The present invention relates to a method and apparatus for cleaning an oil and gas well riser section or assembly on location offshore that includes a larger diameter central pipe and a plurality of smaller diameter pipes that are spaced radially away from the central larger diameter pipe. Even more particularly, the present invention relates to an improved method and apparatus for cleaning oil and gas well riser sections wherein a specially configured cap or pair of caps are fitted to the ends of the riser which enable pressure washing cleaning tools (or a camera) to be inserted into and through a selected one of the pipes including either a smaller diameter of the pipes or the central larger diameter pipe and wherein the cap continuously collects spent cleaning fluid and debris, allowing the cleaning process to be done on location without transporting the riser section back onshore. In one embodiment, a specially configured back out preventer can be used to prevent inadvertent backwards travel of the cleaning tool during cleaning.