Tripping pipe can be defined as the operations of running in hole or pulling out of hole with the drill string. In this article we have prepared a complete guide for who needs to learn its operations on surface and who needs to understand its procedures to prevent any unforeseen problems.
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During such trips, the potential exists for a significant reduction in bottom hole pressure due to the following effects:
- Reductions of the Equivalent Circulating Density as the mud pumps are stopped.
- Swab pressures due to pipe motion.
- Reduction in height of the mud column as drill pipe is removed from the well.
The procedures required to deal with an influx when the pipe is off the bottom are not as straight- forward as when the pipe is on the bottom, therefore it shall be ensured that the well is stable prior to initiating a trip out of the hole and that correct tripping procedure is strictly adhered to.
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PPE Necessary for Tripping PiPe Operations
A high proportion of accidents on the drill floor while pipe tripping involve the use of tongs. Injuries result from being caught between tongs and being struck by swinging tongs. All tongs should be securely attached and anchored. Tong safety lines should be of sufficient length, preferably allowing a 90° breakout angle between the lines, but short enough to prevent over rotation of the tongs.
The tong jaws including the dies should be inspected regularly for size and condition. Tongs shall be maintained and replaced well before they become worn to the point of being unsafe. Hinge pins should be secured by a nut which should itself be locked in place by a device such as a split pin.
All tong counter balances and parts thereof shall be so restrained, guarded, or located as to prevent them falling or striking crew members if the suspension line breaks. Remember that suspension lines are classified as lifting devices and should be inspected, certified and colour coded in the same manner as slings. Their history should be recorded in the sling register.
While changing tong/slip dies or tripping operations ensure that:
- goggles are worn as hammer blows can cause splinters to develop
- gloves are used
- usual PPE as coverall & safety shoes
- working area is clear and unaffected by other operations in progress.
Back-up post failure has potentially serious consequences. A systematic approach to back-up post-inspection and planned preventive maintenance is required which includes MPI inspections on a six monthly basis. They should also be included in the weekly/monthly drilling equipment inspection checklist
Pre-Trip Checklist & Safety Meeting Prior Tripping Pipe
Ensure that the crew is up to strength and individuals are aware of tasks to be carried out. A communication system should be operational between the rig floor and monkey board. Note the following:
- before starting a trip, ensure that the equipment required is in a serviceable condition
- avoid using the rotary table to spin out pipes. Use a pipe spinner
- never make up a connection with rotary table while using a tong as back-up, the shock loading could result in breaking of the back-up wire
- never attempt to stab a single into the mouse hole when the Driller is lowering the travelling block
- ensure the crown safety device is correctly installed and adjusted at the start of each tour and following drilling line slipping or cutting operations
- monitoring of mud levels, flow checks and use of the trip tank should be part of the routine tripping procedures in and out of the hole
- do not install wipers until, the Driller is satisfied with the hole condition. they act as obstructions to the visual checking of mud levels
- while pipes are being moved from or to the rig floor the catwalk should be kept free of personnel
- when hole conditions allow, pump a heavy pill to avoid pulling ‘wet’ pipe.
Prior to Tripping
Considerable preparation is required before the trip is commenced.
The following are among the most important actions that should be carried out prior to tripping:
- Circulate the hole
- Circulate bottom up until hole/shakers clean from cuttings at the maximum allowed flow rate, reciprocating and rotating the string avoiding prolonged circulations at a point.
- The mud should be conditioned to ensure that tripping will not cause excessive swab/surge pressures. Conditiong means that the rheological characteristics are those planned for the actual hole section and that the mud weight in and out is even
- Any entrained gas or cuttings should be circulated out.
- The mud weight should be such as to ensure that an adequate overbalance will exist at all times during the trip.
- Determine the maximum pipe speed
- Swab/surge pressures should be calculated at various tripping speeds using the appropriate formulae. Calculation of swab/surge pressures is typically available from the mud logging Service Provider.
- The maximum average pipe speed should be selected bearing in mind the estimated overbalance or trip margin.
- Line-up the trip tank
- A trip tank must be available on every rig and be complete with indicator(s) of the trip tank level, visible from the Driller’s position. The trip tank level must also be monitored from the Mud Logger’s cabin.
- It is considered unsafe to trip without a trip tank and, as such, spare parts for the hole fill pump/motor should be kept at the rig site.
- In order that maximum use is made of the trip tank on trips in and out of the hole, a trip sheet shall be used to record the mud volumes required to keep the hole full.
- Fill in the trip sheet.
- A trip sheet should be filled out by the Driller on every trip. Table 2.1 shows an example of a trip sheet. The basic requirement for a trip sheet is that a clear method of comparing the calculated with actual hole fill volumes is provided. The cumulative discrepancy between the two values should also be recorded. The trip sheet for the last trip out of the hole should be available for comparison.
- Provide the Driller with the necessary information.
- The Driller should be told the reason for the trip.
- The Driller should be told of any indicators of increasing pore pressure or near balance that were identified during drilling, before or since he came on shift.
- Troublesome zones, where stringers, dog-legs, washouts may be present based on the available information should be highlighted.
- The Driller should be fully aware of the procedures to be adopted in the event of a kick while tripping.
- The maximum allowed overpull, also considering the weight of the drillstring, shall be noted to prevent mechanical stuck pipe.
- The Driller should be aware of the procedures to prevent the occurrence of differential stuck pipes.
- An example of the standing orders that should be provided to the Driller is shown in Figure 2.1.
- Drill floor preparation
- Crossovers should be available on the rig floor to allow a full opening drillpipe safety valve to be made up to each tubular connection that is in the hole.
- A drillpipe safety valve (kelly valve) should he available on the rig floor. It should be kept in the open position.
- A back-up safety valve, such as a Gray valve, should be available close to the rig floor. This valve should only be used in the event that the drillpipe safety valve does not hold pressure, or if stripping operation in the hole is required and no dart sub is fitted.
- The rig crew should be completely familiar with and trained in their responsibilities in the event of a kick.
Surface Tripping Pipe Operations
Making pipe Connection
When the kelly, or top-drive system, has drilled all the way down, it is withdrawn and a new length of drill pipe is added. Refer to Figure 2 (a through d) for the following text description.
In (a) the kelly is near the “kelly down” position, where another joint of pipe must be added. The new length of pipe has been placed in the “mousehole”, ready to be connected. The mousehole is used to store the next measured joint of pipe until it is required for threading into the drill string. The crew breaks out the kelly so that it can be swung over to the joint of pipe in the mousehole, as in (b). The kelly is made-up on the joint and tightened with the tongs. In (c) and (d), the new joint of pipe is picked up, swung over to the pipe hanging in the rotary table, and “stabbed” into the drill string. The new pipe is screwed into the drill string, tightened with the tongs, and lowered into the borehole to drill another joint length.
The difference with “top drive in drilling rig” systems is that the kelly is not used and stands of pipe are drilled instead of joints. The next stands to be drilled may be placed in the mousehole or may be left standing in the monkey-boards.
When making a trip, the drill pipe is handled in stands of three joints each (approximately 93 feet). The pipe is removed from the borehole and set back on the rig floor. Refer to Figure 3 for the procedure for pulling the pipe out of the hole (tripping out). The kelly, rotary bushing, and swivel are placed in the rathole during the trip, as seen in (a). With the kelly out of the way, the elevators are latched around the pipe just below the tool joint’s box end.
The tool joint’s box end provides a shoulder for the elevators to pull against. The pipe is then pulled from the borehole, and after being secured in the rotary table (using the slips), the connection is loosened with the break-out tongs. A spinning wrench (power tongs) is commonly used to unscrew the pipe at this time. This is illustrated in Figure 4.
The top of the stand, which has been pulled past the derrickman (standing on the monkey-board, Figure 4-2b), has a rope thrown around it. The bottom of the stand is swung to one side of the drill floor where it is set down (Figure 4-2c), and the derrickman racks the top of the stand in the “fingers” in the monkey board to secure it.
The drill collars and Drilling bit are the last to come out of the borehole. The master bushing may have to be removed to allow the large diameter collars to pass through the rotary table. When the bit appears, the master bushing is replaced and a “bit breaker” is placed in the rotary table. Using the breakout tongs, the bit is loosened and removed from the bit sub.
Tripping in, is just the reverse procedure of tripping out.
Some rigs have a pipe handling system to speed up pipe movement during tripping operations (Figure 5)
Downhole Pipe Tripping Procedures
Having completed the preparations as outlined in the previous section, the trip out of the hole can be started. The following procedure is proposed as a guideline:
- Flow check the well with the pumps off to ensure that the well is static.
- Pump a slug. This enables the pipe to be pulled dry and the hole to be accurately, monitored during a trip.
Note: the slug shall not be pumped until a clear indication of well static has been achieved. It is suggested to slug pipes only after the well has been flow checked following the first 5-10 stands pulled. The following formula can be used to calculate the volume of slug to ensure a length, L, of dry pipe:
Vsl = (MW x L x Cp) / (MWsl-MW)
– Vsl = volume of the slug, L (bbl)
– L = length of dry pipe, m (ft)
– Cp = internal capacity of pipes, L/m (bbl/ft)
– MW = mud weight in the hole, kg/L (ppg)
– MWsl = slug weight, kg/L (ppg).
As a general rule, the slug should be mixed to maintain a minimum of 2 stands of dry pipe. It is important to accurately displace the slug to the pipe. In this manner, the Driller will know the weight, depth, and height of the slug at all times during the trip.
- Well operations Supervisor, the Toolpusher and/or the Directional Driller shall be on the rig floor during the initial stage of the trip out of hole (e.g. first 10 stands at least) and during any expected problematic interval.
- For the first 5-10 stands off bottom, monitor the hole through the rotary. This is to check any fluid drop in the annulus level as pipes are removed from the hole. The pipe wiper should therefore be installed only after these stands have been pulled. The trip tank should not be overfilled at this stage in order to ensure that swabbing is clearly noticed, should it occur. The circulating pump should be switched off and the hole filled from the trip tank.
- Circulate the hole across the trip tank and continue to trip out, monitoring hole volumes with the aid of the trip sheet.
- Conduct a flow check when the BHA is into the casing shoe.
- Conduct a flow check prior to pulling the BHA through the Blow Out Preventers stack.
If unsure of the overbalance, consideration should be given to conducting a short round trip. Once back on bottom, the overbalance can be assessed from the level of the trip gas at bottoms up.
If the hole does not take the correct amount of fluid at any stage in the trip, a flow check should be carried out. If the flow check indicates no flow and the cause of the discrepancy cannot be accounted for at the surface, the string should be returned to the bottom while paying particular attention to displacement volumes. After circulating bottoms up, it may be necessary to increase the mud weight before restarting the trip out of the hole. If the flow check is positive, the well should be shut-in procedures according to the procedure indicated in the standing orders. Subsequent action will be dependent upon the conditions at the rig site.
Tripping Procedures With Oil-Based Muds
When oil-based muds are in use, gaseous fluids have a tendency to go into solution with the mud at high temperatures and pressure. Experience has shown that once an influx has gone into the solution, it will not break out of the solution until the bubble point is reached, typically at 70-105 kg/cm2 or 1000-1500 psi (this will depend on the specific fluids).
The possible consequence of this is that a small influx that is undetected at depth may suddenly break out of solution close to the surface, suddenly releasing the gas phase and thus resulting in a significant reduction in hydrostatic pressure in the well and possibly representing a danger for persons working on the rig floor.
Consideration should also be given to the possibility of thermal expansion of the mud at high temperatures. This can cause a reduction in effective mud weight and hence in the overall hydrostatic head. It is, therefore, recommended that tripping procedures are modified to take account of this potential problem when oil-based muds are used, in the following situations:
- When drilling or coring in a potential pay zone;
- On prediction of an increase in pore pressure;
- On detecting significant levels of gas in the mud.
In these circumstances the following procedure is recommended prior to pulling out of the hole:
- Flow check the well.
- Circulate bottoms up.
- Check trip to the shoe monitoring hole volumes.
- Flow check at the shoe and run back to bottom.
- Circulate bottoms up.
- If necessary increase the mud weight and perform a further check trip.
- In case, while tripping out, close in the BOP and circulate through the choke when the potential influx is at about 500 m (1600 ft) below the stack, watching for any pit gain.
This procedure can be relaxed if sufficient experience and confidence in the operations have been achieved.
Tripping Procedures for HT/HP Wells
The mud will be circulated and conditioned until the Mud Engineer is satisfied that the mud weight is consistent and the rheology will ensure minimum swab and surge pressures. The final mud properties will be reviewed by the Well Operations Supervisor before the trip starts.
- Prior to tripping, perform a 15 minute flow check across the trip tank to ensure that the well is stable. On floaters, ensure that there are no crane movements while performing the flow check.
- Conduct a short (5-10 stands) check trip, do not pump a slug and do not pump out of hole. This may be as little as 2 or 3 stands, if tight hole has been experienced further up the well.
- Start pulling out of hole monitoring the fluid level in the trip tank. Do not fit a pipe wiper.
Note: Do not persist with the check trip, if the hole starts to pull tight. The pipe normally will be pumped out of hole on a full trip. RIH to bottom and carry out the flow check procedure.
- If the hole does not take the correct volume of fluid:
- Stop tripping.
- Install a full opening valve.
- Flow check the well for 15 minutes across the trip tank. If in doubt, shut in and observe pressures.
- Once back on bottom, circulate bottoms up.
- If there are no problems while pulling out of the hole on the check trip:
- Perform a flow check for 15 minutes across the trip tank.
- RIH to bottom monitoring hole volumes and taking into account the surge pressures. Circulate bottoms up.
- If gas is breaking out, the trip margin may be too low. If necessary, increase the trip margin and perform a further check trip, as outlined previously.
- Once a safe trip margin has been established, drop the survey barrel, if so required, and start the first stage of the trip out of the hole to the shoe.
- Pump out of hole to the shoe. This will prevent any problems with a slug, if any circulating and back reaming have to take place, before the shoe is reached.
- The pump rate for the trip should be at a slower rate than that of drilling.
- At the shoe, perform a further 15 minute flow check across the trip tank.
- Pump a heavy slug to avoid a wet trip and resultant uncertainty concerning fluid volumes.
- Calculate the slug size and return volume to give two stands of dry pipe.
- Allow the slug to stabilize with the top drive disconnected and ensure that the correct volume of fluid returns, as the slug equalizes.
- Continue to trip out of hole, following the trip schedule as calculated in the trip margin calculations.
- Carry out another flow check, before the BHA is pulled through the BOP stack.
- If the trip is interrupted for any reasons, install the drill pipe safety valve.
- If the hole fill pump fails during the trip, do not fill the hole through the drill pipe. Whilst the drill string is out of the hole, the shear rams will normally remain open. The well will be monitored by circulating with the trip tank. If the shear rams are closed for any reasons, the well should be circulated down the kill line and up the choke line with the trip tank.
- If a 7” liner has been run, the reduced clearance between the drill string and the liner will increase the likelihood of swabbing, while tripping. For this reason, the length of the trip, performed by pumping with the top drive, should extend up to the top of the 7” liner. When pulling out of the hole with a tapered string, i.e. 3½”-5”), 15 additional minutes of flow check will be carried out:
- When the bit is at the liner overlap.
- Prior to the smaller diameter pipe entering the BOP.
Ref: Shell EP 95-0210 HSE manual, Baker Hughes Oil Field Familiarization Book & Eni Drilling operations manual