13. Depth sensor

Depth is monitored through an intrinsically safe encoder wheel mounted on the crown sheaves on land rigs, jackups and platforms. On semi-submersible rigs either a wire line retriever is mounted on the rig floor to provide very accurate kelly height or a combination of compensator opening sensor and crown wheel is used. The depth system provides continuous monitoring of depth, rate of penetration, running speeds, kelly, block, compensator and riser position. The system is fully operative during tripping and other rig activities and also calculates bit off-bottom depth and running speed. This allows the calculation and display of swab and surge pressures while tripping.

The crown type :

Th e depth wheel sensor assembly is mounted against the slow sheave of the crown wheel. It consists of a rotating wheel, twenty four inches in circumference. Two proximity switches detect the wheel’s movement. The proximity switches produce a quadrate signal which can identify the direction of movement of the wheel. The digital pulses from the switches are converted to up or down digital signals for computer.

When monitoring depth on a floating rig, a second depth wheel is mounted on either the riser tensioners or the guide base tensioners to monitor rig heave. A wireline retriever assembly and encoder are mounted on the rigfloor and a line is run to the kelly to provide heave compensated kelly movement. A compensator opening sensor utilizing a digital encoder can alternatively be used in conjunction with the crown wheel.

The depth system measurements are combined to provide very accurate depth monitoring even during severe roll and heave conditions.

The Draw work type

This is smaller and more simple type of depth sensors which is mounted on the draw work. It is used only with the IRIS data aquisetion and is not used in the LS 2000 system.

14. Pump stroke sensor

The pump rate is monitored by non-contact magnetic proximity switches, mounted over the pony rod end clamp. The BLS 2000 can monitor up to six pumps continuously. The system can assign any combination of the pumps to the active and the auxiliary pump counters, which then monitor individual pump rates and total strokes. It calculates the total volume pumped, bottoms up, well circulation times and lags.

In certain installations, a whisker type limit mechanical swhich sensor may be installed if the pump configuration is unsuitable for the non-contact proximity switch.

The sensor is supplied with a 24 VDC excitation voltage and feeds back a digital. This output is isolated by a signal conditioner card passed to the computer for processing.

15. Digital rotary speed sensor

Rotary table revolutions are normally measured with a digital encoder mounted in the rotary drive system. The sensor is driven by a flexible belt around the rotary motor shaft.

The sensor uses an encoder which gives one hundered counts per revolution of the encoder shaft. On most installations this is equivalent to 200 counts per rotary RPM giving high resolution and accuracy.

Two types of encoder are used. For the North Sea sector Pepper and Fushs transformer isolated barriers is used to provide a certified L.S system. Other units use an American Optical encoder which is supplied with a 24 VDC excitation and delivers back a 24 VDC digital signal to the digital signal conditioner board then to the computer.

The signal is calibrated by entering a calibration factor which converts the digital counts into RPM.

A Pepper and Fuchs proximity switch can also be utilised on the drive shaft to provide a non-contacting measurement system.