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Final project

  1. 1. PIPING BASICS <ul><li>Piping may be defined as a mode of transportation for the liquids, gases, and fluidized solids from one place to another. A pipe is basically a tubular structure which is specified by its nominal bore diameter and its Schedule number which generally gives the standards of its wall thickness. The piping is laid and designed according to some standards laid by some of the standard organizations of the world. It forms the heart of industries such as power plants, petrochemical projects etc. </li></ul><ul><li>Hence it becomes very important for us to know the designing of such structures. The sector where piping is used as the backbone are :- </li></ul><ul><li>Oil & Gas Industry </li></ul><ul><li>Refineries </li></ul><ul><li>Petro Chemicals </li></ul><ul><li>Chemical Plants </li></ul><ul><li>Power Plants </li></ul><ul><li>Water Treatment Plants </li></ul><ul><li>(vii) Pharmaceuticals & Food Industry </li></ul><ul><li>(viii) Paper plants </li></ul><ul><li>In other few pages we shall gain the complete knowledge on piping and its basics. </li></ul>
  2. 2. OFFSHORE FUNDAMENTALS INTRODUCTION Offshore industry is basically concerned with the exploration, drilling and production of oil and gas from the sea bed in shallow as well as deep water. Oil and gas are derived almost entirely from decayed plants and bacteria. Energy from the sun, which fuelled the plant growth, has been recycled into useful energy in the form of hydrocarbon compounds - hydrogen and carbon atoms linked together. Offshore and gas originates from two sources. Gas from beneath the southern North Sea and the Irish Sea formed from coals which were derived from the lush, tropical rain forests that grew in the Carboniferous Period, about 300 million years ago. Oil and most gas under the central and northern North Sea and west of the Shet land Islands formed from the remains of planktonic algae and bacteria that flourished in tropical seas of the Jurassic and Cretaceous Periods, about 140 to 130 million years ago (a significant amount of the Kimmeridge Clay Format ion is Cretaceous in age) . They accumulated in muds, which are now the prolific Kimmeridge Clay source rock. Crude oil is a complex mixture of hydrocarbons with small amounts of other chemical compounds that contain sulphur, nitrogen and oxygen. Traces of other elements, such as sulphur and nitrogen, were also present in the decaying organic material, giving rise to small quantities of other compounds in crude oil. Hydrocarbon molecules come in a variety of shapes and sizes, (straight -chain, branched chain or cyclic) , this is one of the things that makes them so valuable because it allows them to be used in so many different ways.
  3. 3. <ul><li>Oil and gas form as the result of a precise sequence of environmental condit ions: </li></ul><ul><li>· The presence of organic material </li></ul><ul><li>· Organic remains being t rapped and preserved in sediment </li></ul><ul><li>· The material is buried deeply and then slowly &quot;cooked&quot; by increased temperature and pressure. </li></ul><ul><li>Offshore platforms are used for exploration of Oil and Gas from under Seabed and processing. The First Offshore platform was installed in 1947 off the coast of Louisiana in 6M depth of water . Today there are over 7,000 Offshore platforms around the world in water depths up to 1,850M </li></ul><ul><li>Platform size depends on facilities to be installed on top side eg. Oil rig, living quarters, Helipad etc. </li></ul><ul><li>Classification of water depths: </li></ul><ul><ul><li>< 350 M- Shallow water </li></ul></ul><ul><ul><li>< 1500 M - Deep water </li></ul></ul><ul><ul><li>> 1500 M- Ultra deep water </li></ul></ul><ul><ul><li>US Mineral Management Service (MMS) classifies water depths greater than 1,300 ft as deepwater, and greater than 5,000 ft as ultra-deepwater. </li></ul></ul>
  4. 4. OFFSHORE PROCESS EXPLORATION DISCOVERING THE UNDERGROUND STRUCTURE Large-scale geological structures that might hold oil or gas reservoirs are invariably located beneath non-productive rocks, and in addition this is often below the sea. Geophysical methods can penetrate them to produce a picture of the pat tern of the hidden rocks. Relatively inexpensive gravity and geomagnetic surveys can identify potentially oil-bearing sedimentary basins, but costly seismic surveys are essential to discover oil and gas bearing structures. Sedimentary rocks are generally of low density and poorly magnetic, and are often underlain by strongly magnetic, dense basement rocks. By measuring ’anomalies’ or variations from the regional average, a three-dimensional picture can be calculated. Modern gravity surveys show a generalized picture of the sedimentary basins. Recently, high resolution aero-magnetic surveys flown by specially equipped aircraft at 70 - 100m altitude show fault t races and near surface volcanic rocks. Shooting seismic surveys More detailed in format ion about the rock layers within such an area can be obtained by deep echo sounding, or seismic reflect ion surveys. In offshore areas these surveys are undertaken by a ship (F52) towing both a submerged air or water gun array, to produce short bursts of sound energy, and a set of streamers of several kilometers length. Each streamer contains a dense array of hydrophone groups that collect and pass to recorders echoes of sound from reflecting layers.
  5. 5. The depths of the reflecting layers are calculated from the time taken for the sound to reach the hydrophones via the reflector; this is known as the two-way t ravel time (F50a & b) . The pulse of sound from the guns radiates out as a hemispherical wave front , a port ion of which will be reflected back towards the hydrophones from rock interfaces (F50a) . The path of the minute port ion of the reflected wave- front intercepted by a hydrophone group is called a ray path. Hydrophone groups spaced along the streamer pick out ray paths that can be related to specific points on the reflector surface (F50c) . Graphs of the intensity of the recorded sound plot ted against the two-way time are displayed as wiggle t races (F50b) . Seismic recording at sea always uses the common depth point (CDP) method (F50c & d) . A sequence of regularly spaced seismic shot s is made as the survey vessel accurately navigates its course. Processing Processing recordings involves many stages of signal processing and computer summing. Firstly, wiggle t races from a single CDP are collected into groups. Displayed side by side in sequence they form a CDP gather (F51a & b) . Reflections from any one reflector form a hyperbolic curve on the gather because the sound takes longer to t ravel to the more distant hydrophones. This effect is called normal move out (NMO) . Correct ion is needed to bring the pulses to a horizontal alignment , as if they all came from vertically below the sound source (F51c) . The separate wiggle t races are added together, or stacked (F51d) . Stacking causes t rue reflect ion pulses to enhance one another, and hopefully, random noise will cancel out . This process is repeated for all the CDPs on the survey line. The stacked and corrected wiggle t races are displayed side by side to give a seismic sect ion (F51e)
  7. 7. Interpretation Seismic sect ions provide 2-dimensional views of underground structure. By using special shooting techniques such as spaced air gun arrays or towing the streamer slantwise, or by shooting very closely spaced lines, it is possible to produce 3-dimensional (3D) seismic images (F59) . These images comprise vertical sect ions and horizontal sect ions ( ’time-slices’) . DRILLING DEALING WITH THE UNDERGROUND STRUCTURE There are two basic types of drilling rigs - fixed plat form rigs and mobile rigs. Fixed platform rigs are installed on large offshore plat forms and remain in place for many years. Most of the large fields in the North Sea such as Forties and Brent were developed using fixed plat form rigs. Drilling fluid (also called &quot;mud&quot;) , which is mainly water-based, is pumped continuously down the drill string while drilling. It lubricates the drilling tools, washes up rock cuttings and most importantly, balances the pressure of fluids in the rock format ions below to prevent blowouts. In offshore drilling, the first step is to put down a wide-diameter conductor pipe into the seabed to guide the drilling and contain the drilling fluid. It is drilled into the seabed from semi-submersible rigs, but on product ion plat forms a pile-driver may be used. As drilling continues, completed sections of the well are cased with steel pipe cemented into place. A blowout preventer is attached to the top of the casing. This is a stack of hydraulic rams which can close off the well instantly if back pressure (a kick) develops from invading oil, gas or water. Drilling grinds up the rock into tea- leaf-sized cuttings which are brought to the surface by the drilling mud. The drilling mud is passed over a shale shaker which sieves out the cuttings .
  8. 8. DRILLING
  9. 9. In exploration drilling, the cuttings are taken for examination by a geologist known as a mud logger who is constantly on the lookout for oil and gas. PRODUCTION THE OFFSHORE CHALLENGE Production facilities had to be designed to withstand wind gusts of 180 km/ hour and waves 30 met res high. Other problems included the ever-present salt -water corrosion and fouling by marine organisms. Dealing with the many underwater construction and maintenance tasks falls to divers and remotely operated vehicles. Giant floating cranes (F83) designed to lift ever greater loads were commissioned and many other specialized craft had to be developed to establish and service the offshore industry. Huge helicopter fleet s were needed to ferry workers to and from the plat forms and rigs. Product ion Plat forms Most oil and gas product ion plat forms in offshore Britain rest on steel supports known as ’jackets’, a term derived from the Gulf of Mexico. A small number of plat forms are fabricated from concrete. The steel jacket , fabricated from welded pipe, is pinned to the sea floor with steel piles. Above it are prefabricated units or modules providing accommodation and housing various facilities including gas turbine generating sets. Towering above the modules are the drilling rig derrick ( two on some plat forms) , the flare stack in some designs (also frequently cantilevered outwards) and service cranes. Horizontal surfaces are taken up by store areas, drilling pipe deck and the vital helicopter pad. Concrete gravity plat forms are so- called because their great weight holds them firmly on the seabed. They were first developed to provide storage capacity in oilfields where tankers were used to transport oil, and to eliminate the need for piling in hard sea beds.
  10. 10. The Brent D plat form (F87) , which weighs more than 200 000 tonnes, was designed to store over a million barrels of oil. But steel plat forms, in which there have been design advances, are now favored over concrete ones. Several plat forms may have to be installed to exploit the larger fields, but where the capacity of an existing plat form permits, subsea collecting systems linked to it by pipelines have been developed using the most modern technology. They will be increasingly used as smaller fields are developed. For very deep waters, one solution was the Hut ton Tension Leg Plat form: the buoyant plat form, resembling a huge drilling rig, is tethered to the sea-bed by jointed legs kept in tension by computer- cont rolled ballast adjustments. Alternatively, a subsea collect ion system may be linked via a product ion riser to a Floating, Production, Storage and Offloading (FSPO) vessel (F88) ; either a purpose built ship or a converted tanker or semisubmersible rig. The oil is offloaded by a shut t le tanker. Product ion Wells To develop offshore fields as economically as possible, numerous directional wells radiate out from a single plat form to drain a large area of reservoir (F94) . For directional drilling special weighted drill collars are used with a ’bent sub’ to deflect the drill bit at a certain angle in the required direct ion (F93) . Wells which deviate at more than 65 degrees from the vertical and reach out horizontally more than twice their vertical depth are known as extended reach wells. More than one horizontal sect ion can be drilled in one well as a multilateral well (F96) . This technique is used to reduce drilling costs and to maximize the number of wells that can be drilled from small plat forms.
  11. 11. Land Rig Platform Jack-up Semi-Submersible Drill Ship T.L.P. PRODUCTION
  12. 12. GETTING OIL AND GAS ASHORE Most offshore oil and all offshore gas are brought to shore by pipelines which operate in all weathers. Pipeline routes are planned to be as short as possible. Slopes that could put stress on unsupported pipe are avoided and seabed sediments are mapped to identify unstable areas and to see if it will be possible to bury the pipe. Pipeline construction begins onshore, as lengths of pipe are waterproofed with bitumen and coated with steel- reinforced concrete. This coating weighs down the submarine pipeline even when it is filled with gas. The prepared pipe- lengths are welded together offshore on a lay barge (F101) . As the barge winches forward on its anchor lines, the pipeline drops gently to the seabed, guided by a ’stinger’. The inside of pipelines need to be cleaned regularly to remove wax deposits and water: to do this a collecting device known as a pig is forced through the pipe. Where tankers transport oil from small or isolated fields, various oil storage systems may be used. These may range from cylindrical cells contained in some of the massive concrete structures, to seabed storage units such as that employed at the Kittiwake field, or integral storage such as that contained in the various Floating, Product ion, Storage and Offloading vessels. In essence these FPSOs are floating storage tankers, as well as product ion and processing installations. FPSOs provide an important option for developing fields which may be remote from existing infrastructure or where the field recoverable reserves are uncertain, for example because of difficult geological conditions.
  13. 13. <ul><li>An oil platform or oil rig is a large structure used to house workers and machinery needed to drill and/or extract oil and natural gas through wells in the ocean bed. Depending on the circumstances, the platform may be attached to the ocean floor, consist of an artificial island, or be floating. Generally, oil platforms are located on the continental shelf, though as technology improves and crude oil prices increase, drilling and production in deeper waters becomes both feasible and profitable. A typical platform may have around thirty wellheads located on the platform and directional drilling allows reservoirs to be accessed at both different depths and at remote positions up to 5 miles (8 kilometres) from the platform. Many platforms also have remote wellheads attached by umbilical connections, these may be single wells or a manifold centre for multiple wells. </li></ul><ul><li>Offshore platforms can broadly be categorized </li></ul><ul><li>into two parts :- </li></ul><ul><li>Structures that extend to the sea bed </li></ul><ul><li>Jacketed or Fixed Steel platform </li></ul><ul><li>Concrete Gravity Structures </li></ul><ul><li>Compliant Tower </li></ul><ul><li>Structures that float near the water surface- Recent Development </li></ul><ul><li>Tension Leg Platforms </li></ul><ul><li>SPAR </li></ul><ul><li>Ship shaped Vessels (FPSO) </li></ul>OIL PLATFORMS
  14. 14. <ul><li>STRUCTURES THAT EXTEND TO THE SEA BED </li></ul><ul><li>FIXED STEEL PLATFORMS </li></ul><ul><ul><li>A Fixed Platform is a type of offshore platform used for the production of oil or gas. These platforms are built on concrete and/or steel legs anchored directly onto the seabed, supporting a deck with space for drilling rigs, production facilities and crew quarters. Such platforms are, by virtue of their immobility, designed for very long term use (for instance the Hibernia platform). Various types of structure are used, steel jacket, concrete caisson, floating steel and even floating concrete. Steel jackets are vertical sections made of tubular steel members, and are usually piled into the seabed. Concrete caisson structures, pioneered by the Condeep concept, often have in-built oil storage in tanks below the sea surface and these tanks were often used as a flotation capability, allowing them to be built close to shore (Norwegian Fjords and Scottish Firths are popular because they are sheltered and deep enough) and then floated to their final position where they are sunk to the seabed. Fixed platforms are economically feasible for installation in water depths up to about 1,700 feet (520 m). Space framed structure with tubular members supported on piled foundations. Used for moderate water depths up to 400 M. Jackets provides protective layer around the pipes. Typical offshore structure will have a deck structure containing a Main Deck, a Cellar Deck, and a Helideck. The deck structure is supported by deck legs connected to the top of the piles. The piles extend from above the Mean Low Water through the seabed and into the soil. Underwater, the piles are contained inside the legs of a “jacket” structure which serves as bracing for the piles against lateral loads. </li></ul></ul>
  16. 16. <ul><ul><li>The jacket also serves as a template for the initial driving of the piles. (The piles are driven through the inside of the legs of the jacket structure).Natural period (usually 2.5 second) is kept below wave period (14 to 20 seconds) to avoid amplification of wave loads. 95% of offshore platforms around the world are Jacket supported. </li></ul></ul><ul><ul><li>CONCRETE GRAVITY BASE STRUCTURES </li></ul></ul><ul><ul><li>Whilst the vast majority of fixed offshore platforms employ a tubular jacket to support the topside facilities, a number of installations have been constructed using a base manufactured from reinforced concrete. They are Fixed-bottom structures made from concrete . They are heavy and remain in place on the seabed without the need for piles. They are widely used for moderate water depths up to 300 M. Part construction is made in a dry dock adjacent to the sea. The structure is built from bottom up, like onshore structure. At a certain point , dock is flooded and the partially built structure floats. It is towed to deeper sheltered water where remaining construction is completed. After towing to field, base is filled wi1th </li></ul></ul><ul><ul><li>water to sink it on the seabed. Its main advantage is its less maintenance. The first concrete structure to be installed in the North Sea was constructed by the Norwegians in 1973 and used to develop the Ekofisk field. Since then those people have installed a steady string of concrete structures and it came as no surprise when their government elected to develop the other fields with the same concrete structure which stands in 350ft. of water and is currently the largest offshore structure in Europe and the largest concrete platform in the world. </li></ul></ul><ul><li>  </li></ul>
  18. 18. COMPLIANT TOWER A compliant tower (CT) is a fixed rig structure normally used for the offshore production of oil or gas. The rig consist of narrow, flexible ( compliant ) towers and a piled foundation supporting a conventional deck for drilling and production operations. Compliant towers are designed to sustain significant lateral deflections and forces, and are typically used in water depths ranging from 1,500 and 3,000 feet (450 and 900 m). With the use of flex elements such as flex legs or axial tubes, resonance is reduced and wave forces are de-amplified. This type of rig structure can be configured to adapt to existing fabrication and installation equipment. Compared with floating systems, such as Tension leg platforms and SPARs, the production risers are conventional and are subjected to less structural demands and flexing. This flexibility allows it to operate in much deeper water, as it can 'absorb' much of the pressure exerted on it by the wind and sea. Despite its flexibility, the compliant tower system is strong enough to withstand hurricane conditions. The first tower emerged in the early 1980s with the installation of Exxon's Lena oil platform. Narrow, flexible framed structures supported by piled foundations. It has no oil storage capacity. Production is through tensioned rigid risers and export by flexible or catenary steel pipe. It undergos large lateral deflections (up to 10 ft) under wave loading. Used for moderate water depths up to 600 M. Natural period (usually 30 second) is kept above wave period (14 to 20 seconds) to avoid amplification of wave loads.
  19. 19. STRUCTURES THAT FLOAT NEAR THE WATER SURFACE TENSION LEG PLATFORMS A Tension-leg platform or Extended Tension Leg Platform (ETLP) is a vertically moored floating structure normally used for the offshore production of oil or gas and is particularly suited for water depths greater than 300 meters (about 1000 ft). Also proposed for wind turbines. The platform is permanently moored by means of tethers or tendons grouped at each of the structure's corners. A group of tethers is called a tension leg. A feature of the design of the tethers is that they have relatively high axial stiffness(low elasticity), such that virtually all vertical motion of the platform is eliminated. This allows the platform to have the production wellheads on deck (connected directly to the subsea wells by rigid risers), instead of on the seafloor . This makes for a cheaper well completion and gives better control over the production from the oil or gas reservoir. The first Tension Leg Platform was built for Conoco's Hutton field in the North Sea in the early 1980s. The hull was built in the dry-dock at Highland Fabricator's Nigg yard in the north of Scotland, with the deck section built nearby at McDermott's yard at Ardersier. The two parts were mated in the Moray Firth in 1984. Tension Leg Platforms (TLPs) are floating facilities that are tied down to the seabed by vertical steel tubes called tethers. This characteristic makes the structure very rigid in the vertical direction and very flexible in the horizontal plane. The vertical rigidity helps to tie in wells for production, while, the horizontal compliance makes the platform insensitive to the primary effect of waves. It has large columns and Pontoons and a fairly deep draught.
  20. 20. TLP has excess buoyancy which keeps tethers in tension. Topside facilities , no. of risers etc. have to fixed at pre-design stage. It is used for deep water up to 1200 M. It has no integral storage. It is sensitive to topside load/draught variations as tether tensions are affected. SPAR A SPAR, named for logs used as buoys in shipping and moored in place vertically, is a type of floating oil platform typically used in very deep waters. Spar production platforms have been developed as an alternative to conventional platforms. A Spar platform consists of a large-diameter, single vertical cylinder supporting a deck. It contains a deep-draft floating caisson, which is a hollow cylindrical structure similar to a very large buoy. Its four major systems are hull, moorings, topsides and risers. About 90% of the structure is underwater. The spar design is now being used for drilling, production, or both. The distinguishing feature of a spar is its deep-draft hull, which produces very favorable motion characteristics compared to other floating concepts. Water depth capability has been stated by industry as ranging up to 10,000 ft. The first Spar platform in the was installed in September of 1996. It follows the concept of a large diameter single vertical cylinder supporting deck. These are a very new and emerging concept: the first spar platform, Neptune, was installed off the USA coast in 1997.Spar platforms have taut catenary moorings and deep draught, hence heave natural period is about 30 seconds.
  22. 22. FPSO {Floating Production Storage and Offloading} A Floating Production, Storage and Offloading vessel (FPSO; also called a &quot;unit&quot; and a &quot;system&quot;) is a type of floating tank system used by the offshore oil and gas industry and designed to take all of the oil or gas produced from a nearby platform (s), process it, and store it until the oil or gas can be offloaded onto waiting tankers or sent through a pipeline.     History Oil has been produced from offshore locations since the 1950s. Originally, all oil platforms sat on the seabed, but as exploration moved to deeper waters and more distant locations in the 1970s, floating production systems came to be used. The first oil FPSO was the Shell Castellon, built in Spain in 1977. The first ever conversion of a LNG carrier (Golar LNG owned Moss type LNG carrier) into an LNG floating storage and regasification unit was carried out in 2007 by Keppel shipyard in Singapore. The last few years concepts for LNG FPSOs has also been launched. An LNG FPSO works under the same principles as an Oil FPSO, but it only produces natural gas, condensate and/or LPG, which is stored and offloaded.
  23. 23. Working principles Oil produced from offshore production platforms can be transported to the mainland either by pipeline or by tanker. When a tanker solution is chosen, it is necessary to accumulate oil in some form of tank such that an oil tanker is not continuously occupied while sufficient oil is produced to fill the tanker. Often the solution is a decommissioned oil tanker which has been stripped down and equipped with facilities to be connected to a mooring buoy. Oil is accumulated in the FPSO until there is sufficient amount to fill a transport tanker, at which point the transport tanker connects to the stern of the floating storage unit and offloads the oil. An FPSO has the capability to carry out some form of oil separation process obviating the need for such facilities to be located on an oil platform. Partial separation may still be done on the oil platform to increase the oil capacity of the pipeline(s) to the FPSO. Advantages Floating Production, Storage and Offloading vessels are particularly effective in remote or deepwater locations where seabed pipelines are not cost effective. FPSOs eliminate the need to lay expensive long-distance pipelines from the oil well to an onshore terminal. They can also be used economically in smaller oil fields which can be exhausted in a few years and do not justify the expense of installing a fixed oil platform. Once the field is depleted, the FPSO can be moved to a new location.
  24. 24. FPSO
  25. 25. Specific types A Floating Storage and Offloading unit (FSO) is a floating storage device, which is simplified FPSO without the possibility for oil or gas processing. Most FSOs are old single hull supertankers that have been converted. An example of this is the Knock Navis, the world's largest ship, which has been converted to an FSO to be used offshore Qatar. A LNG floating storage and regasification unit (FSRU) is a floating storage and regasification system, which receives liquefied natural gas(LNG) from offloading LNG carriers, and the onboard regasification system provides natural gas send-out through flexible risers and pipeline to shore.
  26. 26. <ul><li>MODU’s { Mobile Offshore Drilling Units } </li></ul><ul><li>The basic work of the mobile units is to drill the well in the sea bed and prepare the line for production. Offshore drilling is divided into two parts i.e shallow water and deep water. In shallow water, jack-up rigs, standing with their feet on the seabed are used to drill the oil wells. In deeper water, floating drilling units are used. There are two basic types; Drill ships and Semi-submersible drilling rigs. This is a very important process and is very hard in nature as the environmental conditions are always unfavorable for such a process to accomplish. There are basically three type of drilling units that are widely used over the world. They are :- </li></ul><ul><li>Semi-submersible drill rigs </li></ul><ul><li>Self elevated Jack up rigs </li></ul><ul><li>Drill ships </li></ul>
  27. 27. SEMI SUBMERSIBLE DRILLING RIG <ul><li>A semi-submersible drilling rig is one in which: </li></ul><ul><li>Sea water is pumped into the hull of the vessel causing the vessel to submerge to the desired depth. </li></ul><ul><li>The submerged vessel maintains its position over the well location by means of anchor chains. </li></ul><ul><li>A semi-submersible is not bottom-founded and can work in much greater water depths than a jack-up. </li></ul><ul><li>The maximum water depth is a function of the length of the rig's riser, a bundle of utility tubes through which drilling fluids and other material is conducted, enclosed in an outer tube, suspended to the seafloor. </li></ul>DRILL SHIPS Drill ships, a maritime vessel that has been fitted with drilling apparatus. It is most often used for exploratory drilling of new oil or gas wells in deep water but can also be used for scientific drilling. It is often built on a modified tanker hull and outfitted with a dynamic positioning system to maintain its position over the well.
  30. 30. ELEVATING JACK UP RIGS INTRODUCTION A Jack Up is an offshore structure composed of a hull, legs and a lifting system that allows it to be towed to a site, lower its legs into the seabed and elevate its hull to provide a stable work deck capable of withstanding the environmental loads. A typical modern drilling Jack Up is capable of working in harsh environments (Wave Heights up to 80 ft, Wind Speeds in excess of 100 knots) in water depths up to 500 feet. Because Jack Ups are supported by the seabed, they are preloaded when they first arrive at a site to simulate the maximum expected leg loads and ensure that, after they are Jacked to full air gap and experience operating and environmental loads, the supporting soil will provide a reliable foundation. Jack Up Units have been a part of the Offshore Oil Industry exploration fleet since the 1950’s. They have been used for exploration drilling, tender assisted drilling, production, accommodation, and work/maintenance platforms. As with every innovative technology, Jack Up Units have been used to their operational and design limitations. These limitations include deck load carrying limits when afloat, load carrying capabilities when elevated, environmental limits, drilling limits, and soil (foundation) limits.
  31. 31. The reasons for pushing these limits include the desire to explore deeper waters, deeper reservoirs in harsher environments, and in areas where soils and foundations may be challenging or even unstable. There are three main components of a Jack Up Unit: the Hull, the Legs & Footings, and the Equipment. Each of the component are described below :- HULL The Hull of a Jack Up Unit is a watertight structure that supports or houses the equipment, systems, and personnel, thus enabling the Jack Up Unit to perform its tasks. When the Jack Up Unit is afloat, the hull provides buoyancy and supports the weight of the legs and footings (spud cans), equipment, and variable load. Different parameters of the hull affect different modes of operation of the Unit. In general, the larger the length and breadth of the hull, the more variable deck load and equipment the Unit will be able to carry, especially in the Afloat mode (due to increased deck space and increased buoyancy). Also, larger hulls generally result in roomier machinery spaces and more clear space on the main deck to store pipe, 3rd Party Equipment, and provide for clear work areas. The larger hull may have larger preload capacity that may permit increased flexibility in preloading operations. Larger hulls generally have the negative effects of attracting higher wind, wave and current loads. Since Jack Ups with larger hulls weigh more, they will require more elevating jacks of larger capacity to elevate and hold the Unit.
  32. 32. The large weight also affects the natural period of the Jack Up Unit in the elevated mode. The draft of the hull, or the distance from the afloat waterline to the baseline of the hull, has a direct effect on the amount of variable deck load that can be carried and the stability when afloat. The draft of the hull has an opposing relationship with the hull’s freeboard, or the distance from the afloat waterline to the main deck of the hull. Every incremental increase in the draft of a Jack Up decreases the freeboard by the same increment. LEGS AND FOOTINGS The legs and footings of a Jack Up are steel structures that support the hull when the Unit is in the Elevated mode and provide stability to resist lateral loads. Footings are needed to increase the soil bearing area thereby reducing required soil strength. The legs and footings have certain characteristics which affect how the Unit reacts in the Elevated and Afloat Modes, while going on location and in non-design events. The legs of a Jack Up Unit may extend over 500 ft above the surface of the water when the Unit is being towed with the legs fully retracted. Depending on size and length, the legs usually have the most detrimental impact on the afloat stability of the Unit. The heavy weight at a high center of gravity and the large wind area of the legs combine to dramatically affect the Unit’s afloat stability. For Units of the same hull configuration and draft, the Unit with the larger legs will have less afloat stability. When in the Elevated Mode, the legs of a Jack Up Unit are subjected to wind, wave, and current loadings. In addition to the specifics of the environment, the magnitude and proportion of these loads is a function of the water depth, air gap (distance from the water line to the hull baseline) and the distance the footings penetrate into the seabed.
  33. 33. Generally, the larger the legs and footings, the more load wind, wave, and current will exert on them. Legs of different design and size exhibit different levels of lateral stiffness (amount of load needed to produce a unit deflection). Jack Up stiffness decreases with increases in water depth (or more precisely, with the distance from the support footing to the hull/leg connection). Furthermore, for deeper water depths, flexural stiffness (chord area and spacing) overshadows the effects of shear stiffness (brace). Leg stiffness is directly related to Jack Up stiffness in the elevated mode, thereby affecting the amount of hull sway and the natural period of the Unit (which may result in a magnification of the oscillatory wave loads). EQUIPMENT The equipment required to satisfy the mission of the Jack Up Unit affects both the hull size and lightship weight of the Unit. There are three main groups of equipment on a Jack Up Unit, the Marine Equipment, Mission Equipment, and Elevating Equipment. “Marine Equipment” refers to the equipment and systems aboard a Jack Up Unit that are not related to the Mission Equipment. Marine Equipment could be found on any sea-going vessel, regardless of its form or function. Marine Equipment may include items such as main diesel engines, fuel oil piping, electrical power distribution switchboards, lifeboats, radar, communication equipment, galley equipment, etc. Marine Equipment, while not directly involved with the Mission of the Jack up Unit, is necessary for the support of the personnel and equipment necessary to carry out the Mission. All Marine Equipment is classified as part of the Jack Up Lightship Weight.
  34. 34. “ Mission Equipment” refers to the equipment and systems aboard a Jack Up Unit, which are necessary for the Jack Up to complete its Mission. Mission Equipment varies by the mission and by the Jack Up. Two Jack Up Units which are involved in Exploration Drilling may not have the same Mission Equipment. Examples of Mission Equipment may include derricks, mud pumps, mud piping, drilling control systems, production equipment, cranes, combustible gas detection and alarms systems, etc. Mission Equipment is not always classified as part of the Jack Up Lightship Weight. Some items, such as cement units, are typically classified as variable deck load as they may not always be located aboard the Jack Up. MODES OF OPERATION OF A JACK UP Jack Up Units operate in three main modes: transit from one location to another, elevated on its legs, and jacking up or down between afloat and elevated modes. Each of these modes has specific precautions and requirements to be followed to ensure smooth operations. A brief discussion of these modes of operations along with key issues associated with each follows. TRANSIT FROM ONE LOCATION TO ANOTHER The Transit Mode occurs when a Jack Up Unit is to be transported from one location to another. Transit can occur either afloat on the Jack Up Unit’s own hull (wet tow), or with the Jack Up Unit as cargo on the deck of another vessel (dry tow). .
  35. 35. Main preparations for each Transit Mode address support of the legs, support of the hull, watertight integrity of the unit, and stowage of cargo and equipment to prevent shifting due to motions. Though the Unit’s legs must be raised to ensure they clear the seabed during tow, it is not required that the legs be fully retracted. Allowing part of the legs to be lower than the hull baseline not only reduces jacking time, but it also reduces leg inertia loads due to tow motions and increases stability due to decreased wind overturning. Lowering the legs a small distance may also improve the hydrodynamic flow around the open leg wells and reduce tow resistance. Whatever the position of the legs during tow, their structure at the leg/hull interface must be checked to ensure the legs can withstand the gravity and inertial loads associated with the tow. Field Tow corresponds to the condition where a Jack Up Unit is afloat on its own hull with its legs raised, and is moved a relatively short distance to another location. For a short move, the ability to predict the condition of the weather and sea state is relatively good. Therefore, steps to prepare the Unit for Field Tow are not as stringent as for a longer tow. Most Classification Societies define a “Field Tow” as a Tow that does not take longer than 12 hours, and must satisfy certain requirements with regards to motion criteria. This motion criterion, expressed as a roll/pitch magnitude at a certain period, limits the inertial loads on the legs and leg support mechanism. For certain moves lasting more than 12 hours, a Unit may undertake an Extended Field Tow. An Extended Field Tow is defined as a Tow where the Unit is always within a 12-hour Tow of a safe haven, should weather deteriorate. In this condition, the Jack Up Unit is afloat on its own hull with its legs raised, similar to a Field Tow. The duration of an Extended Field Tow may be many days. The motion criteria for an Extended Field Tow is the same as for a Field Tow.
  36. 36. The main preparations for a Unit to undertake an Extended Field Tow are the same as those for a Field Tow with the additional criteria that the weather is to be carefully monitored throughout the duration of the tow. A Wet Ocean Tow is defined as an afloat move lasting more than 12-hours which does not satisfy the requirements of an Extended Field Tow. In this condition, the Jack Up Unit is afloat on its own hull with its legs raised as with a Field Tow, but, for many Units, additional precautions must be made. This is because the motion criteria for a Wet Ocean Tow are more stringent than for a Field Tow. The additional preparations may include installing additional leg supports, shortening the leg by cutting or lowering, and securing more equipment and cargo in and on the hull. A Dry Ocean Tow is defined as the transportation of a Jack Up Unit on the deck of another vessel. In this condition, the Jack Up Unit is not afloat, but is secured as deck cargo. The motion criteria for the Unit is dictated by the motions of the transportation vessel with the Unit on board. Therefore, the precautions to be taken with regard to support of the legs must be investigated on a case-by-case basis. Generally, though, the legs are to be retracted as far as possible into the hull so the Jack Up hull can be kept as low as practicable to the deck of the transport vessel and to reduce the amount of cribbing support. The other critical precaution unique to Dry Ocean Tow is the support of the Jack Up hull. The hull must be supported by cribbing on strong points (bulkheads) within the hull and in many cases, portions of the hull overhang the side of the transportation vessel. These overhanging sections may be exposed to wave impact, putting additional stress on the hull, and if the overhanging sections include the legs, the resultant bending moment applied to the hull (and amplified by vessel motions) can be significant. Calculations should be made to ensure that the hull will not lift off the cribbing with the expected tow motions.
  38. 38. ARRIVING ON LOCATION Upon completion of the Transit Mode, the Jack Up Unit is said to be in the Arriving On Location Mode. In this Mode, the Unit is secured from Transit Mode and begins preparations to Jack Up to the Elevated Mode. Preparations include removing any wedges in the leg guides, energizing the jacking system, and removing any leg securing mechanisms installed for the Transit thereby transferring the weight of the legs to the pinions. SOFT PINNING THE LEGS If an independent leg Jack Up Unit is going to be operated next to a Fixed Structure, or in a difficult area with bottom restrictions, the Jack Up Unit will often be temporarily positioned just away from its final working location. This is called “Soft Pinning” the legs or “Standing Off” location. This procedure involves lowering one or more legs until the bottom of the spud can(s) just touches the soil. The purpose of this is to provide a “Stop” point in the Arriving On Location process. Here, all preparations can be checked and made for the final approach to the working location. This includes coordinating with the assisting tugs, running anchor lines to be able to “winch in” to final location, powering up of positioning thrusters on the Unit (if fitted), checking the weather forecast for the period of preloading and jacking up, etc.
  39. 39. FINAL GOING ON LOCATION Whether a Unit stops at a Soft Pin location, or proceeds directly to the final jacking up location, they will have some means of positioning the Unit so that ballasting or preloading operations prior to jacking up can commence. For an independent leg Jack Up Unit, holding position is accomplished by going on location with all three legs lowered so the bottom of the spud can is just above the seabed. When the Unit is positioned at its final location, the legs are lowered until they can hold the rig on location without the assistance of tugs. Mat type Jack Up Units are either held on location by tugs, or they drop spud piles into the soil. These spud piles, usually cylindrical piles with concrete fill, hold the Unit on location until the mat can be ballasted and lowered. JACKING A mat Unit will jack the mat to the seabed in accordance with the ballasting procedure. Once the mat has been lowered to the seabed, the hull will be jacked out of the water. The Unit then proceeds to Preload Operations . All Independent leg Units must perform Preload Operations before they can jack to the design air gap. Most independent leg Units do not have the capacity to elevate the Unit while the preload weight is on board. For these Units, the next step is to jack the hull out of the water to a small air gap that just clears the wave crest height. This air gap should be no more than five (5) feet. Once they reach this position, the Unit may proceed with Preload Operations.
  40. 40. PRELOAD OPERATIONS All Jack Up Units must load the soil that supports them to the full load expected to be exerted on the soil during the most severe condition, usually Storm Survival Mode. This preloading reduces the likelihood of a foundation shift or failure during a Storm. The possibility does exist that a soil failure or leg shift may occur during Preload Operations. To alleviate the potentially catastrophic results of such an occurrence, the hull is kept as close to the waterline as possible, without incurring wave impact. Should a soil failure or leg shift occur, the leg that experiences the failure loses load-carrying capability and rapidly moves downward, bringing the hull into the water. Some of the load previously carried by the leg experiencing the failure is transferred to the other legs potentially overloading them. The leg experiencing the failure will continue to penetrate until either the soil is able to support the leg, or the hull enters the water to a point where the hull buoyancy will provide enough support to stop the penetration. As the hull becomes out-of-level, the legs will experience increased transverse load and bending moment transferred to the hull mostly by the guide. With the increased guide loads, some braces will experience large compressive loads. During normal preload operations it is important to keep the weight of the hull, deck load, and preload as close to the geometric center of the legs as possible, as this will assure equal loading on all legs. Sometimes, however, single-leg preloading is desired to increase the maximum footing reaction of any one leg. This is achieved by selective filling/emptying of preload tanks based on their relative position to the leg being preloaded. Preload is water taken from the sea and pumped into tanks within the hull. After the preload is pumped on board, it is held for a period of time.
  41. 41. The Preload Operation is not completed until no settling of the legs into the soil occurs during the holding period while achieving the target footing reaction. The amount of preload required depends on the required environmental reaction and the type of Jack Up Unit. Mat Units normally require little preload. JACKING TO FULL AIR GAP OPERATIONS Once Preload Operations are completed, the Unit may be jacked up to its operational air gap. During these operations it is important to monitor the level of the hull, elevating system load and characteristics, and for trussed-leg Units, Rack Phase Differential (RPD). All of these must be maintained within design limits. Once the Unit reaches its operational air gap, the jacking system is stopped, the brakes set, and leg locking systems engaged (if installed). The Unit is now ready to begin operations. ELEVATED OPERATING CONDITION When the Unit is performing operations, no particular differences exist between the various types of Units. Likewise, there are no particular cautionary measures to take other than to operate the Unit and its equipment within design limits. For Units with large cantilever reach and high cantilever loads, extra care must be taken to ensure that the maximum footing reaction does not exceed a specified percentage of the reaction achieved during preload.
  42. 42. ELEVATED STORM SURVIVAL CONDITION When the Unit is performing operations, the weather is to be monitored. If non-cyclonic storms which exceed design operating condition environment are predicted, Operations should be stopped and the Unit placed in Storm Survival mode. In this mode, Operations are stopped, equipment and stores secured, and the weather and watertight enclosures closed. If cyclonic storms are predicted, the same precautions are taken and personnel evacuated from the Unit. This is how a jack up rig is bought from the shore to the required location for drilling.
  43. 43. ELEVATING SYSTEM All Jack Ups have mechanisms for lifting and lowering the hull. The most basic type of elevating system is the pin and hole system, which allows for hull positioning only at discrete leg positions. However, the majority of Jack Ups in use today are equipped with a Rack and Pinion system for continuous jacking operations. There are two basic jacking systems: Floating and Fixed. The Floating system uses relatively soft pads to try to equalize chord loads, whereas the Fixed system allows for unequal chord loading while holding. There are two types of power sources for Fixed Jacking Systems, electric and hydraulic.Both systems have the ability to equalize chord loads within each leg. A hydraulic-powered jacking system achieves this by maintaining the same pressure to each elevating unit within a leg. Care must be taken, however, to ensure that losses due to piping lengths, bends, etc., are either equalized for all pinions or such differences are insignificant in magnitude. For an electric powered jacking system, the speed/load characteristics of the electric induction motors cause jacking motor speed changes resulting from pinion loads, such that if jacking for a sufficiently long time, the loads on any one leg tend to equalize for all chords of that leg.
  45. 45. UPPER AND LOWER GUIDES All Jack Ups have mechanisms to guide the legs through the hull. For Units with Pinions, the guides protect the pinions from “bottoming out” on the rack teeth. As such, all Units are fitted with a set of upper and lower guides. Some Jack Up Units, which have exceptionally deep hulls or tall towers of pinions, also have intermediate guides. These guides function only to maintain the rack the correct distance away from the pinions and are not involved in transferring leg bending moment to the hull. Guides usually push against the tip (vertical flat side) of the teeth, but this is not the only form of guides. There are also other forms of guides such as chord guides, etc. Depending on accessibility, some guides are designed to be replaced and are sometimes known as “wear plates.”In addition to protecting the pinions and hull, all upper and lower guides are capable of transferring leg bending moment to the hull to some degree determined by the design. The amount of moment transferred by the guides to the hull as a horizontal couple is dependant on the relative stiffness of the guides with respect to the stiffness of the pinions and/or fixation system (if any).
  46. 46. DRILLING RIG COMPONENTS 1. Crown Block and Water Table 2. Catline Boom and Hoist Line 3. Drilling Line 4. Monkeyboard 5. Traveling Block 6. Top Drive 7. Mast 8. Drill Pipe 9. Doghouse 10. Blowout Preventer 11. Water Tank 12. Electric Cable Tray 13. Engine Generator Sets 14. Fuel Tank 15. Electrical Control House 16. Mud Pumps 17. Bulk Mud Component Tanks 18. Mud Tanks (Pits) 19. Reserve Pit 20. Mud-Gas Separator 21. Shale Shakers 22. Choke Manifold 23. Pipe Ramp 24. Pipe Racks 25. Accumulator
  47. 47. Crown Block and Water Table An assembly of sheaves or pulleys mounted on beams at the top of the derrick. The drilling line is run over the sheaves down to the hoisting drum. Catline Boom and Hoist Line A structural framework erected near the top of the derrick for lifting material. Drilling Line A wire rope hoisting line, reeved on sheaves of the crown block and traveling block (in effect a block and tackle). Its primary purpose is to hoist or lower drill pipe or casing from or into a well. Also, a wire rope used to support the drilling tools
  48. 48. Monkeyboard The derrickman's working platform. Double board, tribble board, fourable board; a monkey board located at a height in the derrick or mast equal to two, three, or four lengths of pipe respectively. Traveling Block An arrangement of pulleys or sheaves through which drilling cable is reeved, which moves up or down in the derrick or mast. Top Drive The top drive rotates the drill string end bit without the use of a kelly and rotary table. The top drive is operated from a control console on the rig floor.
  49. 49. Mast A portable derrick capable of being erected as a unit, as Distinguished from a standard derrick, which cannot be raised to a working position as a unit. Drill Pipe The heavy seamless tubing used to rotate the bit and circulate the drilling fluid. Joints of pipe 30 feet long are coupled together with tool joints. Doghouse A small enclosure on the rig floor used as an office for the driller or as a storehouse for small objects. Also, any small building used as an office or for storage.
  50. 50. Blowout Preventer A large valve, usually installed above the ram preventers, that forms a seal in the annular space between the pipe and well bore or, if no pipe is present, on the well bore itself. Water Tank Is used to store water that is used for mud mixing, cementing, and rig cleaning. Electric Cable Tray Supports the heavy electrical cables that feed the power from the control panel to the rig Motors.
  51. 51. Engine Generator Sets A diesel, Liquefied Petroleum Gas (LPG), natural gas, or gasoline engine, along with a mechanical transmission and generator for producing power for the drilling rig. Newer rigs use electric generators to power electric motors on the other parts of the rig. Fuel Tanks Fuel storage tanks for the power generating system . Electric Control House On diesel electric rigs, powerful diesel engines drive large electric generators. The generators produce electricity that flows through cables to electric switches and control equipment enclosed in a control cabinet or panel. Electricity is fed to electric motors via the panel.
  52. 52. Mud Pits A series of open tanks, usually made of steel plates, through which the drilling mud is cycled to allow sand and sediments to settle out. Additives are mixed with the mud in the pit, and the fluid is temporarily stored there before being pumped back into the well. Bulk Mud Components in Storage Hopper type tanks for storage of drilling fluid components . Mud Pump A large reciprocating pump used to circulate the mud (drilling fluid) on a drilling rig.
  53. 53. Reserve Pits A mud pit in which a supply of drilling fluid has been stored. Also, a waste pit, usually an excavated, earthen - walled pit. It may be lined with plastic to prevent soil contamination. Mud-Gas Separator A device that removes gas from the mud coming out of a well when a kick is being circulated out. Shale Shaker A series of trays with sieves or screens that vibrate to remove cuttings from circulating fluid in rotary drilling operations. The size of the openings in the sieve is selected to match the size of the solids in the drilling fluid and the anticipated size of cuttings. Also called a shaker.
  54. 54. Choke Manifold The arrangement of piping and special valves, called chokes, through which drilling mud is circulated when the blowout preventers are closed to control the pressures encountered during a kick. Pipe Ramp An angled ramp for dragging drill pipe up to the drilling platform or bringing pipe down off the drill platform. The storage device for nitrogen pressurized hydraulic fluid, which is used in operating the blowout preventers. Accumulator
  55. 55. BASIC SYSTEMS IN JACK UP RIGS <ul><li>Drilling Systems </li></ul><ul><li>Drilling system is the heart of the jack up rig. Drilling is carried out at the drill floor which is at certain elevation from the main deck. There is a mouse hole and rat hole in which the drill stems are assembled. Once the stem is assembled it is placed concentric with the rotary table which is either driven from the top or from the bottom. Kelly bushing is provided to support the drill stem and prevent it from buckling. </li></ul><ul><li>Mud Systems </li></ul><ul><li>These are of two types – high pressure mud system and low pressure mud system. </li></ul><ul><li>Mud is a mixture of raw water, clay, bainite and some other minerals. Working of the jack up rig depends upon the power of the mud system. It is a cyclic procedure which is used to convey the crushes from the bottom of drill bits to the top of mud tanks. This is called high pressure mud system. when we punch through the reservoir a high pressure builds up due to the presence of gases, to prevent our system from blowing out high viscosity mud is used to lower the pressure. When the mud returns from the BOP and goes to shale shaker assembly, it is called low pressure mud system </li></ul>
  57. 57. <ul><li>Power Generation Systems </li></ul><ul><li>Power is must to run a jack up rig. Jack up rigs are installed in deep seas thereby we have no other provision of getting power. Hence the internal combustion engines are used to supply power. They are high capacity engines which are located in engine rooms at the main deck. Once the electricity suddenly shuts off then the crude oil in the annular should be at the same level of the mud which is present in the drill stem below the return line. </li></ul><ul><li>Cementing Systems </li></ul><ul><li>Cementing system is a provision which is used to cement the sides of the well forbidding the soft soil to enter the drill well. </li></ul><ul><li>In cement system there are four tanks which have an accumulator on the top which mixes the cement continuously with raw water. On both sides of the rig there are centrifugal pumps which sucks marine water from the sea and supply it to the tank which is used to dilute the cement. </li></ul><ul><li>Living quarters & Landing Systems </li></ul><ul><li>Living quarters are provided for the officials and a helideck is also commissioned for some external support needed for the persons working on the rig. Complete care of the people is taken to ensure a safe working environment. </li></ul>
  58. 58. Diesel Electric Generator BOP
  59. 59. <ul><li>BOP & Well Completions </li></ul><ul><li>BOP stands for blow out preventer. It is the most important part of the jack up rig. When the crude oil comes from the well it at such pressure which can send a rocket to space i.e about 10,000 psi so it can destroy the jack up rig in one blow ,hence to prevent the rigs we have BOP’s which cuts the line when a limiting pressure value is reached , hence saving the rig. </li></ul><ul><li>Well completion process, when the drilling and cementing is done a plug is placed at the cement sleeve which at some clearance from the sea bed. The Christmas tree is placed on this plug and the plunger on the bottom of Christmas tree is used for the production of oil. </li></ul><ul><li>Jacking and Hydraulic Systems </li></ul><ul><li>Care must be taken when positioning a new jack up rig at a site previously occupied by another jack up because of the tendency of the spud cans of the new rig to slip into the spud cans holes or “foot prints” left on the sea floor by the previous rig. If there is an overlap of a spud can over an old spud can hole, there is a tendency for the spud can not to penetrate straight into the soil, but instead to slip into the old spud can hole. This movement of a spud can, without a corresponding movement of all the other spud cans in the same direction, will impose a bending moment on the legs. This bending moment can be quite severe and may damage the leg in the preloading or jacking up process or it may reduce the allowable storm environmental load of the rig due to the resulting bend of the leg. When selecting a rig for a platform, it is always best to choose a rig with the same leg spacing as a rig that previously drilled at the platform. </li></ul>
  60. 60. However, the effect of previous spud can holes can be mitigated if the new rig is positioned so that the centers of its spud cans are positioned either at the center of the holes left by the previous rig or about 1.5 spud can diameters away for the edge of the holes left by the previous rig. If the rig selected for the platform does not have the same leg spacing as a rig previously at the platform and it is not possible to position the new rig so that its legs either are centered over old holes or 1.5 diameters away from old holes while still reaching all of the required drilling positions, there are two techniques which can be used to minimize the effects of old holes. These techniques are “Reaming” and “Swiss Cheesing”.“Reaming” is a technique by which the leg or legs are sequentially raising and lowering the spud can in the hole left by the previous rig in an attempt to wear away the side of the hole, thereby elongating the hole and creating a new hole center location at the spacing of the legs of the new rig. “Swiss Cheesing” is a method in which a number of large diameter holes (24 to 30 inch diameter) are drilled at the side of an existing can hole in order to degrade the strength of soil at the side of the can hole, effectively enlarging the hole.
  61. 61. SOME SENSIVITIES OF JACK UP RIGS LEG PUNCH THROUGHS When a Jack Up is being preloaded, it is important to be prepared to act in the event of rapid penetration of one or multiple legs. Because of the increased demands on Jack Ups (i.e., larger water depths and higher environmental loads) resulting in higher elevated weights during preload, the consequences of a punch through are increasingly more pronounced. A typical soil’s bearing capacity increases with depth. When a soil layer is underlain by a weaker soil layer, there is a rapid reduction of soil strength. When the spud can reaches this interface, the weaker soil gives way and the support of the leg moves downward at a faster rate than the jacking system is capable of lowering the leg to maintain the hull level. As such, the hull rotates, the legs tilt and bend, causing the hull to sway. This results in a weight shift relative to the supports, thereby increasing the required footing reaction needed to maintain equilibrium. This process continues until either the soil’s bearing capacity or any hull buoyancy arising from the hull entering the water increase sufficiently to reach equilibrium. Jack Ups of all design types experience punch throughs and their resulting damages to braces, chords and jacking units.
  62. 63. The accidental loading resulting from a punch through can lead to several types of leg damage including buckling of the braces, buckling or shearing of the chord, punching shear and joint damage. The extent of possible damage is dependent on the magnitude of the punch through and, more importantly, on the actions taken before, during, and immediately after the punch through. Punch through is an extreme event; therefore, proper management of this event is necessary. Modern rigs with a better guide design along with a proper punch through management system, can minimize some of the risks. OTHER SOIL ISSUES (SCOUR, EARTHQUAKE) There are soil issues other than poor bearing capacity to consider when reviewing a Jack Up’s suitability for a given location. This section presents just a few of the main issues. The first is the case where the soil is extremely hard or calcareous. In these cases, the penetration of the spud can will be minimal allowing only a portion of the spud can bottom plate to be in contact with the seabed. In this condition, only that part of the spud can structure in contact with the soil will be supporting the environmental loads, deadweight, and operational weight of the Jack Up. It is extremely important to verify that such partial bearing will not cause damage to the spud can structure. In cases like these, an adequately reinforced tip on the spud can may be advantageous compared to flat bottomed footings.
  63. 64. Scour is another problem that occurs in certain locations such as areas with sandy bottoms and high bottom current. In this case, the footing is originally supported over a certain portion of its bottom area during the initial preload operation. Over time, however, high currents may cause erosion under a portion of the footing. When this happens, the bearing pressure increases over the preload value due to loss of support area. Depending on the bearing capacity of the soil, additional penetration or spud can rotation may occur. Additionally, if the footing is not structurally adequate, structural damage may occur. Finally, if scour is severe and over a large enough area, the footing may slide into the depression created. Any of these scenarios can be extremely severe, especially since they occur with the hull at full air gap. GUIDE / RACK TEETH WEAR The legs are restrained in horizontal movement and in rotation by the leg guides. Leg guides may also maintain the allowable position of the elevating pinions with respect to the leg rack. Over time, it is normal to experience wear in both the guides and in the part of the leg that is in direct contact with the guides. This wear in both the leg guides and the leg should be monitored. When the leg guides are excessively worn, they should be replaced. If leg wear should become excessive, the leg should be repaired.