# The Evolution and Development Trends of LNG Loading and Unloading Arms
## 2 Development of Marine LNG Loading and Unloading Arms
### Overall Structure of the Marine Loading Arm  
The marine LNG loading arm consists of five major components: riser pipe, trunnon box, inner arm, outer arm, and counterweight [13]. It is a loading and unloading device for tankers to load and unload liquid media. Each loading system of an LNG receiving  
Figure 1: The development history of LNG loading and unloading arms.  
terminal is composed of three liquid-phase arms and one gas-phase arm. The sizes of the loading and unloading arms include 12-inch, 16-inch, 20-inch, etc. arms. Currently, the 16-inch loading and unloading arm is the most widely used [14]. The marine loading and unloading arm in working condition is shown in Figure 2.  
There are three types of balance methods for marine loading and unloading arms: the full-balance type, rotary-balance type, and double-balance type. The full-balance-type loading and unloading arm adopts a self-supporting structure; the case is depicted in Figure 3a. The internal and external loads of the loading and unloading arm as well as all bending moments are borne by the rotary joint with a hard raceway, reducing the dependence on external support. The rotary-balance-type loading and unloading arm adopts an independent supporting structure; the case is depicted in Figure 3b. The loading pipe is relatively independent of the supporting structure, and the weights of the inner and outer arms are balanced through a single counterweight system. The double-balance-type loading arm balances the inner and outer arms through two independent balance systems; the case is depicted in Figure 3c. It can respond quickly in case of an emergency disconnection of the loading arm and has more advantages in preventing the unbalanced dynamic movement of the loading arm. In addition, there are four commonly used structures for loading and unloading arms: the self-supporting double-counterweight single-tube type, mixed-supporting single-counterweight single-tube type, independent-supporting single-counterweight single-tube type, and independent-supporting single-counterweight double-tube type. The double-tube structure is widely used due to its higher safety and flexibility. Taking the most widely used single-counterweight loading arm as an example, by placing counterweight blocks at specific positions, the weight of the loading arm is distributed reasonably, so that the center of gravity is near the hinge point in the middle section of the inner arm. The counterweight swings synchronously with the outer arm, which can balance the weights of the outer arm and the three-dimensional joint, achieving the self-weight balance of the loading arm [16].  
Figure 2: Marine loading and unloading arm in working condition. Adapted from [15].  
The loading arm has three core components: the cryogenic swivel joint, the emergency release system (ERS), and the quick-connect/disconnect device (QC/DC). The cryogenic swivel joint is used to connect different components of the fluid loading arm, ensuring that the loading arm can rotate freely during the loading process, guaranteeing the stable transmission of the fluid medium and playing a sealing role. The ERS is a protective device used to quickly cut off the connection between the ship and the end of the loading arm in case of emergencies. The QC/DC is installed at the very end of the three-dimensional joint of the loading arm. Its main components include a straight joint pipe, a clamping mechanism, and a driving device, which are used for quick connection with the tanker interface [17]. Currently, the QC/DC has two forms: manual and hydraulic-driven. For example, the Quikcon III quick-connector designed by FMC Corporation adopts a hydraulic drive system. One hydraulic distributor controls five hydraulic fasteners, and these five hydraulic fasteners can complete the fastening work simultaneously when tightening the connecting flange. In case of a failure of the hydraulic or electrical system, the QC/DC should have an automatic protection function to ensure the safe locking with the flange of the LNG carrier.