PROVIDING FOR THE FULFILLMENT BY THE NATIONAL POWER CORPORATION OF ITS OBLIGATIONS UNDER THE AGREEMENT FOR THE SALE AND PURCHASE OF NATURAL GAS DATED DECEMBER 30, 1997 WITH SHELL PHILIPPINES EXPLORATION B.V./OCCIDENTAL PHILIPPINES, INC. AND THE COMPLIANCE OF THE NATIONAL GOVERNMENT, THROUGH THE DEPARTMENT OF FINANCE AND THE DEPARTMENT OF ENERGY WITH ITS PERFORMANCE UNDERTAKING THEREFOR AND OTHER PURPOSES