During hydraulic transport for deep sea mining, polymetallic nodules are transported in various ascending inclined pipes located at the sea floor. These inclined pipes can constantly change their angle of inclination due to moving excavation equipment attached to these pipes. Flow assurance during transport requires a safe transport velocity which takes into account all inclination angles. A study was conducted into safe transport velocities of slurries composed of gravel sized material in ascending inclined pipes. Experimental research was conducted with 4.6, 6.3 and 12 mm diameter gravel in a 100 mm experimental flow loop up to an inclination angle of 52 degrees. Measured parameters include pressure losses, mixture velocity, delivered concentration, deposit limit velocity and velocity profiles from high speed camera footage. During this research, various literature sources have been studied for definitions and models of transition velocities between safe and unsafe transport. These definitions and models are discussed in terms of their relevance for coarse slurries. With these definitions in mind and with the experimental data a recommendation is given for a transportation velocity of coarse slurries in inclined pipes.