System Development of Rig Profiling Current Meter
Authors: Burbine N. 
Published: December 2007

This paper discusses some of the aspects for design of a profiling current meter system. The system is not considered an in situ system since all power will be applied topside and the instruments will be hardwired to a power will be applied topside and the instruments will be hardwired to a console.

Measurements are an integral part of all our lives. Without measurements we would neither know where we are or hope to know where we are going.

Many people actually believe that measurements, especially oceanographic, are somewhat magical or mystical. It is true that measurement systems, regardless of the application, if not thought out beforehand, are indeed magical. But in reality these magical systems produce phenomena which are quite explainable when all factors are accounted for.

A well-known authority on measurements often preaches on "how to obtain valid data on purpose." We should all commit this to memory and strive to see that in fact valid data are obtained on purpose.

The need for a measurement system to aid in determining forces on risers is of major importance to drilling operations. Analysis with only theoretical data leaves many other areas for error. Worst yet is analysis with data upon which a high level of confidence is placed without regard to the acquisition system used.

Specific measurements considered necessary are current speed, direction and depth. Also considered important by the author is time. This is actual real-time recorded simultaneously with the other parameters.

It is also necessary when talking about measurements to remind ourselves that a large difference exists between the two words accuracy and precision. precision. Accuracy is defined as the degree of agreement between a true value or a value accepted as true and the average of multiple recordings of the observed value.

Precision is the degree of agreement within any given set of observations. Precision is the degree of agreement within any given set of observations. In other words, it is possible to obtain the most precise measurements possible and have a set of totally erroneous data. If any degree of possible and have a set of totally erroneous data. If any degree of reliability is to be placed on the data, one should be convinced that the system design and operational parameters will permit the taking of both precise and accurate data. precise and accurate data.

Transducers Speed

A wide variety of transducers is available for measuring the desired parameters. Commercially available for speed measurements are devices parameters. Commercially available for speed measurements are devices based on electromagnetic induction, vortex shedding, and rotational frequency of a rotor.

Electromagnetic devices may be used for rig profiling applications and many are currently in use. However, due to the complexity of the electronics necessary for signal conditioning, this type of transducer, in the author's opinion, may be more than is needed to accomplish the assigned task.

It is not uncommon that an active system for corrosion control be installed on a drilling rig. Such a system utilizes an electric current to bias the rig in such a way as to inhibit corrosion. Generally this method of protection is called an impressed current system. protection is called an impressed current system. Should an impressed current system being used for cathodic protection be activated, the potential for erroneous readings on an EM meter is severe.

Another fairly common device counts vortex formation frequencies. In this type of system, usually a cylindrical rod is placed in the flow field. As the current flows around and past the obstruction, eddy currents will be formed behind it. These currents, "vortexes," can be related to velocity as a function of frequency formation and subsequent shedding behind the obstruction.

Vortex shedding devices also require fairly sophisticated means to measure the rate at which vortex shedding occurs. Commonly accepted methods include sonic techniques and pressure measuring devices.

This leaves rotating type devices still waiting for consideration. Simplicity is really a desired attribute, as in transducers such as a Savonius rotor. No complex signal conditioning electronics are necessary. A reed switch or hall effect device detecting revolutions or some multiple of revolutions is all that is needed. A rotating device, should it malfunction or break, needs no sophisticated parts, thus the potential for being repaired on site is far greater than for other devices.

Direction

The need for a direction sensor is imperative for determining not only relative direction, but an absolute direction with respect to magnetic north or south. By determining absolute direction, no other reference is needed, thus eliminating recording an additional point of information. Also, with a magnetic reference it is not necessary to maintain a specific angular position during the profiling operation.