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STS Operations bare an increased risk compared to other tanker vessel operations because of the elements involved and the fact that the operation takes place underway amongst the participant vessels.

Past experience and literature offers several incident scenarios and "lessons to be learned" with respect to the STS (or lightering) operations.

The Purpose of this section is to focus on data accumulated from reports on various incidents worldwide and present the attributed factors or most likely cause of incidents. There are documented concerns of the maritime community with respect to STS Operations and their impact on environmental safety against oil pollution as well as preservation of existing nature.

Available literature consist of data "caught in the net". Sources are mentioned and acknowledged for their contribution. Should you are interested to post a comment please This email address is being protected from spambots. You need JavaScript enabled to view it.. In the near future comments will be able to be posted on the STS FORUM which will be generated in this web site.



(Melbourne Marine Services Limited, Ship-to-Ship Oil Transfer Operations in the Firth of Forth, Oil Spill Contingency Plan)

Original Document may be found here.

In order to determine which of the oil containing systems could result in an accidental spillage, the initiating events and maximum quantity of oil that could be released has been identified and examined considering the risk reducing measures in place at the STS operations.

Mother Daughter Vessel Collision

The collision risk between the two vessels is low due to the fact that the mother vessel will be stationary and the daughter vessel will be approaching at a very low speed assisted by local tugs and pilot. In the unlikely event of a collision structural damage to both vessels may occur. This may result in an oil spill.

Third Party Collision

Other vessels manoeuvring within the Firth of Forth include cargo vessels, Ministry of Defence (MOD) vessels, cruise ships, gas tankers oil tankers, product tankers, pleasure craft and fishing boats. The anchorage points are located off the existing shipping lanes and entry and estuary passage through the Firth of Forth is controlled and regulated by local bylaws and Forth Ports and FTNS.

Third party collisions could potentially rupture one or more cargo tanks. Collisions are more likely during periods of low visibility. However no unauthorised craft will be allowed alongside either vessel at any time, prior to, during or after the transfer, therefore the risk for third party collision will be minimised.

Incidence of collision is strongly dependant on traffic levels and controls. General collision risks worldwide are considered to be in the region of 2.0 x 10-6 per vessel hour, or a collision every 500,000 hours of vessel operation. On this basis the maximum total risk for the proposed operations would be 9.2x10-4 per annum, or a collision every 1080 years. Given that these activities are being undertaken away from key traffic zones in area under control of the Forth Ports, it would be expected that the risk of an incident in reality would be still lower . The maximum increase in traffic involved in the STS proposal is for 150 vessels. This is less than a quarter of the decline in vessel numbers since 1998 and would only partially offset the decrease in oil related traffic by 40% still producing a net decrease in traffic from recent levels and removing vessel activity from the potentially higher risk area of the inner firth. (Aquatera 2005).

A catastrophic tanker collision was considered but discounted due to the stringent control measures in place within the Firth of Forth including the early notification to the port authority of vessels proposed movements into or out of port/anchorage and statistical figures showing a low probability of such events.


This would directly involve either of the STS transfer vessels either during exit/entry to the transfer area, dragging anchors or an error during manoeuvring.

It is not expected that any grounding events will take place as a result or the proposed operation. In the event that grounding did occur there is a strong likelihood that it would be on a sedimentary rather than a rock seabed. The balance of probabilities suggests a non-catastrophic grounding would occur in over 99% of circumstances. The risk assessment suggested grounding may occur at a probability of 1.3 x10-3 per annum with the proposed level of activity. This can be converted into the possibility of a grounding event every 790 years and the likelihood of a catastrophic grounding event every 80,000. (Aquatera 2005).

Deep-water channels provide access and manoeuvring potential within the Forth and around the anchorage points. The seabed around the anchorage points is sand and shale so any grounding is likely to cause minimal hull damage. Depth limits are clearly marked on all Admiralty Charts along with all fouling risks. Experienced harbour pilots should be able to minimise the risk of groundings.

Anchor Dragging

The mother vessel will be held on her anchor during all transfer operations. It is assumed that under normal operations the anchor will be secure. There is however, a small chance of anchor failure due to dragging of the anchor. A careful anchor watch must be maintained. Main engines and spare anchor to be ready at short notice.

Both moorings are just outside a gas pipeline exclusion zone. If the anchor drags then the ships may drift towards the pipeline. There is a small risk that the pipeline may be damaged by the anchor. Rupture of the gas pipeline would result in the escape of methane gas that would bubble to the surface even after the supply had been shut off until the pressure in the pipeline had equalised. This would result in an explosion risk and reduced buoyancy to shipping due to the gasified water. This has been discounted due to control measures in place and a low statistical probability.

Operational Spillage

Worldwide spill statistics show that operational spills tend to be small, the majority being less than a tonne. Spills occurring through transfer operations may be as a result of hose ruptures, failure of connections or tank overflow. The amount of oil spilled in the event of a hose rupture or flange leak during ship-to-ship transfer operations would be governed by the size of rupture or leak. Spillage from loading hose connection / disconnection is unlikely as the lines are cleared after each operation however this risk has not been discarded. Drip trays that are located under each manifold prevent the escape of any residual oil.

Shipping Accident Spillage

Any shipping accident has the potential to result in the loss of the entire cargo. This could be as a result of an on-board explosion or the breaking up of the vessel. Such accidents however, are rare. The likelihood of the occurrence and impact of the above spills are discussed in the risk assessment section 3.8.

Fire or Explosion

Although considered to be of minimal risk, fires or explosions onboard either vessel could potentially result in the loss of the entire cargo through the tanker breaking up or grounding through loss of controls. Typically, accidents of this nature occur once in every hundred years of tanker operations.

Based upon the level of shipping activities and IMO international industry data, the likelihood of a fire or explosion happening on STS related vessels is predicted to be 2.6-3.2x10-3 per annum or 1 incident in every 312 to 385 years (Aquatera 2005).

Ship-to-Ship Transfer Vessels Drainage Systems

It is proposed that all bilge water produced by the tankers during STS transfer operations shall either be held on the vessels or transferred to shore via the supply vessels.

Spill Probability

This section has used two approaches to establish the spill probability for the identified risks associated with STS operations:

  • Analysis of likely hydrocarbon spill sources from STS operations that may result in hydrocarbon entering the marine environment;
  • Analysis of historical spill data to assess the frequency of spillages. Aquatera (2005b) completed a comprehensive Risk Assessment report for STS operations in the Firth of Forth on behalf of Melbourne Marine Services.

Analysis of likely Spill Courses from STS Operations

Based on the observations shown above a range of potential oil spill causes exist from STS operations that may result in a release of hydrocarbon into the marine environment. These are mainly due to oil handling and tanker loading operations and are highlighted below.

  1. Adverse weather conditions
  2. Collision between a tanker and a support vessel
  3. Collision between STBL and receiving vessel
  4. Communication failures (Including failure to communicate the start of STS operations, emergency plan ect.)
  5. Cow line failure
  6. Electrostatic discharge between the tankers
  7. Error by Harbour Pilot or Mooring Master
  8. Failure of cargo pipes on the tanker
  9. Failure of unmooring operations (ropes wrap around propeller)
  10. Fire/ explosion from VOC in confined spaces
  11. Hose gaskets failure
  12. Manifold valve failure
  13. Mechanical failure on the tanker during mooring (steering or power failure)
  14. Oil spill onto tanker deck (during hose connection / disconnection)
  15. Poor visibility
  16. Tank overflow




MT Saetta / MT Conger (10 AUGUST 2009)


Report on the investigation of the collision between mt Saetta and mt Conger on completion of a ship to ship transfer 9.5 miles south east of Southwold, UK on 10 August 2009. Report No 3/2010. Published 25 March 2010.


(Source: Marine Accident Investigation Branch UK)

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