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17

Topside Installation

17.1 General

Over the past years, almost all topside facilities have been first fabricated into modules

and then transported by barge and set on the platform by an offshore derrick barge. Thecapacity of offshore derrick barges has steadily grown to where 1200-tn. modules arecommonplace and individual lifts of 400011,000 tn. and more have been made.

A further extension of this trend to prefabrication has been the innovative developmentand application of the Float-Over System, in which a complete deck, with all its equipmentand facilities installed, has been floated over a jacket and set down. So far, this has beenrestricted to relatively calm seas, such as the Arabian-Persian Gulf, offshore West Africaand in the Timor and Philippine Seas.

The purpose of using large modules is to enable more of the fit-up and testing to becompleted at the shore site. Not only does this allow the work to be done under optimal

conditions, it also disperses the work so that it can be accomplished concurrently withother modules and other structural work.

17.2 Module Erection

The modules are set by a crane barge onto the module support frame, which is a skeleto-nized deck structure (seeFigure 17.1). Some will be set onto skid beams and skidded andjacked to final position; others may be set directly. The modules must be structurallyadequate in themselves, both for the temporary loads imposed during transport andinstallation and for the permanent loads due to the operations and environment. Thestructure of each module must first support the vessels and the piping within it andthen transfer the forces developed by dead and live loads and environmental loads toother modules or the module support frame.

Lifting of such extreme loads must follow the general principles of heavy offshore liftsoutlined inSection 6.3and must be thoroughly engineered for all stages of the operation.Picking points and padeyes must transfer the forces to the slings. The slings, with theirangles in three-dimensional space, must in turn transfer the loads to the hook. Wheremore than one crane will be involved in the lift, the interaction of loads between thecranes must be considered, including the effect of tolerances in boom position, sea-

induced motion and the change in the derrick barges water planes as the load comesonto them.

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When picking a module with multiple pickup points, deflections must be carefullycontrolled so as not to distort the equipment and piping. Very elaborate rigging systemsoften result. The supporting structure may have to be stiffened.

Picking loads are dynamic; adequate allowance must be made for dynamic amplifi-cation in lifting force, as well as in lateral swing. This latter can be greatly reduced bypower-controlled tag lines. Low-temperature effects, possibly causing embrittlementunder impact loads, need to be addressed and suitable steels and welding procedures

adopted. Many modern heavy lifts of modules are assisted by onboard computers moni-toring the loads, the radii, and the position of the boom. Some crane barges are equippedwith boom tip motion sensors and onboard computer systems to determine the best head-ings and boom angles to minimize boom-tip motion.

Modules are usually loaded onto a barge at a shipyard or shore base by skidding out,much as a jacket is loaded out. Dimensions are smaller and total weight much less, butloads may be more concentrated. Alternatively, they are loaded by transporters. Themodules must then be properly tied down for sea.

Engineered slings are pre-attached to each module so that all that remains to be done asthe lift commences is to raise each sling up over the hook by means of the crane whip line.Meanwhile, the tie-downs are cut loose.

When sea conditions appear favorable, the module is lifted clear of the barge, slowlymoved astern or rotated to position, and set in its place (see Figure 14.3). Auxiliarymeans, such as powered tag lines on the deck of the platform, tapered guides, andfenders are used to help seat the module in correct position. The module needs to be set

FIGURE 17.1

Large module being lifted onto dock of plat-form. (Courtesy of Aker Marine.)

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down smoothly and quickly so as not to expose the operation to higher waves or low-cyclefatigue. It is desirable to incorporate tolerance in initial positioning into the structuraldesign. Once the unit is set, jacks can then move it to its final exact position. Jackingpoints should be provided in the module frame.

The problem of overhaul during set-downthat is, of getting rid of a load from the hookwhen there are up to 24 or more parts of line in the hoist blocksis a difficult one. A freeoverhaul clutch for the crane hoist is the primary solution. In some cases, ballast may betransferred to the stern of the crane barge as soon as the load touches down. The object is toprevent the load from being inadvertently lifted back up if a subsequent wave raises thederrick barges stern before there is enough slack in the falls.

Some rather spectacular lifts of modules have been made by the use of two or even threecrane barges working in concert. The three Statfjord A quarters modules were too high (40 m)to be lifted by a single crane. The weight of each, about 1000 tn., was not too unusual, but theheight and profile required that three crane barges be used. These were moored together withall deck winch controls and dynamic-positioning thruster controls at one control location.

The three barges picked up the module at the dock, transported it to the concrete gravityplatform moored in the fjord, and repositioned the barges whilecarrying the load. At the finallifting site, the barges were moored to the structure. Because of the short length of lines, nylonrope was used in order to have some elastic stretch to accommodate surge as the quartersmodules were then raised and set on skid beams mounted on the deck frame. Each modulewas then skidded sideways to its final position. The pick had to be engineered with extremecare, since the load exceeded the capacity of any one of the crane barges.

On a subsequent platform, similarly high and heavy modules were set by a semisub-mersible crane barge. Since this work was carried out in a fjord, the semisubmersible wasnot selected for minimal response to seas but rather for its extreme height when debal-

lasted to ride on its pontoons. Now it was able to lift the module over the deck structureand to set the module directly in place. Up on the platform deck, each module was thenwelded to the module support frame.

A typical series of modules will include the following:

Utilities modules

Control room module

Quarters modules

Helideck

Wellhead module

Separation module

Dehydration module

Pig-launching module

Generator module

Switchgear module

Metering module

Bulk storage modules

Pedestal cranes

Drilling modules Drilling derrick

Flare stack

Casing and drill string laydown racks.

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17.3 Hookup

The hookup of these modules and their subsequent testing is highly demanding in terms

of both manpower and support (see Figure 17.2). It delays the start of production of oil orgas and hence adversely affects cash flow. In recent years, the complexities of hookup haveled to overruns in cost and time of 100% or more. To reduce these problems, the first step isto use larger and fewer modulesthat is, more self-contained modules. A second step is tospace the modules apart by 1 m or so, to allow a crawl space for access for interconnection.A third step is to use flexible connections to the extent permissible for the high operatingpressures in the pipeline connections.

Careful control in tolerances of all interconnecting points at the time of module fabrica-tion is essential. Templates and pre-matching may be used to ensure compatibility.

Hook-up offshore is very demanding of personnel and very costly. If the hookup work is

supported by a semisubmersible derrick or floatel, a suitable gangplank or walkway isrequired. This must have rollers to accommodate surge of the semisubmersible. It must besupported, for example, by cantilevering, so that it will not fall even if the barge drags ananchor or parts a mooring line. Sophisticated articulation and hydraulic compensators areoften required to accommodate the barges relative motions.

FIGURE 17.2Hook-up of topsides of Condeep Platform.(Courtesy of Aker Maritime.)



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expansion must be considered. Finally, means must be developed to equalize the loadbetween the four or more legs after the deck support has been transferred.

The transfer in the open sea, subject to long-period swells, is a much more complexproblem than that performed in inland waters. In most cases, the transfer operation first

positions the barge between or around the legs (shafts) and dampens out the relativemotions. During this phase, the legs of the jacket must be protected against impact dueto surge and sway of the barge. Then, the lowering process has to consider the impact inheave, as amplified by pitch and roll. Even more critical is the rapid removal of contact ofbarge and deck, so that the barge does not strike the structure in the period immediatelyfollowing transfer. Several methods have been developed and successfully used to carryout a float-over installation in the open sea.

The Float-Over method requires careful consideration of the prevalent waves and swell,not only as to height and period but also as to direction. The range of the tide can be bothfavorable and unfavorable.

Shock loads are developed as the load is transferred to the jacket legs. Shock loads canalso occur once transfer is completed, the swell raising the barge to impact the deck. Rapidremoval of potential contact is essential (see Figure 17.4).

High capacity ballast and deballast pumps should be provided.The barge, with deck loaded, has to have transverse stability but be narrow enough to fit

between the jacket legs. Stability and longitudinal strength must be adequate for bothtransport and at the critical stage when the deck is lifted above the barge.

Lateral impact against the jacket legs is restricted by polyester lines (or a combination ofnylon and polyester) attached to winches on the barge. Fenders are fixed to the jacket legs.

For a gas platform in the Eastern Timor Sea, 8 shock-absorbing leg-mating units wereinstalled on each of two platforms. Each had a capacity of 2000 tn., with a deflection of800 m. Smaller deck-supporting units, to cushion the deck against the heave of the barge,were also employed. The integrated decks weighed up to 13,900 tn. each.

Larger shock-absorbing units, with up to 10,000 tn. vertical capacity, are currently underdevelopment for use in mounting the decks on concrete GBS platforms.

FIGURE 17.3Integrated deck of Statfjord C platform.



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17.5.2 Hi-Deck Method

The Hi-Deck method has been used to install several completely outfitted decks in theNorth Sea, including those for platform Maureen and the Hutton TLP. The deck structurewas transported on a steel frame 17 m high, supported on the barge to clear the top of thejacket legs. The barge carrying the deck was maneuvered in between the legs. Compositemooring lines of steel wire and polyester-sheathed aramid fiber (Kevlar) were run to theplatform legs, in order to dampen out relative motion in the horizontal plane. Large rubberfenders were secured to the jacket legs. During mating of the Hutton deck, relative motionwas limited in design to 200 mm but in the actuality, only 60 mm relative displacementwas experienced.

Vertical lowering was by rapid ballasting. Several separate shock-absorber systems were

installed to absorb the impact as the hydraulic catch probes engaged the cones in the deck.One system consisted of 1.5 m pillars of polyurethane enclosed in telescoping steelcasings. Another used hydraulic jacks, suitably softened by connecting the hydraulictanks to nitrogen-filled bladders. Model tests of relative motions during transfer provedmore accurate than elaborate computational analyses. The operation was successfullycarried out in 1.5 m swells.

17.5.3 French Smart System

The French Smart system has been successfully used for the installation of a deck in thelong-period swells offshore Congo. The deck structure is designed with vertical tubularsthat match the jacket legs of the platform on which the deck is to be installed. Smart legsare pipe columns within the deck tubulars. They are extended by long-stroke hydraulicjacks. Smart struts extend horizontally from the barge, also with hydraulic jacks. These areused to control the relative movement of the barge in relation to the jacket legs in surge,

FIGURE 17.4Deck being lowered onto mating cones of high-capacity shock absorber. (Courtesy of ConocoPhillips andClough-Aker.)



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sway, and yaw. Smart releases are doubly hinged supports which can be collapsed byactivating a hydraulic cylinder to rapidly give 34 m clearance between the barge and theunderside of the deck structure. Rapid removal of the barge is essential to prevent impacton the next heave cycle of the barge.

Operations proceeded by gradually moving the barge-plus-deck combination inbetween the jacket legs until the Smart legs are directly above their top. The Smartstruts engage the jacket legs and gradually dampen out the surge, sway, and yaw motion.Each strut is hinged to accommodate heave while the other end rolls on a reinforced padon the jacket leg. Then the Smart legs are extended down to engage the top of the jacketlegs. The load is gradually transferred. Then the Smart releases are tripped so that thebarge has clearance beneath the deck and can be extracted from between the jacket legs.These operations were successfully carried out in a 1m swell.

17.5.4 The Wandoo Platform

For the Wandoo platform, on the Northwest Shelf of Australia, a deck structure, fullyoutfitted, was successfully transferred by the float-over method. A heavily reinforcedbarge carrying the deck was maneuvered in between the concrete shafts of a GBS. Thedeck structure was supported high enough to clear the shafts. Composite mooring lines ofsteel wire and polyester-sheathed Kevlar were used to position the barge accurately. Thebarge and deck were then ballasted down onto hydraulic jacks set in pairs atop each of theshafts. To enable rapid release of vertical supports, large sand jacks were used. Theiropenings were sized, on the basis of tests, to empty in about 1 min. Then the barge hadample clearance under the deck underside and could be pulled clear.

17.5.5 Other Methods

Other methods of float-over are under development, including a large catamaranconstructed of vertical concrete caissons. The Versatruss method, used for theremoval of complete decks from decommissioned platforms, involves the use of inclinedsteel struts. It has been used to raise a deck which had been skidded onto a trestle, thentransport it across the Caribbean Sea to Venezuela and lower it down onto a pre-installed jacket.

The development of long-stroke hydraulic jacks will continue to be a major factor in theextension of the Float-Over concept. The Float-Over method was recently (2003) usedsuccessfully in the Philippines and has become the current state-of-the-art in the

Arabian Gulf. The successful extension of the Float-Over method in the open sea requiresincreasingly sophisticated engineering.

Or where the Northern Ocean, in vast whirls,Boils round the naked melancholy isles

Of farthest Thule, and th Atlantic surgePours in among the stormy

Hebrides.

James Thomson, In Autumn



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