The new FES CALM Buoy Swivel

8th August 2017

The supply of the first-ever CALM Buoy Swivel supplied to China helps to reduce maintenance downtime

Ian Latimer, FES International


The offshore industry has been using the Catenary Anchor Leg Mooring (CALM) Buoy for many years as a system for importing and exporting oil and gas, used in shallow waters between 20 to 100 meters close to shore,

Acting as an offshore terminal anchored to the seabed, the CALM 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.

Manufacturers have been making standard CALM Buoys for many years and with them they supply their own swivel as part of the unit. When the swivels need to be repaired or replenished however, the owners of the CALM Buoy are not always able to repair or purchase the replacement swivel on its own from the original manufacturer and instead they are forced to remove the CALM Buoy from service or replace the whole CALM Buoy itself.

With the ever increasing demand for more cost effective cargo offtake, there is a huge market potential for CALM Buoy Swivel supply to both new and existing Calm Buoys throughout the world.

A new solution

As a provider of fluid transfer solutions to the offshore industry, at FES we believe that heavy investment in research and development is crucial in bringing new innovative solutions to market and in helping to overcome the challenges facing our customers.

Two years ago, our team of engineers began looking at the CALM Buoy swivel market and realised the need for a new swivel design which could be bought separately from the CALM Buoy unit itself. After focusing our research and development on the challenge, FES came to the market with a new CALM Buoy swivel design.

The design solution 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.

Usually, if there is a mechanical issue the whole swivel needs to come out of the CALM buoy and is required to go back to shore for repair. This new design however, enables the replacement of the main swivel bearing and sealing system on board without the requirement to bring the main buoy into a repair yard – significantly reducing maintenance downtime by avoiding it being out of service for long periods of time.

Additional features

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

This is also 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 can also be supplied to allow for remote and on-site telemetry monitoring of the swivel sealing system.

It 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.

Living up to its name of Flexible Engineered Solutions, the company has delivered a modular solution for this new CALM Buoy swivel allowing for 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 (FAT) 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 provide the CALM Buoy swivel with excellent sealing performance and a minimum design life expectation of 25 to 30 years.

The challenges

As with the design of any new offshore solution, 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. We therefore had to look very closely at the design of the bearing and sealing system to come up with a solution that would fulfil a minimum of the 20/25 year design life for the bearing.

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

Benefits to the operator  

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; from disconnecting the swivel, repairing it (if it can be repaired) and then reconnecting it back to the CALM Buoy. As well as this there are also the indirect costs associated with the unavailability of the terminal.

With the new FES modular design however, the centre 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 and months.

The future  

CALM Buoy swivels are suppled onto CALM Buoys all over the world. With the older CALM Buoys swivels in need of replacement there is huge potential for this new design in the market as it offers a cheaper, robust and effective alternative.

We are also well on the way to 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 sizes swivel units can then be made available on an even quicker turnaround as the design will already be complete.

Going forward, if FES CALM Buoy swivels are purchased with an additional FES modular centre section as a spare sitting in an offshore base, when there is an issue, this centre section can be replaced in one day and then the CALM Buoy swivel can be back up and running straight away.

The FES Calm Buoy swivel is fully certified and has been verified and approved during design, manufacture and test by DNV GL.