Final project

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Final project

  1. 1. PIPING BASICSPiping may be defined as a mode of transportation forthe liquids, gases, and fluidized solids from one place toanother. A pipe is basically a tubular structure which isspecified by its nominal bore diameter and its Schedulenumber which generally gives the standards of its wallthickness. The piping is laid and designed according tosome standards laid by some of the standardorganizations of the world. It forms the heart ofindustries such as power plants, petrochemical projectsetc.Hence it becomes very important for us to know thedesigning of such structures. The sector where piping isused as the backbone are :-(i) Oil & Gas Industry(ii) Refineries(iii) Petro Chemicals(iv) Chemical Plants(v) Power Plants(vi) Water Treatment Plants(vii) Pharmaceuticals & Food Industry(viii) Paper plants In other few pages we shall gain the complete knowledge on piping and its basics.
  2. 2. OFFSHORE FUNDAMENTALSINTRODUCTIONOffshore industry is basically concerned with theexploration, drilling and production of oil and gas from the sea bedin shallow as well as deep water. Oil and gas are derived almostentirely from decayed plants and bacteria. Energy from thesun, which fuelled the plant growth, has been recycled into usefulenergy in the form of hydrocarbon compounds - hydrogen andcarbon atoms linked together. Offshore and gas originates from two sources. Gas frombeneath the southern North Sea and the Irish Sea formed from coalswhich were derived from the lush, tropical rain forests that grew inthe Carboniferous Period, about 300 million years ago. Oil and mostgas under the central and northern North Sea and west of the Shetland Islands formed from the remains of planktonic algae andbacteria that flourished in tropical seas of the Jurassic andCretaceous Periods, about 140 to 130 million years ago (a significantamount of the Kimmeridge Clay Format ion is Cretaceous in age) .They accumulated in muds, which are now the prolific KimmeridgeClay source rock. Crude oil is a complex mixture of hydrocarbons withsmall amounts of other chemical compounds that containsulphur, nitrogen and oxygen. Traces of other elements, such assulphur and nitrogen, were also present in the decaying organicmaterial, giving rise to small quantities of other compounds incrude oil. Hydrocarbon molecules come in a variety of shapes andsizes, (straight -chain, branched chain or cyclic) , this is one of thethings that makes them so valuable because it allows them to beused in so many different ways.
  3. 3. Oil and gas form as the result of a precise sequence ofenvironmental condit ions:· The presence of organic material· Organic remains being t rapped and preserved in sediment· The material is buried deeply and then slowly "cooked" byincreased temperature and pressure.Offshore platforms are used for exploration of Oil and Gas from underSeabed and processing. The First Offshore platform was installed in1947 off the coast of Louisiana in 6M depth of water. Today there areover 7,000 Offshore platforms around the world in water depths up to1,850MPlatform size depends on facilities to be installed on top side eg. Oilrig, living quarters, Helipad etc.Classification of water depths: < 350 M- Shallow water < 1500 M - Deep water > 1500 M- Ultra deep water US Mineral Management Service (MMS) classifies water depths greater than 1,300 ft as deepwater, and greater than 5,000 ft as ultra-deepwater.
  4. 4. OFFSHORE PROCESSEXPLORATIONDISCOVERING THE UNDERGROUND STRUCTURELarge-scale geological structures that might hold oil or gasreservoirs are invariably located beneath non-productiverocks, and in addition this is often below the sea. Geophysicalmethods can penetrate them to produce a picture of the pat ternof the hidden rocks. Relatively inexpensive gravity andgeomagnetic surveys can identify potentially oil-bearingsedimentary basins, but costly seismic surveys are essential todiscover oil and gas bearing structures. Sedimentary rocks aregenerally of low density and poorly magnetic, and are oftenunderlain by strongly magnetic, dense basement rocks. Bymeasuring ’anomalies’ or variations from the regionalaverage, a three-dimensional picture can be calculated. Moderngravity surveys show a generalized picture of the sedimentarybasins. Recently, high resolution aero-magnetic surveys flownby specially equipped aircraft at 70 - 100m altitude show fault traces and near surface volcanic rocks.Shooting seismic surveysMore detailed in format ion about the rock layers within suchan area can be obtained by deep echo sounding, or seismicreflect ion surveys. In offshore areas these surveys areundertaken by a ship (F52) towing both a submerged air orwater gun array, to produce short bursts of sound energy, and aset of streamers of several kilometers length. Each streamercontains a dense array of hydrophone groups that collect andpass to recorders echoes of sound from reflecting layers.
  5. 5. The depths of the reflecting layers are calculated from the time takenfor the sound to reach the hydrophones via the reflector; this isknown as the two-way t ravel time (F50a & b) . The pulse of soundfrom the guns radiates out as a hemispherical wave front , a port ionof which will be reflected back towards the hydrophones from rockinterfaces (F50a) . The path of the minute port ion of the reflectedwave- front intercepted by a hydrophone group is called a ray path.Hydrophone groups spaced along the streamer pick out ray pathsthat can be related to specific points on the reflector surface (F50c) .Graphs of the intensity of the recorded sound plot ted against thetwo-way time are displayed as wiggle t races (F50b) . Seismicrecording at sea always uses the common depth point (CDP) method(F50c & d) . A sequence of regularly spaced seismic shot s is made asthe survey vessel accurately navigates its course.ProcessingProcessing recordings involves many stages of signal processing andcomputer summing. Firstly, wiggle t races from a single CDP arecollected into groups. Displayed side by side in sequence they form aCDP gather (F51a & b) . Reflections from any one reflector form ahyperbolic curve on the gather because the sound takes longer to travel to the more distant hydrophones. This effect is called normalmove out (NMO) . Correct ion is needed to bring the pulses to ahorizontal alignment , as if they all came from vertically below thesound source (F51c) . The separate wiggle t races are addedtogether, or stacked (F51d) . Stacking causes t rue reflect ion pulsesto enhance one another, and hopefully, random noise will cancel out. This process is repeated for all the CDPs on the survey line. Thestacked and corrected wiggle t races are displayed side by side togive a seismic sect ion (F51e)
  6. 6. SEISMIC SURVEYS INTERPRETATION
  7. 7. InterpretationSeismic sect ions provide 2-dimensional views of undergroundstructure. By using special shooting techniques such as spaced air gunarrays or towing the streamer slantwise, or by shooting very closelyspaced lines, it is possible to produce 3-dimensional (3D) seismicimages (F59) . These images comprise vertical sect ions and horizontalsect ions ( ’time-slices’) .DRILLINGDEALING WITH THE UNDERGROUND STRUCTUREThere are two basic types of drilling rigs - fixed plat form rigs andmobile rigs. Fixed platform rigs are installed on large offshore platforms and remain in place for many years. Most of the large fields inthe North Sea such as Forties and Brent were developed using fixedplat form rigs. Drilling fluid (also called "mud") , which is mainlywater-based, is pumped continuously down the drill string whiledrilling. It lubricates the drilling tools, washes up rock cuttings andmost importantly, balances the pressure of fluids in the rock formations below to prevent blowouts.In offshore drilling, the first step is to put down a wide-diameterconductor pipe into the seabed to guide the drilling and contain thedrilling fluid. It is drilled into the seabed from semi-submersiblerigs, but on product ion plat forms a pile-driver may be used. Asdrilling continues, completed sections of the well are cased with steelpipecemented into place. A blowout preventer is attached to the top of thecasing. This is a stack of hydraulic rams which can close off the wellinstantly if back pressure (a kick) develops from invading oil, gas orwater.Drilling grinds up the rock into tea- leaf-sized cuttings which arebrought to the surface by the drilling mud. The drilling mud is passedover a shale shaker which sieves out the cuttings .
  8. 8. DRILLING
  9. 9. In exploration drilling, the cuttings are taken for examination by ageologist known as a mud logger who is constantly on the lookoutfor oil and gas.PRODUCTIONTHE OFFSHORE CHALLENGEProduction facilities had to be designed to withstand wind gusts of180 km/ hour and waves 30 met res high. Other problems includedthe ever-present salt -water corrosion and fouling by marineorganisms. Dealing with the many underwaterconstruction and maintenance tasks falls to divers and remotelyoperated vehicles. Giant floating cranes (F83) designed to lift evergreater loads were commissioned and many other specialized crafthad to be developed to establish and service the offshore industry.Huge helicopter fleet s were needed to ferry workers to and fromthe plat forms and rigs.Product ion Plat formsMost oil and gas product ion plat forms in offshore Britain rest onsteel supports known as ’jackets’, a term derived from the Gulf ofMexico. A small number of plat forms are fabricated from concrete.The steel jacket , fabricated from welded pipe, is pinned to the seafloor with steel piles. Above it are prefabricated units or modulesproviding accommodation and housing various facilities includinggas turbine generating sets. Towering above the modules are thedrilling rig derrick ( two on some plat forms) , the flare stack insome designs (also frequently cantilevered outwards) and servicecranes. Horizontal surfaces are taken up by store areas, drillingpipe deck and the vital helicopter pad. Concrete gravity plat formsare so- called because their great weight holds them firmly on theseabed. They were first developed to provide storage capacity inoilfields where tankers were used to transport oil, and to eliminatethe need for piling in hard sea beds.
  10. 10. The Brent D plat form (F87) , which weighs more than 200 000tonnes, was designed to store over a million barrels of oil. Butsteel plat forms, in which there have been design advances, arenow favored over concrete ones. Several plat forms may have tobe installed to exploit the larger fields, but where the capacityof an existing plat form permits, subsea collecting systemslinked to it by pipelines have been developed using the mostmodern technology. They will be increasingly used as smallerfields are developed. For very deep waters, one solution wasthe Hut ton Tension Leg Plat form: the buoyant platform, resembling a huge drilling rig, is tethered to the sea-bedby jointed legs kept in tension by computer- cont rolled ballastadjustments. Alternatively, a subsea collect ion system may belinked via a product ion riser to a Floating, Production, Storageand Offloading (FSPO) vessel (F88) ; either a purpose built shipor a converted tanker or semisubmersible rig. The oil isoffloaded by a shut t le tanker.Product ion WellsTo develop offshore fields as economically aspossible, numerous directional wells radiate out from a singleplat form to drain a large area of reservoir (F94) . For directionaldrilling 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 thevertical and reach out horizontally more than twice theirvertical depth are known as extended reach wells. More thanone horizontal sect ion can be drilled in one well as amultilateral well (F96) . This technique is used to reduce drillingcosts and to maximize the number of wells that can be drilledfrom small plat forms.
  11. 11. Platform Semi- T.L.P.Land Jack- Submersible DrillRig up Ship PRODUCTION
  12. 12. GETTING OIL AND GAS ASHOREMost offshore oil and all offshore gas are brought to shore bypipelines which operate in all weathers. Pipeline routes areplanned to be as short as possible. Slopes that could put stresson unsupported pipe are avoided and seabed sediments aremapped to identify unstable areas and to see if it will bepossible to bury the pipe. Pipeline construction beginsonshore, as lengths of pipe are waterproofed with bitumen andcoated with steel- reinforced concrete. This coating weighsdown the submarine pipeline even when it is filled with gas.The prepared pipe- lengths are welded together offshore on alay barge (F101) . As the barge winches forward on its anchorlines, the pipeline drops gently to the seabed, guided by a’stinger’. The inside of pipelines need to be cleaned regularlyto remove wax deposits and water: to do this a collectingdevice known as a pig is forced through the pipe. Wheretankers transport oil from small or isolated fields, various oilstorage systems may be used. These may range fromcylindrical cells contained in some of the massive concretestructures, to seabed storage units such as that employed at theKittiwake field, or integral storage such as that contained inthe various Floating, Product ion, Storage and Offloadingvessels. In essence these FPSOs are floating storage tankers, aswell as product ion and processing installations. FPSOsprovide an important option for developing fields which maybe remote from existing infrastructure or where thefield recoverable reserves are uncertain, for example becauseof difficult geological conditions.
  13. 13. OIL PLATFORMSAn oil platform or oil rig is a large structure used to houseworkers and machinery needed to drill and/or extract oil andnatural gas through wells in the ocean bed. Depending on thecircumstances, the platform may be attached to the oceanfloor, consist of an artificial island, or be floating. Generally,oil platforms are located on the continental shelf, though astechnology improves and crude oil prices increase, drillingand production in deeper waters becomes both feasible andprofitable. A typical platform may have around thirtywellheads located on the platform and directional drillingallows reservoirs to be accessed at both different depths andat remote positions up to 5 miles (8 kilometres) from theplatform. Many platforms also have remote wellheadsattached by umbilical connections, these may be single wellsor a manifold centre for multiple wells.Offshore platforms can broadly be categorizedinto two parts :-Structures that extend to the sea bed Jacketed or Fixed Steel platform Concrete Gravity Structures Compliant TowerStructures that float near the water surface- RecentDevelopment Tension Leg Platforms SPAR Ship shaped Vessels (FPSO)
  14. 14. STRUCTURES THAT EXTEND TO THE SEA BEDFIXED STEEL PLATFORMSA Fixed Platform is a type of offshore platform used for theproduction of oil or gas. These platforms are built on concreteand/or steel legs anchored directly onto the seabed, supportinga deck with space for drilling rigs, production facilities and crewquarters. Such platforms are, by virtue of theirimmobility, designed for very long term use (for instance theHibernia platform). Various types of structure are used, steeljacket, concrete caisson, floating steel and even floating concrete.Steel jackets are vertical sections made of tubular steelmembers, and are usually piled into the seabed. Concretecaisson structures, pioneered by the Condeep concept, oftenhave in-built oil storage in tanks below the sea surface and thesetanks were often used as a flotation capability, allowing them tobe built close to shore (Norwegian Fjords and Scottish Firths arepopular because they are sheltered and deep enough) and thenfloated to their final position where they are sunk to the seabed.Fixed platforms are economically feasible for installation inwater depths up to about 1,700 feet (520 m). Space framedstructure with tubular members supported on piledfoundations. Used for moderate water depths up to 400 M.Jackets provides protective layer around the pipes. Typicaloffshore structure will have a deck structure containing a MainDeck, a Cellar Deck, and a Helideck. The deck structure issupported by deck legs connected to the top of the piles. Thepiles extend from above the Mean Low Water through theseabed and into the soil. Underwater, the piles are containedinside the legs of a “jacket” structure which serves as bracing forthe piles against lateral loads.
  15. 15. FIXED STEEL PLATFORMS
  16. 16. The jacket also serves as a template for the initial driving ofthe piles. (The piles are driven through the inside of thelegs of the jacket structure).Natural period (usually 2.5second) is kept below wave period (14 to 20 seconds) toavoid amplification of wave loads. 95% of offshoreplatforms around the world are Jacket supported.CONCRETE GRAVITY BASE STRUCTURESWhilst the vast majority of fixed offshore platforms employa tubular jacket to support the topside facilities, a numberof installations have been constructed using a basemanufactured from reinforced concrete. They are Fixed-bottom structures made from concrete . They are heavy andremain in place on the seabed without the need for piles.They are widely used for moderate water depths up to 300M. Part construction is made in a dry dock adjacent to thesea. The structure is built from bottom up, like onshorestructure. At a certain point , dock is flooded and thepartially built structure floats. It is towed to deepersheltered water where remaining construction iscompleted. After towing to field, base is filled wi1thwater to sink it on the seabed. Its main advantage is its lessmaintenance. The first concrete structure to be installed inthe North Sea was constructed by the Norwegians in 1973and used to develop the Ekofisk field. Since then thosepeople have installed a steady string of concrete structuresand it came as no surprise when their government electedto develop the other fields with the same concrete structurewhich stands in 350ft. of water and is currently the largestoffshore structure in Europe and the largest concreteplatform in the world.
  17. 17. CONCRETE GRAVITY BASE STRUCTURECOMPLIANT TOWER
  18. 18. COMPLIANT TOWERA compliant tower (CT) is a fixed rig structure normally usedfor the offshore production of oil or gas. The rig consist ofnarrow, flexible (compliant) towers and a piled foundationsupporting a conventional deck for drilling and productionoperations. Compliant towers are designed to sustainsignificant lateral deflections and forces, and are typically usedin water depths ranging from 1,500 and 3,000 feet (450 and 900m). With the use of flex elements such as flex legs or axialtubes, resonance is reduced and wave forces are de-amplified.This type of rig structure can be configured to adapt to existingfabrication and installation equipment. Compared withfloating systems, such as Tension leg platforms and SPARs, theproduction risers are conventional and are subjected to lessstructural demands and flexing. This flexibility allows it tooperate in much deeper water, as it can absorb much of thepressure exerted on it by the wind and sea. Despite itsflexibility, the compliant tower system is strong enough towithstand hurricane conditions. The first tower emerged in theearly 1980s with the installation of Exxons Lena oil platform.Narrow, flexible framed structures supported by piledfoundations. It has no oil storage capacity. Production isthrough tensioned rigid risers and export by flexible orcatenary steel pipe. It undergos large lateral deflections (up to10 ft) under wave loading. Used for moderate water depths upto 600 M. Natural period (usually 30 second) is kept abovewave period (14 to 20 seconds) to avoid amplification of waveloads.
  19. 19. STRUCTURES THAT FLOAT NEAR THEWATER SURFACETENSION LEG PLATFORMSA Tension-leg platform or Extended Tension Leg Platform(ETLP) is a vertically moored floating structure normally usedfor the offshore production of oil or gas and is particularlysuited for water depths greater than 300 meters (about 1000 ft).Also proposed for wind turbines. The platform is permanentlymoored by means of tethers or tendons grouped at each of thestructures corners. A group of tethers is called a tension leg. Afeature of the design of the tethers is that they have relativelyhigh axial stiffness(low elasticity), such that virtually all verticalmotion of the platform is eliminated. This allows the platformto have the production wellheads on deck (connected directlyto the subsea wells by rigid risers), instead of on the seafloor .This makes for a cheaper well completion and gives bettercontrol over the production from the oil or gas reservoir. Thefirst Tension Leg Platform was built for Conocos Hutton fieldin the North Sea in the early 1980s. The hull was built in thedry-dock at Highland Fabricators Nigg yard in the north ofScotland, with the deck section built nearby at McDermottsyard at Ardersier. The two parts were mated in the Moray Firthin 1984. Tension Leg Platforms (TLPs) are floating facilities thatare tied down to the seabed by vertical steel tubes calledtethers. This characteristic makes the structure very rigid in thevertical direction and very flexible in the horizontal plane. Thevertical rigidity helps to tie in wells for production, while, thehorizontal compliance makes the platform insensitive to theprimary effect of waves. It has large columns and Pontoons anda 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-designstage. It is used for deep water up to 1200 M. It has no integralstorage. It is sensitive to topside load/draught variations astether tensions are affected.SPARA SPAR, named for logs used as buoys in shipping and mooredin place vertically, is a type of floating oil platform typicallyused in very deep waters. Spar production platforms have beendeveloped as an alternative to conventional platforms. A Sparplatform consists of a large-diameter, single vertical cylindersupporting a deck. It contains a deep-draft floatingcaisson, which is a hollow cylindrical structure similar to a verylarge buoy. Its four major systems are hull, moorings, topsidesand risers. About 90% of the structure is underwater. The spardesign is now being used for drilling, production, or both. Thedistinguishing feature of a spar is its deep-draft hull, whichproduces very favorable motion characteristics compared toother floating concepts. Water depth capability has been statedby industry as ranging up to 10,000 ft. The first Spar platform inthe was installed in September of 1996. It follows the concept ofa large diameter single vertical cylinder supporting deck. Theseare a very new and emerging concept: the first sparplatform, Neptune, was installed off the USA coast in 1997.Sparplatforms have taut catenary moorings and deep draught, henceheave natural period is about 30 seconds.
  21. 21. TENSION LEG PLATFORM SPAR
  22. 22. FPSO {Floating Production Storage and Offloading}A Floating Production, Storage and Offloading vessel(FPSO; also called a "unit" and a "system") is a type offloating tank system used by the offshore oil and gasindustry and designed to take all of the oil or gas producedfrom a nearby platform (s), process it, and store it until theoil or gas can be offloaded onto waiting tankers or sentthrough a pipeline.HistoryOil has been produced from offshore locations since the1950s. Originally, all oil platforms sat on the seabed, but asexploration moved to deeper waters and more distantlocations in the 1970s, floating production systems came tobe used. The first oil FPSO was the Shell Castellon, built inSpain in 1977. The first ever conversion of a LNG carrier(Golar LNG owned Moss type LNG carrier) into an LNGfloating storage and regasification unit was carried out in2007 by Keppel shipyard in Singapore. The last few yearsconcepts for LNG FPSOs has also been launched. An LNGFPSO 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 principlesOil produced from offshore production platforms can betransported to the mainland either by pipeline or by tanker.When a tanker solution is chosen, it is necessary toaccumulate oil in some form of tank such that an oil tanker isnot continuously occupied while sufficient oil is produced tofill the tanker.Often the solution is a decommissioned oil tanker which hasbeen stripped down and equipped with facilities to beconnected to a mooring buoy. Oil is accumulated in the FPSOuntil there is sufficient amount to fill a transport tanker, atwhich point the transport tanker connects to the stern of thefloating storage unit and offloads the oil. An FPSO has thecapability to carry out some form of oil separation processobviating the need for such facilities to be located on an oilplatform. Partial separation may still be done on the oilplatform to increase the oil capacity of the pipeline(s) to theFPSO.AdvantagesFloating Production, Storage and Offloading vessels areparticularly effective in remote or deepwater locations whereseabed pipelines are not cost effective. FPSOs eliminate theneed to lay expensive long-distance pipelines from the oilwell to an onshore terminal. They can also be usedeconomically in smaller oil fields which can be exhausted in afew years and do not justify the expense of installing a fixedoil platform. Once the field is depleted, the FPSO can bemoved to a new location.
  24. 24. FPSO
  25. 25. Specific typesA Floating Storage and Offloading unit (FSO) is a floatingstorage device, which is simplified FPSO without thepossibility for oil or gas processing. Most FSOs are old singlehull supertankers that have been converted. An example ofthis is the Knock Navis, the worlds largest ship, which hasbeen converted to an FSO to be used offshore Qatar.A LNG floating storage and regasification unit (FSRU) is afloating storage and regasification system, which receivesliquefied natural gas(LNG) from offloading LNG carriers, andthe onboard regasification system provides natural gas send-out through flexible risers and pipeline to shore.
  26. 26. MODU’s { Mobile Offshore Drilling Units }The basic work of the mobile units is to drill the well inthe sea bed and prepare the line for production. Offshoredrilling is divided into two parts i.e shallow water anddeep water. In shallow water, jack-up rigs, standing withtheir feet on the seabed are used to drill the oil wells. Indeeper water, floating drilling units are used. There aretwo basic types; Drill ships and Semi-submersibledrilling rigs. This is a very important process and is veryhard in nature as the environmental conditions arealways unfavorable for such a process to accomplish.There are basically three type of drilling units that arewidely used over the world. They are :-Semi-submersible drill rigsSelf elevated Jack up rigsDrill ships
  27. 27. SEMI SUBMERSIBLE DRILLING RIGA semi-submersible drilling rig is one in which:Sea water is pumped into the hull of the vesselcausing the vessel to submerge to the desireddepth.The submerged vessel maintains its positionover the well location by means of anchor chains.A semi-submersible is not bottom-founded andcan work in much greater water depths than ajack-up.The maximum water depth is a function of thelength of the rigs riser, a bundle of utility tubesthrough which drilling fluids and other material isconducted, enclosed in an outer tube, suspendedto the seafloor.DRILL SHIPSDrill ships, a maritime vessel that has been fittedwith drilling apparatus. It is most often used forexploratory drilling of new oil or gas wells in deepwater but can also be used for scientific drilling. Itis often built on a modified tanker hull andoutfitted with a dynamic positioning system tomaintain its position over the well.
  28. 28. SEMI SUBMERSIBLE DRILL RIGS DRILL SHIPS
  29. 29. SELF ELEVATED JACK UP RIGS
  30. 30. ELEVATING JACK UP RIGSINTRODUCTIONA Jack Up is an offshore structure composed of a hull, legsand a lifting system that allows it to be towed to a site, lowerits legs into the seabed and elevate its hull to provide astable work deck capable of withstanding the environmentalloads. A typical modern drilling Jack Up is capable of workingin harsh environments (Wave Heights up to 80 ft, WindSpeeds in excess of 100 knots) in water depths up to 500feet. Because Jack Ups are supported by the seabed, theyare preloaded when they first arrive at a site to simulate themaximum expected leg loads and ensure that, after they areJacked to full air gap and experience operating andenvironmental loads, the supporting soil will provide areliable foundation. Jack Up Units have been a part of theOffshore Oil Industry exploration fleet since the 1950’s. Theyhave been used for exploration drilling, tender assisteddrilling, production, accommodation, and work/maintenanceplatforms. As with every innovative technology, Jack UpUnits have been used to their operational and designlimitations. These limitations include deck load carryinglimits when afloat, load carrying capabilities whenelevated, environmental limits, drilling limits, and soil(foundation) limits.
  31. 31. The reasons for pushing these limits include the desire to exploredeeper waters, deeper reservoirs in harsher environments, and inareas where soils and foundationsmay 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 thecomponent are described below :-HULLThe Hull of a Jack Up Unit is a watertight structure that supportsor houses the equipment, systems, and personnel, thus enablingthe Jack Up Unit to perform its tasks. When the Jack Up Unit isafloat, the hull provides buoyancy and supports the weight of thelegs and footings (spud cans), equipment, and variable load.Different parameters of the hull affect different modes ofoperation of the Unit. In general, the larger the length and breadthof the hull, the more variable deck load and equipment the Unitwill be able to carry, especially in the Afloat mode (due toincreased deck space and increased buoyancy).Also, larger hulls generally result in roomier machinery spacesand more clear space on the main deck to store pipe, 3rdParty Equipment, and provide for clear work areas. The largerhull may have larger preload capacity that may permit increasedflexibility in preloading operations. Larger hulls generally havethe negative effects of attracting higher wind, wave and currentloads. Since Jack Ups with larger hulls weigh more, they willrequire more elevating jacks of larger capacity to elevate and holdthe Unit.
  32. 32. The large weight also affects the natural period of the Jack UpUnit in the elevated mode. The draft of the hull, or the distancefrom the afloat waterline to the baseline of the hull, has a directeffect on the amount of variable deck load that can be carried andthe stability when afloat. The draft of the hull has an opposingrelationship with the hull’s freeboard, or the distance from theafloat waterline to the main deck of the hull. Every incrementalincrease in the draft of a Jack Up decreases the freeboard by thesame increment.LEGS AND FOOTINGSThe legs and footings of a Jack Up are steel structures thatsupport the hull when the Unit is in the Elevated mode andprovide stability to resist lateral loads. Footings are needed toincrease the soil bearing area thereby reducing required soilstrength. The legs and footings have certain characteristics whichaffect how the Unit reacts in the Elevated and AfloatModes, while going on location and in non-design events. Thelegs of a Jack Up Unit may extend over 500 ft above the surface ofthe water when the Unit is being towed with the legs fullyretracted. Depending on size and length, the legs usually havethe most detrimental impact on the afloat stability of the Unit.The heavy weight at a high center of gravity and the large windarea of the legs combine to dramatically affect the Unit’s afloatstability. For Units of the same hull configuration and draft, theUnit with the larger legs will have less afloat stability. When inthe Elevated Mode, the legs of a Jack Up Unit are subjected towind, wave, and current loadings. In addition to the specifics ofthe environment, the magnitude and proportion of these loads isa function of the water depth, air gap (distance from the waterline to the hull baseline) and the distance the footings penetrateinto the seabed.
  33. 33. Generally, the larger the legs and footings, the more loadwind, wave, and current will exert on them. Legs of differentdesign and size exhibit different levels of lateral stiffness(amount of load needed to produce a unit deflection). Jack Upstiffness decreases with increases in water depth (or moreprecisely, with the distance from the support footing to thehull/leg connection). Furthermore, for deeper waterdepths, flexural stiffness (chord area and spacing) overshadowsthe effects of shear stiffness (brace). Leg stiffness is directlyrelated to Jack Up stiffness in the elevated mode, therebyaffecting the amount of hull sway and the natural period of theUnit (which may result in a magnification of theoscillatory wave loads).EQUIPMENTThe equipment required to satisfy the mission of the Jack UpUnit affects both the hull size and lightship weight of the Unit.There are three main groups of equipment on a Jack UpUnit, the Marine Equipment, Mission Equipment, and ElevatingEquipment. “Marine Equipment” refers to the equipment andsystems aboard a Jack Up Unit that are not related to theMission Equipment. Marine Equipment could be found on anysea-going vessel, regardless of its form or function. MarineEquipment may include items such as main diesel engines, fueloil piping, electrical power distribution switchboards,lifeboats, radar, communication equipment, galleyequipment, etc. Marine Equipment, while not directly involvedwith the Mission of the Jack up Unit, is necessary for the supportof the personnel and equipment necessary to carry out theMission. All Marine Equipment is classified as part of the JackUp Lightship Weight.
  34. 34. “Mission Equipment” refers to the equipment and systemsaboard a Jack Up Unit, which are necessary for the Jack Up tocomplete its Mission. Mission Equipment varies by the missionand by the Jack Up. Two Jack Up Units which are involved inExploration Drilling may not have the same MissionEquipment. Examples of Mission Equipment may includederricks, mud pumps, mud piping, drilling controlsystems, production equipment, cranes, combustible gasdetection and alarms systems, etc. Mission Equipment is notalways classified as part of the Jack Up Lightship Weight.Some items, such as cement units, are typically classified asvariable deck load as they may not always be located aboardthe Jack Up.MODES OF OPERATION OF A JACK UPJack Up Units operate in three main modes: transit from onelocation to another, elevated on its legs, and jacking up ordown between afloat and elevated modes. Each of these modeshas specific precautions and requirements to be followed toensure smooth operations. A brief discussion of these modes ofoperations along with key issues associated with each follows.TRANSIT FROM ONE LOCATION TO ANOTHERThe Transit Mode occurs when a Jack Up Unit is to betransported from one location to another. Transit can occureither afloat on the Jack Up Unit’s own hull (wet tow), or withthe Jack Up Unit as cargo on the deck of another vessel (drytow)..
  35. 35. Main preparations for each Transit Mode address support of thelegs, support of the hull, watertight integrity of the unit, andstowage of cargo and equipment to prevent shifting due tomotions. Though the Unit’s legs must be raised to ensure theyclear the seabed during tow, it is not required that the legs be fullyretracted. Allowing part of the legs to be lower than the hullbaseline not only reduces jacking time, but it also reduces leginertia loads due to tow motions and increases stability due todecreased wind overturning. Lowering the legs a small distancemay also improve the hydrodynamic flow around the open legwells and reduce tow resistance. Whatever the position of the legsduring tow, their structure at the leg/hull interface must bechecked to ensure the legs can withstand the gravity and inertialloads associated with the tow. Field Tow corresponds to thecondition where a Jack Up Unit is afloat on its own hull with itslegs raised, and is moved a relatively short distance to anotherlocation. For a short move, the ability to predict the condition ofthe weather and sea state is relatively good. Therefore, steps toprepare the Unit for Field Tow are not as stringent as for a longertow. Most Classification Societies define a “Field Tow” as a Towthat does not take longer than 12 hours, and must satisfy certainrequirements with regards to motion criteria. This motioncriterion, expressed as a roll/pitch magnitude at a certainperiod, limits the inertial loads on the legs and leg supportmechanism. For certain moves lasting more than 12 hours, a Unitmay undertake an Extended Field Tow. An Extended Field Tow isdefined as a Tow where the Unit is always within a 12-hour Towof a safe haven, should weather deteriorate. In this condition, theJack Up Unit is afloat on its own hull with its legs raised, similarto a Field Tow. The duration of an Extended Field Tow may bemany days. The motion criteria for an Extended Field Tow is thesame as for a Field Tow.
  36. 36. The main preparations for a Unit to undertake an ExtendedField Tow are the same as those for a Field Tow with theadditional criteria that the weather is to be carefully monitoredthroughout the duration of the tow. A Wet Ocean Tow isdefined as an afloat move lasting more than 12-hours whichdoes not satisfy the requirements of an Extended Field Tow. Inthis condition, the Jack Up Unit is afloat on its own hull with itslegs raised as with a Field Tow, but, for many Units, additionalprecautions must be made. This is because the motion criteriafor a Wet Ocean Tow are more stringent than for a Field Tow.The additional preparations may include installing additionalleg supports, shortening the leg by cutting or lowering, andsecuring more equipment and cargo in and on the hull. A DryOcean Tow is defined as the transportation of a Jack Up Unit onthe deck of another vessel. In this condition, the Jack Up Unit isnot afloat, but is secured as deck cargo. The motion criteria forthe Unit is dictated by the motions of the transportation vesselwith the Unit on board. Therefore, the precautions to be takenwith regard to support of the legs must be investigated on acase-by-case basis. Generally, though, the legs are to beretracted as far as possible into the hull so the Jack Up hull canbe kept as low as practicable to the deck of the transport vesseland to reduce the amount of cribbing support. The other criticalprecaution unique to Dry Ocean Tow is the support of the JackUp hull. The hull must be supported by cribbing on strongpoints (bulkheads) within the hull and in many cases, portionsof the hull overhang the side of the transportation vessel. Theseoverhanging sections may be exposed to wave impact, puttingadditional stress on the hull, and if the overhanging sectionsinclude the legs, the resultant bending moment applied to thehull (and amplified by vessel motions) can be significant.Calculations should be made to ensure that the hull will not liftoff the cribbing with the expected tow motions.
  37. 37. DIGRAMETIC VIEW OF THE OPERATION OF JACK UP RIG
  38. 38. ARRIVING ON LOCATIONUpon completion of the Transit Mode, the Jack Up Unit is saidto be in the Arriving On Location Mode. In this Mode, theUnit is secured from Transit Mode and begins preparations toJack Up to the Elevated Mode. Preparations includeremoving any wedges in the leg guides, energizing the jackingsystem, and removing any leg securing mechanisms installedfor the Transit thereby transferring the weight of the legs to thepinions.SOFT PINNING THE LEGSIf an independent leg Jack Up Unit is going to be operatednext to a Fixed Structure, or in a difficult area with bottomrestrictions, the Jack Up Unit will often be temporarilypositioned just away from its final working location. This iscalled “Soft Pinning” the legs or “Standing Off” location. Thisprocedure involves lowering one or more legs until the bottomof the spud can(s) just touches the soil. The purpose of this is toprovide a “Stop” point in the Arriving On Location process.Here, all preparations can be checked and made for the finalapproach to the working location. This includes coordinatingwith the assisting tugs, running anchor lines to be able to “winchin” to final location, powering up of positioning thrusters on theUnit (if fitted), checking the weather forecast for the period ofpreloading and jacking up, etc.
  39. 39. FINAL GOING ON LOCATIONWhether a Unit stops at a Soft Pin location, or proceeds directly tothe final jacking up location, they will have some means ofpositioning the Unit so that ballasting or preloading operationsprior to jacking up can commence. For an independent leg JackUp Unit, holding position is accomplished by going on locationwith all three legs lowered so the bottom of the spud can is justabove the seabed. When the Unit is positioned at its finallocation, the legs are lowered until they can holdthe rig on location without the assistance of tugs. Mat type JackUp Units are either held on location by tugs, or they drop spudpiles into the soil. These spud piles, usually cylindrical piles withconcrete fill, hold the Unit on location until the mat can beballasted and lowered.JACKINGA mat Unit will jack the mat to the seabed in accordance with theballasting procedure. Once the mat has been lowered to theseabed, the hull will be jacked out of the water. The Unit thenproceeds to Preload Operations . All Independent leg Units mustperform Preload Operations before they can jack to the design airgap. Most independent leg Units do not have the capacity toelevate the Unit while the preload weight is on board. For theseUnits, the next step is to jack the hull out of the water to a smallair gap that just clears the wave crest height. This air gap shouldbe no more than five (5) feet. Once they reach this position, theUnit may proceed with Preload Operations.
  40. 40. PRELOAD OPERATIONSAll Jack Up Units must load the soil that supports them to the fullload expected to be exerted on the soil during the most severecondition, usually Storm Survival Mode. This preloading reducesthe likelihood of a foundation shift or failure during a Storm. Thepossibility does exist that a soil failure or leg shift may occurduring Preload Operations. To alleviate the potentially catastrophicresults of such an occurrence, the hull is kept as close to thewaterline as possible, without incurring wave impact. Should a soilfailure or leg shift occur, the leg that experiences the failure losesload-carrying capability and rapidly moves downward, bringingthe hull into the water. Some of the load previously carried by theleg experiencing the failure is transferred to the other legspotentially overloading them. The leg experiencing the failure willcontinue to penetrate until either the soil is able to support theleg, or the hull enters the water to a point where the hull buoyancywill provide enough support to stop the penetration. As the hullbecomes out-of-level, the legs will experience increased transverseload and bending moment transferred to the hull mostly by theguide. With the increased guide loads, some braces will experiencelarge compressive loads. During normal preload operations it isimportant to keep the weight of the hull, deck load, and preload asclose to the geometric center of the legs as possible, as this willassure equal loading on all legs. Sometimes, however, single-legpreloading is desired to increase the maximum footing reaction ofany one leg. This is achieved by selective filling/emptying ofpreload tanks based on their relative position to the leg beingpreloaded. Preload is water taken from the sea and pumped intotanks within the hull. After the preload is pumped on board, it isheld for a period of time.
  41. 41. The Preload Operation is not completed until no settling of thelegs into the soil occurs during the holding period whileachieving the target footing reaction. The amount of preloadrequired depends on the required environmental reaction andthe type of Jack Up Unit. Mat Units normally require littlepreload.JACKING TO FULL AIR GAP OPERATIONSOnce Preload Operations are completed, the Unit may bejacked up to its operational air gap. During these operations itis important to monitor the level of the hull, elevating systemload and characteristics, and for trussed-leg Units, Rack PhaseDifferential (RPD). All of these must be maintained withindesign limits. Once the Unit reaches its operational air gap, thejacking system is stopped, the brakes set, and leg lockingsystems engaged (if installed). The Unit is now ready to beginoperations.ELEVATED OPERATING CONDITIONWhen the Unit is performing operations, no particulardifferences exist between the various types of Units.Likewise, there are no particular cautionary measures to takeother than to operate the Unit and its equipment within designlimits. For Units with large cantilever reach and high cantileverloads, extra care must be taken to ensure that the maximumfooting reaction does not exceed a specified percentage of thereaction achieved during preload.
  42. 42. ELEVATED STORM SURVIVAL CONDITIONWhen the Unit is performing operations, the weather is to bemonitored. If non-cyclonic storms which exceed design operatingcondition environment are predicted, Operations should bestopped and the Unit placed in Storm Survival mode. In thismode, Operations are stopped, equipment and stores secured, andthe weather and watertight enclosures closed. If cyclonic stormsare predicted, the same precautions are taken and personnelevacuated from the Unit.This is how a jack up rig is bought from the shore to the requiredlocation for drilling.
  43. 43. ELEVATING SYSTEMAll Jack Ups have mechanisms for lifting and lowering thehull. The most basic type of elevating system is the pin andhole system, which allows for hull positioning only atdiscrete leg positions. However, the majority of Jack Ups inuse today are equipped with a Rack and Pinion system forcontinuous jacking operations. There are two basic jackingsystems: Floating and Fixed. The Floating system usesrelatively soft pads to try to equalize chord loads, whereasthe Fixed system allows for unequal chord loading whileholding. There are two types of power sources for FixedJacking Systems, electric and hydraulic.Both systems havethe ability to equalize chord loads within each leg. Ahydraulic-powered jacking system achieves this bymaintaining the same pressure to each elevating unit withina leg. Care must be taken, however, to ensure that losses dueto piping lengths, bends, etc., are either equalized for allpinions or such differences are insignificant in magnitude.For an electric powered jacking system, the speed/loadcharacteristics of the electric induction motors cause jackingmotor speed changes resulting from pinion loads, such thatif jacking for a sufficiently long time, the loads on any oneleg tend to equalize for all chordsof that leg.
  44. 44. ELEVATING SYSTEM GUIDES
  45. 45. UPPER AND LOWER GUIDESAll 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 fittedwith a set of upper and lower guides. Some Jack Up Units, whichhave exceptionally deep hulls or tall towers of pinions, also haveintermediate guides. These guides function only to maintain therack the correct distance away from the pinions and are notinvolved in transferring leg bending moment to the hull. Guidesusually push against the tip (vertical flat side) of the teeth, butthis is not the only form of guides. There are also other forms ofguides such as chord guides, etc. Depending onaccessibility, some guides are designed to be replaced and aresometimes known as “wear plates.”In addition to protecting thepinions and hull, all upper and lower guides are capable oftransferring leg bending moment to the hull to some degreedetermined by the design. The amount of moment transferred bythe guides to the hull as a horizontal couple is dependant on therelative stiffness of the guides with respect to the stiffness of thepinions 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 TableAn assembly of sheaves or pulleysmounted on beams at the top of thederrick. The drilling line is run overthe sheaves down to the hoistingdrum.Catline Boom and Hoist LineA structural framework erectednear the top of the derrickfor 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. MonkeyboardThe derrickmans workingplatform. Double board, tribbleboard, fourable board; a monkeyboard located at a height in thederrick or mast equal totwo, three, or four lengths of piperespectively.Traveling BlockAn arrangement of pulleys orsheaves through which drillingcable is reeved, which moves up ordown in the derrick or mast.Top DriveThe top drive rotates the drill stringend bit without the use of a kellyand rotary table. The top drive isoperated from a control console onthe rig floor.
  49. 49. MastA portable derrick capable of beingerected as a unit, as Distinguishedfrom a standard derrick, whichcannot be raised to a workingposition as a unit.Drill PipeThe heavy seamless tubing used torotate the bit and circulate thedrilling fluid. Joints of pipe 30 feetlong are coupled together with tooljoints.DoghouseA small enclosure on the rig floorused as an office for the driller or asa storehouse for small objects.Also, any small building used as anoffice or for storage.
  50. 50. Blowout PreventerA large valve, usually installedabove the ram preventers, thatforms a seal in the annular spacebetween the pipe and well boreor, if no pipe is present, on the wellbore 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 SetsA diesel, Liquefied Petroleum Gas(LPG), natural gas, or gasolineengine, along with a mechanicaltransmission and generator forproducing power for the drillingrig. Newer rigs use electricgenerators to power electric motorson the other parts of the rig.Fuel TanksFuel storage tanks for the powergenerating 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 PumpA large reciprocating pump used tocirculate the mud (drilling fluid) ona drilling rig. Bulk Mud Components in Storage Hopper type tanks for storage of drilling fluid components. 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.
  53. 53. Reserve PitsA mud pit in which a supply ofdrilling fluid has been stored.Also, a waste pit, usually anexcavated, earthen - walled pit. Itmay be lined with plastic toprevent 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 ShakerA series of trays with sieves orscreens that vibrate to removecuttings from circulating fluid inrotary drilling operations. The sizeof the openings in the sieve isselected to match the size of thesolids in the drilling fluid and theanticipated size of cuttings. Alsocalled 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 RampAn angled ramp for dragging drillpipe up to the drilling platform orbringing pipe down off the drillplatform. Accumulator The storage device for nitrogen pressurized hydraulic fluid, which is used in operating the blowout preventers.
  55. 55. BASIC SYSTEMS IN JACK UP RIGS Drilling SystemsDrilling system is the heart of the jack up rig. Drilling is carried outat 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 areassembled. Once the stem is assembled it is placed concentric withthe rotary table which is either driven from the top or from thebottom. Kelly bushing is provided to support the drill stem andprevent it from buckling.Mud SystemsThese are of two types – high pressure mud system and lowpressure mud system. Mud is a mixture of raw water, clay, bainite and some otherminerals. Working of the jack up rig depends upon the power ofthe mud system. It is a cyclic procedure which is used to conveythe crushes from the bottom of drill bits to the top of mud tanks.This is called high pressure mud system. when we punch throughthe reservoir a high pressure builds up due to the presence ofgases, to prevent our system from blowing out high viscosity mudis used to lower the pressure. When the mud returns from the BOPand goes to shale shaker assembly, it is called low pressure mudsystem
  56. 56. MUD SYSTEMCATERPILLAR GENERATOR
  57. 57. Power Generation SystemsPower is must to run a jack up rig. Jack up rigs areinstalled in deep seas thereby we have no other provisionof getting power. Hence the internal combustion enginesare used to supply power. They are high capacity engineswhich are located in engine rooms at the main deck. Oncethe electricity suddenly shuts off then the crude oil in theannular should be at the same level of the mud which ispresent in the drill stem below the return line.Cementing SystemsCementing system is a provision which is used to cementthe sides of the well forbidding the soft soil to enter thedrill well.In cement system there are four tanks which have anaccumulator on the top which mixes the cementcontinuously with raw water. On both sides of the rigthere are centrifugal pumps which sucks marine waterfrom the sea and supply it to the tank which is used todilute the cement.Living quarters & Landing SystemsLiving quarters are provided for the officials and ahelideck is also commissioned for some external supportneeded for the persons working on the rig. Complete careof the people is taken to ensure a safe workingenvironment.
  58. 58. BOPDiesel Electric Generator
  59. 59. BOP & Well Completions 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. 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. Jacking and Hydraulic SystemsCare must be taken when positioning a new jack up rig at a sitepreviously occupied by another jack up because of the tendency ofthe spud cans of the new rig to slip into the spud cans holes or “footprints” left on the sea floor by the previous rig. If there is an overlapof a spud can over an old spud can hole, there is a tendency for thespud can not to penetrate straight into the soil, but instead to slipinto the old spud can hole. This movement of a spud can, without acorresponding movement of all the other spud cans in the samedirection, will impose a bending moment on the legs. This bendingmoment can be quite severe and may damage the leg in thepreloading or jacking up process or it may reduce the allowablestorm environmental load of the rig due to the resulting bend of theleg. When selecting a rig for a platform, it is always best to choose arig with the same leg spacing as a rig that previously drilled at theplatform.
  60. 60. However, the effect of previous spud can holes can be mitigatedif the new rig is positioned so that the centers of its spud cansare positioned either at the center of the holes left by theprevious rig or about 1.5 spud can diameters away for the edgeof the holes left by the previous rig. If the rig selected for theplatform does not have the same leg spacing as a rig previouslyat the platform and it is not possible to position the new rig sothat its legs either are centered over old holes or 1.5 diametersaway from old holes while still reaching all of the requireddrilling positions, there are two techniques which can be used tominimize the effects of old holes. These techniques are“Reaming” and “Swiss Cheesing”.“Reaming” is a technique bywhich the leg or legs are sequentially raising and lowering thespud can in the hole left by the previous rig in an attempt towear away the side of the hole, thereby elongating the hole andcreating a new hole center location at the spacing of the legs ofthe new rig. “Swiss Cheesing” is a method in which a number oflarge diameter holes (24 to 30 inch diameter) are drilled at theside of an existing can hole in order to degrade the strength ofsoil at the side of the can hole, effectively enlarging the hole.
  61. 61. SOME SENSIVITIES OF JACK UP RIGSLEG PUNCH THROUGHSWhen a Jack Up is being preloaded, it is important to beprepared to act in the event of rapid penetration of one ormultiple legs. Because of the increased demands on JackUps (i.e., larger water depths and higher environmentalloads) resulting in higher elevated weights duringpreload, the consequences of a punch through areincreasingly more pronounced. A typical soil’s bearingcapacity increases with depth. When a soil layer is underlainby a weaker soil layer, there is a rapid reduction of soilstrength. When the spud can reaches this interface, theweaker soil gives way and the support of the leg movesdownward at a faster rate than the jacking system is capableof lowering the leg to maintain the hull level. As such, thehull rotates, the legs tilt and bend, causing the hull to sway.This results in a weight shift relative to thesupports, thereby increasing the required footing reactionneeded to maintain equilibrium. This process continuesuntil either the soil’s bearing capacity or any hull buoyancyarising from the hull entering the water increase sufficientlyto reach equilibrium. Jack Ups of all design types experiencepunch throughs and their resulting damages tobraces, chords and jacking units.
  62. 62. The accidental loading resulting from a punch through can leadto several types of leg damage including buckling of thebraces, buckling or shearing of the chord, punching shear andjoint damage. The extent of possible damage is dependent on themagnitude of the punch through and, more importantly, on theactions taken before, during, and immediately after the punchthrough. Punch through is an extreme event; therefore, propermanagement of this event is necessary. Modern rigs with a betterguide design along with a proper punch throughmanagement system, can minimize some of the risks.OTHER SOIL ISSUES (SCOUR, EARTHQUAKE)There are soil issues other than poor bearing capacity to considerwhen reviewing a Jack Up’s suitability for a given location. Thissection presents just a few of the main issues. The first is the casewhere the soil is extremely hard or calcareous. In these cases, thepenetration of the spud can will be minimal allowing only aportion of the spud can bottom plate to be in contact with theseabed. In this condition, only that part of the spud can structurein contact with the soil will be supporting the environmentalloads, deadweight, and operational weight of the Jack Up. It isextremely important to verify that such partial bearing will notcause damage to the spud can structure. In cases like these, anadequately reinforced tip on the spud can may be advantageouscompared to flat bottomed footings.
  63. 63. Scour is another problem that occurs in certain locations suchas areas with sandy bottoms and high bottom current. In thiscase, the footing is originally supported over a certain portionof its bottom area during the initial preload operation. Overtime, however, high currents may cause erosion under aportion of the footing. When this happens, the bearingpressure increases over the preload value due to loss ofsupport area. Depending on the bearing capacity of thesoil, additional penetration or spud can rotation may occur.Additionally, if the footing is not structurallyadequate, structural damage may occur. Finally, if scour issevere and over a large enough area, the footing may slide intothe depression created. Any of these scenarios can beextremely severe, especially since they occur with the hull atfull air gap.GUIDE / RACK TEETH WEARThe legs are restrained in horizontal movement and in rotationby the leg guides. Leg guides may also maintain the allowableposition of the elevating pinions with respect to the leg rack.Over time, it is normal to experience wear in both the guides andin the part of the leg that is in direct contact with the guides. Thiswear in both the leg guides and the leg should be monitored.When the leg guides are excessively worn, they should bereplaced. If leg wear should become excessive, the leg should berepaired.

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