L4-Platform Installation [Compatibility Mode]

Lecture 4Lecture 4Platform Installation

Arun S ChandelAssistant [email protected]

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Project PhasesA steel jacket installation usually consists of the followingproject phases:Loadout - Comprises the movement of the completedt t t th b hi h ill t t it ff hstructure onto the barge which will transport it offshore.

Seafastening - Comprises fitting and welding sufficientstructure between the structure and the barge to prevent thestructure between the structure and the barge to prevent thejacket shifting during transit to the offshore site.

Offshore Transportation - Comprises the tow to the locationOffshore Transportation Comprises the tow to the locationoffshore and arrival of the barge at the offshore site with theseafastened structure.

Installation - Comprises the series of activities required toplace the structure in the final offshore location. Theseactivities include jacket lift and upending, positioning, pile

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installation, jacket levelling and grouting, together withsupport services for these activities.

Installation PlanningConsists of loading out, seafastening and transporting the structure

to the installation site, positioning the jacket on the site and achievinga stable structure in accordance with the design drawings anda stable structure in accordance with the design drawings andspecifications, in anticipation of installation of the platform topsides.

Avoidable of unexpected risk during offshore activities from loadoutt l tf i t ll tito platform installation.

Contractor is obliged to produce procedures for these activitieswhich demonstrate that the risk of failure has been reduced toacceptable levels. Also required to demonstrate that, prior to thecommencement of an activity, all the necessary preparations havebeen completed.

The plan will include the method and procedures developed for theloadout, seafastening and transportation and for the completeinstallation of the jacket, piles, superstructure and equipment.

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Installation Planning

Installation drawings, specifications and procedures must beprepared showing all the pertinent information necessary for

t ti f th t t l f ilit l ti t Th d iconstruction of the total facility on location at sea. These drawingstypically include details of all inspection aids such as lifting eyes,launch runners or trusses, jacking brackets, stabbing points, etc.

The engineering input into an offshore installation project alsoinvolves the design of all temporary bracing, seafastenings, rigging,slings, shackles and installation aids, etc. These must be designed inaccordance with an approved offshore design codeaccordance with an approved offshore design code.

Quality management is a vital and integral component of all offshoreinstallation projects.

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PROJECTPROJECT

PHASESPHASES

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Loadout

Loadout entails the movement of the completed structure onto thebarge which will transport it offshore. The jacket must be loaded insuch a manner that the barge is in a balanced and stable condition.

For load-out three basic methods are applied:

1. Skidding2. Platform trailers3. Shearlegs

1 Skidding1. SkiddingSkidding is a method feasible for items of any weight. The systemconsists of a series of steel beams, acting as track, on which a groupof skids with each approximately 6 MN load capacity is arranged. Each

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skid is provided with a hydraulic jack to control the reaction.

Loadout

2. Platform TrailersSpecialized trailerunits (see Figure 10)can be combined toact as one unit forloads up to 60 - 75loads up to 60 75MN. The wheels areindividually suspendedand integrated jacksll dj t tallow adjustment up

to 300 mm.

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Loadout

3. ShearlegsLoad-out by shearlegs is attractive for small jackets built on the

S ll d k ( t 10 12 MN) b l d d t thquay. Smaller decks (up to 10 - 12 MN) can be loaded out on thedecklegs pre-positioned on the barge, thus allowing deck anddeckleg to be installed in one lift offshore.

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Loadout Checklist

1. Is the jacket complete? Has the structure been analysedfor loadout stresses on the basis of the actual structure aso oadou s esses o e bas s o e ac ua s uc u e asfabricated at the time of loadout?

2. Is the launch barge securely moored to the loadout dock,so that it won't move out during the loading? Is the bargeproperly moored against sideways movement?

3 If i t t d b t th b kid3. If compression struts are used between the barge skidways and those on shore, are they accurately aligned andsupported so they won't kick out during loadout? Have thepull lines shackles and pad eyes been inspected to ensurepull lines, shackles, and pad eyes been inspected to ensurethey are properly installed and can't foul during loadout?

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Loadout Checklist4. Can the barge be properly ballasted? If the tide will vary

during loadout, are ballasting arrangements adequate? Willballast be adjusted as the weight of the jacket goes ontoballast be adjusted as the weight of the jacket goes ontothe barge? Are there proper controls? Is there an adequatestandby ballast system? Are there back-up systems to pullthe jacket back to shore if anything goes wrong duringthe jacket back to shore if anything goes wrong duringloadout? If the ballast correction is to be made iteratively,step-by-step as the jacket is loaded, are there clear paintmarks so that each step can be clearly identified?p y

5. Have clear lines of supervision and control beenestablished? Are the voice radio channels checked? Havethe marine surveyors been notified so that they can bepresent? Owner's representatives? Certifying Authority?Have their approvals been received?

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Loadout ForcesThese are forces generated when the jacket is loaded from theThese are forces generated when the jacket is loaded from the

fabrication yard onto the barge.

When loadout is done by skidding the structure onto the barge, anumber of static loading conditions must be considered, with thejacket supported on its side. Such loading conditions arise from thedifferent positions of the jacket during the loadout phases, (asshown in Figure) from movement of the barge due to tidalshown in Figure), from movement of the barge due to tidalfluctuations, marine traffic or change of draft, and from possiblesupport settlements.

S f h k l ll l d dSince movement of the jacket is slow, all loading conditions can be taken as static.

Typical values of friction coefficients for calculation of skidding forces :Typical values of friction coefficients for calculation of skidding forces :

Steel on steel without lubrication : 0.25 Steel on steel with lubrication: 0.15

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Steel on steel with lubrication: 0.15 Steel on teflon: 0.10 Teflon on teflon: 0.08

Loadout Forces

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Loadout

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Loadout

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SeafasteningSeafastenings are installed after loadout and must be completed

prior to sailaway. They are major structural systems, subjected toboth static and dynamic loads.

When the barge is on the high seas it must be assumed that it canencounter conditions which are "as bad as could have beenstatistically foreseen". Accordingly, the gravity and inertial forcesinvolved must be calculated for all anticipated barge accelerationsand angles of roll and pitch during the design sea conditions adoptedfor the tow.

Seafastenings should be attached to the jacket only at locationsapproved by the designer.

Sh ld b tt h d t th b t l ti hi h bl fShould be attached to the barge at locations which are capable ofdistributing the load to the barge internal framing.

Should also be designed to facilitate easy removal on location.

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ou d a o b d g d o a a a y o a o o a oSeafastenings are normally subject to the same code requirementsfor fabrication as the jacket.

Seafastening

The module requires fixing to the barge (see the barge (see

Figure 9) to withstand barge motions in rough

Th seas. The sea fastening concept is determined by the

positions of the pos t o s o t eframing in the

module as well as of the "hard points"

in the barge

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in the barge.

Offshore TransportationThe transportation of heavy components from a fabrication yard to

the offshore site is a critical activity.

It i i ll i th f th j k t i th b h i fIt is especially so in the case of the jacket since the behaviour ofthe unit usually influences the verification of barge strength, thedesign of seafastenings, and indeed the design of the jacket itself.

There are the practical aspects of tug selection, tow route, etc. tobe considered.

Tug selection involves such considerations as length of tow routeTug selection involves such considerations as length of tow route,proximity of safe harbours and the anticipated weather conditionsand sea states. As a minimum the tugs should be capable ofmaintaining station in a 15 metre/second wind with accompanyingg p y gwaves.

The behaviour of the jacket seafastened to the barge must besatisfactory both from the point of view of static and dynamic

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satisfactory both from the point of view of static and dynamicstability. Both are verified by means of numerical analyses.

Offshore Transportation

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Transportation ForcesThese forces are generated when platform components (jacket,

deck) are transported offshore on barges or self-floating. Theydepend upon the weight, geometry and support conditions of thestructure (by barge or by buoyancy) and also on the environmentalstructure (by barge or by buoyancy) and also on the environmentalconditions (waves, winds and currents) that are encountered duringtransportation.

In order to minimize the associated risks and secure safe transportfrom the fabrication yard to the platform site, it is important to planthe operation carefully by considering, according to API-RP2A [3], thefollowing:following:

Previous experience along the tow routeExposure time and reliability of predicted "weather windows"A ibilit f f hAccessibility of safe havensSeasonal weather systemAppropriate return period for determining design wind, wave and

current conditions, taking into account characteristics of the tow such

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, gas size, structure, sensitivity and cost.

Jacket & Barge Motions

In considering the motions of the jacket and barge it is intuitivelyplausible that roll will be the most problematical motion (from thepoint of view of body accelerations) and that the largest roll will bep y ) gcaused by a beam sea.

It may be less obvious, but nevertheless true, that if the bargewidth and to a lesser extent the length are reduced the roll willwidth and, to a lesser extent the length, are reduced, the roll willdiminish and if the barge is set at a (much) deeper draft, the roll willalso diminish.

All of these considerations reflect static properties of the jacketand barge. Improvements can occasionally be made by choosing anarrower barge (although obviously stability will suffer) orincreasing the draft (although in this case stability may again sufferincreasing the draft (although in this case stability may again sufferand parts of the structure which were previously 'dry' may now besubjected to 'slamming').

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Incorrect "balancing" of these aspects can have very seriouscost/risk implications in overall project terms.

BargeBarge

MotionsMotions

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Offshore InstallationThe jacket must be removed from the transportation barge. There aretwo basic methods used:

LAUNCH

LIFT

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LAUNCHThe launch site is normally at or near the installation location.

Steps

1. Immediately prior to launch, the seafastening securing the jacketto the barge is cut. The jacket is then pulled along the barge skidways (which were used for loadout) by winchesways (which were used for loadout) by winches.

2. As the jacket moves towards the stern of the barge, the bargestart to tilt and a point is reached when the jacket is self sliding.An initial tilt to the barge may have been provided by ballastingimmediately prior to launch. A stern trim of approximately 5° isusually aimed for.

3. The skid ways terminate in rocker arms at the stern of the barge.As the jacket moves along the skid ways its centre of gravityreaches a point where it is vertically above the rocker arm pivot.

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Further movement causes the rocker arm and jacket to rotate. The

jacket will then slide under its own self weight into the water.

Steps

4. Once in the water the self floating jacket is brought under controlwith lines from tugs and/or the installation vessel.

5. Once launched the jacket must float with a reserve of buoyancy inorder to stop the downward momentum of the jacket. Thisrequires the jacket to be water tight.

Jacket Launch Naval AnalysisA jacket launch naval analysis is required in order to:

• Ensure that an adequate sliding velocity is maintained during therocker arm rotation;• Verify that the trajectory followed has a safe seabed clearance;• Determine the jacket behaviour during launch;• Define operational requirements during launch, including ballastconfiguration;• Check the stability of the jacket during launch and when free floating

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• Check the stability of the jacket during launch and when free floating.

Offshore Installation by Launching

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LIFT

An increasing number of jackets are being installed by direct lift.This trend has been encouraged by the availability of large cranevessels such as the Micoperi 7000vessels such as the Micoperi 7000.

In a direct lift the jacket is lifted off the barge completely in air. Asecond form of lift is the buoyancy assisted lift. In this case they ybarge is flooded and hence submerged. This results in part of thejacket being buoyant, reducing hook loads. Buoyancy tanks may beadded to the jacket if required.

Shallow water jackets may be lifted in the vertical position. In thiscase no up-ending is required and installation is straight forward.

Deep water jackets will in general be lifted on their side. Twocranes will normally be used, noting that large derrick barges suchas the Micoperi 7000 are fitted with two cranes as standard.

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LIFTWhen considering a tandem lift it should be noted that it is

unlikely that both hooks will carry the same load, and that themaximum permissible jacket weight will be less than the sum of thet ititwo crane capacities.

When the jacket is to be removed from the transportation bargeby lifting, it is normal for the installation vessel to be correctlyy g, ymoored and positioned so that up-ending and set-down may proceedas one integral lift operation.

The selection of a suitable installation vessel is clearly essential InThe selection of a suitable installation vessel is clearly essential. Inaddition to lift capacity, it is also necessary to consider stability andmotion response characteristics. The large semi-submersible cranevessels used in the North Sea have full dynamic positioning systemsfor locating themselves on site. They also have sophisticatedcomputer controlled ballast systems to keep the vessel level duringlifting operations. In the harsh North Sea environments installationvessels are usually semi-submersibles such as the Micoperi 7000

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vessels are usually semi-submersibles such as the Micoperi 7000.

Lifting Steps1. The first stages in lifting a jacket from the transportation barge

involve positioning the barge and connecting the slings to thehook. The barge will normally be controlled by tugs.

2. Once everything is ready for lift to proceed the seafastenings willbe cut.

3. The next stage is to transfer the weight of the jacket from thebarge to the crane. The general requirement here is to lift asrapidly as possible. However, careful control and phasing withbarge and crane vessel motions is required in order to ensure thatbarge and crane vessel motions is required in order to ensure thatonce the jacket is lifted clear of the barge it does not hit the bargeas a subsequent wave passes through.

4. The same lift procedure is adopted in both a direct and buoyancyassisted lift.

5 Once the jacket is lifted clear of the barge the barge is removed

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5. Once the jacket is lifted clear of the barge, the barge is removedby tugs. Up-ending of the jacket will then normally proceeddirectly.

OffshoreOffshore 

InstallationInstallation

by Liftingby Lifting

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Up‐ending 

&& 

Set‐DownSet Down

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Up‐ending by ballast control & FloodingA large crane vessel will not normally be required for either aA large crane vessel will not normally be required for either alaunched or self-floating jacket. Upending is therefore achieved bycontrolled flooding. A small installation vessel will usually berequired for the installation of piles once the jacket has been set-down, so this is used as the platform from which to control thevarious flooding operations. This installation vessel will also be usedto help position the jacket.

Steps1. In step 1 the waterline compartments at one end of the jacket

are flooded.2 M li k fl d d I 2 il b 3 h2. More water line tanks are flooded In step 2 until by step 3 the

upper frame of the jacket reaches waterline and may also beflooded.

3. The jacket is then allowed to rotate until all legs are equally3. The jacket is then allowed to rotate until all legs are equallyflooded as in step 4.

4. The jackets natural position will then be floating upright as instep 5.

5 Further flooding of the jacket as in step 6 will enable the jacket to

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5. Further flooding of the jacket as in step 6 will enable the jacket tobe lowered onto the sea bed in a controlled manner.

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Up‐ending using Crane vessel

A horizontally lifted jacket may be upended in one of two ways:

1 Perhaps the most straight forward is to lower the jacket into1. Perhaps the most straight forward is to lower the jacket intothe water so that it floats. Slings can then be removed andnew slings attached at the top of the jacket. The jacket maythen be up-ended using the same steps as in Lauching Thisthen be up ended using the same steps as in Lauching. Thismay require closures to legs and some additional buoyancy.

2. A second method is to up-end directly, as done in Lifting.p y, gThis requires special padears so that the necessary rotationbetween slings and jacket can occur. Careful naval analysis isalso required in order to carefully determine hook loads andto ensure that the jacket remains stable. Once up-ended thejacket can be set-down on the sea-bed. Since the liftingpoints are submerged divers may be required to disconnectth li f th j k t

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the slings from the jacket.

On‐bottom Stability

It is necessary for the jacket to be stable and level duringpiling. A separate on-bottom stability analysis is thereforecarried out Three conditions need to be met:carried out. Three conditions need to be met:

(1)vertical resistance to jacket weight and piling loads;

(2) stability against sliding under wave/current loading;

(3) stability against overturning under wave/current loading.( ) y g g / g

In carrying out the above analyses it is necessary to use anappropriate sea-state to generate hydro-dynamic loading.This should be the maximum statistical wave which may occurprior to piling being completed. Assuming installation to occurin the summer months, a typical criteria may be a 1 year

t

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summer storm wave.

Installation

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Installation

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