Transient simulation of two-phase gas-liquid flow in pipes requires considerable computational efforts. Until recently, most available commercial codes are based on the two-fluid model which includes one momentum conservation equation for each phase. However, in normal pipe flow operation, especially in oil and gas transport, the transient response of the system proves to be relatively slow. Thus it is reasonable to think that simpler forms of the transport equations might suffice to represent transient phenomena. Furthermore, these types of models may be solved using less time-consuming numerical algorithms.