A Study of Riser Oscillations Caused by Slug Flows During Subsea Petroleum Production

Abstract

The theoretical influence of the internal slug flows on the motion of suspended slender pipes is presented here through two examples of computer simulations applied to two types of risers employed in offshore petroleum production – a steel catenary riser (SCR) and a steel lazy wave riser (SLWR) hanging from a floating platform. The results presented here provide strong evidence that slug flows may induce hazardous oscillation, even in the absence of other excitations. The exploitation of hydrocarbons from underground reservoirs in open sea is undertaken amidst very adverse environmental conditions, below ultra deep waters and usually very deep reservoirs, as in the pre-salt and carbonate formations discovered in the brazilian southeast atlantic coast basin. In this scenario, large production rates of oil and gas are required to achieve economically feasible operations; which means successfully maintaining an uninterrupted flow of reservoir fluids under safe conditions from many subsea wells. The multiphase petroleum flowing from the wellhead at the sea bottom to the floating process facility at the sea surface is conducted by lengthy pipelines lying on the seabed up to the point where the flow ascends to the sea level through very long and slender pipes – named risers – suspended by its top end at the floating production unit. Risers may be flexible or made from steel rigid pipes and are often subjected to sea current and wave loads and to the movements of its supporting ship or platform, which are dangerous to the pipe integrity. Risers are also excited to oscillate due to the multiphase flow inside them. It is of paramount importance to understand and determine the dynamic behavior of the suspended pipes due to those loads, in order to achieve a service life as extensive as economically possible. The method employed in the present research consists of modifying a computer dynamic simulator of pipeline movement, adding the internal fluid loads arising due to the weight and momentum of the flowing masses of liquid and gas. The simulations show that a steady slug flow may alone produce oscillatory displacements of the riser of rather significant amplitudes, without any other external cause; particularly when the slug’s excitation is near the natural frequencies of the riser. The results indicate that the oscillations are strongly dependent on the slug frequency. The modified simulator is flexible enough to compute the effect of a wide range of slugs’ configurations (geometry, fluids, flow rates etc.), allowing the analysis of fatigue damage and risk of failure under many production conditions.