Dynamic behaviors of a marine riser with variable length during the installation of a subsea production tree

In subsea production tree (SPT) installation, the length of risers is time variable. To analyze the dynamic behaviors of risers, a novel analysis model with varying riser length had been established at two installation stages, and a Keller-box method was subtly applied to solve the differential control equations of riser with varying lengths. Specifically, the riser was discretized into multiple rigid segments, and the dynamic model was established under different boundary conditions at two stages. With a transformation between two adjacent segments to vary the length of the riser, the model was calculated. The upper-end tension and lower-end displacement with different launch velocities were studied in the time domain. Moreover, the influences of the drag coefficient, wind velocity, tide velocity, and SPT weight on the stress and lateral displacement of the riser were also discussed. Then, some interesting and meaningful conclusions were found, such as a launch velocity keeping at similar to 1.5 m/s was the better choice, the effect of the normal drag coefficient weakens as its value increases, a larger value of the tide velocity yields a smaller depth of the maximum displacement point, and so on. This work was not only used for guiding the installation of SPT, but also could provide a new method to research the slender with variable length.