What could be the biggest U.S. IPO week in 18 months, expected to raise up to $9.1 billion, took shape on Monday after Spanish bank Santander , insurance risk specialist Verisk Analytics Inc and biotech Omeros Corp set terms of initial public offerings next month.

Santander said the IPO for its Brazilian unit, Banco Santander (Brasil) SA, could raise up to $7.3 billion when it prices on October 6, the same date as the Verisk deal. It is set to list on the New York Stock Exchange and in Sao Paulo.

Verisk, a New Jersey-based company that collects actuarial and underwriting data related to U.S. property and casualty insurance risks, said in an updated prospectus filed with U.S. regulators that it expects to sell 85.3 million shares for $19 to $21 apiece, for expected proceeds of $1.7 billion, and list them on Nasdaq.

Omeros, which makes central nervous system anti-inflammation products, said it planned to sell 6.82 million shares for between $10 and $12 in a $75 million IPO to be managed by Deutsche Bank Securities. The Omeros IPO is set to price on October 7 and list on Nasdaq.

The IPOs could total $9.1 billion in the week. The last time U.S.-listed IPOs raised that much in one week was in March 2008 when credit card operator Visa Inc went public in a $19.6 billion IPO.

U.S. IPOs began to come back this spring after six months of near drought amid the world financial crisis. This week, eight IPOs -- the most since December 2007 -- are expected to raise $3.5 billion, the highest amount so far this year.

Santander, Spain's biggest bank, set out plans to list 16.2 percent of its Brazilian unit in the South American country's biggest ever public offering, selling 525 million units in New York and Sao Paulo, each representing 55 common shares and 50 preferred shares, according to a prospectus.

All the shares in the Verisk IPO are being sold by Verisk's existing shareholders, which include insurance companies American International Group Inc , The Hartford Financial Services Group , ACE Group Holdings and Warren Buffett's Berkshire Hathaway Inc .

Verisk Analytics' revenue rose 15 percent to $503.7 million in the first six months of 2009 over the year-earlier period, while net income rose 12 percent to $90.9 million over the same period.

Recent IPOs by risk analytics firms have included the January 2008 offering by RiskMetrics Group Inc and the November 2007 IPO by MSCI Inc , a spinoff of Morgan Stanley.

RiskMetrics closed 18.1 percent below its IPO price on Monday while MSCI was 52 percent above its IPO price.

(Reporting by Phil Wahba; Editing by Richard Chang, Gary Hill)