Accidents will happen, and having a captain who had been drinking vodka before sailing perhaps made the Exxon Valdez especially accident-prone.

But to see the tanker's grounding on 24 March 1989 as a matter of chance would be wrong. The verdict of the Alaska Oil Spill Commission makes that clear:

"The grounding of the Exxon Valdez was not an isolated, freak occurrence.

"It was simply one result of policies, habits and practices that for nearly two decades have infused the nation's maritime oil transportation system with increasing levels of risk.

"The Exxon Valdez was an accident waiting to happen."

Neglected remedies

There were recommendations afterwards intended to make sure there would never be another incident like it.

There were calls for tankers to be fitted with transponders, so their progress could be tracked by satellite.

There were calls for all tankers using the cold, clear waters of Prince William Sound to be double-hulled, to lessen the chances of a damaging spill.

Under MARPOL 73/78, the International Convention for the Prevention of Pollution from Ships, all tankers worldwide will have to have double hulls - from 2015.

Not much has happened. But the US Interior Department's Minerals Management Service (MMS) predicts they will.

There is growing pollution from rigs

It believes there will be a series of significant spills in Prince William Sound and the Gulf of Alaska as a result of Arctic oil development, totalling about 60 million gallons.

The MMS says these will come from tankers - and other sources. And it is the other sources that are of greater concern globally than the occasional dramatic and massive spill.

Certainly around British coasts - and almost certainly worldwide - it is the daily insidious flow of oil into the sea that does more damage than the Exxon Valdez, or the Braer or Sea Empress.

Illegal waste disposal

There is a growing problem with oil escaping from drilling rigs.

But a large source of contamination is shipping (of all sorts, not just tankers), in the form of engine room waste, discharged ballast, and water used for cleaning both fuel and cargo tanks.

MARPOL rules require ports to provide facilities for vessels to get rid of oily wastes.

Thoughtless tourists add oil to the oceans

But unscrupulous masters will dump their wastes far out to sea (often by night, to avoid detection), because it is cheaper and quicker to do so.

The industry is trying to clean up its act, and has made progress.

The International Association of Independent Tanker Owners (Intertanko) says: "It is land-based waste which contributes most to sea pollution from oil - from industry, sewage, tourism, and dumping into rivers, harbours, bays and the open sea.

"1.48 million tonnes per year, or 61% of the total annual oil pollution of the sea, comes from the land.

"Just 11.25% comes from oil tankers, a percentage that has fallen dramatically over the past decades, but one the industry continually strives to improve upon.