Compared with the onshore LNG receiving terminals, the floating storage and regasification unit (FSRU), as one of offshore receiving terminals, integrates LNG receiving, storage and regasification functions, and has advantages such as low initial investment, short construction period and flexible movement to adapt to market changes. Thus, FSRU has gradually become an important application form of regional gas supply, seasonal peak shaving or gas supply service in the early stage of onshore terminal construction. FSRU can be converted from LNG carriers or newly built, and its storage capacity generally ranges from tens of thousands of cubic meters to 300,000 cubic meters. It is permanently fixed at the designated positions through a single-point mooring system or berthed at port/shielded water jetty position for operation. When the use of FSRU scheme is limited by factors such as jetty operation water depth and operation sea area, the scheme of matching floating storage unit (FSU) with floating regasification unit (FRU) can be adopted to realize FSRU function .

Although LNG carriers can realize the function of FSU, its application as FSU by mooring at the jetty is different from the design purpose as a transport ship. Herein, the applicability of LNG carrier as FSU is evaluated according to the actual situation of layout scheme and the characteristics of service water area . Aiming at the characteristics of frequent cargo transfer and continuous unloading in FSU, the risk of layout scheme and LNG transmission system is also evaluated, and thus appropriate mitigation measures are put forward, which can provide useful guidance for the safe operation of LNG carrier as FSU .

Figure 1: Typical Layout of "FSU + FRU" Design Scheme

1. Technical scheme

Taking the 30,000m3   "M" LNG carrier as an example (see Table 1 for the main parameters), this project adopts the FSU + FRU design scheme. The small LNG carrier is moored at the jetty for a long time, functioning as FSU. Specifically, the shuttle LNG carrier regularly approaches the small LNG carrier to make up the cargo through ship-to-ship (STS) transfer, and the small LNG carrier unloads the cargo to the FRU through the jetty boarding ladder and

hose lifting unit. Subsequently, the LNG in the FRU  storage tank is gasified and transported to the jetty adjustment and control unit, and then the natural gas with appropriate flow rate, pressure  and  temperature  is continuously supplied to the onshore power plant or natural gas pipeline network through the onshore pipeline network .

Figure 2: Mooring Plan I

2. Feasibility analysis

(1) Selection of fender

Fixed arch fenders and four Yokohama movable fenders have been provided between FSU and jetty, while the equivalent displacement coefficient C of fenders between FSU and shuttle LNG carrier can be calculated according to OCIMF equivalent deadweight method, and suitable fenders can be selected based on its schedule. The scheme is equipped with four Yokohama movable fenders. If a 22,000m3  LNG carrier is selected as the shuttle LNG carrier, and its maximum draft is 6m, the berthing collision energy is calculated to be 26.11 tonne·m, which is less than the maximum absorption energy (68.9 tonne·m) of the selected fender. The lateral load and tidal load generated by wind are 114.9t and 1.03t, respectively, which are assumed to be evenly distributed in the four impacts. The lateral pressure of a single fender is 2.90t/m2 , which is less than the pressure limit (14.08t/m2) of the fender.

Table 1 Mooring Calculation Results of Plan I

Figure 3: Mooring Plan II

(2) Mooring analysis

As an FSU berthing jetty, the carrier "M" is moored with 12 cables, including 6 bow cables and 6 stern cables. The jetty is equipped with a quick cable-removing hook device. The mooring cables are numbered 1~12 from stern to bow, and the fenders are numbered A, B, C and D in order. The mooring cable is composed of eight polyethylene ropes with a diameter of 32mm, a length of 200m and a breaking limit of 66t. The hydrodynamic analysis software AQWA is used to check the existing mooring schemes, and the extreme working environment conditions are selected: wind speed of 15kn, wave height of 0.8m and flow velocity of 3kn. Considering the normal tidal level = 0m and the high tidal level = 2.7m, the cable tension is calculated and analyzed in 8 different wind directions (at 45° interval) and 8 working conditions.

Table 2 Mooring Calculation Results of Plan II

From the calculation results, it can be seen that the cable tension in mooring plan I is within the breaking limit, but the safety factor is low (less than 1.75), the stress on the fender is uneven, and the ship moves greatly. Therefore, the scheme is partially optimized to form plan II, which adopts 6 stern cables and 8 bow cables for mooring. The cables are numbered 1~14 from stern to bow, and the fenders are numbered A, B, C and D in order. The calculation and analysis results of AQWA software are shown in Table 2. The safety factor of each cable in the optimized mooring scheme is high, and the force on the fender is uniform.

3. Risk assessment

(1) HAZID

As the LNG carrier has its use and nature changed in case of using as FSU, there are many risks in berthing, mooring, connection, ship-to-ship transfer, ship-to-shore unloading and other operations when FSU is fixed at the jetty as a floating storage unit. Therefore, the hazard sources shall be identified and appropriate control measures shall be put forward. HAZID analysis is carried out for the carrier "M" as FSU, which is divided into three nodes:

Node 1: Design scheme of "M" as FSU;

Node 2: Ship-to-ship transmission system between the shuttle LNG carrier and FSU;

Node 3:  "M" and FRU unloading transmission system 5.

According to the division of the above three nodes, nearly 20 guide words are selected, and 54 hazard sources are identified. The existing measures of the design scheme are analyzed, and suggestions on countermeasures are given.

Figure 4: LNG Pipeline Transmission System

(2) Scenario selection

According to the statistical analysis of LNG accidents over the years, 75% of the accidents occurred during loading and unloading operations, and 43% of the accidents were leakage of pipeline joints and valves. From the above HAZID analysis, it can be seen that the risk level of LNG leakage during LNG transmission is high, and the harm caused by leakage at LNG cargo connection manifold on both sides of FSU shall be paid attention to. Generally, probability analysis of LNG pipeline joint and valve leakage is needed, and risk value calculation of LNG disaster after leakage is carried out by combining event tree analysis. In order to simplify the program, four scenarios, namely, leakage of connection manifold between FSU and Shuttle LNGC, leakage of LNG to the water surface between two ships, breach of 10% connection manifold (pipe diameter 250) and 50% connection manifold (pipe diameter 250), and pipeline pressure of 13bar and 12bar, are directly selected for consequence simulation analysis.

(3) Consequence simulation

The three-dimensional computational fluid dynamics (CFD) software FLACS developed by Norwegian consulting company GexCon is used to calculate the leakage consequences.

Scenario 1: LNG leakage occurs in the connection manifold between FSU and Shuttle LNGC, with a breach of 10% pipe diameter and duration of 60s. The calculation results are shown in Figure 7. After leakage, the LNG diffuses along the length direction, and the maximum diffusion impact range is 95.86m.

Scenario 2: LNG leakage occurs in the connection manifold between FSU and Shuttle LNGC, with a breach of 50% pipe diameter and duration of 60s. After leakage, the LNG diffuses along the length direction, and the maximum diffusion impact range is 175.82m.

Figure 5: LNG Leakage between FSU and Shuttle LNGC (Breach of 10% Diameter)

Figure 6: LNG Leakage between FSU and Shuttle LNGC (Breach of 50% Diameter)

Scenario 3: LNG leakage occurs in the connection manifold between FSU and jetty, with a breach of 10% pipe diameter and duration of 60s. After leakage, the LNG diffuses along the length direction, and the maximum diffusion impact range is 100.6m.

Scenario 4: LNG leakage occurs in the connection manifold between FSU and jetty, with a breach of 50% pipe diameter and duration of 60s. After leakage, the LNG diffuses along the length direction, and the maximum diffusion sweep range is 147.55m . Any fire source shall be strictly prohibited in the above four different scenario areas, and irrelevant personnel shall be prevented from entering.

For LNG plants arranged in open places, because the possibility of diffusion disaster after LNG leakage is the highest, and the diffusion impact range is also the largest compared with other disasters of pool fire, the diffusion concentration range of 2.5% is usually set as the safe operation distance. Based on the simulation analysis results of disaster consequences in various scenarios, it is advised that the FSU connection manifold extend outward within 176m as safety zone.

4. Conclusions and suggestions

As FSU matched with FRU, small LNG carrier can form a small LNG receiving terminal with FSRU function, which is suitable for the energy supply scheme of jetty with shallow water depth, small scale and limited operating conditions. Through the research, the following conclusions and suggestions are put forward:

Figure 7: LNG Leakage between FSU and Jetty (Breach of 10% Diameter)

Figure 8: LNG Leakage between FSU and Jetty (Breach of 50% Diameter)

(1) The small LNG carrier can berth at the jetty to provide services as FSU, and its own LNG cargo system can meet the requirements of LNG receiving, storage and unloading operations, but compatibility evaluation and confirmation shall be carried out according to the actual situation on site;

(2) Due to the limitation of jetty infrastructure, frequent ship- to-ship transfer, continuous ship-to-shore operation and long-term berthing, some additional risks will be brought to FSU. Thus, FSU berthing and mooring, hose connection, operation procedures and safety management shall be considered emphatically;

(3) When FSU is berthed at the jetty for operation, the operation safety area shall be determined by QRA method, and eye-catching signs shall be set. Irrelevant ships and personnel are strictly prohibited from entering the safety zone.