This paper describes a case study or the sitting of a jack-up rig in calcareous soils prevalent in the Gulf of Suez. This study illustrates the importance of a rational interpretation of soil conditions in calcareous material in predicting spud can penetrations and rig stability. Contrary to some belief, comparison of actual and predicted spud can penetrations shows that grain crushing of calcareous soils does not have any significant influence on spud can penetrations.
The calcareous soil formations encountered in the Gulf of Suze are often complex, stratified and highly variable. These formation are typically very porous, exhibit varying degrees of cementation and are susceptible to grain crushing under foundation loads. Sitting of an independent leg jack-up rig in such variable soil formations is of great concern both to rig operators and oil companies from a standpoint of sit stability. Spud cans are prone to sudden additional penetrations (punch-through) and most punch-throughs occur during preloading operations or during a major storm. The resulting damage to a rig can range from minor structural failure of the leg braces to a major catastrophe such as loss of rig and life.
The aim of this paper is to evaluate rationally the important parameters that must be considered in successfully predicting spud can penetrations and rig stability in calcareous material. Comparison of actual full-scale field test results and precited spud can penetrations shows that with proper soil interpretation, conventional bearing capacity theories can be used to analyze spud can penetration problems. The study also shows that grain crushing of calcareous soils is not an important consideration.
The study area, shown in Figure I, is approximately 25 miles (40 km) southeast of Ras Gharib, Egypt, in the Gulf of Suez. The geotechnical investigation program for the independent leg jack-up rig sitting consisted of a soil boring and a cone penetration test boring. These borings were 100 feet (30.5 m) deep and were located within 36 feet (11 m) of the proposed rig location. Soil samples were obtained at 3-foot (1 m) intervals with a hydraulic push sampler and 3-inch O.D. thin walled tube samples were taken. Data from a deep boring, performed as a part of another investigation, was also available. This boring was 78m feet (24 m) northwest of the rig location. The water depth in this general area is 200.5 feet (61.1 m).
The shallow boring and the cone penetration (shown on Figure 2) revealed fairly complex stratigraphy. The surficial sediments consisting of very soft to soft carbonate clayey silt, extend to a depth of 31 feet (9.5 m). These are underlain by 10 feet (3 m) of medium dense fine to medium carbonate sands which are weakly cemented. Below 41 feet (12.5 m), firm to stiff carbonate silty clay was encountered to a depth of 58 feet (17.7 m).
