STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Bits and Downhole Tools 815
Nomlnal
sln
Tool
(A)
12
10
8
7
6%
'Sugg..l.d .
Sp.cn1-m
CJolnt
C.mr
HobSlt.
RacomNndad
Makaup
iM 0m.t
OD
Pmrformance Bar.
R-lb)
17YlmrU30 2'h 2'910 Spec@ TO80 160 30 120 3070 70.000
12hlhru 15 2% 2'14
SpecifW 148 23 116 2OXI 55.000
athru 12V4 2% 2'4 Sizeand SameOD 149 23 110 1635 41.000
BMlhru 11 2'sis 2'14 AsDrill 145 23 112 1400 35.m
mmrus 2v4 2 Type Collar 143 23 111 1100 27.000
6%. thru 9 2'/. 1 'h
144 23 111 890 2c.m
Nota 1 All dimensions are given in Inches. unless otherwse staled
'2 Recommended lor optimum IwI life
'Courtesy Smith International, Inc
deviation problems, the shock absorber should be place as shown in Figures 4-171B
and 4-171D.
For turbine drilling, it is recommended that the shock absorber be placed
on top of the first stabilizer above the turbine as in Figure 4-171A.
Jars
Jars provide an upward or downward shock (or jar) to the entire drill string.
Early attempts to recover stuck drill pipe motivated the development of jars.
Types of Jars. There are two general classes of jars: fishing jars and drilling jars.
A fishing jar is used to free stuck drill string, and is added to the drill string
only when the string becomes stuck.
The drilling jar is used as a part of the drill string to work any time it is
needed. With modern drilling requiring more safety and less cost per foot, it
has become more economical to use drilling jars. In areas where possible sticking
conditions exist, the drilling jar is ready to free a stuck pipe through calculated
string over-pull or slack-off. The jars are used immediately when the string
becomes stuck, which prevents excessive downtime and costly tripping. Unlike
the fishing jar, the drilling jar has the additional function of transmitting the
high amount of drilling torque to the bit.
Drilling Jar Design. The jarring and bumping characteristic of drilling jars is
determined by their specific type of release elements and stroke. There are three
basic types of release elements: (1) hydraulic, (2) mechanical, and (3) a combination
of hydraulic and mechanical. Hydraulic mechanism employs a sleeve, or
valve that is pulled through a restricted area that allows only a small amount
of hydraulic oil to pass through. Once the sleeve, or valve passes through the
restricted area and enters the larger chamber, it is free to travel upwards until
reaching the anvil that creates a sudden stop and sends a shock throughout
the string.
Mechanical mechanisms involve the following types of dynamic action:
1. Adjustable spring pressure against locking system.
2. T-slots system. A combination of upward overpull or string weight, and
right-hand torque is required. When torque is released, shots disengage for
jarring.