Arnaldo Papaleo’s research while affiliated with U.S. Army Engineer Research and Development Center and other places

Response based approaches are not common in riser design. Due to the high computational costs associated to these methodologies, it is usual to replace the calculation of extreme long term responses by the calculation of responses to a few number of artificial sea states, supposed extreme. However, this hypothesis may not always be applicable. The extreme response of a riser is influenced by several factors. For instance, vessel response motions resonance can occur for waves of periods lower than the ones associated to the desired long term period. In this way, this work has two main objectives. The first is to propose a computationally feasible methodology to calculate long term extreme responses; the second is to calibrate loading conditions, based on the long term responses, to be used when designing catenary risers. The parameter selected to represent the response is the centenary (100y) riser top tension. The utilization of the proposed methodology is illustrated by a case study where three possible positions for a turret in a FPSO hull were compared. The obtained results indicate that this methodology can contribute to substantial changes in the way risers are designed, focusing on the response instead of on the occurrence of extreme sea states.

Even though hydraulic collars are largely used to install flexible risers, neither the loads imposed by this equipment nor the response of the risers to these loads have been previously studied. Hence, this paper presents a three-dimensional nonlinear finite element (FE) model to predict the response of flexible risers to loads imposed by hydraulic collars and also provides a set of equations to predict these loads. The FE model relies on an analogy between helical tendons and orthotropic shells to simulate the inner carcass and the pressure armour of flexible risers. Shell elements are used to represent the polymeric layers and three-dimensional beam elements simulate the wires of the tensile armours. Material, geometric and contact nonlinearities are addressed. Contact interactions between the layers of the riser are handled by surface to surface contact elements with a contact detection algorithm based on the pinball technique and contact forces evaluated with the augmented Lagrangian method. A 9.5″ flexible riser is analyzed and the numerical results are compared to those from the experimental tests. The agreement between all results indicates that the proposed FE model is an efficient approach to predict the response of flexible risers to loads imposed by hydraulic collars and, moreover, may be used to analyze the response of such structures to other types of loads.

The joint probabilistic models (JPM) of the environmental parameters of wave, wind and current are nowadays extremely needed in order to perform reliability analyses of offshore structures. These JPM are also essential steps for the design of offshore structures based on long-term statistics and to perform dynamic response analysis of floating units that are strongly dependent on the directionality of the environmental actions, such as turret-moored FPSOs. Recently, some JPM have been proposed in the literature to represent the joint statistics of a reduced number of environmental parameters. However, it is difficult to find a practical and fully operational model taking into account the complete statistical dependence among all the environmental parameters intensities and their correspondent directions. In this paper, it is presented a straightforward methodology, based on the Nataf transformation, to create a JPM of the environmental parameters taking into account the dependence between the intensity and direction of all variables. The proposed model considers the statistical dependence of ten short-term variables: the significant wave height, peak period and direction of the sea waves, the significant wave height, peak period and direction of the swell waves, the amplitude and direction of the 1-h wind velocity and, finally, the amplitude and direction of the surface current velocity. The statistical dependence between them is evaluated using concepts of linear-linear, linear-circular and circular-circular variables correlation. Some results of the proposed JPM methodology are presented based on simultaneous environmental data gathered in a location offshore Brazil.