As operator of Block 17 offshore Angola, Total E&P Angola has developed three oil fields, all in about 1300m of water and with uniform soil conditions made of soft highly plastic clays. With Sites A and B floating production storage and off-loading (FPSO) piles and Sites A and C riser tower foundations having similar dimensions, the comparison of their installation behaviour is particularly relevant for highlighting some specific features. The Site A installation results were significantly below predictions, because of (a) reduced friction along partly painted steel piles and (b) overestimation of the penetration resistance for the first suction piles ever installed in such West Africa deepwater clays. The installation results for Sites B and C were in agreement with the predictions, thus confirming the validity of the penetration analyses.


As operator of Block 17 offshore Angola, Total E&P Angola has developed three oil fields in about 1300m of water (named Sites A, B and C), all located in the same area of Block 17, which represents a 90km x 60km rectangle with a bathymetry ranging between approximately 300 and 1900m. Both Site A and Site B developments are based on subsea production systems, associated with a large FPSO unit and an export buoy, respectively with 40 and 70 wells spread over an area of about 50km2. The Site A subsea wells and flowlines are linked to the FPSO by means of three riser bundle towers, while flexible catenary risers are utilised for Site B. The Site C development includes about 20 wells and the produced oil is sent by flowlines to the Site A FPSO via a fourth riser tower. The distance between the Site A FPSO and the Site B and Site C fields is approximately 10 and 15km, respectively.

Suction piles are used to anchor both FPSO vessels (Figures 1 and 2) and export buoys, and suction caissons are utilised as foundations for the four riser towers (Figures 3 and 4) as well as for all subsea wells manifolds1. The geotechnical design of the suction piles for both FPSO vessels and of the caisson foundations for the riser towers was carried out by the Norwegian Geotechnical Institute (NGI), and the moorings of the two FPSO vessels were certified by Bureau Veritas (BV).

Soil Conditions and Geotechnical Design Profiles

In the central part of Block 17, the seabed slopes gently to the south west with a gradient of about 1:50 (1.2°). On the basis of high resolution, 3D seismic data, the thickness of the clayey sedimentary infill is estimated to be equal to approximately 300m. From about 40m below the sea bottom, numerous small faults or fractures are present, some of them probably extending to the seabed. Locally, specific features are encountered, such as salt diapirs, pockmarks, cold seeps or carbonate concretions.

The soil conditions, being consistent and relatively uniform, are composed of high plasticity clays, characterised by a plasticity index, Ip', over 100%, a low effective unit weight

This content is only available via PDF.
You can access this article if you purchase or spend a download.