Investigation, repairs underway after ships drop two risers

Two recent equipment failures on new deepwater drillships resulted in dropped risers and damage to the vessels.

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Two recent equipment failures on new deepwater drillships resulted in dropped risers and damage to the vessels. As a result, drilling contractors are taking precautions to ensure against future incidents. There was concern earlier that deepwater riser programs could be pushing the technical limits of design, even though ultra-deepwater drillships are establishing benchmarks with each record well drilled. One industry source said it is frustrating knowing all the testing and evaluation that goes into these systems, only to see accidents still happen.

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The Deepwater Pathfinder, is moored in Galveston as it undergoes repairs.
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  • Deepwater Pathfinder: R&B and Conoco are assessing the damage and looking for the specific cause of a failure aboard the Deepwater Pathfinder. The rig is in Galveston, Texas after an accident on October 22 caused the drillship to drop 20 joints of riser and the blowout preventer stack in 7,000 ft water depths. The contractor plans to make quick repairs and put the vessel back on line. R&B spokesman Charles Ofner said the lost riser joints can be replaced from identical equipment manufactured for the Deepwater Millennium, which is still under construction. "That's the beauty of having this big construction program. We have identical equipment in the pipeline for other rigs," he said.

At press time, there was not yet a decision on whether the company would attempt to retrieve the BOP stack. Industry sources said the stack shot down into the seabed up to 150 ft and would be difficult to recover. Ofner said a BOP stack could also be borrowed from the Millennium without setting back the construction schedule. The riser joints apparently sheered off near the seabed and are lying on the ocean floor, but experience has shown such a drop would deform them, requiring replacement. Ofner said repairs will take 3-5 months. There are no plans to modify the damaged or lost equipment, and identical components will be used in the repairs, Ofner said.

  • Pride Africa: The drillship Pride Africa experienced a similar event on November 11 off Angola in 5,400 ft water depth. The unit dropped its BOP stack and 2,000 ft of riser, damaging the top drive and drill line in the process. The accident took place as the BOP was being pulled and not during drilling. Pride International, the operator, was still investigating the event at the time of this writing.

While the braking system on the drawworks might have played a role in the accident on the Pathfinder, a parting of the drill line appears to have caused the dropped riser on the Africa. Despite this difference, R&B's Ofner said the braking system on the Pathfinder is similar to that on all deepwater vessels.

In the wake of the Pathfinder event, another drilling contractor, Global Marine, is conducting an audit of its preventive maintenance procedures for the drawworks system. Jeff Shepard, of Global Marine, said the company was very confident in the safety of the BOP riser handling system. Without knowing specifically what caused the problems on the Pathfinder, and Africa, Shepard said Global Marine has consulted with the manufacturer of the drawworks braking systems installed on its deepwater rigs and is satisfied with the design rating of the equipment for the service intended.

It is probably a credit to the level of automation aboard the two new-generation vessels that no one was injured in either incident.

Micro-rig workover tractor ready

A potential source for millions of barrels of oil literally remains untapped because of the workover costs on depleted fields. In many cases, these fields are abandoned with no secondary or tertiary recovery program because the economics of such a program are cost prohibitive.

In and effort to lower the cost of working over depleted fields or revitalizing secondary or tertiary programs, IIC has developed a wireless tractor that is an option to many activities now performed with a slick-line workover. The wireless tractor or "micro rig" is a tool that transports logging equipment downhole with no connection to the surface. According to its designers at IIC, the micro rig would be carried to the seafloor by a small ROV, launched from a dive-support vessel rather than a workover rig. The micro rig can be screwed into a subsea wellhead using a modified lubrication joint. After installing the tool, the ROV and support vessel would actually go off station, only to return when it is time to remove the tool.

Once in place, this autonomous tool will follow a pre-programmed series of activities without contact from the surface. Tom McIntyre, IIC Director of Technology, explained that the tool would be capable of transporting a memory production logging tool, caliper log, or video equipment downhole. It could perform a shut-in well test and has a range of 10,000 total vertical ft or a combination of vertical and horizontal travel of 25,000 ft. The ratio of depth to stepout is about 5 to 1 using current battery technology. This is an advantage over conventional slick line, which is limited to about 15,000 ft horizontally because of cable management issues.

McIntyre said his company has assembled a consortia of major oil companies including BP Amoco and Marathon and has plans to install the first of these tools by next spring. McIntyre said the artificial intelligence technology being applied here is at least 12 years old, although it is new to the oil industry. He is predicting cost savings in the range of 60-90% over a conventional workover program. Such a savings would in effect create its own market by making depleted wells economically viable.

Subsea robots for pipeline tie-ins

A team of three oil field companies has been assembled by gas transport giant Williams to develop a subsea robot capable of performing pipeline tie-ins in up to 10,000-ft water depths. Williams Energy Services division is leading a Houston team made up of Oceaneering, Oil States, and TD Williamson. With a budget of around $6 million, the team hopes to develop system equipment that could be available by the middle of 2000. The goal is to avoid the need for building new transmission lines for deepwater fields by tying in new lines to the existing infrastructure. This would save oil companies an estimated $100 million per year, according to Williams.

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