The vibro-acoustic behavior of elastic structures coupled with cavities filled with a heavy fluid can be modeled by using the Finite Element Method. In order to reduce computing time, the Patch Transfer Function (PTF) approach is used to partition the global problem into different sub-problems. Different types of problem partitioning are studied in this paper. Partitioning outside the near field of structures to reduce the number of patches of the coupling surface for frequencies below the critical frequency is of particular interest. This implies introducing a non- standard modal expansion to compute the PTF accurately enough to guarantee the convergence of the PTF method and reduce computation time in comparison to a direct Finite Element resolution. An application on a submarine structure illustrates the interest of this approach.