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The sum type combination resonance response of a classic spar platform is studied under a long periodic regular wave in this paper. The nonlinear differential equations for heave and pitch are established considering the time-varying incident wave elevation which is regarded as a parametric excitation. The first order steady-state response is solved by the method of multiple scales when the incident wave frequency approaches the sum of heave natural frequency and pitch natural frequency. The heave and pitch responses of a practical spar platform are obtained by solving the bifurcation equation using a numerical method. It is observed that the large amplitude sub-harmonic motion of heave and pitch mode are tripped when the wave height exceeds a certain value. The result shows that the spar platform may exhibit complicated dynamic behaviors when the sum type combination resonance occurs and indicates that the available damping plays a very important role in suppressing the instable motion responses.

Unburied subsea pipelines operating under high-temperature and high-pressure conditions tend to relieve their axial compressive force by forming lateral buckles. Uncontrolled lateral buckling may lead to pipeline failure. In order to control lateral buckling, a sleeper is often employed as a buckle-initiation technique. In this study, analytical solutions of lateral buckling for unburied subsea pipelines with sleeper are derived. An energy analysis is employed to investigate the stability of the buckled pipeline. The influence of sleeper height and sleeper friction on pipeline buckled configurations and typical lateral buckling behaviour is illustrated and analysed. The results are shown to be in very good agreement with experimental data in the literature. We also discuss the effect of imperfections and conduct an error analysis of one of the main assumptions of the proposed analytical method. Our results show that increasing the height of the sleeper or decreasing the friction between pipeline and sleeper can all be used to decrease the minimum critical temperature difference. However, only the sleeper height is effective in substantially reducing the maximum compressive stress.

Unburied subsea pipelines operating under high temperature and high pressure (HT/HP) conditions tend to relieve their axial compressive force by forming lateral buckles in an uncontrolled manner. In order to control lateral buckling, a distributed buoyancy section is often employed. In this study, analytical solutions are deduced for lateral buckling of unburied subsea pipelines with a distributed buoyancy section. An energy analysis is employed to investigate the stability of the buckled pipeline. The influence of the length and weight of the distributed buoyancy section on pipeline buckled configurations, typical lateral buckling behaviour and the minimum critical temperature difference is illustrated and analysed. The results are shown to be in good agreement with experimental data in the literature. The effect of imperfections is also discussed and an error analysis is conducted for one of the main assumptions of the proposed analytical method. The results show that increasing the length or decreasing the weight of the distributed buoyancy section can both be used to decrease the minimum critical temperature difference. The maximum compressive stress will decrease with decreasing weight of the distributed buoyancy section. However, the influence of the length of the distributed buoyancy section on the maximum compressive stress is complicated.

Buried subsea pipelines under high temperature conditions tend to relieve their axial compressive force by forming localised upheaval buckles. This phenomenon is traditionally studied as a kind of imperfect column buckling problem. We study upheaval buckling as a genuinely localised buckling phenomenon without making any ad hoc assumptions on the shape of the buckled pipeline. We combine this buckling analysis with a detailed state-of-the-art nonlinear pipe-soil interaction model that accounts for the effect of uplift peak soil resistance for buried pipelines. This allows us to investigate the effect of cover depth of subsea pipelines on their load-deflection behaviour. Furthermore, the influence of axial and uplift peak soil resistance on the localised upheaval behaviour is investigated and the maximum axial compressive stress during the buckling process is discussed. Parameter studies reveal a limit to the temperature difference for safe operation of the pipeline. Localised upheaval buckling may then occur if the pipe is sufficiently imperfect or sufficiently dynamically perturbed.

Parametric instability of a riser is caused by fluctuation of its tension in time due to the heave motion of floating platform. Many studies have tackled the problem of parametric instability of a riser with constant tension. However, tension in the riser actually varies linearly from the top to the bottom due to the effect of gravity. This paper presents the parametric instability analysis of deepwater top-tensioned risers (TTR) considering the linearly varying tension along the length. Firstly, the governing equation of transverse motion of TTR under parametric excitation is established. This equation is reduced to a system of ordinary differential equations by using the Galerkin method. Then the parametric instability of TTR for three calculation models are investigated by applying the Floquet theory. The results show that the natural frequencies of TTR with variable tension are evidently reduced, the parametric instability zones are significantly increased and the maximum allowable amplitude of platform heave is much smaller under the same damping; The nodes and antinodes of mode shape are no longer uniformly distributed along the axial direction and the amplitude also changes with depth, which leads to coupling between the modes. The combination resonance phenomenon occurs as a result of mode coupling, which causes more serious damage.