On Dynamic Compression of Risers: An Analytical Expression for the Speed of Compression Waves

Several aspects of risers’ dynamics still demand extensive researches as to improve the understanding about physical phenomena involved, so check the adequacy among analytical and numerical results in offshore applications. Dynamic compression of risers is one of these subjects, which has extreme practical relevance in the sense of both design and structural analysis. In this work a riser is anchored at its top end and the other one remains resting on the seabed. Consider that the riser has quasi-vertical configuration. Thus, the angle between the tangent line at the top end and the vertical is small, and its tension on the vicinity of the touchdown point (TDP) is also small. The riser is statically subject to its own submerged weight and on it can act a horizontal ocean current. Furthermore, it supposed a harmonic motion imposed at the top in the tangent direction to the line, caused by local waves on the floating unit. Since the dynamic tension τ(s) changes cyclically in time and the tension T(s) in the TDP is small, it is possible that in this region, the first exceeds the second one in part of cycle’s wave. In these situations occurs a phenomenon known as dynamic compression, widely discussed in Aranha; Pinto and Silva (2001), Aranha; Pinto (2001), Ramos; Pesce (2003) and Fujarra; Simos (2006). During the dynamic compression occurs riser buckling-like. In a strict sense, the behavior of a riser is similar to a cable, although its bending stiffness is not null. Hence, risers have some resistance to buckling-like and can support a certain level of compression. The maximum loading of compression that can be measured in a riser segment is known as critical load (Pcr). Aranha; Pinto and Silva (2001) proposed a simple analytical formulation for the critical load. The aim of the present paper is to propose a simple analytical equation for the speed of compression waves that propagate from the TDP to the riser’s top end. This formulation depends on physical characteristics of the riser and the levels of tension to which it is submitted. As a result, it is possible to compare the theoretical results obtained with numerical simulations and physical experiments. The outcomes provide more understanding to the phenomenon of dynamic compression of risers.