HOW IT WORKS
How do Shuttle Tankers Work?

Petrobras will make history in late 2011 when it starts production from its Cascade and Chinook fields 160 miles from shore in 8,000 feet of water in the Gulf of Mexico. But it won't be the distance of the field from shore, nor the water depth that makes it special. Rather, Petrobras' field development plans include the use of the first floating, production, storage and offloading (FPSO) vessel in the GOM and subsequently the first shuttle tankers. Oil will be transported to shore via shuttle tanker while gas will travel through pipelines.

News of the first FPSO and shuttle tanker in the GOM made waves through the industry when Petrobras disclosed its plans in 2007. Since 1920, the Merchant Marine Act, better known as the Jones Act, has prohibited entry of foreign ships working between US ports. The vessel must be built in the US, owned and crewed by US nationals and be flagged in the US. A FPSO is recognized as a US port, but it does not have to follow the Jones Act. However, any shuttle tanker working in the GOM has to be built or converted in one of the few US shipyards that meet United States Federal maritime qualifications.

In 2002, the Minerals Management Service announced its approval for FPSOs to be considered for use in the GOM. Several ultra-deepwater field discoveries as well as severe hurricane damage to pipelines sparked rumors of the first FPSO in the GOM, but it wasn't until 2006 that Petrobras announced plans to use an FPSO at its Greater Chinook Area development. In March 2011, BOEMRE gave final approval for Petrobras to start oil and natural gas production at its Chinook-Cascade project using the BW Pioneer FPSO.

What is a Shuttle Tanker?

A shuttle tanker is a specialized ship designed to transport oil from offshore oil fields to onshore refineries. Shuttle tankers are often used as an alternative to pipelines in harsh climates, remote locations or deepwater.

Shuttle Tanker
Shuttle Tanker
Source: shetlandtimes.co.uk

Shuttle tankers were initially used in the North Sea in the 1970s. Since then, the value of using a shuttle tanker instead of pipelines has spread worldwide. Operators have recognized the flexibility of loading oil and transporting it to any destination they want as opposed to pipelines that are fixed to one receiving terminal.

Operators also favor the ability of shuttle tankers to segregate the oil. Pipelines often blend crude from various fields. Shuttle tankers are also easier to maintain and have a back-up-system in place if they require maintenance. If a pipeline requires maintenance, the whole network is likely to be shut in.

Shuttle tankers have also proven to be safer than conventional crude oil tankers. Conventional tankers can only be used in calm waters. They load from offshore installations with assistance from tug boats to stay on location; whereas shuttle tankers operate independently in all water and weather conditions. Shuttle tankers are equipped with two or three bow thrusters and stern thrusters as well as dynamic positioning to keep the tanker on location. Flap rudders and controllable pitch propellers also help to increase low speed maneuverability.

Field operators often award shuttle tanker owners long-term contracts to offload and transport oil from the field installation to the receiving terminal. The crew is highly trained to maneuver the tanker safely when close to offshore installations. They also go through extensive training to operate the offshore loading system and the dynamic positioning system safely.

Types of Systems

The offshore loading system transfers oil from a FPSO. The loading system is on the bow of the ship, which allows the shuttle tanker to load safely from various platforms, regardless of weather conditions. A special cone-shaped loading system can also be installed on the keel of the ship.

The offshore platform has a field export system that connects to the loading system on the shuttle tanker. The type of export system onboard varies based on safety, weather conditions, water depth, seabed conditions and cost.

The single point mooring system (SPM) is an offshore loading buoy where tankers can connect to load and offload oil. SPMs can handle any size ship. The body of the buoy, mooring and anchoring elements, product transfer system and other components make up an SPM. Together, these parts allow shuttle tanker operators to favorably position the vessel around the mooring point so it is less affected by weather conditions. A hose from the export system is then connected to the loading system on the vessel. Dynamic positioning systems have significantly reduced wear-and-tear on SPM equipment.

After the introduction of dynamic positioning in the 1980s, the Ugland Kongsberg Offshore Loading System was introduced. This less expensive option can only be used with dynamically positioned shuttle tankers.

Eight years later, the Submerged Turret Loading (STL) system hit the market in 1993. An STL has a submerged buoy that comes through a cone-shaped turret on the keel of the shuttle tanker. The STL is used in the most extreme weather conditions with sea states up to 6 meters.

Introduced in the late 1990s, Single Anchor Loading (SAL) is used primarily in shallow water and in good weather conditions. The SAL is a simple, cost effective alternative to the STL.

The Tandem Loading System, first carried out in 1991, requires the shuttle tanker to connect to the stern of the FPSO or FSO. Dynamic positioning is necessary to monitor the positions of two units.

Dynamic Positioning Systems

Explained above, dynamic positioning systems are an essential component in today's shuttle tanker technology. These computerized steering and positioning systems keep the vessel on position in some of the harshest environments.

The computer monitors wind, currents, tides and location of thrusters and adjusts the position of the vessel using pitch propellers and lateral thrusters.

There are three classes of DP systems.

  • Class one vessels have one set of maneuvering and computer systems. There is no redundancy, which means that there is no back-up system in place. If a single component fails, the whole system fails. These vessels primarily work in calm weather conditions.

  • DP2 vessels have redundancy. If one component fails, there is a backup component for all active systems. The North Sea requires DP2 vessels or greater.

  • New vessels are equipped with DP3. These vessels have to withstand fire or flood in one compartment while keeping the system running.

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