This paper reviews the potential for conductor and shallow well drilling operations to affect the safety of offshore structure foundations. It is written from a geotechnical perspective and draws both on the Authors? experience and on a confidential study carried out for the UK Health and Safety Executive (HSE) which included a literature survey and discussions with operators? drilling and geotechnical engineers, drilling contractors, structural designers and geotechnical consultants.
Major changes have taken place which could potentially increase any detrimental effects of conductor and shallow well drilling on offshore structure foundations, in particular, on driven piles. Firstly, conductors and wells are often installed from mobile units through drilling templates, prior to jacket structures to become narrower and to have vertical piles, with a consequent reduction in pile to conductor spacing.
Conductors may be installed by drilling, by driving, or a combination of both. Where drilling is undertaken, problems can occur due to hydraulic fracture or to wash-out. Hydraulic fracture occurs where drilling fluid pressure is too high and fluid is lost into the formation, possibly softening the surrounding soil. Wash-out generally occurs in granular soils and may, in part, be induced by high circulation rates of the drilling fluid, Wash-out leads to increased stress relief in the soils. Where such problem occur, the drilling solution is often to use ?lost circulation material? to stem fluid losses, or fill voids with a cement grout. Damage to surrounding soils by softening or stress relief is not drilling consideration. Since lost circulation is not unusual, drilling engineers tend to alert geotechnical engineers only where a problem is severe. Therefore, foundation soils may be weakened without those responsible for the design or verification of the foundations being made aware of this Most operations do not have dedicated geotechnical engineers and so there may be no in-house expertise to evaluate potential conductor installation problems. It is clear from the Authors? research that many drilling engineers are not aware that their activities may affect foundation stability, although some operators do have close, albeit informal liaison between drilling and geotechnical engineers.
While structure designers or geotechnical engineers are likely to consider pile penetration requirements when recommending conductor setting depths, the final decision regarding depth and method of installation is likely to be established by drilling engineers in isolation. It is rare for geotechnical engineers to monitor conductor installation or shallow well drilling. Furthermore, there are many differences in the way operators establish and undertake these operations.
Case histories of problems encountered during, or as a result of, conductor installation or shallow drilling activities include:
Deviating and clashing conductors;
Weakening of foundation soils due to significant loss of circulation of drilling fluid;
Drilling fluid coming up around a jacket mudmat;
Interconnection between adjacent drilled holes;
Drilling-induced seabed craters around conductors;
Blowouts; and
Loss of contact beneath a gravity base structure
The major finding is that problems of occur, during these operations, which could have a significant impact on the safety of foundations.
