The volume of oil spilled from a tanker with double sides and a double bottom in a grounding or collision is likely to be one third less than from a conventional tanker involved in a similar incident.
That's the finding of a study conducted by the Norwegian classification society Det Norske Veritas (DNV).
DNV findings also favor retrofits of vacuum systems to the tanks of conventionally built tankers to reduce the volume of oil spilled from a ruptured tank.
Separately, DNV plans to implement a system to audit the safety and pollution records of tanker owners. Tentative rules covering those areas have been published for the first time.
DNV Executive Vice Pres. Tor-Christian Mathiesen said the first two owners, one in tankers and the other operating tankers and ferries, have signed up for the audit system.
TANKER DESIGN
The DNV study of oil spill potential compared different combinations of double hulls and double bottoms with a conventional ship.
Mathiesen said the extra protection of a doubled side/double bottom would add 8-15% to fabrication costs. Based on new building steel at $1,700/metric ton, that works out to an added $1.6 million for a 60,000 dwt vessel, $3.4 million for a 135,000 dwt vessel, and $8.7 million for a 250,000 dwt ship.
For double sides alone, there would be no added cost for a 60,000 dwt vessel, $700,000 for a 135,000 dwt vessel, and $1.7 million for a 250,000 dwt tanker.
Added costs for double bottoms range from $1 million at 60,000 dwt to $2.7 million at 135,000 dwt to $5.7 million for a 250,000 dwt ship.
Ten tanker designs in various hull and tank configurations were evaluated.
Apart from concluding that double hull/double side designs are superior in collision or grounding, the study showed that narrow, long cargo tanks and increased double bottom height reduce oil outflow.
The study also assessed the relationship between inner and outer skins of a double bottom and the probability of spilling oil in a grounding. With a 2.4 m space between inner and outer bottoms there would be a 52.6% chance of no leakage. But to obtain a 99.8% chance of no leakage in a grounding the space would have to be 6.6 m.
In a collision, a 3 m space between inner and outer skin of a doubled sided vessel would yield a 20.4% probability of no leakage, while a 6.3 m space would increase the probability to 42%.
DNV said in about 85% of all groundings, the double bottom designs studied leaked no oil, while some oil will leak from single bottom designs regardless of tank size and the use of a vacuum tank system.
A vacuum system would significantly reduce the volume of oil escaping in a grounding of the single bottom designs analyzed. However, the volume of oil escaping from a modern, conventional, very large crude carrier with a vacuum system would still be about twice the volume escaping from a double side/double bottom design in the case of collision and grounding.
VACUUM SYSTEMS
Mathiesen said the vacuum issue has spawed opposition and confusion.
Some operators wrongly assume that a vacuum would be introduced into tanks every time a port or critical area was approached.
Opposition is based on a misunderstanding of how the system would work.
When the bottom of a ship opened there would be an immediate outflow of oil, creating an underpressure. If a vacuum were created by valves on the top of the tank, this underpressure would reduce the outflow because a hydrostatic balance would result.
Mathiesen said conversion of a tanker would require installation of a level detector in the tank and a vacuum ejector to maintain underpressure. A portable pump also would be needed to reduce the overall level in the tank.
On a VLCC, a spill of 600-700 metric tons could not be avoided in the first 50 sec after a tanker was penetrated. This is the time it would take to activate and close various valves.
Although the system would be cheaper than fitting a double bottom to an existing vessel, the vacuum system would require extra maintenance to ensure it was in order.
AUDIT PROGRAM
More charterers, including Exxon, Shell, and BP, have started to introduce programs to evaluate management practices of ships and owners, Mathiesen said.
DNV will provide a standard for evaluating performance but will not tell a company how to operate. It will only oversee procedures in the first stage, then conduct regular audits to check implementation and operation.
Although in the short term, problem shipping companies will not want this sort of voluntary audit, Mathiesen said, in the long run more charterers will ask for this type of documentation.
A DNV study of 111 tanker accidents showed 72% were due to human errors that included procedural failures, 17% stemmed from technical failures, and 11% were due to external causes such as a tropical storm or an earthquake.
Mathiesen said behind the man on the deck who was not performing properly was a manager who had hired him without checking qualifications and ensuring the man was properly trained.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.