On Choice of Finite Element Type for the Filled Bodies in Umbilicals for Deep Water Application

Filled bodies are often built into umbilicals to support other key components such as tubes and electric elements. These bodies play an important role in transferring the contact load between bodies when the structure is loaded. The geometrical profile can be arbitrary to fill the voids within the umbilical cross section and this causes difficulties with respect to implementation into a general finite element model. Common practice is to omit the filled bodies in cross section modeling by enabling direct contact between components. However, it has been found that the friction stress will be over estimated by this method and cause over-conservative fatigue calculations. This may be critical specially for deep water dynamic umbilicals and more accurate estimation of the friction stress is therefore needed. UFLEX2D is a non-linear finite element computer program for stress analysis of complex umbilical cross sections, see [3] and [5]. The model can handle arbitrary geometries wound in an arbitrary order including filled bodies. Contact elements are used to handle the contact between bodies due to external loading. Thin-wall shell elements were used to model the steel tubes while beam elements were used for the filled bodies in the earlier version of UFLEX2D. A beam element is treated as a rigid body incapable of deforming under external loading. It has been found that the formulation of the beam element for the filled bodies yields relatively large contact pressure for the neighboring element due to its rigidity. As a consequence, friction stress owing to the contact pressure is overestimated by the choice of the beam element for the filled bodies, however, it will be smaller than the direct contact modeling technique mentioned above. A new element type, i.e. a beam-shell element, has been developed to represent the filled bodies so as to improve the contact formulation between the filled bodies and the other surrounding structural elements. Unlike the beam element, the beam-shell element is able to deform, therefore the contact area is varying while the external load updates. The friction stress will be accordingly affected by the redistribution of the contact pressure on an updated contact area. The paper outlines how different implementations of the filled bodies will affect the distribution of the contact pressure as well as the friction stress under cyclical loading. The effect of the original contact area, as well as the development of the contact area is also a part of the study fot the three alternative models investigated.