Hybrid Mooring System

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Brazilian Application No. BR1020240225503 filed on Oct. 29, 2024, the disclosure of which is expressly incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention falls within the technical field of naval and offshore engineering, with an emphasis on mooring systems for floating oil production platforms (FPSO-Floating Production, Storage, and Offloading), as well as operations related to the maintenance, demobilization, and operation of such systems in challenging maritime conditions.

BACKGROUND OF THE INVENTION

Mooring systems onboard oil production platforms, commonly called mooring systems, have the function of moving the installation mooring outward (paying the chain), allowing the mooring line to be connected to the platform, and then retracting the installation chain and excess top chain until the mooring line reaches working tension. On modern FPSO platforms, the mooring system typically consists of a fairlead (a pulley that diverts the chain from its top angle to vertical), a chain-stopper (an articulated hydraulic device that “bites” the chain, locking it), and a chain-jack (a device responsible for effectively moving and applying tension to the chain, similar to a hydraulic jack).

On a modern FPSO, the mooring lines are divided into four clusters, and accordingly, the mooring system of the FPSO is divided into four mooring balconies. A typical balcony consists of one chain-stopper per line and a pair of rails, which move the chain locker or box, and the chain-jack, which is attached to it. The four balconies are powered by two high-capacity hydraulic power units (HPUs), one located in the utility area of the FPSO and the other on the forecastle. The chain-jack and its HPU are designed to provide maximum pulls of between 400 and 600 tons-force.

Although highly safe, current mooring systems are significantly complex and, therefore, subject to failures that lead to downtime during hook-up operations. Hook-up is the operation of connecting a new platform to its mooring system. In a typical hook-up operation, the AHTS (Anchor Handler and Tug Supply) vessel approaches the FPSO carrying the mooring line, receives a messenger cable from the FPSO, through which it pulls the installation chain, which is then connected to the mooring line and released into the water. Finally, the mooring winch of the FPSO retrieves the installation chain until the top chain can be set on the chain-stopper. Tensioning of the mooring lines usually occurs after hook-up.

Maintaining this equipment during the operational life of the platform brings inherent challenges due to its complexity, and the operability of the mooring winch can become critical to the safety of the platform in the event of mooring line failure. Furthermore, the lack of operability of the mooring winch is a typical problem during mooring line maintenance and demobilization at the end of the service life of the unit.

In recent years, some smaller platforms have been equipped with innovative technology that eliminates the need for a mooring winch and all its associated equipment. In this solution, the fairlead is replaced by an articulated universal joint (uni-joint), the mooring winch by a device consisting of a pulley similar to a fairlead, and a spring-loaded ratchet-like latch (in some models, opening and closing are performed by a ROV (Remotely Operated Vehicle-remotely operated underwater vehicle). In the mooring operation with the in-line tensioner, the first section of the top chain can be installed offshore or pre-installed at the shipyard. For typical Brazilian coastal conditions, a section of chain must be pre-installed at the shipyard. This section is connected to the uni-joint and temporarily accommodated on the side of the platform. The AHTS can then connect this chain directly (in shallow water) to the rest of the mooring line and release it into the water, or install an intermediate section (in deep water) and then connect this second section to the rest of the mooring line.

However, the in-line tensioner impacts the mooring operations of a new platform (hook-up), such as chain accommodation and release. In deep water, it would be necessary to pre-install approximately 70 meters of chain per line, which may be impractical on a modern FPSO. Furthermore, there is no reliable method for replacing the chain connected to the uni-joint, which poses a significant risk to mooring system maintenance throughout the service life of the platform. Demobilization can also show similar difficulties to units with a non-operational winch.

The invention solves the technical problem of optimizing both the mooring and demobilization processes of floating oil production platforms by combining features of a traditional mooring winch system and in-line tensioners to create a hybrid system that leverages the strengths of each. By utilizing a low-capacity rotary winch in conjunction with other devices, the invention allows for faster mooring operations compared to the system currently used on large FPSOs (the same features that make the chain-jack safe limit its speed). Additionally, the invention reduces the quantity and complexity of mooring equipment onboard the platform.

The document CN 117227901 B describes a telescopic anchor hoisting device for tugboats. This device features a design aimed at optimizing mooring operations. It consists of a base body fixed to the bow of the deck of the tugboat, where a horizontal telescopic anchor support is inserted and can slide horizontally in and out. This movement is enabled by a hinge-connected hydraulic cylinder fixed below the base body bulkhead. The chain guide roller, located on top of the anchor support, ensures smooth flow of the anchor rope, contributing to the safety and efficiency of operations. Therefore, it is clear that the document CN 117227901 B fails to show a hybrid mooring system, as proposed by the present invention.

The document WO 2021262980 A1 describes a mooring assembly used to secure structures such as wind turbines or oil platforms to the seabed using mooring lines. These lines consist of chains anchored to the seabed, coupled to ropes at the top, forming a link between the structure and the seabed. In-line tensioner systems, such as the one detailed, play a crucial role in maintaining the tension and length of these mooring lines. They consist of structures assembled to provide stability, with one end connected to a fixed chain (passive chain) and the other to an adjustable chain (active chain). These tensioners allow precise control over the tension of the mooring lines, vital for the stability of the structure in changing conditions. However, the main contribution of this document lies in the tensioning device itself, distancing itself from the proposal of the present invention for a hybrid mooring system. Furthermore, the system proposed in the document WO 2021262980 A1 would be unfeasible for application in non-ideal environmental conditions, such as those off the Brazilian coast. For the tail chain to disconnect, an ROV must be docked to the equipment, which can pose serious risks to the ROV due to the movement generated by the disconnection of the mooring line.

The document GB 2572252 A describes a mooring system that utilizes a first tensioning means to connect the floating structure to the line and a second tensioning means, such as intermediate tensioners along the line. This second tensioning means may include devices such as guides, locking arrangements, and hydraulic mechanisms to adjust the tension in the line. In general, the document GB 2572252 A combines a system with an in-line tensioner with a high-capacity hydraulic system, with high pulling capacity and very low movement capacity, to allow fine adjustments to pulling the mooring system when necessary. From the perspective of mooring (Hook-up), un-mooring (Hook-out), or component replacement operations during service life, this system has the same difficulties as systems with in-line tensioners and universal joints, requiring ideal sea conditions, thus deviating from the problem solved by the present invention.

Therefore, it is clear that the documents from the state of the art completely fail to show the hybrid mooring system of the present invention. None of the documents show: the use of a low-capacity rotary winch (typically about 10 times faster than a chain-jack); the use of a removable winch, meeting the main appeal of the in-line tensioner system, which is to reduce onboard equipment; the use of a sacrificial cable; the repositioning of the tensioner close to the seabed (enabled by the presence of the sacrificial cable), reducing the negative effect it has on the efficiency of the mooring line (because it is positioned close to the seabed and thus has a reduced impact on the top angle of the mooring line); use of KS hook to allow disconnection of the in-line tensioner from the fixed point.

The proposed invention addresses all phases of the service life of the unit, unlike current systems (chain-jack), which, due to the complexity of the equipment used, are generally inoperative near the end of the service life of the unit. This also differs from the state of the art, as they are not adapted to suboptimal maritime conditions.

SUMMARY OF THE INVENTION

The present invention relates to a hybrid mooring system that combines elements of traditional mooring winch systems with in-line tensioners for floating oil production platforms (FPSOs). The hybrid mooring system comprises a reduced-size mooring balcony equipped with chain-stoppers and fairleads positioned below the balcony. A standard rotary winch with a drum and steel cable, with a pulling capacity of approximately 30 tons in the last layer (varying depending on the diameter of the chain used), is mounted on the balcony, along with a hydraulic power unit (HPU).

The rotary winch is responsible for bringing the chain from the anchor handling tug supply (AHTS) to the chain-stopper without applying tension beyond the chain weight. Traction is applied by an in-line tensioner positioned close to the seabed, which minimizes the negative impact on the top angle of the mooring line and improves the ability of the SPU to maintain its position.

The system includes a short synthetic fiber (typically polyester) sacrificial cable connected between the active chain and the in-line tensioner. This sacrificial cable allows the mooring line to be opened during maintenance or demobilization, mitigating risks associated with the loss of in-line tensioner operability during the service life of the SPU.

During hook-up operations, the rotary winch of the balcony pulls the top chain from the AHTS to the chain-stopper, after which the in-line tensioner applies the necessary tension. The use of a KS hook allows the in-line tensioner to be safely connected and disconnected from the fixed point of the mooring line, facilitating subsequent operations.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

The present invention will be described below with reference to its typical embodiments and with reference to the accompanying drawings.

FIGS. 1A and 1B are representations of the SPU balcony of the system of the present invention.

FIG. 2 is a representation of the mooring line of the system of the present invention.

FIGS. 3A to 3F illustrate the hook-up procedure using the system of the present invention.

FIG. 4 illustrates the AHTS actuating the in-line tensioner, applying tension to the active mooring line, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The hybrid mooring system of the present invention comprises three main structures: the SPU balcony, the mooring line, and a fixed mooring point. As shown in FIGS. 1A and 1B, the balcony is small in size and is equipped with chain-stoppers (10), with their respective fairleads below the balcony. Above the balcony, there should be a standard rotary winch (12), with a drum and steel cable, and a pulling capacity in the last layer adequate to handle a portion of approximately 70 m of the top chain (approximately 30 t), in addition to a HPU (16).

Above each chain-stopper (10) there must be a gantry (14) with an eyelet, to which a pulley (18) is connected when operating the chain-stopper (10). The apparatus above the balcony (HPU (16), rotary winch (12), gantry (14), and pulley (18)) serves only to bring the chain (20) from the AHTS to the messenger cable support (22) of the chain-stopper (10), without any traction other than the own weight of the chain. Traction must be provided by an in-line tensioner located on the mooring line and closest to the bottom. The chain-stopper (10) also has a load cell (24) to measure the force exerted on this structure.

To complete the assembly of the hybrid system of the present invention, the fixed point of the mooring line must be provided with a chain section with a waiting shackle for a KS hook at the end. The KS hook is of the type described in U.S. Pat. No. 6,915,754, reference numeral 34. Possible fixed points for the mooring line include: torpedo piles, suction piles, or VLA anchors (vertical load anchors).

As can be seen in FIG. 2, which depicts the mooring line (2), a KS hook (34) is connected underwater to the waiting shackle (32). A section of bottom chain (30) is incorporated into the eyelet of the KS hook (34), which is, in its turn, connected to the eyelet of an in-line tensioner (36).

The chain that passes through the in-line tensioner (36) (also known as the active chain (52)), which is pulled by the AHTS to provide traction), must be connected to a short polyester rope (38), between 10 m and 15 m, an intermediate chain section (42), and the main polyester rope (40), followed by the assembly of the line with an arbitrary number of polyester ropes and intermediate chain sections, up to the top section.

It should be noted that the short polyester rope is a sacrificial rope (38), and its purpose is to provide a means of opening the mooring line by cutting it. Since the means to open the mooring line, in the event of maintenance or demobilization of the unit, is guaranteed by the sacrificial rope, the risks associated with the loss of operability of the in-line tensioner during the useful life of the Stationary Production Unit (SPU) cease to exist.

The top chain (44) is divided into two sections, so that the AHTS can leave the first section mounted on the upper polyester rope and the second section can be delivered to the SPU, closing the mooring line between these two chain sections.

The weight of the tensioner (36) and the chain downstream of the tensioner (52) (the chain that has already passed through the tensioner) has a negative influence on the top angle of the mooring line, and consequently, on the ability of the mooring line to maintain the position of the SPU. For this reason, the tensioner (36) should be positioned in the deepest section of the mooring line, below the polyester ropes, as shown in FIG. 2. In this position, the negative effect is minimized.

Furthermore, since the tensioner (36) does not have the function of opening the mooring line, the active chain downstream of the tensioner can be cut by an ROV, relieving weight on the mooring line and further reducing the negative influence on the top angle. In this case, the shackle at the end of the active chain, suitable for KS hook connection, would be lost, leaving only a common link at the end, unsuitable for subsea connections with KS hooks.

More specifically, the invention relates to a hybrid mooring system between a stationary production unit (SPU) and a fixed mooring point (50), wherein the balcony of the SPU (1) comprises: at least one hydraulic power unit (HPU) (16); at least one rotary winch (12); and one or more chain-stoppers (10) equipped with a gantry (14) with an eyelet.

Furthermore, the hybrid mooring system features an mooring line (2) comprising: at least two bottom chain sections (30) connected to the fixed mooring point (50), wherein one of the bottom chain sections comprises a waiting shackle (32); a KS hook (34); an inline tensioner (36); an active chain (52) followed by a sacrificial line (38); a main polyester line (40); at least two chain tails (42); a top chain line (44); and an end assembly connection (46).

As depicted in FIGS. 3A to 3F, the hook-up operation occurs much like the traditional operation-on the balcony, the steel cable of the winch is passed through the pulley and connected to the messenger line. The AHTS approaches carrying the bottom section of the mooring line and receives the other end of the messenger line; the AHTS retracts the messenger line, bringing the 30 t winch cable of the SPU to its deck. The AHTS connects the 30 t cable to the top chain and begins lowering it into the water, while the 30 t winch pulls the top chain to the balcony. It is worth noting that the rotary winch is about 10 times faster than the chain-jack.

FIG. 3A depicts the procedure for passing the messenger line, also called the “retained” (represented by the blue and orange sections). Next, FIG. 3B shows the transfer of the synthetic fiber messenger line (represented in green). FIG. 3C depicts the AHTS retracting the synthetic fiber messenger line, bringing the steel messenger line. As shown in FIG. 3D, the AHTS retracts the steel messenger line connected to the winch cable of the SPU until the rotary winch cable (represented in orange) reaches the AHTS.

FIG. 3E shows the procedure for connecting the top chain to the platform cable (PSC). The AHTS “pays the chain” and the PSC retracts the rope, bringing the chain to the balcony until the chain-stopper can be closed.

In FIG. 3F, the AHTS brings the bottom section aboard, connects the first section of the top chain (connected to the polyester rope) to the second section (seated on the chain-stopper), and releases the mooring line into the water.

FIG. 4 shows the AHTS (100) actuating the in-line tensioner, applying tension to the active chain of the mooring line (2) through its working rope (106), connected to the active chain through a second KS hook (102) associated with a second shackle (104).

Since the rotary winch employed has a complexity similar to that of a single subsystem of a chain-jack, fewer incidents are expected during operation.

Furthermore, this type of winch is off-the-shelf, mass-produced equipment, and therefore typically has superior reliability than complex, custom-designed equipment such as a chain-jack.

The speed of the rotary winch is much higher than that of a chain-jack, and the AHTS winch that drives the in-line tensioner is also significantly faster and more reliable. Thus, both hook-up and tensioning operations are faster than existing alternatives.

An important detail is that in hybrid balcony operation, to service the next mooring line, simply reposition the winch pulley and cable, whereas with the traditional system, it is necessary to move the entire chain-jack structure from one chain-stopper to the other.

With the hybrid system, the operation of the mooring balcony becomes as simple as the operation of the pull-back winches, which are used during hook-up to connect the positioning AHTSs. Thus, the mooring operation can be performed 12/12 by the same team that operates the pull-back winches. In other words, the hybrid system eliminates the need for additional personnel onboard, as does the system with a pure inline tensioner.

The combination of all these factors makes the hybrid mooring system of the present invention extremely desirable for operations in rough waters and challenging marine scenarios, facilitating everything from the hook-up process to decommissioning at the end of the service life of the unit.