A Measuring the pipe stretch under tension

The method is based upon Hooke's Law. Knowing the stretch under a particular tensile load enables the unstretched length to be calculated. This equals the length of pipe between the stuck point and surface. The stretch in the pipe is found with reference to some fixed point which will not move when extra pull is applied

(SI UNITS)

26.374 x Wdp x e where L = Length free pipe in metres

Wdp = Plain end pipe weight in kg/m e = Differential stretch in mm p = Differential pull in kN

(API UNITS)

735294 xWcpxe P

where: L = Length free pipe in ft

Wdp — Plain end pipe weight in lb/ft e = ins p — lbs

Reasonable estimates of the depth of the stuck point can be obtained in this way. The values obtained are less reliable asthe deviation increases due to down hole friction. Other minor inaccuracy is introduced by neglecting the changing crosssection of the string at the tooljoints.

A stuck or free-point indicator service is offered by Schlumberger and Dia-log. A sensitive electronic strain gauge is run on the logging cable. The cable transmits measurements of torque and stretch in the pipe to the surface read-out instruments.

The indicator is lowered to various depths and at each depth tension and torque are applied to the string at the surface. The strain gauge indicates whether the pipe reacts at that depth to the applied tension and the applied torque. Pipe which appears to be free in tension does notalways react to applied torque. A back-off can only succeed if the pipe is free in both senses

Seperate slim acoustic logs are designed to indicate intervals of stuck, partially stuck or free pipe which may exist below the upper stuck point.

Drillpipe or collars can be unscrewed downhole by exploding a charge inside a selected tooljoint connection, just above the stuck point.

A succesful back-off depends upon having the following.

Select a connection which has been broken during the round trips prior to the pipe becoming stuck.

For a safe operation carry out the following checks.

  • ensure that tong and slips dies are clean, sharp and the proper size for the string above the rotary
  • check that tong, snub and jerk lines are in excellent condition
  • ensure that slip handles are tied together with strong line, to prevent the slips being kicked out of the table and thrown clear when the pipe breaks out
  • ensure that elevators are latched around the pipe and slackened off under a tool joint with the hook locked when torque is being applied to the string
  • ensure that no torque remains in the string when it is picked out of the slips, unless the pipe is properly held with a back-up tong.

Particular care should always be taken when applying torque or releasing it from the string. Keep the forces involved fully under control and keep men out of the potentially dangerous area.

Note: - Torque should be worked down the string before the string shot is fired, this may take some time.

If the string fails to backoff after the firing of the charge, continue to work the torque down the string before trying another string shot.

sufficient left-hand, or reverse torque at the joint zero or slightly positive tension at the joint a sufficiently large explosive charge, accurately located at the joint

The ideal tensile load is zero, ie with the threads subject to neither compression nor tension. However, since a zero tensile load is difficult to achieve, pull is applied which will develop a slight tension rather than compression. Over the years there has been some debate regarding the surface pull required to achieve this condition. Since the pipe is held down then it can be assumed that buoyancy does not affect the pipe above the stuck point. However, as soon as the joint is cracked buoyancy will act on the freed pipe.

If buoyancy does not apply then the pull required to maintain the drillpipe in tension will be the total weight of pipe above the stuck point plus the weight of other equipment such as blocks.

An alternative method for finding the required pull is to use the actual hook load observed by the Driller just before getting stuck:

c -ir-,.. Hookload-wt, of blocks-weight of fish in mud , ,. , , Required Pull = y + wt. of blocks

Bouyancy Factor

In deviated wells with excessive drag and pull it will be difficult to develop the correct tension at the joint and more than one attempt may be necessary before a successful backoff is achieved. In a highly deviated well the pipe weight may be partially supported.

If the hookload while moving the string slowly up has been observed prior to becoming stuck, the following method can be used to estimate the required pull:

  1. Calculate the theoretical weight otthe whole string in air (using approximate weight for drillpipe)
  2. Subtract from this the observed weight of the string (hookload-blocks)
  3. This gives the weight loss due to buoyancy, friction and wall support which can be expressed as a percentage.
  4. Calculate the theoretical weight of pipe in air down to the stuck point (using approximate weights) then subtract the percentage weight loss due to buoyancy and wall support etc.
  5. Add the weight of the blocks etc. and this will be the tension prior to backoff.

Listed below are fishing tools often kept on the rig site for various hole sizes drilled.

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Responses

  • Rufneck
    does bouyancy affect stuck pipe?
    9 years ago
  • Ansegar
    How to calculate drill pipe stretch on the rig?
    8 years ago
  • Amanuel
    How to calcuate pipe stretch?
    7 years ago
  • Sini
    How to figure srill pipe atretch?
    7 years ago
  • jarmo
    How to calculate tubing stretch?
    7 years ago
  • GRANT
    How can I calculate the strech on stuck pipe?
    1 year ago
  • Adalfredo Trevisan
    How is drill pipe stretch calculated?
    1 year ago
  • libby
    How to figure pipe stretch?
    1 year ago
  • biniam
    How to calculate stretch for tention?
    9 months ago
  • donata
    How much can steel stretch?
    6 months ago
  • BISRAT
    How to determine free point pipe?
    4 months ago
  • massimo trevisano
    How do you figure stretch on hung tubing?
    4 months ago
  • William Edwards
    How to measure the length op oilfield pipe?
    3 months ago
  • Pandora
    How to caluclate the pipe strech in completion tools?
    2 months ago
  • madoc
    How to measure pipe stretch?
    2 months ago
  • aamos hyypi
    Can you calculate how much steel will stretch in tension?
    2 months ago

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