Hydrogen adsorption in porous materials is one of the methods actively being studied to store hydrogen on board vehicles driven by fuel cells. Recent experiments and numerical simulations have shown that adsorbent materials appropriate to hydrogen storage could be made up of carbon nanotubes. In this work we study the properties of such materials. By Monte Carlo simulations, we compute the hydrogen adsorption in two arrangements of opened single-walled carbon nanotubes at a temperature of 77 K and pressures up to 15 MPa. A quantitative comparison is made between simulation data and experimental results. The studied nanotube arrangements seem able to adsorb hydrogen gas in amounts required for their use in fuel cell vehicles.