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ABSTRACT Severe tilting and overturning of caisson breakwaters due to wave loading is well documented. Current simplified methods of caisson analysis are either fully elastic analyses which do not account for permanent displacement or stability analyses that determine whether overturning will occur. In reality, caisson can incur significant tilt without overturning; such phenomenon cannot be replicated by either of the above analyses. Elastic response was simulated using a lump-mass-spring model. Permanent caisson tilt was analyzed based on the assumption of the caisson tilting over a circular slip surface beneath its base. Comparison with centrifuge model data shows that the dynamic slip circle is able to capture the progressive build-up of tilt reasonably well. Finally, a parametric study was presented which shows that the computed permanent caisson tilt can be reasonably well-correlated to a function of four dimensionless groups of caisson parameters. INTRODUCTION Nowadays, more attention is being paid to the catastrophic failures of caisson breakwaters induced by impulsive wave loads which are transferred to the foundation soil through swaying and rocking of the structure. Owing to higher strength and shorter duration of the impact loading as shown in Fig. 1(a), Oumeraci et al. (2001) reported that such loading might induce soil as well as structural failures of a caisson breakwater. As such, the caisson movements and excess pore water pressure build-up of the foundation soil are expected to be more severe under breaking wave impact loads. Fig. 1(b) shows that when the waves break directly on a caisson body, the resultant impulsive force illustrated can be represented by a series of quasi-static component termed "church roof load". Sekiguchi and Ohmaki (1994) proposed an overload factor beyond which a given caisson may be overturned, but the specific caisson movement was not obtained.
ABSTRACT As is generally known, the response of a jack-up unit can be amplified significantly if the natural period of jack-up drilling unit falls within the range of wave excitation. It has been the industry practice to account for such amplified responses by means of Most Probable Maximum Extremes (MPME) and Dynamic Amplification Factor (DAF). Presently, there are four methods recommended by SNAME T&R Bulletin 5–5A (hereafter referred to as "SNAME 5–5A") for predicting the DAFs of overturning moment and base shear, namely the Drag/Inertia parameter method, the Gumbel fitting method, the Weibull fitting method and the Winterstein/Jensen method. The aim of this paper is to further investigate these four methods by means of re-evaluating extreme response values and the corresponding DAFs. The characteristics of the respective methods are discussed and the sensitivity of random seed selection on DAFs is investigated. INTRODUCTION Recent trends have seen the application of jack-up rigs extended into deeper waters; hence the dynamic effect becomes a significant concern. In order to investigate the dynamic behavior of a jack-up rig in terms of MPME and DAF, nonlinear dynamic analysis in time domain has been carried out with the created random wave surface history. The DAF is defined as the ratio of the dynamic and static responses to account for dynamic effects. Normally, the DAFs are computed for the two basic global design parameters, i.e. base shear and overturning moment. The dynamic effect magnifies the hydrodynamic loads on a jack-up which lead to greater sideways movement and then more P-Δ Effect. For a single-degree-of-freedom (SDOF) system vibrating in sinusoidal waves, (equation shown in paper) where Ω is the ratio of the jack-up natural period to wave excitation period, andε is the damping ratio.