Contract for the purchase and sale of natural gas entered into between Petrobrás and Coperbrás, which, by means of another contract, resells the product to Termopernambuco S.A. Contracts with arbitration clauses. Dispute between Coperbrás and Termopernambuco. Inclusion of Petrobrás in the arbitration proceedings as a "necessary co-defendant" denied in a partial award, with only simple participation in a proceeding in support of plaintiff or defendant admitted. Existence of a network of contracts and connection between the contracts that does not give rise to compulsory unitary joinder. Absence of Petrobrás from the arbitration proceedings that is not a sufficient ground for its annulment: