For some unfathomable reason,
the fact that almost every tanker in existence
is one failure away from being adrift has not penetrated the public consciuosness.
A body politic that is obsessed with single skin hulls
has not even noticed that all but a handful of the 3600 odd large
tankers in existence have a single engine.
Some time this myopia reaches bizarre proportions.
In November 2000, the fully loaded 88,000 ton tanker Westchester
had a main engine crankcase explosion in the Mississippi River.
Without power, she drifted aground, holed a tank,
and spilled 2000 cubic meters of her cargo.
This was very high profile spill, receiving extensive media coverage.
Almost all these reports decried the fact that the ship was a single hull.
Not one of them, at least not any that has come to the attention of the CTX,
even noted that the real problem was that the ship was single screw with nil propulsion redundancy.
In 1996, there was an even more high profile casualty
when the bulk carrier Bright Field lost power
and rammed into the crowded Poydras Street wharf in New Orleans.
Miraculously, no one was killed but 62 were injured.
This generated a 99 page report
by the National Transportation Safety Board.
The proximate cause was that the main engine tripped due to low lube oil pressure.
This report goes into great detail about the engine room deficiencies
(see the horrifying list in the Appendix B of the NTSB report [you will have to download the PDF]),
but never even mentions the fact that the casualty would have been prevented by twin screw.
The Amoco Cadiz was lost because the ship's only steering gear failed.
This gigantic spill generated a remarkable amount of regulation
but, with the minor exception of the requirement for a reserve hydraulic tank,
none of it had anything to do with the real cause of this spill.
Part of the reason for this blind spot
in that many casualties that resulted from lack of machinery redundancy
are listed as groundings (eg Wafra) or collisions (eg Baltic Carrier) or the like in most spill compendiums.
The CTX Tanker Casualty Database attempts to get to the real cause.
In many cases, we have not yet been successful,
but here is a list of casualties for which we have both spill volume
and been able to ascertain that lack of twin screw was a primary cause.
SPILLS IN TCD FOR WHICH INITIAL CAUSE IS MACHINERY FAILURE
Frankly, I am surprised it has not been much worse.
My own experience with a much better than average quality fleet
leads me to believe
that newly built motor tankers are experiencing
at least one total loss-of-power incident every ship-year.
Most of these loss-of-power incidents last
only a few minutes to an hour
and are rarely reported to the owners,
let alone made public.
However, roughly one in ten results
from a major casualty immobilizing the ship
for at least several hours,
and often a day or more.
The owners usually know about these casualties;
but once again they are never made public
unless they result in a spill that becomes public.
Worldwide there are currently about 3600 tankers
with a deadweight of 10,000 tons or more afloat.
All but a handful of these ships are single screw.
If my once a ship-year number is correct,
this means that on average there are ten "minor"
loss-of-power incidents every day,
even if you are crazy enough to call any
loss-of-power that risks a major oil spill "minor".
If my once every ten year number for "major"
loss-of-power incidents is correct,
then worldwide we are averaging one major tanker loss-of-power
incident every calendar day.
Even if my time between loss-of-power incidents is low by a factor of ten,
which I am prepared to argue is extremely unlikely,
then we are still talking about one "minor" loss-of-power per calendar day,
and a major loss-of-power every ten days.
Of course, only a small percentage of tanker loss-of-power incidents
actually end up in a spill (Amoco Cadiz, Braer, etc.).
But, given the consequences,
any sane person has to regard these numbers as unacceptable.
They become totally unacceptable, as soon as one realizes
that total loss-of-power incidents can be reduced
by several orders of magnitude or more by simply going twin screw.
To get a feel for the power of redundancy,
then we must make some assumptions about the length of the loss-of-power.
For the sake of argument, let's assume that
a "minor" loss-of-power lasts one hour,
and a "major" loss-of-power lasts a day.
If we have a twin screw ship,
then to have a total loss-of-power,
the second loss of power must occur
while the first incident is still happening.
If twin screw is properly implemented,
so that loss-of-power incidents on-board a single ship are independent,
then using my numbers the probability per at-sea day
of the second engine room going down in a second "minor" incident
while the first is down in a "minor" incident
is 1/2,160,000 or on average once every 6000 ship-years.
The probability of the second engine room going down in a "minor"
incident while the first is down in a "major" incident
is 1/900,000 or once every 2500 ship-years on average.
The probability of the second engine room going down in a "major"
incident while the first is down in a "minor" incident
is 1/21,600,00 or once every 59,000 ship-years
The probability of the second engine room going down in a "major"
incident while the first is down in a "major" incident
is 1/9,000,000 or once every 25,000 ship-years.
There are a number of academic caveats required here:
Poisson distribution, independence, etc, etc.
But the point is crystal clear.
Propulsion redundancy -- properly implemented --
can reduce tanker total loss-of-power incidents
not by 20%, not by 50%,
but by a factor of 1000 or more.
And twin screw brings us not only propulsion redundancy,
but steering redundancy as well.
Of course, this is just simple common sense.
Airplane engines are orders of magnitude more reliable
than tanker engine rooms.
Yet no one in his right mind would use
a single engine airplane across the Atlantic
on a routine, commercial basis.
In fact, one would probably be regarded
as a bit of a dare devil to cross the Atlantic once on a single engine plane.
You'll probably make it; but, if 3600 people try it,
it is nearly certain that someone will not.
Right now there are 3600 sizable tankers out there
routinely playing daredevil.
Given all this, it is nuts that any tanker is single screw.
The single screw tanker developed at a time
when the only losers from a loss of a tanker
due to machinery failure were the ship owner,
the cargo owner, and the crew.
The ship owner and the cargo owner could
-- the owner still can --
buy their way out of the risk
in a very imperfect insurance market,
a market which gives only the most modest credit for ship quality and reliability.
Nobody gave a damn about the crews.
Now we have a situation in which we recognize that
the cost to society of an oil spill can easily be orders of magnitude
larger than the loss of a ship or a cargo.
This is reflected in multi-billion dollar risks
somehow spread among the charterers, insurers, governments,
and that portion of mankind that lives or plays beside the sea.
(Interestingly and significantly, the owner of the ship
and the yard that built the ship bears almost none of this risk.)
It blows my mind
that the environmentalists are not screaming for twin screw.
There are at least three issues here:
|19780316||AMOCO_CADIZ ||267000||MR||off Portsall, FR |
|19930105||BRAER || 99600||MY||Shetlands |
|19750110||BRITISH_AMBASSADOR || 52400||MY||333m w of Iwo Jima |
|19710227||WAFRA || 47000||MY||off Cape Aghulas |
|19760204||ST_PETER || 44300||MY||30M W Cabo Mangalares |
|19770527||CARIBBEAN_SEA || 35200||M_||S of El Salvador |
|19891229||ARAGON || 29400||MY||north of Madiera |
|19810329||CAVO_CAMBANOS || 24300||MY||4 mi off Tarragona |
|19700131||GEZINA_BROVIG || 18800||MY||NW Puerto Rico |
|19740926||TRANSHURON || 18600||MB||Kiltan Is, India |
|19870623||FUYOH MARU || 11900||MR||Seine R |
|19680307||GENERAL_COLOCOTRONIS || 6000||MY||off Eleuthera, Bahamas|
|19761227||OLYMPIC_GAMES || 5880||MY||Marcus Hook,Del R. |
|20010329||BALTIC_CARRIER || 2900||MR||Baltic Sea, Denmark |
|19770327||ANSON || 2330||MR||Orinoco River |
|20001128||WESTCHESTER || 2030||MC||Miss R mile 38 |
|20020814||GOLDEN_GATE || 1520||MY||entering Karachi |
|19760124||OLYMPIC_BRAVERY || 842||MY||near Ushant |
|19840319||MOBILOIL || 624||MR||Columbia River |
|19780321||AEGIS_LEADER || 586||MY||off Sumatra |
|19760119||IRENES_SINCERITY || 582||MY||Baltic Sea |
|19730624||CONOCO_BRITANNIA || 500||MY||approaching Humber SBM|
|19810725||AFRAN_ZENITH || 302||M_||Elbe |
|19990523||PARNASO || 151||MR||60 mi s of cuba |
|19981207||TABRIZ || 117||MY||Bandar Abbaas |
|19990227||HYDE_PARK || 15||MY||Miss R, mm 92-79 |
Issure (1) has to be addressed on at least three levels.
Almost all these machinery failures never become public.
The Bright Field (Appendix B of the NTSB report) is a rare exception
because the Coast Guard grabbed the ship's paperwork
before it could be sanitized.
There has been no coherent reliability analysis of tanker
How much redundancy should be required.
Once a failure database begins to be collected, then
it will be possible to develop a model of overall system
reliability, identify the weak spots, and come up with
requirements for improvements either in the form of improved MTBF,
MTTR, and/or redundancy.
However, there is one machinery issue that should not wait
until this reporting and analysis system is in place.
That is the question of twin screw.
Enough data exists to make some estimates
of the mean time between failure of the main engine
and the main propulsion train.
The CTX should use this to compare the reliability
of single screw with twin screw.
Secondly, the CTX should develop a best practice twin screw hull form
and estimate the hydrodynamic performance
of this form and its propulsion system.
This hull form will be a varient of the very successful
Swedish design first used on the Nanny.
This hull form involves a low L/B ratio
and a twin skeg cataraman-like stern.
This resulted in a overall hull/propeller performance
which approached the best single screw behavior.
This form also has some commercial advantages
with respect to draft and length.
Thirdly, the CTX needs to address the unique hull structural
problems associated with this form,
an issue with which the Swedes dealt a lot less successfully.
A hydrodynamically efficient twin screw big tanker
requires a fundamentally different aft body structure
from that of a single screw.
To date these problems have not been properly examined and resolved.
A lot of work with the CTX HULL finite element model will be required.
Secondary but still important issues include the
vibration characteristics of the very large long strokes
versus the smaller, higher RPM engines
which the twin screw would use, the relative maintenance costs,
and the issue of maneuverability on a single screw.
The goal here is an overall comparison of the costs and
benefits of twin screw versus single screw.
This information can then serve as guidance to regulators
as to whether or not requiring twin screw is indicated.
Email about this project should be sent to email@example.com.
- a) Regulation requiring owner reporting
IMO needs to develop regulation which requires all loss of power incidents
to be reported to the port state,
with significant penalties for owner and officers
for failure to do so.
The owners will still be able to cover up many of the problems,
but at least he will have to think about it, case by case.
Most crews involves at least one disgruntled person and some
crews contain principled individuals who cannot always be relied
on to cover up for the owner,
especially if they are risking their own neck at the same time.
My experience is that the American requirement to report
the slightest spillage has been a total success.
Technology can help here as well.
Most modern engine room control systems automatically
log all alarms, trips and other non-routine events.
These logs should be publically available.
- b) Regulation requiring class reporting
Many of the more severe failures will bring in the Class surveyor
if only to validate the insurance claim.
IMO must write legislation that requires class to report
any and all defects to IMO.
This requirement should be retroactive.
Class has a vast database of past failures
which needs to be analyzed.
- c) Regulation requiring underwriter reporting
The other database is with the insurers.
IMO must write legislation making this valuable data available
- 1989-07-31, Mobil Arctic
- Gyrocompass failed in fog. Ship returned to berth,
- 1989-09-20, Atigun Pass
- Lost power between Bligh Reef and Glacier Island.
Escort vessel held ship in shipping lane
until power was restored one hour later.
- 1990-06-20, Southern Lion
- Lost power at about the same spot as Atigun Pass.
Ship did not drift out of shipping lanes before regaining power.
Sailed to Knowles Head for repair.
- 1990-08-04, Kenai
- Lost power near Rocky Point,
Stayed in shipping lanes.
Did not require help from escort vessels.
- 1990-11-14, Arco Prudhoe Bay
- Gyrocompass failed while still in Port Valdez.
Went to container dock for repair.
- 1991-04-01, Arco Sag River
- Discovered a possible mechanical problem
with its propulsion system
while passing through Valdez Arm.
Sailed under own power to an anchorage at Knowles Head.
- 1992-03-04, Exxon North Slope
- Bad propeller vibrations after leaving the Sound.
Returned to Sound and escorted to anchorage at Knowles Head.
Divers checked prop, found nothing.
When engine restarted, vibrations were gone.
Probably fouled fishing net.
- 1992-09-09, Brooks Range
- Lost power in Valdez Arm.
Regained power before it required aid from escort vessels.
- 1992-10-20, Kenai
- Problem with steering system and headed toward Middle Rock.
USCG estimates ship was about 100 yards from the rock,
when escort vessel turned the ship back on course.