Jack-up Spud Can Foundation Fixity for Various Clay Strength Profiles

Templeton, J. S. (Sage USA, Inc.)

OnePetro 

For site specific assessment of jack-up rigs, existing standards and practices require establishment of certain spud can fixity parameters. Among the required parameters are the maximum moment capacity and the maximum horizontal capacity. These parameters are established by use of site-specific soils data, spud can geometry and preload values. Logically the parameter formulations should provide for improvement in horizontal and moment capacity with increased penetration. Prior to the issue of ISO 19905-1 there was no such provision. However, work sponsored by the International Association of Drilling Contractors’ Jack-Up Committee (IADC) provided technical basis for inclusion of penetration-dependent foundation performance parameters to rectify this situation, particularly in the case of clay soils. The result was substantial increase in foundation parameter values for deep penetrations in clay. Subsequent hindcast studies using the new penetration-dependent parameters have reported very good agreement with actual field performance of instrumented rigs, and the IADC-recommended parameter formulations were subsequently adopted in ISO 19905-1 and in SNAME Bulletin 5-5.

The IADC recommendations were based, in part, of results of finite element analysis of spud-can/soil interaction. Those analyses modeled part of the spud can installation as well as the subsequent moment and horizontal loading. Both parts were done largely with both small strain and finite strain assumptions. More recent finite element work by on Zhang, Biennen and Cassidy (2013, 2014) used newer modeling techniques to substantiate more conservative recommendations.

The current revision draft of ISO 19905-1 prescribes the original IADC-based recommendations only for the case of uniform soil strength, and it prescribes recommendations based on Zhang, Biennen and Cassidy (2013, 2014) for the case of a strength profile increasing strictly linearly from negligible strength at the mudline. This situation leaves a gap in recommendations for more commonly encountered strength profiles intermediate to these two extremes.

The present paper reports results of new IADC-sponsored finite element analyses done with full Eulerian modeling for both the spud can installation and subsequent moment and horizontal loading. The new results support recommendations needed for the more commonly encountered strength profiles intermediate to the extreme cases now covered by the current revision draft of ISO 19905-1.