PRESSURE BUILD-UP IN MULTI LAYER FLEXIBLE PIPE BARRIER DURING SHUT DOWN

摘要

Flexible risers for high temperature service involve a multi layer design for the barrier. A sacrificial layer is extruded on top of the anti collapse carcass before the barrier is applied. In some cases a second sacrificial layer is extruded on top. These layers are not bonded. During operation of the flexible riser, gas (e.g. CH_4, CO_2) dissolves in the sacrificial layers and the barrier until these polymer layers are saturated. Upon shut down of the pipe, the dissolved gas diffuses out of the polymer into the annulus and the bore of the pipe and into the gaps between the different layers. If the gaps are not vented this leads to a pressure build up which may cause collapse of the carcass. A two-dimensional diffusion model has been developed to predict the pressure build up in multi layer pipes between the sacrificial layers and the barrier. The model accounts for temperature gradients across the layers and shielding effects of steel wires. In a first step the concentration profiles across the polymer layers are calculated as a function of time for a particular gas until saturation is reached. In the second step the boundary conditions are changed to shut down conditions and the concentration profiles are again calculated as a function of time.