Welding using what process:- Gas welding / Arc / Mig ?

With confined space you have to provide the usual emergency recovery plan, but say Oxygen / Acetylene, you may have an oxygen rich atmosphere and or Acetylene rich atmosphere (both), Arc welding you will need to consider isolation of equipment electrically from outside confined space so as recovery of person does not allow inadvertent arcing. Mig will have shield gas which may reduce the oxygen ratio etc. Depending on amount of welding there may also be local temperature increase, to environment (Possibly max working time). Blowing fresh air in may help with both oxygen depletion / heat, but you may also want to have air monitoring equipment.

Never take Oxy-Acetylene Cylinders inside a confined space. Confined space may be suitably illuminated.Continuous air monitoring is required.Workers may carry a Multigas monitor. Welding would reduce oxygen concentration inside the space. So mechanical ventilation is necessary.Venturi blowers may be used to remove toxic fumes.