The UKDOT link is the official Department of Transport report by P. B. Mariott.
It is a model of what all spill investigation reports should be.
Knowledgeable, objective, thorough;
far superior to anything I've ever seen from the USCG.
Anyone with serious interest in the Sea Empress, or tanker spills in general,
should carefully read Captain Mariott's report.
The initial grounding was caused by pilot error.
The pilot was relatively inexperienced in handling ships this size.
He chose to cut a corner in approaching the entrance to Milford Haven.
(This would have been explicitly against the port rules
for ships over 150,000 dwt.
The Sea Empress was 147,000 dwt
but, because she was a Marpol ship,
her real size was larger than a 150,000 ton pre-Marpol ship.
The port rules were written when there were no Marpol tankers.)
The deep water part of this entrance is only 160 m wide
and the tidal currents in this area are strong and notoriously unpredictable.
The pilot reacted to an unexpected tidal set too late,
and the ship grounded.
He testified that the reason they did not put the helm hard over
when they knew they were in trouble was stern swing.
There concern may have been well-founded.
The subsequent damage survey showed the highest damage
was at the aft end of the starboard paralled midbody.
Pretty clearly she had experienced some stern swing.
When she hit the ship was loaded to 15.9 m draft even keel
and at 10 knots probably had about 0.75 m squat.
She hit in a area where the charted depth is about 16 m and shoaling rapidly.
She suffered damage all along the starboard side to a maximum height of about 3 m.
The initial effect of a powered grounding is that the sea bottom
pushes into the tanks which vent violently.
The crew forward said it sounded like a jet plane taking off.
The mate forward could not use his walkie-talkie it was so loud.
The damage was so extensive that by the time the tugs reached her 15 minutes later
the ship already had large trim by the head and 18 degrees list to starboard.
The forward starboard deck was awash.
Her max draft was 23 m.
Fortunately, she bounced off the ledge and ended up in deeper water
about 0.5 mi NE of where she initially hit.
where the depth was 20 to 23 m.
This area of deeper water is called The Pool.
Even more fortunately she had grounded at very close to low tide.
She reached hydrostatic balance within a few minutes
with a loss of 2500 tons and then stopped leaking.
The ship was counterflooded which reduced the list to 10 degrees.
This could have caused more spillage but apparently did not.
With the tide coming in, the ship refloated herself;
and at 0127 on the 16th she was moved to a deeper portion of The Pool
The weather was deteriorating.
Somehow the pump room had flooded to a depth of 5 m
and had a flammable atmosphere.
The liquid in the PR was described as a mixture of oil and water.
The rest of the 16th was wasted pumping out and purging the pump room.
The pump room flooding to this depth could not have occurred
from external damage; and if the damage had been external,
the liquid would not have contained oil,
nor they could not have pumped it out.
Despite this, this flooding was blamed in subsequent legislation
on external damage.
On the 17th, they still had not started any lightening.
The forecast was for strong gale force winds from the WSW.
The salvors etc knew that the position was precarious.
The Pool is exposed to the west, subject to very strong tidal currents,
and the holding ground is not good.
The decision was made to turn the ship into the forecast wind.
This decision is very hard to defend.
By deballasting and, if necessary,
accepting a modest amount of additional spillage,
the ship could have been brought into the shelter of Milford Haven.
The salvors knew this was possible;
but the port authority had put an unnecessarily restrictive
draft limit on bringing the ship in.
Relations between the salvors and the port authority were not good,
and the salvor decided not to argue the case,
in part because they knew there might be some spillage
associated with decreasing the ship's draft.
Anyway the decision was a disaster.
It proved difficult to turn the ship,
the manuever was not completed until 1555 in the afternoon.
A few hours later the ship broke loose, drifted to the west
and went aground again at 1805.
In the process, she lost her anchors.
Much worse, she grounded about 40 minutes hours after high tide.
The tidal range in this area is about 6 m.
She grounded in an area that has a low water depth of about 14 m (stern)
to 17 m (bow), with the port side on the shoal side.
At the time, she probably had a max draft in excess of 20 m.
When the tide went out, she came out of the water by some 5 m.
And she holed a number of tanks, port side aft.
The CTX believes a large part of the oil was lost
during this ebb tide.
In an attempt to hold the Empress in place as the tide came back in,
the FP and 2P were ballasted.
Good idea but this failed
and at 0820 on the 18th she came lose again
and grounded on the east side of the channel at 0840.
The charted water depth in this area is about 18 m.
This grounding was at mid-tide with the port stern
in the shallowest water.
Unless she experienced a lot of new damage in the grounding,
it is hard to see how she could have spilled
a lot of new oil at this point.
Through the 18th and the 19th the ship kept moving around
before once again grounding at 1815 on the 19th
on the north side of the channel
at the peak of the tide in about 11 m of water.
As the tide went out, the casualty grounded along
her enter length, and the list reduced from 10 degrees to 7.
She must have lost a massive amount of oil on this ebb tide.
An attempt too refloat her on the 20th at high tide (1839)
by blowing out the ballast tanks failed.
However, the salvor were about to get the ship down to
an draft of about 16.6 m.
On the 21st more severe pressurization was used,
the ship floated clear two hours at 1900
an hour before high tide, and she was towed into
Milford Haven at a surprisingly low draft of 11.95 m.
The Sea Empress ended up losing about 71,800 tons
of the original 130,000 tons on-board.
It turned out that by properly counter-ballasting
they could have got the draft down to 20.5 m
and moved the ship into protected waters
on the first high tide after the initial grounding.
This would have resulted in small amount of spillage
as the damaged cargo tanks were elevated.
Nobody in charge had the guts to do this.
This ship is a Marpol Suezmax with only 12 cargo tanks (plus two slops).
All the ballast tanks are outboard, but the wing tanks are very narrow.
A ship with better sub-division would have spilled less.
The single screw Sea Empress was also underpowered at 13000 KW
(max speed of 14 knots).
If she were twin screw,
the pilot would have been able to correct his course more rapidly
and without stern swing.
If the ship had had GPS/ECDIS,
the pilot would have known about the tidal set sooner.
Much has been made of the fact the Borga, a double hull ship,
grounded in nearly the same spot a few months earlier and there was no spill.
(Actually the Borga is double bottom, not a double hull.)
But the key factor that saved the Borga
was the weather remained good until they completed lightering.
If the Borga had been single bottom,
she would have spilled some oil,
but she also would have required less lightering to refloat.
Also she was small enough (112,000 dwt) so that once refloated,
it would have been obvious that she could be moved into protected waters.