By Scott Murdoch SYDNEY (Reuters) -Nearly 40billionworthofbigticketbuyoutshavecollapsedinAustraliathisyear−themostinfifteenyears−asregulatoryriskandmisalignedvaluationsaddtothegrowingchallengesinnavigatinganincreasinglystringentregulatoryenvironment.AnADNOC−ledconsortium′sdecisiontowalkawayfromits18.7 billion bid for Santos , Australia's second largest gas producer, is the latest in a string of high-profile deals to collapse in Australia this year. The ...