Effect of the ordering of cyclic loading on the response of suction caissons in sand

Abstract

Wind and wave action on offshore wind turbines causes irregular cyclic loading on the foundation that can lead to rotation accumulation of the structure. Such loading is generally considered in design using ‘counting methods’ that decompose the time history of irregular cyclic loads into a series of cyclic load parcels of uniform amplitude. These parcels are ordered in magnitude, with the rotation due to previous cyclic load parcels accounted for by including an equivalent number of cycles of the current cyclic load parcel. Hence, this superposition approach adopts Miner's rule, as the accumulated rotation is considered to be independent of the ordering of the cyclic load parcels. This paper examines the validity of this assumption for moment cyclic loading of suction caissons in sand, through a series of model tests involving both constant and varying amplitude cyclic loads, with each test involving at least one million cycles. In the varying amplitude cyclic tests, the accumulated rotation approximately doubled when the load ordering changed from ascending to descending. This is considered to be due to changes in grain contacts and beneficial densification effects from lower amplitude cyclic loads that are absent when the cyclic loads are arranged in descending order.