Pathfinder riser incident leading to drill floor changes
The 1999 Deepwater Pathfinder drillship drilling riser incident is already producing a number of drillfloor equipment and procedural changes in the offshore drilling business. R&B Falcon announced findings and some of its own changes at the recent Deepwater Riser System Management Forum, following several months of forensic investigation. The following is a summary of those findings and a description of events leading up to and following the vessel's loss of its riser and blowout preventer (BOP).
The incident occurred on the morning of October 22, 1999 in preparation for drilling an ultra-deepwater well. At the time, the modern Deepwater Pathfinder was reportedly in a controlled drift with the current in the deepwater Gulf of Mexico off the coast of Louisiana. The vessel, owned 50-50 by R&B Falcon and Conoco, was drilling a one-well farm-out for Chevron after completing two wells under a long-term contract with Conoco.
Before dawn, the Pathfinder had driven several miles off station, against the current, in order to drift back toward the well site, while running its riser and subsea BOP stack. By drifting with the current, the vessel was able to run the riser straight down, rather than trying to hold the vessel on station and running a riser that is affected by the current.
As it drifted, the vessel was slightly out of trim, which is not unusual, but makes it more difficult to run the riser through the rotary table. The driller onboard used his radio headset to contact the bridge from the driller's cabin and ask that the vessel be trimmed level with the sea. As he spoke with the bridge, the driller noticed his headset needed new batteries.
Running riser
The driller was overseeing the riser running operation from a state-of-the-art control console in the driller's cabin. By dawn, the crew had run 19 bare joints of riser above the BOP and was in the process of picking up the first buoyant joint in the string. It was important that the ship be level for this procedure since the syntactic foam module encasing the buoyant joint made it almost as big around as the rotary table.
Even with this minor adjustment in trim, operations had been progressing very smoothly. The driller planned to use the few minutes it would take to trim the rig as an opportunity to set the drawworks parking brake and change the batteries in his headset.
Like other ultra-deepwater rigs, the Pathfinder has a fully automated drillfloor with advanced pipe handling equipment and an iron roughneck. When a rig carries almost two miles of riser joints, automation is the only way to handle these large tubulars quickly and safely. With all of the automation, it was nevertheless necessary that morning for four men to be working on the drillfloor and another two in the moonpool.
The automated drillfloor allowed the roughnecks and driller to handle the heavy riser joints quickly and make them up efficiently. But it takes human intervention to ensure the process runs efficiently. The rig crew had a good rhythm going that morning and was making fast progress as the captain guided the rig toward the wellsite.
Brakes engaged
At about 8:40 a.m., the first buoyant joint was in place and made up. The driller engaged the parking brake on the drawworks, transferring the load from the drawworks' motors to the brakes. After the system confirmed the brakes were set, he de-energized the motors. The drawworks was rated for 750 tons, and carried a 2-in. drill line that ran through a 14-segment block reeve. The brake acted on the drawworks drum to hold it stationary against the tension of the drill line.
This parking brake is a considerable change from the old hand brake used for years onshore and offshore. Steve Leppard, Drilling Equipment Engineer with R&B Falcon, describes the braking systems as "intricate as a Swiss watch." The shoes of these disc brakes require very fine adjustment so they float 1-3 mm off the surface of the disc. The brake consisted of 8 spring-set calipers with the capacity to engage 3.45 million lb. Tests on the sister ship Deepwater Frontier showed the system could engage 4.20 million lb which is beyond the manufacturer's rating.
With the brake engaged, the driller changed the batteries in his headset. The headsets were a new safety feature brought over from R&B and Conoco's experience in the North Sea, where their use is standard practice. Using this equipment, the driller could communicate with everyone on the drillfloor from inside the air-conditioned driller's cabin. By monitoring the transmissions, anyone on the vessel could keep up with the activities on the drillfloor.
Drill line slips
As the driller prepared to change his headset batteries, the brakes began to slowly creep. It is believed it took several seconds to notice the movement, because as the drill line passed through the 14-part reeving, the riser string movement was very slight. After several seconds, the riser string crept down several inches. The movement was very slight. The movement alerted the floor crew, and because the driller had not advised them he was doing this, they recognized that something was wrong. Through use of the headsets, the driller confirmed there was a problem and the floor hands moved to a safe area.
Estimates indicate the initial slippage began at 8:40:40 a.m. The driller immediately returned to his chair and fought to save the riser. The driller remained at his post fighting to recover control, as the riser-handling tool, elevators, traveling block, and top drive slid out of the derrick toward the drillfloor.
Getting no response from his joystick controlling the drawworks, the driller hit the emergency shutdown button at 8:40:51. The button sets off an almost instantaneous series of actions designed to disengage the drawworks motors and engage the brake. In this case, hitting the button made no difference, since the motors were already shut down and the brakes engaged.
Losing control
Sliding on the brakes, the riser running tool, elevators, traveling block, and top drive raced downward out of the derrick and slammed into the now abandoned drillfloor. The top drive became wedged, crushing the spider gimbal. Unable to pass through the rotary table, the top drive sheared in two, releasing the 1.3 million lb string of riser joints and BOP into the sea.
When the top drive slammed into the drillfloor, the inertia kept the drawworks drum spinning, spooling off drill line. In little more than a second, hundreds of feet of 2-in. fast line disintegrated. The disintegration of the drill line was so intense, pieces of wire rope were found by the stern of the ship, 350 ft away.
At that same moment, within the drawworks cabinet, the drill line damaged a lube oil tank. The ruptured storage tank sprayed hot lube oil all over the drawworks. Witnesses said the brake shoes and remaining lengths of drill line glowed cherry red from the heat of the friction. The resulting oil vapor was ignited by the hot brake discs and a fire broke out. Three of the eight brake shoes were knocked off the spool by the tail of the spinning drill line.
Crew members reacted, quickly extinguishing the small fire. The vessel was never in any danger, and no one was injured. Even as the crew worked to restore order, the 620,000 lb BOP sank into darkness, trailing behind it 20 joints of riser and one-half of the top drive. This 2,000-ft. train of equipment made an impact 7,300 ft below, creating a crater into the soft seabed.
In the wake of the event, the vessel was towed to Galveston, where it would be repaired over a four-month period. The riser and BOP would have to be replaced.
What happened
An investigation was launched onboard the Pathfinder. Teams were appointed from R&B Falcon, partner Conoco, and manufacturers National Oilwell Dreco and Hitec. An independent third party was also appointed from the certification group Det Norske Veritas.
Several avenues were pursued in an effort to nail down exactly what caused the drill line to slip. As the damaged drill floor and drawworks were disassembled, the five remaining intact brake calipers were recovered and tested. It was noted at the time that the shoes had contamination on them. Because of the fire and ruptured lube oil tank, it was difficult to determine where the contamination had come from and what was deposited on the shoes after the fire started.
To make this determination, a testing fixture was built. Clean, dry shoes were tested to determine their friction characteristics. Then, different contaminates were tested, including hydraulic fluid, water, drilling fluids, drill line lubricant, and lube oil. In particular, the drill line lubricant made a dramatic difference. Applied to the shoe, this lubricant eliminated almost all friction.
Leppard said that once this initial test was completed, the brake disc sample was washed, but even after applying a number of cleaners and solvents to the disc, a film of the lubricant remained, and that surface drastically reduced the braking friction. On the Pathfinder's sister ship, Deepwater Frontier, a traditional bitumen-based drill line lubricant was used and there were no similar problems on this vessel. The decision was then made to discontinue using the new lubricant and return to the heavier bitumen lube on all R&B Falcon vessels.
Other changes
In addition, shields were installed around the brake assembly to protect it from contamination and a new computer protocol was introduced. The new computer program was designed to automatically re-engage the drawworks and alert the driller at the first sign that the brakes began slipping. Further training on the maintenance and adjustment of the brake system was also implemented. A testing routine was initiated where the brakes would be tested for brake-holding capacity, prior to any heavy lift by the drawworks.
During the investigation, it was thought the brake hydraulic system might have failed. Shuttle valves, actuated by hydraulic pressure, activate the brakes. Possibly, the hydraulic components could have stuck, or been blocked by contamination particles. This proved not to be the case.
As an additional safeguard, double hoses were installed on the hydraulic and oil lines in and around the drawworks to prevent contamination of the brake shoes by a leak in a line.
Not a design failure
Leppard noted that there was no mechanical or design failure in any of the systems used on the Pathfinder. Instead, it was contamination of the brake friction materials with wireline lubricant that led to the accident. Although, the Pathfinder suffered quite a bit of damage, Leppard said the vessel was fortunate in many ways, most importantly that no one was injured. R&B Falcon had a new BOP and riser system on order at the time of the accident, allowing the Pathfinder to return to work once repairs were completed. In all, the incident set the vessel's drilling program back only six months.
