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pipe installation (Trenchless)
#1
In recent years there have been large advances in trenchless technology for pipe installation and more can be expected in the next few years. In addition to tunnelling, there are four other methods of trenchless technology used for new pipelines in New Zealand.These are:

• Horizontal boring

• Directional Drilling

• Pipe Jacking

• Micro Tunnelling

The limits and advantages of each of the technologies are discussed below. This is intended as a general guideline only as rapid advances are being made and some equipment and some contractors may be able to obtain more distance or better accuracy.
Horizontal Boring

This is boring with normal drill rods in a horizontal (and hopefully straight) direction. A drill bed is up at the correct depth and aligned at the required direction and grade and a small hole drilled (typically about 70mm diameter). This hole is later reamed out to the required diameter. This can be done either with a cutter that back cuts a short length of the line and then pushes the cut material to the far end of the hole for removal or by pulling a pig or similar through the hole to expand the hole to the required diameter. This method can be used with pipelines up to about 900mm diameter or larger provided the ground conditions are suitable, and holes will remain open safely.

Advantages

The advantage with this method is that the drilling equipment is relatively cheap and it can drill rock if required. It can also be usedpipe installation at any depth. Over short distances relatively accurate line and grade bores can be achieved. Claims of accurate grades of 0.5% over 30 – 50 m are common in the industry. My personal experience is that this is generally not obtained and that to get 0.5% lengths have to be less than 30m.

Disadvantages

The disadvantages are that the drill head cannot be controlled after drilling starts. The head generally slowly falls below the grade that it started at. It can however go in any direction if a hard layer is encountered at a shallow angle to the bore direction. Because of the tendency of the drill to slowly fall off the design grade a maximum length of about 60m is recommended for firm consistent materials. In hard materials where the drilling head is likely to be deflected by varying strengths of materials and in soft materials where it is likely to “fall” more rapidly this distance may have to be reduced to as little as 10m.

A drilling pit is required to the depth of the line and in the location of the line. The pit has to be long enough to fit the machine in. This is generally not a problem but can be on some confined sites.

As the original hole is reamed out after drilling the final hole can at times finish up with it’s centre not in the same location as the original and some variation additional to that caused by inaccuracies in the original drilled hole can occur.

Pipe Materials

With this method polyethylene pipes would be the most common material. However, concrete, steel, PVC or Hobas GRP can all be used. With these materials the pipe jointing needs to be considered. Rubber ring joint pipe should not be used as the installation will push the joints in past the witness marks causing failure of the pipe collars. If the pipe is not the same outside diameter over its full-length consideration needs to be made of if and how the annulus between the drilled hole and the pipe is to be filled.
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