The SZ36-1 is the largest oil field in Bohai Bay, which has been in production for over 20 years. To increase the field production, more wells are going to be drilled at the site of the WHPJ jacket platform, and a new method for adding slots was designed for such a marginal field development. Two new legs are added for supporting the new decks, and special connections are designed to avoid underwater operation during the whole process of installation. This paper presents the design considerations, including structure transportation and installation. The ultimate strength analysis, earthquake analysis, fatigue assessment and impact strength calculation are performed and the results show that new structure and the existing platform meet the minimum requirements for yielding, fatigue criteria during the service period of another 20 years.