gdy 105 million jv origin energy, page-70

Geojac,
Just to clarify,
the bottom hole assembly, often referred to as the BHA,
is a heavy section of the drill string the function of which is to provide weight, to keep the drill string in tension.
We often think of drilling like a drill bit drilling into a piece of wood, ie it is pushed down. But in deep drilling, the drill string is kept in tension with the BHA acting like a plumb bob weight, the idea is to apply weight to the bit by easing it down bit by bit (excuse the pun) but all the time keeping the drill string in tension.

The BHA normally consists of two major sections, the lower part Drill collars, in this case 8" OD and 2 7/8" ID, and each 10m long, making up a total length typically 80-100m.
These provide most of the weight and stiffness. The upper part of the BHA consists of Heavy Weight Drill Pipe, 5" OD and 3"ID, also in 10m lengths, they basically provide extra weight only. Normal drillpipe is 5" OD and about 4.2" approx ID.

In addition there are stabilisers 2 or 3 of these which are about 1m long and 12.25" OD (the same as the drill bit)and are with in the Drill collar section of the BHA, these keep the drill collars in the centre of the hole and provide additional stiffness to the BHA.

The spiral flutes running up the side are to minimise surface contact area of the collars with the wall of the hole. In some formations it is possible for the drill collars to suck against the wall of the hole, differential sticking, and the flutes minimise this.

Thats basically it. There is more strain on these joints than elsewhere, and it's normally a collar joint that fails.

A turbine or motor maybe at the bottom to provide additional rotation to the bit. This is powered by the drilling fluid passing through it.

Hope that helps to visualise it.