Study Of Group Effect And Its Application In Multi-Wellhead Conductor Casing Platform

With functionalities of isolating seawater, forming the back annulus for drilling fluids, setting the wellhead, suspending casings and providing vertical capacity, conductor casing is essential used in fixed offshore platform for the safety of wellhead equipments. Currently, single platform in conjunction with multi- wellhead is often utilized in shallow water areas to improve economic benefit while reducing platforms needed. However, the theory of pile foundation is used for the conductor casing without taking consideration of its operating conditions. Besides, capacity of each conductor is varied because of the compaction effect and group effect among these conductors.Conductor casing and open-end pile are alike in structure, but different in load-bearing mechanism. Moreover, there is no such a "pile cap" structure in the conductor group. Compaction stress is caused when surrounding soil is pushed away by the penetration of conductors, and compaction effect arouse when nearby conductors are influenced by these stress. Group effect arouse when shaft friction spreads through soil among these conductors.Conductor casing in subsea saturated granular fine sand soil is studied in this paper. Firstly, ABAQUS combining large deflection method is used to simulate the penetration of conductor casing to study the loading-bearing mechanism. Effect of factors such as center-to-center distance of conductors, numbers, plane layout to the compaction effect and group effect are then studied.A vertical capacity calculation method accounting for both the compaction effect and group effect are proposed. Finally, a small-scale indoor experiment is carried out to evaluate this method.Study shows that the interaction among conductors can be defined by compaction effect and group effect. Conductor number is the key factor affecting compaction effect and group effect. When conductor number is fixed, the better squareness of the plane layout, the stronger will the compaction effect and group effect is. Compaction strength decreases logarithmically, while group effect decreases linearly with the increase of conductor distance. Finally, a calculation method accounting for both the compaction effect and group effect are proposed based on the above analysis.