When manufactured, new pipe will be subjected by the manufacturer to a series of mechanical, tensile and hydrostatic pressure tests in accordance with API Specification 5A and 5AX. This will ensure that the pipe can withstand specified loads. A joint of drillpipe will however be used in a number of wells. When it has been used it will undergo some degree of wear and will not be able to withstand the same loads as when it is new.

It is extremely difficult to predict the service life of a drillstring since no two boreholes experience the same drilling conditions. However, as a rough guide, the length of hole drilled by a piece of drillpipe, when part of a drillstring will be :

soft drilling areas: 220000 – 250000 ft
hard or deviated drilling areas: 180000 – 210000 ft

This means that a piece of drillpipe may be used on up to 25 wells which are 10,000 ft deep

During the working life of the drillpipe it will therefore be necessary to determine the degree of damage or wear that the pipe has already been subjected to and therefore its capacity to withstand the loads to which it will be exposed in the future. Various non-destructive tests are periodically applied to used drillpipe, to assess the wear and therefore strength of the pipe, and to inspect for any defects, e.g. cracks. The strength of the pipe is gauged on the basis of the remaining wall thickness, or if worn eccentrically, the average minimum wall thickness of the pipe. The methods used to inspect drillpipe are summarised in Table 4.

table 4

Following inspection, the drillpipe is classified in terms of the degree of wear or damage which is measured on the pipe. The criteria used for classifying the drillpipe on the basis of the degree of wear or damage is shown in Table 6. The ‘Grade 1 or Premium’ drillpipe classification applies to new pipe, or used pipe with at least 80% of the original wall thickness still remaining. A classification of Grade 2 and above indicates that the pipe has sustained significant wear or damage and that its strength has been significantly reduced. The strength of some typical drillpipe sizes when new, and when worn, is shown in tables 11 and 12.

Drillpipe will generally be inspected and classified before a new drilling contract is started. The operating company would require that the drilling contractor provide proof of inspection and classification of the drillstring as part of the drilling contract.In general, only new or premium drillpipe would be acceptable for drilling in the North Sea.

The term drillstring is used to describe the tubulars and accessories on which the drillbit is run to the bottom of the borehole. The drillstring consists of drillpipe, drillcollars, the kelly and various other pieces of equipment such as stabilisers and reamers, which are included in the drillstring just above the drillbit (Figure 1). All of these components will be described in detail below. The drillcollars and the other equipment which is made up just above the bit are collectively called the Bottom Hole Assembly (BHA). The dimensions of a typical 10,000 ft drillstring would be :


The functions of the drillstring are:
•     To suspend the bit
•     To transmit rotary torque from the kelly to the bit
•     To provide a conduit for circulating drilling fluid to the bit

It must be remembered that in deep wells the drillstring may be 5-6 miles long.

drill string component