Centrifuge Modelling And Parametric Analyses Of Drag Anchor Behavior

Abstract

ABSTRACT A centrifuge and analytical modelling study was performed to gain new insight into the behavior of drag anchors in more-or-less normally consolidated clay soils. The conditions investigated are pertinent to deep-water MODUs and FPSS in "soft clay" regions such as the Gulf of Mexico. The study results show that soil shear strength gradient and lead line type (wire rope vs. chain) significantly impact anchor capacity. Cyclic loading and soil consolidation (anchor soaking) are of lesser importance. Also, it was found that modern, high-capacity drag anchors can provide significant resistance to vertical upIift in soft clay. Standard industry guidelines for predicting anchor capacity do not consider lead line type or soil shear strength gradient and do not permit vertical loads on anchors. It is recommended that anchor sizing guidelines be modified in light of this studyâ??s results. INTRODUCTION Existing industry procedures for predicting the capacity of drag anchors [1,2] are based in large part upon the results of field tests performed by the U. S. Navy [3]. These (and other) results have been used to develop empirical charts that relate anchor weight to anchor holding capacity. These charts have some limitations when applied to sizing anchors, particularly those for deep-water moorings:The anchors needed for deep-water mobile offshore drilling units and floating production systems commonly weigh 15 to 80 kips, while the design curves are based primarily upon tests of anchors that weighed 0.5 to 6 kips. Extrapolation therefore is a source of uncertainty.The tests used to develop the curves were performed at a limited number of sites. As a result, the curves are for use in generic soil types such as "soft clay", "sand", and "stiff clay". Site-specific conditions may not be properly addressed.Since the standard curves are based on historical test data, they may not reflect the performance characteristics of new anchor types or enhanced versions of older anchor types.The standard design curves have been developed using short-duration tests in which the anchors were subjected to monotonically increasing loads. Other conditions of potential importance, such as cyclic loading and the period of anchor embedment, generally have not been addressed. The work reported here was a study of the performance of two types of modern drag anchor in soft clay. The study investigated how anchor capacity is influenced by anchor size, soil shear strength gradient, cyclic loading, anchor soaking, and lead line type (wire rope vs. chain). It included the first tests of properly scaled model drag anchors in a geotechnical centrifuge and limit equilibrium analyses. The results showed, first, that these methods can be used successfully to investigate the behavior of very large anchors in soft clays. Second, the results revealed the significant impact that soil shear strength gradient and lead line type have on anchor capacity.