THE EFFECT OF JARRING ON HOISTING EQUIPMENT

Summary:When jarring, a static pull load is applied (to fire the jar) over and above the weight of the drillstring and hoisting equipment. This additional pull load is referred to as “overpull”. Due to dynamics, it is possible to have an effective overpull load (on the hoisting equipment) that is a multiple of the static overpull load. The worst case, when the jar is located near the surface, is for the dynamic overpull to be twice its static value. When the jar is located deep in the well, and/or when an intensifier is used, the dynamic overpull is only marginally greater than its static value. It is recommended to keep the total loads (static and dynamic) below the API static rating of the hoisting equipment.

Discussion:Jarring is a frequent operation in drilling and workover. While much has been written about the effects of downhole forces and jar placement, there are no established guidelines to account for the effects of jarring on hoisting equipment.

When the jar is located deep in a well or in wells with high frictional drag (such as directional wells) almost all of the jarring impact forces are dissipated before reaching the surface. When the jars are located shallow in the well, there are effects on the hoisting equipment. The largest effect occurs for large overpulls when jarring up.

Once the jar fires, there is a compressive stress wave which travels up toward the surface at approximately 19,000 ft/sec. When this stress wave hits the hoisting equipment it sudden relieves some or all of the overpull load.. The stress wave reflects off the hoisting equipment and instaneously becomes a tensile stress wave which reapplies the overpull load. The sudden reapplication of the overpull causes an effective load which can or be up to twice the overpull. The stress wave bounces back and forth between the jar and surface. For each round trip, the end of the drillstring attached to jar moves up a small amount, reducing the overpull load. Eventually the overpull load is relieved by the amount of free stroke of the jar times the effective spring rate of the drillstring.

The effective loads on the hoisting equipment, arising from the events described above, may be approximated mathematically as follows:

F = H + COF = Effective load on the hoisting equipment due to jarringH = Hook load due to the weight of the drillstring (in mud) plus the weight of the hoisting equipmentO = Overpull load ( additional static pull load at the surface, used to fire the jar )C = A factor ranging from 1.0 for deep or directional wells to 2.0 for very shallow jar location.

Note: The value for C varies according to frictional losses, drillstring design, and many other factors.

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What F should be allowed?

API Specifications 8A and 8C provides design standards, ratings and safety factor for hoisting equipment. The intent is for the user to limit application loads to be no greater than the equipment ratings. Safety factors, which range from 2.25 to 3.0 depending on load rating, are designed to account for possible material flaws and unusual loading condition which are not taken into account in the calculation methods.

One could agree that the safety factor accounts for any extra loads from jarring. In other words, F should be less than the equipment rating: but take C = 1.0 whether jarring or not.

A conservative view is to keep F less than the equipment rating: but take C= 2.0 when jarring. This worst case approach would unnecessarily limit overpulls in all but the shallow hole cases.

It is up to the user to decide on the criteria for maximum F allowed. However, we feel that reasonable approach is as follows:

n take C = 2.0 for case when the jar is under 5,000 ft from the surface

n take C = 1 .33 for cases when the jar is between 5,000 and 10,000 ft from the surface

n take C = 1.0 for cases where the jar is greater than 10,000 ft from the surface

Examples:Consider

n 5” , 19.50 lb/ft Grape E Drillpipe (22.26 adjusted lb/ft)n 10 lb/gal mudn 6-1/4” jar with 150,000 lb overpulln 40,00 lb BHA weight in mudn 30,000 lb of hoisting equipment

For jar position at 5,000 ftH = 164,000 lbO = 150,000 C = 2.0F = 464,000 or 232 tons

S0, 250 ton rated hoisting equipment is acceptable.

For jar position at 10,000 ftH = 258,000O = 150,000C = 1.33F = 458,000 lb or 229 tons

For jar position at 15,000 ft2

H = 283,000O = 150,000C = 1.0F = 433,000 or 217 tons

Jarring with Top Drives

With respect to jarring, top drives are basically no different than other hoisting devices. Their deign for lifting should be in accordance with API Specification 8A or 8C. As such, the discussion above applies to top drives as well. After jarring, it is recommended to check any electrical, hydraulic, or mechanical connection for any possible loosening from the jarring.

The thrust bearings of swivels and top drives with integrated swivels have an API dynamic load rating. This dynamic rating is always less than the hoisting equipment static load rating. However, the bearings static load rating is usually over 4 time its API dynamic rating. Thus the thrust bearings of swivels are generally unaffected by jarring.

J. Steve Williamson Bowen Tools 9/97

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