New Cargo Transfer Vessels Allow Offshore Loading for All

Offshore loading operations from Floating Production Storage Offloading vessels (FPSOs) in Brazil are usually carried out by dedicated dynamically positioned (DP) shuttle tankers. The shuttle tanker’s bow loading system connects to the FPSO’s offload hose and its dynamic positioning system is used to position the shuttle tanker during the loading operation.

It has not been possible for conventional tankers to load directly from the FPSOs in Brazilian waters, which is in contrast with West Africa where such operations are common. This is because:

• Brazilian waters typically feature strong and shifting currents
• the offload hoses are not long enough to reach a midships manifold
• the offload hoses have a specialised hose end valve which is not compatible with a standard manifold

But now, the development of the Cargo Transfer Vessel (CTV) allows conventional tankers to load directly from the FPSO.

Introducing the CTV

This is a dynamically positioned vessel that has the ability to deploy a mooring hawser and an offload hose to connect to a conventional tanker and then connect to the FPSO’s offload hose.

The initial connection process is very similar to a tanker connecting to a single buoy mooring and is co-ordinated by a mooring master onboard the CTV. The CTV deploys a hawser for the tanker to moor and a hold-back tug is used to keep the tanker in position. The hose is then deployed from the CTV and connected on the tanker’s midship manifold. With the hawser and hose connected, the CTV tows the tanker and tug towards the FPSO and takes up a position in the offloading sector of the FPSO.

At this point, the CTV performs the same role as a shuttle tanker and maintains position within the FPSO’s offloading sector. The offloading hose is passed from the FPSO to the CTV and it is connected.

The cargo is then pumped to the tanker via the CTV which uses booster pumps to supplement the flow rate. The specialised DP software on the CTV tracks and follows the movements of the tanker while keeping the CTV in the offloading sector of the FPSO.

This new approach allows larger parcels to be exported directly to tankers rather than a shuttle tanker performing the offshore loading operation and then transferring the cargo via an STS.

The risks

Introducing a CTV into a loading operation introduces different risks when compared to a vessel loading directly from an FPSO.

While the CTV will be able to perform an emergency disconnection from the FPSO in the same way a dedicated shuttle tanker can, any disconnection between the CTV and tanker will be required to be performed manually. As a safety feature, the offload hose between the CTV and tanker is fitted with a dry break coupling. This will operate in the event of any excessive strain to the offloading hose.

As the tanker is mooring to a dynamically positioned vessel, it will be relying on the station-keeping ability of the CTV rather than a secure mooring directly to the FPSO. Any excessive loads from the hold-back tug or environmental forces could result in the CTV not maintaining position and being forced to perform an emergency disconnection from the FPSO.

Additional vessels in the operation brings with it a greater risk of miscommunication amongst the vessels. This could lead to incorrect or misunderstood instructions or delays in stopping cargo operations.

Safe operations

To ensure the operation is carried out safely, the detailed field specific offload procedures should be strictly followed by all parties.

This will include testing communications, applying weather limitations for the connection and loading operation and making contingency plans. Masters on tankers undertaking this operation should be familiar with these procedures. Address any concerns with the mooring master prior to commencing the connection process.

Lars Einar Rosenhaug Bjørset of Kongsberg Maritime, the designers and providers of the CTV’s dynamic positioning software and systems, explains “The personnel performing this operation should be well trained. The operation is a new way of thinking, not like any other operations before. There are new DP software functions to be learned together with the external forces from the VLCC and the tug.

Training in simulators will help in crew training for this new scenario: “As the operation is so new to everyone involved, the CTV crew should practise whole scenarios in full scale bridge simulators taking all the vessels into account. This will provide personnel  with a situational awareness of the operation, real-time communication protocols and an opportunity to follow procedures during normal operation and emergency situations.”