Flow Assurance Indicators for the Design of Long Subsea Tiebacks

Abstract

Abstract Failure to model representative pipeline geometries during the design phase of new multiphase pipeline projects can result in poor pipeline operating flexibility which could lead to deferred production and reduced ultimate hydrocarbon recovery. Detailed analyses of high resolution bathymetric data and out-of-straightness pipeline surveys have revealed that indicators can be practical flow assurance tools for the modeling of multiphase pipeline profiles:The Terrain Indicator can be used to quickly quantify the roughness of pipeline topographies and to anticipate the influence of route selection on hydrodynamic phenomena such as liquid accumulation and slugging.When only low resolution bathymetry is available, regional regressions of roughness indicators can be used to create pipeline geometries that are representative of the actual seabed roughness for a better prediction of operating envelopes.Roughness indicators can be used to fingerprint the roughness of existing pipeline topographies and to characterize geomorphic features of the seafloor. Reducing noise in raw survey data is essential for pertinent characterizations of the seafloor and pipeline profiles. Noise in data from seabed and out-of-straightness surveys can be reduced through a smoothing technique combining a moving average and a linear least squares fit. According to high resolution survey data, the span of this moving average should be equivalent to the length of one pipe joint; i.e. 40 feet or 12.2 meters. Before the completion of onerous Finite Element simulations for on-bottom roughness analyses, the same smoothing technique can be applied to the transformation of bathymetric data into representative pipeline geometry. The span of the moving average required for this transformation appears to be equivalent to the length of one double pipe joint. Two examples are presented to illustrate the application of flow assurance indicators to the design of long subsea tiebacks.