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      Subsea Control Systems


    Process IT's engineers have been involved with a number of subsea control systems in the North Sea and Australia:
    • Shell, Gannet (UK Sector, North Sea)
    • Norsk Hydro, Troll B (Norwegian Sector, North Sea)
    • Esso, Blackback (Australia, Bass Strait)
    • Woodside Energy, Laminaria (Australia, Timor Sea)
    • Norsk Hydro, Tune (Norwegian Sector, North Sea)
    We provide expertise in developing solutions for various aspects of subsea production:

    • Well Flowrate Calculations
    • Hydrate Alarming

     Well Flowrate Calculations 
    Calculating the well mass flowrate is usually performed using one of the following techniques:

    • Table Lookup (Well Performance Tables)
    • Curve-fitting (Well Performance Curves)
    In both cases, the main input to the calculation is the wellhead pressure (also known as the tubing head pressure). If wellbore gas lift is being employed then the flowrate of the lift gas is also required.

    Performance Tables
    These plot tubing head pressure versus mass flowrate. If gas lift is being used, then the massflow-THP curves are given for a number of different gas lift flowrates.

    The data shown in the table is derived from well tests, although there are usually only a few data sets obtained through normal well testing. To generate data for the well performance tables, the well test data is typically used to model the well, for example with Pipesim.
    The well performance table can be generated in a straightforward manner.

    Performance Curves
    As an alternative to using a table, it it is possible to use a simple equation to calculate mass flowrate as a function of wellhead pressure. This often involves using a least-squares minimisation to fit a quadratic, or a function of the form yn, to the available well test data.

    While this can be easy to implement, it is really only suitable for wells which are not gas lifted. Once gas lift is introduced, well performance tables provide a much better solution.

     Hydrate Alarming 
    Hydrates are solid, semi-stable water-based compounds which can form in oil and gas pipelines (1). When formed, they can block the pipeline and can be difficult to remove.

    From an operational perspective, hydrates are best avoided. The usual means of avoiding hydrate formation is to alert the operator when the operating conditions in the pipeline are approaching a region conducive to hydrate formation.

    As hydrates tend to be formed under high pressure, hydrate removal often involves depressuring the pipeline. If the pipeline is only partially blocked, another possibility to inject methanol upstream of the hydrate.

    1. Pedersen et al, Properties of Oils and Natural Gases, Gulf Publishing

    The hydrate formation temperatures are typically stored in the Distributed Control System (DCS), so that an alarm can be raised when the actual subsea temperature (typically a manifold temperature) approaches this hydrate formation temperature.