Within a continuum model for magnetic systems with competing interactions introduced by Hubert the classification of all possible modulated structures for axial easy-plane systems has been carried out, and the phase portraits for these solutions are constructed. Analytic solutions for the structure of isolated domain walls between helicoids of different chirality are presented. The equilibrium structures of the modulated phases in an applied magnetic field and the critical field for transitions from helical to fan phase are in qualitative agreement with results obtained within the corresponding discretized model and various experimental data.