Successful Reduction of North Sea Drillstring Failures
Authors: Horbeek J.H. , Birch W. , McMahon M.J. , Fearnley K. 
Published: December 2007

Shell Expro was confronted with an unacceptable high cost attributable to drillstring failure in 1991. One specific drilling unit experienced 5 failures within a two month period. The cost of these failures alone was estimated to be more than 2 MM.

A Drillstring Failure Prevention Quality Improvement Project (QIP) was instigated with the objective of reducing drillstring failure frequency, and hence Non Productive Time (NP') costs, by at least 50% in 2 years. This goal has been achieved by carrying out detailed autopsies of failed components to determine 'root cause' of failures.

Analysis of the root causes and symptoms resulted in a series of improvement strategies which have been implemented to successfully reduce drillstring failure. NPT costs associated with drillstring failure have been dramatically reduced from 6.5 MM in 1992, to less than 1.0 MM in 1994.

Introduction

Starting in April 1992 the QIP, using Juran problem solving methodology was created to tackle drillstring failures in Shell Expro's North Sea operations. A multi-disciplined team was formed to pursue the drive towards continuous quality improvement.

Prior to the start of the QIP, drillstring integrity failures accounted for 6.5 MM per year with 0.67 incidents per 10,000 ft of hole drilled, and an average ratio of twist-offs to total failures of 0.64.

To reduce drillstring failure costs the following strategies were implemented: Inspection, Design, Procurement, Personnel Awareness, Contracts, and Operations. These strategies and their implementation are discussed in this paper.

By the end of 1994 Twistoffs had been reduced by 75%. Total failures (washouts and twist-offs) had been reduced by 55%. This has resulted in a cost saving of some 8.5 MM to year end 1994. This success has attracted the interest of not only other Shell operating companies, but also other North Sea operators.

Definition of Project Boundary Limits

A diversity of failure types contributed to the individual events which led to NPT. Electronic failures in MWD systems as well as bearing failures in mud motors/turbines both contributed to the problem, although from very different root causes.

It was clear that these problems could not be tackled in the same way as the mechanical failure of the connections on BHA components, or drillpipe tubes and tool joints. The QIP team therefore decided, for this reason, to 'park' problems that were specific to mud motor/turbine and MWD and concentrate on BHA/Drillpipe failures.

For the purposes of this project the drillstring extends from the drill bit connection up to, but not including the top drive or kelly. Failure of drillstring integrity arises when there is a failure in the pressure containing conduit. This usually is manifested in the form of a catastrophic separation of the drillstring (twistoff) or a leak in the fluid circulation path through the drillstring (washout). Failure costs include IPT and equipment lost in hole, but do not include deferred production revenues.

As part of the QIP team's learning process, a literature search was carried out to establish what work had already been done within the industry and where this could be of use to prevent unnecessary repetition of work.

The QIP team suspected that deficiencies may exist in the way in which drill string components were acquired. An understanding was therefore required of the 'as is' process. This information was obtained by 'interviewing' individuals across the departments in Shell Expro and contractors, who are involved in buying, renting, certifying and maintaining drillstring components. This information was flowcharted and showed what specifications were requested and by whom, how compliance was checked and whether Shell Expro's own standards were met.

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