A single point mooring system allows the FPSO to rotate around a designated base point, creating a weathervane effect. This ensures the FPSO’s bow always faces the direction of minimum net external load, reducing the total mooring force required by the system. Because single point mooring systems are adaptable to a wide range of water depths, can moor very large FPSOs, and are capable of operating in harsh sea conditions, they are widely used. single point mooring systems primarily perform two functions: first, positioning and mooring, securing the FPSO to the oilfield operation site using the mooring force provided by a single point, serving as the offshore terminal for transporting refined crude oil; second, serving as a crucial connection point between the subsea production system and the FPSO, providing fluid transfer, power, and optical control transmission to the subsea production system. It can also transfer wellhead fluids from the oilfield to the FPSO for processing.
SOFEC Single Point Mooring System
SBM (Separable Single Point Mooring System)
Some single point mooring systems also have an emergency release function, which can disconnect from the mooring system and evacuate to a safe area in extremely rough sea conditions. However, the reconnection operation of this system is subject to strict sea condition requirements and is not suitable for use in sea areas where sea conditions are rough most of the time.
World-renowned single point mooring (SPM) design and service companies include SBM, Bluewater, SOFEC, APL, Prosafe, and BW Offshore. My country’s CNOOC Engineering has also completed the construction of a float-type SPM system.
Buoy-type Single Point Mooring System
Buoys are anchored in waters far from the coast, serving as mooring points for tankers loading and unloading gases or liquids, and acting as connection points between subsea manifold systems. The primary function of the buoys is to transport oil and gas resources from onshore or underwater to the moored tankers.
Buoy Mooring System
Buoy Appearance
Buoys typically consist of multiple watertight compartments. Even if one is damaged by an external impact, the remaining compartments can still provide sufficient buoyancy.
There are generally two types of buoys: turntable and turret.
Left side: Turret type; Right side: Turntable type
Turntable buoys allow the upper part to rotate freely, placing the FPSO in a position of minimum stress. They have no upper compartments.
SBM
Turret buoys, developed and designed by Bluewater, consist of a turret and deck buoys. The buoys rotate around the turret, allowing them to be made in a square shape.
Bluewater buoy-type single point mooring system
Common buoy mooring systems include: chain-assisted buoy mooring systems (CALM) and single-leg mooring systems (SALM). Chain-assisted buoy mooring systems are commonly used on shuttle tankers and FSOs, and are the most economical and efficient method for mooring and loading/unloading tankers. Single-leg mooring systems are similar to chain-assisted buoy mooring systems but are typically used on shuttle tankers or FSOs, not FPSOs.
Single-anchor leg mooring system diagram
Turret-type Single Point Mooring System
The turret-type single point mooring system evolved from the CALM system, but because FPSOs have a much greater weight and buoyancy than buoys, they offer superior performance in terms of motion and stress distribution. Currently, over 70% of FPSOs worldwide use turret-type single point mooring systems, distributed across the northern North Sea, Norwegian Sea, Gulf of Mexico, and South China Sea. The turret-type single point mooring system integrates mooring, gas, hydraulic, and electrical transmission, making it complex in design and requiring high stability. Currently, turret-type systems are mainly divided into two types: internal turret mooring systems and external turret mooring systems.
Internal Turret Mooring System
The internal turret mooring system is the most widely used mooring system for FPSOs, commonly used in medium and deep-water areas. Internal turret mooring systems are generally located at the bow, and their characteristics include: the turret diameter can be designed to be very large, leaving sufficient space for equipment and manifolds; the internal turret is embedded in the hull, providing good protection, but its position affects the FPSO’s “wind vane effect.”
Internal turret mooring system under construction
Internal turret mooring systems include two types: passive and active.
Passive internal turret mooring systems refer to FPSOs without propulsion power; the turret system allows the FPSO to rotate 360 degrees around its center. Currently, most internal turrets worldwide are of the passive type.
Passive internal turret mooring systems
Active internal turret mooring systems are typically located at the bow third of the hull. Due to their proximity to the midships, they effectively reduce mooring forces and riser fatigue damage. However, this mooring method has a weak “wind vane effect,” requiring propeller assistance to adjust the bow, and is mainly used in the North Sea.
Active internal turret mooring systems
Because the active mooring system is located close to the midships, living quarters can be placed at the bow, placing the personnel concentration area upwind of the oil and gas pipelines and combustion boom, greatly improving the safety of the FPSO.
However, due to the large size of FPSOs, the auxiliary power requirements are substantial, and the reliability requirements of the propulsion system are high. The risks of propulsion system failure must be considered during the design phase.
SOFEC internal turret system components
Main Components of the Turret:
The internal turret mooring system mainly consists of four parts:
Turret and Sleeve: Connected by bearings, the FPSO can rotate freely.
Turret from various manufacturers
Turret construction
The bearing connecting the turret and the sleeve
Anchor Arrangement: Anchors can be arranged according to actual conditions.
Anchoring diagram
Turntable: The turntable is a structural platform located at the top of the turret, accommodating various equipment such as winches for tensioning anchor chains, riser winches, and electrical control facilities.
Liquid Transfer System: The liquid transfer system is the center of the entire turret mooring system. Subsea pipelines and risers need to use it to transport crude oil to the FPSO. The submersible (STL) internal turret is moored to the seabed via conical buoys. During installation, it needs to be lifted into the conical space reserved in the hull and hydraulically locked.
Submerged internal turret
Submerged Turret Production(STP)
External Turret Mooring System
The external turret single point mooring system (ETS) operates on the same principle as the internal turret single point mooring system, the main difference being that the mooring device is installed outside the bow.
External turret mooring system
External turret mooring systems reduce the necessary constraints on the hull and can be installed at the dock, while internal turret mooring systems must be installed in a dry dock. Additionally, the number of risers required for external turrets is significantly less than that for internal turrets.
FPSO installation of external turret mooring system
Generally, converted FPSOs are suitable for using external turret mooring systems.
Tower-type Single Point Mooring Systems
Tower-type mooring systems are fixed to the seabed by rigid tower structures, serving as permanent anchor points for oil tankers loading and unloading oil and other products. Their main characteristics are: the tanker is connected to the tower via a permanent fork-shaped structure or mooring cables; multiple riser systems can be installed; construction is simple and cost-effective. They are suitable for use in shallow to medium-depth waters. Common tower-type single point mooring systems include the yoke system, submerged tower mooring systems, and mooring cable systems.
Yoke System
The yoke system is the most common type of tower mooring system.
Yoke System
Submerged Tower Mooring Systems are similar to conventional tower mooring types, except the tower is completely submerged underwater, unaffected by wave and tidal changes.
Mooring Cable Tower Mooring Systems are similar to conventional tower mooring systems, except that mooring cables replace the fork-shaped structure, reducing complexity and increasing economy.