This paper presents a method for determining the fatigue behaviour of Drill String (DS) components under given environmental conditions. Over the past few years. field data for DS failures have been gathered from certain wells drilled in Italy. West and North Africa. Clearly, fatigue phenomena were identified as the main cause of most of such failures. Among the several factors that can affect DS element fatigue resistance are tensile load. makeup torque. material properties. corrosive environment. abrasive wear and so on. However, simply on the basis of the existing guidelines it is difficult to define the real cause of a failure under given operating conditions. Based on a comparative study of full-scale fatigue test results and field data. a new approach is suggested for the analysis of some of the drilling technology factors that can affect the DS fatigue behaviour.
DS is subject to great stress variations during service such as different kinds of dynamic and static loading. mud corrosion. temperature and so on. This, in combination with high stress concentrations. makes DS susceptible to fatigue damage which is still a major concern for Oil Companies and Drilling Contractors. On the other hand. in the available literature it is possible to find recommendations on how to take fatigue into account. As a matter of fact. according to, DS fatigue resistance analysis, for drilling directional wells. is carried out by means of drill pipe (DP) combined loading curves or given equations. Published curves display stress as a function of combined tension and dogleg, and show regions of "no fatigue damage" and "fatigue damage". However. this approach is based on the knowledge of the cyclic load history which is usually very difficult to quantify.
Current methods for evaluating the drilling equipment fatigue failure risk as well as analysing failures are usually based on full-scale fatigue test results. The results of such investigations show that all factors affecting the DS element fatigue resistance can be divided into two main groups that can be defined as construction factors and drilling technology factors. Construction factors refer to fatigue characteristics of the tubular goods as they leave the manufacturer, such as the following:
- type and size of DS components;
- material characteristics;
- design parameters;
- strengthening methods (SRF, CW and so on).
None of these factors can be changed at the rig site. On the other hand. drilling technology factors can be defined as those that can be "worked on" at the rig site such as:
- tensile load;
- makeup torque:
- abrasive wear;
- mud corrosion;
- tripping technique, equipment and so on.
In other words, drilling technology factors depend on the drilling technology itself. In this paper, the terms construction and drilling technology factors, as described above, will be used extensively.
Full-scale fatigue tests on DS elements have been performed for years and represent the basis for evaluating the effect of these factors on DS fatigue resistance. However. the existing specifications limit the possibility of changing the DS element construction thus making any change of construction factors while operating "unpractical".