3.4 offshore drilling - treccani

3.4.1 Introduction

The techniques and equipment for drilling offshorewells (i.e. offshore drilling) are very similar to thoseused for onshore drilling. The main differencesconsist in the arrangement of the drilling rig and ofthe equipment, and in certain particular methods ofcarrying out the operations, which have to be adaptedto the requirements dictated by far more difficult andoften extreme environmental conditions. Thisobviously entails considerably higher costs, to whichmust also be added considerable investments toprovide the facilities and plants for subsequent fielddevelopment.

The earliest episodes of offshore drilling tookplace just after the turn of the Nineteenth century,when numerous oil fields were discovered along thecoastline of southern California, and were exploitedwith wells drilled up to the shoreline. In the attemptto follow the fields out into the open sea, it wasdecided to extend operations offshore by positioningthe drilling rigs on piers which stuck out for about ahundred metres into the sea. However, the bigdevelopment in offshore drilling did not start until thelatter half of the Twentieth century.

In Europe, the first offshore well was drilled in1959 in an oil field off Gela, in Sicily. Thedevelopment of the gas fields off the coast ofRavenna started in 1960 with the drilling of the firstoffshore well in Europe for the production of gas. Inthe early Seventies, the discovery of the large fieldsin the North Sea and in the Gulf of Mexico gave afinal boost to the development of increasingly refinedtechnologies for offshore hydrocarbon explorationand production.

In the last few decades, in spite of the hostileenvironment, the difficulty, the higher investments and

the risks involved in carrying out drilling andproduction operations in a marine environment,exploration for hydrocarbons in the open sea hasundergone unprecedented development. In fact,compared with onshore areas, by now explored almosteverywhere in such detail that it is considered unlikelythat any new large-scale fields can be discovered, theoceans, and above all deep-water areas (at depths ofmore than 1,000 m), still contain zones where littleexploration has yet taken place and where thepossibility of discovering large fields of hydrocarbonsstill seems promising. The costs entailed in offshorehydrocarbon exploration and production are increasingrapidly, due to the great depths of water concerned andthe hostile nature of the environmental andmeteorological conditions. For this reason, the volumeof the hydrocarbon reservoirs that can be developedand which justify investments in offshore developmentprojects is usually very large, and depends both on theinvestment capacities of the oil companies, and on oilprices on the international market.

An offshore drilling rig has to create the sameworking conditions as for onshore rigs which canmove from one point to another without anydifficulty. It must therefore be a mobile unit equippedto contain an autonomous drilling site, including thederrick, the technical personnel and all the serviceequipment. This can be done with a supportingstructure (or platform) which rests on the seabed andrises above sea level, or with a floating structure, keptvertical above the well by means of anchors or withdynamic positioning systems (see below). Very oftenthese are isolated structures which have to house notonly the personnel necessary for ordinary operations,but also the equipment of the service companies (e.g.for cementing and logging) which in the case ofonshore drilling, on the other hand, are transported to

373VOLUME I / EXPLORATION, PRODUCTION AND TRANSPORT

3.4

Offshore drilling

the rig site and used only for the time strictlynecessary. These conditions increase the complexityof the offshore supporting structures, and justify theirhigher daily rate, which might be up to 5-10 timesgreater than that for an onshore drilling rig of thesame capacity.

From an operational point of view, offshoredrilling may be subdivided into two main categories,depending on the water depth.

Drilling with the rig standing on the seabed. Thesafety equipment, i.e. ordinary Blow-Out Preventers(BOPs) located permanently above sea level andaccessible from the supporting structure; in this casedrilling operations are practically identical to thosecarried out in onshore drilling.

Drilling with floating rigs. The wellhead and thesafety equipment (i.e. special submarine BOPs) areplaced on the seabed, and are not therefore directlyaccessible from the supporting structure. In this case,a number of sequences of the drilling operationsdiffer from onshore ones, as the plant is not immobilein relation to the wellhead but, because it floats, issubject to the action of the wind, currents and waves,which cause it to make small horizontal and verticalmovements. Naturally, in this case, too, the drillingfluid has to rise to the floating rig, through a specialpipe connecting the subsea wellhead with the rig.

The use of floating rigs is necessary for exploratorydrilling in water depths of more than 100 m, while thegreatest depth to which it is possible to operate under

safe conditions exceeds 3,000 m. Clearly, this referssolely to exploratory drilling operations, and not tothe subsequent development drilling. Thetechnological limit for developing an offshore fieldand bringing it into production is a water depth ofaround 1,700 m. However, this limit is bound toincrease within the next few years as technologicalinnovation in this sector is extremely active: by wayof example, it is recalled that in 1995 this limit wasless than 1,000 m.

The main types of drilling rigs for offshoreexploratory wells, with equipment designed for thesole purpose of drilling the well, are described below.If one or more exploratory wells discover a field withreserves that justify its development, it is necessary todesign and prepare the permanent productionstructures. These also either rest on the seabed orfloat, and are very often able to accommodate even arig for drilling the development wells. The permanentoffshore structures for the production ofhydrocarbons are hi-tech engineering and structuralcomplexes based on architectural concepts whichvary according to the water depth (see Chapter 5.2.).

3.4.2 Rigs standing on the seabed

Submersible drilling pontoonsSubmersible drilling pontoons were designed

in the Thirties in Louisiana, where they were used

374 ENCYCLOPAEDIA OF HYDROCARBONS

DRILLING AND COMPLETION OF WELLS

1) towing

2) hull flooding

3) pontoons

4) drillingpontoon

posts

barge hull

Fig. 1. Drilling pontoon of the posted-barge type.

for drilling wells in the swampy areas of theMississippi delta, not accessible from ordinaryroads. In concept, the first pontoons consisted ofan ordinary rig on a suitably adapted barge, whichwas transported to the site along channels dredgedfor the purpose. The barge was then filled withwater and the pontoon came to rest on the seabed,and was held firm by means of driven piles. Todaydrilling pontoons consist of a shallow-draft hull(usually 2-3 m), divided into compartments whichcan be flooded to enable the pontoon to rest onthe seabed, and emptied at the end of operationsso as to refloat the pontoon and enable it to move.The hull is covered by one or two decks; in thecase of two, the engine-room, the mud circulationpumps, the storage area for chemical products andthe cementing unit are situated on the lower deck,while the offices, the accommodation, the depositfor tubular materials and the rig are on the upperdeck, the derrick usually being located in thestern.

After drilling in swampy areas, the next stepwas to conquer actual delta areas, characterized byperiodic variations in the sea level, which made itnecessary to raise the level of the deck for the rigand the various items of equipment. To be able tooperate in these conditions, a pontoon wasdesigned with a hull that could be flooded, with themain decks raised over the hull using a series ofposts. In a working position, the pontoon wastransformed into a sort of pilework structure,which allowed drilling to take place in up to 8-10m of water. The typical posts used for raising thedeck gave their name to this particular type ofvessel (posted barge), usable only in extremelycalm waters (Fig. 1). They are still in use today, andthe drilling techniques applied are identical tothose of onshore wells.

Subsequently this type of rig was furthermodified, to be able to operate in deeper anddeeper waters. The piling was transformed into asteel reticular structure, formed by big pipeswelded together. The outside piles, consisting oflarge-diameter pipes, were enlarged to enable themto be flooded and then emptied to make thestructure float, and allow it to be moved. Theplatform placed over the reticular structure, onwhich the whole of the drill rig was housed, wascalled a submersible bottle platform because of itsdesign. These units, the first ones to operate in theshallow waters of the Gulf of Mexico in the 1950sand 1960s, were able to drill in depths of just afew dozen metres. The largest unit of this type,built in the early Sixties, could operate in a depthof 50 m.

Jack-up drilling platformsOffshore drilling continued to be performed in

deeper and deeper waters, and to do this it wasnecessary to use a different type of plant. To limit thehigh costs necessary to construct higher and highersubmersible drilling pontoons, drilling platformscommonly known as jack-ups were devised (Fig. 2).

Jack-ups are triangular or rectangular floating hullsfitted with long mobile legs (usually 3 or 4) at thecorners of the hull, which are able to move verticallyup and down. Thanks to lifting systems using jacks, orrack-and-pinion mechanisms, it is possible to rest thefeet (also known as spud cans) on the seabed and thusto lift the hull above sea level. The first jack-up wasconstructed in 1954, and its design made it animmediate success, thanks to its stability and


OFFSHORE DRILLING

conductorpipe

cement

open uncased hole

spud can

Fig. 2. Jack-up drilling platform and cemented conductor pipe.

efficiency. In fact, the whole structure rests firmly onthe seabed, and the big platform can house even themost complex drill rigs without difficulty. In theensuing decades these plants were considerablyimproved and enlarged, this applying in particular tothe size and length of the legs. The largest jack-upscan operate at considerable water depths: some ofthem have legs 150 m in length and are able to drill inwater depths of 90-110 m, the maximum depthdepending on the mechanical strength of the seabed.All modern jack-ups are also provided with a sideplatform for the use of helicopters.

When moving from one well to another, the legsare raised and the rig floats on the surface. Once onsite, the legs are lowered until the spud cans touchthe seabed. By continuing to lower the legs, thespud cans slowly penetrate the bed, compacting itso that it can bear the weight of the entire hull.When the hull has been raised about one metreabove sea level, preloading takes place, using theseawater as a ballast for the hull, so as to simulatethe maximum loads foreseeable in the operativephases. In this way the spud cans are furtherembedded, giving stability to the whole structureuntil drilling operations have been completed.Finally, the hull is raised some 10-15 m above sealevel, according to the maximum wave heightforeseen in case of storm. The height that the hull israised has to be such that its underside cannot bereached by the crest of the waves, which coulddestabilize the structure. Jack-ups can haveindependent legs or legs that are connected togetherat the base by a loading plate which replaces thesingle spud cans (mat-supported rigs). Thisconfiguration enables the supporting area to beincreased, reducing the specific weight acting onthe seabed. This is necessary where the seabed haslittle bearing capacity.

To be moved for short distances, jack-ups arefloated and towed by tugs (although some jack-upshave their own means of propulsion). For longerdistances ships equipped with a submersible loadingdeck are used: after the jack-up has been positioned onthe deck, with its legs raised, the ship proceedsnormally, transporting the rig even from one continentto another in a relatively short time.

The drilling technique used on jack-ups is thesame as that used onshore. Ordinary BOPs are used,located on the conductor pipe, which must be self-supporting. This is possible for depths of around 50-60 m, beyond which a fixed structure has to beinstalled on the seabed, to be able to withstand boththe lateral stresses generated by the sea currents, andthe vertical loads due to the dead weight of theconductor pipe and of the BOPs. The conductor pipe

is embedded with the use of a pile driver, if theseabed is sufficiently soft. Otherwise the bedrockhas to be drilled with a drill string fitted with a bit,using seawater circulation. In this case, theconductor pipe has to be completely cemented (seeagain Fig. 2).

Jack-ups are most frequently used to drill wellsfrom fixed jacket-type platforms constructedspecifically for the development of a field (seeChapter 5.2). After having positioned the jacket firmlyon the seabed, and having prearranged the variousconductor pipes of the wells, the jack-up is transportedalongside the jacket for the drilling of the developmentwells. For this purpose, the jack-up must be of thecantilever type, with the derrick placed on a skid thatslides along two axes, so that it can be positionedvertical to every single conductor pipe of each well. Inthese cases, the centre-to-centre spacing of the wells isusually about 2-3 m.

Lastly, it is recalled that in shallow watersdrilling can be carried out with a tender rig; thissystem was formerly common also in the Adriaticoffshore area. It is used for drilling developmentwells from light jackets, which are unable to housea full-scale drill rig. In this case, just the derrick isinstalled on the fixed platform, while all the otherequipment (generators, pumps, mud tanks,materials, etc.) are housed on the tender, which is aservice craft suitably equipped and mooredalongside the fixed platform. The tender and theplatform are connected by flexible pipes for thefluids and by cables for power supply. An inclinedplane with one end on the tender and the otherhinged on the drilling floor is used for the passageof the drilling crew and material. This type of rigcan be used only in relatively calm waters, as thetender has to be disconnected (and drilling thereforesuspended) even when the wave motion is notparticularly strong.

3.4.3 Floating rigs

TypesThe offshore drilling of exploratory wells is

strongly conditioned by the depth of the water:beyond about 100 m, the use of rigs standing on theseabed is no longer possible, and it is thereforenecessary to use floating units, i.e. buoyantstructures on which a complete drill rig is installed.Such structures are designed to be kept in positionas firmly as possible above the well being drilled,by means of anchoring or dynamic positioningsystems. The main problem in such operations isthat of obtaining a sufficiently rigid connection



between the seabed and the floating unit, enablingthe drilling equipment to be lowered into the welland guaranteeing hydraulic continuity forcirculation of the drilling fluid, which has to returnto the rig. The connecting element between thefloating unit and the wellhead (through submarineBOPs) is a special pipe called the marine riser (seebelow). On account of the movement of the sea, thewind and the tides, floating units, not being rigidlyconnected with the seabed, can move vertically andhorizontally in relation to the well axis: thesemovements, although very modest compared withthe water depth, must never exceed the limitsimposed by the design conditions, compatible withthe operations to be performed. As a rule, theadmissible horizontal movement during drilling isabout 3-5% of the water depth. During operations,the movement or displacement has to be constantlymonitored so as to prevent the occurrence ofexcessive stresses on the structures connecting thesubsea wellhead with the floating unit; if theweather and marine conditions cause thedisplacement to exceed the safety limits, thestructures have to be disconnected.

Generally speaking, a floating craft possesses sixdegrees of freedom, as it is able to move and rotatealong three main axes. The rotary movement aroundthe transversal axis of the craft is called pitching, thataround the longitudinal axis is called rolling, and thataround the vertical axis is known as yawing. Motionalong the transversal axis is swaying, that along thelongitudinal axis is surging and that along the verticalaxis is called heaving. Rolling, pitching and heavingare mainly influenced by the distribution of the massesof the structure and by the load of the craft. Surging,swaying and yawing are, on the other hand, influencedby the natural oscillation period of the anchoringsystem.

Floating drill rigs can be divided into two mainclasses: semi-submersible rigs and drilling ships. Inboth cases they are basically vessels constructed tocontain an autonomous drilling site, a platform forhelicopters, quarters for all the personnel and spacesfor the materials and equipment. Floating rigs areproper vessels, and therefore they have a captain anda crew of seamen. In general, drilling ships can travelat a reasonably high speed and have a considerablecarrying capacity but, under equal weather and seaconditions, they are less stable than semi-submersible rigs, that are able to operate in a stablemanner even in difficult environmental conditions.Both types of rig, not being firmly connected withthe seabed, need to use far more complex wellheadsand submarine BOPs than those used in onshoredrilling operations.

Semi-submersible rigsSemi-submersible drill rigs consist of a large

triangular, rectangular or pentangular platform,connected with submerged hulls by means of largecolumns which vary in number from 3 to 8, accordingto the shape of the vessel (Fig. 3); they are kept verticalover the site by means of mooring or dynamicpositioning systems.

The first rigs, moved by tugs, were constructedtowards the end of the 1950s, and led to thedevelopment of the semi-submersible types that nowexist. When being moved from one site to another, thesubmerged hulls are emptied and the rig becomes afloating unit, similar to an ordinary vessel. Some semi-submersible rigs have to be towed by tugs, whileothers have an autonomous propulsion system. In itsworking position, the height of the platform above sealevel can be regulated by filling the hulls and thecolumns with seawater as ballast. By appropriatelyregulating the amount of ballast water, the draft of thevessel is varied, optimizing its stability during drillingoperations. Moreover, when the state of the seabecomes particularly severe, the safety of the vesselcan be improved by increasing the ballast, whichlowers the vessel’s centre of gravity.

Semi-submersible rigs are constructed with anatural period of rolling and pitching different fromthe period of the waves normally encountered in theopen sea, and they thus have considerable stability,which is little affected by the wave motion, andpermits comfortable working conditions. In fact, as alarge part of the mass of the vessel is submerged, it ishardly subject to rolling or pitching. However, it isharder to control the heave, i.e. its vertical movement.By way of example, the heave of a large semi-submersible rig, in the presence of 30-m-high waves,is about 6 m. In spite of this, semi-submersible rigshave only short WOW (Waiting On Weather) times,namely the periods when drilling has to be suspendeduntil weather and sea conditions improve. Anchoredsemi-submersible rigs are used for drilling in waterdepths of up to about 1,000 m. At greater depthsdynamic positioning systems are required.

Drilling shipsThe first offshore well was drilled in 1947 in the

Gulf of Mexico by a drilling ship, at a water depth of 6 m. The first drilling ships were usually old colliers,whalers or cruisers, with their hulls suitably adapted tomake an opening, still known as the ‘moon pool’,vertically above the centre of gravity. The derrick wasinstalled above this, together with the relevantequipment. The deck was organized to accommodatethe tubular materials, while the pumps and the mudtreatment plant were housed in the hold. Modern


OFFSHORE DRILLING

drilling ships are designed and built specifically to actas drilling sites and they are equipped with particularlycomplex technological systems. Drilling ships are usedfor operating in deep waters, often under extremeenvironmental conditions, such as drilling in arcticareas. To this day it is the best means of drillingexploratory wells in remote areas, far removed fromsupply points, as it can carry all the material necessaryfor drilling even a particularly difficult well. Just as forsemi-submersible rigs, drilling ships are kept in avertical position over the well by means of mooring ordynamic positioning systems (Fig. 4). These shipswhen moored can be used for drilling in depths of upto about 1,000 m, while for greater depths dynamicpositioning systems must be used, and with these theship is capable of operating in 3,000 m of water. Inthis case, the depth limit depends only on the weightand the mechanical strength of the connecting systemwith the subsea wellhead.

The mooring systemThe traditional positioning system for a vessel

foresees the use of mooring lines with cables orchains which run from the hull and become fixed tothe seabed by anchors, arranged according toschemes depending on the geometry of the vessel andon the expected sea and weather conditions. Ingeneral, drilling ships have three or four pairs of

mooring lines – at least two lines in the stern, two inthe bows and one on each side – while semi-submersible rigs have at least one pair on eachcolumn at the apexes of the platform. The mooringlines are usually made of various parts, an upper partconsisting of a steel cable connected to the vessel,and a lower part, consisting of a chain fastened to theanchor. Should just a single anchor not be sufficientto grip the seabed, two or more anchors in series areused, connected by another chain. The anchors arelowered vertically by a tug using a special cable. Thetug tows the anchor to the anchorage, stretching themooring line, and when the right position has beenreached, it lowers the anchor to the seabed so that theflukes become embedded in the bottom. Verticallyabove each anchor there is a buoy marking itsposition and facilitating its retrieval when operationsare over. In the case of very deep waters (more than1,000 m), the traditional mooring system requireslong, heavy lines, more powerful tugs and lengthy,difficult positioning and retrieval operations, whichinvolve considerably higher costs.

The mooring of vessels is programmed accordingto the force exercised by the wind and the stressesinduced by the sea. For example, drilling ships arenormally moored with their bows towards the wavesand the prevalent winds, if possible, or at least towardsthe strongest expected force. If the direction of the



column

drilling draft main deck

survivalcapsule

drill floor

hull

Fig. 3. Semi-submersibledrill rig.

wind and of the waves changes, the vessel’s stabilitycan suffer, causing the rolling and the pitching toincrease, as also the tension in the mooring lines andthe pull on the anchors. If these movements exceed agiven limit, drilling has to be suspended and it may benecessary to detach the connection with the subseawellhead, allowing the vessel to move away safely.Regarding the wind direction, it is as well to rememberthat it must be possible to safely ignite the flare of thedrilling rig at any moment, and that the helicopterlanding pad must be accessible during the majority ofweather and marine events (the helicopter has to takeoff into the wind, and the derrick must not hinder thisoperation). As mentioned, these problems are lessserious in floating rigs with dynamic positioningsystems, which are able to rotate more easily thananchored rigs.

The dynamic positioning systemAn offshore rig can be kept in a relatively fixed

position vertically above the well also by means of thedynamic positioning system. This technology isnecessary when the water depth is such that it is nolonger possible to use traditional mooring systemsdue to the weight of the mooring lines and theexcessive elasticity of the system. For this purpose,

the vessel must have pairs of screw propellers in thestern, in the bows and on both sides, which are alwayskept working (see again Fig. 4). The wellhead, whichis always positioned on the seabed when drilling fromfloating rigs, is fitted with a device that sends anacoustic signal to the vessel, and under the keel thereis a series of hydrophones which pick up the signalarriving from the seabed. This signal is then relayed toan electronic control device, which identifies in realtime the position of the vessel in relation to thewellhead and, depending on its movement, it restoresits vertical position by varying the speed of one ortwo pairs of propellers. Compared with a mooringsystem, dynamic positioning has the advantage ofpermitting a certain possibility of rotation on the partof the vessel, and therefore permits the bestorientation vis-à-vis the direction of the wind, thecurrents and the waves. In some cases various systemsof measuring the vertical position are used, as thepresence of gas bubbles in the water or theinterference of the sound of the screws can falsify thehydrophone recordings. It is possible, by means ofspecial devices, to measure the angle of inclination ofa cable, connected to a fixed point on the seabed andkept at a constant tension. More refined methods usemodern satellite positioning systems, called GPS(Global Positioning System).

3.4.4 Drilling from floatingoffshore rigs

Preliminary drilling operationsThe techniques for drilling offshore wells from

floating rigs are basically the same as those used foronshore wells. The few differences stem from the factthat a number of additional elements are required inorder to connect the wellhead safely to the rig. Ingeneral, the important factors involved in drillingwells from floating rigs are due to the followingcircumstance: a) the wellhead is located on the seabed;b) the submarine BOPs are located on the subseawellhead and are controlled hydraulically orelectrically from the surface; c) the BOPs areconnected to the rig by means of a pipe known as themarine riser which enables the drilling fluid tocirculate upwards; d) the marine riser, connected at thetop of the BOP stack, has a ball joint on its base and aslip joint above sea level to offset the horizontal andvertical movements of the rig; e) the lines forpreventing blowouts (kill lines for the introduction ofmud, and choke lines for mud recovery purposes) runfrom the surface manifold on the rig to the subseawellhead, as independent lines fixed to the outside ofthe marine riser.


OFFSHORE DRILLING

thrusters

well

subsea beacon

taut wire

Fig. 4. Drilling vessel with dynamic positioning systems.

Drilling starts after the rig has been positionedvertically over the well, by means of mooring ordynamic positioning systems. The first operationconsists in placing on the seabed, by means of apipe string, what is called the temporary guidebase (Fig. 5), a strong steel framework with acentral hole which has a tapered inlet at the top,provided with four guidelines and a number ofsteel pins that become embedded in the seabedand prevent displacement.

The pipe string is then disconnected, leavingonly the temporary guide base on the seabed,with the four guidelines that connect up with therig. Usually at the end of this operation, atelevision unit is sent down along a guideline, tocontrol that the temporary support base is restingproperly on the seabed. At this point the drillingphase proper can begin, so as to place the firstcasing, which in offshore drilling is called thefoundation pile. The hole is drilled using anordinary drill string, guided inside the hole of thetemporary guide base by a frame and held inposition by the four guidelines. This first drilling phase is carried out using the circulation of seawater, and the cuttings donot rise to the surface but are scattered over theseabed. The foundation pile is taken down to adepth of a few tens of metres, generally between30 and 50, as for the conductor pipe of onshorewells. At this point the foundation pile is broughtinto operation, being lowered into the hole again

using a light frame and the four guidelines. The light frame is made in such a way that it issevered when the foundation pile enters the hole.The foundation pile ends with a special itemknown as the permanent guide structure,characterized by four robust tubular columnsplaced at the apexes, 3-6 m long, through whichrun the guidelines (see again Fig. 5). The fourcolumns serve in the subsequent phases to guidethe submarine BOPs to the wellhead withprecision. The permanent guide structurecontains the housing for the wellhead, to whichthe successive casings will be anchored. This is aparticular wellhead which, compared with thoseused on land, has a different system of flangingand anchoring the casings. The subseawellhead is shaped in such a way as to enable thelock-up of the hydraulic connector to which theBOPs are coupled.

Next, the foundation pile is fully cemented bymeans of a drill string. Once the cement has set,drilling continues, boring the second section ofthe hole inside the foundation pile, in which thesecond casing is inserted (in offshore drilling thisis called the conductor pipe, analogous to thesurface casing in onshore wells). Once the conductor pipe has also been fullycemented, the well has a stable structure and it ispossible to install the submarine BOPs. After thisdrilling continues with the sequence of operationstypical of onshore wells.



perforazioneMare fig. 5, 6, 7

temporary guide base temporary guide base temporary guide base

guideline

guide frame

bit

hole opener

permanentguidestructure

foundation-pilecasing

Fig. 5. Drilling from floating vessels: A, temporary guide base; B, guide frame for the drill string;C, foundation-pile casing and permanent guide structure.

A B C

Subsea BOPsIn offshore drilling BOPs have the same function

as those used in onshore wells, but are connected in asingle complex (the BOP stack) before being mountedon the wellhead, so as to reduce assembly times at thesea bottom. They are lodged in a square-section cagestructure with female columns at the apexes, intowhich the male tubular columns of the permanentguide structure fit (Fig. 6).

The BOP stack is lowered and fastened to thewellhead by means of a hydraulically controlledconnection, ensuring hydraulic sealing. In the upperpart there is the annular BOP, followed by a series ofram BOPs. In the event of temporarily abandoning thewell because of adverse weather and sea conditions, itis possible to suspend the pipes on the shaped rams ofthe lower BOP, to unscrew the pipes and to close thewell with the upper blind rams. At this point it ispossible to disconnect also the marine riser andpossibly to abandon the site. The marine riser can bereconnected when the sea and weather conditionsimprove.

The hydraulic lines controlling the variousfunctions of the BOP stack converge in a connectorblock, to which is connected the bundle of flexiblepipes for their control from the surface. The BOPscan be operated in a similar way to that used inonshore wells, known as the direct system. In thiscase, the operating system and the pressureaccumulator are installed at the surface, and thecontrols are connected to the BOPs with flexiblelines. The direct system has the advantage of beingsimple, cheap and easy to maintain, but it becomesimpossible to use with increased water depth (beyond100 m), due to the longer operating times. At greatdepths an indirect system is used, in which theoperating fluid from the surface accumulators isconveyed to the seabed in a single high-pressureflexible pipe, to which the other lines for operatingthe distributing cock and the regulation valve are alsoconnected.

The marine riser for returning mud The marine riser, or simply the riser, connects

the top part of the subsea BOP stack with thefloating rig. It is a heavy-duty steel tube, verysimilar to casing, and its purpose is to guide thetools into the well and to bring the drilling fluid tothe surface (Fig. 7). It is mounted above the BOPstack by means of a special connector fitted with aball joint. The connections are operatedhydraulically from the surface to permit rapiddisconnection (in the event of bad weather and seaconditions) and easy connection of the riser. The balljoint enables the riser to displace by a few degrees to


OFFSHORE DRILLING

floating rig

riser pipe

subsea BOPstack

permanent guide structure

Fig. 6. The subsea BOP stack.

adjust to the horizontal movements of the vessel.During drilling, to increase the speed at which thecuttings reach the surface inside the riser, mud ispumped through a special booster line into the riserabove the ball joint.

The proper body of the riser starts above the balljoint. Risers are seamless pipes, usually connectedby non-screwed joints. In parallel and firmly fixed

by means of clampers to the body of the riser are thechoke line, the kill line and the service lines (boosterline, BOP controls), subdivided into sections of thesame length, to make assembly easier.

The top of the riser is connected to the floatingrig by means of a telescopic joint, in order tocompensate for the vessel’s vertical displacement.The inner part of the joint is connected to the rig andmoves with it, whilst the outer part is integral withthe riser and is firm in relation to the seabed. Thehydraulic seal between the two parts in relativemotion is provided by packing that is activatedpneumatically. Above the telescopic joint there is adiverter, connected to the riser by means of aknuckle and socket joint, the function of which is todrive any gas flow from the well to a safe position.

For drilling operations in very deep waters, thedead weight of the body of the riser can causestability problems. It is recalled that the body of atypical riser with a diameter of 22 inches (55.88cm) has a linear mass in water of about 240 kg/m.In such cases, it is possible to provide the sectionsof the riser with external floats made of synthetic,plastic-based foam. If the use of floats is notsufficient, to limit the compression stress that tendsto destabilize the pipe, it is necessary to put theriser under tension from the surface. The requisitetension is supplied and kept constant by pneumatictensioners on the vessel, located at the corners ofthe moon pool, and anchored by cables to the riserunder the telescopic joint.

Mechanisms for motion compensationA floating rig must be able to operate with the

vessel in motion. In fact, it has been seen that themooring and dynamic positioning systems are notrigid, and permit even quite considerablemovements, both horizontally and vertically.Vertical motion is particularly prejudicial todrilling operations, as it modifies the tensionsacting on the drill string. Motion compensators aretherefore necessary, to guarantee a constanttension both on the drill string and on the marineriser. If mechanisms to offset the motion were notforeseen, the vertical movement of a floating rig,generated by waves and tides, would transmitdangerous stresses to the drill string and to the bit.During the upward motion of the vessel, the bitwould become detached from the bottom hole,making drilling impossible, while during thedownward motion it would bounce against thebottom of the hole, causing damage to the bit andtransmitting an anomalous compressive force tothe drill string. Two variants of motioncompensators these exist, based on different



telescopic joint

choke and kill lines

ball joint

marine riserconnector

marine riserjoint

guideline

Fig. 7. The marine riser.

principles: the bumper sub (or telescopic joint)and the heave compensator.

The bumper sub, little used nowadays, is a slidingjoint in a hydraulically sealed oil bath, located in thedrill string above the drill collars. It also allows forrotation, thanks to its grooved profile. The elongationof each single joint is around 1.5 m, which means thatseveral joints are necessary to offset the expectedvertical movements of the rig.

The system to offset heave, on the other hand,involves the use of oleodynamic or pneumatictensioners which balance the vertical shifts of the rigby means of special pistons, acting both in tension andin compression, which keep the upper part of the drillstring at a constant tension (Fig. 8).

Bibliography

Baker R. (1998) A primer of offshore operations, Austin (TX),Petroleum Extension Service.

Bradley H.B. (editor in chief) (1992) Petroleum engineeringhandbook, Richardson (TX), Society of PetroleumEngineers.

Fay H. (1990) Dynamic positioning systems, Paris, Technip.

Gerwick B.C. Jr. (2000) Construction of marine and offshorestructures, Boca Raton (FL), CRC Press.

Holand P. (1997) Offshore blowouts. Causes and control,Houston (TX), Gulf.

Nguyen J.P. (1996) Drilling, Paris, Technip.

Puech A. (1983) La technique des ancres dans l’exploitationpétrolière en mer, Paris, Technip.

Paolo MaciniDipartimento di Ingegneria Chimica,

Mineraria e delle Tecnologie AmbientaliUniversità degli Studi di Bologna

Bologna, Italy


OFFSHORE DRILLING

travellingblock

cylindersto offset heave

hook

drill stem

bit

Fig. 8. The system to offset heave.

3.4 offshore drilling - treccani

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