Lecture No 08

8/16/2019 Lecture No 08

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8/16/2019 Lecture No 08


that there is no dent, crimp, or buckle more than the small annular space. The squeegeesrestrict the loss of pressure yet allow the pig to move. The pig is usually equipped with anacoustic transponder or radioactive marker so that if it does get stuck, its position can bedetermined. For short lines, an umbilical line “fishing line” can be unreeled behind it.

Fig. 2 . Pipeline “pig”

The three normal installation methods for pipelines/risers are as follows: S-layingmethod; J-laying method; reel method.

S-laying method

S-laying is a method in which the pipe is laid from a near-horizontal position on a lay barge using a combination of horizontal tension and a stinger (bend-limiting support). The S-lay installation methodology is the traditional method of deployment from a pipe lay barge.Pipe lengths are loaded onboard and installation operations conducted via a series a workstations on the after deck of the barge. With the pipe lengths in the horizontal, the pipe lengthsare welded together, inspected and coated in each of the work stations. The welded pipe isoverboarded under tension and supported by a guide arrangement (stinger). This stinger limitsthe overbend in the pipe by limiting the curvature of the pipe as it exits the vessel and thetensioners prevent pipe damage due to buckling. The size of the stinger arrangement isdirectly related to the pipe diameter, coating thickness and water depth. As the pipe isoverboarded, the pipe lay barge is moved ahead and the sequence is repeated. The S-laymethodology is characterised by the double bend in the pipe as it is released to the sea,forming an 'S' track.

The S-lay method has been used for many years to lay offshore pipelines. An S-lay vesselhas a broad deck operating area. The pipe joint assembly line is deployed in the middle or atthe side of the main deck, which includes pipe conveyors units, welding, NDT inspection andcoating stations etc. The stern of the vessel is constructed into a sloping slide way with astinger, which is used to modify and control the configuration (stress distributions along the

pipeline) of the pipeline in an “S” shape down to the seabed. Figure 1 shows a picture of aconventional S-lay installation vessel.

Fig.3. Conventional S-lay vessel

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Fig. 4. Offshore S-pipelaying Vessel

Figure 4 shows an offshore pipelaying vessel- 1 for laying a pipeline- 7 on a seabed. Thevessel- 1 comprises a pipeline-laying installation, in this case of the S-lay type. The vessel is

provided with three tensioners- 50 for carrying a weight of the pipeline hanging downwardsfrom the rear side of the vessel, and a stinger- 11 which is shown in two positions: a raised

position and a lowered position. In the lowered position, the stinger is able to guide the pipeline towards the seabed. The raised position is convenient when no pipeline is laid.

The vessel- 1 comprises a superstructure- 9 at a front side of the vessel for crewaccommodation. Extending from the superstructure towards a rear side of the vessel, a maindeck- 3 is provided. On this main deck- 3 a hoisting crane- 13 is located at the rear side of thevessel used to transport objects from and to the vessel, or to transport objects around the maindeck. Shown on the main deck are racks- 20 filled with single length pipe sections which actas a pipe section storage on the vessel. The hoisting crane- 13 comprises a hollow column- 15 and a jib- 17 attached to the column in such a way that the jib can be pivoted up and down andis able to revolve around the column. The stinger is attached to the column by cables- 12 ,which can be hauled in or paid out to change the position of the stinger. The hoisting crane- 13 is shown in a resting position.

The pipeline-laying installation comprises an assembly line- 40 located on a workingdeck- 5 in line with the tensioners- 50 , said assembly line having at least one pipeline assemblystation to assemble the pipeline- 7 to be launched from the vessel- 1 by joining multi length orsingle length pipe sections end to end. The multi and/or single length pipe sections aretransported from the pipe section storage on the main deck to the assembly line- 40 by aconveyor system.

Fig. 5. S-pipelaying Vessel ’s Stinger

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The stinger- 1 is an S-lay stinger for use with an offshore S-lay pipelaying vessel forlaying a pipeline on the seabed. Such a vessel comprises a hull- 2, the elongated stinger havinga hull end- 1a and a free end- 1b, said hull end- 1a here being attached to the hull- 2 of thevessel, commonly pivotally, said stinger being adapted to support a pipeline- 10 to be laidfrom the vessel.

The stinger comprises a stinger frame- 3, which is in this embodiment composed of three

pivotally interconnected rigid stinger frame sections- 3′, 3″ and 3 ′″, which are possiblyarticulated sections. Distributed over the length of the stinger – multiple pipeline supportassemblies- 5 are mounted that provide support for the pipeline. In the shown situation, the

pipeline is held by tensioners- 50 , provided on the hull of the vessel. Also, a clamp- 20 is provided to alternatively support the weight of the suspended pipeline. A gantry construction-21 is provided on the hull, which in the shown embodiment supports the hull end stingerframe section 3′.

J-laying method

The J-lay vessel uses a J-lay tower/ramp to install the pipeline instead of the stingers used

in the S-lay vessel. The J-lay method differs from the S-lay method in that the pipe departsfrom the lay vessel at a near-vertical angle (e.g., 60 to 87). There is no over bend to maintainas in S-lay, thus no stinger is required. This type of operation is developed to cater todeepwater pipeline installations J-laying is a method in which the pipe is laid from anelevated tower on a lay barge using longitudinal tension without an over bend configuration atthe sea surface.

Load-out and transportation of pipe joints will be performed by the transportation bargeat the same time pipe is being laid by the pipe-laying vessel for the S-laying and J-layingmethod. The J-lay installation methodology utilises a vertical or near vertical tower structure.The pipe lengths are lifted from the storage location on deck, pre-jointed into triple orquadruple joints and placed into the tow' structure where welding, inspection and coating iscompleted.

The use of this methodology eliminates the potential for overbending of the pipe whichcan occur in S-lay operations, as the pipe exits the vessel in a near vertical aspect. Laying the

pipe at near-vertical angles also reduces the distance to the touchdown point. The mainadvantage of the J-lay method in comparison with S-lay is for deepwater installations wherethe near vertical deployment minimises tensions and allows the installation to be under morecontrol. The tensions associated with the S-lay method become unfeasible for deeper waterinstallations.

The J-lay methodology is associated with vertical deployment and characterised by asingle bend in the pipe just above the seabed, forming a 'J' track.

Fig.6. Types of J-lay vessel

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Fig. 7. Offshore J-pipelaying Vessel

TOWER of a J-lay vessel

Fig. 8. Tower of a J-pipelaying Vessel

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Reel-laying method

Reel-laying is a method in which the pipe is made up at some remote location onshore,spooled onto a large radius reel aboard a reel-lay vessel, and then unreeled, straightened, andlaid down to the seabed at the offshore installation location. The reel lay installationmethodology can be utilised to deploy continuous lengths of pipe. The pipe is loaded onboard

the vessel on deck or under deck mounted reels or carousels and minimises the need forwelding offshore, allowing increased deployment speeds. Offshore, the pipe is spooled offfrom its storage location, via straighteners and tensioners to the seabed, as the vessel movesahead. The reel lay method can include both J-lay and S-lay operations dependent on thearrangement of the straighteners, tensioners and associated equipment.

A typical reel-lay vessel usually provides an economical tool for installing long, small-diameter pipelines (typically smaller than 0.406m). The pipeline is made up onshore and isreeled onto a large drum on the middle deck of a purpose-built vessel in a spool base. Duringthe reeling process in the spool base, the pipe undergoes plastic deformation on the drum.During the offshore installation, the pipe is unreeled and straightened using a special straightramp. The pipeline on drum will be unreeled accompanying with vessel speed at e.g.

12km/day (usually 3-15 km depending on pipe diameter). The pipe is then placed on theseabed in a configuration similar to that used by S-lay vessels, although in most cases asteeper ramp can be used and over bend curvature is eliminated as with a J-lay. This kind of

pipe-laying vessel is easily manipulated with simple pipe-laying devices; in addition, it has agood pipe laying speed. Figure 10 shows a typical reel-lay vessel.

Fig. 9. Reel-laying Vessel

Figure 10 is a side view of a reel laying system- 10 mounted on a floating pipe layingvessel- 2. Pipe- 20 , having been previously fabricated onshore, is wound on a reel- 30 andmounted on vessel- 2. As the pipe- 20 is installed or laid onto the seabed- 3, as in conventional

practice, after being unwound from the reel- 30 , the pipe- 20 passes through a pipe aligner- 40 ,followed by a pipe straightener- 50 , a pipe tensioner- 60 and a hang off clamp- 80 before

entering the water- 14 . The pipe laying equipment, i.e., the aligner- 40 , straightener- 50 ,tensioner- 60 and hang off clamp- 80 , are supported by ramp- 34 mounted on the deck ofvessel- 2. The pipe straightener- 50 can take the form of a series of rollers or tracks which are

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capable of applying force to the pipe- 20 in a three-point configuration to create reverse bending in order to remove residual curvature from the pipe- 20 . In other words, the rollers ortracks of the straightener contact the pipe at three points, two on one side and one on theopposite side, in order to impart reverse bending.

Fig. 10. Reel-laying Vessel Components

According to the present disclosure, the straightener- 50 has one or more settings whichcan be adjusted. These settings include the relative positions of the rollers or tracks, thespacing of the rollers or tracks, and the amount of pressure to be applied by each of the rollersor tracks to the pipe- 20 at the points of contact with the pipe. Conventionally, these settingsare determined onshore during unwinding of the pipe- 20 from the reel- 30 mounted onto thevessel- 2. In the presently disclosed methods and systems, non-straightened sections of pipecan be provided over predetermined lengths at predetermined locations by adjusting thesettings of the pipe straightener- 50 . For instance, the pressure applied by each of the rollers ortracks of the straightener can be reduced in a way that results in greater residual curvature inthe pipe and therefore non-straightened sections of pipe. Alternatively, the pressure applied bythe rollers or tracks of the straightener can be eliminated by disengaging the rollers or tracksfrom the pipe completely over predetermined lengths at predetermined locations along thelength of the pipe- 20 . Alternatively, the relative positions and/or spacing of the rollers ortracks can be modified in a way that results in non-straightened sections of pipe.

The pipe aligner- 40 can take the form of a series of rollers, a conveyor belt or a wheelwhich supports and aligns the pipe- 20 after being unwound from the reel- 30 and guides the

pipe- 20 as it enters the straightener- 50 . In one embodiment, non-straightened sections of pipecan be provided over predetermined lengths at predetermined locations by modifying theradius of the aligner- 40 during pipe laying operations. For instance, the modified radius of thealigner can result in greater residual curvature in the pipe- 20 .

A control system can be used to control the methods and systems of the presentdisclosure. For instance, a control system can be embodied in a control station- 70 used tocontrol the engagement and disengagement of the rollers or tracks of the pipe straightener- 50 ,to modify the relative spacing and positions of the rollers or tracks of the pipe straightener- 50 ,to control the amount of pressure applied by the rollers or tracks of the pipe straightener- 50 ,

and/or to control the radius of the pipe aligner- 40 . The control station- 70 can include a programmable processor which can be programmed with the predetermined location and the predetermined length along the length of the pipe- 20 at which to adjust the pipe straightener

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and/or pipe aligner settings, thus creating the non-straightened sections of pipe. Alternatively,the control station- 70 can include manual controls by which an operator can adjust the pipestraightener and/or pipe aligner settings. The control station- 70 can be connected to the pipestraightener- 50 and/or the pipe aligner- 40 by any suitable control line, e.g., hydraulic fluidcontrol line. The control station- 70 can be located on the vessel- 2 in any convenient locationas would be apparent to one skilled in the art.

The pipes laying principle

The lay barge is a system that comprises the following principal operations and systems:1. Seaborne work platform vessel2. Mooring and positioning systems, either lines or dynamic positioning3. Pipe delivery, transfer, and storage facilities4. Double-ending of pipe, conveying to lineup station, and lineup equipment5. Welding of joints6. X-ray7. Joint coating

8. Tensioning of line during laying9. Support of line into water either by “stinger” or cantilevered ramp 10. Survey and navigation11. Anchor-handling boats12. Communications13. Personnel transfers — helicopter and crew boat14. Diver or ROV for underwater inspection15. Control center16. Crew housing and feeding17. Power generation18. Repair facilities and shops

Fig.11. Typical Lay barge operation

The basic operations of the lay barge can be outlined as follows (it is equally valid for all pipe laying methods):

1. The lay barge is positioned on its anchors, eight to twelve in number, holding it aligned

with the pipeline route, with a “crab” or slight orientation angle as needed to accom modatethe effects of the current. Its position is determined by an electronic positioning system orGPS, augmented by laser in some cases. Its orientation is by gyroscope.

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2. The anchors will be progressively moved forward as the laying takes place, usually in500 – 600 m jumps. One anchor-handling boat on the starboard side will move each anchorahead in succession; another anchor-handling boat will move each of the port anchors aheadin succession

Fig. 12. Typical anchor position

Typically, the anchor-handling boat maneuvers close to the anchor buoy to enable thedeckhand to hook an eye in the end of the pendant. The deckhand attaches a wire line fromthe deck engine of the tug, which either pulls the buoy aboard or pulls the pendant through the

buoy, thus lifting the anchor clear of the bottom. The boat then runs forward, setting theanchor as directed in its new position and releasing the buoy. The boat turns outboard andgoes back for the next anchor in the cycle. The new position of the anchor is given by voiceradio command from the control house, which is based on radar, gyro, and the reading on theremote mooring line length counters, reading the line length paid out by the winch. Anchorhandling is a very dangerous operation. Hydraulically operated ramps and booms have beendeveloped to enable these operations to be carried out safely. The proper paying out andtaking in of each mooring line on the winch drum is monitored by video in the control houseto ensure against crossed lines on the drum or fouling of the line.

3. From a supply boat or barge alongside the port side, the crawler crane on the lay bargesnags (picks) one pipe length (12 m) at a time, turns, and sets it in storage. From storage, thecrane picks a pipe length and sets it on the end-O conveyor, which moves it to the transverseconveyor at the bow. This conveyor feeds it onto the lineup station, where it is positioned,usually semi-automatically, in correct alignment and then run forward to the end of the

preceding segment

4. The internal lineup clamp positions it in exact spacing and holds it for the hot passweld.5. The hot-pass weld is made and ground or gouged.

Fig. 13. Welding procedure

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6. The segment moves forward successively to weld stations 2, 3, and 4, with one or more passes being applied at each station, and then chipped or gouged.

7. The fully welded line now passes through the tensioner, where it is gripped by polyurethane cleats on caterpillar-like treads. Hydraulic rams push the pads against thecoating, adjusting their pressure so as not to deform the pipe or crush the coating, while stilldeveloping frictional resistance. The tensioners run on torque converters or similar devices to

pay out under a set tension. This tension or tensioner typically has a rather wide tolerance onexternal pipeline diameter.

Fig.14. Pipeline tensioners

8. The joint now goes to the x-ray station, where it is x-rayed, and the films are developedand checked. If a flaw is found, it must be cut out, re-welded, and re-x-rayed. For a cutout, the

barge must be moved astern and the line brought back up on board one or two lengths so thatthe cutout is forward, i.e., on the untensioned side, of the tensioner.

9. The pipe section now moves astern, where the joint is coated with the specialcorrosion-protective coating. A bracelet of zinc – aluminum or other anode is affixed. Concretemortar coating is applied to protect the corrosion-protective coating at the joint. This freshconcrete is protected by a sheet-metal wrap-around.

Fig.15. Coating the joint

10. The completed pipeline now passes down the ramp and over the stern of the bargeand bends downward. This downward bend is called the “over bend” .

Fig.6. Pipeline in over bend, passing down stinger

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11. The line rides down the stinger or ramp to a point of departure, where it leaves thestinger due to the tension in the line. The stinger has a hinged connection to the barge. It has

built-in flotation to support the pipeline while still allowing a downward inclination and someflexibility to accommodate surge. The stinger may be articulated to permit continuouscurvature or may have a fixed vertical curve. Load cells on the roller supports, plus depthindicators such as bubble gauges, enable the stinger to be ballasted for optimum support.

12. The line now moves downward through the water and bends back to the horizontal atthe seafloor. This bend is called the “sag” bend. At this bend, the pipeline is usually subjectedto its maximum stresses and potential buckling due to the combined axial tension, vertical

bend, and circumferential hydrostatic pressure.13. As the line lays out on the seafloor, its integrity is checked either by divers and video

or by ROV.

Functional Requirements

A pipe-laying vessel should include the following additional requirements for somespecific equipment:

• Capacity of tensioner: This is required based on the water depth and pipeline unitweight and buoyancy.• Abandon and recovery winch: This is required a t the end of the pipe-laying process

and if emergency conditions arise.• Davits capacity: This is required once davit activity is necessary for the offshore

connection of pipeline or umbilicals.• Product storage capacity: This is required if the pipe joi nts or umbilical reels cannot

be transferred from the storage barge to the laying vessel due to bad weather.From the above sequence, it can be seen that the typical lay barge system described at the

beginning of this section has the following physical components: lay barge; anchor-handling boats (usually two); supply boats (usually three) or supply barges (usually two) with tug;

helicopter service and/or crew boat; shore base; pipe storage racks; pipe conveyors; lineupstation; internal line up device and clamp; welding stations; tracked tensioner; X-rayequipment; joint-coating equipment; constant-tension winch for abandonment and recovery;stinger and stinger control; winches with mooring lines; control room; radio circuits to shoreand boats; voice and indicator circuits to welding stations, stinger control, and X-ray;gyrocompass; radar; DGPS and/or electronic positioning; tensioner force readout; mooringline tension readout and video display; mooring line length-out readout; diver shack;decompression tank; heliport; quarters for crew; mess hall and kitchen; office; first-aid andmedical facilities; owner’s q uarters and office; repair shop; power plant; fuel and waterstorage; store rooms for slings, shackles, etc.

Tension is maintained in the pipeline from the barge to the seafloor in order to reduce thevertical bending and the tendency to buckle. Values of applied tension range from a low of

perhaps 100 – 150 kN in shallow water and calm seas to 300 kN or more in deep water andrough seas.

The lay barge is subject to dynamic surge motions, depending on the relationship between wavelength, barge length, and depth of water. This surge is usually too fast for thetensioner and the welder to follow. Thus, under low sea states, the pipe is locked in fixed

position in relation to the barge. Therefore, the tension in the pipeline varies cyclically aboutthe steady-state force. Typical ranges of variation are of the order of 100 kN each way in amoderate sea. Heave and pitch also have some effect on the tension, but generally to a muchlesser degree than surge. This tension must also be introduced and maintained during thestartup and lay-down of the pipe. The skill of the welders is critical to the operation. They areworking on a rolling and heaving barge, often with a moving joint, yet must produceessentially perfect welds. If the X-ray discovers a flaw in the weld, the resultant cutout repairstops the entire operation until it is completed.

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The actual performance of the welds is also of serious concern to the pipeline installationcontractor, because of the responsibility to ensure a sound, leak-free pipe on completion. Thecombination of axial tension and overbend stresses on the weld are very severe, especiallysince the latter is dynamic. Not only the toughness of the weld itself is involved, but also thatof the heat-affected zone (HAZ), which in turn is influenced by the parent steel quality as wellas the welding procedures. The constructor may therefore find it prudent to test the pipe steel

and welding procedures under dynamic tension loads prior to finalizing procedures.In a typical offshore operation, the barge will move one pipe length every fifteen minutes.On the most modern, using advanced welding techniques and double- or triple-ended pipe

joints, rates of a mile per day are achieved. This means that all the work must be completed ateach station within that same time frame. This translates to 100 or more meters lengths pertwenty-four-hour day. These performances have been exceeded by topnotch crews on gooddays, even with manual welding.

Stresses in the pipe in the laying operation are controlled not only by axial tension butalso by the net submerged weight of the pipe. This latter is the difference between two largenumbers, the one being the air weight of the pipe, the other being the buoyancy due to thedisplaced volume. The major variable is the thickness of mortar coating, which affects bothair weight and displacement, but not equally. In a typical case, a pipe may have an air weightof 15 kN/m and a displacement of 14.3 kN/m leaving a net (buoyant) weight of 0.7 kN/m. Ifthe coating increases the weight by 5%, the displacement may increase by only 2%. Thesenumbers may sound small, but they develop an increase in net buoyant weight of 0.45 kN/mor a 60% increase in the force causing the bending. Thus, while weight control is normally notas critical with lay barge operations as it is with bottom pulls, it nevertheless is of greatimportance and must be monitored.

The pipe is generally furnished in double-random lengths, which are normally 12 m.Most sections will run 11.4 – 12.6 m.

As previously noted, rates of progress with modern generation lay barges may reach onemile per day or more. This means that one hundred or more sections must be loaded out eachday from the shore base, transported to the site, and then unloaded to the deck of the barge.This last is a critical operation when the seas are running high and may, along with anchorhandling, be the controlling operation. The transfer at sea of the pipe is a typical case ofoperations involving two vessels alongside each other, of different characteristics, eachresponding in its own way to the seas, in each of six degrees of freedom. The relative

positions in plan can usually be maintained in a moderate sea state by tying the transport barge alongside the lay barge, with suitable fendering, so that the major individual responsesare limited to heave, roll, and pitch. In heavier sea states, barges can no longer be keptalongside, and so supply boats are used. By running a line from the boat and keeping poweron, a good skipper can hold the boat in reasonably close position, although now the boat willdevelop some relative sway, surge, and yaw motions as well as heave, pitch, and roll.

The typical lay barge is restrained from lateral motion by the mooring lines; it is alsomoved periodically one pipe length ahead. These lines, while catenary in scope in deep water,are kept under tension by the winches. The line tensions are measured by tension meters onthe wire rope or on the winch drum or both. In the typical lay barge, the tension may be 400kN with some variance. The mooring lines must provide the horizontal and longitudinalrestraint against wave drift, wind drift, and current drift. They also react against one anotherand especially must counter the tension on the pipe, which in effect is like a mooring line ofrelatively equal tension, leading directly astern. Balancing out the tensions in eight to twelvemooring lines plus one pipeline is a complex problem, especially when these line forces arenot steady but subject to the significant ranges introduced by the long-period excursions.

Typically, the tensions in the mooring lines are set so that under the maximum designsurges, the force will not exceed 50% – 60% of the guaranteed minimum breaking strength. Tooffset the pipeline tension requires additional mooring line forces in the lines leading forward.

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Lecture No 08

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