Terra Nova switches technologies for excavating glory holes

Glory holes will protect the subsea wellhead and manifolds of the Terra Nova field.

• The trailer suction dredge is mounted on the 173-meter vessel The Queen of The Netherlands. [20,272 bytes]
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After falling behind in the excavation of the four glory holes required to protect the Terra Nova subsea wellheads from scouring icebergs, the Terra Nova alliance stepped back and re-evaluated its approach to this problem.

The Terra Nova oil field located on the Grand Banks 350-km east-southeast of St. John's Newfoundland, is the second largest field off Eastern Canada. The environment offshore Newfoundland is one of the most challenging in the world. Harsh sea states tell just a part of the story. Down below, the seafloor is a combination of sands, cemented sands, and rock. Scoured by iceberg which move through the area each year, the sea floor is a difficult area to dredge.

Because the Terra Nova project must contend with the possibility of scouring icebergs moving through the field, the subsea wellheads and templates will be located in enormous glory holes, the largest will measure 16 meters by 56 meters at the base. In this way, the wells will be protected from icebergs even if the bergs are so large they drag the bottom of the sea.

Reverse circulation

The excavation of these glory holes was initially started last spring using a reverse circulation drilling unit. A large diameter-drilling device bored into the seafloor and used vacuum pressure to lift cuttings to the surface. According to Terra Nova's Manager of Marine Operations, Mark Williams, this process worked, but proved much too slow to meet the project's time line.

During the off season, last fall, a task force of experts in marine construction, dredging, and geotechnology was assembled to consider a variety of alternatives for dredging these holes. The team has settled on a trailer suction dredger to complete the job.

Williams said this type dredge is used commonly in the marine construction industry for deepening channels and so forth. The dredge is mounted on the 173-meter vessel The Queen of The Netherlands. This dynamically positioned ship-shaped vessel offers a number of advantages over the previous system and is one of only a few vessels capable of dredging in water 106 meters deep.

Dredge excavation

The excavation process involves trailing a large 6.5-meter wide "dredgehead" across the seabed. This dredgehead breaks up the seabed material and then uses suction to draw the material up the 1.2-meter diameter dredgehead deployment arm. The cuttings move through the arm into a storage container in the ship.

The vessel has an inclined pipe attached near the bow. A series of winches lower the pipe attached to the dredgehead. The dredgehead is dragged along the sea floor, vacuuming up the sand and rocks that is loosened by high-pressure water jets and metal teeth. The vessel will intermittently move off location to deposit cuttings away from the excavation site.

Trailer Suction Dredge vessels have been in development over the last few years, but were not advanced enough at the time to be considered a viable alternative during the initial analysis of the glory hole project.

Williams said these sorts of vessels have worked in similar environments in the North Sea and other parts of the world. Dredging is scheduled to resume in May. The team hopes to have anywhere from one to all four of the holes dredged over the summer.

The first of the glory holes is the most critical because it must be completed before drilling on the field can begin. If everything goes as planned, Williams said all four of the glory holes, a total of 300,000 cu meters of dredge material, should be completed in this first season and drilling will commence soon after the first glory hole is completed.

If all four of the holes can be dredged in this first season, it would put the Terra Nova project back on schedule. "If things work out as we're expecting, then we haven't been held up with the Terra Nova alliance."

Giant dredge

Boskalis Offshore of the Netherlands has been hired to dredge the glory holes using its purpose built vessel The Queen of the Netherlands, which is the largest vessel of its kind in the world. Williams said Boskalis was hired on a performance-based contract.

"While the task force looked at a number of options, it recommended using trailer suction dredging as the most efficient and cost effective approach for Terra Nova," Gary Bruce, Chairman of the Terra Nova Management Committee said.

Terra Nova Alliance member Coflexip Stena Offshore Newfoundland is responsible for subsea construction on the project and will manage the contract with Boskalis. While the dredging will begin in May and wrap up in October of this year, drilling is currently scheduled to begin sometime in July.

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