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On The Record

Real time ground information can be produced by equipping rigs with sensors and data loggers. Jeff Williams tells GE about the innovation.

Instrumented drilling has long been applied in the oil and gas industry, where it is referred to as “monitoring while drilling”, and was first adopted in France in the early 1970s for onshore geotechnical use.

Drilling parameter recording (DPR) is routinely used as part of ground investigation projects in many countries of continental Europe, and is gaining acceptance in the US.

However, the technique has rarely been used in the UK.

After reviewing current technology, Soil Engineering concluded that there was a potential for using it in the UK, and in April 2009 equipped a number of its rotary rigs with the latest version of DPR equipment.

A DPR set-up consists of a control panel and data logger unit that is typically configured to record five drilling parameters, namely thrust, torque, flush pressures, rotation speed and drilling rate (ie rate of penetration).

The first three parameters are monitored using calibrated pressure sensors on a rig’s hydraulic circuits and flush supply line respectively, with rotation being recorded using an electromagnetic proximity sensor mounted on the rotary head.

A depth encoder mounted on the mast records the drilling rate.

The logger unit is attached to a rig’s control panel so the driller can operate the system and monitor the data.

Power for the system is provided by the rig’s electrical supply.

The rate of data acquisition can be controlled by the logger and can be adjusted to take readings at depth intervals of between 10mm and 90mm, depending on the expected ground conditions and the purpose of the investigation.

DPR boreholes are typically formed using destructive drilling methods, using either drag or tricone (rock roller) bits, or even down-the-hole hammers (including simultaneous casing systems), depending on the ground conditions and purpose of investigation.

The flush can be water, mud or air. DPR can also be used with rotary coring to supplement the information obtained using either conventional or wireline equipment.

After reviewing current technology, Soil Engineering concluded that there was a potential for using it in the UK, and in April 2009 equipped a number of its rotary rigs with the latest version of DPR equipment.

To minimise the effects of the driller’s influence on the data recorded, the settings for thrust, torque, rotation and flush pressure are maintained as constant as possible for the ground conditions encountered.

The data recorded, particularly the drilling rate, is therefore a reflection of the nature of the ground being drilled.

During drilling the data is instantaneously displayed on the screen of the control panel, either numerically, or as plots against depth, so the driller can monitor the data as the borehole is progressed.

The data is stored on the unit’s internal memory and is downloaded when required onto a memory stick for transfer to a laptop or PC in the field or by GPRS transfer to an email address.

The field data is then processed using PC-based software to produce a graphical record. If required, data can also be provided in CSV format for importing into a spreadsheet.

On sites where there is a computer and a printer available, field records can therefore be provided to the engineer as soon as a borehole is completed.

Used in isolation, DPR, allied with destructive drilling methods, cannot be used to determine soil or rock types beyond a basic description obtained from cuttings brought up in the flush return.

Some fully sampled or, better still, cored, control boreholes should always be used to establish sub-surface profiles across a site, with DPR boreholes drilled close to them to develop parameter characteristics for the various layers.

This comparative information can then be used to add geological descriptions to the logs of DPR boreholes formed elsewhere over a site.

The common perception is that DPR is useful only for probing for cavities, both natural and man made.

While it is an excellent tool for this task, the potential uses for DPR are much more extensive, and include:

  • Establishing the depth to top of competent rock where overlain by a weathered or highly fractured zone of variable thickness.
  • Proving the thickness of a particular rock type across an area, or below a proposed structure. Useful when piling into rock containing weaker layers, or where piles or ground anchors have to be formed into a specific horizon.
  • Establishing soil/rock profiles below large structures such as storage tanks, or water treatment structures. Also very useful for linear structures such as pipelines, roads railways and tunnels.
  • Providing accurate, rapid and reliable information on grouting projects, such as helping to delineate areas where grouting is, or is not, required.
  • Establishing the effectiveness of grouting by comparing DPR records before and after.
  • Determining whether or not a cavity contains backfill/collapse material. This procedure involves recording baseline parameters for each section of hole drilled (usually each 1.5m or 3m rod) and comparing these against the values recorded while drilling that section.

Although other methods are available, DPR drilling can provide the information much faster and with better accuracy. In comparison to rotary cored boreholes DPR drilling is also more cost effective.

Shortly after purchasing the equipment, Soil Engineering used a DPR-equipped rig for a probing project in Buckinghamshire to investigate the cause of subsidence of a house.

This was followed by multi-rig projects in Berkshire and Kent to probe for potential chalk mines and/or solution features adjacent to and under existing buildings
Soil Engineering’s equipment has recently been used with wireline rotary coring on a project in London.

It is thought this is the first time DPR recording has been used in this manner in the UK and therefore the interpretation of the data is in its infancy.

However, based on the limited amount of data collected to date, Soil Engineering believes that DPR improves the completeness of the data set with little or no time penalty.

There is the potential for developing more accurate borehole logs, especially in difficult ground.

Part of this improvement is greater accuracy of depth measurement, such as for core runs, which the driller can obtain from the data displayed on the screen of the logger.

On this project Soil Engineering used a DPR-equipped Boart Longyear Deltabase 520 rotary rig, set up to record information at 10mm depth increments.

Geobore S wireline equipment with polymer mud flush was used to obtain cores of the Harwich group, the underlying Lambeth group and the Thanet sand.

Some core loss was experienced in the granular soils, so accurately identifying boundaries between the various soil types was at times difficult.

This is illustrated by the logs (above), where (a) is the log produced from the engineering geologist’s examination of the core recovered, (b) is the DPR record over the same depth range, and (c) is the same log as shown in (a) amended on the basis of the information from the DPR record.

On log (a) there is a zone of assumed core loss from 10.19m to 10.98m depth, and the boundary for the base of the Harwich formation cannot be identified.

Fig_1_revised

However, based on the DPR record, the base of the Oldhaven member can be inferred as occurring at a depth of around 10.6m.

There is a second zone of core loss in the core run from 13.19m to 14.69m and here the geologist has placed the thin band of clay of the laminated beds recovered as being from 13.19m to 13.51m.

With sand being encountered in the top of the next core run, the inference is that the core loss is in a granular layer from 13.51m to 14.69m.

However, the DPR record suggests there are two layers of granular soils, probably sands, occurring between about 13.19m and 15.0m, with a thin band of clay present from about 14.1m to 14.45m.

It is therefore considered that the clay layer of the laminated beds occurs over this depth range, as shown on log (c), not at the shallower depth shown in (a).

With greater use it is hoped to be able to refine procedures, for example by mathematically combining individual DPR parameters to obtain values such as specific energy or Somerton index, and then relating these to various soil and rock types.

Initially this is likely to be on a site specific basis, and to improve accuracy it may be necessary to introduce additional sensors into DPR systems in order to record net thrust and torque pressures.

It may also be necessary to calibrate rigs in order to convert the pressures measured in the hydraulic systems to pull down forces and torque.

A further refinement that is possible with current equipment is measurement of the flow rate in the flush delivery line, although at the present time it is not possible to record the return flow rate.

A working group of CEN Technical Committee 341 is proposing a new work item that deals with the technical principles for measuring, recording and reporting DPR information as geotechnical data.

The standard is expected to list which measuring methods may be used for what measurements.

Technical specifications will be given for the logging of DPR for different ground masses and the best way to report them for the purpose of subsurface characterisation.

  • Jeff Williams is principal engineer at Soil Engineering (formerly Norwest Holst Soil Engineering.

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