In situ and laboratory tests on a novel offshore mixed-in-place pile for oil and gas platforms

For the majority of offshore jackets, driven piles are the preferred foundation solution; however difficult soil conditions may preclude conventional pile driving. In such circumstances, methods of grouting a pile into an oversized hole or drilling and underreaming for a cast-in-place pile have received considerable attention. Drilled-and-grouted (D&G) piles are normally used in challenging geologies, such as very dense sands, very stiff clays and in calcareous deposits. However, D&G piles are normally time-consuming to construct. To address this challenge, the MIDOS pile system, which minimizes the number of offshore operations, was created and subjected to a suite of field tests. Cyclic tests were conducted on a pile installed in a silica sand deposit, where over 1000 loading cycles in between 1000 kN and 5000 kN (in tension) were applied. The evaluation of the cyclic test data clearly shows that the serviceability of the pile is unaffected by the cyclic loads. A feasibility study of the MIDOS pile in carbonate sands (Dog's Bay sand) was also explored in a laboratory test program. Carbonate sands are particularly relevant for drilled foundation solutions as the shaft friction of driven piles is typically very low due to the high radial contraction of such material adjacent to the pile shaft during impact driving. A finite element model (FEM) developed for this calcareous sand was used to compare the lab test results. The outcome of the laboratory testing program demonstrated that the MIDOS pile would have similar geotechnical and structural properties in both calcareous and siliceous deposits, suggesting that this novel offshore foundation type is suitable for a range of sand conditions.