Ultra-long subsea gas condensate tie back - Pseudo dry gas - Liquid handling system

Abstract

The Pseudo Dry Gas (PDG) technology / concept has been demonstrated for transporting wet gas in a long subsea tieback pipeline (200 km) in deep water depths (1.8 km) under wet gas conditions (water saturated gas) [Ref.1] along with a state of the art technology review of existing solutions. When a multiple of these in-line / piggable liquid removal units are used, they help to reduce the well back pressure by reducing the liquid content to an extent where 'dry gas' pressure losses are seen. Therefore, this mitigation of the gravitational pressure drop allows the use of larger pipelines to minimise the frictional pressure drop. This in turn increases recovery of reserves and allows tie back distances to be enhanced. The objective of this paper is to investigate a Pseudo Dry Gas System (PDGS) for an ultra-long deepwater gas condensate development, building upon the research and development already conducted with Strathclyde University. This work was undertaken using non-standard flow assurance methodologies and simulations recycling data and results with the advanced Computational Fluid Dynamics simulations of the liquid removal units behaviour, over various operational boundary conditions. Engagement with subsea equipment suppliers based on the flow assurance results has been undertaken. This paper describes how gas condensates within a subsea tieback system behave very differently to condensed water from a wet gas system and therefore a pseudo dry gas system needs to be configured differently for gas condensate developments. These differences include how and where the liquid drops out of the gas phase, where and if the free liquid is reabsorbed back into the gas stream and how the bubble point of condensate is equal to or very close to liquid removal units operating pressure; this greatly impacts the liquid handling system compared to a wet gas (water) design. Therefore, to ensure controlled liquid only transportation, careful examination of the liquid removal units performance, the liquid pump selection criteria and optimisation of the system needs to be undertaken. This results in a trade-off between maximum reserve recovery and system complexity. The paper demonstrates that the liquid condensate system will remain as a single liquid phase pipeline, where the number of pumps can be reduced and the pump power requirements are very low and within the existing technically qualified subsea pumps.