Using penetrometer in situ and in box-core testing to obtain design information for lazy wave riser-soil interaction

Geotechnical assessment of vertical seabed stiffness during small vertical cycles of riser movement is important for riser fatigue analysis. These stiffness values change both with the amplitude of the cycles (because of non-linear stiffness) but also as the seabed consolidates under the small amplitude loading during long-term operation. To encapsulate both effects in design, a series of site-specific small displacement ball penetrometer testing was conducted both at seabed and in box-core samples on deck during a geotechnical survey for a recent deep water project. The paper describes the cyclic load and displacement controlled in situ testing, the results obtained, and how this data aided design. Of particular note were the long duration cyclic tests which were carried out for sufficiently long so as to not only capture secant stiffness reduction due to remoulding, but also subsequent stiffness increases due to consolidation. This novel test program performed on intact soil (as opposed to reconstituted samples in the laboratory or geotechnical centri-fuge) confirms that consolidation hardening can occurs in naturally structured near-seabed soils.