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From the same paper as the previous image
The flexible pipes are subject to various loads during installation and operation. Especially, they are exposed to dynamic loads induced by dynamic motions of platforms and interaction with soil at touch down zone. In particular, the maximum net pressure is imposed during installation because there is no internal fluid, but the high hydrostatic pressure is acting on the outside of flexible pipes. At the touch down zone, the flexible pipe experiences the highest pressure-unbalance. As shown here, when the flexible pipe receives high external pressure, it tends to elongate due to the Poisson effect. However, because of the friction with the seabed, the lower end acts like a fixed-end and a reaction force is applied in the axial direction. Since the flexible pipes are not designed to have high axial compressive strength, relatively low loads can trigger buckling failure. A typical failure mode caused by the axial compression is radial buckling of tensile armor layers as shown in the next image.
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