It is widely recognised that hydrocarbon well drilling from offshore platforms affects the surrounding ground. In clays, the level of disturbance can be severe when sections of open well collapse during drilling, impeding fluid return to the seabed (termed ‘packing-off’), and drilling fluid pressure is increased in order to obtain a break-through return path for the fluid. Significant swelling and hydraulic fracturing can take place in the surrounding soil mass. With time the excess pore pressures will dissipate and could cause increases in pore pressures far from the wells, potentially affecting areas occupied by the platform’s foundation piles. There is no established procedure to quantify the impact of such processes on foundation performance.
GCG developed a numerical approach that involves a series of finite element analyses in which drilling disturbance is treated as an idealised fluid injection process. In this approach the entry of pressurised drilling water into progressively enlarging fractured disturbed zones, and hence into the surrounding soil mass, is simulated with two and three dimensional non-linear finite element (FE) models, without modelling the actual hydraulic fracturing processes. The analyses carried out by GCG made use of key observations made in the field at a piled offshore platform. The first key practical prediction was that despite assuming an aggressive mode of disturbance, GCG could assure the client of relatively modest overall reductions in axial capacity for the closest foundation piles at the platform under consideration. However, another key result was more significant than anticipated ground movements around the foundation piles. GCG’s client was able to apply the “best estimate” ground movement predictions in separate soil-structure interaction analyses that assessed the potential impact of the drilling induced movements on the foundations and jacket structure.
Schroeder, F. C., Jardine, R. J., Kovacevic, N. & Potts, D. M. (2007). Potential effects of well drilling operations on foundation piles in clay. Proc. 6th Int. Conf. on Offshore Site Investigation and Geotechnics – OSIG 6, pp.271-276
Schroeder, F. C., Jardine, R. J., Kovacevic, N. & Potts, D. M. (2008). Assessing well drilling disturbance effects on offshore foundation piles in clay. ASCE J. of Geotechnical and Geoenvironmental engineering, 134, No. 9, pp.1261-1271