The pipe configuration and internal loads along the pipeline during the pipeline laying process have long been the focus of engineers. Most researchers simplify the seabed to be rigid and the water to be calm, ignoring the pipe embedment into the seabed and the influence of ocean currents. In this paper, a novel numerical approach is proposed for the laying of pipelines in the so-called J-lay method, taking into account the importance of both pipe embedment and ocean currents. The pipeline is divided into two parts, one part suspended in water, and the other laid on the seabed. The continuity of the two parts at the touch down point (TDP) is guaranteed to make a whole. The feasibility of the model is proved by the comparison between the present model and an analytical model, which shows good agreement in both pipeline configuration and bending moment distribution. Finally, parametric study was performed to consider the influence of current velocity, water depth, top inclination angle, and seabed stiffness, and conclusions are drawn.

J-lay is widely accepted as a favorable method for deepwater pipeline installation. The configuration and internal force of the pipeline during the laying process are key factors for its safety, and the embedment induced during pipeline installation plays an important role for pipeline stability in service. Traditionally, the seabed in the analysis of J-lay is assumed to be elastic, whereas deepwater deposits are usually very soft, exhibiting low strength and obvious plasticity. This paper presents an analytical model for pipelaying on plastic seabed. This model simplifies the pipeline as the combination of four segments: a natural catenary in water, a boundary-layer segment, a beam on the seabed (touchdown zone) where the soil deforms plastically, and a freely laid horizontal segment. The comparison with a traditional elastic-seabed model reveals that both models have similar pipeline configurations, although the elastic-seabed model slightly underestimates the maximum bending moment. In contrast, the pi...

Multicomponent taut mooring lines are widely used to secure floating facilities to anchors embedded in the seabed to restrict motions. Optimal design of the mooring line system requires a realistic model of the combined performance of all segments of the mooring line, including the separate segments contained within both the water column and the soil column. This paper presents a two-dimensional quasi-static model, which can analyze mooring lines comprising multiple types of mooring lines or chains, taking into account the effects of ocean currents, soil resistance, and elastic elongation of mooring line. An example analysis is carried out to predict the responses of multicomponent mooring line during pretension and under service conditions. The example analysis puts special focus on conditions where the floating facility undergoes a series of motion away from its original position to assess the effect of the vertical offset is studied in detail. Finally, based on the presented model, the performances of different components of the mooring system are thoroughly investigated and some useful conclusions are drawn.

The S-lay method has been widely used in pipeline installation from shallow water to deep water for decades. In this paper, a novel numerical model for analyzing pipelines in the S-lay problem is proposed to investigate the overall configuration, internal forces, and strain of the pipeline taking into account the influence of ocean currents and seabed stiffness. The influence of many important factors, including the variation position of the liftoff point, the change of stinger radius, ocean currents, seabed stiffness are investigated in detail. Some useful results are obtained: the stress state of the pipeline is found to vary greatly during the whole laying process; the train of the pipeline at both the upper and the lower sides is very important; ocean currents have negligible influence on the pipeline; traditional “touchdown factor” is not suitable to predict the real pipe embedment; and soil stiffness plays an important role in pipeline behavior on the seabed. The illustrative examples and comparison with a previous work demonstrate the widespread applicability of this model. Moreover, the solution process of this model is easy and fast, so it is suitable for engineering applications.

Silty sand is a kind of typical marine sediment that is widely distributed in the offshore areas of East China. It has been found that under continuous actions of wave pressure, a mass of fine particles will gradually rise up to the surface of silty sand seabeds, i.e., the phenomenon called wave-induced soil erosion. This is thought to be due to the seepage flow caused by the pore-pressure accumulation within the seabed. In this paper, a kind of three-phase soil model (soil skeleton, pore fluid, and fluidized soil particles) is established to simulate the process of wave-induced soil erosion. In the simulations, the analytical solution for wave-induced pore-pressure accumulation was used, and Darcy flow law, mass conservation, and generation equations were coupled. Then, the time characteristics of wave-induced soil erosion in the seabed were studied, especially for the effects of wave height, wave period, and critical concentration of fluidized particles. It can be concluded that the most significant soil erosion under wave actions appears at the shallow seabed. With the increases of wave height and critical concentration of fluidized particles, the soil erosion rate and erosion degree increase obviously, and there exists a particular wave period that will lead to the most severe and the fastest rate of soil erosion in the seabed.