Drill String Design BHA Design
Drill String Design BHA Design
Drill String Design BHA Design
Design
• I. Introduction to Drill String
Design
Multiply 5.0417 by 10 C
50.417 Choose first two digits
C 50 Hence NC 50
NC Drill string
•
Connections
There are 17 NC’s in use : NC-10 (1
1/16”) through NC-77 (7 3/4”)
• Typical sizes: NC 50 for tool joints with 6
1/2” OD for 5” pipe and NC 38 for 4 3/4”
tool joints and 3 1/2” pipe.
• Seal is provided by shoulder not threads. A
clearance exists between the crest of one
thread and the root of the mating thread
• Use of Lead based dope vs Copper based
dope for DCs. Not for sealing but for
lubrication, to help make-up and prevent
galling
Drill
•
Collars
Functions
– To put weight on bit (WOB)
– To keep the drill string from buckling
• Types
– Typically 4 ¾” to 9 ½” OD
– Most commonly in lengths of 30-31
feet
– Square collars where the holes tend to
be crooked
– Spiral collars where there is
chance of getting stuck
(differentially, etc..)
– Collars with elevator and slip recesses
More functions of Drill
Collars
1. Protect the Drill string from Bending and
Torsion
2. Help to control direction and inclination of
wells
3. Drill straighter holes or vertical holes
4. Provide Pendulum effect at low WOB
5. Reduce dog legs, key seats and ledges
6.Improve the probabilities of getting casing in
the hole.
7. Increase bit performance
8. Reduce rough drilling, sticking and jumping
9. As a tool in fishing, testing, completing
More Types of Drill
Collars Slick Drill Spiral Drill
Collar Collar
In areas where
differential sticking is a
possibility spiral drill
collars and spiral HWDP
should be used in order to
minimize contact area
with the formation.
Drill Collars
Strapping
API Drill Collar
Sizes
Drill Collar
Connections
Characteristics
• DC connections are rotary shouldered
connections and can mate the various DP
connections
• The shoulder provide the only positive seal
against fluid leakage
• The lubricant is Copper based dope
• The connection is the weakest part of the entire
BHA
• The DC connections go through cycles of
tension- compression and are subject to
bending stresses
• Improper M/U torque, improper or
insufficient lubricant, galling can all lead to
connection failure
Drill Collar
•
Connections
Stress Relief Features
• Stresses in DC connections are concentrated
at the base of the pin and in the bottom of the
box (stronger)
• DP body bends easily and takes up the majority
of the applied bending stress, DP connections are
therefore subjected to less bending than the DP
body.
• DCs and other BHA components are however
much stiffer than the DPs and much of the
bending stresses are transferred to the
connections.
• These bending stresses can cause fatigue failure
at the connections Stress Relief Groove / Bore
Back
Stress Relief Pin
Feature
Stress Relief Pin & Box
Features
Drill Collar
Connections
• The stress relief groove is to mitigate the fatigue
cracks where the face and threads would have
otherwise joined
• The Bore Back serves the same purpose at the
bottom of the box
• Stress relief features should be specified on all
BHA connections NC-38 or larger.
• Pin stress relief grooves are not recommended on
connections smaller than NC-38 because they may
weaken the connection’s tensile and torsional strength.
• Bore Back boxes could be used on smaller connections.
• The Low-Torque face is to increase the compressive
stress at normal M/U torque above that of a regular
face
Lo- Torq
Feature
• The low torque feature
consists in removing
part of the shoulder
area of the pin and box.
• This allows for lower
make up torque
maintaining
adequate shoulder
loading.
• It is a common feature
in large OD
connections.
Torsion limits for
DC
Torque is rarely limited by the DC connection
because rotary torque is usually higher in the
DP at surface and lower in the DC at deeper
depths.
• DF for excess
BHA=1.15
• Neutral Point
(NP) to tension
should be in
drill collars
BHA
Design
• Procedure For Selecting Drill
Collars:
• 1. Determine the buoyancy factor for the
mud weight in use using the formula
below:
• where
– BF =Buoyancy Factor, dimensionless
– MW =Mud weight in use, ppg
– 65.5 =Weight of a gallon of steel, ppg
BHA
•
Design
2. Calculate the required collar length to achieve
the desired weight on bit:
DC Length = 1.15* WOB / (BF*Wdc)
• where:
• WOB=Desired weight on bit , lbf (x 1000)
• BF =Buoyancy Factor, dimensionless
• W dc =Drill collar weight in air, lb/ft
• 1.15 =15% safety factor.
• The 15% safety factor ensures that the neutral point
remains within the collars when unforeseen forces
(bounce, minor deviation and hole friction) are
present.
BHA
Design
• 3. For directional wells:
• Stiffness Coefficient :
= Moment of Inertia x Young’s Modulus of Elasticity
= л (OD4 − ID4) / 64 x 30.000.000
Bending Strength
of a given Ratio
•BSR is the relative stiffness of the box to the pin
connection.
•Describes the Balance between two members of a
connection and how they are likely to behave in a
rotational cyclical environment
Where:
Zbox = box section
modulus Zpin = pin
section modulus
D = Outside diameter of pin and
box b = thread root diameter of
box
threads at end of pin.
R = Thread root diameter of pin
threads ¾ of an inch from
shoulder of pin.
d= inside diameter or bore.
Section Modulus for
Connections
BSR in DC
•
Connections
A Connection is said to
be balanced if the BSR
is 2.5
– When BSR is higher tend
to see pin failures
– When BSR is lower tend
to see more box failures
• However, field
experience has shown
that:
– 8” Dc having BSR’s of
2.5 usually fail in the
box
– 4-3/4” DC having BSR
as low as 1.8 very rarely
fail in the box.
BSR in
Connections
Additional BSR
Guidelines
• High RPM, Soft Formation Small DC (8
in in
12.25 hole or 6 in in 8.25 hole) 2.25-2.75
• Low RPM Hard Formations Large DC
(10 in in 12-1/4 hole 2.5-3.2 (3.4 if using
lo-torq connection)
• Abrasive formations 2.5-3.0
• New DCs 2.75 − more wear resistant
API BSR
Charts
• Fortunately for you
API have worked the
problem!!!
• Pages 39-44 of Spec
7G list the BSR of
Connections by OD
and ID of the collar
T.H.Hill BSR
Tables
Stiffness
Ratio
•The SR measures the stiffness of a connection in a
transition between 2 types of pipe
• • Based on field experience, in a
transition from one collar or
pipe to another the SR should
not exceed
– 5.5 for routine drilling
– 3.5 for severe or rough drilling
Given that we will drill a vertical 12 ¼” hole, with 9.5 ppg mud and 65000
pounds in a relatively hard formations, what API collar would you
recommend?
• • Step 2
– Drill collars and bottom drillpipe act as the
weight carried by top section…effectively the
drill collar
– Apply the equation for top drill pipe last
Other
Loads
• Collapse under Tension
• Burst
• Other loads not covered here
– Shock Loads
– Bending Loads
– Buckling Loads
– Torsion
– Torsion with Simultaneous
Tension
Biaxial
Collapse
• The DP will collapse if:
External Pressure Load > Collapse pressure
rating
• A Design factor of 1.15 is used:
External Pressure Load < Collapse rating /
1.15
• When the string is in tension, the Collapse
rating is further de-rated:
Biaxial
Collapse
• Collapse load is worst when For dry test
work where pipe is run in empty