Subsea mooring systems – the quiet revolution
  from: Asian Oil & Gas
  by: Brian Green
  Thursday, June 12, 2008

Installing floating platforms and offloading buoys has traditionally relied on chain and shackles methods of mooring. However, this approach is often costly in terms of the numbers and types of vessels needed for pile installation and buoy tow-out and connection. Moreover, the surface connection of the chains under-tension, can pose a significant health and safety risk.

Ball and taper technology mooring systems, on the other hand, operate on the simple principle of a ball engaged in a taper. As the male connector’s balls roll up the connector’s tapers, the tightness of the grip increases in direct proportion to the load applied. In the field, the male tool connected to the mooring chain is aligned into position and inserted to the female connector attached to the mooring pile. The connectors are fully self latching without the need for diver intervention, and with minimal ROV intervention for visual checks. The connector’s selfactivating mechanism allows rapid deployment. Typically a ball and taper connection can take loads up to 19,000kN MBL (minimum breaking load) and is suitable for shallow water and deepwater depths of over 2000m. Once engaged the connectors cannot be released until the load is removed. This ease of disengagement would allow floating platforms in the Gulf of Mexico moored using ball and taper connections to be moved out of the way of an oncoming hurricane, for example.

So how are the subsea mooring connectors deployed in shallow and deepwater environments?

Ball and taper connection is being used for driven and suction pile mooring applications where a mooring system requires a long length of ground chain to be connected to the mooring pile. Unless the water depth allows the connection of the ground chain to the mooring pile subsea with divers, then the mooring pile and ground chain must be installed pre-connected. Only large installation vessels have the capability of handling the piles and chain simultaneously, and where these vessels are not available numerous vessels and barges to carry out the operation.

Ball and taper connection offers an alternative approach that is simpler and more cost-effective. It involves splitting the chain, enabling the pile and chain to be installed separately in different campaigns, and using smaller installation vessels. Moreover the self-locking assembly means there is no requirement for diving operations resulting in a significantly safer mooring installation. Ball and taper technology has been successfully used in mooring spars, FPSOs, buoys, PLET tie-backs and drilling applications. Nexen Petroleum UK’s Ettrick project demonstrates how the mooring system can be applied to a FPSO retractable buoy. The installation called for a 160m length of ground chain to be connected to an 84in diameter x 50m long mooring pile. The installation vessel did not have diving capabilities and was not capable of handling the mooring pile with the ground chain pre-connected. Additional barges and/or vessels would have had to be utilised, resulting in a difficult operation with numerous simultaneous operations.

To overcome the handling problems, the ground chain was spilt into a 40m section and a 120m section. One end of the 40m section was connected to the mooring pile padeye and the other end had a female subsea connector attached to it. The chain and female subsea connector were then secured to the mooring pile.

A male subsea connector was fitted to one end of the 120m chain with the free end being left for future connection to the mooring wire.With only 40m of chain connected to the mooring pile the operation was well within the installation vessel’s capabilities. This resulted in one installation vessel installing the mooring pile and 40m of chain first then, at a later date, another installation vessel connecting the 120m of chain using the subsea connector. The installation vessel then continued to lay the remainder of the mooring line.

Although one commonly thinks of mooring in terms of the connection to the seabed, there is no reason why the mooring connection cannot be made on the platform or buoy itself. One example, where shallow water depths do not allow the top mooring wire section to hang beneath the buoy during towout, is the introduction of subsea connectors on the buoy’s mooring uni-joint. The tow becomes a far simpler and safer operation. All final mooring line connections can be made subsea without the need for divers.

An instance of this was the STP (submersible turret production) buoy used for Woodside Energy’s Vincent project offshore Australia, where pre-installed mooring wires were not permitted to hang beneath it during its towout through shallow waters containing coral reefs. There was a concern that the reef and mooring lines would be damaged during the tow. To overcome the problem, female connectors were secured to the mooring uni-joint connectors on the underside of the buoy. This meant that the buoy could be towed to the field with no wires connected and hence required fewer vessels in the fleet. Having no wires connected to the underside of the buoy also allowed the buoy to be lifted into the water in a far more simplistic way, without the need for numerous cranes to support the wires as the buoy is being lifted. Once the Vincent STP buoy had been towed to the field all the mooring line connections were carried out subsea using the subsea connectors.

Shaping the architecture

Increasingly crowded subsea production fields have allowed the ball and taper connection technology to play a more active role in subsea field architectures. The two-part installation of the mooring system referred to earlier can be used to enable the more accurate positioning of mooring piles away from pipelines during tieback operations. Alternatively the connection system provides a simpler way of installing mid water arches (MWA).

Where a MWA buoy is deployed as part of a subsea riser catenary system they are usually installed using a foundation clump weight to counteract the buoyancy of the buoy. The clump weight mass has to be greater than, the uplift buoyancy. For example, a 500t uplift buoy needs a clump weight greater that 500t. This clump weight may require a heavy lift vessel to lift and place it subsea. The use of a driven pile as a foundation can eliminate the clump weight and the need for a heavylift vessel.

On the OMV Maari project in New Zealand, a driven pile was installed beneath the point where the MWA buoy was required. Because the pile driving vessel was already in the field installing the mooring piles, the MWA buoy foundation could be installed by a vessel already rigged up to carry out the operation, and without the need for a separate heavy lift vessel or rigging.

The operation was made easier by the use of a special female subsea connector mounted within the pile that was driven into place. This allowed the pile to be placed before the MWA buoy and NKT riser system was ready to be deployed. The MWA buoy was later connected to the pile by a special male subsea connector attached to the mooring wire. The MWA buoy was then sunk to allow the subsea connector to be made up, guided by a ROV, and then de-ballasted to the uplift buoyancy rating.

In summary, where facilities for clump weight deployment are not available, a subsea connector mooring connector can be used, attached to a driven pile, to act as a MWA buoy foundation. By adopting this approach the MWA installation engineer eliminates the need for a heavy clump weight on the seabed. This allows the pile driving vessel to install the foundation, instead of a heavy lift vessel, enabling the MWA buoy and riser to be installed at a later stage.

Mooring systems integrity

Mooring lines are often subject to extreme loading conditions. For this reason it is important that ball and taper connection systems are designed and tested in accordance with certifying authority standards including DNV, ABS, BV and Lloyd’s Register, and extensively tested to ensure a long-term consistent fatigue life. Each project has unique parameters of dynamic load and design life criteria; and so all mooring connectors are designed to meet specific project criteria.

The characterisation of the metals used in subsea mooring is now a ‘live’ issue for offshore companies. This follows concerns about the engineering integrity of forged metals and their implications for the integrity of shackles, triplates, cable sockets and H-links raised in January this year by the US Department of the Interior’s Minerals Management Service, Gulf of Mexico OCS Region, issuing of a safety alert headed ‘Catastrophic failures in mooring systems possibly put floating structures at risk’. The alert followed the failure of a forged mooring shackle; where the shackle manufacturing process was found to be at fault. Heat treatment after casting and forging had resulted in a metal unable to meet certifying authority ‘Charpy’ standards for material toughness. This failure has highlighted a lack of knowledge about what happens to metals during the forging process. Surprisingly this lack of knowledge is shared by a number of industries that rely on forged metals.

To meet growing concerns about the integrity of mooring systems among oil and gas operators, First Subsea is undertaking the industry’s first investigation into shackle and mooring systems forging in collaboration with the department of engineering materials at Sheffield University in the UK. The objective of the project is to fully understand what happens during the forging process and in so doing characterise the micro structure of forged metals and its relation to Charpy toughness values and relate to the design and integrity of ball and taper connectors and shackles.

Ultimately it is expected that the project will lead to the production of a new standard for characterising the forged materials used in a ball and taper subsea connector that can be used by offshore engineers when specifying their mooring connection systems.

Ball and taper subsea connection systems have proved their worth in providing offshore engineers with a flexible, safer and cost-effective alternative to traditional mooring systems.

As offshore operations become more demanding in terms of the environment and complexity of production, ball and taper mooring systems are set to play an increasing role in shaping subsea architecture. AOG

About the author

Brian Green has clocked up 30 years' experience in the offshore industry since graduating as a scientist from Aberdeen University. Today, as general manager of First Subsea, he is responsible for the continued growth of the Ballgrab subsea connection systems business and development of new niche markets such as riser technologies and marine renewables.

Click here to register to receive your own copy of Asian Oil & Gas each month.