Choking lines

If a situation arises whereby formation fluid or gas enters the bore bole the driller will notice an increase in the total volume of mud. Other indications such as a sudden increase in penetration rate and a decrease in pump pressure may also indicate an influx. Much depends on a quick response of the driller to close in the well before substantial volumes of formation fluid have entered the borehole. Onoe the BOP is closed, the new mud gradient required to restore balance to the system can be calculated. The heavier mud is then circulated in through the kill line and the lighter mud and influx is circulated out through the choke line. Once overbalance is restored, the BOP can be opened again and drilling operations continue. 

Figure 17-1. The wrong (a) and right (b) ways of clearing a choked line.

Gas pressure should never be used for clearing choked lines. 

The choke lines are attached to the BOP to enable controlled circulation of the drilling and formation fluids out of the hole. 

When the well is circulated through the choke line, a rapid loss in hydrostatic pressure is seen when the kick fluid begins to enter the choke line. Hydrostatic pressure is lost because low density gas is displacing the drilling mud from the small volume of choke line. Small kick volumes can result in long columns of gas in the choke line. Surface choke response must be rapid enough to prevent new kick fluid from entering the well due to the reduction in bottomhole... 

Drilling Rig encountered gas and choke line developed leak causing vapor cloud, explosion and fire 1 Fatality, Rig totally destroyed... 

Amoco used nitrogen to confirm a safe method to remove gas trapped in a subsea BOP. The gas can be recovered up the choke line by displacing the kill mud with water and then allowing the gas to expand against the lower hydrostatic gradient of the water. 

The increase in compressor pressure ratio decreases the operating range of the compressor. The operating range of the compressor stretches from the surge line at the low flow end of the compressor speed line to the choke point... 

The DC filter choke is used for ripple voltage and current reduction on the output of the switching power supply immediately following the existing filters. It is also used as an EMI filter on switching power supplies that have a single power line on the input, such as battery and distributed power systems. 

In ae off-line applications the common-mode choke is typically used and its design procedure can be found in Appendix E. The Alters resemble pi Alters, but are actually bi-directional L-C Alters. The important function is to Alter the noise generated by the switcher before it exits via the input power lines. 

Sometimes the high-frequency attenuation is insufficient to meet the specifications and a third pole needs to be added to the EMI filter. This filter is typically a differential-mode filter and will share the Y capacitors from the common-mode filter. Its corner frequency is typically the same as the commonmode filter. This filter is made up of a separate choke on each power line, and is placed between the input rectifiers and the common-mode filter. 

Health Hazards Information - Recommended Personal Protective Equipment Self-contained breathing equipment, air-line mask, or industrial canister-type gas mask rubber-coated gloves, apron, coat, overalls, shoes Symptoms Following Exposure Inhalation of fumes results in coughing and choking sensation, and irritation of nose and lungs. Liquid causes bums General Treatment for Exposure ... 

Indirect fired heaters (sometimes called line heaters) heat the gas stream before and/or after the choke so that the gas is maintained above the hydrate temperature. Indirect fired heaters can also be used to heat crude oil for treating, heat a hot fluid circulating medium (heat medium) that is used to provide process heat, etc. 

It is perfoclly acceptable for a line heater to have an L[ equal to 0. In this case all the heat is added downstream of the choke. It is also possible to have equal to 0 and do all the heating before the choke. Most fre quently it is found that it is better to do some of the heating before the choke, take the pressure drop, and do the rest of the heating at the low er temperature that exists downstream of the choke. 

The previous discussion focused on the use of indirect fired heaters as line heaters to provide the necessary heat to avoid hydrate formation at wellstream chokes. Indirect fired heaters have many other potential uses in production facilities. For example, indirect fired heaters can be used to provide heat to emulsions prior to treating, as reboilers on distillation towers, and to heat liquids that are circulated to several heat users. The sizing of indirect fired heaters for these uses relies on the same principles and techniques discussed for wellstream line heaters. 

The flow of a compressible fluid through an orifice is limited by critical flow. Critical flow is also referred to as choked flow, sonic flow, or Mach 1. It can occur at a restriction in a line such as a relief valve orifice or a choke, where piping goes from a small branch into a larger header, where pipe size increases, or at the vent tip. The maximum flow occurs at... 

How should we clear chokes in small bore lines ... 

A sample point on the suction line of two water pumps became choked, and a maintenance worker was asked to clear it. He was not told how to do so—craftsmen dislike people from other departments telling them how to do their Jobs—but the operators assumed he would use water under pressure or a rod. Instead he used compressed air at a gauge pressure of 115 psi (8 bar), and a pocket of air caused the pumps to lose suction (Figure 1-19). 

In another similar case, chokes occurred in the line between the two vessels. 

Some vertical drain lines in a building were no longer needed, so they were disconnected and capped but left connected to the horizontal main drain below. The caps were fixed with tape but were not made watertight as there was no way. it seemed, that water could get into them. Fifteen years later a choke developed in the main drain, water backed up into the disused legs and dripped into an electrical switch box. All power was lost, and some of the switch gear was damaged beyond repair [23]. 

Many operators find it hard to grasp the power of compressed air. Section 2.2 (a) describes how the end was blown off a pressure vessel, killing two men, because the vent was choked. Compressed air was being blown into the vessel, to prove that the inlet line was clear. It was estimated that the gauge pressure reached 20 psi (1.3 bar) when the burst occurred. The operators found it hard to believe that a pressure of only twenty pounds could do so much damage. Explosion experts had to be brought in to convince them that a chemical explosion had not occurred. 

Actually the air was not vented. The 1-in. vent line on the air supply was choked by a wasp nest. Whenever possible we should measure directly what we need to know and not some other parameter from which it can be inferred [1]. 

Rodding out narrow bore lines is sometimes necessary. But before doing so, a ball valve or cock should be fitted on the end (Figure 17-lb). It is then possible to isolate the flow when the choke has been cleared, even if the original valve will not close. 

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