def estimate_u_value(d_i):
    """
    Estimate U-value (in W/mK) depending on inner pipe diameter d_i.
    Estimation based on values by: U-values: C. Beier, S. Bargel,
    C. Doetsch, LowEx in der Nah- und Fernwaerme. Abschlussbericht, 2010.

    Parameters
    ----------
    d_i : float
        Inner diameter of pipe in meters

    Returns
    -------
    u_pipe : float
        U-value of pipe in W/mK
    """

    u_pipe = 0.9264 * d_i ** 0.501

    return u_pipe