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NEW MULTIPURPOSE VESSEL CONCEPT

Jean-Marc Cholley, Nicolas Tcherniguin,Technip.

This paper was presented at the Offshore Mediterranean Conference and Exhibition in Ravenna, Italy, March 25-27, 2009. It was selected for presentation by the OMC 2009 Programme Committee following review of information contained in the abstract submitted by the authors. The Paper as presented at OMC 2009 has not been reviewed by the Programme Committee. ABSTRACT For deep water developments, the installation of production decks onto fixed or floating substructure using the heavy lift vessel method is now well established, though, for high payloads, this requires multiple lifts and hence extensive offshore hook-up. As demand for lift vessels increases, their availability to match a specific project’s schedule cannot be guaranteed. Consequently, an alternative solution by using this new multipurpose vessel has been developed for not only installing decks onto floaters, but also for the large sub-sea package installation. This paper will present this new design. The new vessel design consists of a linked catamaran shaped vessel with dimensions that permit it to go around the spar hull so that the deck can be lowered and stabbed. The procedure for lowering is based on motorized “legs” to achieve a rapid weight transfer in severe sea-states, but, it also permits the deck to be raised up once at site to achieve a high air gap installation. This new vessel design greatly extends the geographical range for deck installation using the float-over method and offers a cost effective alternative to relying on crane vessels for installation. Additionally, the vessel can perform a range of other tasks, particularly relating to the deep water installation of or large sub-sea processing package, decommissioning of facilities, or offshore wind-farm turbine installation. INTRODUCTION Topsides for all spar platforms prior to Kikeh were installed by heavy lift derrick barges in single or multiple modules lifts ranging from about 3,000 MT (metric ton) to over 10,000 MT. New offshore developments may include several floaters with topside installed weights of over 20,000 MT. There are some disadvantages in a lifted installation, particularly for large topsides requiring multiple module lifts, such as : Schedule and cost for an offshore integration and commissioning campaign. The facility’s delivery schedule is tied to the availability of relatively few heavy lift derrick barges. The first of these disadvantages could be eliminated if the topside could be fully integrated at the fabrication facility and installed using the float-over method, and the second, by the availability of a dedicated vessel. Regarding float-over installation methodology, Technip has developed a jacking assisted float-over installation system, called “UNIDECK®”, which has been successfully used for decks up to 18,000 MT (Ref 1). This technology uses hydraulic jacking to provide a safe and quick transfer of the deck weight onto the pre-installed jacket. However, the height to which jacks can lift the deck is constrained by the sensitivity of the jacks to lateral forces (at high elevation) and their limited stroke. Hence it is suited to low to medium air gap fixed platforms. Additionally, the “catamaran” style float-over onto a floating body has been used previously, but generally in protected waters of Norwegian fjords for deck installation onto floating concrete substructures.. In November 2006, the catamaran configuration using two independent barges was combined with on-site (open-ocean) installation by Technip to

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complete the first spar topside float-over for Murphy’s Kikeh Dry Tree Unit (DTU). This installation was performed in 1330 m water depth in the South China Sea, offshore East Malaysia. The topside installation weight was 4000 MT and the swell at the time of installation was Hs of 0.7m at periods of 7 - 8 seconds. This was achieved by de-ballasting the spar hull to lift the deck of the catamaran (though ballasting of the barges was also used). The newly developed DSIV solution consists of replacing the limited stroke jacks of the “UNIDECK®” concept by a rack and pinion system that has no stroke limitations within all conceivable air gap requirements. The non-limited stroke permits high air gaps to be achieved when installing a deck onto a fixed substructure. With a floating hull, lowering of a deck onto it either requires de-ballasting of the hull (a slow process) or the hull being ‘pushed’ into the water so that its buoyancy is increased to compensate for the deck weight. Being able to lift the deck to a high elevation and then lowering it onto a floater hull which gains buoyancy as it is ‘pushed’ into the water ensures a rapid load transfer and avoids, or reduces to a minimum, the ballasting of the floating platform during the operation. This elevating system is fitted on a catamaran made by two pontoons linked together by a truss beam and a “rear gate” (see Fig 1).

Fig.1: “Deck Salvage & Installation Vessel” The main characteristics of the system are that, during transportation, the deck is at a minimum elevation from the vessel in order to maximize the floating stability and reduce horizontal acceleration applied on the deck. Once arrived at the installation site, the deck is elevated to a sufficient height using a rack and pinion system for the deck to be above the substructure. The load transfer on to the substructure is then achieved by operating the rack and pinion system to rapidly lower the deck, thereby minimizing the duration of the critical load transfer operation. By the use of this method, topsides can be installed onto either floating platforms or conventional jackets at low or high air gap. Additionally, the unit is designed for the removal of existing decks from platforms at the end of their operational life by reversing this process. Therefore, the unit is designated as a “Deck Salvage and Installation Vessel” (DSIV). The unit is also capable of other functions including jacket removal and installation of wind farm turbines.

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INSTALLATION METHOD Like the established float-over method, the concepts consists of installing the substructure (floater or other) and then transporting the deck to the site on the DSIV which has dimensions that permit the vessel to go around the floater (or jacket) with the deck simply supported by each pontoon so that it can be lowered and stabbed onto the floater (or jacket). The environmental conditions at the site (i.e. the wave, wind and current statistics) and the weather window required for a safe operation allow evaluation of the risk of weather stand-by and selection of the optimum platform orientation. It should be noted that the hydraulic rack & pinion elevating system is fully reversible which provides both flexibility and safety. Once arrived at the installation site and with a prediction of a suitable weather window, the deck is elevated on shuttles to a sufficient height for the vessel to enter around the pre-installed substructure with the deck above it (Fig. 2, the shuttles being shown in purple)

Fig.2: Entrance around the substructure The load transfer onto the substructure is obtained by lowering the deck (Fig. 3). The fast lowering velocity ensures a limited duration of this critical phase (constant speed 0.9 m/min during lowering and load transfer). Installed on each substructure pile is a transition piece called a “shock absorber”. The purpose of the elastic shock absorber is to progressively control the engagement of the deck into the floater/jacket legs without generating too high an impact load During the load transfer operation a slight adjustment of the vessel’s ballast will be done to compensate for the eccentricity of the deck’s center of gravity (COG) to avoid a heel or trim angle at the end of this phase. After completion of the load transfer, the lifting/lowering systems are fully lowered in order to withdraw the vessel from the substructure (Fig. 4). Installation of a deck of 20,000 MT located at an air gap of 20 m requires 1.5 hours to raise the deck to its final elevation for vessel entry around the substructure and 20 min. for lowering and transfer of 100% of the deck weight.

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Fig.3: Deck lowering and load transfer

Fig.4: Load transfer achieved, ready for withdrawal

SYSTEM DESCRIPTION The rack and pinion components utilized in the DSIV’s jacking system are derived from the tried and tested components used in the TPG 500 jack-up production platform, of which two are installed in the North Sea and one in the Caspian Sea. The DSIV system has been fully designed to install a deck of 20,000 MT with a 20 m air gap and includes the following main elements (Fig.5):

• Two pontoons both equipped with a rear gate and two elevating systems. • One transversal truss beam linking each pontoon.

Fig.5: Main structural parts of the system. The capacity of 20,000 MT presented in this paper can be upgraded to 25,000 MT by addition of more jacking elements, all the component of the vessel having been designed to this higher value. In order to fit the deck and floater (or jacket) width, the pontoons clearance is adjustable by modifying the truss beam and rear gate lengths. The elevating system is designed to transfer topside static and dynamic loads to the hull taking into account longitudinal and transversal COG offsets. In addition, the reaction redistribution due to the relative out of plane displacement of support points remains acceptable for both topside and elevating system.

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Each elevating system includes the following elements: • A jack house fixed on the top of the pontoon. It supports the jacking systems

and guides the leg at its top level. • The jacking systems are inserted in the jack house and support the leg. • The leg is a plated triangular shape with rack plate and chord shell on the two

motorized corners. The leg is moving through the hull in a leg well and is guided at the bottom of the hull and the top of the jack house.

• The shuttle is supporting the deck reaction during installation phase and is resting on the top of the leg. The shuttle is laterally guided on the jack house.

• Hydraulic equipment associated with each jacking system. The rack and pinion system is basically a structural jack-case housing six similar mechanical assemblies, each of them including the following components (Fig. 6):

• One seven-tooth pinion that matches with the leg rack teeth • One bearing at each extremity of the shaft of the seven-tooth pinion • One gearbox/reducer • One hydraulic motor on the high-speed side of the gearbox

Each jacking system is inserted in the jack house with a top and bottom shock pad to ensure equal distribution of vertical load on all the jacking systems. The standard capacity of the individual jacking systems is 1,980 MT and 7,920 MT per elevating system (including leg and shuttle weight).

Fig.6: Rack and pinions system on an existing jack-up platform The legs are designed to transfer vertical and horizontal loads from the deck to the hull via jacking system and guides. The main dimensions are:

• Width of 10 m between motorized chords. • Depth of 8 m to un-motorized chord. • The motorized chord is 180 mm thick rack plate with a tooth pitch of 250 mm. • The un-motorized chord is 140 mm thick plate 1500 mm wide. • The plating is 20 mm between motorized chord and 25 mm for other sides.

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The shuttle is a “fork” of a box girder construction and, rests on the hull deck during transport. Prior to deck elevation to the entrance air gap, the legs are lowered and the shuttles are locked on the top of the legs by a shear key. During deck lifting and lowering phase, the shuttle is resting on the top of the leg and is laterally guided by the jack house (Fig. 7). The shuttle is designed to transfer vertical and horizontal loads from the deck to the leg and jack house. Once the deck load is fully transferred to the substructure, the legs are further lowed to bring the shuttles to rest on the pontoons and hence provide suitable clearance for the removal of the DSIV from around the substructure. After the legs are fully lowered, the shuttles are unlocked from the legs and, once the DSIV is clear of the platform, the legs are raised into their elevated transit configuration.

Fig.7: Elevating system (jack house, leg and shuttle).

During transit with or without a deck, the rear gate is closed and links both pontoons at their stern. Each gate leaf is adjustable to match with the required pontoon clearance. The gate is designed to limit the relative displacement of the pontoons’ stern during a transoceanic tow. The transversal truss beam links both pontoons at their bow. The beam is also adjustable to fit with the required pontoon clearance (Fig. 8). The beam is designed to withstand:

• Static bending moment due to the torsion in the pontoons generated by the eccentricity of the deck reaction.

• Dynamic bending and torsion moment due to vessel motion and relative deflections between pontoons.

The design of the rear gate and the transverse beam is made in such a manner that transportation (i.e. relative vertical displacement of support points) is not a governing condition for the topside.

Fig.8: Adjustable transversal beam.

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LOAD OUT OPERATION The load-out of a deck onto the DSIV can be performed in several ways:

• The DSIV is perpendicular to the quay and the deck is pulled onto the vessel along the skid ways using a strand jack system connected to the end of skid ways (Fig. 9).

• The DSIV is in sheltered sea. A vessel supporting the deck enters between the twin pontoons and the deck is transferred onto the DSIV by ballasting or using the elevating system in a similar way to the installation phase (Fig. 10).

Fig.9: Topside loadout from quay onto DSIV

Fig.10: Topside loadout from barge onto DSIV To facilitate a topside loadout by skidding, the top of steel of the skid ways are equipped with low friction material to limit the friction force during the operation. The skid ways distribute the moving loads to the longitudinal bulkheads of the vessel. The loadout is achieved by continuously adjusting the vessel ballast to keep it level at the interface with the quay, including adjustments for tide changes, in the same manner as a conventional single transport vessel. The rear gate is closed in calm water before sail-away TRANSPORT 1 INSTALLATION One of the main advantages of using motorized legs (instead of a high support frame) is that the centre of gravity of the system is low during transportation as the topside is directly resting on skid ways. Due to the catamaran shape of the vessel, the standard criteria for stability during transport and installation are easily met. The “standard” float-over installation of a heavy topside at a 20 m air gap onto a fixed substructure requires a wide vessel of more than 40 m. The main disadvantage of such a vessel is that the jacket must be designed to have a huge open bay to allow the vessel to enter and install the topside. The dynamics during transport for two typical transport vessels and the DSIV have been analyzed by a linear diffraction method. Significant wave heights between 5 and 7 m and periods between 3 and 15 s have been used. Fig 11 illustrates the advantage of the smaller dynamics at the deck center of gravity by transport at a low elevation on a DSIV by plotting barge capability in terms of topside deck weight against air gap based on achieving the stability criteria during transport.

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Fig.11:Barge capability The motions of the DSIV in the elevated installation configuration have been studied in detail with a range of wave periods and directions for a significant wave height (Hs) of 1 m and for deck weights of 15,000 MT to 20,000 MT (Tab 1). Deck accelerations and reactions were also studied before, during and after the load transfer.

Tab. 1: Allowable installation Hs 20 000T topside Tp < 8 s

Tp < 10 s

Tp < 13 s

Tp < 15 s

Surge motions (m) 0.11 0.19 0.43 0.56 Surge accelerations (m/s²)

0.14 0.14 0.14 0.14

Heave motions (m) 0.50 0.70 0.85 0.89 Heave accelerations (m/s²)

0.30 0.32 0.32 0.32

Allowable Hs (m)

1.60 m 1.14 m 0.94 m 0.90 m

15 000T topside Tp < 8 s

Tp < 10 s

Tp < 13 s

Tp < 15 s

Surge motions (m) 0.12 0.20 0.45 0.58 Surge accelerations (m/s²)

0.16 0.16 0.16 0.16

Heave motions (m) 0.43 0.59 0.75 0.81 Heave accelerations (m/s²)

0.29 0.29 0.29 0.29

Allowable Hs (m)

1.86 m 1.35 m 1.07 m 0.99 m

The analytical results for installation Hs have been related to prevailing sea conditions for various regions of the world taking account of scatter diagrams to estimate the percentage of time that the installation could be performed (Fig 12). The sites used for the scatters diagram are:

• Gulf of Mexico and Australia (Timor Sea) year around • South China sea four months (Aug, Sept, Oct, Dec) • North Sea in four seasons, year around period distribution • Sakhalin area, July

The results of additional analysis of allowable significant wave heights (Hs) against associated periods (Tp) are presented in Fig 13. The results of the analysis demonstrates that the limiting conditions with head seas for the installation are better than Hs = 1.5 m for Tp = 4.5 to 7.5 s and Hs = 1.0 m for the rest. This limiting condition provides a large

Barge capabilities

0,05,0

10,015,020,0

25,030,035,040,0

45,050,0

15000 20000 25000 30000

Topside transport weight (T)

Inst

alle

d ai

rgap

ie w

ater

line

to B

OS

tops

ide

(m)

Grillage BlackMarlinGrillageTransshelf

23 m airgap atentranceDSIV

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probability of installation in tropical and sub-tropical cyclonic regions such as Australia’s North West Shelf.

Allowed wave heights

0,00

0,20

0,40

0,60

0,80

1,00

1,20

1,40

1,60

1,80

3,5 4,5 5,5 6,5 7,5 8,5 9,5 10,5Wave peak period (s)

Hs

(m)

HeadQuarteringBeam

Fig 12: Model & measurement gauge Fig 13: configuration installation

IMPACT ANALYSIS The impact analysis is performed in two steps: 1) Hydrodynamic analysis in time domain is performed with DIODORE. The objective is to determine the motion for the phases before and during the jacking down and to ensure that the size of guide cone (shock absorber) is adequate. And finally to generate the hydrodynamic load for the Abaqus analysis. 2) Non linear analysis with Abaqus, the aim is to determine the impact load before and during the first jacking down. A non-linear (contact) analysis is performed. The hydrodynamic loadings are applied to the vessel while the jacking systems lower the topside onto the jacket or floater. The impact loads during the simulation are determined at the conical interface between the substructure and the topside. Linear and non-linear springs have been used in association with the contact elements to model the stiffness of the shock absorbers, jacket/floater legs and substructures. Dynamic analysis is done using a direct-integration method, with automatic time increment.

Two models are used in this study: • First model: Catamaran model, fitted with elevating system, • Second model: Barge model.

FASTRUDL model of catamaran and support

with topside

Modal analysis

ABAQUS model

Modal analysis

Comparison

Impact Dynamic Analysis

DIODORE analysis Two bodies

Sea states: wave load on hulls

Installation possibility worldwide

01020304050607080

Austra

lia all y

ear

North S

ea sp

ring

North Sea

summer

North Sea

autumn

North S

ea w

inter

South

China S

ea A

ug.

South

China S

ea Sept.

South

China S

ea O

ct.

South

China S

ea D

ec.

GOM all y

ear

Sakha

lin Ju

ly

Perc

enta

ge o

f ins

talla

tion

poss

ibili

ty

(%)

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The vessel motions are applied on the hulls cog. The wave selected corresponds to the maximum velocities at cone in the transition piece. The catamaran model is presented fig. 14 (first analysis).

Fig. 14: Catamaran model description The barge model is presented fig. 15 (second analysis).

Fig. 15: Barge model description

The contact element representing the shock absorber is modeled by contact element (see fig. 16)

Fig. 16: model description

Deck (grey)

DSIV Catamaran (blue)

Elevating system (green)

Damping hull elements (red)

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BASIN TEST A tank testing campaign was carried out in Oceanide at La Seyne-sur-Mer, France. The model consisted of hulls scaled at 1/50. The beam and gate connecting the hulls were modeled with the correct representative stiffness for the moments about X and Y. (Fig 17 & 18).

Fig 17: Model & measurement gauge Fig 18: configuration installation

These moments as well as the vertical forces were measured at each of the connections (fig 18). The model topside was supported on the four movable shuttles with the correct horizontal and vertical stiffness Vertical reactions were measured at the four supporting points(fig 19).

TWT Mx beam average range 1/3 double amp

0

5000

10000

15000

20000

25000

30000

35000

90 112,5 135 157,5 180Heading (°)

Rang

e 1/

3 do

uble

am

p (T

m)

Meas Mx beam 8sMeas Mx beam 12sMeas Mx beam 15sCalcul Mx beam 8sCalcul Mx beam 12sCalcul Mx beam 15s

Fig 18: Beam Mx benchmarking config. Towing with topsides

TWT Fz topside average range 1/3 double amp

0

200

400

600

800

1000

1200

1400

1600

1800

2000

90 112,5 135 157,5 180Heading (°)

Rang

e 1/

3 do

uble

am

p (T

)

Meas Fz 8sMeas Fz 12sMeas Fz 15sCalcul Fz 8sCalcul Fz 12sCalcul Fz 15s

Fig 19: topsides Fz _ benchmarking config. Towing

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A total of 61 tests have been performed for four different configurations: installation with and without topside, transport with and without topsides (fig 20 & 21). The result analysis confirms the general behavior of the structure and provides confidence as to the reliability of the coupled hydrodynamic and structural analysis.

Fig 20:: Roll benchmarking _ in towing with topsides HS=5m

Fig 21: Pitch benchmarking _ in towing with topsides HS=5m

Conclusion The DSIV installation method greatly extends the viability of deck installation onto a floater and the geographical range for deck installation onto fixed structures using the float-over method. It offers a cost effective alternative to relying on derrick barges for installation with the additional benefit for large decks of a significantly reduced integration, hook-up and commissioning cost, and shorter schedule to start-up. The main advantages can be summarized as follows:

• The ability to perform float-over deck installation on to a floater or pre-installed jacket at high air gap.

• The ability to perform topside removal for decommissioning on a floater or pre-installed jackets that were not designed for float-over operations.

• The ability to perform trans-oceanic transport increases the choice of construction yards around the world with potential fabrication cost optimization.

• Minimal sea fastening due to the low clearance between the deck and the vessel. • Quick retractable legs allow rapid load transfer and avoid potential shocks. • No floater de-ballasting for deck mating. • Flexible and reusable system. • The substructure design (e.g. leg spacing) is not influenced by the

transport/installation vessel and hence leads to fabrication cost optimization. • Proven components (rack, pinions, jack-cases and hydraulic system all derived from

TECHNIP’s standard TPG 500 design) Acknowledgments The authors would like to thank the management of TECHNIP for granting their permission to publish this paper. The authors express their special appreciation to their colleagues for their support and advice

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References 1- C. Tribout and D. Emery, Technip, “Float-Overs Offshore West Africa”, paper OTC 19073, presented at the Offshore Technology Conference held in Houston, Texas 30 April – 3 May, 2007. 2- J-M Cholley, and M. Cahay, Technip “Float Over High Air Gap: New Solution” paper OTC 18683, presented at offshore Technology Conference held in Houston, Texas 30 April – 3 May, 2007. 3- J-M Cholley, Technip “Vessel adapted for deck spar installation” paper ID 32 presented at Deep Offshore Technology held in Stavanger, October 2007. 4- J-M Cholley, A. Vanneste, Technip; F. Petrié, B. Rousse, Océanide, “Comparison of tank testing and numerical analysis for the design of a catamaran for deck installation by the float-over method”, paper OTC 19835, will be presented at the Offshore Technology Conference, Houston, Texas May, 2009.