All mudmotors, non-magnetic drill collars, stabiliser mandrels and subs shall be inspected as per Company inspection programme. All tools supplied to the rig shall be accompanied by the appropriate documentation. All pin connections shall be stress-relieved and all boxes bored back.

Bottom Hole Assembly

In general, only the bit, drill collars, and stabilisers are taken into account for trajectory control. The specific operation will determine the rest of the BHA components including:

• shock subs
• hammers
• mudmotors
• MWD
• jars
• accelerators
• heavy weight drill pipe
• transitional drill collars, etc.

BHA Stabilisation

Bending Moment

When it is desirable to prevent bending moment from being carried through to the bit from the BHA above, a packed hole or stiff BHA should be selected where a large diameter, short drill collar is placed between the first string stabiliser and the near bit stabiliser.

The stiffness of this arrangement below the first string stabiliser provides a rigid BHA section just above the bit to ensure a straight bit trajectory.

This arrangement can be used either in vertical wells or in deviated wells where it is desirable to hold azimuth and inclination.

Packed Hole Assemblies and Full Gauge Stabilisers

In packed hole assemblies, the near bit stabiliser must be full gauge and it is sometimes necessary to include tandem near bit stabilisers where severe bit walk might be expected.

The first string stabiliser should normally be full gauge; however, under-gauge first string stabilisers are sometimes used to hold angle in deviated holes where there is a tendency to drop.

BHA Standoff

In addition to improving the steerability of the drilling assembly as described above, stabilisers are also important in providing stand off of the BHA from the wellbore - thereby decreasing the possibility of differential sticking.

Wellbore Gouging

In general, full 360° wall contact stabilisers should be used to prevent gouging the wellbore.

In a highly abrasive field, sleeve type stabilisers are used; these keep down replacement costs and are particularly advantageous when frequent adjustments are required for directional reasons.