Offshore oil spills FAQ

Offshore oil spills
Frequently Asked Questions

Oil spills at Offshore Loading and Discharging Terminals

1. What are the risks of oil spills at offshore Loading and Discharging Terminals such as Single Point Moorings (SPM), Conventional Buoy Moorings (CBM), Multi Buoy Moorings (MBM), Floating Production Storage and Offloading (FPSO) and Articulated Loading Platforms (ALP)?

The most common and potentially most dangerous causes of offshore oil spills at terminals are:

Tanker breakout

Tanker breakouts can be caused by storms, sudden squalls, strong currents, variable tides, tanker manoeuvring errors or failure of deck equipment that results in unscheduled hawser failure. The consequences can be catastrophic, with the possibility of substantial pollution and damage to hoses and transfer systems. Tanker and terminal downtime also has immeasurable cost implications. Unwelcome negative media attention often follows.

Incident where Gall Thomson MBCs were not fitted

An offshore terminal operator in Southern Europe experienced a disastrous and fatal accident during a sudden storm in 1998.

The subject company has a refinery where crude oil is imported through an offshore CBM (Conventional Buoy Mooring) tanker discharging terminal. Additionally, refined products are exported via an adjacent terminal of similar type.

On this occasion a tanker was offloading crude oil at the import terminal and another tanker was loading gasoline at the export terminal. A sudden storm occurred with force 8 to 9 (Beaufort Scale) winds and both tankers parted their mooring hawsers and drifted out of control.

Extreme tensile loads were applied to the transfer systems resulting in ruptured hoses, crude oil and refined product spill, damaged PLEMs and fractured submarine pipelines. A serious fire resulted and four support vessel crew lost their lives.

This company now has Gall Thomson Marine Breakaway Couplings fitted to all of its offshore hose strings.

Example where Gall Thomson MBCs were fitted

In 2012 a tanker breakout occurred at a SPM Terminal offshore to India during which two 16” NB Gall Thomson Standard Double Closure Marine Breakaway Couplings parted successfully. The activation safely relieved the extreme tensile load in the floating hose strings and shut down product flow. This negated transfer system damage and minimised pollution.

Extreme Pressure Surge

An excessive and damaging pressure surge is normally generated by the inadvertent sudden closure of the export tanker manifold valve or the failure and slamming shut of the hose end butterfly valve (if fitted) during pumping operations.

Example without Gall Thomson MBCs being fitted

In 2007 an emergency spill response plan was put into operation after as much as 27,700 barrels (4,400 mts3) of oil leaked into the sea from a platform in the Norwegian sector of the North Sea.

The offshore oil spill occurred during crude oil loading from the platform to the shuttle tanker.

Upstream Oil and Gas newspaper reported the following:

“The incident started with a gash, caused by corrosion and external damage, in a hydraulic hose leading to the coupler valve on the tanker’s bow loading system. The ruptured hydraulic hose caused the valve to snap shut in half a second, as opposed to the normal 25 to 28 seconds, creating a pressure surge that ruptured the loading hose about 10 metres underwater.”

Upstream 15th February 2008

The 20”NB flexible subsea hose was completely severed by the extreme pressure surge which was generated by the failure of a hydraulic line to the Quick Connect/Disconnect Coupling at the shuttle tanker bow, causing the integral disc valves to slam shut against full flow.

The installation of a Gall Thomson Marine Breakaway Coupling in the 20” hose string would almost certainly have negated the hose rupture and avoided the pollution and everything else associated with a this extreme pressure surge event.

Example with Gall Thomson MBCs fitted

In March 2012, an extreme pressure surge incident was experienced at a SPM terminal offshore to Nigeria due to the sudden closure of the hose end butterfly valve. The Gall Thomson MBCs in the floating hose strings successfully activated, immediately dissipating the intensity of the pressure surge. No system damage was experienced in the line and serious pollution was prevented.

2. How can Gall Thomson MBCs prevent major offshore oil spills?

The simple installation of an MBC will protect the system against tanker breakouts and extreme pressure surge incidents. The device will part at a predetermined load and shut off the product flow should excessive load be applied to the hose string.

The Marine Breakaway Coupling separates when its pre-set parting load is exceeded. This relieves tension in the hose system before it can rupture. On parting, the MBC’s unique Petal Valve shuts off the line on both sides of the parting point. This is either a controlled or instantaneous closure depending on the specification of the MBC as determined by operational requirements.

The result of the coupling’s function is a massive reduction in offshore oil spill and pollution along with a big reduction in costly vessel downtime. Prevention of damage to extremely expensive capital equipment such as the hose string, mooring buoy structures (or storage vessel), off-take arm or sub-sea PLEM (in the case of a CBM system) and shuttle tanker or barge manifold is also avoided.

3. What is the average volume of oil spilled following a tanker breakout or extreme pressure surge incident when no MBC is installed?

This is a very difficult question to answer as there are so many determining factors to be considered. As can be seen from the Norway offshore oil spill example – the oil spill amounted to approximately 4,400 mts3 ; however, some offshore oil spills have been much larger and others have been less.

The volume of offshore oil spills depend, among other things, upon the time taken to shut down the pumps, the flow velocity at the time of the incident and the size of the pipeline.

4. Why do the vast majority of Engineering Contractors and Oil Companies insist on employing a Gall Thomson MBC?

The Gall Thomson Marine Breakaway Coupling has become the world-wide “Industry Standard” for offshore marine terminal applications. The Gall Thomson MBC is also the only proven and reliable safety device; backed by a long and successful track record of 40 years.

Recent major offshore oil spills have heightened awareness of oil companies for the need to take environmental protection precautions and avoid incidents that incur substantial costs and negative media attention: consequences that can severely damage any organisation.

Gall Thomson Marine Breakaway Couplings enhance the safety, reliability and integrity of offshore oil and gas terminal operations and this is why engineering contractors and oil companies insist on the inclusion of Gall Thomson MBCs.

5. What are the benefits to be gained from operating with a Gall Thomson MBC?

  • prevention of offshore oil spills
  • environmental protection
  • prevention of damage to transfer system
  • reduced terminal operating insurance premiums
  • reduced terminal downtime
  • minimal costly tanker downtime
  • substantial cost savings
  • no negative media attention
  • enhanced safety for operations personnel
  • a demonstration of due diligence in the commissioning of transfer system equipment.

For additional information please contact Gall Thomson:

Tel: +44 1493 857 936


Also see:

Marine Breakaway Couplings FAQ

Petal Valve Marine Breakaway Couplings

Flip-Flap Valve Marine Breakaway Couplings