## 1 Introduction  
A horizontal Christmas tree wellhead connector links the Christmas tree body to the subsea wellhead. Its working performance directly affects the stability and reliability of a subsea production system [1]. Sealing performance is the most important index for evaluating a connector's working capability, and attempts to improve the connector's sealing performance are underway. Research is currently focused primarily on improving the seal's form and optimizing its structural parameters. To address seal form improvement, [PERSON] et al. [2] designed a double sealing structure comprised of a lens gasket and an O seal ring. When applied in deepwater pipelines, the sealing structure had a certain temperature compensation capability, and the axial pretension force on the collar's flange was also reduced, indicating an improvement in the sealing performance of the subsea connector. [PERSON] et al. [3] designed a collect connector that was applied in the end of the subsea pipeline. Testing results showed that the highest sealing strength of the connector is 65 MPa. With respect to sealing structure parameter optimization, to improve the sealing performance of a novel subsea pipeline connector, [PERSON] et al. [4] established a multiobjective connector optimization model focused on minimum plastic strain and contact pressure variance. In this model, the radial basis function method was used to construct the objective functions. The model optimization significantly improved the connector's sealing performance. [PERSON] et al. (2016), considering the lens gasket of a collect connector of an object, identified the gasket's spherical radius and flange taper as two main parameters affecting sealing performance after the relationships between the contact performance and the compacting force were revealed. After establishing the sealing structure of the optimization model, an optimized structure of the connector was obtained, taking the optimization objective as the minimum compacting force. [PERSON] and [PERSON] (2016) analyzed the sealing mechanism of a deepwater pipeline connector. In their study, the authors used the contact pressure and its variance on the primary and secondary sealing surfaces as the optimization objectives to obtain an optimized connector sealing structure after the multiobjective optimization model was established. Previous research following different approaches has resulted in improved connector sealing performance mainly via improving the seal's form or optimizing the sealing structure. However, in all approaches, the working load was regarded as a stable value, whereas a connector's actual working load is a random variable that fluctuates with the well's production pressure. Studies have shown that it is easier for the sealing structure to fail under a random working load condition ([PERSON] and [PERSON], 2016), indicating that the obtained optimized sealing structure is not optimum if the randomness of the working load is not considered. Meanwhile, time consumption remains a problem during sealing structure optimization, which affects a subsea connector's optimization efficiency. A complete sealing structure's optimization process for a subsea connector requires 500 h, even though multidisciplinary virtual simulation methods are used ([PERSON] and [PERSON], 2016). Therefore, optimizing the sealing structure while taking into consideration the randomness of the working load is necessary to further improve the connector's sealing performance. In this context, new optimization algorithms are required for improving the optimization efficiency of a subsea connector. Modern intelligent algorithms are becoming popular optimization approaches for their global search abilities and relatively high solution speed. For example, in the field of structural optimization, modern intelligent algorithms are often combined with surrogate models for engineering optimization problems with a high computational cost. [PERSON] et al. (2016); [PERSON] et al.