OSV-sized transfer vessel simplifies crude oil transfer

Offshore loading operations from floating production, storage and 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 DP system is used to position the shuttle tanker during the loading operation.

It has not been possible for conventional tankers to load directly from 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, and the offload hoses are not long enough to reach a midship manifold and have a specialised hose-end valve which is not compatible with a standard manifold.

But thanks to the development of the cargo transfer vessel, conventional tankers in Brazilian waters can now load directly from the FPSO.

Dynamically positioned, the cargo transfer vessel – which is the size of an offshore support vessel – has the ability to deploy a mooring hawser and an offload hose to connect to a conventional tanker and then 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 on board the cargo transfer vessel. The cargo transfer vessel 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 vessel and connected on the tanker’s midship manifold. With the hawser and hose connected, the vessel tows the tanker and tug towards the FPSO and takes up a position in the offloading sector of the FPSO.

At this point, the cargo transfer vessel 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 cargo transfer vessel, where it is connected.

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

This new approach allows the operator to export larger parcels directly to tankers rather than having a shuttle tanker perform the offshore loading operation and the cargo transferred via an STS.

The risks

Introducing a cargo transfer vessel into loading operations poses different risks compared to having a vessel load directly from an FPSO.

While the cargo transfer vessel is able to perform an emergency disconnection from the FPSO in the same way a dedicated shuttle tanker can, any disconnection between the cargo transfer vessel and tanker has to be performed manually. As a safety feature, the offload hose between the cargo transfer vessel and tanker is fitted with a dry break coupling, which activates in the event of any excessive strain to the offloading hose.

Since the tanker is moored to a dynamically positioned vessel, it relies on the station-keeping ability of the cargo transfer vessel rather than a secure mooring directly to the FPSO. Any excessive loads from the hold-back tug or environmental forces could result in the cargo transfer vessel moving out of position and being forced to perform an emergency disconnection from the FPSO.

The introduction of additional vessels into the operation increases the risk of miscommunication among the vessels. This could lead to inaccurate or misunderstood instructions or delays in stopping cargo operations.