Factoring to Improve Conservatism of the Drag-Inertia Method When Determining Dynamic Amplification of Jackup Units

Abstract

The Drag-Inertia method is one of the most popular time domain methods for estimating the contribution of dynamics to the response of a Jackup. The method calculates the Dynamic Amplification Factor (DAF) using the simulated response of the Jackup to a short, usually one hour, random storm. The Drag-Inertia method has been subject to some criticism as it produces results which are, in some cases, less conservative than other methods. Despite this, we feel the benefits of the Drag-Inertia method; efficient and repeatable assessment, are essential to a site assessment. Other methods, such as the Winterstein method, require much longer storm durations, while the variability in DAF results is considerably more. In this study, we propose that factoring the DAFs produced by the Drag-Inertia method is a practical way to improve conservatism, bringing the DAFs in line with the mean DAF results produced by the Winterstein method. The factor is proposed based on simulations of 18 different Jackup configurations and eight different wave conditions. The simple factor is a function of period ratio, Tn/Tp, allowing for efficient and repeatable factoring of DAF values without additional simulation time. When the factor is applied to the Drag-Inertia DAF values the resulting DAFs are within 0.05 of the mean Winterstein DAF in 61% of cases and in only 7% of cases is the Winterstein DAF more than 0.10 above the factored Drag-Inertia result.