DRILLING STRING

Drilling String

Today I would like to share you about some information of Drilling String from a Schlumberger article. The objects of this article will help you getting:

• Funtions of Drill Pipe, Drill Collar and BHA selection.
• Grade of Drill Pipe and Strength Properties
• Thread types and Tool joints
• Drill Collar weight and neutral point.

Funtion of Drilling String:

The drilling string is the mechanical linkage connecting the drill bit on bottom to the rotary drive system on the surface.

Figure 1 - Drilling String  Components

The drilling serves the three main following functions:

• Transmit and support axial loads - WOB
• Transmit and support torsional loads - Torque
• Transmit hydraulics to clean the hole and cool the bit.

Drilling String Components:

The Drill String includes all tubular equipment between the Kelly Swivel and the bit

• Kelly
• Surface Safety Valves
• Drill Pipe
• Heavy Weight Drill Pipe
• Drill Collar
• Jars - Shock Subs - Bumper Subs - Junk Baskets - Accelerators ect...

The Kelly/Top Drives:

Strictly speaking, Kelly/Top Drive are not components of the drill string; however, they provide the essential requirements for drilling a well:

• Transmit rotation to the drilling string.
• Provide access to the drilling fluid into the drilling string.
• Support the weight of the string.

The Kelly:

The Kelly is the rotating link between the rotary table and the drilling string.

• Transmits rotation and weight-on-bit to the drillbit

Supports the weight of the drilling string

• Connects to the swivel and allow circulation thru pipe

Figure 2 - Drilling operation with using a kelly and rotary table

The Kelly comes in lengths ranging from 40 to 54 ft with cross sections such as hexagonal (most common), square or triangular.

Connected to a Kelly Saver Sub 

Kelly Cock :

The Kelly is usually  provide with two safety valves, one at the top and one at the bottom, called Kelly Cock.

The Kelly cock is used to close the inside of drilling string in the event of a kick

The upper and lower Kelly cocks operate manually

IBOP / DPSV / FOSV are not run in the drilling string but kept handy on the rig floor.

Figure 3.a - Position of kelly cock on Kelly

Figure 3.b - Kelly cock

Top Drive:

The top drive is basically a combined rotary table and kelly.

It is powered by a separate motor and transmits rotation to the drilling string directly without the need for a rotary table.

Advantages over the kelly system:

• Efficient reaming and back reaming
• Circulating while running in hole or pulling out of hole in stands
• The kelly system can only do this in singles; ie 30 ft

Figure 4 - Top Drive rotates the drilling string without using a kelly and rotary table

Stabilizers:

Figure 5. Types of stabilizer

Reasons for Using Stabilizers:

• They are used as a fundamental method of controlling the directional behavior of most BHAs.
• Help concentrate the weight of BHA on the bit.
• Minimize bending and vibrations which cause tool joint wear and damage to BHA components such as MWDs.
• Reduce drilling torque by preventing collar contact with the side of the hole and by keeping them concentric in the hole.
• Help preventing differential sticking and key seating.

Roller Reamers:

Roller Reamers are designed for reaming and stabilization in any type of formation. All parts of the tools are made of special alloy steel and heat treated for hardness. 

Figure 6a. Roller Reamer 3 points

Figure 6b. Cutters

Drill Collars:

Drill collars are used to apply weight to the bit. Their large wall thickness give them a greater resistance to buckling than DP. The neutral point should be located at the top of the drill collars section (never in the DP section), 75 to 85% (maximum) of the drill collars section should be under compression (this will include HWDP if utilised).

The lower part of drill collar section is under compression, therefore subject to buckling. This generates high stresses and potential fatigue failure, particularly at the connections.

The clearance between drill collars and the wellbore is smaller than with DP, therefore increasing the possibility of differential sticking. If it is thought to be a potential problem, spiral DC can be used to reduce the contact area with the wellbore and consequently the chance of differential sticking.

In deviated holes, to avoid sticking of the drilling assembly, the minimum of number of drill collars should be used (and these should be of the spiral type). Heavy Weight Drill Pipe in conjunction with undergauge stabilisers, should be used to substitute the drill collar weight.

Figure 7. Slick DC & Spiral DC

Descriptions

They are heavy walled metal tubes

The ends are threaded (box and pin)

Functions

• To put weight on bit (WOB)
• To keep the drilling string from buckling
• Protect the Drill string from bending and Torsion
• Control direction and inclination of wells
• Drill straighter holes or vertical holes, Packed Hole BHA (Gun Barrel BHA)
• Provide Pendulum effect, Pendulum BHA
• Increase bit performance

Types

Comes in many OD and ID sizes

Typically 3 1/8" to 11" OD

Most commonly in lengths of 30 - 31 ft 

Square collars where the holes tend to be crooked 

Spiral collars where there is chance of getting stuck

Collars with elevator and slip recesses

Some type of drill collars

Both slick and spiral drill collars are used.

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.

Figure 8a. Slick Drill Collar

Figure 8b. Spiral Drill Collar

Figure 8c. Integral Spiral HWDP

Drill Collars Connections

• DC connections are rotary shouldered connections and can mate the various DP connection
• 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 Connection

• 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

• 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
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 ==> Stresses Relief Groove / Bore Back