The Saudi Aramco Mobil Refinery Company (Samref) has invited contractors to bid for an engineering, procurement and construction modification project, which will be carried out on a cost-reimbursable basis.

Samref is required to make the changes as part of rules introduced by the US' Environmental Protection Agency in 2006, making it mandatory for refiners to reduce the sulphur content of the gasoline they produce.

The bulk of the kingdom's refineries escape the legislation because they do not sell refined products to the US. However, Samref exports 46,000 b/d to the US, meaning it must comply with the rules.

The law change requires gasoline to have a sulphur content of less than 10 parts per million (ppm) and 1 per cent benzene by 2013. Diesel must have less than 50 ppm of sulphur by 2013 and 10 ppm by 2016.

The project is in to two phases, the first covering process units and the second offsites and utilities. The process unit package covers the creation of facilities, including a hydrogenation unit, which reduces sulphur from diesel supply, a splitter to divide naptha and a heavy naptha hydrotreater.

Other facilities include a 40,000-b/d distillate hydrotreater, a sulphur recovery unit and handling facilities, and a sour water stripper.

A 70 million cubic feet a day gas supply also forms part of the scope and will either be owned and operated by Samref or supplied by a third party. A decision is to be made on this by the end of the year.

The offsites and utilities package covers tie-ins for all process units, connections to the control system and the relocation of the refinery's laboratory.

Europe has introduced similar measures to the US, including an emissions trading system to meet targets set by the Kyoto agreement.

The tough cap on sulphur dioxide emissions marks a big change for the refining sector. In the 1970s, emissions levels were about 10,000 ppm, and 2,000-5,000 ppm a decade later.

Last year, South Korea's Samsung Engineering beat three of its national rivals for the $500m contract to install a diesel hydro-treater at Aramco's Ras Tanura refinery.

The project aims to reduce sulphur content in the diesel produced at the 550,000-b/d refinery, although it is not thought to have been prompted by the regulatory changes in the US.

The $2bn upgrade at Yanbu comes at a testing time for Aramco, as it grapples with massive cost increases at its two planned export refineries in Jubail and Yanbu.

The contractor that is appointed for the modification project will also carry out the engineering, procurement and construction management (EPCM) part of the scheme in addition to the front-end engineering and design deal.