Detecttion and maintaince of offshore structures under sea water are difficult to implement especially in the ChenDao Oil Field Yellow River Delta. This article presents several equipments to make detects and maintains easily to be actualized. A shallow water submersible umbrella is introduced to check the scour of piles and damnification of production lines which is not easily closed up. An underwater maintenance complex node cover of offshore platform is developed for dry welding under sea water. The feasibility of two new auxiliary equipments is tested by small size model. The results of experimentation show that the assumption of partial water insulation around work area by uninterrupted air input is reasonable. And theoretic analysis is given.
Most of offshore structures for oil exploitation are composed of components which are framed by welding many steel elements. Though it can change microstructure around welding zone, welding is the best effective maintenance technology of ordinary steel structure. It is the normal method for effecting subsea weld repairs and has been used as an underwater maintenance technique since about 1970. Wet welding also can repair underwater structures, but the quality of repaired elements is not good as dry welding. So dry welding is preferred to wet welding in many conditions. A lot of equipments are developed for underwater dry welding. Dry welding can be implemented in one atmosphere or high pressure above one atmosphere under sea water. Two methods are available which can provide a dry environment at the repair site underwater. The one is cofferdam, the other is pressureresistant chamber. But one-atmosphere environment repair methods are uneconomic for depths greater than 10 meters because of sealing and amount of steelwork. Cofferdam attracts more environmental forces and it is heavy even for smaller depths.