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Note 30 - Principal Subsidiaries - Percentage of Ownership of Each Controlled Company (Details) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
HYDRIL COMPANY and subsidiaries [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership |  | 100.00% |  |
CANADA | ALGOMA TUBES INC. [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 100.00% | 100.00% | 100.00%
CANADA | PRUDENTIAL STEEL LTD. [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership |  | 100.00% | 100.00% | 100.00%
CANADA | TENARIS GLOBAL SERVICES (CANADA) INC. [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 100.00% | 100.00% | 100.00%
BRAZIL | CONFAB INDUSTRIAL S.A. and subsidiaries [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 100.00% | 100.00% | 100.00%
ITALY | DALMINE S.p.A. [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 100.00% | 100.00% | 99.00%
UNITED STATES | HYDRIL COMPANY and subsidiaries [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1],[2] | 100.00% | 100.00% | 100.00%
UNITED STATES | MAVERICK TUBE CORPORATION and subsidiaries [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 100.00% | 100.00% | 100.00%
UNITED STATES | TENARIS BAY CITY, INC. [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 100.00% | 100.00% | 100.00%
UNITED STATES | TENARIS GLOBAL SERVICES (U.S.A.) CORPORATION [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 100.00% | 100.00% | 100.00%
JAPAN | NKKTUBES [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 51.00% | 51.00% | 51.00%
INDONESIA | PT SEAMLESS PIPE INDONESIA JAYA [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership |  | 89.00% | 77.00% | 77.00%
ROMANIA | S.C. SILCOTUB S.A. [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 100.00% | 100.00% | 100.00%
ARGENTINA | SIAT SOCIEDAD ANONIMA [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 100.00% | 100.00% | 100.00%
ARGENTINA | SIDERCA S.A.I.C. and subsidiaries [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership |  | 100.00% | 100.00% | 100.00%
PORTUGAL | TALTA—TRADING E MARKETING SOCIEDADE UNIPESSOAL LDA. [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 100.00% | 100.00% | 100.00%
URUGUAY | TENARIS FINANCIAL SERVICES S.A. [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 100.00% | 100.00% | 100.00%
URUGUAY | TENARIS GLOBAL SERVICES S.A. and subsidiaries [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1],[3] | 100.00% | 100.00% | 100.00%
UNITED KINGDOM | TENARIS GLOBAL SERVICES (UK) LTD [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 100.00% | 100.00% | 100.00%
LUXEMBOURG | TENARIS INVESTMENTS S.àr.l. [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 100.00% | 100.00% | 100.00%
SWITZERLAND | TENARIS INVESTMENTS SWITZERLAND AG and subsidiaries [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 100.00% | 100.00% | 100.00%
NETHERLANDS | TENARIS CONNECTIONS BV [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership |  | 100.00% | 100.00% | 100.00%
COLOMBIA | TENARIS TUBOCARIBE LTDA. [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 100.00% | 100.00% | 100.00%
MEXICO | TUBOS DE ACERO DE MEXICO S.A. [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Percentage of ownership | [1] | 100.00% | 100.00% | 100.00%
[1] | All percentages rounded.
[2] | Tenaris Investments S.a.r.l. holds 100% of Hydril's subsidiaries shares except for Technical Drilling & Production Services Nigeria. Ltd where it holds 80% for 2017, 2016 and 2015.
[3] | Tenaris holds 97,5% of Tenaris Supply Chain S.A, 60% of Gepnaris S.A. and 40% of Tubular Technical Services and Pipe Coaters, and 49% of Amaja Tubular Services Limited.

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