Sedgwick Group P.L.C. is expanding its South American operations. It has acquired Brazilian broker Portominas Administracao e Corretagem de Seguros Ltda., from the industrial group Companhia Siderurgica Belgo-Mineira. Portominas will be merged into the joint venture Sedgwick is setting up in southern Europe and Latin America with Nikols Group of Italy; the merger will create the third-largest international broker by revenue in Brazil. In Chile, Sedgwick has increased its shareholding in Andueza & Compania, which also will become part of the Nikols Sedgwick joint venture, to 80% from 50% . . .Hiscox Group has become the first London insurance market underwriter to join the New York-based Catastrophe Risk Exchange. Hiscox syndicate 33 has subscribed for CATEX membership, which it will use to buy and sell catastrophe risk. The move is expected to be followed by other Lloyd's syndicates and by the Corporation of Lloyd's itself joining CATEX. . . .Standard & Poor's Corp. has raised the claims-paying ability ratings of Gerling-Konzern Globale Ruckversicherungs A.G. and its London subsidiary, Gerling Global General & Reinsurance Co. Ltd., to AA- from A+. S&P said the upgrade is based on Gerling-Konzern Globale's "strong global market presence, excellent capitalization and good financial flexibility as a member of the Gerling Group" and consistently improved earnings since 1992. In another rating move, S&P has assigned an A- claims-paying ability rating to Arig Insurance Co. Ltd., the London subsidiary of Bahrain-based Arab Insurance Group, which also is rated A-. S&P said the rating reflects the fact that Arig U.K. is a core part of Arab Insurance Group and so deserves the same rating as its parent. Factors in the ratings include the group's "very strong" capitalization, Arig U.K.'s improved profitability since 1993 in line with the group's performance, and Arig U.K.'s good business position. . . .London-based Eastgate Group Ltd., backed by New York-based private equity fund Greenwich Street Capital Partners Inc., has launched a company, Dedalian Investments Ltd., to take over and recover reinsurance debts. Dedalian intends to purchase up to $500 million of reinsurance debt from companies. It said its acquisition of debt will provide a cost-effective solution to the London market's cash flow problems and enable insurers to exchange non-performing assets for cash. . . .Lloyd's of London has granted Octavian Syndicate Management Ltd. approval to form non-marine syndicate 1239 for the 1998 year of account. Headed by Malcolm Warrington, formerly underwriter for syndicate 1069, the new syndicate will write mainly commercial and industrial risks, concentrating on the specific areas of financial institutions, professional indemnity and property. It also will write contingency and personal accident business.