As the Royal Opera House. Covent Garden, closes down for a two-year refit, it is not alone. Around the world, managements in the throes of redevelopment are negotiating with artists, architects, bankers and politicians - and wondering how best to keep their companies and audiences intact while the house is closed. Daniel Snowman visits a selection of opera houses in San Francisco, Barcelona, Washington DC and Venice (all in the hands of the builders) to find out. Producer Daniel Snowman