Design Development of a Soft-Yoke Mooring System for the S236-1 Field in Liaodong Bay, China

ABSTRACT The Suizhong 36-1 (SZ 36-1) Oil Field in northern Bohai Bay in about 30m. water depth is a giant offshore oil fie1d with estimated reserves of 1.2 billion barrels of oil. The development plan requires a Floating Production Storage Unit (FPSU) and a Single Point Mooring (SPM). The SPM is designed for the 10 a-year return non-ice storm and the 25-year storm with 0.24m. first year ice sheet with the FPSU connected. It is also designed for the 50-year ice event with 0.55m. rafted first year ice with the FPSU disconnected. The engineering design of the fixed tower soft-yoke SPM developed for these conditions is presented. Information on system performance analysis, design, procedures for predicting ice loads on SPM and FPSU and on model test results is included. INTRODUCTION Field Location The SZ 36-1 oil field is located in Liaodong Bay in the eastern part of the Bohai Gulf (Fig. 1). The field is about 50 kilometers (km) from shore. It is characterized by heavy, high Viscosity oil, low oil/gas ratio and a loose reservoir which makes the development of the field a technical challenge. It is being explored and developed by Bohai Oil Corporation (BOC) of the China National Offshore Oil Corporation (CNOOC). The discovery and development of the SZ 36-1 oil field was reported in Reference 1. BOC has prepared a field development plan, shown in Fig. 2. Conceptual design of the engineering facilities conducted by BOC in conjunction with foreign consulting companies has been completed and contracts for fabrication and insta11ation of the facilities are being awarded. The field is expected to be on stream by 1992. This paper focuses on the preliminary design of the SPM system. Topics specifically addressed are the system design basis, evaluation of alternate SPM systems and selection of a preferred system, system description, mooring analysis and design and model test. The preliminary design may differ from the final design, which will be engineered by an SPM Contractor. DESIGN BASIS SPM Location The coordinate location of the SPM is shown in Figure 2. The mean sea level at the location is 32.80 m. above the seabed with an extreme tidal variation of +2.16, -2.73 m. Environmental Conditions SPM environmental design criteria are summarized in Table 1. Of particular concern is the presence of sea ice in the winter. Ice conditions are categorized into light, ordinary and heavy ice years. In 1ight ice years there will be no ice in the field. In ordinary ice years about half the fie1d will be covered with drift ice with level ice thickness ranging from 5 to 15 cm. and speeds from 0.3 to 0.5 m/s and a maximum of 1.0 m/s. This condition lasts for about 30 days in January and February. The FPSU must remain connected in ordinary ice years. Heavy ice years have taken place three times since 1936 (1936, 1947 and 1949) when the entire field was covered with land fast ice, with level ice thickness ranging from 20-30 cm.