Need help in render rdlc in multiple worksheets

Question

protected void BtnSubmitGSFC_Click(object sender, EventArgs e) { if (ddlGFSCCargo.SelectedValue == "00") { ClientScript.RegisterStartupScript(this.GetType(), "SAVE", "alert('Please Select Cargo');"); return; } string url; int CargoGSFC = int.Parse(ddlGFSCCargo.SelectedValue); System.Data.DataSet DS = null; System.Data.DataSet DS1 = null; SqlDataAdapter Cont = new SqlDataAdapter(ConfigurationManager.AppSettings["VslID"], ConfigurationManager.ConnectionStrings["Connectionstring"].ConnectionString); System.Data.DataTable dt0; dt0 = new System.Data.DataTable(); Cont.Fill(dt0); Microsoft.Office.Interop.Excel.Application app = null; app = new Microsoft.Office.Interop.Excel.Application(); app.Workbooks.Add(""); int TotalRow = dt0.Rows.Count; string[] res = new string[TotalRow];
        for (int i = 0; i < TotalRow; i++)
        {
            try
            {
                ReportViewer viewer = new ReportViewer();
                LocalReport report = new LocalReport();
                SQLHelper objSQL = new SQLHelper();
                SqlParameter[] SqlParam1 = new SqlParameter[2];
                SqlParameter[] SqlParam2 = new SqlParameter[1];
                SqlParam1[0] = new SqlParameter("@VesselID", dt0.Rows[i][0].ToString());
                SqlParam1[1] = new SqlParameter("@Cargo", CargoGSFC);
                DS = objSQL.ExecuteDataSet("SP_GFSC_Daily_Rpt", SqlParam1);
                viewer.LocalReport.ReportPath = Server.MapPath("~/RDLC/GFSC_Rpt.rdlc");
                SqlParam2[0] = new SqlParameter("@VesselID", dt0.Rows[i][0].ToString());
                DS1 = objSQL.ExecuteDataSet("Sp_GSFC_Reg_Select", SqlParam2);
                viewer.LocalReport.ReportPath = Server.MapPath("~/RDLC/GFSC_Rpt.rdlc");
                ReportDataSource reportDSDetail1 = new ReportDataSource("GFSC", DS.Tables[0]);
                viewer.LocalReport.DataSources.Add(reportDSDetail1);
                ReportDataSource reportDSDetail2 = new ReportDataSource("Rpt_Gspc", DS1.Tables[0]); viewer.LocalReport.DataSources.Add(reportDSDetail2);
                FileStream fs = null;
                Warning[] warnings;``
                string[] streamIds;
                string contentType;
                string encoding;
                string extension;
                //create file stream in create mode
                fs = new FileStream(DS.Tables[0].Rows[0]["VesselID"] + "_GSFC.xls", FileMode.Create);
                byte[] bytes = viewer.LocalReport.Render("EXCEL", null, out contentType, out encoding, out extension, out streamIds, out warnings);
                           
                //create Excel file
                fs.Write(bytes, 0, bytes.Length);
                fs.Close();
                fs = null;