Hydraulic Evaluation of Transport Gas Pipeline on Offshore Production
Authors: Nava Zhaira Elena , Rojas Marisela , Martinez Marcano Nelson Gabriel , Trujillo Jorge Nicolas , Rigual Yobiris Celia , Gonzalez Camilo 
Published: May 2016

Gas production has increased due to increasing in demand of energy worldwide. Flow assurance studies in transport systems related to offshore pipelines must be performed since the design in order to reduce operational costs and failures. This paper presents the flow assurance study of a hydrocarbon pipeline of 120 km, which transports 650 MMscfd of gas production from offshore fields to onshore facilities. Fluidynamics parameters were studied in order to optimize internal diameter and process equipment required for handling of liquids present in the transport systems.

Design parameters were evaluated, such as: fluid, gas superficial velocity and liquid erosion rate, pressure drop and temperature. The selection of the optimal diameter pipe from the point of hydraulic and thermal focused on one export system that meets the most technical aspects of flow assurance established for the study, with the 28" internal diameter pipe that presented those operational conditions.

The designed and applied methodology in this paper was aimed to optimize the transport scheme of offshore gas production based on a compositional and thermodynamic analysis of the feed gas and liquids streams flowing throught the system as well as a compilation and analysis of information required to define the numerical simulation model in steady and transient states. In this sense, a sensitivity analysis was conducted at operating conditions: gas and liquid flow, pressure, temperature, pipe diameters, and the hydrate formation curve to establish safe operating limits in order to minimize potential flow restriction problems, plugging and damage to the pipeline and process equipment, thus ensuring reliability in the desing of the pipeline.

Introduction

Transportation of natural gas through subsea pipelines could be affected by problems of flow blockage. Low temperature, long transport distances, fluid composition bathymetry at offshore environments influences multiphase flow behaviour. The economic viability of an offshore project is mainly subject to the costs of drilling and pipelines. The subsea pipelines represent at least 25% of the total project cost and this is one of the reasons for the flow assurance studies.

Prediction and mitigation of flow blockage in pipelines is part of a flow assurance philosophy and should be applied on from the beggining phase of a project in order to minimize operational problems, increasing guarantee of having a continuous transport of a multiphase flow throughout the productive life of wells.

Flow assurance is a thermodynamic and hydraulic analysis used to develop control strategies to avoid flow stoppage by solid formations such as hydrates, asphaltenes, scale, paraffin, and corrosion problems. Furthermore, this analysis allows establishing operational strategies, based on thermal behavior of the system, at different stages of production: startup, restart, shutdown, warm up, cooldown, etc.

In this study, flow assurance philosophy is applied in an export pipeline of gas and liquid with the aim of selecting the internal diameter that optimize hydraulic and thermal fluid behaviour allowing achieve maximum recovery of gas and liquid as possible by maximizing the energy reservoir. The analysis takes into account the accumulation of liquid, the erosion ratio, superficial gas velocity, superficial liquid velocity, pressure drop and temperature. Sensitivities are also performed with the production profile (gas / water / condensates) of fields and different pipe diameters.