UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                                  SCHEDULE 13G
                    Under the Securities Exchange Act of 1934

                               (Amendment No. __)*

                                  Tenaris S.A.
--------------------------------------------------------------------------------
                                (Name of Issuer)

                                 Ordinary Shares
--------------------------------------------------------------------------------
                         (Title of Class of Securities)

                                  88031 M 10 9
               -------------------------------------------------
                                 (CUSIP Number)

                                December 31, 2002
--------------------------------------------------------------------------------
             (Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule
is filed:

[ ]   Rule 13d-1(b)

[ ]   Rule 13d-1(c)

[X]   Rule 13d-1(d)

*The remainder of this cover page shall be filled out for a reporting person's
initial filing on this form with respect to the subject class of securities, and
for any subsequent amendment containing information which would alter the
disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of
1934 ("Act") or otherwise subject to the liabilities of that section of the Act
but shall be subject to all other provisions of the Act (however, see the
Notes).




------------------------
 CUSIP NO. 88031 M 10 9
------------------------

--------------------------------------------------------------------------------
 1.   NAMES OF REPORTING PERSONS

      I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)

      ROBERTO ROCCA

--------------------------------------------------------------------------------
 2.   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)

                                                       (a) [ ]

                                                       (b) [ ]

--------------------------------------------------------------------------------
 3.   SEC USE ONLY

--------------------------------------------------------------------------------
 4.   CITIZENSHIP OR PLACE OF ORGANIZATION

      ITALY

--------------------------------------------------------------------------------
                        5.    SOLE VOTING POWER: 176,070

 NUMBER OF SHARES       -------------------------------------------
BENEFICIALLY OWNED      6.    SHARED VOTING POWER: 710,747,187
 BY EACH REPORTING
  PERSON WITH:          -------------------------------------------
                        7.    SOLE DISPOSITIVE POWER: 176,070

                        -------------------------------------------
                        8.    SHARED DISPOSITIVE POWER: 710,747,187

--------------------------------------------------------------------------------
 9.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING
      PERSON: 710,923,257

--------------------------------------------------------------------------------
10.   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES
      CERTAIN SHARES (SEE INSTRUCTIONS)
     ....                                              [ ]

--------------------------------------------------------------------------------
11.   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9): 61.25%

--------------------------------------------------------------------------------
12.   TYPE OF REPORTING PERSON (SEE INSTRUCTIONS): IN
--------------------------------------------------------------------------------


                               Page 2 of 22 pages


------------------------
 CUSIP NO. 88031 M 10 9
------------------------

--------------------------------------------------------------------------------
 1.   NAMES OF REPORTING PERSONS
      I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)

      ROCCA & PARTNERS S.A.

--------------------------------------------------------------------------------
 2.   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)

                                                       (a) [ ]

                                                       (b) [ ]

--------------------------------------------------------------------------------
 3.   SEC USE ONLY

--------------------------------------------------------------------------------
 4.   CITIZENSHIP OR PLACE OF ORGANIZATION

      BRITISH VIRGIN ISLANDS

--------------------------------------------------------------------------------
                        5.    SOLE VOTING POWER: NONE

 NUMBER OF SHARES       -------------------------------------------
BENEFICIALLY OWNED      6.    SHARED VOTING POWER: 710,923,257
 BY EACH REPORTING
  PERSON WITH:          -------------------------------------------
                        7.    SOLE DISPOSITIVE POWER: NONE

                        -------------------------------------------
                        8.    SHARED DISPOSITIVE POWER: 710,923,257

--------------------------------------------------------------------------------
 9.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING
      PERSON: 710,923,257

--------------------------------------------------------------------------------
10.   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES
      CERTAIN SHARES (SEE INSTRUCTIONS)
                                                       [ ]

--------------------------------------------------------------------------------
11.   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9): 61.25%

--------------------------------------------------------------------------------
12.   TYPE OF REPORTING PERSON (SEE INSTRUCTIONS): CO

--------------------------------------------------------------------------------


                               Page 3 of 22 pages


------------------------
 CUSIP NO. 88031 M 10 9
------------------------

--------------------------------------------------------------------------------
 1.   NAMES OF REPORTING PERSONS
      I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)

      SAN FAUSTIN N.V.

--------------------------------------------------------------------------------
 2.   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)

                                                       (a) [ ]

                                                       (b) [ ]

--------------------------------------------------------------------------------
 3.   SEC USE ONLY

--------------------------------------------------------------------------------
 4.   CITIZENSHIP OR PLACE OF ORGANIZATION

      NETHERLANDS ANTILLES


--------------------------------------------------------------------------------
                        5.    SOLE VOTING POWER: NONE

 NUMBER OF SHARES       -------------------------------------------
BENEFICIALLY OWNED      6.    SHARED VOTING POWER: 710,923,257
 BY EACH REPORTING
  PERSON WITH:          -------------------------------------------
                        7.    SOLE DISPOSITIVE POWER: NONE

                        -------------------------------------------
                        8.    SHARED DISPOSITIVE POWER: 710,923,257

--------------------------------------------------------------------------------
 9.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING
      PERSON: 710,923,257

--------------------------------------------------------------------------------
10.   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES
      CERTAIN SHARES (SEE INSTRUCTIONS)
                                                       [ ]

--------------------------------------------------------------------------------
11.   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9): 61.25%

--------------------------------------------------------------------------------
12.   TYPE OF REPORTING PERSON (SEE INSTRUCTIONS): CO

--------------------------------------------------------------------------------


                               Page 4 of 22 pages


------------------------
 CUSIP NO. 88031 M 10 9
------------------------

--------------------------------------------------------------------------------
 1.   NAMES OF REPORTING PERSONS
      I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)

      I.I.I. INDUSTRIAL INVESTMENTS INC.

--------------------------------------------------------------------------------
 2.   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)

                                                       (a) [ ]

                                                       (b) [ ]

--------------------------------------------------------------------------------
 3.   SEC USE ONLY

--------------------------------------------------------------------------------
 4.   CITIZENSHIP OR PLACE OF ORGANIZATION

      BRITISH VIRGIN ISLANDS

--------------------------------------------------------------------------------
                        5.    SOLE VOTING POWER: NONE

 NUMBER OF SHARES       -------------------------------------------
BENEFICIALLY OWNED      6.    SHARED VOTING POWER: 710,923,257
 BY EACH REPORTING
  PERSON WITH:          -------------------------------------------
                        7.    SOLE DISPOSITIVE POWER: NONE

                        -------------------------------------------
                        8.    SHARED DISPOSITIVE POWER: 710,923,257

--------------------------------------------------------------------------------
 9.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING
      PERSON: 710,923,257

--------------------------------------------------------------------------------
10.   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES
      CERTAIN SHARES (SEE INSTRUCTIONS)
                                                       [ ]

--------------------------------------------------------------------------------
11.   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9): 61.25%

--------------------------------------------------------------------------------
12.   TYPE OF REPORTING PERSON (SEE INSTRUCTIONS): CO

--------------------------------------------------------------------------------


                               Page 5 of 22 pages


------------------------
 CUSIP NO. 88031 M 10 9
------------------------

--------------------------------------------------------------------------------
 1.   NAMES OF REPORTING PERSONS
      I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)

      SIDERTUBES S.A.

--------------------------------------------------------------------------------
 2.   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)

                                                       (a) [ ]

                                                       (b) [ ]

--------------------------------------------------------------------------------
 3.   SEC USE ONLY

--------------------------------------------------------------------------------
 4.   CITIZENSHIP OR PLACE OF ORGANIZATION

      LUXEMBOURG

--------------------------------------------------------------------------------
                        5.    SOLE VOTING POWER: NONE

 NUMBER OF SHARES       -------------------------------------------
BENEFICIALLY OWNED      6.    SHARED VOTING POWER: 710,923,257
 BY EACH REPORTING
  PERSON WITH:          -------------------------------------------
                        7.    SOLE DISPOSITIVE POWER: NONE

                        -------------------------------------------
                        8.    SHARED DISPOSITIVE POWER: 710,923,257

--------------------------------------------------------------------------------
 9.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING
      PERSON: 710,923,257

--------------------------------------------------------------------------------
10.   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES
      CERTAIN SHARES (SEE INSTRUCTIONS)
                                                       [ ]

--------------------------------------------------------------------------------
11.   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9): 61.25%

--------------------------------------------------------------------------------
12.   TYPE OF REPORTING PERSON (SEE INSTRUCTIONS): CO

--------------------------------------------------------------------------------


                               Page 6 of 22 pages


Item 1.

      (a)   Name of Issuer:

            Tenaris S.A.

      (b)   Address of Issuer's Principal Executive Offices:

            23, Avenue Monterey
            L-2163 Luxembourg

Item 2.

      (a)   Name of Person Filing:

            Roberto Rocca
            Rocca & Partners S.A.
            San Faustin N.V.
            I.I.I. Industrial Investments Inc.
            Sidertubes S.A.

      (b)   Address of Principal Business Office or, if none, Residence:

            Roberto Rocca
            Av. Leandro N. Alem 1067, 29th floor
            Buenos Aires
            Argentina

            Rocca & Partners S.A.
            Vanterpool Plaza, 2nd Floor,
            Wickhams Cay I, Road Town, Tortola
            British Virgin Islands

            San Faustin N.V.
            Berg Arrarat 1
            Curacao
            Netherlands Antilles

            I.I.I. Industrial Investments Inc.
            Vanterpool Plaza, 2nd Floor,
            Wickhams Cay I, Road Town
            British Virgin Islands

            Sidertubes S.A.
            23 Avenue Monterey
            L-2086 Luxembourg

            Citizenship:

            See item 4 on page 2
            See item 4 on page 3
            See item 4 on page 4
            See item 4 on page 5
            See item 4 on page 6

      (c)   Title of Class of Securities:

            Ordinary Shares


                               Page 7 of 22 pages


      (d)   CUSIP Number:

            88031 M 10 9

Item 3. If this statement is filed pursuant toss.ss.240.13d-1(b) or 240.13d-2(b)
or (c), check whether the person filing is a: (Not Applicable)

      (a)   [ ]   Broker or dealer registered under section 15 of the Act (15
                  U.S.C. 78o);
      (b)   [ ]   Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);
      (c)   [ ]   Insurance company as defined in section 3(a)(19) of the Act
                  (15 U.S.C. 78c);
      (d)   [ ]   Investment company registered under section 8 of the
                  Investment Company Act of 1940 (15 U.S.C. 80a-8);
      (e)   [ ]   An investment adviser in accordance with
                  ss.240.13d-1(b)(1)(ii)(E);
      (f)   [ ]   An employee benefit plan or endowment fund in accordance with
                  ss.240.13d-1(b)(1)(ii)(F);
      (g)   [ ]   A parent holding company or control person in accordance with
                  ss.240.13d-1(b)(1)(ii)(G);
      (h)   [ ]   A savings association as defined in Section 3(b) of the
                  Federal Deposit Insurance Act (12 U.S.C. 1813);
      (i)   [ ]   A church plan that is excluded from the definition of an
                  investment company under section 3(c)(14) of the Investment
                  Company Act of 1940 (15 U.S.C. 80a-3);
      (j)   [ ]   Group, in accordance withss.240.13d-1(b)(1)(ii)(J).

Item 4. Ownership.

      Provide the following information regarding the aggregate number and
percentage of the class of securities of the issuer identified in Item 1.

      (a)   Amount beneficially owned:

            See item 9 on page 2
            See item 9 on page 3
            See item 9 on page 4
            See item 9 on page 5
            See item 9 on page 6

      (b)   Percent of class:

            See item 11 on page 2
            See item 11 on page 3
            See item 11 on page 4
            See item 11 on page 5
            See item 11 on page 6

      (c)   Number of shares as to which the person has:

(i) Sole power to vote or to direct the vote:

            See item 5 on page 2
            See item 5 on page 3
            See item 5 on page 4
            See item 5 on page 5
            See item 5 on page 6


                               Page 8 of 22 pages


(ii) Shared power to vote or to direct the vote:

            See item 6 on page 2
            See item 6 on page 3
            See item 6 on page 4
            See item 6 on page 5
            See item 6 on page 6

(iii) Sole power to dispose or to direct the disposition of:

            See item 7 on page 2
            See item 7 on page 3
            See item 7 on page 4
            See item 7 on page 5
            See item 7 on page 6

(iv) Shared power to dispose or to direct the disposition of:

           See item 8 on page 2
           See item 8 on page 3
           See item 8 on page 4
           See item 8 on page 5
           See item 8 on page 6


                               Page 9 of 22 pages


Item 5. Ownership of Five Percent or Less of a Class.

If this statement is being filed to report the fact that as of the date hereof
the reporting person has ceased to be the beneficial owner of more than five
percent of the class of securities, check the following [ ].

      Not applicable.

Item 6. Ownership of More than Five Percent on Behalf of Another Person.

      Not applicable.

Item 7. Identification and Classification of the Subsidiary Which Acquired the
Security Being Reported on By the Parent Holding Company.

      See Exhibit A.

Item 8. Identification and Classification of Members of the Group.

      Not applicable.

Item 9. Notice of Dissolution of Group.

      Not applicable.

Item 10. Certification.

      Not applicable.


                               Page 10 of 22 pages


                                    SIGNATURE

      After reasonable inquiry and to the best of my knowledge and belief, I,
Fernando Mantilla, attorney duly authorized by ROBERTO ROCCA to sign this
statement, certify that the information set forth in this Schedule 13G is true,
complete and correct.

      Pursuant to Rule 13d-1(k)(1), the undersigned joins in the filing of this
Schedule 13G on his own behalf and on behalf of ROCCA & PARTNERS S.A., SAN
FAUSTIN N.V., SIDERTUBES S.A. and I.I.I. INDUSTRIAL INVESTMENTS INC.

January 23, 2003

                                                       /s/ Fernando Mantilla
                                                       ---------------------
                                                         Fernando Mantilla
                                                         Attorney-in-fact


                               Page 11 of 22 pages


                                    SIGNATURE

      After reasonable inquiry and to the best of my knowledge and belief, I,
Fernando Mantilla, attorney duly authorized by ROCCA & PARTNERS S.A. to sign
this statement, certify that the information set forth in this Schedule 13G is
true, complete and correct.

      Pursuant to Rule 13d - 1(k)(1), the undersigned joins in the filing of
this Schedule 13G on his own behalf and on behalf of Roberto ROCCA, SAN FAUSTIN
N.V., SIDERTUBES S.A., I.I.I. INDUSTRIAL INVESTMENTS INC.

January 23, 2003

                                                       /s/ Fernando Mantilla
                                                       ---------------------
                                                         Fernando Mantilla
                                                         Attorney-in-fact


                               Page 12 of 22 pages


                                    SIGNATURE

      After reasonable inquiry and to the best of my knowledge and belief, I,
Fernando Mantilla, attorney duly authorized by SAN FAUSTIN N.V. to sign this
statement, certify that the information set forth in this Schedule 13G is true,
complete and correct.

      Pursuant to Rule 13d-1(k)(1), the undersigned joins in the filing of this
Schedule 13G on his own behalf and on behalf of Roberto ROCCA, ROCCA & PARTNERS
S.A., SIDERTUBES S.A., and I.I.I. INDUSTRIAL INVESTMENTS INC..

January 23, 2003

                                                       /s/ Fernando Mantilla
                                                       ---------------------
                                                         Fernando Mantilla
                                                         Attorney-in-fact


                               Page 13 of 22 pages


                                    SIGNATURE

      After reasonable inquiry and to the best of my knowledge and belief, I,
Fernando Mantilla, attorney duly authorized by I.I.I. Industrial Investments
Inc. to sign this statement, certify that the information set forth in this
Schedule 13G is true, complete and correct.

      Pursuant to Rule 13d-1(k)(1), the undersigned joins in the filing of this
Schedule 13G on his own behalf and on behalf of Roberto ROCCA, ROCCA & PARTNERS
S.A., SAN FAUSTIN N.V., and SIDERTUBES S.A.

January 23, 2003

                                                       /s/ Fernando Mantilla
                                                       ---------------------
                                                         Fernando Mantilla
                                                         Attorney-in-fact


                               Page 14 of 22 pages


                                    SIGNATURE

      After reasonable inquiry and to the best of my knowledge and belief, I,
Fernando Mantilla, attorney duly authorized by SIDERTUBES S.A. to sign this
statement, certify that the information set forth in this Schedule 13G is true,
complete and correct.

      Pursuant to Rule 13d -1(k)(1), the undersigned joins in the filing of this
Schedule 13G on his own behalf and on behalf of Roberto ROCCA, ROCCA & PARTNERS
S.A., SAN FAUSTIN N.V. and I.I.I. INDUSTRIAL INVESTMENTS INC.

January 23, 2003

                                                       /s/ Fernando Mantilla
                                                       ---------------------
                                                         Fernando Mantilla
                                                         Attorney-in-fact


                               Page 15 of 22 pages


                                  EXHIBIT INDEX

      Exhibit     Description

      A           Item 7 Information.
      B           Power of Attorney for Roberto Rocca, dated January 17, 2003.
      C           Power of Attorney for Rocca & Partners S.A., dated January 20,
                  2003.
      D           Power of Attorney for San Faustin N.V., dated January 17,
                  2003.
      E           Power of Attorney for I.I.I. Industrial Investments Inc.,
                  dated January 17, 2003.
      F           Power of Attorney for Sidertubes S.A., dated January 17, 2003.


                               Page 16 of 22 pages


                                                                       EXHIBIT A

                               ITEM 7 INFORMATION

Roberto Rocca controls a majority of the voting power of Rocca & Partners S.A.
Roberto Rocca and Rocca & Partners share majority voting power over San Faustin
N.V. Sidertubes S.A. is a wholly-owned subsidiary of I.I.I. Industrial
Investments Inc., which is a wholly-owned subsidiary of San Faustin N.V.

The shares of Tenaris S.A. to which this statement relates are held directly by
Roberto Rocca, San Faustin N.V. and Sidertubes S.A.


                               Page 17 of 22 pages


                                                                       EXHIBIT B

                          [LETTERHEAD OF ROBERTO ROCCA]

                                POWER OF ATTORNEY

The undersigned, Roberto Rocca, an Italian citizen resident in the Argentine
Republic, hereby constitutes and appoints Carlos Manuel Franck, Fernando R.
Mantilla and Carlos E. Zandona, and each of them, as attorneys or attorney of
the undersigned with full power of substitution and resubstitution, to sign on
its behalf a Statement on Schedule 13G relating to the Common Stock, of TENARIS
S.A., including any agreements attached as exhibits thereto, and to sign any or
all amendments to such Statement, and to file the same, with all exhibits
thereto, and other documents in connection therewith, with the Securities and
Exchange Commission, granting unto said attorneys or attorney, and each of them,
full power and authority to do and perform each and every act and thing
requisite and necessary to be done in and about the premises, hereby ratifying
and confirming all that said attorneys or attorney, or any of them or their
substitute or substitutes, may lawfully do or cause to be done by virtue hereof.

By: /s/ Roberto Rocca
    -----------------
       Roberto Rocca

Dated: January 17, 2003


                               Page 18 of 22 pages


                                                                       EXHIBIT C

                      [LETTERHEAD OF ROCCA & PARTNERS S.A.]

                               POWER OF ATTORNEY

      The undersigned, Roberto Rocca, President of ROCCA & PARTNERS, a company
organized under the laws of the British Virgin Islands, hereby constitutes and
appoints Carlos Manuel Franck, Fernando R. Mantilla and Carlos E. Zandona, and
each of them, as attorneys or attorney of the undersigned with full power of
substitution and resubstitution, to sign on its behalf a Statement on Schedule
13G relating to the Common Stock, of TENARIS S.A., including any agreements
attached as exhibits thereto, and to sign any or all amendments to such
Statement, and to file the same, with all exhibits thereto, and other documents
in connection therewith, with the Securities and Exchange Commission, granting
unto said attorneys or attorney, and each of them, full power and authority to
do and perform each and every act and thing requisite and necessary to be done
in and about the premises, hereby ratifying and confirming all that said
attorneys or attorney, or any of them or their substitute or substitutes, may
lawfully do or cause to be done by virtue hereof.

By: /s/ Roberto Rocca
    -----------------
       Roberto Rocca

Dated: January 20, 2003


                               Page 19 of 22 pages


                                                                       EXHIBIT D

                        [LETTERHEAD OF SAN FAUSTIN N.V.]

      POWER OF ATTORNEY

The undersigned, Roberto Bonatti, President of SAN FAUSTIN N.V., a company
organized under the laws of Curacao, hereby constitutes and appoints Carlos
Manuel Franck, Fernando R. Mantilla and Carlos E. Zandona, and each of them, as
attorneys or attorney of the undersigned with full power of substitution and
resubstitution, to sign on its behalf a Statement on Schedule 13G relating to
the Common Stock, of TENARIS S.A., including any agreements attached as exhibits
thereto, and to sign any or all amendments to such Statement, and to file the
same, with all exhibits thereto, and other documents in connection therewith,
with the Securities and Exchange Commission, granting unto said attorneys or
attorney, and each of them, full power and authority to do and perform each and
every act and thing requisite and necessary to be done in and about the
premises, hereby ratifying and confirming all that said attorneys or attorney,
or any of them or their substitute or substitutes, may lawfully do or cause to
be done by virtue hereof.

By: /s/ Roberto Bonatti
    -------------------
    Roberto Bonatti
    President

Dated: January 17, 2003


                               Page 20 of 22 pages


                                                                       EXHIBIT E

               [LETTERHEAD OF I.I.I. INDUSTRIAL INVESTMENTS INC.]

                                POWER OF ATTORNEY

      The undersigned, Roberto Bonatti, President of I.I.I. INDUSTRIAL
INVESTMENTS Inc., a company organized under the laws of the British Virgin
Islands, hereby constitutes and appoints Carlos Manuel Franck, Fernando R.
Mantilla and Carlos E. Zandona, and each of them, as attorneys or attorney of
the undersigned with full power of substitution and resubstitution, to sign on
its behalf a Statement on Schedule 13G relating to the Common Stock, of TENARIS
S.A., including any agreements attached as exhibits thereto, and to sign any or
all amendments to such Statement, and to file the same, with all exhibits
thereto, and other documents in connection therewith, with the Securities and
Exchange Commission, granting unto said attorneys or attorney, and each of them,
full power and authority to do and perform each and every act and thing
requisite and necessary to be done in and about the premises, hereby ratifying
and confirming all that said attorneys or attorney, or any of them or their
substitute or substitutes, may lawfully do or cause to be done by virtue hereof.

By: /s/ Roberto Bonatti
    -------------------
        Roberto Bonatti
        President

Dated: January 17, 2003


                               Page 21 of 22 pages


                                                                       EXHIBIT F

                         [LETTERHEAD OF SIDERTUBES S.A.]

                                POWER OF ATTORNEY

      The undersigned, Paolo Rocca, President of SIDERTUBES S.A., a company
organized under the laws of Luxembourg, hereby constitutes and appoints Carlos
Manuel Franck, Fernando R. Mantilla and Carlos E. Zandona, and each of them, as
attorneys or attorney of the undersigned with full power of substitution and
resubstitution, to sign on its behalf a Statement on Schedule 13G relating to
the Common Stock, of TENARIS S.A., including any agreements attached as exhibits
thereto, and to sign any or all amendments to such Statement, and to file the
same, with all exhibits thereto, and other documents in connection therewith,
with the Securities and Exchange Commission, granting unto said attorneys or
attorney, and each of them, full power and authority to do and perform each and
every act and thing requisite and necessary to be done in and about the
premises, hereby ratifying and confirming all that said attorneys or attorney,
or any of them or their substitute or substitutes, may lawfully do or cause to
be done by virtue hereof.

By: /s/ Paolo Rocca
    ---------------
        Paolo Rocca
        President

Dated: January 17, 2003


                               Page 22 of 22 pages