Twin Screw


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. 1

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.

19780316AMOCO_CADIZ 267000MRoff Portsall, FR
19930105BRAER 99600MYShetlands
19750110BRITISH_AMBASSADOR 52400MY333m w of Iwo Jima
19710227WAFRA 47000MYoff Cape Aghulas
19760204ST_PETER 44300MY30M W Cabo Mangalares
19770527CARIBBEAN_SEA 35200M_S of El Salvador
19891229ARAGON 29400MYnorth of Madiera
19810329CAVO_CAMBANOS 24300MY4 mi off Tarragona
19700131GEZINA_BROVIG 18800MYNW Puerto Rico
19740926TRANSHURON 18600MBKiltan Is, India
19870623FUYOH MARU 11900MRSeine R
19680307GENERAL_COLOCOTRONIS 6000MYoff Eleuthera, Bahamas
19761227OLYMPIC_GAMES 5880MYMarcus Hook,Del R.
20010329BALTIC_CARRIER 2900MRBaltic Sea, Denmark
19770327ANSON 2330MROrinoco River
20001128WESTCHESTER 2030MCMiss R mile 38
20020814GOLDEN_GATE 1520MYentering Karachi
19760124OLYMPIC_BRAVERY 842MYnear Ushant
19840319MOBILOIL 624MRColumbia River
19780321AEGIS_LEADER 586MYoff Sumatra
19760119IRENES_SINCERITY 582MYBaltic Sea
19730624CONOCO_BRITANNIA 500MYapproaching Humber SBM
19810725AFRAN_ZENITH 302M_Elbe
19990523PARNASO 151MR60 mi s of cuba
19981207TABRIZ 117MYBandar Abbaas
19990227HYDE_PARK 15MYMiss R, mm 92-79

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. 2

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:

  1. 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.
  2. There has been no coherent reliability analysis of tanker machinery systems.
  3. How much redundancy should be required.
Issure (1) has to be addressed on at least three levels.
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 for analysis.
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. 3 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.

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1. Sometimes even the most spectacular machinery failure will receive little or no attention unless there's a big spill or a lot of non-crew deaths. In February, 1999, the Hyde Park lost power at Mile 92 on the Mississippi River. She was fully loaded with 25,000 tons of gasoline. The Hyde Park then went on a 13 mile rampage, drifting downriver, causing multiple collisions including sinking a crew boat and a barge containing caustic soda, before power was restored. But the only damage to the tanker itself was a holed bunker tank. This miraculous escape received almost no publicity. The CTX has no evidence that USCG tried to figure out why the ship lost pwoer.

2. There are a few exceptions. After the Exxon Valdez spill in 1989, ships loading at Valdez, Alaska were subject to unusual scrutiny. Based on Alyeska records, the Anchorage Daily News reported on 1992-11-22 (page A13) that in the two plus years since the Exxon Valdez there had been four total loss of power incidents on laden tankers in Prince William Sound alone. Since Valdez loads roughly one tanker a day and these ships are in laden passage in Prince William Sound for less than one day per trip, we are talking four total loss of power incidents in at most 2.5 ship-years. This is roughly consistent with the numbers from my own fleet. Here's a brief summary of the casualties in this article. Remember these are outbound (loaded) tankers only.

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.

3. There are a number of big spills in which the low speed maneuverability of the twin screw would probably have been a big help. These include the Diamond Grace in Tokyo Bay and possibly the Tasman Spirit entering Karachi. Another important issue here is the amount of power a tanker should have. Many existing tankers have so little power that they have difficulty manuevering in bad weather. Yet installed power is quite cheap, less than $200 per KW. A 20% increase ininstalled power on a VLCC would cost about a million dollars or a little more than 1% of the initial cost. Part of this project will be a comparision of twin screw and single screw manueverability.