ABSTRACT This paper describes the replacement of an underwater brace which had been torn from MGS Platform A in Cook Inlet, Alaska. The damage apparently occurred during abnormal winter ice conditions and was detected during a subsequent underwater inspection of the platform. The replacement operation, consisting of the underwater placement and structural attachment of a six foot diameter, sixty-five foot long brace between the platform legs, was conducted in an environment where thirty foot tides and twelve fps currents are not unusual. Diver operations are normally possible only during twenty minute slack tide periods that occur every six hours. Diver"activities are further hampered by the lack of visibility resulting from a heavy sediment load in the swift currents and low water temperatures. A bolted connection, reinforced with grout, was used to attach the replacement brace to the structure. The extensive underwater work included: trimming the remaining stubs of the original brace to expose the undamaged vertical gusset plates, placing the brace and providing a temporary attachment, drilling twenty-four 1-1/2 inch bolt holes in each of the gusset plates, installation of bolts, sealing the gap between the replacement brace and the trimmed stubs of the original brace, placing reinforcing bars across each joint and grouting the connections. Most diver activities could be conducted from inside the replacement brace, independently of the tidal current, by providing access tubes at either end of the replacement brace for diver communication with the surface. The brace was replaced within one hundred days after the damage was detected at a cost of about $500,000. Installation was completed in three weeks. INTRODUCTION Cook Inlet is a tidal estuary on the southern coast of Alaska. Middle Ground Shoal Platform A, owned by the Shell/Arco/Socal group, is the first drilling/production platform installed in Cook Inlet, Alaska. The platform site, Figure 1, is between the East and West Forelands in about ninety feet water depth and is exposed to severe tidal ranges up to about thirty feet. Swift tiday currents, particularly through the constriction caused by the Forelands, are normal and can exceed seven knots.' The current abates only during short slack tide periods which occur about every six hours as the tidal flow reverses. This period of relatively still flow averages about twenty minutes. During winter months the Inlet is' covered with ice which may exceed several feet in thickness. 2 Ice stamukhi, tens of feet in thickness,3 can be formed as layers of ice are frozen together on the beaches and subsequently broken loose by receding high tides. The ice, driven by the tidal current, applies considerable force to the Inlet platforms and represents the major design load.