OTC 17299 Launching

8/4/2019 OTC 17299 Launching

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Copyright 2005, Offshore Technology Conference

This paper was prepared for presentation at the 2005 Offshore Technology Conference held inHouston, TX, U.S.A., 25 May 2005.

This paper was selected for presentation by an OTC Program Committee following review ofinformation contained in a proposal 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 a proposal of not

more than 300 words; illustrations may not be copied. The proposal must contain conspicuousacknowledgment 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 Holstein truss spar hull, the largest ever built, was

fabricated in two sections in Finland and the U.S. The Truss

section with the soft tank was fabricated in the U.S. and

required transferring from land to water at the fabrication yard

before it could be mated with the hard tank section. The

challenges due to the size, schedule and government

regulations resulted in selecting a unique controlled launch

submergence method to offload the truss section into the

water. The offloading operation involved inclining the H-627

launch barge to unusually high angles while ballast operatorswere onboard the vessel, moving the truss down the barge in a

controlled fashion with winches rigged to skid shoes and

finally submerging the barge to create separation. The truss

section was loaded on the barge from the bow and launched

over the tilt beam at the stern. Discontinuous removable skid

shoes were used instead of traditional continuous permanent

launch cradles.

The truss section travel over the barge, and speed control

during launch was achieved by employing two sets of

winches, Pull Winches and Hold Back Winches, located on

the Barge Deck. Structural modifications had to be made to

the barge, skid beams and soft tank structure to make theoperation feasible. Additional barge marine systems had to be

added for the unusual ballasting and floating configuration

required for the operation. An active mooring system along

with the winch line tensions was used to control the barge and

the truss during the operation.

IntroductionThe Holstein truss spar hull (Ref. 1) due to its large diameter

of 45.5 m (about 150 ft.) and 227.38 m (746 ft.) length, and

weight could not be carried as a single piece, from the

fabrication yard in Finland to the offloading site in Gulf of

Mexico, on the largest heavy lift vessel available at the time o

fabrication. As a result, the Spar hull was fabricated in two

pieces and in two different fabrication yards. The truss and

the soft tank (referred to only as truss structure from here on)

were fabricated on land as one-piece in Technips GMF yard

in South Texas. The hard tank of the hull was built in

Technips TOF yard in Finland and transported to the Gulf ofMexico on a heavy lift vessel and offloaded in sheltered water

near the truss fabrication yard. The two floating sections were

joined (mated) and welded together in the water next to the

fabrication yard (Ref. 2).

The truss structure had to be offloaded into the water next

to the fabrication yard for mating with the hard tank. The

truss structure with the skid shoes weighed approximately

11,600 short tons. Various options were investigated to

offload the truss and soft tank. Some of the options

investigated are as follows:

Submersible barge.

Dry Dock - either by bringing the dry dock next to thefabrication yard or loadout of the structure on a barge

transport to a dry dock, offload the truss and wet tow the

structure back to the yard for wet mating with the hard

tank.

Lift Loadout the structure on a barge and use land basedcranes or crane barges to lift and lower into water.

Use a combination of submersible barges and liftingequipment.

VersaTruss system.

Launch barge and controlled launch.

Fabricate truss and soft tank sections separately and jointhem over water.

Due to the size, weight and floating draft of the structure

most of these options were not feasible using the existing

facilities in the U.S. New build or modification of existing

vessels in the U.S. was also investigated and ruled out because

of cost and project schedule. Submersible barges available to

perform truss offload operation were not employed due to

various legal considerations. The option selected, about seven

months before the execution of the offload operation, was to

OTC 17299

Controlled Launch Offloading of Holstein Spar Truss Section

A. Sablok, J. Gebara, and T. DeMerchant,Technip Offshore, Inc.; S. Piter,EdMar Engineering, Inc.;S. Perryman,BP America Inc.


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use a launch barge and controlled launch the structure in the

waters next to the truss structure fabrication yard (GMF) in

South Texas. The launch barge selected for this operation was

Heeremas H-627. There were restrictions regarding the

offloading configuration and procedure in order to satisfy the

Jones Act. The barge could not be moved but only yawed

about its center to offload after the truss structure was loaded

on the barge. Furthermore, the barge geometric center and thetruss geometric center were to coincide at the time of rotating

the barge for the yaw operation not to be considered a

transport of the truss. This requirement resulted in performing

a bow loadout and offloading the truss over the stern where

the tilt beam is located. The offloading operation involved

inclining the H-627 launch barge to an unusually high angle,

moving the truss down the barge in a controlled fashion with

winches rigged to the skid shoes and finally submerging the

barge to create separation. Figures 1 and 2 shows the initial

and final configuration of the offloading operation

respectively. This paper describes: the challenges faced, the

offloading system, the barge modifications required, other fit-

for-purpose structures used for this operation, the offload

analysis and the offload procedure.

Description of Truss StructureThe truss structure positioned on the H-627 barge is shown in

Figure 1. The truss structure included three Truss bays with

three heave plates and a soft tank at the bottom of the Truss.

The overall dimension and the weight of the truss structure are

as follows:

Length, overall 130.62 m (428.5 ft.)

Breadth 45.50 m (~150 ft.)

Depth 45.50 m (~150 ft.)

Outfitted Loadout Weight 10,572 mt (11650 s. tons)

The truss was supported at the three heave plates and under

the soft tank with discontinuous skid shoes on two sides inside

the vertical plane of the Truss Legs. The nominal height of

the skid shoes was 9 ft. including 1 ft. high timber. The

mating tank (Ref. 2) required for making the truss float during

offloading and mating operations was also supported on the

skid shoe under the aft most heave plate. The six skid shoes

under the heave plates were suspended from the heave plates

with steel wire and offloaded with the truss. Restraints

provided on the skid shoes prevented the skid shoes from

moving longitudinally, laterally and rotating. The skid shoe

under the soft tank stayed on the barge when the Truss wasoffloaded. The truss structure longitudinal movement during

the offloading operation was done through the two skid shoes

under the soft tank as explained below. The mating tank was

placed on the skid shoes of the aft most heave plate with

restraint provided on the skid shoes to prevent the mating tank

from accidentally sliding longitudinally off the skid shoes

when the barge is pitched down. Laterally, the truss legs were

supported on the cradle of the mating tank. During loadout, a

small gap was left between the mating tank cradle and the

truss legs. This gap closed once the mating tank became

positively buoyant during the offloading operation. This was

required to reduce the concentrated loads on the mating tank

during loadout and offload.

Figure 1: Truss Structure on H-627 Barge at Start of OffloadOperation

Figure 2: Configuration at End of Offload Operation

During the offloading operation, the truss rotated relative to

the barge about the end of the soft tank. Two hinge structure

were installed at the bottom of the soft tank in line with the

skid shoes as shown in Figure 3. The hinge structure wa

welded to the soft tank with a half tubular at the bottom. The

half tubular was placed in a cup structure welded on top of the

skid shoe. During loadout, a small gap was kept between thehalf tubular and the cup to ensure that the loadout loads were

not passed through the hinge but distributed along the soft

tank skidshoe. The hinge system was also designed to allow

the truss structure to yaw relative to the skid shoe due to

environmental load. Side plates were added toprevent latera

sliding. The maximum total force that each hinge had to carry

during the offload operation was 3000 metric tons.

Skid Shoes

Soft Tank

Mating

Tank


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Figure 3: Hinge Structure at Bottom of Soft Tank

Barge H-627 DescriptionThe dimensions of the Launch Barge H-627 used for the float-

off operation are as follows:

Length, hull 176.78 m (580 ft.)

Length, overall 192.35 m (631 ft.)

Breadth 48.77 m (160 ft.)

Depth 10.97 m (36 ft.)

Skidway Height 1.54 m (5 ft.)

The barge has eight rows of tanks along its length and

generally four compartments per row except at the stern wherethere were five compartments across the width. There is a

central tunnel compartment where the pumping system and

machinery are located. There are two pumps that can pump

simultaneously or individually to all the ballast compartments

except the compartment located at the stern, aft of the tunnel.

This compartment, normally not used for launching

operations, had to be manually ballasted and deballasted with

external pumps. The system also allows transferring water

between two compartments. The valves for opening or

closing the water supply to the compartments had to be

manually operated. These valves were located next to the

compartments and therefore personnel had to walk along the

length of the tunnel to control the valves.

The barge had two adjustable tilt beams and skidbeams

running from the stern aft of the tilt beam to short of the bow.

Figure 4 shows the top view of the barge before the truss

structure was loaded out on the barge.

Offload Analysis

Three primary analyses were performed for the offload

operation: Hydrostatic/Structural analysis, Winching analysis

and Mooring analysis. The Hydrostatic analysis results served

Figure 4: Barge H-627 before Truss Loadout

as partial input to the other two analyses. Based on theanalyses, all the systems required to execute the operation

were designed. The offloading systems and the challenges to

execute the operation are discussed later in the paper.

Hydrostatic/Structural Analysis. This analysis helped to

develop the offload procedure steps with regards to the

ballasting and skidding operation. The analysis was done

using a quasi-static approach. Spring compression connector

were used at the supports. A hinge connector was modeled a

the bottom of the soft tank. Structural models of the truss and

the barge were used for the analysis. The stiffness of the

connectors was based on the stiffness of the barge skid beams

and the skid shoes. Two different analyses were done. Rigid

body analysis was performed using the truss section and theH-627 models. This analysis treated the bodies as rigid

connected by flexible generalized springs. The second

analysis used the same model of the truss section, but a new

model was created with a complete structural model of the

barge. In this analysis, after the equilibrium position wa

found, a structural analysis was performed considering the

flexibility of the barge and truss. Flexible connectors were

also used between the two bodies. Figure 5 shows the analysis

steps used to develop the offload procedure. Sensitivity

analyses with weight and C.G. variation of the truss structure

and barge were also performed. The following

assumptions/restraints were used for the purpose of

performing the offload analysis:

Maximum submergence depth of the barge of 76 ft.

The maximum loads were not to exceed the strengthdesign of the truss or barge.

The offload was to be done under favorable weatherconditions.

Ballast tanks full at 98%.

Ballast tanks empty at 2.25% for the No. 1 tanks and4.50% for all others.

Ballasting and Winching are sequential operations, nosimultaneous.

Bow

Stern

Welded to

SkidShoe

Welded toBottom ofSoftTank

Half Tubular


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Separation of the truss and barge of at least one foot wasrequired at the separation point.

Barge must have at least 1.0 m of metacentric height(GM) during all stages of offload.

The following limits maintained the truss section and skidshoes within allowable limit states:

Reactions at the truss section support points not to

exceed 1.5 x their reaction when the truss issupported with all supports on a level plane.

Hinge Reactions during and after rotation to be 3000mT or less

The barge/truss system deflections at the skid shoesupports must not exceed plus or minus 32 mm (1.25

inches) during skidding.

Figure 5: Offload Analysis Procedure

A simplified symmetrical mooring system was included in

the hydrostatic analysis to help in the process of finding

equilibrium. The individual mooring line tensions were

adjusted at each step to 30 kips. The purpose of this system

was to constrain lateral motions of the H-627 during the

offload analysis. It had negligible effect on the analysis. The

detail mooring analysis of the actual mooring configuration

used is described later. Figure 6 shows the truss and the barge

model with simplified mooring at one of the final steps of theoffload operation.

Figure 6: Computer analysis model of Truss, Barge and Mooring

Figure 7 summarizes the configuration of the barge and the

position of the truss for the various steps. Results of these

studies indicated that the truss section could be safely

offloaded from the H-627. The sensitivity studies showed tha

there was sufficient reserve in the system to accommodate the

variances of the base parameters within the boundaries set as

criteria. An independent study was carried out by Det Norske

Veritas to verify the offload analysis and found very similarresults.

Figure 7: Offload Procedure Summary

Winch Load Analysis. The net loads required to either push

or hold the truss structure during various steps of the analyses

above were estimated based on the barge/truss pitch angle and

net vertical loads on the truss structure including buoyancyloads. The following criteria were used to determine the

winching requirements and design the rigging required for the

operation:

Friction coefficients (Sealed, greased wood on lubricated

Teflon):

Static friction = 0.10

Dynamic friction range = 0.04 - 0.08

Friction losses within sheaves and blocks were

considered.

Step 0

Step 1

Step 2

Step 3

Step 4

Step 5

Step 6

Step 7

Step 8

Step 9

Step 10

Step 11

Step 12

Step 13

Step 14

Step 15

Step 16

Step 17

Step 18

Step 19

Step 20


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The fleet angle on winches, blocks and sheaves did not

exceed 2.5 degrees.

No skew load was considered owing to the fact that the

loads in the pulling systems were measured and could be

adjusted.

The winch and rigging systems selected based on the analyses

are described below.

Offload SystemThe quasi-static launch operation consisted of a combination

of winching the truss structure along the length of the barge

and ballasting of the barge. The winching system, shown in

Figure 8, consisted of a Pull Winch System and Hold-Back

Winch system.

The pull system initially was used to overcome the friction

load and push the truss structure towards the stern of the

barge. After ballasting the barge and reaching certain pitch

angles always accounting for the truss structure buoyancy, the

weight component of the truss parallel to the barge deck

becomes higher than the friction force between the truss

structure and the barge skid beams and theoretically no pull

force is required to move the truss. The truss, therefore,

would slide without any external load and required restraint

from launching dynamically. Dynamic launch of the truss

structure was not a feasible option because of the potential

damage to the truss and the barge due to limited water depth

and proximity of the barge to the bulkhead. Any restraint on

the barge to keep it in place and not move backwards towards

the bulkhead may result in the truss structure colliding with

the barge tilt beam as it separated from the barge. The Hold

Back system was used to control the launch of the structure

Since the skid shoes on the truss were not continuous, the

pivoting of the barge tilt beam would have damaged the truss

while moving the truss and finally separating. Therefore, the

Tilt Beam was fixed and not allowed to pivot.

Each of the winching systems consisted of a pair of 200 tonwinches. All four winches were spooled with 6000 ft. of 2

inch wire rope reeved through a pair of 8-sheave 850 ton

blocks. A 12-part rigging was used for each of the winches.

The pull and the hold back loads were applied to the truss

structure through the skid shoe under the soft tank of the truss

structure. Due to the limit on the maximum submergence of

the barge, the offload procedure required that the soft tank

skid shoe be moved almost to the tip of the Tilt Beam for the

separation to occur. To make this happen, a deadman for the

pull system had to be located outside the barge and aft of the

stern. This was not feasible with the offload procedure

therefore a skid frame connected to the soft tank skid shoe was

used to make offload of the truss structure feasible. The skid

frame is shown in Figure 9. Essentially, the skid frame

extended the length of the soft tank skid shoe. The Pull and

the Hold Back systems were connected to the soft sank skid

shoe through the Skid Frame. The two Pull System deadmen

were placed at the aft most possible location on the raked end

of the barge deck. The traveling blocks of the Pull and Hold

Back systems were connected to the Skid Frame. The Skid

Frame was designed to stay parallel to the barge deck and was

sliding on the barge skid beams.

Figure 8: Offload System, Skid Frame and Winch Lines shown in final position

Hold back winch

Hold back linesPulling lines

Skid frame

Deadmen

Blocks in pulling lines

Blocks in hold back lines

Blocks in hold back lines Blocks in pulling linesPullin winch

DeaBow

Bollard (Typ)


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The Winch control system was designed to synchronize the

winches or control them individually. The synchronization

option was to ensure that the load and payout length of wire

was within acceptable ranges on both sides of the truss. A

minimum nominal tension was kept in the wires during the

entire operation to prevent the lines from becoming slack and

to control the alignment of the truss longitudinal axis with the

barge longitudinal axis.

Figure 9: Skid Frame used for Offload Operation

Mooring SystemMooring control systems were used for the offload of the truss

section of the Holstein truss spar from the H-627 launch barge.

They consisted of the following three major sub-systems:

Barge Mooring System, Truss Mooring System and the

Onboard Winch System. The Barge Mooring system was

designed for a directional environment with wind speeds (one

minute average) up to 26 knots and current speeds up to 1.0

m/s. The truss section control system was designed for an

omni-directional environment with maximum wind speeds

(one minute average) of 18 knots and current speeds of 0.6

m/s, respectively. The Barge Mooring system was designed

for a higher speed due to its longer exposure to the

environment when the barge is prepared for the offload afterthe loadout, and possibly need to wait for a favorable weather

window for the offload operation. The Truss Mooring system

was used to control the truss after it separates from the launch

barge. Waves during the operation were assumed benign and

were notconsidered in the mooring analysis.

1) Barge Mooring System: An 8-line barge mooring systemthat was used to maintain the position of H-627 barge

during the offload operation. The mooring lines were

double-parted 1-1/2 diameter, 6x41 IWRC EEEIPS wire

ropes which were connected to land-based winches

through the sheaves on the mooring dolphin or fairleads

on land. Spring lines were also included in each line to

soften and minimize the shock on the offloading system

in case any mooring line was damaged. The winches o

moving deadmen were used to adjust the barge position

and mooring line pretensions. The same mooring system

was also used for the truss section load-out. The loads in

each line were monitored by attaching an in-line load cell

Figure 10 shows the barge mooring configuration. Themooring line pretensions during the offload operation

tightened up as the barge submerged and had to be

adjusted by paying out lines at intermediate float-over

steps. Since the direction of mooring line loads relative to

the barge change due to submergence of the barge during

the offload operation, special bollards had to be designed

and installed on the barge. These bollards helped the

mooring lines to stay connected to the barge and also take

high mooring line loads. The existing barge bollards were

of the open type and not suited for the mooring line loads

experienced during the operation. Figure 11 shows the

tubular bollards used for the offload operation.

Figure 10: Barge Mooring System

Figure 11: Barge Mooring Bollards

Connected to

Pull Lines

Connected to

Soft Tank Skid

Connected to

Pull Lines

Connected to

Hold Back Lines

Bollard


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2) Truss Mooring System: A 6-line control system on thetruss section that was used to control the position of the

truss section during and after the offload operation. Each

line was a single 2 diameter, IWRC EIPS wire rope

connecting the truss to land-based winches through the

mooring dolphin and fairleads on land. This system was

activated when the truss started to rotate upward from thebarge at the hinge on the soft tank. It assisted the

winching system on the barge in keeping the orientation

of the truss so that the truss could continuously skid along

the skid beams before the final separation from the barge.

Tugs were also deployed to control the position of truss

section during and after the offload.

3) Onboard Winching System: An on-board winchingsystem that consisted of two pulling lines and two hold

back lines. Each line consists of a 2 IWRC EIPS wire

rope running between two blocks and controlled by an on-

board winch. It is used to control the movement of the

truss section as it is skidded off from thebarge.

Analysis of the winching system was performed for the

stage at which the truss was partially floating and the soft

tank end was still supported on the barge. At this stage,

only the last pair of skid shoes under the soft tank were in

contact with the barge. Wind and current loads in the

transverse direction (beam direction) would force the

truss to rotate (yaw) around the soft tank end of the truss.

Excessive yaw of the truss would prevent the truss from

being continuously skidded off along the skid beam. The

results showed that the winching system alone (without

the help of the mooring lines on the truss) could maintain

the truss position during the operation in the design

environment of 18-knot wind and 1.2-knot current. Themaximum expected relative (to the barge) yaw of the truss

was 0.16 degrees which would occur in the beam

environment. To achieve such low yaw motion of the

truss, the pulling winch lines had to maintain a minimum

of 200 kips of tension in each line during the operation.

PlanningThe planning and design for the offload operation started

about seven months before actual execution of the operation.

Many tasks had to be completed during the short duration.

These included performing the analyses, developing the

procedures, designing and fabricating the different systems

(Mooring, Pull and Hold Back System) required for theoperation, making modifications to the barge after arrival at

the fabrication yard, determining the dredging requirements,

dredging the area, testing various systems and training

personnel for the operation. The float-off operation was

among the most challenging of all operations performed on the

Holstein project. Various risks were identified during risk

reviews of the procedure and were eliminated or mitigated by

changing the design, procedure or performing additional

verification and tests. Following are some of the tasks that

were performed to mitigate risks:

1) FMECA analysis of the entire operation performed todetermine the most vulnerable component or

procedure that may cause failure of the operation.

2) New wires used with higher than normal margin onsafety factors.

3) All sheaves and blocks inspected and load tested.4) Soft tank skid shoes tied off with wires to avoid

lateral motion or to prevent shoes rolling over due tolateral loads.

5) Multiple redundant systems used to keep the truss in-line with the barge during winching and while truss

was over hanging beyond the stern of the barge.

6) Barge inclination test performed to the maximuminclination of the barge without the truss onboard

The test was performed to ensure that: a) all barge

systems would operate safely to the required

inclination angles, b) to train ballasting crew and c)

to perform a safety drill which ensured that safety

procedures for retrieving an injured person were

tested and an evacuation was possible at any stage of

the offload operation.

7) Introduced soft lines in the mooring system.8) Third party verification of the analyses.9) Trained personnel before the actual operation.

The execution of this first-of-a-kind operation was very

successful and smooth to a large extent, as a result of the steps

taken to identify and minimize the risks.

Barge ModificationsSeveral modifications to the launch barge had to be made to

make the truss structure offload operation possible and safe.

The major modifications made are listed below:

1) Bow Extension Beams: The barge was designed toloadout structures and launch them over the tilt beams

As a result the skid beams are not required for the entire

length of the barge. For the Holstein truss structure, a

Bow Loadout had to be done, followed by a controlled

launch over the stern. Additional skid beams had to be

added on the forward end to make the skid beams span

the entire length of the barge and also cantilever over the

bow for loadout.

2) Marine System: The barge compartment vents are locatedat the top of each compartment. Figure 2 shows the

floating configuration of the barge near the end of the

offload operation with about half the barge deckunderwater. Also, there are other openings for pump

exhaust, fuel intake and ventilation hatches aft of the

midships. To avoid water going through the vents and

other openings, they either had to be closed or routing

them forward above the final waterline on the bargedeck

Thevents for all the aft compartments were modified by

adding vent pipes to the existing vents and routed forward

to be above water all through the offload operation. The

vent extensions for some compartments were combined

taking into consideration the ballast plan for all the

offload steps. The pump room exhausts were also

extended and routed forward. The extension pipes were


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run along the sides of the barge and along the longitudinal

center of the barge away from the winching system and

mooring lines as shown in Figure 12.

3) Cattle Chute: It was very critical to maintain thelongitudinal axis of the truss aligned with that of the barge

while the truss was skidded down the barge. Otherwise

there was a risk of the truss falling off the skid beams.Guide rails were provided along the skid beams to help

guide the skid shoes during the winching operation.

However the risk of the truss going off line was very high

after the passage of the soft tank skid shoe past the barge

skid beams and while on the tilt beam. At this point the

truss structure was supported only on two points on the

soft tank hinges. As a safeguard, a bumping structure was

built on top of the tilt beams. The bumping structure,

fabricated with tubulars and resembling a cattle chute was

used to guide the skid shoes as it was being skidded. The

cattle chute structures also helped to guide the skid shoes

under the heave plates. Figure 13 shows the cattle chute

structures on top of the Tilt Beams.

4) Mooring Bollards: As explained in the mooring sectionand shown in Figure 11, new bollards were designed and

installed for mooring the barge.

5) Deadmen: As mentioned above in the winching systemsection, one deadman was required for each of the pull

and hold back lines. The deadmen for the pull lines were

located on the aft rake of the barge. A sheave was welded

on top of this deadmen. The deadmen for the Hold Back

lines were welded on top of the new Bow Extension

beams. The locations of the Deadmen are shown in

Figure 8.

Figure 12: Barge Modifications Vent and Exhaust Lines Extended Forward

Figure 13: Cattle Chute Structure on Top of Tilt Beam

Bow

Vent and Exhaust Lines

Extended Forward

S


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OTC-17299 9

Barge Inclination Test. The H-627 barge had never been

used or designed to perform an operation similar to the

Holstein truss offload operation. The barge structure and its

systems were being pushed to their design limits and to

conditions for which the systems were never tested. The barge

had never been submerged statically to the depth required for

the offload operation of 76 ft. or statically pitched to about 9

degrees. The barge pumping and marine system had neverbeen subjected to the heads required for this operation. The

worst case for the pumping system was to pump water from

the aft most compartments to the foremost during the barge re-

float operation at the end of the offload operation.

To understand and minimize risks related to the launch barge

and mooring during the actual offload operation, it was

decided to perform a barge inclination test by simulating the

truss offload procedure without the Truss. The barge

inclination test was performed for the following purposes:

Test the barge pumping system because of extensivepumping required for ballasting the barge to a high pitch

angle, submergence and also for barge re-float.

Verify theoretical predictions related to bargeperformance with actual operation.

Train the personnel working in the pump room operatingthe barge pumps to familiarize and operate at high barge

pitch angles.

Verify no leaks through the manholes or vents includingthe vent extension system added for this operation.

Test the active mooring system to control the barge.

Train the land mooring winch personnel with theprocedure to adjust line tensions required for active

mooring of the barge during actual offload operations

Run safety evacuation drills with actual conditions duringoffload operations.

Test the access system to the barge from the land withvarying freeboard of the barge and with boats during

actual operations.

Verify the dimensional control procedure and familiarizepersonnel with taking dimensional control measurements

with changing barge floating configuration.

Test the communication system between the Pump Room,Barge Deck, Mooring Winch operators and the Control

Room during the operation.

Understand any uncertainties and surprises with theoperation.

The inclination test helped the personnel understand the

magnitude of the task required to perform the actual operation

and what to expect as far as the barge and mooring are

concerned. The inclination test revealed a few important

issues, mainly with the mooring, and raised confidence among

the personnel to perform the actual operation with the truss

loaded on the barge. The important findings or lessons

learned from the inclination test are as follows:

Barge pump system is well suited to ballast and de-ballastthe barge.

Helped understand the ballasting and deballasting timerequired for the actual operation. Personnel developed a

better understanding regarding different filling rates when

water is pumped in different compartments

simultaneously and how to control the barge roll during

ballasting/de-ballasting.

Few underwater manhole seals had to be changed thatleaked during the test. One of the welds at the exhaus

extension line had to be repaired since water leaked into

the Pump Room. One of the mooring lines broke during adjustment of pre-

tensions. Mooring Line load cells need to be replaced

with in-line load cells. The mooring line pre-tension

adjustment procedure had to be modified and additiona

mooring personnel were required during the actua

offload operation.

It was possible to maintain the barge in position asrequired and there was sufficient clearance between the

barge and the dredged pit.

Figure 14 shows the picture of the barge being ballasted

during the inclination test.

Figure 14: Barge Inclination Test Photo

Measurements during Offload OperationIt was very important to understand and maintain the globa

position of the barge during the entire operation to maintain

sufficient clearance all around and between the barge and the

dredged pit, and to ensure conformance with the Jones Ac

requirements. Also knowing the floating position of the barge

the truss, and the relative position of the truss on the barge wasimportant to understand and control the operation and to make

any adjustments in case of any unforeseen events. The loads

in the mooring lines and the pull/hold back lines were required

to be measured constantly to monitor and control the

operation. Generally at least two independent measurement

were made to determine the floating configuration of the

barge. Various instruments/tools were used to measure

different quantities during the operation. These included:

Optical targets on the barge and truss

Additional draft measuring sticks at bow and sternbelow keel and above deck respectively (Barge draf


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marks at the bow were above water line and at the

stern completely underwater during most of the

operation)

Barge bubble gage

Paint marks on barge deck

Barge compartment soundings

Mooring line Load Cells

Load Links in-line with the hold back/pull system, Length of wire on the winch spools.

The frequency of the measurements depended on the

operation step.

The measurements from the optical targets installed on the

barge, truss and land were used as a primary means of

understanding the position and configuration of the barge and

truss. The optical targets provided accurate and high

resolution readings. The readings from different targets were

fed to a computer program and corrected for the rotation of the

barge/truss to provide:

Position and orientation of barge relative to land andthe dredged pit.

Barge floating configuration (Draft, Roll, Pitch,Yaw).

Deflection of the barge.

Position of the truss on the barge and configurationof the truss (Draft, Roll, Pitch, Yaw).

Offload ProcedureThis section briefly describes the offload procedure. Only one

major task such as ballasting, winching or mooring

adjustment, was done at any given time to understand and

control the operation at the critical stages of the operation.The offload operation was completed within 24 hours of

starting the ballasting to separation of the truss from the barge.

Post Load-out Re-rigging. At the end of load-out operation,

the truss section was seated at the middle of barge with the

keel of the soft tank located 23.0 m from the barge bow. After

the loadout operation, the loadout pulling system was

disconnected from the barge, loadout compression struts

removed, cantilevered portion of the extension beam cut and

the barge rotated approximately 5 deg. about its geometric

center to line up with the deepest section of the deep pit.

Following the rotation and mooring adjustment, re-rigging on

the barge was done. During the re-rigging phase, the Pulling

Winch system was hooked to the Skid frame connected to the

soft tank. Figure 15 shows the picture of the truss loaded on

the barge.

Winching and Connecting Holdback System. The truss

section was winched sternward 70.0 m [230 ft]. During the

winching, the H-627was ballasted at each individual step to

make the system even keel. At the end of winching, the

Pulling Winch HPU was transferred from a cargo barge to the

Deck of the H-627; the Hold Back System (winches, HPU and

deadmen) was installed on the H-627 and connected to the

Figure 15: Truss Structure Loaded Out on Barge

skid frame. The Holdback system was pre-tensioned followed

by ballasting of two mating tank compartments to correct for

transverse eccentricity in the truss section and to ensure tha

the truss and mating tank would float at even roll. The tie-bars

connecting the skid shoes during loadout were disconnected

and removed from the barge before the following steps

Figure 16 shows the location of the truss at the end of this

procedure.

Figure 16: Truss Structure Winched for Initial Ballasting

Initial Ballasting. The H-627 was ballasted until it had a trim

of approximately 6.6 degrees by the stern. Figure 17 show

the configuration of the truss at the end of initial ballasting.

Winching. The truss section was winched sternward 112.78m [370 ft] (until Deck 1 of the soft tank was 182.83 m or 600

ft. from the barge bow). As the truss section moved, the

mating tank continued to gain buoyancy and the truss section

rotated upward with respect to the barge. The winching

continued until the forward end of the soft tank skidshoes

were 178.56 m [586 ft] aft of the barge bow. The H-627 then

had a trim of about 6.25 degrees by the stern and the truss had

a trim of about 1.50 degrees by the mating tank. The

winching speed was about 1 ft/min or less. Figure 18 show

the configuration at the end of the winching operation.


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OTC-17299 11

Survey information was continuously updated at regular

intervals during the offload. The distance from the H-627 bow

to the soft tank keel was reported along with barge and truss

section list, trim and yaw at each interval. This survey

information was used to verify that the operation was within

the allowable tolerances.

Figure 17: Truss Structure at End of Initial Ballasting

Figure 18: Configuration at End of Winching Operation

Final Ballasting. The H-627 was then ballasted until the

truss section floated off from the hinge. The barge had a trim

of approximately 8.1 degrees by the stern at this point and the

truss had a trim of approximately 0.43 degree by the soft tank.

The barge was further ballasted to about 8.5 deg. to provide

sufficient clearance for the truss to be safely pulled away from

the barge. At the moment of separation the actual truss draft

was within 2 inches of the predicted draft. Figure 19 shows

the pictures of the truss and the barge at the end of finalballasting.

Post Float-off. The truss section was transferred to thecontrol of the tugs and wires connected to winches on land.

The mooring lines were slacked and disconnected. The tugs

then maneuvered the truss section to the mating site where it

was moored and made ready for mating.

The execution of this first-of-a-kind offload operation was

very successful and smooth and worked according to plan.

The complete operation was coordinated well between the

ballasting, winching and mooring operations. The operation

was completely controlled at all times. The detailed design

planning, preparations and good communications helped to

successfully execute the operation while mitigating the

associated risks.

Figure 19: Barge and Truss at the End of Offload Operation.


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Conclusions1) The truss offload operation, the first-of-a-kind was

designed and executed over a period of about seven

months.

2) The truss structure was loaded out from the bow andoffloaded over the stern of the barge by winching the truss

and ballasting the barge to cause total separation.

3) The barge inclination test performed without the trussonboard helped to verify most of the systems used for the

offloading operation and helped to improve the execution

procedure.

4) Various systems were used to address the critical issue ofwinching the truss parallel to the barge longitudinal axis.

5) Proper planning and detail planning was important insuccessfully executing the challenging operation of

offloading the truss structure from H-627 barge while

mitigating the associated risks.

6) The operation was completed conforming to the Jones Actregulation.

References[1] Perryman, S., Gebara, J., Botros, F. and Yu, A. (2005),

Holstein Truss Spar and Top Tension Riser System DesignChallenges and Innovations, OTC Paper No 17292.

[2] Sablok, A., Liu, C., Gebara, J., Cattell, A. and Perryman, S.(2005), Mating of Holstein Hard Tank and Truss Challenges,Execution, Dimensional Control and Analysis, OTC Paper No

17297.

AcknowledgementsThe authors wish to thank BP America Inc., Shell Offshore

Inc. and Technip Offshore, Inc. for their permission to publish

this paper. The authors would also like to acknowledge their

colleagues at the Houston engineering office and at the

fabrication yard for their contribution to the offload design and

operation.

OTC 17299 Launching

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