The subject of this paper is the mechanical and acoustical behaviour of an infinite shell with any number of layers and composed of orthotropic material. In each layer, we take into consideration a ''3-D field displacement'' and the bending, membrane, shear, longitudinal and rotational inertia effects. As compared to previous work, the new approach includes the introduction of the transverse deformation in the thickness. By introduction of forces of excitation on the external faces of each layer, it is possible to establish the interface conditions: structure-structure and fluid-structure. The transmission loss of the shell excited by oblique plane waves is calculated. In this first part the formulation of the problem is presented.