Drilling String

The drill tools transfers the driving force from the rig to the drill bit through rotational energy and thrust. The rods are also hollow which enables the supply of drilling fluid to keep the components cool and to transport the cuttings back to the surface. Some level of directional control is possible by stopping the rotational motion and pushing on the string where the drill bit will then shift it's alignment whereby the pressure of the soil on the slanted edge will cause a change in direction. Cables are also routed along the drilling tools which communicate with sensors.

We can supply drilling tools as follow:

Drill Pipe

Heavy Weight Drill Pipe (HWDP)

Drill Collar

Kelly

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The drill pipe has two tool joints, one female is called a box and the other male is called pin. The outer diameter of the tool joint is larger than the body of the drill pipe to accommodate the threads.

One drill pipe is called a single or a joint. The dimensions of the drill pipe are specified by the API (American petroleum Institute). There are three ranges of length; range 2 is most common on rig sites. The drill pipe must be measured on rig site to get accurate length of drill pipe. The drill pipe is manufactured in verity of weight, diameters and also materiel grades.

The drill pipe is characterized by burst, collapse, tensile and torsional strength. These specifications are used to select the appropriate drill pipe for a particular drilling operation.

The weight of the drill pipe mentioned in the manual is called weight in air. When drill pipe is in the well, it has to be taken in consideration the buoyancy force which is related to density of the drilling fluid. The weight of the drill pipe when run in the well can be calculated as follows:

Buoyant weight= weight of pipe in air x buoyancy factor

Drill pipe stress and failure:

The drill pipe can be exposed to many stresses:

Tension: due to hole problems, an over pull can be exerted causing extra tension on the drill pipe (ex: stuck pipe). This tensile load can lead to drill pipe failing.

Torsion: Bad hole conditions can increase the twisting force and torque on each joint leading to poor rotation transmission from the surface to the bottom.

Cyclic fatigue: The wall of the drill pipe while drilling deviated wells is exposed at point of bending to tensile and compressive forces. While rotating the drill string, the same point on a drill pipe sustains a cycle tensile and compressive forces. This cyclic stresses can result in fatigue of the drill pipe.

Also there are other causes of fatigue like abrasive friction, vibration and bit bouncing off bottom.

Corrosion is also another issue which can affect drill pipe strength; corrosion can be due to presence of dissolved gases and acids. Carbone dioxide can form acid dioxide which can lead to steel corrosion.

The hydrogen sulfide can be present in the formation. It can cause hydrogen embrittlement or sulfide stress cracking. The surface of the steel absorbs the hydrogen in the presence of the sulfide. When the concentration will be greater than a certain level (less than 13 ppm), cracks can appear on pipe body. The combinations of stress and cracks leads to pipe failure.

Tool joints:

The tool joints provide screw threads in order to connect drill pipes together. The seal is guaranteed by the shoulder/shoulder connections between pin and box. Hard facing material is welded on the surface to protect the tool joint from abrasive wear when rotating drill string in the borehole or when making connection by rig tongs. This layer of hard facing material can be replaced in workshops when it becomes depleted due to excessive wear. The internal diameter of the tool joint is less than the internal diameter of main body. The same stresses applied on the drill pipe during drilling operations are subjected on the tool joint, but also there are other additional issues:

- During operations, frequent engagement of box and pins can be done harshly which can damage the threads.

- When tripping out of the hole, the last tool joint support all the weight of the string beneath it.

We provide a complete line of drill string products, including a full range of drill pipe in nominal sizes from 2-3/8” to 6-5/8” and in a wide range of wall thicknesses.

Heavy weight drill pipe has a greater wall thickness if we compare it with the wall thickness of ordinary drill pipe. It is used instead of drill pipe where the stresses concentration is important. These stresses are due to:

- The sharp difference in cross section between the drill pipe and drill collars.

- The difference in stiffness between the drill pipe and drill collars

- The bouncing caused by the bit while drilling

The main benefit of using the HWDP is absorbing the stresses being transferred from the drill collars to the drill pipe. The use of HWDP between drill collars and drill pipe can minimize the stresses caused by the high level of difference in stiffness.

The heavy weight drill pipe is characterized by many features:

High wall thickness

Long tool joint

More hard facing

For operations, the heavy weight drill pipe should be operated in compression; in order to maintain the compression while drilling highly deviated wells.

The Heavy Weight Drill Pipe (HWDP) is made from one piece of AISI 4145H solid bar, fully heat-treated, all physical properties confirm with API Spec.7 Latest Editions.

Heavy Weight Drill Pipe (short for HWDP) is an intermediate weight drill stem component which is used in conjunction with the drill pipe and drill collars. In some applications, heavy-weight also can be used instead of the drill collars.

Features and Benefits

A center upset or wear pad to increase tube life, reduce hole drag and differential sticking problems;

Connections are completed phosphate coated to protect them from the elements after machining and to help prevent galling upon initial make-up;

Thread roots are cold rolled on API and H-90 connections. And pressed steel thread protectors are supplied for standard connections;

Hard banding and Internal Coating can be provided on customer’s request.

Specifications - Heavy Weight Drill Pipe

Drill collars have larger outer diameter and smaller inner diameter than drill pipe. The functions of drill collars are:

- Provide the required weight on bit while drilling

- Maintain drill string in tension which reduces bending and fatigue failures.

- Provide stiffness for directional control

Size: 3-1/8”, 4-1/8”, 4-3/4”, 5”, 6”, 6-1/4”, 6-1/2”, 6-3/4”, 7”, 7-1/4”, 8”, 8-1/4”, 9”, 9-1/2”, 9-3/4”, 10”, 11”. Length: 9150mm, 9450mm.

The connection thread of a drill collar can be machined directly on the body due to the large wall thickness. In order to prevent failure, correct make up torque has to be applied. It is very important to handle drill collars carefully.

Shoulders and threads should be lubricated with lubricant containing 40% to 60% of metallic-zinc powder.

Similarly, to drill pipe, drill collars are subjected to different stresses caused by:

- Bending and buckling forces

- Tension

- Vibration

- Alternate tension and compression

There are other types of drill collars used for special operations:

- Spiral drill collars are used to avoid stuck while drilling through highly porous formation. The stuck can occur when high overbalanced of mud pressure is applied or while drilling high deviated wells. Among the methods used to minimize the risk of differential sticking is reducing the contact area between the collars and wellbore. In order to reduce the surface contact, spiral grooves are cut on collars.

- Nonmagnetic collars: this type of collars is used to isolate survey instruments from magnetic distortion caused by the steel of the drill string. The magnetic distortion can affect the directional instrument functioning

Features and Benefits

A hardness range of 285 to 341 BHN and a Charpy impact value of 40 ft-lbs are guaranteed at evenly distributed 16 points in any sections at room temperature;

Connections are completed phosphate coated to protect them from the elements after machining and to help prevent galling upon initial make-up;

Thread roots are cold rolled on API and H-90 connections;

Pressed steel thread protectors are supplied for all drill collar that are equipped with standard connections When ordering Please specify;

Drill collar OD and ID;

Overall length;

Connections required (size and type);

Special features desired, for example:

Slick or Spiral;

Stress Relief Features;

Slip and/or Elevator Recess;

Hard banding;

Product Type: Square Kelly/ Hexagon Kelly

Standard: API Spec 7-1, API RP 7G

Specification:

Steel Grade: AISI 4145H, 4145HM

OD: 2-7/8’ – 6-5/8’

Connection: LH, RH

Kelly is used to connect the topmost joint of drill pipe for drill stem turning when the rotary table runs. Petrosadid supplies both square kelly and hexagonal kelly.

Square Kelly