ESSO Chad Development Project

Location: Near Kome, Chad

Client: ExxonMobil

ExxonMobil is developing a major oilfield in the African country of Chad. Ultimately, the field will consist of more than 200 wells with gathering lines, pumping stations and a pipeline from the field in Chad to tanker loading facilities on the coast of Cameroon. Chad is an underdeveloped country with minimal infrastructure. Engineering for the development project is being completed by a consortium led by Kellogg Brown & Root Inc. Burns & McDonnell was retained by KBR to design the electric transmission and distribution network.

The line design poses several unique engineering, construction and design challenges. The electric system consists of a backbone of 132- and 33-kv radial transmission and 33-kV distribution to the wells and other facilities from the field’s power plant. Oil will be pumped from the wells with medium voltage submersible pumps of up to 1,500 hp. The pumps are designed for continuous operation and will tolerate only a few restarts before requiring a rebuild that can cost up to $250,000. Additionally, if oil flow is stopped, there is no guarantee it will resume once the pump is reinstalled and started. A highly reliable electric system is required to allow sustained and economical operation of the field.

Chad has a very high level of lightning activity, about 180 thunderstorm days a year, and extremely high soil resistivity of up to 9,000 ohm-meters. The extremely high soil resistivity caused an unacceptably high incidence of back-flashovers. Increasing the insulation level from 66-kV to 345-kV reduced the number of back-flashovers to 6/100 mile-year but still did not reduce them to an acceptable level. Additionally, designing the lines for such high insulation levels greatly increased the structure height and foundation requirements with the associated increase in material and construction costs.

Burns & McDonnell engineers determined the best method to economically obtain the desired level of reliability was to utilize a separate shielding structure adjacent to the structure supporting the conductors. The shielding structure would shield the line from direct lightning strikes the same as a traditional shield wire design. The increase in potential of the shielding structure will not cause a back-flashover to the phase conductors on the adjacent structure. The two-pole system results in comparable overall steel and concrete quantities as the 345-kV design while achieving an acceptable level of direct and back-flashovers.