ORS § 777.470
Upon issuance of any refunding bonds the port board shall levy a tax sufficient to pay the principal and interest of such bonds at maturity. If the bonds are serial bonds the tax shall provide an amount of money sufficient to pay the next maturing installment of principal and the interest on the entire issue. If the bonds are issued as term bonds the tax shall be sufficient to raise an amount of money, which, if the same amount were raised each year thereafter for the life of the bonds, would produce a sum equal to the principal amount of the bonds so issued. The proceeds of taxes levied under this section shall be set aside when collected into a special fund and used for no other purpose than the payment of the bonds so issued.
ORS 777.470