Techshore Institute Blogs about Best Technical and Management Courses: Inspection

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Sunday, February 5, 2017

Techshore Inspection Services : Half Cell Electrical Potential method

Principle

The half-cell electrical potential method is used to measure the potential of an embedded reinforcing bar relative to a reference half-cell placed on the concrete surface. The half-cell usually used is a copper/copper sulphate or silver/silver chloride cell but other combinations are also used. The concrete functions as an electrolyte and the risk of corrosion of the reinforcement near the test location may be related empirically to the measured potential difference. ASTM C876 - 91 gives a Standard method for testing Half-cell potential of uncoated reinforcing steel in concrete.

Equipment

The testing apparatus consists of the following components:

Copper-Copper Sulphate Half-Cell:

The cell consists of a rigid tube made of dielectric material which is non-reactive with copper or copper sulphate, a porous wooden or plastic plug which remains wet by capillary action, and a copper rod which is immersed within the tube in saturated copper sulphate solution. The solution is prepared by dissolving reagent grade copper sulphate crystals in distilled or de ionized water.

The inside diameter of rigid tube should not be less than 25 mm and that of porous tube should not be less than 13 mm. The immersed copper rod should have inner diameter of not less than 6 mm and length 50 mm.

Electrical junction device:

An electrical junction device is used to provide a low electrical resistance liquid path between the concrete surface and the half-cell. It consists of a sponge pre-wetted with a low electrical resistance contact solution. The sponge can be attached to the tip of the half-cell so that it provides electrical continuity between the porous tip and the concrete member.

Electrical contact solution:

Electrical contact solution is used to wet the electrical junction device so that the potential drop through the concrete portion of the circuit is standardized. It is a prepared by thoroughly mixing a 95 ml of wetting agent or a liquid household detergent with 19 L of potable water.

Voltmeter:

A battery operated voltmeter is used with ± 3% end of scale accuracy at the voltage ranges in use. The input impedance should be not less than 10 MW while operating at a full scale of 100 mV.

Electrical lead wires:

The electrical lead wire should be such that its electrical resistance for the required length does not disturb the electrical circuit by more than 0.0001 V.

Procedure

Measurements are made in a grid pattern. The spacing between the measurements is generally chosen such that adjacent readings are less than 150 mV with the minimum spacing so that there is at least 100 mV between readings. An area of high corrosion activity is indicated by the area with greater than150 mV. A direct electrical connection is made to the reinforcing steel using a compression clamp.

To get a low electrical resistance connection, the rod must be scraped before connecting it to the reinforcing bar. The reinforcement bar is connected to the positive terminal of the voltmeter. A lead wire is used to connect the half-cell to the negative terminal of the voltmeter. Before the test begins, the concrete surface has to be pre-wetted with a wetting agent. Areas in the member where corrosion activity may be occurring can be represented using an equipotential contour map.

The potential risks of corrosion based on potential difference readings are shown in Table 1 below:

Table 1. Risk of Corrosion against the potential difference readings

Potential difference levels (mv)	Chance of re-bar being corroded
Less than –500	Visible evidence of corrosion
-350 to -500	95%
-200 to -350	50%
More than -200	5%

Monday, January 2, 2017

TECHSHORE INSPECTION SERVICES : WELL TESTING

WELL TESTING

More and more importance is being placed upon efficient performance of oil and gas wells. Generally some kind of testing should be done to determine the performance of an oil or gas well . There are many types tests ,to check certain parameters about the well. Many tests are done often and some are done rarely.

Some of the well tests are simple and some are complicated .Standard production equipments are only needed for many tests, but special equipments are required for others. These tests are to be done carefully because the results reflects the health of the well. These tests are done for the company to plan proper production procedures and to meet regulatory requirements . It is very important that the test be done accurately since well test data documents the performance of a well and the reservoir in which the test is completed . Very often these tests are performed by the producer to help in establishing proper production practices and to comply with regulatory requirement .

There are many types of well tests done by lease operators , well testers, or specialist companies. Most commonly used well tests are discussed below.

DRILL-STEM TEST

When well is being drilled ,a drill- stem test is often run on a formation of unknown performance to determine if a formation contains oil , water , gas , or a combination of the three .

These testing require special tools and packers which have to be run through the drill pipe to isolate the problem areas. Fluid is then allowed to flow from the formation to pass through the tester to the drill pipe. Fluid is then collected in a system of valves in the tester which is then collected for further examination. If there is sufficient pressure to cause the flow to the ground level, then productivity and capacity can be assessed.

Up This procedure requires special tools and packers to be run on the drill pipe to isolate the interval to be tested .Fluid is thus allowed to flow from the formation ,through the tester, and finally to the drill pipe . Fluid which is trapped in the drill pipe is collected by the system valve and its taken for further testing. If the formation pressure is sufficient to cause flow to the surface , a sustained flow test through the drill pipe will provide a measure of the productivity and capacity of the interval tested.

The test result shows the bottom hole shut- in pressure , bottom – hole flowing pressure,and bottom-hole temperature, and the type of fluid in the well might be capable of producing .

The result of drill-stem tests on a well will often h if a casing string should be run to complete the well as a producing well , or if the well is a dry hole .This information is used as the design basis for a production system .

POTENTIAL TEST

In potential test output of a well for a day (24 hrs) is measured in rigid conditions. This test is done one very newly completed well. Sometimes this test is repeated when state or federal regulatory agency or the well-operator needs a potential report.

PRODUCTIVITY TEST

Productivity tests are made on both oil and gas wells and include the well potential and bottom-hole pressure tests .

Flow from reservoir to the well is caused by the pressure difference.High pressure in the reservoir will force the fluid to flow to the low pressure area in the well. Flow rate is proportional to the pressure difference..

During productivity test shut in pressure of the bottom hole is measured first .This pressure is known as the static/reservoir pressure.The well is then tested in different stable rates and the bottom pressures are carefully recorded.From this data an engineer can calculate the output from the well.In gas wells the output potential is determined by measuring the atmosphearic pressure at the well mouth and the gas output..

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Friday, December 30, 2016

TECHSHORE INSPECTION SERVICES : REBAR LOCATOR

Principle

The Principle of Rebar Locator is that it can be worked either by utilizing Eddy current effect or Magnetic induction effect.

In cover meters using eddy current effect, current in the search coil set up eddy currents in the reinforcement which in turn causes the measured impedance of the search coil to change. Instruments using this principle operate at above 1 kHz frequencies. They are thus sensitive to the presence of any conducting material in the vicinity of the search head.

In cover meters using magnetic induction effect, a multi coil search head is used with a lower operating frequency of less than 90 Hz compared to the eddy current type device. Here the principle used is similar to that of a transformer. In that one or two coils (acting as primary coil) carry the driving current while one or two further coils (acting as secondary coil) pick up the voltage transferred through the magnetic circuit formed by the search head and embedded reinforcing. Such instruments are less sensitive to the presence of magnetic materials in the vicinity of search head compared to those using the eddy current principle. When there is a change in the amount of ferromagnetic material under the search head due to the presence of reinforcing bar or other metal object, there is an increase in the field strength. This in turn, causes an increase in the voltage detected by the secondary coil, which can be displayed after amplification by the meter.

In both types of instruments the meter reading is affected by both the orientation and the proximity of the metal to the search head. Thus it can be used to locate reinforcing bars and determine their orientation. The cover to a bar can also be find out if a suitable calibration can be obtained for the particular bar size and the materials under investigation. Most instruments have a procedure to estimate both bar size and distance from the probe to the bar when neither is known.

Equipment

Equipment mainly consists of a number of suitable battery or mains operated cover meters. Each cover meter is comprised of a Search head, meter and an interconnecting cable. The concrete surface is scanned by keeping the search head in contact with it while the meter indicates the proximity of reinforcement by analogue or digital means.

Procedure

The Rebar Locator equipment is switched on. Then start the main menu function and select the single point test. Then select approximate diameter 16. After that fix the probe in vacuum and scan the probe with 90 degree to rebar direction. Whenever the beep sound comes, move the probe slowly and it means we are near the rebar. When the scroll bar gets maximum points stop the probe and select the OK button. The value shown is the total depth of concrete. Then move the probe slowly again and get maximum point in scroll bar and press menu button. Here we get two values. The first value is the diameter of rebar and second value is the rebar thickness.

Applications

Rebar Locator can be used for following applications:

· For quality control by ensuring correct location and cover to reinforcing bars after the placement of concrete.

· Investigation of concrete members whose records are not available or need to be rechecked

· Location of reinforcement as a preliminary test to some other tests in which reinforcement should be avoided or its nature taken into account.

· Location of buried ferromagnetic objects like water pipes, steel joints, lighting conduits etc.