Risk-informed design of new anchor concept for floating energy production systems

Abstract

The motivation for this work was to (1) improve the performance of permanent mooring systems; (2) pursue a novel anchor concept that minimizes installation effort and exhibits a ductile failure, (3) meet system reliability targets while optimizing resources during installation and operation. The objectives of this research were to (1) develop a new concept for an anchor, (2) optimize the ratio of structural capacity to weight for different configurations of the anchor that are stable during freefall through the water; (3) develop a mechanism to free the shank of the anchor during pitch rotation; (4) develop understanding about how interactions between mooring lines, the anchor and the soil affect the performance of mooring systems; (5) assess the reliability of mooring systems and relate it to the reliability and design factors of safety of individual components in the system, (6) implement a risk-based framework to improve the reliability and efficiency of mooring systems for floating production systems and facilitate implementation of the new anchor concept. This work has produced a new anchor concept. The anchor (1) is liberated at a considerable height above the seafloor, (2) freefalls in a vertical position and trajectory through the water column, (3) embeds vertically into the soil, (4) rotates in places to align itself with an optimized direction and (5) slides laterally to embed deeper into the soil. This anchor has a theoretical efficiency, defined as the maximum holding capacity divided by the anchor weight, of more than 75 and significantly greater than most conventional foundations and anchors. The anchor also could be used to significantly increase the reliability of mooring systems by acting as a fuse to protect lines from breaking. The movement of the new anchor introduces ductility, meaning that the limit state potentially reached by the anchor is always related to excessive deformation and not a complete loss of restoring force. A new design philosophy for mooring systems where anchors have more probability of failing than the lines could be developed to improve the reliability of the system without increasing the capacity of the mooring lines.