A partnership between Boskalis Offshore BV a subsidiary of Royal Boskalis Westminster NV group and Tideway a subsidiary of Belgian group DEME has dredged two glory holes 120 meters (394 feet) below the water’s surface. The southernmost hole was completed on July 22 and the northern hole on August 10. They are 350 kilometers (217.5 miles) off the Newfoundland coast in Husky Oil Company’s White Rose Field.
A third hole will be excavated by Jan de Nul.
Using the dynamic positioned fall pipe vessel Seahorse the companies created the holes to protect subsea oil wells from iceberg scour in the Labrador Current.
This is the first time a contractor has excavated glory holes at this northern latitude and at such a depth. The southern hole required excavation of 46000 cubic meters (60168 cy) of material and 19000 cubic meters (25852 cy) were excavated from the northern hole.
The contractor used an innovative underwater grab and drag system in the 15 to 20-million euro project which was awarded to Boskalis/Tideway in August 2001.
Each glory hole contains several drill centers with production and water and gas injection wells located at each center. Each glory hole is nine meters deep (29.5 feet) and measures 25 meters (82 feet) by 25 to 65 meters (213 feet) in size. Material excavated from the holes was deposited on the nearby seabed.
The partners decided not to use a trailing suction hopper dredge because enormous boulders were found during an earlier assignment. The |!|grab and drag|!| system close to the seabed has a double advantage: the work is less weather dependent and the holes can have steeper slopes. The proposed solution has now proved to be successful with the completion of the first two glory holes.
Excavation of glory holes on the ocean floor requires exceptional precision. The high precision dynamic positioning (HPDP) devices on board the Seahorse allow for a 10 to 20 centimeter (four to eight inch) accuracy. The challenges of working in this environment are rapidly changing weather conditions high seas storm surges and poor visibility. The Husky contract was spread over two working seasons each one restricted from late April until September. Ice drift was still hampering operations in April and deteriorating weather conditions in this region become serious handicaps in late summer.
Innovative engineering was required for making this deep-sea excavation possible at all. The Seahorse was equipped with a very heavy clamshell on her afterdeck. Two different clamshells are used (see cover) with capacities of 10 or 16 cubic meters (13 to 21 cy). The first one is as heavy as the second but its smaller dimension allows for digging in the hardest ground layers. The ROV-operated clamshell transports the excavated material in a lateral way shuttling at a short distance above the sea-bottom between dredging and dumping sites in five to seven-minute cycles.
The grab and drag system addresses the need for a new deep dredging solution for offshore projects. Beyond glory hole excavation this precision dredging system is suitable for pipeline-related projects and other deep seabed works with the potential of working as deep as 1000 meters (3300 feet).
The soil conditions at the location were also challenging consisting of dense sand stiff clays large boulders and cemented organics. The asphalt-like |!|hardpan|!| was broken up and crushed prior to lifting.
Husky Oil Operations Ltd. is a wholly owned subsidiary of the Canadian-based Husky Energy Group. The White Rose oil field is located on the eastern edge of the Jeanne d’Arc Basin on the Grand Banks.
The White Rose oil field is the third major oil field in the Jeanne d’Arc Basin at 50 km (31 miles) north of the Hibernia and Terra Nova fields and will yield an estimated 230 million barrels of recoverable oil on a 40 square kilometer (15 square mile) area. Production is scheduled to begin before the end of 2005. Excavating the glory holes is the first major step of the oil field development.