Installation and Removal Records of Field Trials for Two Mooring Dolphin Platforms with Three Suction Caissons

Abstract

In 1999, two mooring dolphin platforms (MDP1 and MDP2), each with three suction caissons, were installed in the JZ9–3 field of the Bohai Sea of China. Before installation, penetration and retrieval tests for MDP1 were conducted near the design location site. In 2006, after being in service for 7 years, partial removal and repenetration prototype trials were performed for MDP2. The MDP1 platform has the same geometric structure and weight as MDP2, in which each suction caisson is 6 m in diameter and 9 m in height. Typical soil profiles at the site mainly consist of organic silt, silty clay, and clay. Records from the transducers were relevant to the installation and retrieval of the suction caissons, which were performed by controlling the pump discharge into/out of the caissons. The results show that skin friction decreases because of the seepage effect that occurs after applying the suction pressure inside the caisson. However, clayey or silty soils with low permeability required more time and suction to create a seepage flow to lower resistance in the penetration or removal phases. The setup effect caused by suction significantly increases the skin friction resistance by 62–84%, and the pump-out pressure equals 1.79–2.04 times the pump-in suction after 7 years. The on-site data are valuable for further study on the design and construction of platforms with suction caissons, especially regarding external skin friction.