Recovered joints from the drilling riser of the Pride Africa.
After more than two months stuck in 80 ft of seabottom muck, in a water depth of 5,400 ft, the blowout preventer (BOP) stack of the Pride Africa was pulled back to the surface. The event took place on January 13. Recovery of the BOP topped off an intense and successful salvage program, which Gary Casswell, Pride Vice President for the Eastern Hemisphere, calls a triumph for the industry.
On November 11 last year, the ultra-deep water drillship Pride Africa experienced a parting in the drill line that sent the 132-ton subsea stack and 2,000 buoyant and bare-joint riser sections crashing into the seabed. The incident occurred offshore Angola.
Testing is still underway to determine the exact cause of the failure. Although a disaster, the situation could have been much worse. Remotely operated vehicle (ROV) reconnaissance showed that the BOP stack landed 150 ft from the subsea wellhead.
The weight of the unit was such that it plunged 80 ft into soft soil below the mudline. The broken riser joints lay on the seabed in a loose pattern. Some riser joints were bent and others broken, but none damaged the wellhead during the collapse.
The Pride Africa was initially brought into Cape Town South Africa to undergo immediate repairs, then sent back out to recover the lost equipment. The recovery program began on December 24, 1999.
Riser joints were brought to the surface in single joints or stands of one and two. In some cases, auxiliary lines had to be cut, but no explosives were needed, according to Pierre LeMouroux, Pride Operations Manager for the Eastern Hemisphere. By January 13, all the joints were recovered. It appears that none will be reusable, but the joints will be examined to help determine the cause of the accident and its effect on the equipment.
Once the tubulars were safely out of the way, the operation progressed to the most serious phase - recovery of the BOP. Again, Pride was relatively fortunate. When the BOP broke loose from the riser string on its way to the seabed, it sheared off a joint and a half of riser, which remained attached to the top of the unit. This event may account for the fact that when the sunken BOP was located, it was sitting only 5° off vertical. This made the recovery much simpler because the slings and riser joints were pointing straight up.
Excavation of the seabed was undertaken to expose the BOP. The original plan was to dredge a 20,000-ft conical section of the seabed around the BOP. While the surface of the seabed was silty and easily removed using a Mercur jet and a jet prop, the mud below proved more difficult.
The gumbo consistency of the seabed soil clogged the pumps and was not easily displaced. After several hours, it was clear that progress had stalled and another approach was needed. While the gumbo complicated the recovery process, Casswell said the soft foundation was actually a blessing is disguise. Because the seabed was so absorbent, it cushioned the impact of the BOP when it initially fell. Had the surface been hard rock, recovery might have been more simple, but the BOP most likely would have sustained extensive damage.
Using an ROV equipped with a hydraulic cutter, the half section of riser was sheered off clean. LeMouroux said it was necessary to have a clean-cut and perfectly round surface. The auxiliary lines were cut using a tungsten driver and the broken section of riser removed.
With the full joint of riser exposed like a stovepipe at the seabed, a conventional, bull-nose-washing tool was run on drillpipe. The tool was used to clean out the inside of the BOP, down below the BOP connector. This tool was pulled and a conventional 18 5/8-in. casing spear with a stinger on the end was run using 9 5/8-in. casing as a fishing string.
The spear was engaged into the fish (BOP), then hung off on the tensionors. The casing string was pressured up using the mud pumps until pressure was broken below the sunken BOP. While circulating water through the BOP, a conventional string of drillpipe with a specific bull-nose for shearing clays was made up.
Using the dynamic positioning system of the vessel and the eyes of the ROV, a series of pilot holes were drilled all around the sunken BOP. These holes were expertly positioned, LeMouroux said, within 4 ft of the BOP itself. Pride President Paul Bragg said in a press statement that "This operation was like threading a needle with a robot from a mile away..."
The goal was to vent the pressure building up below the BOP, freeing it from the muck. One after another, the holes were drilled as the tension on the casing string was closely monitored. Finally, after 18 hours of drilling, the tension dropped 100-140 tons, and the BOP rose 50 ft.
The elevators were used to move the casing string back to the traveling block, and as the joints were broken out, the BOP began its rise to the surface. As the BOP rose, the ROV was used to remove as much mud as possible below the surface. On January 25, the BOP was back on deck. LeMouroux said it looked a little dirty, but appeared to be in excellent condition, based on an initial external examination.
The Pride Africa then headed back to Cape Town where it was to drop off the BOP and riser joints, have its new topdrive and BOP installed, and load on the replacement joints of riser.
The BOP will be shipped back to the manufacturer to undergo complete recertification. Pride hopes the unit will be re-usable, so it will have a full set of replacement equipment. Casswell said the vessel would be ready to recommence drilling operations off Angola in April.