Offshore installations designed to withstand extreme ice actions, such as the multi-leg structures in Cook Inlet, the gravity based Molikpaq during its mobilization in the Beaufort Sea, lighthouses and channel markers in the Baltic Sea, jackets and mooring poles in Bohai Bay and multi-leg structures offshore Sakhalin, have experienced ice-induced vibrations (IIVs). Full-scale data from Bohai Bay also demonstrate that a conical waterline geometry of the structure does reduce the magnitude of the ice forces, but it still experiences IIVs that can be treated as a stochastic process. ISO 19906 recommends that the dynamic ice actions and the corresponding IIVs shall be considered in the design as the fatigue limit state (FLS). ISO 19906 provides the guidance for the time-domain random dynamic ice action on conical structures. The dynamic structural response to such ice action can take the form of a random vibration. As an alternative to the time-domain approach, random vibration analysis can also be done in the frequency domain by the spectral approach. In addition to the time-domain random dynamic ice action on conical structures provided in ISO 19906, a type of ice-force spectrum on conical structures has been developed. In this paper, a simplified single-degree-of-freedom system (SDOF system) and the ice-force spectrum are used to derive an analytical random solution to assess the IIVs of conical structures. As ISO 19906 points out that particular attention shall be given to dynamic actions on narrow structures and flexible structures, the developed random solution can be useful for designers to make a fast estimate of IIVs (i.e., displacement, velocity and acceleration) and to efficiently screen out the key design parameters of a conical ice-resistant structure.
SafeHull (SH) is a system that comprises Rules and software programs, as depicted in Figure 1. The ABS Guide for Building and Classing Floating Production Installations [2] (herein after called ABS FPI Guide) and the associated SH-FPSO software system have been in use for over 3 years in the design classification of FPSO new builds and conversion from existing tankers. This paper provides information about the new FPI Rule change (April 2004) regarding the classification process of FPSO conversions as an ongoing effort to address the needs of clients. It highlights the impact of the changes on the two typical conversion processes – the basic Ordinary Conversion and the optional SH (CS) Conversion. The paper focuses on the new requirements for fatigue assessment, i.e., the Environmental Severity Factors (ESF), and strength including loading cases that apply to SH phase A and B as a requirement for the structural review process, which is carried out by the classification society. Class requires not only the certification of the project but also future surveys of the vessel. FPSOs are also considered offshore structures, therefore, this paper also discusses the application of the ABS Guide for Buckling and Ultimate Strength Assessment of Offshore Structures [4] applied to FPSOs and makes comparison with the ABS Rules for Building and Classing Steel Vessels , which uses the net scantling concept to determine buckling of structural members.
A series of sensitivity analyses is carried out against flow models (i.e. inviscid, laminar and turbulent models), mesh size and scaling factor based on Froude scale laws on the time history of sloshing impact pressures and computational requirements. In order to capture the complex wave breaking and gas trapping phenomena associated with the violent liquid sloshing motion, a free surface tracking approach called Volume of Fluid (VOF) method is employed. The computational results show that the inviscid flow model based on Euler equation would provide a reasonably accurate impact load prediction and with efficient computing time. This is because the inertia force is the dominant factor in the sloshing movement compared to the localized viscous effect. For mesh sensitivity analysis, 5mm and 10mm mesh size models have been simulated for 20 cycles. It is found that 5mm resolution provides better results than 10mm. Further mesh refinement is expected to provide better results, but with significant computational power requirement. Finally, the full-scale simulation results have also been compared against the model scale results. The simulated impact pressures derived from model scale based on the Froude scale law agree with those of calculated from the full scale simulation.
Spudcan rotational fixity under combined vertical, horizontal and moment loading is often assumed to be invariant with time. In reality, the actual rotational fixity of spudcan footing is likely to change with time as excess pore pressure builds up and dissipates. This paper describes a series of centrifuge experimental tests conducted at 100-g acceleration using a small spudcan model and specimens of normally consolidated reconstituted kaolin clay. Using a servo-motor, belt-driven actuator system, loading episodes comprising one thousand cycles of combined loading were applied to model foundation. The PPTs are installed in soil specimens to measure the excess pore pressure and degree of saturation of soil. One small-rotation test is conducted just after spudcan’s penetration; while another one small-rotation is conducted when the excess pore pressure is fully dissipated after the spudcan’s penetration. The results show bending moments at four locations along the spudcan shaft, which indicates that the lattice confers a significant lateral soil resistance, and the presence of the lattice will also cause the location of maximum bending moment to be up-shifted along the leg, towards to the soil surface. The rotational fixity of the spudcan shows distinct changes over time, which is attributed to consolidation and settlement effects. Comparison of fixity of spudcan with and without lattice leg indicates that the lattice leg can lead to a large increase in lateral resistance of deeply penetrated spudcans. This lattice leg effect has been largely ignored in both academic study and industrial design.
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