Impact analysis of flexible riser

Abstract

Research shows 35% of damage to flexible pipes occurs within the first year. A minimum of these failures are from poor design or manufacturing problems, since factories conduct Factory Acceptance Tests (FAT). Additionally, the installation phase is a critical stage of the service life. For instance, in the installation phase the flexible pipe can be exposed for impact from dropped tools. In worst case an impact of flexible pipes result in an environmental pollution. Hence it is important to assess the impact resistance of flexible pipes. The thesis work is devised for NOV Flexibles in Denmark, which produces flexible pipes. An experiment performed by NOV Flexibles shows the current simplified analytical method utilized by NOV Flexibles is significantly conservative. Hence NOV Flexibles has requested a less conservative method to estimate the impact resistance of their flexible pipes. Through the thesis work three different approaches have been utilized to assess the impact capacity of the 16" Production Jumper manufactured by NOV Flexibles. All the analyses are a part of the accidental limit state design. First a simplified analytical analysis is utilized. The analysis is based on the theory for dented tubular members. A diamond shaped yield line model is utilized in the analysis. The experiment performed by NOV Flexibles is regenerated by finite element models. Two commercial programs are utilized for the simulations; BFLEX2010 and LS-DYNA. The model in LS-DYNA is more detailed compared to the model conducted in BLFEX2010. All the layers except the tapes are included in the LS-DYNA model. The carcass and the pressure armour are simplified. They are modeled as a shell structure and special orthotropic material is utilized for these layers. The BFLEX2010 model is modeled with beam elements. In this model the layers are assembled in three layers; two tensile layers and the core. In both models the indenter, the 6" Multi Purpose Riser, is modeled as a rigid pipe. In harsh weather accidents to the whole semi-submersible may occur. As a result of harsh weather the semi-submersible may get an inclination. With an inclination of the semi-submersible and forces from the waves and current, the flexible riser can clash into the pontoon. This impact scenario is analyzed and is referred to as "platform impact scenario" in the thesis. A beam model in BFLEX2010 is utilized for the analysis. The principle of the model is similar to the BFLEX2010 model of the experiment.