New Generation of Soft Formation TCI Bits Reduces Drilling Costs in High-Cost Environments
Authors: Scott D.E. , Zahradnik A.F. 
Published: October 2008

Steady improvements in bearing and seal technology coupled with more durable carbide shapes and the latest tungsten carbide and steel technology have afforded the drilling industry an excellent opportunity to expand the use of the softest IADC classes of bits into many formations previously reserved for steel tooth or PDC bits. This technology also has allowed softer class bits to be used in firmer rocks with excellent penetration rates and lower drilling costs. This is particularly important offshore where reliability and penetration rates are absolute essentials to reducing drilling costs.

The soft 16" and 17 1/2" tungsten carbide insert (TCI) bit, for example, can consistently drill at cost effective penetration rates in North Sea and Pacific Rim programs. Additionally, soft 12 1/4" TCI bits are taking over many of the soft formation applications formerly reserved for steel tooth and PDC bits while supplanting IADC Class 5-1-7 bits in the firmer rocks.

This paper describes the latest in design improvements and discusses the development and evaluation of carbide grades and steel technology which has opened this opportunity for the drilling industry. Case histories from high-cost drilling areas will be reviewed.

Introduction

Upon their introduction in 1951, TCI rolling cone bits featured blunt and durable ball nose inserts that were designed chiefly for drilling the extremely hard and abrasive limestone and chert of West Texas. The carbide grades used had been adopted from metal-forming applications. Over the next 15–20 years, cutting structure development on TCI bits focused on the medium and hard formations where the superior wear resistance and durability of the carbide inserts yielded substantial benefits over the steel tooth bits of the day. New carbide grades were developed specifically for rock bits and the shapes evolved to include blunt conicals and blunt chisels.

The introduction in 1969 of the long-life "0 Ring" sealed journal bearing bit paved the way for the application of TCI technology in softer formations. The designated IADC Class 5-1-7 bits quickly gained wide acceptance in the early 1970'5. Designers then concentrated on developing a TCI bit with twice the insert projection of the previous generation. The longer projection would enable the bit to yield cost-effective penetration rates when compared to steel tooth bits, while also exhibiting longer life.

The first generation of these long projection, soft formation bits encountered lost and rotated inserts, while the second generation experienced broken inserts. To ensure consistency of performance, the emphasis shifted tometallurgical developments in steel and carbide technologies, which will be discussed later.

The long projection and large diameter chisel inserts in the early IADC 4-3-7 bits featured sharp corners and narrow crests, which were intended to simulate the geometry of steel tooth bits. The development of the IADC 4-3-7 class of bits was regarded as a significant milestone in the evolution of the rolling cone rock bit.

Development of the scoop insert1,2 further widened theapplication of the IADC 4-3-7 bit, because of the improved durability and faster penetration rates it yielded.