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cutting head or drill bit to facilitate the removal of cuttings, stabilise the borehole, cool the cutting head, and lubricate the passage of the product pipe.


5.5.2.1.1 Drilling the profile 280. A small diameter pilot hole is drilled under directional control to a predetermined path using a mud-motor or jet bit on the end of the pilot string. The pilot string is drilled up to 80m in length, and then the wash over pipe is advanced in rotary mode until it is approximately 30m behind the drill bit. Alternate pilot string and drilling operations take place until the exit point is reached. Then the smaller pilot string is removed.


5.5.2.1.2 Enlarging the hole 281. Pre-reaming operations are carried out to enlarge the drilled hole to a size suitable for accepting the duct. Pull-back pipe is added behind the reamer. Depending upon the pipe diameter to be installed, several pre-reaming operations may be necessary, each progressively enlarging the hole.


5.5.2.1.3 Installing the pipe 282. The pull-back pipe is connected to a 'cleaning' reamer, which in turn connects to a swivel joint (to prevent pipe rotation) that is attached to the pipeline towhead. The drill rig is then used to pull the duct into the preformed hole. The drilling fluid consisting of water and clay minerals would remain in the annulus and protect the duct. Once the duct is installed, the ends would be covered or plugged until the cable is ready to be installed. The cable is then passed through the duct (see Diagram 5.10).


Preliminary Environmental Information May 2014


East Anglia THREE Offshore Windfarm


Chapter 5 Description of the Development Page 62


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