work with the helical shape of the Bowl interior. Its design also allows engagement with the in-hole fish. Bowen manufactured 3 types each one has a different application than the other which are:
- The Plain Basket Grapple is standard & it works with any plain single diameter fish.
- The Basket Grapple with Long Catch Stop is designed to stop and catch an upset, or box section of a tool joint. At the same time, it allows the joint or upset to be packed off with a Basket Mill Control Packer due to the sufficient length below the grapple.
- The Basket Grapple with Short Catch Stop works with a ruptured piece of pipe engaged in its upper end.
The Grapple Controls
Grapple Controls are used as a special key, to allow the Grapple to move up and down during operation while simultaneously transmitting full torque from the Grapple to the Bowl.
There are a lot of types for Grapple Controls which are:
- Plain basket control
- The plain control packet (no mill teeth)
- Mill control packer (mill teeth and tapered to fish OD)
- Spiral grapple control.
One problem with an overshot fishing tool and grapple assembly is bulldogging. This is when the grapple will not release the fish because it has gone over a coupling or other large OD area that has a smaller OD immediately below. You should always include the grapple stop in the grapple assembly to prevent bulldogging.
There are main 3 types which are:
- First, The built-in basket grapple stop prevents bulldogging but reduces the strength of the overshot by 25%.
- Second, loating stop grapple does not reduce the strength of the overshot.
- Third, all stops for spiral grapples are top-loaded and do not reduce the overshot strength.
In operation, whether the overshot is dressed with any type of grapple, it will work the same way ( apply right rotation to expand the grapple) the Overshot functions in the same manner, whether dressed with Spiral Grapple parts or Basket Grapple parts. During the engaging operation, as the Overshot is rotated to the right and lowered, the Grapple will expand when the fish is engaged, allowing the fish to enter the Grapple. Thereafter with rotation ceased and upward pull exerted, the Grapple is contracted by the
The Bottom Hole Assembly Design For Overshot Fishing Tool
You will always have the following components in the bottom hole assembly (BHA): drilling overshot tool, bumper jar, oil jar, drill collars or heavy weight drill pipe, accelerators, drill pipe or tubing
When you are designing an overshot assembly, you will need to select the:
- overshot size
- connection on the top sub
- grapple type
- grapple size
- pack-off type
- grapple control type
- overshot guide type
- overshot extension size.
Read more about its design in our article overshot fishing assembly design.
Making Up Overshot Assembly
Assemble the overshot components in a clean and thoroughly doped (Thread Compound) bowl using the appropriate tightening torque.
Note: A bumper sub is usually run above an overshot.
When making up the overshot assembly, it is important not to exceed the recommended tightening torque for each of the components. It is essential that you apply the torque evenly on the OD, avoiding OD collapse. Additionally, while you may reference the maximum recommended torque value for a component, you should use a lower torque value in order to minimize wear and tear to the threads.
The below table lists the maximum recommended tightening torques for the most popular Series 150 Bowen Overshots drilling tools. The listed makeup torques are for the top sub to bowl and bowl to guide connections. Any extension would be made up to the same torque as the top sub is to the bowl. These values are the maximum torque values recommended and are set at 50% of yield.
Note: Torque is measured in ft./lbs.
|Cut Lip Guide||Torque|
Running an Overshot Fishing Tool
- Measure everything, including the bump up depth on the overshot top sub or stop.
- Pipe tripping In the Hole (TIH) with the overshot to within 10 to 15 feet of the top of the fish.
- Pick up the Kelly (Check also: Top Drive System) and any pup joints needed for space out and circulate long enough to displace the fishing string.
- Record the pick up and down weight of the string and then tag the fish with the overshot.
- Slow the pump down and work the overshot over the fish as gently as possible. Note: In some cases it will be necessary to rotate in order to get the fish to enter the overshot. However, be careful not to rotate the grapple on the fish.
- When the fish has been swallowed and the fish bumps up in the stop or top sub, apply straight over pull to engage the overshot. As long as there is an over pull, either right or left hand torque can be applied. Note: This is where knowing your exact measurements pay off.
- Maintain mud pump pressure at a low rate (500 to 700 psi), and watch for pressure bleed off as cuttings are being washed from around the fish. Caution: Do not rush this procedure. Allow plenty of time for the mud to wash through the bit. If circulation can be established, circulate long enough to displace mud around the fish and fishing string. Continually move the fishing string the distance of the free travel in the bumper drilling jars (check also in the drill string neutral point).
- Pick up on the fish at this time. Note: Do not pull up more than 20,000 pounds and do not allow the jars to hit.
- Continue pumping.
- If the fish moves, THEN continue to work the string with only enough pull to get movement.
- But, if it does not move, THEN commence jarring on the fish. Note: If the fish is moving, it will nearly always come out of the hole if you exercise patience and common sense.
Releasing an Overshot Tool In Drilling
There are several methods for releasing overshot fishing tools, including releasing:
- Downhole on a production rig
- Down hole with the Kelly in the string and bushing in the rotary table l down hole with drill pipe in the rotary table
- On the surface with the fish in the rotary table
- Down hole on a production rig.
Releasing The Overshot Fishing Tools
- Bump or jar down on the string to break the freeze between the overshot bowl and the grapple. Note: You will know that the freeze is broken when you are able to turn to the right.
- Once rotation has been established, slowly pick up while continuing to rotate. This will allow the left hand wicker in the grapple to walk off the fish.
Caution: If any overpull (check margin of overpull) is exerted during this operation, you will have to repeat the operation from the beginning.
Releasing downhole on a production rig
- Bump down on the overshot and turn it with the tongs. Note: For the most part, you will be depending on hydraulic tongs for rotation.
- Pick up slowly and turn the string until it is free of the fish, being careful not to pull up to hard on the grapple.
Note: Most fish on a work over rig can be removed from the overshot drilling tool on the ground more easily and safely than in the slips.
Overshot fishing tools are very versatile and may be fitted for a combination of problems. In some situations, we may need mill controls to dress the area that the grapple will catch in order to remove burrs and splinters on the pipe. When the pipe has been “shot off” or parted in such a way to heavily damage it, it may be necessary to fit a mill extension or mill guide, to the overshot bowl so that extensive milling (check also: Milling In Drilling Operations) can be accomplished for the catch to be made on the same trip in the hole. These extensions, or guides, are “dressed” inside with tungsten carbide and can mill off a substantial amount of material so that the “fish” is trimmed down to the grapple size.
Controls may also be designed with a pack-off, or packer, that seals off around the fish and allows the circulating fluid to be pumped through the fish to aid in freeing the stuck fish.
To properly engage an overshot tool on a fish in drilling, slowly rotate the overshot as it is lowered onto the fish. The pump may be engaged to help clean the fish and also to indicate when the overshot goes over the object. Once this has been indicated by an increase in pump pressure, stop the pump, as there may be a tendency to kick the overshot off the fish. Set the grapple with gradually increasing, light upward blows. An excessively hard upward impact may strip the grapple off the fish and cause the wickers to be dulled, resulting in a misrun and trip to replace the grapple.
To release overshot in drilling, it is first necessary to free the two tapered surfaces, bowl tool, and grapple, from each other. This freeing of the grapple or “shucking” can be accomplished by jarring down with the fishing string. Usually a bumper sub is run just above the overshot and is used for this purpose. After bumping down on the overshot the grapple is usually free and the overshot can be rotated to the right and released from the fish. If a large amount of the fish has been swallowed, it may be necessary to free or ” shuck” the grapple more than once.