Suppose you had a modest little upholstery shop in Halifax. How would you feel if someone asked you to slipcover the pontoons of a semisubmersible rig? After 7,500 yards of material, your old Singer would never be the same again. Introduction Scour protection is often required to prevent scour from occurring around marine structures. This was the concern when it was proposed to set the semisubmersible drilling rig Sedco H on the sea floor to drill the Mobil-Tetco Thebaud P-84 well in 90 ft of water 6 miles southwest of Sable Island. Fig. 1 shows the location of the well. Normal tidal currents in the area reach a maximum of about 1.5 knots. Scour was expected since it was known that the sea floor was fine sand and that the 80-ft-diameter Sedco H pontoons would cause the current velocities around the pontoons to be accelerated considerably. However, it was impossible from existing information to predict the amount of scour that would occur or the amount of scour protection that would be required. This is mainly protection that would be required. This is mainly because the relationship of current velocity, rate of scour, scour depth, and grain-size distribution is not yet well defined. Also, the geometry, spacing, and size of the Sedco H pontoons are such that correlations from other experimental work could not be made. It was therefore decided to analyze the problem with model tests in a hydraulic tank model basin. Delft Hydraulics Laboratory of The Netherlands, who had previously performed similar scour studies, was selected for the model work. The results from the tests were used to design the scour protection and to plan a successful field installation. The Scour Process A sand bed exposed to ocean currents in a sea-floor environment is usually in a condition of equilibrium. In this condition the bottom material is transported but no net deposition or erosion occurs. However, when an object is placed on the sea-bed, current velocities near the object increase and this equilibrium condition is disturbed. Additional turbulence results and there is an increase in the transport capacity of the fluid near the object. The sea bed around the object starts to erode. This erosion is known as scour. The magnitude of scour depends onthe geometry of the object,the characteristics of the seabed material, andhydrodynamic conditions. Current velocity influences both the rate and depth of scour. The influence of current velocity on the depth of scour around structural piles, bridge piers, and abutments has been studied by Delft Hydraulics Laboratory, among others. The influence of the combination of wave-induced currents and tidal current on scour around structural piles was studied by Abou-Seida and by Delft Hydraulics Laboratory. The above references indicate that scour can be predicted for simple cases such as for a predicted for simple cases such as for a small-diameter isolated circular pile. The Sedco H pontoons, however, are 80-ft in diameter. pontoons, however, are 80-ft in diameter. JPT P. 381