2. Oil Pollution

Case Study: The Sea Empress Oil Spill

The Sea Empress aground off St. Anne's Head at the entrance to Milford Haven on 15th February 1996.
Because of the strong winds (>15 knots -7.5 m s^-1) the ship could not be refloated, but remained aground and leaking oil until it was brought into the harbour six days later. Photo: International Tanker Owners Pollution Federation (ITOPF).

On 15th February 1996 the Sea Empress went aground at St. Anne's Head near the entrance to Milford Haven in Pembrokeshire in Wales. Over the next 7 days 72,000 tonnes of light crude oil from the Forties oil field in the North Sea leaked into the sea. The fuel tanks also sprang leak.

Aerial photograph of the cliffs at St. Anne's Head, where the Sea Empress went aground.
Photo: Countryside Council for Wales.

For several days strong winds prevented the Sea Empress from being re-floated. On February 21st the wind calmed, and the ship was towed into Milford Haven, where she continued to leak cargo and fuel oil until the tanks could be emptied. Much of this oil ended up on the mudflats to the south of the harbour, in an area of high environmental value.

Surface oil near St. Anne's Head
Section of a CASI image from St. Anne's Head where the Sea Empress went aground taken on February 21st, the day the ship was refloated and brought in to Milford Haven. Thick oil drifting southwards from the accident area show as brown streaks, aligned along the current. CASI (Compact Airborne Spectrographic Imager) measures reflected sunlight at visible and near infrared wavelengths.
Source: UK Environment Agency / National Oceanography Center Southampton

The Pembrokeshire coast is highly valued for its wildlife and outstanding natural beauty, and is popular with both tourists and locals. The time of year, the wind direction and effective clean-up at sea reduced the environmental impact, but there were never-the-less adverse effects on fisheries and tourism, as well as on wildlife. Overwintering birds feeding on the mudflats and at sea, were particularly badly affected.

Fate of the oil

About 40% of the light oil evaporated and was carried away by the wind. A further 28% was dispersed naturally by waves and currents. Mechanical recovery was hampered by the strong winds and recovered only about 1-2% of the oil. However, dispersant spraying from aircraft was very successful and resulted in about 24% of the oil being chemically dispersed. The remaining 5-7% of the oil reached the shore, and spread over a 200 km long shoreline. By then the 4-5 thousand tonnes of oil had been transformed into about 11-16 thousand tonnes of water-in-oil emulsion.

Remote sensing of the oil spill

Airborne CASI image of the Sea Empress at the quay in Milford Haven.
This image was obtained 21 February soon after the ship was brought into the harbour. A boom alongside is helping to contain some of the oil, but has escaped and is drifting towards the southern shore of the Haven. Thicker oil shows up as brown or red streaks, thin surface oil appears bright blue (sheen). Source: UK Environment Agency / V.Byfield, National Oceanography Center Southampton

Airborne support for clean-up at sea

Aerial surveillance of the area used visible observations by trained personnel. These were supported by Sideways-Looking Airborne Radar (SLAR) and ultraviolet and thermal infrared video cameras. The video output was displayed on monitors in the aircraft cabin. In this way the survey aircraft was able to direct the clean-up effort to areas of thick oil, where mechanical recovery or dispersant spraying would be most effective.

Environmental impact assessment

Airborne CASI image of the Sea Empress at the quay in Milford Haven 27 February 1996.
The tanks have been emptied and the ship is no longer leaking oil. Streaks of surface sheen (very thin oil) are still visible in Angle Bay to the south.
Source: UK Environment Agency / V.Byfield, National Oceanography Center Southampton

For two days (22 and 27 of February) additional surveys were carried out from a second aircraft belonging to Environment Agency, equipped with thermal video and CASI (Compact Airborne Spectrographic Imager) - an instrument that records light reflected from the sea at optical and near-infrared wavelengths.

The CASI data was not used to direct clean-up, but was later combined with data from the Coastguard's aircraft and satellite images from ERS and RADARSAT to assess the geographical extent of the oil spill and the severity of the pollution in affected areas. This impact assessment also made recommendation for how to improve the use of remote sensing techniques in future oil spills.

Satellite radar images (SAR)

RADARSAT image 22 February.

Oil is visible in radar images because it dampens ripples and reduces the radar signal reflected back towards the sensor. As a result oiled areas appear dark. However, areas of low wind also appear dark, because there is less wind to ripple the sea surface.

In this RADARSAT image the two main areas of such 'dark' water are found in Milford Haven and in Carmarthen Bay. We know from other observations that there was surface oil in Milford Haven, but the dark water there could also be a result of the harbour being sheltered from the winds. Without extra information it is hard to tell if this is oil or wind shadow.

Source: RADARSAT / UK Environment Agency

Satellite images were not used to support the clean-up. In 1996 satellite radar sensing of oil spills were still at the experimental research stage, and processing of the data took too long for the images to be of use to those planning the oil spill response.

ERS-1 SAR image from 26 February 1996.

In this ERS-1 SAR (synthetic aperture radar) image 'dark water' is again found in Milford Haven and Carmarthen Bay. We know from other observations that there was still surface oil in Milford Haven. However, without such prior knowledge, the most likely explanation would be that the harbour appears dark because it is sheltered from the wind.

This was the day before the remaining oil came ashore on the western beaches in Carmarthen Bay. The shape of the dark lines away from the shore make them likely to be oil, but the dark areas in the bay itself are probably due to low wind conditions.

Source: ESA / UK Environment Agency

A small number of radar images from the European Space Agency's ERS satellite and the Canadian RADARSAT did show the surface oil. These were used for later studies of the spill. Two of these images are shown on the left.

Once the clean-up operation was over, the Environment Agency commissioned a study of all available remote sensing data to develop recommendations for how to use remote sensing effectively in future incidents. This study showed that the best information is obtained with data from different sensors, with satellite images giving an overview, and airborne data giving more immediate and detailed information.

CASI image of surface oil in the Tenby area in Carmarthen Bay on 27 February 1996.
This airborne image from the Tenby area in west Carmarthen Bay was obtained by the UK Environment Agency's CASI sensor on the day that the surface oil was being blown ashore in the area. The image shows a large swirl of thick, emulsified oil (red and black) as well as areas of dispersed oil (green). Two clean-up vessels can be seen as small dots, trying to recover the emulsion before it reaches the shore.
Source: UK Environment Agency / V.Byfield, National Oceanography Center Southampton

Thermal infrared image from the Tenby area obtained at the same time as the CASI image.
This thermal video image has been rotated so that north is upwards, to allow comparison with the corresponding CASI image. Here the thick oil is visible as bright swirls with edges of dark grey - darker than the surrounding water. The dispersed oil seen in the CASI image cannot be detected by the thermal sensor.
The bright colour is the result of very thick emulsion being warmed up by sunlight to a temperature higher than that of the water. Thinner areas of oil lose heat more quickly and are not warmer than the water. They appear dark because oil has lower emissivity (emits less radiation at the same temperature).
Source: Environment Agency / V.Byfield, National Oceanography Center Southampton

Comparing airborne sensors

Radar, ultraviolet, optical (visible and near infrared) and thermal sensors are sensitive to different thicknesses of surface oil.

Radar sensors are sensitive to very thin oil
Ultraviolet sensors are sensitive to oil thicker than 0.01-0.05 micron. Oil appears brighter than water, increasing in brightness with increasing oil thickness.
Optical sensors can measure relative oil thickness from about 2-500 micron, and can also detect oil dispersed in the water.
Thermal infrared sensors measure relative thickness from about 50 micron.

The images on the right show the same area of surface oil (near Tenby) measured with both optical and thermal infrared sensors. Current oil spill response in the UK now has access to all these different types of airborne sensors, which may be used not only in post-spill impact assessment, but also combined with oil spill models to help predict where the oil may be transported by wind and currents.

Map of South Wales and the Bristol Channel.
The point of the accident near St. Anne's Head is marked with a cross. Almost immediately oil drifted into Milford Haven. Oil was observed with airborne radar, stretching from Skomer southwards across the Bristol Channel towards Lundy. Later in the end of the week oil also entered Carmarthen Bay.
Source: P.Dyrynda, School of Biological Sciences, University of Wales Swansea, UK

CASI image of the clean-up at sea in the Tenby area of Carmarthen Bay on 27th February.
This section of a CASI (Compact Airborne Spectrographic Imager) image shows thick oil emulsion at the surface as black and green. Thinner surface oil and oil dispersed in the water looks green. To clean-up vessels can be seen near the tip of the oil slick, using booms and skimmers to recover the oil-in-water emulsion.
Source: UK Environment Agency / National Oceanography Center Southampton

Oil spill response

The oil spill response was coordinated by the Marine Pollution Control Unit (MPCU) of the UK Coastguard. At sea a surveillance aircraft directed the clean-up effort to areas with thick oil, where dispersant spraying and recovery with booms and skimmers would be most effective.

Cleaning up the beach in the Saundersfoot area of Carmarthen Bay.
Most of the oil in this area came ashore on the 27th of February. By then it was more than 50% water-in-oil emulsion, and had more than doubled in volume.
The oil was very stiff because of the cold, and was relatively easy to scrape off sand and rocks.
Source: P. Dyrynda, University of Wales, Swansea

The shoreline clean-up involved over a thousand workers (not counting volunteers who were trying to save wildlife). The effort concentrated on removing oil from beauty spots and public beaches of importance to the tourist industry. Within six weeks of the accidents these beaches were visibly clean and available for use.

Map showing the shoreline most affected by oil.
The coastline where oil drifted ashore is shown in red. Some areas were affected already on the first day; this included Milford Haven and the areas south of Skomer and around St. Anne' s Head, where the accident occurred. The winds then changed to northerly driving the oil southward across the Bristol Channel. Most of the oil came ashore when the winds changed again on February 27th, driving the oil onto the shoreline of western Carmarthen Bay. Source: P.Dyrynda, University of Wales, Swansea

By Easter only about 500 tonnes of oil was left on rocks and beaches. Some clean-up was still required over the next 18 months, when pockets of oil appeared on beaches or were released from offshore sediments.

In general the oil spill response was considered successful, and the damage not as severe as it might have been. There were several reasons for this. The time of year meant tourism and birdlife was less affected than if the spill had happened later in the year. Strong winds drove the oil away from the coast during the first week, and created waves that helped to disperse the oil at sea. This meant there was time to mount an effective clean-up operation at sea, reducing the amount of surface oil that finally drifted ashore when the winds changed.

A look at two areas affected by the oil

Angle Bay, Milford Haven

Angle Bay grasslands and intertidal mudflats.
This image from Angle bay on the south side of Milford Haven was taken well after the Sea Empress oil spill. The grasslands of this peaceful bay are rich in wildlife, and home to many different species of nesting birds. The intertidal mudflats seen in the distance are important feeding grounds for waders.
Photo: Countryside Council for Wales

Angle Bay on the south side of Milford Haven was among the first beaches to be affected. Oil came ashore on the evening of the original grounding, and over the next few days much of the coastline within the Haven was oiled. When the Sea Empress was brought into the Haven on 22nd February, oil continued to leak from the cargo and fuel tanks until these could be emptied (see CASI image on the left).

Angle Bay is well protected from wind and waves, with a shoreline over 3km long. The beaches and intertidal mudflats are important as feeding and roosting area for wintering and migrant wildfowl and waders. The bay also supports beds of the nationally rare dwarf eelgrass (Zostera noltii), and has been declared a Site of Special Scientific Interest (SSSI).

Cleanup in West Angle Bay.
The clean-up began on 16th February and continued for over a month. Oil was removed by digging trenches to collect oil and water, a sucking it up into containment tanks.
Photo: Countryside Council for Wales

In places the shoreline is accessible from the road or the Pembrokeshire coast path, and these areas are of high value for leisure and tourism. In the past a considerable amount of shellfish gathering and bait digging also took place in the bay.

Cleaning began on 16th February with up to 20 people and continued until 20 March. Bulk oil was removed by trenching and suction, rock surfaces were cleaned with high-pressure washing. When clean-up resumed in mid- May it was reported that oil had penetrated into muddy sediments and mobile fuel oil was present below a surface covering of stones, some of this oil still remained after cleaning was stopped in autumn 1996.

After the spill there were some criticism of the clean-up in Angle Bay. Mudflats are very sensitive to being trampled and disturbed by people walking or by vehicle traffic. The clean-up crews tried to avoid damaging the area, but some ecologists were of the opinion that the clean-up effort did more damage than the oil itself, and that the mud-flats should have been left to recover undisturbed.

Surface oil in the Tenby area of Carmarthen Bay.
In this aerial photograph the surface oil appear a brownish red against the blue of Carmarthen Bay. The oil has already started to reach the shoreline north of the headland.
Photo: Countryside Council for Wales

Saundersfoot Bay

Most of the oil that ended up on the beaches in West Carmarthen arrived on February 27 when the wind changed direction and brought large quantities of emulsion ashore in the Tenby area. Many of the beaches in this area are popular with tourists, particularly between the holiday resorts of Tenby and Saundersfoot Bay.

View from Saundersfoot Leisure Park across the Bay.
This photo, taken much later in the summer shows why the Saundersfoot Bay area is so popular with locals and tourists.
Photo: Saundersfoot Bay Leisure Park

A major clean-up operation began, which involved over 650 people (not including those involved in wildlife rescue). The coastline in Carmarthen bay has a wide range of beach types: from popular sandy beaches to rock, coble and shingle beaches with cliffs. Many have difficult access and several are environmentally sensitive.

Surface oil around the rock in Saundersfoot Bay.
Reddish brown emulsified surface oil washes around the rock in Saundersfoot Bay after the wind changed direction at the end of February. Where the oil has reached the sand, it is black. Several oiled birds came ashore along with the oil. Hundreds of volunteers took part in the wildlife rescue effort.
Photo: P.Dyrynda, University of Wales, Swansea

Most of the oil that reached the coast was water-in-oil emulsion. In the cold of winter it was viscous and stiff, and relatively easy to scrape off rocks and sandy beaches. This allowed the beaches to be cleaned of oil in time for the tourist season, and minimising impact on this industry.

Wildlife was more severely affected, especially birds. Hundreds of volunteers organised by the RSPCA helped to rescue seabirds, waders and other wildlife. Most of the oiled birds did not survive. Despite the loss, studies indicate that populations had largely recovered after a few years.