When it came to his fleet of superyachts, the Russian oligarch Roman Abramovich was nothing if not generous.As 2011 drew to a close, his biggest boat yet – the 162-metre (533ft) Eclipse – lay moored in the glittering waters of St Barts, the picturesque Caribbean island where billionaires like to gather to see in the new year.Then the largest pleasure cruiser in the world, insured for €390m (£330m), the ship was all set to welcome guests for Abramovich’s own VIP celebration.Perhaps arriving via one of the yacht’s two helipads, they could cool their toes in the onboard swimming pools, unwind in the whirlpool, or take in a film in the private cinema before heading to the disco lounge.As evening fell, guests including Rupert Murdoch and the now disgraced film producer Harvey Weinstein assembled for a no-expenses-spared party. The star attraction? A private concert by the US rock band the Red Hot Chili Peppers, hired to play a set of at least 75 minutes for a cool $1m.Abramovich had always been magnanimous with his yachts, documents show, welcoming guests including Chelsea football players Frank Lampard and John Terry, and the club’s manager José Mourinho.While Abramovich lavished money on enjoying the high life onboard his boats, he appears to have been less forthcoming when it came to paying tax on their expenses.Today, an investigation by the Guardian, the BBC, the Bureau of Investigative Journalism and international partners Der Spiegel and ZDF uncovers how an offshore scheme appears to have helped companies owned by the billionaire avoid duties potentially amounting to tens of millions of dollars.Experts have said the arrangements could amount to deliberate tax evasion.Details came to light thanks to documents contained in Cyprus Confidential, the largest ever leak of information from the Mediterranean island’s offshore financial industry.The files show how offshore companies were used to create a fiction: that five of Abramovich’s superyachts were being hired out to customers for commercial purposes.This allowed the boats’ operating companies to avoid paying VAT on a long list of expenses, such as the $2m-a-tank cost of refuelling the Eclipse. The files suggest VAT may have been due in Italy, Cyprus and a number of other EU countries.Lawyers for Abramovich said he had always acted in accordance with professional tax advice and denied he had knowledge of or was personally responsible for any alleged tax evasion.The yacht schemeIn April 2005, Abramovich was surfing a wave of success.Chelsea FC were heading towards their first Premier League title, delivered on a plate by the staggering wealth of its free-spending new owner.What’s more, he was on the verge of a landmark deal, the $13bn (£10.5bn) sale of his remaining stake in the oil and gas company Sibneft, to state-backed rival Gazprom.Now, as the oligarch closed in on glory, his advisers were hard at work. Their focus was Blue Ocean Yacht Management, a Cyprus-based company