how do you fit a new turbo to a nissan primastar 2.5 dci 140?

do you need to remove the engine to change the turbo ? i have looked and it seems very tight,the van will loose all its oil occasionally has blown lots of blue smoke on start up twice i have taken the pipe from the intercooler off and there was oil present the top of the engine is covered in oil, it does still have power but on checking the air filter it looks as if the engine has sucked the filter up and ripped it apart !