2005 citroen picasso 1.6 hdi?

on the rear of the engine there are two hard plastic water hoses,( I think they maybe supplied as one unit ) they run all the way across the rear of the engine stating from a plastic fitting on the o/s rear of the engine block then accross under the air filter box then around the n/s end of the engine, has anyone any info on replacing this pipe ie best way of doing the job. thanks Rod