Extensive experience

NGI is a significant contributor to a large number of offshore structures with gravity based foundations (GBS), starting as early back as 1969 with the discovery of the Ekofisk oil field in the North Sea. We were also heavily involved with geotechnical design for most of the Condeep GBS platforms developed by Norwegian Contractors/Aker, icluding design and follow-up of instrumentation system to control and monitor the installation phase, and the pålatform behaviour during normal operation.

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Illustration of Condeep GBS concrete platforms in the North Sea - structural design by Dr. Techn. Olav Olsen

A recent example of a GBS application for offshore wind is the DolWin 2 HDVC converter platform in which NGI undertook the foundation design. A novelty of the DolWin 2 platform foundation is that it consists of two parallel pontoons, rather than a base slab. The DolWin 2 platform was installed in 2015 in the North Sea (picture below).

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Comprehensive technical expertise

Detailed understanding of the geotechnical characteristics of the soil, including the cyclic behaviour and the consolidation properties is an important aspect for the GBS foundation analysis. NGI has extensive expertise in undertaking and interpreting soil tests to obtain these details, as well as implementing the results into foundation analysis, including advanced numerical modelling (top picture).

A rock bed layer may be pre-installed on the seabed to improve the performance of GBS structures. NGI is experienced with field testing to measure characterizes and the behaviour of such rock material. This was used for the ExxonMobil ALT platform (top heading picture).