3,605 ton module lies in 1,754 ft of water
- The Petronius compliant tower was installed in four sections: foundation, bottom section, top section, and module support frame in Viosca Knoll 786. [3,658 bytes]
The three sections of the Petronius compliant tower had already been installed in 1,754 ft of water on Viosca Knoll Block 786. The tower itself rises 2,002 ft off the sea floor making it the highest freestanding structure in the world.
The Modular Support Frame had been installed. It acts as a transition piece between the six loading points of the tower top section to the eight loading points required by two deck modules.
The north topside module which weighs more than 4,000 tons had to be stripped down to be lifted by the J Ray McDermott DB 50, which has a lift capacity of 3,800 tons. This module contains the 21 well bays for the platform, the power generation module, the compression module, four 4,000-hp turbine generators, and three 3,200-hp compressors. It is now secured safely to the tower.
At 10:30 p.m. on December 3 the DB 50 was in the process of lifting the south topside module into place when it suddenly broke free from its supports. The 3,605 ton module struck its transport barge as well as the DB 50 as it fell into the sea. It now rests on the bottom in 1,754 ft of water.
Gulf Island Fabricators built the module, which measures 140 ft by 110 ft. It includes the processing equipment, water flood equipment, and crew quarters for Petronius. At press time there was no decision on what would happen next. A Texaco spokesman said there are no plans at present to recover the damaged module and estimated it could take months to get the half-billion dollar project back on track. The construction of the south module took two and a half years.
InstallationThe installation project began last summer with the laying of the foundation template. At that time there were three wells drilled on Petronius, the discovery well and two appraisal wells. A spacer template was laid over these wells to provide an index for future wells. Leveling piles were then driven through guides in this template. Once this was set and leveled, Texaco installed a 42-ft tall deep template. The twelve 96 in. diameter foundation piles were driven 465 ft through the deep template. The piles measure 530 ft long. Three were driven at each corner of the template, then the deep template was pulled. A rig was brought in to drill three additional wells.
On November 9, 1998 the bottom tower section was launched from the Intermac 650 launch barge. The bottom section self-righted and was then lowered onto the foundation piles using a specially designed underwater block and winching system.
According to Settoon the key to installing a compliant tower is to get the verticality right because there is so much weight balanced on such a narrow structure. If the tower is not perfectly vertical then it can create eccentricities that will increase the bending moment on the structure. The goal was to install the structure within 0.1 degree of vertical. Jacks were available to level the tower if it was not vertical. Settoon said these jacks were not needed as the initial reading was only 0.06 degrees off vertical.
The top section of the tower was installed on December 2, from the DB 50. The section was lowered into place by the vessel's main crane. It was set down on top of the bottom section and grouted into place.
Petronius projectThe total development costs of this project are estimated at $500 million. Once on line, the tower would handle 80 to 100 MMb/d. The Petronius project is a 50-50 partnership between Texaco and Marathon with Texaco as the operator. J. Ray McDermott fabricated the 21-slot tower in two pieces, not including the foundation. The bottom section measures 110 ft by 110 ft. The top section tapers from 110 ft by 110 ft at the connection point, to 110 ft by 60 feet at the surface. The bottom section of tower measures 1,040 ft. It includes a series of flexpiles that frame the first 800 ft of the bottom section. It is these pilings that give the tower its compliant nature.
In contrast to a traditional skirted platform, a compliant tower does not have battened legs. This creates a more slender profile that requires less steel to construct. The key to the compliant tower is that it does not resist wind and wave forces, as a traditional fixed bottom platform would. Instead the compliant tower moves with these forces. This means that there are motions felt on the deck of the facility even though it is connected to the seafloor via 12 foundation pilings. The flexpiles are attached to the lower section of the tower via case nodes and sheer plates to transfer the environmental loads into the tower jacket. Deborah Settoon, Lead Project Engineer with Texaco, compared these flexpiles to giant shock absorbers that allow the tower to move with the wave and wind forces. The design premise for the structure includes 100 hurricane force conditions, meaning 93 mph winds and 74 ft waves, and a maximum offset from vertical of 24 ft, generating .05 g forces. Settoon points out that the structure would be evacuated in such conditions, but the structure itself would recover to the vertical position.
The platform will have four production risers, two will be used for the oil and gas production on Petronius and two will be planned risers. Because this tower sits on the edge of the Gulf of Mexico blocks leased by MMS, Texaco hopes to take advantage of the structure's location as the eastern most production facility in the Gulf.
TransportationTo export the gas from Petronius, Texaco built a 12 in.-12 mile gas export pipeline that ties in subsea to its platform in Main Pass 252. This line was installed and operated by Texaco. The oil export line will be at the Odyssey pipeline operated by Texaco, Marathon, and Shell it is a 16-in. 42-mi. pipeline.
Record setting paceIf the current schedule holds, Texaco expects to have its hook ups completed in the first quarter of next year and first oil by the fourth quarter of 1999. If this is the case, Texaco said it would be an industry record for the amount of time from the spudding of the first well to first oil, less than four years.
Copyright 1998 Oil & Gas Journal. All Rights Reserved.