'Pluto' was the British Pipe Line Under The Ocean (otherwise Pipeline Underwater Transportation of Oil) undertaking by scientists, oil companies and the armed forces to construct undersea oil pipelines on the bed of the English Channel between the southern coast of England and the northern coast of France (1942/31 August 1945).

As it started to plan 'Overlord' to begin the Allied campaign to retake north-western Europe and pave the way for the defeat of Germany, the War Office estimated that petrol, oil and lubricants would constitute some 60% or more by weight of the supplies required by the forces that were to be landed and then fight in north-western Europe. The construction of pipelines would reduce the otherwise total reliance on coastal tankers, whose operations could be affected by bad weather and air attack, and which had to be offloaded into vulnerable storage tanks ashore. A special kind of pipeline was required, and two types were developed as Hais and Hamel. Hais arose from a proposal originated by Clifford Hartley, the chief engineer of the Anglo-Iranian Oil Company, for a flexible pipe with a lead core that could be laid by a cablelayer ship. It was known as Hais after its developers, Hartley-Anglo-Iranian-Siemens, and was tested in December 1942 by running a cable pipeline across the Bristol Channel from Swansea to Watermouth. The design was so successful it was decided to upgrade it in diameter from 2 to 3 in (51 to 76 mm).The Hamel pipe made use of readily available thin mild steel instead of scarce lead, and had to be laid by special Conundrum floating drums. Most of the Hais pipe was manufactured in the UK, although some came from the USA, and all of the Hamel pipe was made in the UK.

Early in April 1942, the Chief of Combined Operations, Vice Admiral the Lord Louis Mountbatten, approached the Secretary for Petroleum, Geoffrey Lloyd, and asked if an oil pipeline could be laid across the English Channel. Mountbatten was currently tasked with planning the Allied invasion of German-occupied Europe, and had concerns about the supply of petroleum products, since it was considered unlikely that a port with oil reception facilities could be secured both quickly and effectively undamaged. The War Office estimated that 60% or more by weight of the supplies of the expeditionary forces would consist of petrol, oil and lubricants. In the first stages of the assault, packaged fuel was to be supplied in 4.4-Imp gal (20-litre) cans and 44-Imp gal (200-litre) drums. To supply the 20 million cans required, a complex manufacturing plant was shipped from the USA to the London area, where it was operated by the Magnatex firm under the supervision of the Ministry of Supply. By 1944, a stockpile of 250,000 tons of packaged petrol and Diesel fuel had been accumulated in the UK.

It was hoped that after the first few days of the invasion, petroleum products could be supplied in bulk. Pipelines were not the sole or even the primary means by which Combined Operations was contemplating supplying petroleum, for it intended to rely mainly on small shallow-draught coastal tankers, of which 30 were under construction. US 600-ton 'Y' tankers began to reach the UK in the spring of 1944, and in 1943 the British had initiated a programme to construct 400-ton Channel tankers, although only 37 had been completed by May 1944. It was hoped that petroleum products might also be supplied by large 'T2' ocean-going tankers lying offshore through ship-to-shore pipelines. The project to develop these pipelines was 'Tombola', and the pipelines themselves became known as Tombolas. The submarine pipeline had sufficient advantages to make it worthwhile to explore as a backup means of supply, however. Submarine pipelines were less vulnerable to air attack and the frequently stormy English Channel weather, and their use would reduce the forces' dependency on vulnerable storage tanks ashore.

Lloyd took expert advice from Brigadier Sir Donald Banks, the director-general of the Petroleum Warfare Department, Sir Arthur Charles Hearn, a former director of the Anglo-Iranian Oil Company and the oil advisor to the Fourth Sea Lord; and George Martin Lees, an eminent geologist. At that time, submarine pipelines were in use in ports and over short distances, but no pipeline had ever been laid across such a great distance or under the currents and tidal conditions typical of the English Channel. Moreover, to minimise German attack and the tidal effect, the entire pipeline would have to be laid in a single night. The British therefore came to the initial conclusion that a cross-Channel pipeline was not feasible using any known construction method for pipelines with a diameter of 6 in (152 mm) or more.

The chief engineer of Anglo-Iranian, Clifford Hartley, was visiting the Petroleum Warfare Department at this time and learned of the proposal, and he was sure that a long underwater pipeline could be created. In the hilly terrain of Iran, Anglo-Iranian had laid and used a 3=in (76-mm) pipeline, and operating at a pressure of 1,500 lb/sq in (10,000 kPa) this delivered 100,000 Imp gal (450,000 litres) per day, the equivalent of more than 20,000 cans. On 15 April he proposed a continuous length of pipeline similar to a submarine communications cable without the core and insulation, but with armour to withstand the internal pressure, and capable of deployment by a cablelayer ship. Additional capacity was to be obtained by the laying of several pipelines. By using high pressure, the line could carry different kinds of fuel: at low pressure different fuels would mix, but at high pressure they would stay separate. Thus the pipeline could be used for aviation spirit and then switched to Diesel fuel.

The resulting project was designated 'Pluto', and placed under the control of the chief-of-staff to the Supreme Allied Commander, Designate (COSSAC). The logistics section of the COSSAC staff, which assumed responsibility for 'Pluto', was headed by a British officer, Major General N. C. D. Brownjohn, with US officers, Colonel F. L. Rash, Colonel Frank M. Albrecht, and Major General Robert W. Crawford successively, as his deputy. A Royal Navy officer, Captain J. W. Hutchings, of the Admiralty’s Department of Miscellaneous Weapons Development, was placed in command of 'Pluto'. By the time of Germany’s surrender in May 1945, Hutchings’s command would comprise several ships, more than 100 merchant navy officers and more than 1,000 other men.

Hartley received support for his proposal from the chairman of Anglo-Iranian, Sir William Fraser, who was also the petroleum advisor to the War Office, and from Henry Wright, the managing director of Siemens Brothers. Fraser agreed to fund the trials, in the hope that the government would subsequently reimburse the company. Siemens Brothers developed the cable in conjunction with the National Physical Laboratory based on their existing undersea telegraph cable. The Hais (from Hartley, Anglo-Iranian and Siemens) was a inner pipe of 2-in (51-mm) diameter to carry the petroleum, and was made of extruded lead. This was surrounded by a layer of asphalt and paper impregnated with vinylite resin. Steel tape was wound around this to give it strength and flexibility. Around this was a layer of jute tape and asphalt-impregnated paper. Finally, it was covered by a protective layer of 50 galvanised steel wires and camouflaged canvas cover. The pipe could deliver 3,500 Imp gal (16000 litres) per day at a pressure of 500 lb/sq in (3400 kPa), and withstand an underwater pressure of 1,950 lb/sq in (13400 kPa). The 2-inch size was selected to keep down the weight and the fact that a larger cable would have required a larger ship to lay it.

A 120-yard (110-m) prototype was laid across the Medway river in Kent by the Post Office cable ship Alert on 10 May 1942, and there followed a pumping test using pumps borrowed from the Manchester Ship Canal Company. After two days of pumping, a failure occurred. The cable was pulled up, and the problem was found to have been caused by extrusion of the lead through gaps in the steel tape, so the steel tape was increased from two to four layers. At Siemens’s suggestion, a second supplier, Henleys, was brought in to increase manufacturing capacity. A second test was carried out in June across the Firth of Clyde, with lengths of pipe manufactured by both Siemens and Henleys. The pipe was laid by the Post Office cable ship Iris. Both pipes worked successfully. Of the 820 miles (1320 km) of Hais cable produced for the operation, 655 miles (1055 km) were manufactured by British companies, and the rest by US companies including Phelps Dodge and the General Electric Company.

Full production of the 2-in (51-mm pipe began on 14 August 1942, using steel from the Corby Steelworks, and on 30 October 30 miles (48 km) of it was loaded on board the naval cable-layer Holdfast, which was to be used for a full-scale rehearsal of 'Pluto'. This trial took place o 29 December 1942, when a 30-mile (48-km) length was laid across the Bristol Channel in rough weather at a rate of 5 kt between shore ends at Swansea and Ilfracombe. The sturdiness of the cable pipe was further tested when two German bombs were dropped on Swansea and impacted 100 ft (30 m) from the cable, which was not damaged. Later a ship’s anchor dragged the cable pipe, but Holdfast was able to locate and repair the damage. To prove the reliability of the cable pipe, pumping operations were carried out continuously, first at the original design pressure of 750 lb/sq in (5200 kPa) and then at 1,500 lb/sq in (10000 kPa), with 56,000 Imp gal (250000 litres) of fuel delivered per day.

The trial was sufficiently successful that it was decided to develop a pipe of 3-in (76-mm) diameter. This reduced the number of pipelines required to pump the same volume of petrol, as each 3-in (76-mm) pipe had more than twice the carrying capacity of the 2-in (51-mm) pipe. A merchant ship, Algerian, was acquired, and converted to carry 30 miles (48 km) of 3-in (76-mm) cable pipe. Two more vessels, Sancroft and Latimer (later renamed Empire Baffin and Empire Ridley respectively), could each handle 100 miles (160 km) of 3-in (76-mm) pipe weighing approximately 6,400 tons. Anglo-Iranian Oil personnel supervised the erection of pumping equipment by personnel of the Royal Army Service Corps, Pioneer Corps and Royal Engineers, and an RASC bulk petroleum company was specially trained to operate the equipment. A Port of London Authority factory at Tilbury was requisitioned and converted into a cable pipe factory at which 3.5 to 4.6 miles (5.6 to 7.4 km) of cable pipe per day was tested, welded into 4,000-ft (1220-m) lengths and stored.

As lead was a material that was in short supply, the Petroleum Warfare Department decided to look for an alternative that made use of cheaper and more readily available materials as a back-up system to Hais, itself a back-up system. Bernard J. Ellis, the chief engineer of the Burmah Oil Company, was convinced that a flexible pipeline could be fabricated of mild steel, which was more readily available than lead. His pipe was 3.5 in (89 mm) in diameter, with 0.212-in (5.4-mm) walls. The prototype was fabricated by J & E Hall in 30-ft (9.1-m) lengths designed to be flash-welded into longer elements. Normally welded pipe offered problems as a result of the rings of residue that formed around each weld, but Ellis designed a special broaching tool to remove the metal swarf. Ellis teamed with H. A. Hammick, the chief engineer of Iraq Petroleum Company, and the pipe became known as Hamel after their surnames, although after the war Ellis successfully asserted his claim to be recognised as the sole inventor.

Unlike Hais, Hamel pipe was too stiff to be coiled in a ship’s hold as it could not withstand the twist along the longitudinal axis that came with each turn of the coil. The Petroleum Warfare Department proposed that it be wound around a buoyant steel drum that could be towed by tugs or fitted on a hopper barge. The resulting steel drum was 60 ft (18 m) long and 40 ft (12 m) in diameter, and was known as a Conun or Conundrum. Tests were carried out at the National Physical Laboratory to verify that Conundrums could be towed at speed without yawing.

Stewarts & Lloyds undertook to design, construct and operate two factories at Tilbury where 40-ft (12.2-m) lengths of pipe were welded into 4,000-ft (1220-m) segments. Six Conundrums were constructed at a unit cost of £30,000, and named Conundrum 1 to Conundrm 6. A Conundrum was towed to a special dock where it was held by two steel arms as a sprocket chain driven by an electric motor rotated the it as pipe was wound around it. At the end of each 4,000-ft (1220-m) segment, the next segment was welded, the swarf cleaned out, and the process continued until the Conundrum held 90 miles (144.8 km) of pipe. A hopper barge named Persephone was converted to carry a Conundrum, and in testing this successfully laid a dozen Hamel pipes across the Solent to the Isle of Wight. It was not known how long the Hamel pipe would last, but it was assumed to be about six weeks. Fluorescent dye was added to the fuel to allow patrol aircraft to detect leaks. Given this success, it was decided to employ both Hais and Hamel.

In the spring of 1943, the Petroleum Warfare Department selected the sites for the pumping stations: these were Sandown on the Isle of Wight and Dungeness on the Kent coast. Construction was carried out at night and in secret, and equipment was delivered under tarpaulins. The pumping stations and storage tanks were camouflaged to look like villas, seaside cottages, old forts, amusement parks and other innocuous features. Strict instructions were issued that neither Petroleum Warfare Department nor its initials should appear on any letter or package, the locations were erased from maps and the drivers of trucks carrying deliveries had to phone from a public phone booth for instructions.

Each pumping station was equipped with 30 Diesel-powered reciprocating pumps with a capacity of 180 tons per day, and four large Byron Jackson Company electric centrifugal pumps capable of 3,500 tons per day, which worked out to 400,000 Imp gal (1.8 million litres) at 1,500 lb/sq in (10,000 kPa). Both stations were fed from the Avonmouth-Thames pipeline, which had a capacity of 135,000 tons per month. There was also a 70-mile (112.7-km) branch line connecting Dungeness with the primary line’s eastern terminal at Walton-on-Thames. Sandown was connected to the system through a 22-mile (35.4-km) link between the Isle of Wight and the refinery at Fawley. The pipeline connections to 'Pluto' had been completed by March 1944.

The corresponding sites in France were selected in June 1943: Sandown was to be connected to the port of Cherbourg, a distance of more than 75 miles (120 km), while Dungeness was to be connected to the little port of Ambleteuse. In keeping with the animated film theme suggested by 'Pluto', the former was codenamed 'Bambi' and the latter 'Dumbo'. Thames barges were converted to handle connecting the cable at the shore ends, where the waters were too shallow for ships to operate.

As part of the 'Fortitude' deception undertaking for 'Overlord', a fake oil dock was created at Dover. Designed by the architect Basil Spence, and constructed of camouflaged scaffolding, fibreboard and old sewage pipe, the fake facility covered 3 acres (1.2 hectares) and included fake versions of pipelines, storage tanks, jetties, vehicle parks and anti-aircraft emplacements. Wind machines were used to create clouds of dust to simulate activity, and the site was guarded by the military police. At night it was obscured by a smoke screen. German aircraft were allowed to overfly the facility, but only at altitudes of more than 33,000 ft (10060 m), where high-resolution imagery was not possible.

According to the original 'Overlord' plan, Cherbourg was to be captured within eight days of the start of the operation with the 'Neptune' (iii) assault landings and, despite the expectation that the Germans would carry out systematic demolitions, be opened within three days. Pipe-laying was to be begun four days later, with the 'Bambi' system fully operational by 75 days after 'Neptune' (iii). The discovery in May of an another German division in the area led to the expected capture being pushed back 10 days from 'Neptune' (iii), but in the event the port of Cherbourg was captured only on 27 June, 21 days after 'Neptune' (iii) and, as a a result of the port’s extensive damage, the first petroleum-laden tanker did not discharge there until 25 July. In the meantime, fuel was supplied through the small port of Port en Bessin by coastal tankers, and from ocean-going tankers using two Tombola lines at Port en Bessin for the British forces and five at Ste Honorine des Pertes for the US forces. The Tombola lines had a tendency to break, and the Chants fared poorly in the rough weather of the English Channel. By 28 July 16 of them were laid up for repairs at a special tanker repair facility that had been established at Hamble le Rice.

Consideration was given to the cancellation of 'Pluto', but it was finally decided to proceed. Time was wasted in deciding whether to terminate the line inside or outside the harbour; eventually the latter was chosen. The first Hais pipeline was laid by Latimer in just 10 hours on 12 August, but the pipeline failed when an escorting destroyer caught it with her anchor and damaged the pipeline beyond repair. A second effort was made by Sancroft two days later, but this pipeline also failed when the pipe became wrapped around the propeller of the support ship Algerian. An attempt to lay Hamel pipe instead failed on 27 August when it was discovered that tons of barnacles had attached themselves to the bottom of Conundrum 1, which prevented it from rotating. The barnacles were scraped off, and another attempt was made a few days later, but the pipeline broke about 33.4 miles (53.75 km) out.

The expert technicians had been able to lay pipelines across the Bristol Channel and the Solent under the supervision of the designers, but it was another matter for the naval pipeclaying parties to achieve the same degree of proficiency under wartime conditions and across the much wider English Channel. Finally, on 22 September a Hais cable was laid successfully, and delivered 56,000 Imp gal (250000 litres) per day. This was followed on 29 September by the successful installation of a Hamel cable by Conundrum 2. However, when on 3 October the pressure was increased by 1.4 to boost the quantity of fuel pumped, both pipelines failed: the Hais as a result of a faulty coupling, and the Hamel when it encountered a sharp edge on the ocean floor. 'Bambi' was terminated on the next day after only about 3,300 tons of fuel had been transferred.

Meanwhile, the port of Rouen on the Seine river had been captured on 30 August, and Le Havre on the same river’s estuary on 12 September. Le Havre was badly damaged in the 'Astonia' fighting and by demolitions. Rouen, 75 miles (121 km) up the river, was in better condition with its quays largely intact, although demolitions had been carried out and the river channel to it was blocked by mines and sunken vessels. Even when it was cleared, the channel from Le Havre was shallow, but coastal tankers from the UK were able to steam up it and discharge their petroleum products in Rouen. Farther to the north-east along the English Channel coast, Boulogne was captured on 22 September in 'Wellhit', and its port facilities were opened on 22 October.

A Hais pipeline was laid by Sancroft. This began to deliver fuel on 26 October, and remained in action until the end of the war. Lines were run to a beach in the outer harbour of Boulogne, 26.5 miles (42.6 km) across the Strait of Dover, instead of Ambleteuse as originally planned because the beach at the latter was heavily mined. This involved a longer distance and a more difficult approach, but by this time cable-laying techniques had been refined: the ends of the cable were dropped just offshore and picked up by the barges for connection to the shore. The Hamel pipe gave more trouble, but after some trial and error, it was laid with sections of Hais pipe at each end. Boulogne also had poor railway facilities, so the pipeline was extended to Calais where better railway connections were available to transport the fuel. This extension was completed in November.

By December, nine 3-in (75-mm) and two 2-in (51-mm) Hamel pipelines and four 3-in (76-mm) and two 2-in (51-mm) Hais pipelines had been laid, and 'Dumbo' was providing 1,300 tons of petrol per day. Not one of the Hais cable pipelines broke, and the mean time between repairs of the Hamel pipelines varied between 52 and 112 days, with 68 days the average. The pipelines could not be run at the intended pressure, so they carried only petrol and plans for the pipelines to deliver aviation spirit were dropped.

In December there was reconsideration of whether to continue with 'Pluto' as by this time Antwerp was unloading one ocean-going tanker per day, and coastal tankers were delivering 2,500 to 3,000 tons per day to Ostend, and a similar quantity to Rouen. On the other hand, only Antwerp and Cherbourg were capable of handling the large tankers, but Antwerp was under attack from V-1 flying bombs and V-2 rockets, and it was considered inadvisable for it handle more than one tanker at a time. Moreover, the coastal tankers were in demand for service in the Far East. In the circumstances it was decided to continue 'Pluto'.

As the fighting moved farther to the east and into Germany, 'Dumbo' was connected to an inland pipeline system that was extended from Boulogne to Antwerp, Eindhoven and ultimately Emerich. 'Dumbo' surpassed its target of some 3,000 tons per day on 15 March 1945, and by 3 April the 'Dumbo' lines were delivering 4,500 tons per day to the Rhine river. The laying of new lines continued, the last on 24 May.

The system was closed on 7 August in order to save manpower, by which time the pipelines had carried 180 million Imp gal (820 million litres) of petrol, and 'Pluto' was officially brought to an end on 31 August. The Tilbury plant was transferred to the Admiralty, and all remaining stores to the Ministry of Supply. No post-war use of the technology was contemplated.

It has been estimated that nearly 5.4 million tons of petroleum products were delivered to the Allied Expeditionary Force. Of this, 826,000 tons came directly from the USA and 4.3 million tons from the UK, of which 'Pluto' contributed 370,000 tons or 8%.