Other than the drill floor itself, the solids control system requires more attention than any other single rig system whether onshore or offshore. For that reason, it is most important that the solids control system is designed to be as efficient and as maintenance free as possible. This section will discuss that pan of the mud system extending from the bell nipple (or diverter housing outlet) to the mud pump suction piping.
Flowline from the bell nipple (or diverter housing) to the shale shakers.
This flowline is generally a closed pipe ranging from 12 to 24 inches in diameter or a large open trough. The flowline should slope as much as possible to prevent cuttings from accumulating in the line. A slope of one inch or more per running foot is recommended to ensure that the flowline does not plug. The larger size lines are used on rigs having greater circulating rates and/or drilling soft reactive shales called gumbos. These gumbos drill fast,
absorb water, swell, and emerge from the bell nipple in large clay-like masses. This gumbo can plug even a properly sloped flowline causing the mud to spill over the top of the bell nipple or trough. Often, roughnecks must continuously clean out the gumbo with shovels and high pressure hoses until this section of hole is completed. The only means to reduce the gumbo problem is to reduce the drilling rate or use an oil base mud. Either of these means may be unacceptable. Many rigs which drill gumbos frequently have installed a separating device called a gumbo box in the flowline close to the bell nipple. The open top of this steel box has parallel steel bars which catch the gumbo balls while letting the liquid mud fall through the bars and continue down the flowline. Roughnecks man the gumbo box and rake the gumbo off the bars continuously while gumbo sections are being drilled. Figure 1 and Figure 2 show the typical location and detail of a gumbo box.
Most of the gumbo must be removed before the shale shakers to prevent the back tanks and screens from being plugged.
Adequate access should be provided around all sides for ease of operation and repair. Additional space may be required at the screen end of the shaker to allow for screen removal. Manufacturer’s information should give the required screen removal clearance. Valves or gates should be fitted in the flowline so that the back tank of any shaker may be isolated temporarily for cleaning or repair.
Desanders, desilters, mud cleaners
Adequate access should be provided around all sides to maintain the equipment. This equipment should be close to and on the same level as the shale shakers, if possible, so the roughneck can watch the shakers while he is working on these items.
Again, adequate access should be provided. A clean water source should be provided. The centrifuge must be located on a stiff sub-foundation to prevent vibration and on its dedicated tank top for ease of returning the liquid phase or solids phase to the active mud system. Thought should be given on how solids will be discarded. Operators may require as many as three centrifuges with a tank under each. Special motor starters may be required to prevent a small or heavily loaded power system from blacking out on start up. The inrush current of four or five times running amps may last for 20 seconds or more. A qualified electrical technician or engineer should evaluate each installation to avoid blackout problems.
Sand trap and mud processing tanks
The first mud processing tank at, er the shale shakers is typically named the sand trap. Mud enters high in the tank and leaves high in the tank via a weir. Some sand settles by gravity and the tank is periodically bypassed and dumped. This inefficient method of separating some sand (and barite) is becoming less and less popular because of the loss of valuable mud and the problem of direct disposal to the environment. Today’s sand trap is often no more than the first mud processing tank having a bottom suction to the first mud processing pump.
All mud processing tanks should have V-bottoms with the pump suction being at or very near to bottom. The tank bottom will stay very clean with this configuration. The tanks should be interconnected via high level weirs or low level equalizers so a tank cannot be pumped dry or overflow if a pump fails. Individual mud processing tank capacity ranges from 15 to 75 barrels on today’s rigs. Tanks on the order of 30 to 50 barrels each are most common.
A tank should contain enough mud to supply its pump for at least one minute with no makeup entering the tank.
Mud processing and mixing pumps
These pumps should be located as close to their respective tanks as possible. The pumps should be located several feet below the liquid level in the tank, but in all cases have a flooded suction. No valve, elbow, or tee should be fitted within two pipe diameters of the suction flange (and, if possible, the discharge flange) of any centrifugal pump. Anti-vortex spools are a prudent addition at the pump suction. Only pumps designed to pump abrasive fluids should be used in this service. The shaft sealing system should also be for use with abrasive fluids rather than just clear water. Several shaft sealing techniques are being used successfully in mud service which greatly surpass the life of chevron packing sealing on a steel shaft.
If pipe misalignment or flexing is possible, flex joints should be installed between the pump flanges and the pipe flanges. The pump base should be sufficiently rigid to prevent flexing or torquing while the pump is being used. Otherwise, bearing failures will occur in the pump pedestal or drive motor.
To minimize the possibility of pump suction lines from clogging up with sand, silt, and barite use the following recommendations:
- Suction lines should be kept as short and straight as possible.
- Avoid large, complicated suction manifolds.
- Slope the suction lines at least 1/4 inch per foot from the tank suction to the pump.
- Avoid barite traps.
Size the suction piping so that the fluid velocity is at least 4 ft./sec., but no more than 8 ft/sec. (larger diameter pipe may be 10 ft/sec).
Note the suction line entrance details given on Figure 3.
A suction at the V-bottom looking up is best, but a trash guard over the entrance is recommended. A bell mouth entrance looking down is good. A pipe suction flush with the tank wall is fair. A pipe suction extending some distance into the tank is poor.
Active mud pits (or tanks)
At least two active pits, two reserve pits, and a slugging pit are typically provided after the mud processing system. The mud weight and chemistry are modified at this part of the mud system. Proper mud pit design and agitator selection and placement are required to keep the solid phase of the mud in suspension. Several tons of barite can settle to the bottom in poorly designed mud pit systems. Long suction pipes should not run through mud pits, but under or beside them. Chamfered corners (deflector plates) in mud pits promote mud agitator efficiency and prevent quiescent areas in the corners that allow barite to settle to bottom. The optimum size and placement of deflector plates will depend on the number and type of agitators and the size of the mud pits.
A loosely fit bottom bearing is recommended on longer agitator shafts to prevent the shaft from whipping and bending on start up. Mud guns are relatively expensive devices to operate from the standpoint of pump maintenance and barite degradation. Good mud tank design and good agitation are a more cost effective means of keeping barite in suspension.