A calibrated trip tank in drilling is normally used for the accurate measurement of mud volumes and changes to mud volumes while tripping. It must be available on all rigs & be equipped with a level indicator easily readable from the driller’s position. Such an indicator must be accurate to within one-half barrel volume.
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
Drilling Trip Tank Definition
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A trip tank is a low-volume, calibrated tank that can be isolated from the remainder of the surface drilling fluid system and used to accurately monitor the amount of fluid going into or coming from the well. A trip tank may be of any shape provided that it is calibrated accurately and a means is provided for reading the volume contained in the tank at any liquid level.
The readout may be direct or remote, preferably both. The size of the tank and readout arrangement should be such that volume changes in the order of one-half barrel can be easily detected. Tanks containing two compartments with monitoring arrangements in each compartment are preferred as this facilitates removing or adding drilling fluid without interrupting rig operations.
Trip Tank Application In Drilling
The primary use of the trip tank is to measure the amount of drilling fluid required to fill the hole while pulling pipe to determine if the drilling fluid volume matches pipe displacement. Other uses of the trip tank include measuring drilling fluid or water volume into the annulus when there is lost circulation & returns are lost, monitoring the hole while logging or following casing cement job, calibrating drilling fluid pumps, etc. The trip tank is also used to measure the volume of drilling fluid bled from or pumped into the well as the pipe is stripped into or out of the well.
If the fluid level in the hole falls as the pipe is removed a reduction in bottom hole pressure will occur. If the magnitude of the reduction exceeds the trip margin or safety overbalance factor a well kick may occur (this is considered one of the major causes of kicks). The hole must be kept full with a lined-up trip tank that can be monitored to ensure that the hole is taking the correct amount of mud. If the hole fails to take the correct mud volume, it can be detected. A trip tank lineup is shown in Fig 1.
- While pipe tripping, for the first 5 to 10 stands off bottom monitor the hole through the rotary. This is to check that the annulus is falling as the drill pipe is removed from the hole. The pipe wiper should therefore be installed only after the first stands have been pulled. The trip tank should not be overfilled at this stage to ensure that swabbing is clearly indicated, should it occur. The circulating pump should be switched off at this stage and the hole filled from the trip tank, after each stand.
- At any stage while tripping out of the hole, and if the hole does not take the correct volume of drilling mud, the pipe should be run back to bottom using the trip tank, and bottoms-up circulated.
- In stripping operations, line up choke manifold so that returns are taken to the trip tank allowing accurate measurements to be recorded. Preference is to have a calibrated stripping tank take off from the trip tank. If this is not installed, the worksheet will reflect any net gain over calculated/predetermined closed-end pipe displacement.
- A trip tank and pit watcher should be considered if vessel movement creates any problem in measuring drilling fluid requirements on trips.
- It is recommended for floating drilling units that flow checks be performed on the trip tank with the hole fill pump circulating across the bell nipple to eliminate rig motion as much as possible.
- In any shut-in procedure, it is prudent to line up the annulus to the trip tank above the annular BOP or rams. This will assist in double-checking to see if they are leaking.
- In Choke Drill, it is desirable to discharge into a trip tank to accurately monitor flow rates for correlation with choke opening, pump rates, and pressure drops in the Drilling circulating system and across the choke. This is particularly important for subsea blowout preventer stacks in deep water, which may have significant circulating pressure losses in the choke lines.
- While drilling, the trip tank should be kept half full of mud when pumps are off. During a connection well should be lined up on trip tank as the most likely time to swab or take a kick is when APL is lost with pumps off.
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