UPDATE 1-Transocean to make Swiss market debut on April 20

* Transocean to keep New York listing

* Seeking to widen and diversify shareholder base

(Adds details)

ZURICH, April 12 (Reuters) - U.S. offshore driller Transocean Ltd (RIG.N) will make its debut on the SIX Swiss Exchange on April 20, adding to its listing on the New York Stock Exchange, the group said on Monday.

Transocean, the world's largest offshore drilling contractor, said it was listing in Switzerland, where it is incorporated, to widen and diversify its shareholder base.

The news comes as Swiss food company Orior prepares for its stock market debut in the first initial public offering on the Swiss exchange in more than half a year. [ID:nLDE62O04G]

The global market for initial public offerings is showing signs of life after a spate of downsizings and postponements, though companies have frequently had to temper their initial pricing expectations.

Transocean, which reported operating revenue of $11.6 billion in 2009, said its shares will be included in the Swiss Performance Index on the second day of trading.

Its shares are expected to be included in the Swiss Market Index .SMIM after the stock market close on Sept. 17, but if the group meets certain conditions, its shares may qualify for early inclusion in the SMI, it said. (Reporting by Katie Reid; Editing by David Holmes)