otc 19072 fpso selection

8/12/2019 Otc 19072 FPSO Selection

1/9

Copyright 2007, Offshore Technology Conference

This paper was prepared for presentation at the 2007 Offshore Technology Conference held inHouston, Texas, U.S.A., 30 April3 May 2007.

This paper was selected for presentation by an OTC Program Committee following review ofinformation contained in an abstract submitted by the author(s). Contents of the paper, aspresented, have not been reviewed by the Offshore Technology Conference and are subject tocorrection by the author(s). The material, as presented, does not necessarily reflect anyposition of the Offshore Technology Conference, its officers, or members. Papers presented atOTC are subject to publication review by Sponsor Society Committees of the OffshoreTechnology Conference. Electronic reproduction, distribution, or storage of any part of thispaper for commercial purposes without the written consent of the Offshore TechnologyConference is prohibited. Permission to reproduce in print is restricted to an abstract of notmore than 300 words; illustrations may not be copied. The abstract must contain conspicuous

acknowledgment of where and by whom the paper was presented. Write Librarian, OTC, P.O.Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

AbstractThe float-over deck installation is in use as an effective

installation method since the eighties. The method hasdeveloped and the applicability has widened to heavier

integrated decks and harsher environments such as West-

Africa and West-Australia.

The paper describes the state of the art in float-over deckinstallations by an overview of a number of typical float-over

operations. Each of these float-overs has a typicality related

to:o Integrated deck weight;o Environment;o Installation concept.

The present state of the art has been reviewed in a SWOTanalysis from a contractors and operators perspective.

Strength, weaknesses, oppurtunities and threats have been

summarized to identify a way forward for futuredevelopments.

Possibilities for the way forward to achieve the following

objectives have been outlined in the paper:

o Improved workability;

o Reduced jacket slot requirements;o Standardization to reduce early commitment

requirements.

IntroductionInstalling an integrated deck onto a jacket structure is an

operation that has been executed by using semi submersible

crane vessels and derrick barges already for many years.

Offshore integration costs can be reduced since the integrated

deck weight can be over above the capacity of (locally)

available crane capacity.

The float-over deck installation is proving to be a competitive

alternative for such an offshore installation operation.

In recent years the concept of float-over deck installation has

matured. With the help of a lengthening track record and

benefits over lift operations, the float-over deck installation istaken into account as a reliable means of installing the assets.

Since the float-over deck installation has been used only for a

limited number of projects compared to the lifting operationsperformed by semi submersible crane vessel, improvements to

the concept are still to be expected.

Float-over Deck Installation: SequenceFor better understanding of the advantages and dis-advantagesof the float-over deck installation over semi submersible crane

vessels, a brief introduction to the float-over concept is

presented.

Load-Out

The load-out is the starting point of a float-over deck

installation. The integrated deck will be build on-shore and

needs to be loaded out onto the installation vessel.

Load-outs can be performed either by self propelled modular

trailers or by the use of skid tracks.Figure 1 presents a skidded load-out of a 15.000 tons module

onto self propelled installation vessel Blue Marlin.

Figure 1: Skidded load-out of 15.000 tons module

Requirements for the load-out stage are governed by thefollowing parameters:

OTC 19072

State of the Art in Float-OversMichel Seij and Henk de Groot, Dockwise Shipping


2/9

2 OTC 19072

o Integrated deck weight;o Tidal range;o Quayside dimensions.

Sea Transportation

After completion of the load-out, the integrated deck has to be

seafastened on board the installation vessel prior to

commencing sea transportation. One aspect of the transportthats always critical for a float-over transport is stability of

the vessel.

The stability is mainly driven by the width and depth of the

vessel:

o Increase in width results in increased initial stabilityand stability range;

o Increase in depth results in increased stability range.

An increase in initial stability results in a reduced roll period.

In general this results in higher acceleration levels andconsequently, increased seafastening loads.

An increase in vessel width results in an increased jacket slot

width requirement. This has unfavourable consequences for

the jacket design.

Therefore it can be concluded that the vessel resulting inoptimum jacket and seafastening design is the vessel having

the minimum width resulting in compliance to stability

requirements.

Float-over Stand-off

After completion of the transit, the vessel needs final

preparations prior to commencement of the actual dockingoperation of the vessel.

During this stage the following preparatory works need to be

executed:

o Cutting/removal of seafastenings;o Start-up of mooring/docking/mating winches;o Start-up of equipment for monitoring motions,

weather etc.;o Start-up of active load-transfer system (if any);o Pre-ballasting of vessel.

For these preparations the vessel needs to be moored at a

stand-off location. The mooring spread for the vessel will be

dependent on the field lay-out and the design environmental

conditions for this stage of the operation.

Docking of Installation Vessel

Upon completion of the preparatory works, the docking

operation of the vessel can commence. During this phase the

vessel is moved into the jacket and transferred from the stand-off location to the correct location in the jacket.

During this phase the following needs to be safeguarded:o Alignment of vessel stern with jacket slot. For this

purpose a guide structure can be attached to the vessel

stern as presented in Figure 2;

o Lateral impact loads on the jacket not to exceed limitloads of jacket and fendering arrangement;

o No vertical impact loads between deck legs and jackejacket legs;

o Control over the movement of the vessel inlongitudinal and transverse direction as well as contro

over the alignment of the vessel.

Figure 2: Stern entry fender on self propelled installation vesseBlue Marlin

Pre-mating Position of the Installation Vessel

Once the vessel is docked, the deck legs need to be aligned

with the jacket legs. The tolerance for this alignment is to a

high extend driven by the diameter of the stabbing cones.

During this stage the clearance between the deck legs and the

jacket legs will be reduced by ballasting the installation vessel

The following aspects need to be taken into account:

o Limited lateral movement of the vessel relative to thejacket to ensure alignment of deck legs and jacke

legs;o Lateral impact loads on the jacket not to exceed limit

loads of jacket and fendering arrangement;

o Vertical impact loads on the jacket not to exceed limiloads of jacket and LMU design.

Mating of Integrated Deck to Jacket

During this stage the installation of the integrated deck on thejacket will be accomplished. The load of the deck will be

transferred from 100% support on the installation vessel to100% support on the jacket legs.

The transfer of the deck weight can be achieved by a varietyof methods such as ballasting of the installation vessel or

active hydraulics in the deck supports.

Post-Mating Position of Installation Vessel

Once load transfer has been completed, there will still be

impact loads between the module and the deck support. As

long as these impact loads occur, un-docking of the vessel isnot feasible. Therefore the clearance between the module and


3/9

OTC 19072 3

the deck support needs to be increased by continuing the

ballasting of the installation vessel.

During this ballasting operation the following issues need to

be taken into account:

o Limited lateral movement of the vessel relative to thejacket to ensure alignment of integrated deck and

supports on the vessel;o Lateral impact loads on the jacket not to exceed limit

loads of jacket and fendering arrangement;o Vertical impact loads on the deck support not to

exceed limit loads of module and DSU design.

Un-Docking of Installation Vessel

After the completion of the ballasting operations to increase

the clearance between the deck supports and the integrated

deck, the vessel can be un-docked from the jacket.

During this phase same issues as for the docking operationneed to be taken into account:

During this phase the following needs to be safeguarded:

o Lateral impact loads on the jacket not to exceed limitloads of jacket and fendering arrangement;

o No vertical impact loads between deck support andintegrated deck;

o Control over the movement of the vessel inlongitudinal and transverse direction as well as control

over the alignment of the vessel;

o Clearance between bottom of installation vessel andjacket bracings;

o Sufficient freeboard of installation vessel during un-docking.

Historical Project ExecutionFloat-over deck installations have become more and more

common in recent years. In the eighties only about 5 float-

overs had been executed, while nowadays about 5 float-oversare executed each year.

The capabilities have developed such that they are competitiveto semi submersible crane vessels from two perspectives:

o Environmental conditions: Both semi submersiblecrane vessels and float-overs have now stringent waveheight restrictions. Especially swell conditions are still

problematic for both installation methods;

o Integrated deck weight: Semi submersible crane

vessels are available having lifting capacities up to14.000 tons, while float-overs have been executed upto 18.000 tons.

Some illustrative projects showing the possibilities of float-

over deck installations have been presented below:

EAP GN-Deck Mobil Producing Nigeria

The GN-deck is part of the East Area Project offshore Nigeria.At 18.000 tons, the module is the heaviest installed in West-

African swell conditions using an active load transfer system.

The float-over has been executed early November 2005. This

date fittted in the West-African installation season running

from early November to end of March.

For the float-over Technip used the UNIDECK system, an

active hydraulic system to achieve an initial load transfer in a

time span of only one minute. The system is also used to

achieve an instant gap after load transfer is completed. Theload transfer sequence is presented in

Figure 3.

The 42.00 m wide selfpropelled installation vessel Black

Marlin has been used for the installation of the module. The

vessel carrying the GN-deck is presented in

Figure 4.

Lun-A Sakhalin

Kikeh

West Australia

Figure 3: Load transfer sequence for Technip UNIDECK system

Figure 4: Selfpropelled installation vessel Black Marlin prior toentering the East Area Project GN-jacket

Lunskoye-A Sakhalin Energy

At 21.800 tons, the Lunskoye-A module is the heaviest

module installed by the float-over installation concept. The

platform was installed in the Sea of Okhotks onto a concretegravity base structure.

In addition to the record weight of the deck, the installation

height was a challenge. The module had to be supported on avery high support frame in order to meet the installation heigh

requirements, resulting in a very high vertical centre of gravity

of the barge carrying the module.


4/9

4 OTC 19072

In order to provide sufficient stability when carrying the heavy

module on the high support frame, a T-shaped barge had beencustom built for this project. The module transported on the

custom built T-shaped barge is presented in Figure 5. The

module during the actual float-over is presented in

Figure 5: Lunskoye-A module transported on T-shaped barge

Figure 6: Lunskoye-A module during float-over on CGBS

EPKE Nigerian National Petroleum Company

The installation of the 4100 tons EPKE module has beenexecuted in 1997 in West-African swell conditions offshore

Nigeria. The float-over has been executed using the ETPM

SMART LEG active load transfer system.

The active load transfer system initiates first contact between

the deck legs and the jacket legs by activating hydraulic jacks

accomodated in the deck legs. These jacks are presented in

Figure 7.

By locking these hydraulic jacks when the installation vesseis on top of the wave, a smooth initial load transfer from the

vessel to the jacket is accomplished.

When the load transfer is close to completion, deck supports

will be instantly removed by using explosives. By removingthese deck supports, two objectives are achieved:

o Instant completion of load transfer;o Instant clearance between installation vessel and

module.

The active deck supports have been presented in Figure 8.

Figure 7: Active hydraulics accomodated in deck legs

Figure 8: Active deck supports on installation vessel

Figure 9: Load transfer sequence for ETPM SMARTLEG system


5/9

OTC 19072 5

Kikeh SPAR Murphy Oil

For the Kikeh SPAR the integrated deck has been installed inopen water using a catamaran float-over concept. The weight

of the deck was about 3200 tons. The operation has been

executed by Technip and OKI in the Malaysian deep water

Kikeh field in November 2006.

For a catamaran float-over, the integrated deck will be loaded

out onto two barges positioned next to each other. A clearancebetween the barges is maintained to fit the permanent support

structure in between. In Figure 10 the support arrangement by

two barges is shown.

Figure 10: Support arrangement of integrated deck on two barges

Figure 11: Float-over on a SPAR

The design of a SPAR does not allow to implement a slot for

the installation vessel in the design and as such an alternative

to a single hull installation vessel had to be used. The

catamaran float-over allows a float-over installation with

minimum modifications to the floater. The fit of the SPAR

between the two catamaran barges is presented in Figure 11.

The concept has the potential to be applied to fixed jackets inopen water as well allowing more flexibility in the design of

the jacket.

Strengths, Weaknesses, Oppurtunities and ThreatsTo further develop the capabilities of float-over deck

installations, one has to know the direction of improvementDefining the present advantages, dis-advantages and marke

developments is an essential starting point.

Strengths

The following strengths of a float-over deck installation can beidentified:

Reduced schedule interf aces:For a float-over, only one asse

is required which is able to perform both the sea transportation

and the float-over deck installation. Risks due to alignment of

schedule of transportation vessel and semi submersible cranevessel are avoided.

I nstall ation vessel avail abili ty: The number of sem

submersible crane vessels capable of lifting heavy topsides is

limited. The number of barges and vessels outnumbers theavailable semi submersible crane vessels and provides the

operator more flexibility from a contractor selection point ofview.

Capacity: The single lift capacity of float-over deck

installation exceeds the maximum capacity of the semi

submersible crane vessels. This makes the need for multiplelifts for big production platforms obsolete and gives the

operator more flexibility from an installation weight

perspective.

Reduced off shore hook-up and commisioni ng:The capability

to install heavy platforms as a single lift instead of multiple

lifts, reduces the time required to execute offshore hook-upand commissioning. This strength is only applicable to

platform sizes that exceed the locally available crane capacityand would otherwise require multiple lifts.

Safety:The float-over deck installation can be designed such

that any single point failure can be dealt with.

Cost savings: The costs of a float-over deck installationcompared to a semi submersible lift are likely to be lower for:

o Lease of transport/installation vessel compared tolease of transport vessel and semi submersible cranevessel;

o Offshore hook-up and commissioning for a singlelift float-over deck installation compared to a

multiple lift installation by semi submersible cranevessel.

Weaknesses

The following weaknesses of a float-over deck installation can

be identified:

Limited workability:Float-over deck installations, much alike

the lifted installations by semi submersible crane vessels aresubject to weather restrictions. Waves, especially swell, resul

in motion responses of the installation vessel. Since the

motions are restricted to ensure alignment of deck legs and


6/9

6 OTC 19072

jacket legs, this has an impact on the maximum allowable

wave heigth.

Also wave loads acting on the vessel and transferred into the

jacket may be subject to restrictions due to limited structural

capacity of the jacket.

Jacket slot requi rement:A basic part of the float-over conceptis the requirement for the installation vessel to be able to sail

into the jacket. This has two implications for the structuraldesign of the jacket:

o The width of the jacket has to be increased such thatthe installation vessel width can be accomodated;

o The upper transverse bracing has to be installed at alevel such that sufficient clearance between vessel

bottom and bracing is maintained during vessel un-

docking.

Transport and I nstall ation support conditi on:An integrateddeck that will be installed by a float-over will be subject to

two structural implications:o The integrated deck cannot be supported on the

permanent installation supports during sea

transportation. This raises the need for the platform to

be structurally sound for two support configurations;

o The jacket slot width requirement will result in apermanent support arrangement having a bigger

spacing compared to a lifted deck.

I nstallation method design impli cations: The float-overinstallation method has a significant impact on design of both

the jacket and the integrated deck. The width of the jacket and

the height of the upper bracing are mainly driven by the choiceof installation vessel. Loadcases resulting from the installation

process might be governing for structural design of both thejacket and the integrated deck. The consequence of these

implications is that a large part of the installation engineering

needs to be executed in the early design phase.

Early commitment to contractor: Since the installation

method and vessel has a significant impact on the design ofthe jacket and deck, the operator is forces to make a decision

on the installation vessel and as a result a commitment has to

be made to the installation contractor to avoid availabilityrisks related to this installation vessel.

Cost increases: Structural weight increase due to the

aforementioned reasons will have cost implications on thefabrication of both the integrated deck and the jacket.

Oppurtunities

The float-over deck installation is still being developed at the

moment. With the increasing number of float-overs of recentyears, the operation has been engineered for many different

applications. Though, many improvements are still to be

expected in the near future.

Improved workability: Workability is driving costs for

contractor, but also schedule for the operator. Improved

workability may result in year round installation for certain

areas such as West-Africa or West-Australia. This is not only

favourable with respect to installation vessel availability, but

also with respect to construction yard availability and theability to cope with delays in construction.

I ntegrated deck weigh t: Float-over deck installations have

been executed for deck weights upto 21.800 tons and are

planned for decks having weights over 30.000 tons. Theseweights are virtually unlimited and only based on the carrying

capacity of the available installation vessels.The drawback of bigger size installation vessels is the

increased width and consequently jacket slot width

Constructing a vessel combining a limited width and a high

stability range would make full use of the advantages of thefloat-over concept and limit the dis-advantages from a

structural design perspective.

I nterface reduction: The float-over deck installation can be

executed from load-out to installation by a single contractorInterfaces between the transport contractor and lifting or float

over contractor can be deleted from the project execution plan

Standardization:The engineering hours required to design a

float-over deck installation exceed the hours required for a

lifted installation. Multiple custom made elements such as

LMUs, DSUs, deck supports, mooring spread etc. arerequired for a succesfull execution.

Standardization of these elements would reduce the number of

hours to be spend on detailed engineering and provides the

oppurtunity to adress interfaces between the physicalcomponents, such as deck, jacket and installation vessel at an

early stage in the design.

Threats

The following threats associated with the execution of a float-over deck installation have been identified.

Lack of earl y commitment:For obvious commercial reasonsthe decision to commit to an installation contractor can be

delayed. Once this decision is delayed to such an extend that

the decision is made only after the main design parametershave been defined, a conflict in jacket/deck design and

installation design might occur. This could be reflected in

reduced workability and consequently in-field delays.

Standardization: With the increasing number of float-overs

the operations may be regarded as run of the mill projects

Though, the succes of the float-over can only be safeguardedby proper engineering and testing of the equipment beingused. The operation can be designed to cope with any single

failure, but local damage to jacket or integrated deck may still

occur in case of such a failure.

Future DevelopmentsBased on the SWOT-analysis, a selection can be made of

developments, technical as well as organisational that have thepotential to strengthen to competitiveness of the float-ove

deck installation compared to lifted installations using sem

submersible crane vessels.


7/9

OTC 19072 7

Stretching the Weather Window

The potential to stretch the weather window for float-over

deck installations has obvious advantages for both installationand fabrication schedule. Cost reductions and schedule

flexibility are the main drivers to invest in stretching the

weather window.

The weather window for float-over deck installations is drivenby a number of aspects, but two main aspects are:

o Relative motions in lateral direction between the decklegs and the jacket legs. When these motions are in

excess of the limitations, proper alignment of deck leg

and jacket leg is no longer feasible and load transfer

cannot commence;o Loads on the jacket structure and on the integrated

deck. The jacket is designed to an optimum steel

weight. The in-place conditions are a given, but when

the installation loads become governing for the design,

this will have a cost impact as a consequence.

Reduced lateral motions

Reducing the lateral motions of an installation vessel in a

jacket prior, during and after the mating operation can be

achieved in a number of ways:

o Increased stiffness of the mooring spread;o Increased damping in the mooring spread;o Reduced wave action on the installation vessel.

Increased stiffness of the mooring spread has the advantage

that lateral motions can be significantly reduced. Drawback ofthis method is that the loads in the mooring system will

increase significantly.

In general winch wires having a diameter that allows for easy

handling are limited to a SWL of about 100 tons. Wincheshaving capacities in excess of 100 tons SWL are often not

practical and require special devices for wire handling and

installation.

Increased damping is an efficient means of reducing lateral

motions. Different concepts can be used to achieve energydissipation by damping.

Damping can be introduced directly into mooring lines by adamping device in line with the mooring line.

An alternative to this approach is to make use of the friction

fender concept. Friction fenders installed on the installationvessel are forced against the jacket by means of pneumaticcylinders. Friction between the jacket and the installation

vessel now dissipates energy from the installation vessel

motions and reduces especially the surge motion and to a

lesser extend the heave and roll motion. The friction fenderconcept is presented in Figure 12. The impact of compression

load on the friction fender on the reduction of surge motions is

presented in Figure 13.

Figure 12: Use of friction fender during a float-over installation

Drawback of the concept of the friction fender is the fact that

to achieve a friction load of X tons a compression load of X/

needs to be applied. Where is the friction coefficient which

can be in the order of 0.4.

Figure 13: Reduction of surge motions as a result of frictionfenders

Reduced wave action on the installation vessel is an efficientway of reducing lateral motions of the installation vessel. Thican be achieved by reducing the allowable wave height, which

is against the objective to improve workability and as such no

feasible or by selecting an optimum shape and draft of theinstallation vessel during the float-over operation.

Reduced loads on the jacket

The loads on the jacket can be divided in two types of loads

Lateral loads and vertical loads. Lateral loads occur during the

docking and the mating phase, vertical loads only occur duringthe mating phase of the float-over deck installation.

Lateral loads on the jacket are mainly driven by the impact of

the vessel. Reducing motions of the installation vessel is aneffective way of reducing jacket loads and can be achieved as

described above.

In case the impact of the vessel on the jacket does occur, a

proper fendering system will help to reduce the impact loads

To have sufficient energy absorbing capacity in the fendering

system, an increased stroke will be helpful. Though increasingthe stroke is in conflict with the lateral motion restrictions.

An option to improve the stroke and the energy absorbingcapacity of fenders is to fill the initial gap between installation

0

20

40

60

80

100

0 250 500 750 1000 125

Compression Load [tons/column]

MaxRelativeSurgeMotion[%]


8/9

8 OTC 19072

vessel and fendering by fenders that can be activated when the

vessel is in the pre-mating phase.

Improving damping characteristics of the fenders is another

effective means to reduce impact loads on the jacket.

Vertical loads on the jacket on the jacket are mainly driven by

the heave, roll and pitch motions of the installation vesselrelative to the jacket. Due to the magnitude of the wave loads

in these directions, they are much more difficult to control.

Longitudinal positioning of the module on board of the

installation vessel is one of the few factors that can be

influenced in the engineering phase of the project.

Reducing wave action is another means, but this is hard to

achieve other than by reducing the allowable wave height,

which is from workability perspective not preferred.

Common practice is to include shock absorbers in the jacket

legs, LMU, and shock absorbers in the deck supports, DSU, inorder to cope with the impacts rather than reducing the

impacts.

Reduced Jacket Slot Width Requirements

The biggest disadvantage of the float-over deck installationconcept is the impact on the design of both the jacket and the

integrated deck. Steel weights of both assets are likely to

increase as a result of the choice for the float-over concept.

The main reason for this increase is the slot in the jacket

required to accommodate the installation vessel. Obvious

choices to reduce the slot requirements are:o T-shape barge, as used for the Lunkoye-A module;

o Catamaran float-over concept, as used for the KikehSPAR float-over.

Some dis-advantages are associated with the above mentionedsolutions to the slot width issue. Most float-overs are designed

for prevailing swell conditions. Orientation of the jacket is

designed such that the installation vessel is heading with itsbow into the prevailing swell.

The frontal area of the vessel is significantly smaller in thisdirection compared to the beam seas frontal area and as such

the wave action on the vessel and consequently surge motions

are reduced.

For the T-shaped barge the frontal is significantly increasedcompared to a normal vessel when subject to bow swell

conditions. This results in higher wave loads and surge

motions, reducing the advantage of designing for prevailing

swell conditions.

Drag of a T-shaped barge is significantly higher compared to a

normal ship when towed at forward speed due to the extremewidth. This will result in higher bollard pull requirements or

reduced tow speed.

Customizing the shape of a T-shaped barge in order to

improve wave loads and drag particulars while maintaining

the improved stability characteristics may prove to have afavorable impact on the overall project execution.

The catamaran float-over concept makes the need for a slot in

the jacket obsolete, allowing more flexibility in the design o

the jacket or floater. Dis-advantage of the catamaran concepis the support by two floating bodies, where the module itself

is the connecting structure. Phase differences in motionsbetween both floating bodies will result in dynamic structura

loads, such as torsion, in the module.

As a consequence the concept is only applicable to float-overs

which require only a short, relatively sheltered transport fromconstruction to installation site.

In order to avoid torsion in the module during sea transport the

catamaran arrangement can be loaded onto a self propelled

transport vessel. This vessel can either load the catamaranfloat-over arrangement by float-on or load the module by skid

on and the catamaran barges afterwards by float-on. Thisconcept is presented in

Figure 14.

Figure 14: Dry tow of catamaran float-over spread to avoid torsionin module

Standardization

Industry standards are virtually non-existent in the float-overmarket. Every contractor operates his own equipment and

installation assets. Every contractor has its own preferences to

execute a float-over deck installation.

Since the equipment and preferences of the various contractors

result in different design implications, the operator needs toinvolve the contractor in an early engineering stage to make

sure these design implications are implemented in an effective

way.

To avoid this early commitment to a certain contractor, the

operator has a benefit in setting standards which can be dealwith by the relevant contractors. This also has its implications

in the engineering workload. Once the commitment to a

contractor is postponed, this means more installation


9/9

OTC 19072 9

engineering has to be delivered by the operator or its design

contractor.

The operator has to decide whether the benefits of a postponed

contractor commitment outnumber the disadvantages in the

engineering process.

ConclusionsThe float-over deck installation has matured rapidly in recent

years. At present it is a competitive method to install platformsin areas where semi submersible crane vessels are a rare

commodity.

Also for platforms which would require a multiple liftoperation by semi submersible crane vessels, the float-over

deck installation is an attractive alternative.

Two important cost factors have a significant impact on the

decision to select a float-over operation as the preferredinstallation method:

1. Steel weight of a jacket and integrated deck to beinstalled by float-over is in excess of the steel weight

of the jakcet and deck of a platform installed by lift;

2. A float-over operation requires only one asset forboth transport and installation. The number of assets

exceeds the number of available crane vessels and theassets are easier to mobilize. As a consequence the

costs of the installation spread is lower compared to

an installation by crane vessel.

A number of alternatives is available in the market to reduce

the cost increase for fabrication of the jacket and the module.

These alternatives are relatively new and the applications canbe optimized to further improve the float-over concept and

make it competitve to lift installations from a steel weightpoint of view.

The float-over deck installation has the potential to improvethe workability and excel in this respect compared to the

installation by a semi submersible crane vessel. Especially for

harsh environments such as West Africa and West Australiathis may improve the competitive position of the float-over

compared to the semi submersible crane vessel.

otc 19072 fpso selection

Documents