Downhole Tool Serviceability Under Dynamic Loads
Authors: Baryshnikov A. , Poli S. , Stewart F. , Ferrara P. 
Published: December 2007

Abstract
Current methods of evaluating equipment failure risk under dynamic load conditions and of analysing failures are usually based on periodic full-scale fatigue test results. At present API does not give effective recommendations for an evaluation of downhole tool fatigue resistance even though their fatigue characteristics need to be known. This paper presents the modern methods for evaluating equipment serviceability under dynamic loads as well as analysing failures based on periodic full-scale fatigue test results and dynamic load measurement data. Recommendations are given for improvement of existing technical specifications and practices for the reliable use of downhole equipment.

Introduction
Serviceability is the state in which all tool parameters fully comply with specified requirements for given operating conditions. The serviceability of downhole tools is one of the critical factors in achieving cost effective drilling and production. These costs grow exponentially for, reservoirs of increasing risk and complexity, failures resulting in fishing operations and, in extreme cases, failures causing well control problems. The severe operating conditions and internal and external diameter restrictions create considerable difficulties for the design and reliable use of downhole equipment in a well. Downhole tools should be able to withstand the main loads and conditions listed below:–axial tension due to weight and pulling;–compression and bending in BHA due to weight on bit;–torsion applied to rotate drillstring and bit;–pin tension and box compression due to rotary shouldered connection makeup;–internal and external drilling mud pressure;–cyclic bending due to downhole tool rotation in dogleg sections and in the presence of buckling;–dynamic loads while tripping;–dynamic loads due to downhole tool vibration.
These load conditions which influence downhole tool serviceability can be subdivided into the following two main groups of strength parameters:–strength under static loads;–strength under dynamic loads.
In accordance with existing practice recommendations for downhole tool design and operating limits, strength is determined under static loads even though this type of failure is quite rare. Resistance to dynamic loads has never been studied closely and usually downhole tools are not analysed under dynamic loads before being qualified for operation. Despite this, from an analysis of the downhole tool failure database it has been shown that fatigue is the primary cause of 60-80% of failures. The development of the Villafortuna-Trecate field, in the northwest of Italy, is an example of such a case.

Case history
This case history is based on failures which occurred during the drilling of some deep HTHP wells in the Po Valley, Italy. Operating conditions while drilling were extremely severe due to the considerable depth of the wells (6000 m - 7000 m), high temperatures (over 180 C), hard formations and an aggressive downhole environment due to the presence of H2S and CO2. In such conditions an abnormally high occurrence of drillstring fatigue failures was recorded. A survey based on the first 7 wells showed that most of the failures took place in the BHA (77%, not including HWDP). A significant number of BHA failures were detected in the drill collars (41%) (FIG. 1).
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