Summary of Drilling Floor Accident:
This is a sample of numerous accidents investigated by the author of this article. The assistant driller on a Jack-up Rig owned and/or operated by an Offshore company, was performing various tasks on the drilling floor, when, one of the floor hands (roustabout) had to leave the drilling floor and the assistant driller replaced him.
After he took the place of one of the floor hands, he was seriously injured when the tong (see Glossary below) he was working with sprung back hitting him.
The IADC Drilling Report (International Association of Drilling Contractors) indicates that at the time of the accident, the drill string was composed of the following: one bit, one bit sub, six joints of drill collars and 3 joints of pipe with a length of about 350 feet.
The assistant driller and the roustabout lined up the kelly's pin (see Glossary below) into the tool joint of the 3 ½", to stab it into the drill pipe.
At this time, the driller activated the kelly spinner.
Then, the assistant driller slid the backup tong (break out tong) on the tool joint of the pipe suspended in the rotary table, closed the backup (breakout) tong around the pipe, pushed the tong as much as possible around the pipe until the snub line was tight and made the tong bite, using both hands.
Suddenly, the kelly spinner, which was rotating fast, made up the face of the box and the face of the pin, creating a sudden jerk, stretching and tensioning the snub line while turning the bottom string in the slips.
The snub line reached the end its elastic stretch and the shock made the tongs become undone. The tong unbite and the stretched snub line pulled the tong back, hitting the assistant driller, while the kelly spinner and pipe string continued rotating.
In order to turn the bottom string in the slips, the kelly spinner must have lifted the bottom string off the slips by the length of the threaded portion of the box, therefore, releasing the weight of the bottom string from the slips. In other words, the driller did not lower the kelly spinner as needed to keep the weight of the bottom string on the slips.
The assistant driller was positioned with his back towards the driller who was at the drilling console, while the roustabout was positioned opposite to him. The driller made the kelly spinner to rotate clockwise very fast. The assistant driller heard the pin of the kelly bind and almost instantaneously the lever of the tong sprung clockwise for a fraction of a second and then immediately swung back counterclockwise and struck the assistant driller on his left arm and shoulder throwing him to the drilling floor.
Glossary and Comments Regarding Equipment and Tools:
Pin: The male threaded section of a tool joint and/or the kelly.
Tool joint: A heavy coupling element for drill pipe, having coarse tapered threads and seating shoulders.
Box: Female section of a tool joint.
Kelly: The kelly is a heavy steel member, four, or six-sided, about 40 feet long, suspended from the swivel and can be connected to the topmost joint of drill pipe.
Kelly Spinner: A kelly spinner is a pneumatically (in the case under consideration) or hydraulic powered device that spins to thread or unthread connections. Crewmembers mount it below the swivel and on top of the kelly.
When connecting the pin end of the kelly to the female or box end of the tool joint, the drill pipe that is being held by the slips about 3 ½ to 4 feet above the rotary, the driller will activate the kelly spinner. The kelly spinner rapidly rotates, or spins, the joint together. The tongs are used to make the joint up to final tightness because the torque produced by the kelly spinner is not enough for the proper torque of the tool joint. Most kelly spinners available in the market are single speed (wide open). This type of kelly spinner is activated by opening a valve that can only supply full airflow to the motors on the kelly spinner and therefore, can only be operated at full power.
There are other kelly spinners, such as the OWI 1500 Kelly Spinner, which allows variable speed control. This kelly spinner gives the driller the ability to increase or decrease the torque being applied.
Tongs: Conventional tongs are hand-latched wrenches that hang from a wire rope. The wire rope (the tong hanger line) runs from the tong's handle or "T" bar up the derrick, over sheaves attached to the derrick, and down to a counterbalance.
Tongs feature a hanger, jaws, inserts (or dies), and a handle or arm, also called lever. The crew matches the tong jaws to the size of the pipe or the drill collar by installing the correct size jaws for each size of pipe.
Rig crews use two sets of tongs to makeup and break out drill pipe. When making up or breaking out drill pipe, they latch one set of tongs on the tool joint of the pipe suspended in the rotary table. This set is the back up tongs, and they keep the pipe from turning in the slips when the snub line is tightened and the makeup tong applies a large amount of torque.
To makeup a joint, crew members first spin it up with the Kelly Spinner. Then, they latch two sets of tongs onto the tool joints of the pipe. Back up tongs hold the bottom joint still. Makeup tongs turn the top joint clockwise. . The driller engages the makeup cathead, which pulls on the chain running to the makeup tongs and turns the connection to final torque.
The driller console is normally designed with two means of control of the rotational speed of the catheads that are connected to the end of the break out tongs by the jerk line. One is a foot pedal and the other is a rheostat. Breakout and makeup tongs must be snubbed with safety lines to anchor posts attached to the sub-structure, in such a fashion that they cannot rotate completely around the drill pipe in the rotary.
Tong jerk line: A wire rope, one end of which is connected to the end of the breakout tongs and the other end of which is attached to the breakout cathead. When the driller activates the cathead, the cathead pulls on the jerk line with great force to apply torque to break out a tool joint.
Snub line: A strong wire rope attached to the end of the tongs and to one leg of the derrick to keep the tongs from turning too far when they are being used to break out, or back up drill pipe or drill collars. Keep the back up tong from turning when torque is applied to the lead tong or, in our case, to the Kelly by the Kelly Spinner. Slips: Slips are wedge-shaped pieces of metal with gripping elements (inserts or dies). Floor hands set them between the drill stem and the master bushing. With the slips set, slips keep the drill stem from falling into the hole.
When crewmembers set the slips around the drill stem in the master bushing, the slips grip the pipe without damaging it. With the slips set, the rotary table assembly suspends the entire drill stem.
When normal drilling is in progress on a conventional rig, the blocks, hook, and swivel support the Kelly and the drill stem attached to it. Floor hands often disconnect the swivel and Kelly from the drill stem to add o remove pipe from the drill stem. To disconnect the swivel and Kelly from the drill stem, crewmembers use slips. The slips take over for the hook, swivel, and Kelly by suspending the drill stem in the hole.
The crew places the slips around the drill pipe, and the driller slowly lowers the pipe. When the pipe stops moving, the rotary helpers set the slips. The driller then eases off the drawworks brake (slacks off). This action puts all the drill stem's weight on the slips. The weight of the drill stem tightly wedges the slips into the master bushing.
The slips hold the drill stem securely because they use the weight of the drill stem itself to transfer the downward (axial) force of the hanging drill string to a sideways (transverse) force. This transverse force wedges the drill stem in the slips against the master bushing.
Slips have several hinged body segments. Typically, rotary slips have three segments and drill collar slips have ten or more segments. The segmented design gives the slips flexibility without sacrificing strength. Draw works: The primary function of the drawwork is hoisting the drill stem, but has several other components such as the catheads that are related to the tongs.
Catheads: The drawworks has a shaft across the top called a catshaft that sticks out of each end. The catshaft's main purpose is to be an axle for small reels called catheads. A drawworks has four catheads. Two help the crew and break out the drill pipe, and two help to move heavy pieces of equipment around the rig floor. The power for turning the catshaft comes from the drum shaft by means of a chain-and-sprocket drive. The catshaft has a clutch with safety locks that keep the clutch disengaged until the driller specifically overrides them. This prevents the driller from accidentally engaging the catheads and injuring workers. The four catheads are of two types: friction catheads and automatic, or mechanical, catheads. The two automatic catheads help the driller and break out drill pipe.
Automatic Catheads: The two automatic catheads are reels on either end of the catshaft, inward from the friction catheads. The makeup cathead is on the driller's end of the catshaft; the breakout cathead is on the end opposite to the driller.
The catline on automatic catheads is usually called a tong pull line. A tong pull line is either a wire rope or a chain. The driller controls the automatic catheads from the console with a lever. The lever connects to built-in friction clutches on the cathead drums. The clutches give the driller precise control over the force of the line pull. Automatic catheads do not need brakes; they are either on or off, depending on whether the clutch is engaged or not.
The makeup cathead helps to screw together joints of pipe or drill string connections.
The breakout cathead, located opposite the driller's position, looks exactly like the makeup cathead except that its line is a wire rope. Rotary helpers use it to unscrew tool joints as they come out of the hole. Just as the makeup cathead pulls on the makeup tongs to tighten and make up pipe, the breakout cathead pulls on the tongs to loosen and break out pipe.
The rotary clutch should never be engaged if drill pipe tongs are latched onto the pipe or anything else in the rotary.
Conclusions and Opinions:
Opinion #1: The accident took place because the driller was not paying proper attention to the ongoing operation and made the mistake of activating the kelly spinner too early as well as allowing the kelly spinner to lift the bottom strings off the slip. He testified that he observed the assistant driller not standing at the exact location where he should have been when the snub line is in tension, and therefore, the kelly spinner should have not been allowed to run until the snub line was tight.
He knew that the load in the slips was light and that the pipe string and the tongs could rotate in the slips if he allowed the bottom string to be lifted by the kelly and if the snub line was slack, but, as per his own testimony, he was not thinking and made the error of not stopping the kelly spinner promptly. As a result, a dynamic elastic stretch of the snub line was created, and this elastic energy was released when the tongs became undone, causing the snub line to recoil and swing the tong back to the assistant driller.
Opinion #2: The Incident Investigation Report indicates as a contributing factor the possibility that the snub line could have been caught on a bolt from the guard. If this occurred, the driller should have observed this condition and should have stopped the kelly spinner until the snub line was free and tensioned.
Opinion #3: This accident could have been avoided if a kelly spinner with means to control the torque was available to the driller.
Opinion #4: If in fact the snub line was caught in an obstruction, giving the assistant driller the false impression that the snub line's slack has been eliminated, the accident could have been avoided if the obstructions would have been removed, or if the snub line was suspended to avoid getting caught in any protruding item.
Hector Pazos, is a Naval Architect, Marine Engineer and a Registered Mechanical Engineer and has been engaged in Accident Investigation/Reconstruction for more than 40 years. He has been retained as an Expert Witness in over 1,200 Maritime cases, related to both commercial vessels and pleasure crafts, for both defense and plaintiff.
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