Garraf oil field is located in the province of Thi Qar, approximately five kilometres north-west of Al-Refaei city and 85km north of the city of Nasiriya. The field is owned by PETRONAS Carigali Iraq Holding (PCIHBV, 45%), Japex (30%) and North Oil Company (25%). Development works for the oil field started in early 2011. Initial production started in August 2013. The onshore oil field was discovered in 1984 and contains oil in Cretaceous reservoirs. The field is 17.5km long and 5.5km wide. It is estimated to hold 1.3 billion barrels of oil reserves. The current production capacity of the field is 35,000 barrels of oil per day (bopd). It is expected to increase to 230,000bopd by 2017. GTL was appointed during FEED as Geotechnical Consultants and later included Engineering Design for piled foundations. GTL also undertook preliminary pile testing and CAT 2 checking for structures. Piles were eventually supplied and installed by the EPC Contractor. GTL is now in discussions regarding their involvement with the plant expansion.
The project was classified as Geotechnical Category 2, in accordance with DMRB (Design Manual Roads and Bridges) Volume 4 Part 1 Section 1 HD22/08, Managing Geotechnical Risk, Chapter 3. Geographically, the FCP site was situated within the regional fertile basin fed by the Euphrates River. The site lies along the fringes of the Garraf River to the east, a tributary of the Euphrates River. The notable features within the local area were the lake and road that formed on the crest of an embankment. Geologically the site was considered young as the ground has been formed by secondary or tertiary geological processes; alluvial clays from the Euphrates River delta and wind-blown sand formations from the desert. Geological records showed the site to be situated within a stable tectonic plate therefore the ground has not been influenced by mountain-building events. As a result, there is the limited thickness (geological memoirs suggest approximately 15 to 30m) of superficial (drift) deposits attributable to the sediment deposits from the adjacent river systems. Sandstone bedrock was recorded beneath superficial deposits of the local region. The exploratory borehole logs recorded numerous layers of clays, which GTL considered to be of alluvial origin, differentiated by changes in colour, strength or variations in the relative proportion of sand. Minor layers at depth of silt were also recorded. Garraf Development is assigned to Seismic Zone 1 and the ground conditions encountered are categorised for seismic design purposes as Soil Profile SD. The facilities are assigned to occupancy category 2.
Over 1000 piles were designed at various depths to accommodate a modular type construction. Piles were designed to The foundation pile design was undertaken as a limit state design, whereby a distinction was made in ultimate limit states (ULS) and serviceability limit states (SLS). Limit states are related to design situations, which was classified in persistent, transient, accidental and seismic design situations. In accordance with the superstructure operation states, accidental design situations are mentioned and classified in the foundation pile design.
Gavin Rixon – Project Director Karsten Richter – Lead Geotechnical Engineer Stephen Whitham– Geotechnical Engineer