Wave-Current Loading on Shallow-Water Caisson: An Evaluation of API Recommended Practice

Results of laboratory model tests are used to assess the accuracy of the American Petroleum Institute (API) method of predicting wave-current loading on a shallow-water production caisson in extreme random waves. Model tests were conducted using a 1-to-20 scale model of a 36-in. (×2.54-cm) diameter caisson in a 50-ft (×3.048-m) water depth. Tests were conducted with and without in-line currents in extreme random waves, many of which were breaking. The experimental results for wave-current kinematics, wave forces, and overturning moments are compared to predictions based on the API procedure using Stream Function wave theory. Based on analysis of 132 extreme random waves from the test program, it is found that the in-line fluid velocities measured just below the wave trough level were 6% larger than measured values on average. Predicted in-line wave forces and moments were, on average, 10% and 15% larger than measured values, respectively. Measured wave loads included dynamic amplification of about 10% due to ringing of the structure following the impact of extreme breaking waves. The standard deviation between measured and predicted values were quite large, however, and measured wave loads for a few waves that plunged directly on the caisson model were between 1.5 and 2.2 times larger than the predicted loads.