The Catenary Anchor Leg Mooring (CALM) Buoy, a system for importing and exporting oil and gas offshore, can be used in shallow waters between 20 m to 100 m (66 ft to 328 ft) close to shore. Acting as an offshore terminal anchored to the seabed, the buoy enables cargo to be offloaded from a tanker offshore rather than the tanker having to sail into dock—something that is not always possible given the size and complexity of some vessels.

CALM Buoys come with their own swivel as part of the unit. When the swivels need to be repaired or replenished, however, the owners of the buoy are not always able to repair or purchase the replacement swivel on its own from the original manufacturer and instead are forced to remove the buoy from service or replace the whole buoy.

To overcome this challenge, FES International has released a CALM Buoy swivel design that can be bought separately from the CALM Buoy unit. The design incorporates the bearing and sealing system into a central modular swivel hub, which is a precision-machined and sealed unit. By doing this, the upper or lower swivel assembly or swivel hub can easily be removed and replaced during the project life or replaced as project requirements change. This new design enables the replacement of the main swivel bearing and sealing system onboard without the requirement to bring the main buoy into a repair yard, significantly reducing maintenance downtime.

The new design also is supplied with a leak detection system mounted on the swivel assembly or at a suitable location on the CALM Buoy. It uses a series of double block and bleed valves to monitor the complete sealing system, allowing constant monitoring of the condition of all seals.

This also is connected to a fluid monitoring assembly, which includes (as a minimum) a sample cylinder and fluid level gauge to enable real-time monitoring. An integrated electronic monitor of the leak detection system also can be supplied to allow remote and onsite telemetry monitoring of the swivel sealing system.

The design also has an integrated pressure transmitter (optional) to monitor the pressure during loading and an integrated electrical slip ring to allow electrical cable connections from subsea to topside.

FES has released a modular option for this new CALM Buoy swivel that allows greater flexibility in the design of swivel interfaces. It accommodates a wide range of pipe configurations with single-, dual- and triple- path swivel assemblies.

The modular system also allows FES to carry out static and dynamic factory acceptance testing of the bearing sealing system while the remaining upper and lower assemblies are being fabricated and machined, decreasing lead times and reducing the risk profile.

Accurate machined interfaces and bearing interfaces are designed to provide the CALM Buoy swivel with excellent sealing performance and a minimum design life expectation of 25 to 30 years.

Challenges

As with the design of any new offshore tool, a number of challenges have to be addressed at the design phase due to the often harsh conditions and remote environment. The remote locations of these CALM Buoys mean they are unmanned and receive very little maintenance. FES had to look very closely at the design of the bearing and sealing system to come up with a solution that would fulfill a minimum of the 20- to 25-year design life for the bearing.

To further extend the life expectancy of the unit, regular maintenance is advised, and FES recommends either removing or replacing the modular section every five years.

Typically, if a standard CALM Buoy swivel started to leak or was damaged in service, it would take weeks if not months to repair and replace. The direct costs associated with this soon add up—disconnecting the swivel, repairing it (if it can be repaired) and then reconnecting it back to the CALM Buoy. There are also the indirect costs associated with the unavailability of the terminal.

With the new FES modular design the center section is modular and not integral to the larger fabrications above and below, so it is easy to install, handle and maintain. Downtime is hours in comparison to weeks or months.

Looking ahead

FES also is creating one design for the modular section, which is unique in that it will fit a number of different sizes of CALM Buoy swivels. Different sized swivel units can then be made available on an even quicker turnaround as the design already will be complete.

Going forward, if CALM Buoy swivels are purchased with an additional FES modular center section as a spare sitting in an offshore base, this center section can be replaced in one day, and then the swivel can be back up and running almost immediately.

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