For buried hot oil pipelines that are either newly constructed or restarted after a long shutdown, a preheating medium should be used before transporting hot crudes to create or to maintain a smooth temperature field between the pipe and the surrounding soil. Hot water usually serves as the preheating medium because of its high specific heat capacity, low viscosity, and easy accessibility. Preheating is a complex process incorporating unstable heat transfer among the preheating medium, pipe walls, soil, and atmosphere, for example. The version of the Olga software - the industry standard for dynamic multiphase flow simulation - including a finite-element method thermal (FEMTherm) module is used to develop a model for simulating the hot water based preheating process of two subsea steel pipelines in an offshore field. The pipelines are connected to each other by one offshore platform and being part of a system transporting oil from offshore platforms to an onshore terminal. The pipelines needed to be preheated before being put into service. The FEMTherm module of Olga comprises a grid generator and a finite-element method (FEM) equation solver. The grid generator creates a 2D unstructured triangular mesh in accordance with the Delaunay criterion for discretizing the computational region. The FEM equation solver is used to solve the heat transfer equations using the finite element method. Three equations govern the mathematical model of dynamic heat transfer for the fluid, pipe walls, and the solid medium, respectively. For the double insulated pipe described in the article, the inner steel pipe, insulation, air gap, and outer steel pipe are taken into account. Other equations describe the interface between the fluid and inner pipe wall, the interface between the pipe wall layers, the interface between the outermost pipe wall and the surrounding soil, and the boundaries of the computational domains. Additional, parameters related to depth of ground cover are mathematically described. The combination of fluid and wall temperature equations along the pipelines and the heat-transfer equations in each of the cross sections results in a three-dimensional temperature field in FEMTherm. FEM is used to discretize the soil region, the heat transfer equations are coupled to the flow equations and solved by the FEM equation solver, and the three-dimensional temperature field is determined. The verification and validation of the used model and a comparison of preheating effects of different operational parameters (e.g. water inlet temperature, water flowrate) allowing for optimizing the preheating plan conclude the article. It was found that a higher flow rate at lower temperatures could more effectively preheat subsea crude oil pipelines than currently used techniques. A 350-cu m/hr flow rate combined with an inlet temperature of 65.2 C was most efficient for the pipelines studied, lasting half as long as a 150-cu m/hr flow rate combined with an 81.6 C inlet temperature.


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    Titel :

    Study improves subsea pipeline preheating


    Beteiligte:
    Yuan, Zongming (Autor:in) / Wang, Yong (Autor:in) / Xie, Ying (Autor:in) / Dai, Kemin (Autor:in) / Ming, Liang (Autor:in)

    Erschienen in:

    Oil and Gas Journal ; 112 , 12 ; 88-95


    Erscheinungsdatum :

    2014


    Format / Umfang :

    8 Seiten, Bilder, Tabellen, 20 Quellen



    Medientyp :

    Aufsatz (Zeitschrift)


    Format :

    Print


    Sprache :

    Englisch




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