Forum469 assumes right. The gas must be dehydrated so no water will drop out in the pipeline and form into hydrates. As the gas travels down the pipeline, it will experience changes in temperature and a drop in pressure that increases with distance travelled. Many subsea pipelines place the gas well within the hydrate formation temperature and so any free water that drops out of the gas due to changes in pressure will form into ice crystals, trapping gas molecules within the crystal structure and hence forming a solid explosive blockage in the pipe. Hydrate formation is quite a well reasearched issue in the oil and gas industry, so a lot of literature exist in the field of flow assurance.
If you have water and carbon dioxide, your problem is then corrosion! Removing the water and injecting a corrosion inhibitor will protect a pipeline from a gas high in CO2. The CO2 will then need to be removed onshore in molesieves if the gas is going into a distribution network.
A brief explaination, but hope it is helpful.
