The purpose of this book is to set out the benefits of using the dense phase gas transport over the two-phase gas transport in OGS projects and to verify that when operating pipelines at high pressures, more mass per unit of volume is transported, and liquid formation risks are mitigated in hostile environments and low temperatures. Moreover, the book contains key data about gas production fields in deep and ultra-deep waters around the world, which serve as a basis for research and provide characteristics for each development to be contrasted with the subsea architecture proposed in this paper. Firstly we will go through an overall introduction about pipelines followed by a brief about the equations and the fluid phases used to do our simulation Finally, using a simulation tool, the gas transport performance in dense phase is analyzed and compared with the two-phase way of transporting gas by underwater pipelines, achieving up to 20 % in cost savings when dense phase is applied.