A SEMI-ANALYTICAL METHODOLOGY TO EVALUATE LONG-TERM RESPONSES AT THE TDZ OF CATENARY RISERS

OSCAR MARROQUIN¹; FERNANDO SOUSA¹; LUÍS SAGRILO¹

¹ UFRJ

doi:10.20906/CPS/CILAMCE2015-0499

Resumo

Response based approaches are not common in the design of risers. 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. For instance, designers commonly use combinations of environmental loads such as i) 100y waves and winds plus 10y currents, and ii) 10y waves and winds plus 100y currents. In 2001, however, the DNV standard OS-F201 presented a methodology based on structural reliability, known as long-term response statistics, which is supposed to be more adequate to fit the riser design to a pre-established level of failure probability. The utilization of this methodology is limited, in most cases, by its high computational costs, because time-domain simulations may be computationally very expensive. An alternative to reduce this cost is the utilization of simplified analytical models; in this way, the objective of this work is to detail a semi-analytical model used to obtain short-term bending moments and tensions at the TDZ, based on models proposed by ARANHA et al. (1997). This analytical model can then be used in the evaluation of long term utilization factors for SCRs. The utilization of the semi-analytical model is exemplified by a case study where a rigid riser connected to a spread moored FPSO is analyzed. The obtained results indicate that this methodology can contribute to simplify the way risers are designed, focusing on the response instead of on the occurrence of extreme sea states.

Palavras-chave: Riser design; bending moments at the TDZ; long-term responses