Mooring system devices can be divided into two series, namely single-point mooring system and multi-point mooring system. Single-point mooring system is aimed at floating bodies, that is, the mooring connection point is a point, which is suitable for floating bodies with large aspect ratio (such as oil tankers, FPSO, transport ships, etc.). ). Its outstanding feature is that it has the function of weather vane, which can make the floating body rotate 360 degrees around the mooring point, so that the floating body is always in the direction with the least stress. Another feature of the utility model is that the rotary joint is adopted, which can transfer flow, electric energy and communication well. Single-point mooring system is widely used in petroleum industry because of its many advantages, but its engineering investment is also high, and it is monopolized by several single-point companies in technology. Multi-point mooring system is different from single point mooring system. There are many mooring connection points on the floating body, which have no function of weather vane and do not need to rotate the joint. It is suitable for projects with similar length, width and size of floating bodies and mild offshore environmental conditions. Dynamic positioning is also a kind of mooring of floating body system, which is managed and operated by computer with the help of thrusters and boosters on the floating body completely or partially, so as to keep the floating body in the direction of minimum environmental force and locate it at a certain point in the sea.
There are various types of mooring systems in the world. Since China Offshore Oil Corporation adopted FPSO for offshore oil and gas development, it has been accompanied by the introduction of mooring system equipment from abroad. The main types are as follows: tower-type soft rigid arm single-point mooring system device (also known as tower-type soft transverse arm 1e mooring system), as shown in figure 13-7, which is mainly used in shallow waters of Bohai Sea, suitable for water depth below 40m, and has anti-icing and detachable functions. At present, there are four sets of such devices in use; The catenary buoy single-point mooring system device (also called calm mooring system) used in Nanhai W 12- 1 Oilfield crude oil export terminal is shown in Figure 13-8. Internal turret mooring system (hereinafter referred to as internal turret mooring system), * * * Seven sets of this device are used in Nanhai Oilfield, which are suitable for water depth of 50 ~ 400 m, of which five sets are used in the old tanker reconstruction FPSO scheme and two sets are used in the new FPSO, as shown in Figure 13-9, which are divided into detachable and non-detachable. Can withstand severe environmental conditions such as typhoons. Figure 13- 10 shows the rotating mechanism of the inner turntable, which is adopted by most deepwater FPSO in the world.
Due to the need of offshore oil development, there are dozens of mooring system design schemes, but only a dozen are actually applied. Since the emergence of FPSO in the world, all relevant companies have devoted themselves to the research and development of various types of mooring system devices, aiming at conducting extensive research on different sea areas, different service periods and different functions and reducing engineering investment. At present, the technology of single point mooring system is monopolized by several single point manufacturers. In recent years, China Offshore Oil Corporation has continuously introduced advanced single-point mooring system from the world to meet the requirements of strong typhoon resistance, permanent mooring and reduction of engineering investment.
Figure 13-7 Single Point Mooring System Device for Soft Rigid Arm of Tower
Figure 13-8 Single Point Mooring System Device of Suspended Chain Buoy
Figure 13-9 Single Point Mooring System Device of Internal Turntable
Since 1980s, China Offshore Oil Corporation has been devoted to the research and development of mooring system. In the mid-1990s, the self-developed two-point mooring system was used in three extended test projects in Bohai Sea, as shown in figure 13- 1 1. In the future marginal oilfield development, we will also use the existing means and technical level to gradually realize the localization of some mooring devices, so as to improve the economic benefits of oilfield development.
South China Sea Lufeng 22- 1 FPSO adopts auxiliary dynamic positioning, as shown in Figure 13- 12.
Fig. 13- 10 rotating mechanism of inner turret
Figure 13- 1 1 Two-point mooring system device
1. Single point jacket
Single point jacket is the stress foundation of single point mooring system. In the design, not only adverse environmental factors such as wind, waves, currents, ice and earthquakes and possible load combinations should be considered, but also the mooring force generated by these environmental forces on FPSO should be considered. Static, dynamic, seismic and fatigue calculations should be carried out to ensure the safety and reliability of a single point during its service life. The force acting on a single point by FPSO is a complex process. In addition to theoretical formula calculation and analysis, the maximum mooring force of single point action and the change process of force with time can be obtained through model test to meet the design needs. When BZ28- 1 single-point conduit is erected, the maximum horizontal mooring force caused by extreme environmental conditions reaches 455t, and the maximum vertical mooring force caused at the same time is 160t.
The single-point jacket is fixed on the seabed by driving steel piles. The size of the steel pile is determined by the design, and the depth into the mud can reach 70 ~ 80m. The top of the jacket is provided with a support column, commonly known as the general column, which is firmly connected with the leg column of the jacket. As the supporting and fixing structure of the rotating mechanism of the mooring head, it is the main component for transmitting the mooring force load. Its interior is used as a channel for arranging oil and gas pipelines.
Fig.13-12 mooring and auxiliary dynamic positioning system of FPSO inner turntable
2. Mooring
Mooring head refers to the part of the general column where the rotating mechanism is upward and can rotate freely with FPSO. This part mainly includes the turntable for fixing the rotary bearing, the oil-gas-water rotary sealing channel and the supporting structure of the jumper. The turntable is made of cast steel, and a connection mechanism is arranged on the turntable side of the mooring head as the connection support between the flexible rigid arm and the mooring head, so as to transfer the mooring force generated by FPSO after the flexible rigid arm is connected with the offshore single point. The top of the mooring head is a steel bracket, which is used as the supporting structure of the jumper pipe and connected with the turntable through the column, so that the jumper pipe bracket rotates with the turntable.
3.a-shaped connecting flexible yoke
A-type connecting flexible arm is the main connecting part between FPSO and single point, which is generally composed of large-diameter circular tube. Its top is connected with the turntable of the mooring head through the universal rotary joint, and both sides of its bottom are hung on the mooring leg through the universal coupling and connected with the mooring bracket on FPSO. Although its structure itself is steel structure, the flexible mooring system composed of rotating mechanism and mooring legs can adapt to various motion states of FPSO without restriction, so this part is also called soft rigid arm. The flexible rigid arm is connected with the turntable of the offshore single-point mooring head. When FPSO is not in place or released, it can be lowered to the sea surface to move freely and bear the environmental forces at sea. There is a ballast tank at the bottom of the flexible rigid arm, and an appropriate amount of antifreeze is injected into it to improve and adjust the motion state of the whole system.
4. Mooring legs
Mooring leg is the connecting part between FPSO and yoke, and there are two mooring legs * * *, which are located on both sides of FPSO mooring bracket respectively. The upper end of the mooring leg is connected with the mooring bracket on FPSO, and the lower end is connected with the flexible rigid arm. Both ends have rotating mechanisms to adapt to the movement of FPSO. Generally speaking, the mooring leg is connected with the mooring bracket of FPSO on land. After the FPSO is towed to the oil field, the bottom of the flexible rigid arm is hoisted and connected with the lower end of the mooring leg, completing the last process of the mooring system of the whole floating production and storage device. If FPSO needs to be released, it can be released from a single point through the reverse operation of this part of the mechanism. The design scheme of this part of the connection mechanism largely determines the time required for FPS0 to connect and disconnect at sea. Mooring legs and flexible rigid arms are the components with the largest range of motion in the whole mooring system.
5. Pipeline system
In order to transport crude oil and natural gas produced by wellhead platform to floating production storage device, or realize reverse transportation of fluid or gas according to production needs, it needs to be completed through submarine pipeline, rotary sealing slip ring, jumper pipe between FPSO and single point and corresponding supporting pipelines. The size and quantity of jumper and sealing channel depend on the production requirements. In order to realize the pigging operation of submarine pipeline, it is necessary to arrange pig ball launching and receiving devices at a single point.
Due to the complexity of single-point mooring system components, the design of FPSO mooring bracket and the whole mooring system is mostly undertaken by foreign professional companies. The rotating part of each component is also provided by foreign companies as patented components. However, the construction and construction of steel structures such as jacket, mooring head, mooring bracket and flexible rigid arm were all undertaken by CNOOC from the beginning.