Over the years a standard set of API sizes have evolved to reflect the materials and manufacturing technology, operating practices, and stress analysis techniques. Drillstring design for slim hole drilling has followed the two distinct paths of destructive and continuous coring although attempts have been made to achieve a compromise design suitable for both. Much engineering effort is put into thread and tool joint design.

1 Traditional oilfield tool joints

Traditional oilfield tool joints are designed to be at least as strong (for tensile burst and collapse rating) as the DP body they are attached to by friction welding and to have good fatigue resistance. The thickness of the tooljoint is about three times the thickness of the pipe by means of internal and external upsets. Traditionally a thread taper of 1:6 has been used to facilitate easy stabbing on the rig floor. The maximum OD of the tooljoint is limited by the catch of the largest overshot which can be run in the smallest hole in which the pipe will be used. A life of several years is expected, which includes several recuts of the thread. The thread has traditionally been a single sealing shoulder with a fixed pitch.

Similar threads are used on collars and other drillstring components.

The importance of smooth tool joint hard facing to minimise casing wear is well known as is the importance of minimising dog leg severity, especially high up the hole.

2 Drillpipe for downsized conventional drilling

Usually when drilling using the downsized conventional approach, liners rather than full strings of casing are used. Thus small high strength drillpipe is often used as a tail under a string of 3 1/2" or larger pipe.

2 7/8" pipe suitable for 4 1/8" and 4 3/4" holes have been developed with double shoulders, tapered thread, a non-symmetrical thread profile and a stress relief groove in the box, for drilling with large clearances where extensive vibrations can be expected.

3 Mining drillpipe and threads

The drillpipe, or drill rod, used in the continuous coring system is different from the traditional oilfield DP in a number of respects. Thousands of wells have been successfully drilled using this pipe. Most of them have been relatively shallow and vertical in stable formations. It is important that the differences between mining rods and traditional pipe be recognised and evaluated before this drill rod is selected for hydrocarbon drilling.

1.The connections are relatively weak in bending and not resistant to fatigue loading. Drilling with large clearance can lead to twist-offs. Thus a separate drillpipe size is required for each hole size.

2.The sizes and grades required for deep drilling are not a commodity item and combined with the above mentioned need for a separate string for each hole size leads to relatively high costs.

3.Due to the very small external upsets, handling of the pipes during trips is by means of lifting plugs.

4.The slender connections dictate the use of thread tapers of 1:12 or 1:20 instead of the more normal 1:6. This has a number of consequences:

a)Stabbing is slower, with more risk of thread damage and cross threading.

b)The thin bottom shoulder of the double shouldered connection is very susceptible to damage while racking a stand, and during transport.

c)If a drillstring is used as a test string at the end of a well there is a slightly greater chance of a connection leaking [638].

d)Stabbing an internal BOP into the string during a kick while tripping or making a connection is more difficult and has a lower chance of success.

5.When a well is shut in with conventional pipe and the well pressure is so great as to start to blow the pipe out of the hole, then contact between the external upset and the pipe rams will restrain the upward movement of the string. With mining strings these upsets do not exist. This problem can be overcome by putting snubbing unit rams in the BOP stack, but this adds to BOP stackup and hence substructure height. Collapse of the thin wall pipe due to gripping forces would need to be considered.

6.The small external upsets might be difficult to hard face without the heat affecting the internal threads.

7.The small annulus between the pipe and the borehole wall would make the system more susceptible to differential sticking and pack off of cuttings from swelling shales during fast drilling.

8.The small annulus between the pipe and the borehole wall combined with the small internal flow channels in the core barrel would impede the passage of conventional lost circulation material (LCM). Running a circulating valve in the BHA may restrict the size of the inner core barrel which can be run.

9.Fishing is limited to internal catch tools. There is less experience of the use of these in open hole compared with overshots. Some misgivings have been expressed as to the chance of getting inside the fish in a washed out hole, which is the most likely place for a twist-off.

10.Mining pipe does not normally have internal plastic coating to prevent corrosion, as this would add to the cost and would be worn off by the wireline retrieval of the core. Mining drilling is usually done with pure water and at relatively shallow depth. Typically a string is only expected to last for a few wells, so corrosion protection is not often considered. Advice on corrosion protection may be found in.

11.The longitudinal stiffness of the pipe located in a tight clearance hole, where buckling is restricted, is high compared with a conventional oilfield drillpipe/hole geometry. This means that the system is very much more sensitive to longitudinal vibrations induced by surface equipment. It is for this reason that mining drilling rigs have very fine hydraulic feed-off compared with the band brake found on oilfield rigs. Some drilling contractors have used mining DP with a conventional rig or workover hoist on account of its low purchase price, but found it to have a very short life.

12.Some times mining pipe is used with conventional rock bits in a high clearance surface hole. The vibrations during this process lead to fatigue failure.

13.Heave compensator performance measurements on floating rigs had shown that they do not fully compensate for heave, and that even with a substantial upgraded heave compensator it was unlikely that the surface feed off system would be sufficiently sensitive to permit a continuous coring system to be used at depth from a conventional semi-submersible rig on a routine basis in the North Sea.