Hydrocarbon well drilling in clay inevitably causes disturbance to the surrounding ground. In circumstances where sections of open well formed in clay collapse during their advancement ("pack-off") the drilling fluid pressure is often increased to obtain a break-through return path. This process may cause swelling and hydraulic fracturing in the surrounding soil mass. With time the excess pore pressures will dissipate and potentially cause increases in pore pressures far from the wells, possibly affecting pre-installed elements such as foundation piles.
The paper presents a series of finite element (FE) analyses in which "packing off" was treated as an idealised fluid injection process and its effects on nearby driven piles were evaluated. The entry of pressurised drilling water into progressively enlarging fractured disturbed zones, and hence into the surrounding soil mass, was simulated with two- and three-dimensional non-linear FE models. The results of the analyses indicate marginal foundation pile capacity reductions, as well as significant global vertical and horizontal movements around the piles. The ground movement predictions are shown to be heavily influenced by the degree of geometric idealisation.
Hydrocarbon well drilling operations often take place in close proximity to driven foundation piles supporting fixed offshore platforms. The inevitable disturbance of the surrounding ground is increased if open wells drilled in clay collapse during their advancement and drilling fluid pressures are increased to enable trapped fluid to break through and escape to the surface. This technique of treating ‘packed-off’ wells in clays may lead to swelling and hydraulic fracturing of the surrounding soil mass. Hobbs and Senner1, 2 reviewed a number of case histories where problems during, or as a result of, conductor installation or well drilling were observed, noting that an excessive head of drilling fluid can affect nearby foundation piles through an increase in pore water pressures around the piles. The Health and Safety Executive (HSE)3 stated in its guidance that: "The effect on a foundation of conductor installation procedures should be reviewed with particular attention given to the possibility of wash-out, hydraulic fracture or other drilling problems endangering the foundations."
However, while the potentially adverse effects of well drilling operations on foundation piles in clays have been recognised, there does not appear to be an established way of predicting their possible impact quantitatively. A numerical approach has been developed by the authors to study drilling disturbance effects on foundation piles in clays as described by Schroeder et al.4 The methodology combines a series of fully coupled two-dimensional (2D) plane strain and axially symmetric finite element (FE) idealisations with an ‘undrained-drained’ simulation using a three-dimensional (3D) FE model. The present paper summarises the work by Schroeder et al.4 describing the application of the approach to a piled offshore production facility and comparing the FE results with available field measurements and observations. The FE analyses were carried out using the FE code ICFEP using an accelerated modified Newton-Raphson scheme with an error controlled substepping stress point algorithm5.
As noted by Hobbs and Senner2, problems encountered during drilling operations
