Engineers Australia

Impact of spudcan footprints on a gravity base structure

Pauline Truong

Abstract

In the offshore oil and gas industry, structures such as gravity base structures and jack-ups are commonly used as drilling and facilities platforms. Installation locations for these structures are usually chosen on flat lying, featureless, undisturbed seabed to reduce installation risks and avoid costly seabed preparation works. In some cases, these structures are installed in close proximity of each other and care is taken to ensure the foundations do not influence the stability of the adjacent structures. This was the case with the skirted gravity base structure discussed in this paper as it was designed by Arup to be installed next to an existing wellhead platform linked with a connecting bridge. However, a jack-up rig unintentionally installed at the designated site of the gravity base structure and therefore significantly changed the seabed profile as well as the strength properties of the underlying soils. The jack-up spudcan footings punched three 20 m diameter craters to about 3 m depth into the seabed. The craters and the disturbance of the soil beneath and around the craters affected the stability of the gravity base structure and increased the risk of installation refusal of the skirted foundation. The craters necessitated the reorientation of the gravity base structure to minimise foundation intersection with the spudcan footprints. In 2011 the gravity base structure was successfully installed and is currently in operation. This paper discusses the investigation process and the analyses that were conducted to assess the impact of the spudcan footprints on the performance of the gravity base structure.