Effect of the Spudcan’s Footprints on Nearby Jacket’s Mudmat in Clayey Soil-Case Study

When a jack-up installed at a clay location and then leaves; it can create several meter deep footprints. In case of soft clay, the spudcan may have actually penetrated much deeper than the observed footprints. When the penetrated spudcan is pulled out, much of the soft remolded clay will flow around it and go back into the hole. This event, leaving a deep region of disturbed soil. The disturbed soil has a lower strength and stiffness in comparing to intact material around. The footprints and the associated remolded soil can potentially present significant hazards for subsequent jack-up or jacket deployments at the same location for example the events  in China Sea and Pesian Gulf of Iran. In this research a case study and numerical simulation (using commercial software ABAQUS) was performed to analyze the effects of spudcan penetration on the adjacent foundations of offshore platforms in clayey soil. Inconsistent with other studies, it was also shown that the penetration of spudcan can affect the soil layer in an annular zone. The maximum width of the affected zone is almost two times of the spudcan diameter; therefore the safe distance for installation of new nearby structures is also affected. In this paper the consequence of deployment of jack-up units in soft to firm clay will be discussed and the safe distance from footprints territory is obtained. All of our cases are located in the Persian Gulf. A jacket location of Assaluyeh/South Pars Gas Field in Persian Gulf was modeled to verify the numerical results. The most important results were the diameter of disturbed soil is ۱.۵-۲ times the spudcan diameter and the safe distance from the territory of footprint is ۳-۴ m in the Persain Gulf zone. These findings will help offshore geotechnical engineers to perform quick preliminary estimates on the severity of footprint-mudmat interaction problems.

Keywords:

Spudcan , Mudmat , Clayey , jack-up , instability