Transducer assembly for an offshore drilling riser, in an example, includes a spool for connecting into the riser and a protrusion extending around the side wall of the spool. A recess extends around the protrusion between upper and lower sloping surfaces. Upper transducer bores are spaced around the protrusion and extend from the upper sloping surface downward and inward into the spool bore and lower transducer bores extend from the lower sloping surface upward and inward into the spool bore. A base of a rigid non-metallic material is located in each of the transducer bores. A seal ring extends around a cylindrical exterior portion of each of the bases and one of the transducer bores. An acoustic transducer element is mounted to the outer end of each of the bases. The transducer assembly can detect drilling fluid and form sealing.