Upload
flowliner
View
219
Download
0
Embed Size (px)
Citation preview
8/4/2019 OTC 17299 Launching
1/12
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.
8/4/2019 OTC 17299 Launching
2/12
2 OTC-17299
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
8/4/2019 OTC 17299 Launching
3/12
OTC-17299 3
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
8/4/2019 OTC 17299 Launching
4/12
4 OTC-17299
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
8/4/2019 OTC 17299 Launching
5/12
OTC-17299 5
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)
8/4/2019 OTC 17299 Launching
6/12
6 OTC-17299
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
8/4/2019 OTC 17299 Launching
7/12
OTC-17299 7
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
8/4/2019 OTC 17299 Launching
8/12
8 OTC-17299
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
8/4/2019 OTC 17299 Launching
9/12
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
8/4/2019 OTC 17299 Launching
10/12
10 OTC-17299
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.
8/4/2019 OTC 17299 Launching
11/12
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.
8/4/2019 OTC 17299 Launching
12/12
12 OTC-17299
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.