Drag and inertia coefficients for offshore risers with strakes have been derived from model tests, and are reported in this paper. The coefficients correspond to flow conditions of forced oscillations with tow in the transverse direction, and without tow. For the forced oscillations the Keulegan-Carpenter (KC) range was 1 to 5. The drag coefficients are higher than generally assumed, and decrease monotonically with KC in the absence of current. In the presence of current the drag coefficient becomes dependent on the ratio of current velocity to wave velocity. Low KC numbers strongly affect the calculated riser fatigue damage caused by vessel motions in storms. The present data fill an important need in riser design since in many situations fatigue damage governs riser design, and frequently the assumed coefficients are overly conservative.