High-viscosity oil, deep waters, complex and unconsolidated reservoirs are among Dalia’s challenges. In response, Total, Sonangol and the partners on Block 17 devised an innovative development scheme and designed installations able to secure a production of 240,000 barrels of oil per day, confirming the Group’s leadership in deepwater conditions.

Complex geological configurations

Dalia, a 1997 discovery, lies 135 kilometers offshore Angola in water depths ranging from 1,200 to 1,500 meters, over an area of about 230 square kilometers. Formed more than twenty million years ago through the accumulation of sediments at the mouth of the Congo River, the Dalia reservoirs are turbidites, like all others in Block 17.
Transported into deep waters under the effects of a powerful canyon river system, these turbidite sediments spread in and along extensive distributary channels on the ocean floor. Dalia is a grouping of four main Lower Miocene and Middle Miocene reservoirs buried 700 to 900 meters below the seafloor and holding heavy, acidic oil.

A gigantic development

Total, Sonangol and the partners on Block 17 have devised a production scheme that makes Dalia one of the largest deepwater developments worldwide. The 71-well subsea production system includes 37 producers that feed into four production loops via nine manifolds.

Stimulated by gas lift,  the produced fluids are transported through eight flexible risers using IPB (Integrated Production Bundle) technology to the FPSO (Floating Production, Storage and Offloading vessel) – one of the largest ever built.

This network, which transports the fluids from the seafloor to the surface, is more than 53 kilometers long. In addition, there are two other networks: one for water injection, one for gas injection. Four flexible risers, each 1,650 meters long, reinject treated produced water and treated seawater into the reservoirs, along 35 kilometers of injection lines that feed into 31 water injection wells — a daily injection capacity of 405,000 barrels of water.

The associated gas produced along with the oil is reinjected into the reservoirs via two flexible risers connected to two injection lines and three gas injection wells, adding up to a subsea gas injection system more than 13 kilometers long, with a compression capacity of up to 8 million cubic meters of gas per day.

A 75-kilometer network of umbilicals transmits the data from a continuous monitoring and control system that will coordinate production and allow this sprawling subsea complex to ramp up to a plateau of 240,000 barrels of oil per day by 2007. Production is exported to a loading buoy moored 2,100 meters from the FPSO.

New technological breakthroughs

The many challenges of producing such difficult reserves in such an extreme environment and under economically viable conditions demanded a series of innovations. For the first time on this scale, the drilling and well completion concepts deployed on Dalia combined the technology of horizontal subsea Christmas trees with light well architecture.

With each manifold designed to connect up to six wells, installing the seabed equipment, with remote guidance from the surface, demanded high-precision: subsea flowlines and connections had to be installed in a congested zone under 1,400 meters of water. Above all, thermal issues called on the Group’s full range of leading-edge expertise and inspired a number of new developments to meet the challenges of flow assurance for Dalia’s inherently cold and heavy oil in such severe pressure and temperature conditions. Advances include the largest flexible production risers ever built — the first using Integrated Production Bundle technology — as well as an insulation system for the flowlines that ranks as one of the most efficient worldwide.

A Project Spanning the Globe

Around 20 industrial facilities in Africa, Europe, Asia and the Americas contributed to the development of the giant Dalia field, with up to 4,600 people working on the project at the same time. Ultimately, design, fabrication and installation of all the parts of this immense industrial jigsaw required more than 15 million man-hours of work.

Local Development
 
Dalia has contributed significantly to industrial expansion in Angola, a priority focus of our long-standing partnership with the country. Many new industrial activities have been created locally thanks to the project’s contractual strategy, and an array of industrial operations have been carried at Angolan yards and sites, including final assembly of manifolds, fabrication of some of the umbilicals, and construction of the FPSO anchor piles and the loading buoy.

A broad training program has supported the transfer of industrial know-how to Angolan engineers and technicians. Around 40 interns have received training from the main project contractors at a variety of industrial facilities, while another 31 gained experience at the Lacq site in France in preparation for their Dalia assignments.

Flexible production risers Eight risers carry the oil from the seafloor to the surface and are no doubt the most spectacular feature of Dalia’s subsea transport system. The biggest production risers ever built, composed of ten superimposed layers, they are 1,650 meters long, weigh nearly 800 metric tons, and have an outer diameter that reaches a record 590 millimeters (23 inches). For the first time, the 12-inch riser core was integrated into a bundle with a built-in gas lift injection system. The stimulus for this performance was thermal constraint: it would have been technically impossible to insulate separate gas lift lines unless they were heated constantly. The alternative of integrating these lines into the production riser provides dual benefits, taking advantage of the heat transferred from the production flow while insulating the system as a whole.