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by Bryan Maher • Large offshore oil-drilling platforms are huge engineering projects, both in their construction and operation. They are electrically self-contained with their own power generation, computer control and robot inspection systems. Large off-shore oil-drilling platforms have become a permanent part of the world scene. These are incredible structures, self-contained and equipped with their own electricity supply, oil drilling equipment, primary refining plant, pumps and compressors. They can drill oil wells through the ocean floor to depths of 6000 metres and more and they they can drill up to 30 oil wells from one position. The oil and gas they obtain is either pumped directly to land refineries or to tanker ships moored alongside. But would you like to work on an oil rig far out in the ocean, perhaps 100km from land? On an oil rig, you live and work on a "platform" supported by long spidery steel legs reaching perhaps 300 metres down to the ocean floor. In the North Sea alone, there are between 180 and 200 such platforms. Many more are in the Gulf of Mexico and off the Pacific coasts of Le~ e large Statsjord-A oil drilling pla m is located in the North Sea. The m ht-time illumination is heightened by the excess gas being burnt on the flare stack. Venezuela, the United States and Alaska. They also sprout in waters off equatorial West Africa and Australia's North West Shelf. The largest oil rigs proliferate in the Persian Gulf. The structures Most platforms are built lying on their side on a land-based slipway and launched like a ship. After being towed to the offshore worksite, watertight sections in the legs are filled with water or concrete to sink the legs to the ocean floor, leaving the whole structure in a vertical attitude. When the steel structure is erected and anchored to the ocean floor, the superstructure is completed. On a typical platform, the lowest deck surmounting the leg structure is up to 20 metres above the ocean surface. Usually, the oil treatment machinery and pumps, being the heaviest, are mounted on the lowest decks. Above this, the "module deck" carries boilers, the power station, workshops, well-heads, gas compressors and gas processing plant. The highest decks carry a number of cranes, the helicopter landing pad and hanger, the gas flare stack, control tower, the 70-metre high oil-drilling derrick and other associated equipment. In between are many decks of air-conditioned control rooms, and sleeping, eating and recreation areas for perhaps 100 workers. This living accommodation can be as high as eight or 10 decks - as big as a large city block of flats. Power station A power station is essential to supply the pumping and electrical load, motors, lighting, airconditioning, water purification, computers and other machinery. Usually four or five alternators rated at 3-5 megawatts each form the primary power station. The electrical load in many cases is from 10-15 megawatts, so that often three alternators are running and one or two sets are on standby. Electrical and mechanical engineers normally prefer to install a number of smaller machines rather than a few larger units. Thus if a serious breakdown occurs, a complete engine or alternator can be airlifted by helicopter to land-based workshops. This is economically reasonable as there is a limit to what can be repaired on the oil platform. A variety of engines will be found driving the alternators. Some early large North Sea and Persian Gulf installations used oil-fired steam boilers and steam turbines. North Sea steam driven plants ran at 3000rpm, driving 2-pole 50Hz alternators, while those under American influence in the Persian Gulf used 3600rpm steam turbines and 2-pole JUNE 1990 5 60Hz alternators. Smaller plants commonly used diesel engines. In later years, the problems of supplying fresh water to oil rigs caused a shift to large diesel engines or gas turbines. Gas turbines would seem a natural for oil-platform power generation as they will run on just about any fuel. However, the efficiency of a gas turbine is very dependent on the ambient temperature; the higher the ambient temperature, the less efficient the turbine. This is such a problem that tropical and Middle East installations now favour large modern diesel engines, while plants in the North Sea tend towards gas turbine drives. Deviation drilling Up to 30 wells are drilled by each platform to maximise access to the oil deposit far below. Wells are drilled at slight angles from the vertical to spread the field. Close computer control of drill bit direction is needed during this deviation drilling to avoid unwanted bends and collisions with existing well pipes. Having drilled a number of wells, oil and gas production is begun. The purpose of the production mach- 6 Computer control is essential for the safe operation of modern oil rigs inery is to separate the wanted oil and gas from the residue. The unwanted water (10%), sand, gravel and other contaminants which always flow up from deep wells must be removed. This process is quite involved and uses heavy, bulky plant. The production technique can be divided into six stages: (1). The oil mixture from all well heads on a platform is combined into two manifolds. Here the pressure is usually between 20 and 35 megapascals (ie, around 3000 to 5000 psi]. (2). The pressure of the stream is reduced by chokes to 10 megapascals and the oil, water and gas are then separated out. Up to three separator stages may be used in series, using large tanks fitted with ViJ,:. I. Upper dffk of a prod11ctio11 platfimn. Fig. 2. Module deck. 1 Living accommodat ion 2 Helideck 3 Heli cop ter hanga r 4 Co ntro l tower 9 Gas injec ti on module 10 Gas compress ion module 11 Well head module 12 Corridor 13 Manifold module 14 Gas treatment SILICON CHIP 5 6 7 8 Cranes Dril lin g derrick Flare stack Drill in g equipment area 15 16 17 16 19 vanes and gas traps. A typical tank is 15 metres long, 3.5 metres in diameter and weighs 150 tonnes (stage 1 tank) when containing 70 kilolitres of oil mix. The mixture takes one minute to pass through each stage. The pressure decreases as the mixture goes through stages 1, 2 and 3, emerging at atmospheric pressure, with water content down to 2% or so. (3). The final water removal stage uses a coalescer, a giant vessel 20 metres long and 4 metres in diameter. The oil mixture rests here for 5 hours while the water separates out (ie, the water sinks to the bottom). The resulting oil now has a water content below 0.1 %. (4). The crude oil passes through the metering stage to the delivery pumps. (5). The delivery pumps are usually rated at 3 to 5 megawatts each and in some North Sea platforms are directly driven by gas turbines. A variation on this method uses double-ended shaft gas turbines driving both an oil delivery pump and a power station alternator. Some recent Norwegian and Swedish oil rigs use 5MW synchronous 11kV 50Hz motors to Main generator Gas treatment Generator module Boiler room Maintenance shop and chemical injection module 20 Emergency generator module 21 Maintenance shop and electrical cont rol room drive the delivery pumps. (6). The natural gas separated out of the mixture is very valuable as fuel. This is dried and compressed in large machines driven by 3-5MW 1 lkV motors or 5MW gas turbines. The pressure required depends on the length of the pipeline to the point of use on land. Long distance land pipelines have 1-5MW relay pumping stations along the way. Some installations highly compress the gas for tanker transport overseas. Computer control Computer control is essential for the safe operation of modern oil rigs. For example, the Danish company ASEA Per Kure has installed computer controlled safety and control equipment on oil drilling rigs in cooperation with the oil companies Maersk of Norway, Mobil of USA and others. Such systems are absolutely vital on all oil platforms, when one considers the volatile nature of the oil and gas, the explosive environment and the very high oil and gas pressures involved. Anyone who doubts the need for stringent control could perhaps recall one rig in the Mexican Gulf Gas compres• ~ - - s,onand dehydration H,O Oil treatment Export pipeline This flow diagram shows the various separation and production processes on an offshore oil rig. All operations are computer controlled. Reservoir which blew its manifold asunder and caught fire. The whole 5000 tonne oil platform literally melted and burned to the waterline with great cost in human life. Sa£ety shut down ASEA Per Kure equipment will safely shut down the whole plant if the oil or gas pressures or temperatures exceed safe limits at any point in the process. Later computer controlled installations, as in the DAN-F platform operated by Maersk Oil and the Gullfaks-A platform in the North Sea, are complete process control systems. Naturally all electric motors, switchgear and control equipment near the oil or gas environment must be housed in explosion-proof casings. This imposes severe design restrictions, particularly where computers, keyboards and control rooms are involved. One solution is to locate some of the electrical and control equipment in sealed airconditioned rooms operating at a slightly positive air pressure. Any gas leak cannot then invade this safe space, so equipment inside does not require explosion proof housings. Pressure relief systems Should the pressure or temperature anywhere in the process exceed safe values, what can be done? Also where does all the water extracted from the crude oil Right: oil-rig process control equipment for Gullfaks A undergoing tests at Asea Per Kure's factory in Oslo. Fig. 3. Lower dee/.:. 22 Ventilation fans 23 Gas condensate treatment 24 Gas treatment 25 Drill pipe area 26 0,1 separators 27 Oil and oily water treatment 28 Ballast water treatment 29 Electrical control room 30 Area above marine riser 31 Pump and auxiliary equipment module 32 Area above equipment shaft 33 Ventilation fan for equipment shaft 34 Heating and ventilation fans 35 Air compressor equipment 36 Main control room ◄ Left: the enormous size of offshore oil rigs can be gauged by the fact that the living accomodation is often eight floors in height. The complete platform is as large as a 5000-10,000 tonne ocean going ship. ]UNE 1990 7 This strange craft is the multi-purpose support vessel Regalia, built by Gotaverken Arendal to provide support for offshore work. The vessel is semisubmersible, has computer control systems and is self-propelled. Note the helicopter landing pad, living area, cranes and large work area. pean waters are politically peaceful, the Persian Gulf is not so lucky. A good friend of the author's, an electrical engineer on a Gulf oil drilling platform between Bahrain and Abadan, tells of constantly living in a war zone. Often under air attack, on one occasion his oil rig became the target for an Exocet missile, which exploded at the oil platform legs, on the waterline. One very large steel leg was blown asunder. But our friend, along with the whole crew, survived unscathed, as the platform continued standing erect. While this is a rather extreme example, it emphasises the need to constantly scrutinise all underwater steelwork. Whether in the North Sea, with its constant gales and storms, or the war-torn Persian Gulf, the original strength of welds in particular can be eroded by fatigue cracks. Such defects, unless repaired by underwater electric welding, can initiate failure of the whole platform under any severe stress, whether caused by man or the sea. (Fatigue in steel is caused by the vibrations of running machinery plus the buffeting strains caused by oceans in turmoil}. Underwater inspection go? You cannot just dump it in the ocean as it still contains some oil and various other contaminants. The computer control system takes care of these problems. Under excess pressure conditions, gas which cannot be pumped ashore or used by the platform boilers or power station may be pumped down another drill pipe. It is thereby returned to the oil deposit far below, the source of the material in the first place. As well as relieving any dangerous pressures on the platform, this practise also helps maintain the flow of oil from the well. Computer control of the gas return pumps regulates the pressure to a value above the oil well pressure. Water removed from the oil is similarly pumped back down the well to the oil deposit. and Saudi Arabia, lies in a war zone bordered by eight independent countries. Beneath lies 60% of the world's proven reserves of crude oil and natural gas. Here, offshore oil drilling platforms, owned and operated by American, Japanese and European companies, supply much of the industrialised nations' fuel needs. The enormous size of these oil platforms and the scope of their electrical engineering is hard to visualise. Their huge oil pumps shift the crude oil to land via underwater pipes big enough for a man to stand erect and walk through. Each pipeline carries one million barrels of crude oil daily right across Arabia to the Mediterranean seaports at Telkalakh and Baniyas. A number of relay pumping stations line the route. Persian Gulf oil War damage The Persian Gulf, between Iran 8 SILICON CHIP Whereas Australian and Euro- Until recent times, deepsea divers continually risked their lives inspecting all submerged steelwork. Particular attention is necessary in the vital joint areas the "K" structure nodes. On oil-rig platforms, all structural work between the ocean surface and the limit of strong sunlight penetration (about 50 metres depth} quickly becomes encrusted with barnacles and various marine growth. This must be removed to enable inspection for hidden fatigue cracks, a cleaning job divers find difficult in the extreme. Momentum reaction while underwater, handling high pressure water jets and hydraulically driven brushes, makes for tedious and arduous work. Furthermore, diving 300 metres to reach the bottom of some structures is a slow, dangerous occupation for which few are suited. Yet inspection for fatigue cracks is absolutely vital. Robots to the rescue Why not design a machine to perform the task? Robots are a natural for performing difficult repetitive tasks, particularly in environments perilous to humans. In June 1986, the Swedish Company ASEA Oil & Gas announced the development of a deep sea computer controlled robot to perform this work. It is called ROCIS which stands for "Remotely Operated Crack Inspection System" . The method of crack detection is based on eddy current techniques. Eddy current testing With a clean piece of flawless homogenous steel on the test bench, eddy current tests are straightforward. An AC magnetic field applied to the steel induces eddy currents therein and these eddy currents set up their own secondary magnetic fields. A field strength sensor will then find a regular magnetic field strength over the surface. Now if we substitute a steel sample containing a surface defect, the results are different. The defect or crack interrupts the surface eddy currents. This produces strong local magnetic poles on either side of the crack. Measurements close to the surface readily detect this abrupt change in field strength, indicating the crack. It's all so easy in the laboratory but it's not so simple at the worksite! Marine growth covering the steel is not a bit constant in thickness. Therefore, a simple magnetic field sensor gives different readings when moving over the steel, riding up and down on the This artist's impression shows the ROCIS inspecting a welded steel joint deep below the ocean (transparent view). Note the claw arms with gripping wheels which drive the robot around the steel beam. The front arm carries the small eddy current crack detector. deposits of marine growth. Furthermore, there is no guarantee of homogeneity in the steel composition in the various members of a structural joint. The welding rods originally used may also be of different alloy content. Although 14 companies worldwide were invited to participate in the development of undersea crack detector, most believed the task too difficult. However, a few did collaborate with ASEA Oil & Gas to produce a highly effective device. As the graph shows, the sensor can detect surface cracks in steel covered by up to 15mm of marine growth. Therefore, only rough cleaning is needed prior to testing. Crack detection responses The detection system cannot avoid responding to normal welds, as any join must create a magnetic discontinuity. These readings must be recognised as welds, not cracks. Oily water Three separator stages are involved in removing the water & gas content from the crude oil. This process occupies much of the space on an offshore oil rig. This diagram shows some of the internal details of the separator vessels. The vessel is typically 15 metres long and 3.5 metres across and can hold 70,000 litres of oil. JUNE 1990 9 Thus, the robot is safe even in the strongest ocean storms. Test display In this drawing, the ROCIS is shown inspecting a K node in the underwater steelwork below the platform. The human diver is shown for size comparison. Normally, the robot works without human intervention but humam divers still have to weld any cracks that are detected. Tests have shown that when the crack detector moves above the steel, maintaining about 10mm separation, the system yields the best distinction ratio in crack/weld responses. In practise, this means that marine growth up to a thickness of 10mm can be tolerated. Results achieved in underwater tests show that the smallest crack detectable in steel members or welds is 15mm long and 2mm deep if the sensor is within 5mm of the steel, or 30mm long and 2mm deep when the sensor is 15mm above the steel. These measurements show a signal/noise ratio of about + lOdB. The above performance is independent of the intercept angle, so the sensor can respond correctly when poked into the corner of a joint. The robot finally developed has a relatively small wrap-around body 10 SILICON CHIP one metre wide, 800mm long and 400mm high. From the body extend claw-like side arms and a telescopic front arm which carries the crack sensor. All joints, the claws and the telescopic arm are electrohydraulically driven, each with its own actuator. Electric motors and hydraulics are located within the main body, while all electronic circuitry is housed in sealed compartments, waterproof to 300 metres. A remotely operated vehicle lifts the robot from its storage area and places it on the structural member to be tested. This action is supervised remotely by the control operator. From this point on, the robot operation is automatic until one joint is completely inspected. The hydraulics wrap the clawarms tightly around any circular steel member measuring between 500mm and 1.2 metres in diameter. Output signals from the robot, the crack sensor and the video cameras are processed in a computer and displayed on two screens. One VDU shows computerreconstructed images of the weld being inspected. This readout highlights any cracks and pinpoints their location even though they may be covered by marine growth. The second display is a 3-dimensional real-time video picture of the robot's position and the area being treated. When first used, the robot was remotely controlled by an operator. Later, the operation was made completely automatic. An adaptive automatic control system was developed in cooperation with Trallfa Robotics A/S of Byrne, Norway. This digital system controls all operations. Once placed on the girder by the Remote Operated Vehicle, the robot finds its own way to the work site and the weld to be inspected. It then proceeds with the complete crack test. In operation, the robot rotates around the steel member 180° clockwise, checking for cracks. Then, to avoid entangling the umbilical cable, the robot returns anticlockwise to its starting position and proceeds around the girder to test the other side. Initially, the rough cleaning required before testing was performed by divers, until scrubbing equipment was later carried by the robot. There is only one problem still to be addressed: once cracks are found, how are they repaired? The answer is that the poor old humans still have to go down and do the job. When that task is finally automated, even larger oil drilling platforms will be able to stand in even greater ocean depths. Acknowledgement The author thanks ASEA Journal and Bill Fitzgerald and Don Smith of ABB for data, photos and permission to publish. ~