Abstract: Multi-footing system composed of multiple separate footings is a common type of offshore foundation. Based on failure envelope theory, the failure mode of multi-footing system on sand is analyzed in this study. Additionally, the corresponding analytical method of the bearing capacity is established and its feasibility is verified. The comparison of the calculated load safety factors between the single footing and multi-footing system is conducted in different loading paths. The analytical method of foundation bearing capacity is also conducted based on the combination of the failure envelope theory and partial factor method. From the analysis of failure mode, head sea side footings with four-legged platform firstly reach the failure envelope as result of increasing horizontal load, and their failure modes further become sliding. However, the sliding of head sea side footings would not occur due to the constraints among other footings. Since the head sea side footings bear less horizontal and vertical loads with increasing the horizontal load, the back wave side footings have to undertake larger load. At last, back wave side footings also reach the failure envelope, which results in the failure of the multi-footing system. It is shown that load paths have critical influence on load safety factors, and thus they should be specified when calculating load safety factors. Since the size and shape of failure envelope depend on a variety of factors, the calculation of bearing capacity needs to consider the failure envelope under particular conditions during design of foundations.

Key words: failure envelope, multi-footing system, failure mode, load safety factor, load path