Design of Gravel Banks: A Way To Avoid Spudcan Punch-Through Type of Failure
Authors: Kellezi Lindita , Stadsgaard Henrik 
Published: April 2012

For the installation of a three-legged jack-up rig at an oil field / drillinglocation in the North Sea conventional and finite element spudcan penetrationanalyses and engineering assessments are carried out. As the location, based onthe preliminary assessments, indicated critical soil conditions with regard torig installation, an improvement / increase in the seabed bearing capacity wasaimed by designing / constructing gravel banks on the seabed, one at eachspudcan location, respectively, as a way to avoid jack-up foundation punchthrough type of failure.

The soil data at the location included three boreholes with sampling andlaboratory testing combined with piezocone penetration tests (PCPTs) one ateach spudcan centre, respectively. Furthermore, a site survey was availablecovering the area of interest. Based on all the available soil data designlower / upper bound soil profiles applicable to penetration analyses at eachspudcan location for virgin seabed conditions were assessed first. Thoseprofiles and the results from the different, conventional and finite elementanalyses carried out together with a validation study are presented. Based onthem the decision for modifying the seabed by constructing gravel banks wastaken.

The gravel banks were designed based on the Plaxis finite element modellingapplying lower bound soil parameters calculating final heights of 4.0 m, 2.5 m,2.0 m for Port, Starboard and Bow legs, respectively. The top diameter and theslope of the banks are chosen to be the same, 50 m and 1:2.5, respectively. Except the small strain (plastic) and updated mesh (UM) analyses an attempt wasmade to adjust the conventional solution for multilayered soil conditions incomparison to finite element results.

For the new conditions, the modified seabed by construction of the gravelbanks, sufficient foundation bearing capacities and no risks for punch-through/ rapid penetrations were calculated. The spudcan-gravel bank-soil interactionsystems able to support the maximum preloads for each leg determined based onthe environmental load analyses. The gravel banks were constructed and shortafter that the jack-up rig was successfully installed measuring shallow spudcanpenetrations as predicted.

Introduction

Prediction of the jack-up spudcan penetrations is an important issue in theprocess of rig installation. Unexpected sudden and rapid penetrations can be ofmajor risk for the stability and equilibrium of the jack-up structures. Classical conventional solutions (Hansen 1970), which are applications ofbearing capacity equations for homogeneous soil conditions and modifiedprocedures for layered soil profiles (Jacobsen et al., 1977), (Hanna &Meyerhof 1980) are normally used for the spudcan penetration prediction. However, for layered soil condition the modified conventional procedures arenot always sufficiently accurate and realistic comparable to fieldobservations.

As analytical procedures have several limitations, alternative analyses basedon numerical modeling are investigated and presented through the years. Applications of those methods mainly based on the finite element analyses incomparison to analytical solutions for the conventional, non-skirted spudcansare among others elaborated in (Kellezi & Strømann 2003), (Kellezi et al.,2005), (Kellezi & Kudsk 2009).