Bit Design

Roller cone bits

Roller cone, or rock, bits are bits made up of cones mounted on rugged bear­ings. Usually there are three cones, so they are also called tricone bits. Each cone houses several rows of steel teeth or tungsten carbide inserts that do the cutting. The quantity, place­ment, and composition of the teeth or inserts and the angle at which the cones meet the for­mation all affect the efficiency with which a cer­tain formation can be drilled.

Steel-tooth roller cone bits

1. Before reading the text, study the vocabulary

Roller cone bit	Шарошечное долото
Steel-tooth, or milled-tooth, roller cone bit	Шарошечное буровое долото с фрезерованными зубьями
To mount	Устанавливать
Rugged bearings	Прочные, выносливые подшипники
Tungsten carbide inserts	Вставные резцы из карбида вольфрама
Milled tooth	Фрезерованные зубья
Marl	Известковая глина
Red beds	Глинистый красный песчаник
Shearing	Скалывание, срезание
Gouging	Выдалбливание
Balling up	Образование сальника
Interfit cones	Взаимопроникающие шарошки
Adjacent cones	Смежные, соседние шарошки
Light journal	Цапфа подшипника
To withstand	Выдерживать
Hardfacing	Твердосплавное покрытие, упрочнение
Cone offset	Смещение осей шарошки
Chipping-crushing action	Скалывающее-дробящие действие

2. Use the dictionary and write down the transcription of the following words:

• rugged bear­ings

• tricone bits

• tungsten carbide

• gypsum

• dolomite

• quartzite

• granite

• abrasiveness

Practise their pronunciation!

3. Read and translate the text

Steel-tooth, or milled-tooth, roller cone bits can be used in a variety of formations; bit specifications vary with forma­tion types. Soft-formation milled-tooth bits are characterized by long teeth set in cones that are offset; that is, they do not rotate around the precise center of the bit, giving the bit a twisting, scraping action that is effective in soft formations. Soft shales, gypsum, marl, red beds, salt, and clays are effectively drilled by the shearing and gouging of these bits. To prevent the bit from balling up, or being unable to function because of interference from cuttings, soft-formation bits have widely spaced, self-cleaning teeth. To accommodate the long teeth, the cones are carefully interfit, allowing the rows of teeth on one cone to project into spaces on adjacent cones as the bit rotates. Light weight on bit and high rotary speed are usually the best combination for soft-formation bits. Their thin cone shells, relatively small bear­ings, and light journals cannot, withstand extreme weights. Too much weight can also result in tooth breakage.

In soft limestone and tough, waxy shales and in soft formations containing streaks, or stringers, of hard or abrasive material, a slight variation in bit type may result in faster penetration rates. Basically, the shearing action is maintained, but more teeth per cutter and the application of tungsten carbide hardfacing to combat abrasion can improve footage per bit.

Harder formations such as dolomite, hard shale, and hard limestone require bits with more teeth, shorter teeth, and less cone offset. These changes the bit's geometry are made to accommodate the added weight required for the bit to overcome the compressive strength of these harder rocks with a greater chipping-crushing type action and a minimum of scrap­ing. The teeth must still be interfit to prevent balling up that may occur if a stringer of softer material is encountered. Hardfacing is bene­ficial in abrasive formations of low compressive strength, but its use in highly compressed for­mations can result in excessive tooth breakage.

Rocks such as quartzite, granite, dolomite, and hard limestone (containing chert and quartz sand) are among the harder formations encountered in drilling. Although the com­pressive strength and abrasiveness of these for­mations vary, the most efficient bit will have the characteristic hard-formation construction and high-capacity bearings suited to the heavy weights that are required.