DRILL PIPE

Drill pipe is used to transmit rotary torque from the rotary table down to the drill bit, and to provide a passage for drilling fluid to the bottom of the hole. It is therefore one of the most important items of rotary drilling equipment, and because of the high standards to which it is manufactured for its rela- tively short but stressful life, it is expensive and forms a large part of the total capital investment of’the drilling contractor.

Drill pipe of various API standard diameters is commonly used, and pipe is also graded according to weight and quality of steel. Standard diameters in use offshore are and with 5” being the most widely Gsed on floaters, Rigs are rated according to the maximum depth to which they can drill, based on the length of drill pipe they carry, and when comparing rigs it is necessary for a client to know what size the rating was based on, since narrower diameter pipe weighs less per foot and would therefore impose less stress on the derrick and hoist equipment than the same length of wider pipe. A typical outfit for a modern semi-submersible might be 25,000 feet of 5” Grade S135 drill pipe weighing 19.51

Pin and box drill pipe tool joints are standard in the industry

high pressure, and if this is not spotted it can eventually cause a connection to fail. Great care is taken not to damage threads when making or breaking connections, and frequent inspections are made for signs of erosion-caused by leaking tool joints and other stresses that weaken the pipe or tool joints. Before use, threads are normally cleaned and inspected, and a special based thread compound is applied to the pipe. Thread protectors made of rubber or rubber-like material are always screwed on to the tool joints when moving and racking pipe.

There are three ranges of length under API standards: 18 to 22 feet, 27 to 30 feet and 38 to 45 feet, but offshore the most common lengths in use are 31 and 4.5 feet. Three joints of pipe, or two joints of pipe are usually ‘made-up’ to form ‘stands’ that cut down the handling time when run- ning in or pulling out of the hole. In addition to these standard lengths there are also short ‘pup joints’ which are used to make up a required length of drill string. These come in lengths such as 5, 10, 1.5 and 20 feet. The ‘wall thick- ness’ of all these joints is about

Each joint of drill pipe is basically a tube with a short, wide section at each end called a ‘tool joint’. One tool joint has an internal or ‘female’ thread, cal- led a ‘box’, while the other has an internal or ‘male’ thread, called a ‘pin’. When two joints are connected, the pin is screwed into the box, tongs being used on the tool joints to apply the necessary torque. The tube section is always manufactured as a unit, and the tool joints are welded on afterwards. They are able to withstand high torsional and tensile stresses, and have ‘shoulders’ under which elevators can be attached for lifting.

Any joint of drill pipe must be expected to occasionally support the entire weight of the drill string when this is hanging from the hook with the bit bottom. At this time the string is held in tension with the greatest tension at its top, since the weight of the rest of the string is below this part. During dril- ling the bit rests on the bottom and weight is applied to it by the drill collars which make up part of the ‘bottom hole assembly’. Some of the tensile load is therefore removed from the top of the string at this time, but when the string is pulled out of the hole the tension again comes on the top joints. Because of this variation in tensile loading, drill pipe must be capable of withstanding great vertical forces in the hole. At the same time, however, it must be pliable. If a ‘drill stem’ (as the string of drill pipe is correctly known) could be seen rotating in a hole, it would appear to bend in many places. This is desirable because the drill pipe must be able to follow the meanderings of the bit as it heads towards the target location; too rigid a drill stem would get stuck in the hole. However, the pipe should be kept permanently in tension by the drill collars at the bottom of the string, and should not be allowed to compress and buckle.

The powerful twisting force, or torque, imparted to the drill pipe by the rotary table and the kelly is another stress which every joint of drill pipe must be capable of enduring. In a crooked hole the pipe may not be able to rotate freely and there may be a tendency for it to break or twist off, necessitating a long and costly fishing operation, so it is vital that only pipe in top condition is used down the hole. The thread of the tool joint is often the first part to fail, due to ‘washing out’, or metal erosion from leaking drilling fluid under

Heavy wall drill pipe is distinguishable from ordinary drill pipe by its thickened centre portion

Heavy drill pipe, called ‘heavy-weight’ or ‘heavy-wate drill pipe’ (HWDP), is also carried by all offshore rigs as an intermediate weight grade of tubular that can be used in the transitional area between the pliable drill stem and the heavier and stiffer drill collars in the bottom hole assembly. Fif- teen or twenty joints of HWDP might be used in vertical holes, and up to thirty in directional wells. The wall thickness is usually about one inch, and while the weight of a particular grade of ordinary drill pipe might be 19.5

that of heavy-weight pipe of the same diameter could be 48

A further type of drill pipe is made from aluminium, but is not commonly used offshore and not accepted as API standard. It is more expensive than steel drill pipe but has applications where its light weight is an advantage, such as where helicopters have to be used to transport the pipe and where extra pipe has to be carried to extend the depth capacity of the rig.