### Literature Review
#### 1.2.1 Safety Distance between Anchoring Ships  
In the literature, [29] analyzes the differences of relevant specifications of the anchorage circle radius under different situations, and an optimal selection of the anchorage circle radius in the regression equation model and algorithm according to different wind conditions was proposed. In [30], the position of the anchor chain hole was taken as the dropping anchor position, converted from the position of GPS antenna to the anchor chain hole with the mathematical model. In [31], the conceptual difference between the ship position and anchor position was pointed out. In [32], the difference and relation between anchor position and dropping anchor position was analyzed, and an estimation method was put forward. In [29; 33; 34], a regression analysis and data mining were applied to calculate the anchorage circle radius and the distance between anchoring ships, which were intelligent to a certain degree. However, as it was a comprehensive analysis of various situations, it could not reflect the maximum safety radius. Some Japanese researchers considered the maneuvering difficulty of a ship passing through anchoring ships and introduced the distance model between two adjacent anchoring ships [29]. The length of the anchoring chain was mainly determined by the external force of wind, wave and current, water depth and the grip force coefficient of the bottom material [35]. A safety evaluation anchorage model of the Tianjin port was introduced by [PERSON] et al., based on the safety distance model of two anchoring ship, and the process of dragging anchor, drift direction and the speed of dragging anchor were studied [25]. [PERSON] put forward a relative safety distance model from the anchoring ship to waterways, including locations such as areas with an intensive traffic flow [28]. In [36], a calculation method was proposed for the anchoring safety distance based on ship drift motion and ship collision risk. A fixed-point anchoring scheme was then proposed, comprehensively considering the basic turning radius of a single anchoring ship and the distance between anchoring ships with maneuvering difficulty [37]. The safety distance between anchoring ships is also influenced by the captain's risk appetite and pressure mode [27].  
In conclusion, the current anchorage circle radius mainly considers factors such as chain length, catenary and the projected length of the anchor chain, horizontal chain and ship length into consideration, which revealed a shortcoming, namely, the insufficient consideration of factors such as ships carrying hazardous goods, ship parameter, the distance from anchor chain hole to the bow and stern centerline, trim angle and water depth. Hence, the existing anchorage circle radius model and radius value cannot fully reflect the engineering practice background. The safety distance between anchoring ships should be set based on the safety radius of two ships, and the navigation impact of passing ships in the anchorage area should also be taken into consideration. Therefore, it is necessary to fully consider factors including ships carrying hazardous goods, ship parameters, ship loading conditions, anchorage depth as well as the risk of ships sailing in the anchorage area. Additionally, the practical background of anchoring engineering will be fully reflected by the distance model.