TMR4225 Marine Operations.
Project work
Load-out, towing to site and installation of
Spool-piece transported on barge
Group members:
Mads Dalane
Petter Søyland
Henrik A. Larsen
Stefan Schlömilch
TMR 4225 Project work
Barge on spool
Preface
This is the result of the project work in the subject TMR 4225 Marine Operations, at the
university of science and technology (NTNU) in Trondheim. The goal of this project was to
enable a group of students to learn about the planning process of a marine operation. The
work was to be carried out by groups consisting of 3-4 members. Each group had the choice
between 7 different operations and this group chose to carry out task number 3: Load-out,
towing to site and installation of a spool piece transported on a barge.
The work was carried out as a group at “marinteknisk senter”. All the members shared an
office during most of the work time. Each member was given specific task but many problems
were discussed amongst all the group members.
A lot of different sources were used as references; those sources were: The internet, books
lend by professors or the library at Tyholt and most of all: communication with people
working in the industry as well as the teaching assistant and professors.
We would like to thank Ken Robert Jakobsen, Rasmus Haneferd of Acergy, Finn Gunnar
Nilsen, Tor Einar Berg and Robert Indegård of Taubåtkompaniet for their kind help.
Trondheim, April 17, 2007
Mads Dalane
Petter Søyland
Henrik A. Larsen
Stefan Schlömilch
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Table of contents
Preface.................................................................................................................................. II
Table of contents.................................................................................................................. III
Table of figures.................................................................................................................... IV
Table of tables ..................................................................................................................... IV
Summary .............................................................................................................................. V
1. Introduction ....................................................................................................................... 1
2. Scope of work .................................................................................................................... 2
2.1 Location ....................................................................................................................... 2
2.2 Weather........................................................................................................................ 3
2.3 Waves .......................................................................................................................... 4
2.4 Current ......................................................................................................................... 6
2.5 Weight of spool piece................................................................................................... 6
3. Operational phases ............................................................................................................. 7
3.1 Load-out and preparations ............................................................................................ 7
3.2 Towing to site .............................................................................................................. 7
3.2.1 Directional stability ............................................................................................... 7
3.2.2 Static stability........................................................................................................ 8
3.3 Installation ................................................................................................................... 9
4. Vessels............................................................................................................................. 11
4.1 Barge.......................................................................................................................... 11
4.2 Towing tug................................................................................................................. 11
4.2.1 Towing force calculations.................................................................................... 11
4.2.2 Tug efficiency factor............................................................................................ 13
4.2.3 Strength of towline and towline connections ....................................................... 13
4.6 Stabilizing tug ............................................................................................................ 14
4.7 Crane vessel ............................................................................................................... 16
5.1 About ......................................................................................................................... 17
5.2 Gantt chart for our project. ......................................................................................... 17
6. Offshore Standard DNV OS H101.................................................................................... 19
7. Forces acting on spool while deploying ............................................................................ 21
7.1 Water entry forces, calm water ................................................................................... 21
7.2 Vortex induced oscillations ........................................................................................ 23
8. Use of Remotely operated vehicle ROV ........................................................................... 25
8.1 Observation ROV....................................................................................................... 25
8.2 Work ROV................................................................................................................. 25
9. Feasibility study of the operation...................................................................................... 27
10. Conclusion ..................................................................................................................... 29
References ........................................................................................................................... 30
Pictures ............................................................................................................................ 30
Appendix ............................................................................................................................. 31
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Table of figures
Figure 1:Map of Haltenbanken............................................................................................... 2
Figure 2:Voyage Kr.sund-Haltenbanken ................................................................................ 2
Figure 3:Kristiansund............................................................................................................. 2
Figure 4: Wind velocities in July and December. ................................................................... 3
Figure 5: Monthly mean values of Hs for the Haltenbanken site ............................................ 4
Figure 6: Current direction at 3 and 100 meter water depth ................................................... 6
Figure 7:Lifting arrangement ................................................................................................. 7
Figure 8:Tow out ................................................................................................................... 9
Figure 9: Landing................................................................................................................. 10
Figure 10: Viking barge 1 .................................................................................................... 11
Figure 11: Tug Pawlina........................................................................................................ 14
Figure 12: stabilizing tug...................................................................................................... 14
Figure 13: Tug Lieni ............................................................................................................ 16
Figure 14: Normand Mermaid.............................................................................................. 16
Figure 15: Gantt chart example ............................................................................................ 17
Figure 16: Gantt chart .......................................................................................................... 18
Figure 17: α-factors.............................................................................................................. 20
Figure 18: Strouhals number vs. Reynolds number............................................................... 23
Figure 19: Observation ROV, Seaeye Lynx.......................................................................... 25
Figure 20: Connection of spool piece with hydraulically powered tool and Work class ROV
............................................................................................................................................ 26
Figure 21: Hercules, Work class ROV.................................................................................. 26
Table of tables
Table 1: Wind data from Haltenbanken. ................................................................................. 3
Table 2: Barge data .............................................................................................................. 12
Table 3: Barge calculations .................................................................................................. 12
Table 4: Towing force on barge............................................................................................ 12
Table 5: Towline pull required related to bollard pull ........................................................... 13
Table 6: Towline pull required and continuous bollard pull .................................................. 13
Table 7:Minimal break loads of towline ............................................................................... 13
Table 8:Towline and towline connection strength................................................................. 14
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Summary
Our task in this project was to investigate different aspects of the load-out, tow-out and
installation of a spool piece with the help of a barge.
Initially we decided that the tow out should take place from Kristiansund to the Njord field
situated at Haltenbanken. Metrologic data about Haltenbanken was gathered and we found out
that it would be preferable to conduct the operation during the summertime, due to wavewind- and lighting conditions.
We need four vessels for the operation. The barge has to be able to carry the 107 meters long
spool, and the selected towing tug requires a towline pull of 65,9 tons. We also need a second
tug for maneuvering through narrow areas, increased directional stability of the barge, as well
as operational aid under lift off. The crane vessel needs to be able to operate the 31 tons spool
with applied heave compensating. This is to make the operation easier and to minimize the
risk. We were able to find 4 suitable vessels and the are presented in the full report.
The main operational phases were defined and those were: the load out and preparations, the
tow out and the installation of the spool. Different aspects of those phases were investigated
such as directional stability of the barge, and forces acting on the spool during descent.
To get a nice overview of the operation a Gantt chart was established and discussed. We also
checked which DNV rules we need to comply with and commented on the most important
rules such as wave height and weather forecast uncertainties.
We also discussed the use of ROVs to assist the operation and a short introduction into
different kinds of feasible ROVs is given.
At the end we once more commented on the feasibility.
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1. Introduction
In this project work we are going to investigate the load out, tow out and installation of a spool
piece with help of a barge. This operation has to be planned and several key parameters such as
locations, distances, water depth and environmental factors as well as vessels and main phases
need to be defined. A Gantt chart should be established, as well as the compliance with DNV
rules. Hydrodynamic and stability analysis should be conducted. At the same time the role of
ROV as an aid should be discussed and last but not least the feasibility of the project should be
checked.
Many of the parameters and options are loosely defined in the exercise so the groups are
encouraged to find solutions and locations etc themselves.
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2. Scope of work
In this chapter we will look into the scope of work and different limitations we have, due to
location, weather, waves, current and weight of structure.
2.1 Location
Our operation will take place at the Njord field in the northern North Sea, Norway. According to
offshore-technology.com: the Njord field is situated on Haltenbanken, in blocks 6407/7 and
6407/10, 130 km north of Kristiansund and 30 km west of the Draugen field. To be more
specific the landing of the spool piece will happen nearby the Njord A platform, and the spool
piece shall connect the Draugen PLEM (Pipe Line End Manifold) and the Njord PLEM.. The
water depth in this area is around 330 meter.
The oil is produced from the Tilje Formation on the East Flank and in the Central Area. The oilbearing Tilje formation is 100-150m deep, covering an area of around 40km². Reservoir
pressure is calculated to be at 390bar, 2,850m below sea level. Temperatures at this depth are
roughly 114°C.
The topography of the sea bed at Haltenbanken is relatively flat .
Figure 1:Map of Haltenbanken
Figure 2:Voyage Kr.sund-Haltenbanken
Figure 3:Kristiansund
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2.2 Weather
All marine operations are weather dependent to some extent. In our operation it is important to
have as little wind and waves as possible during the tow out and most important during the lift
off, water entry and landing of the spool piece. The northern North Sea is well known for its
inhospitable weather during most of the autumn and the winter. Below some wind data from
July and December are listed up. Based on these data the operation will take place during
summertime, most likely in June or July.
Stnr
Month
FFM [m/s]
71850 04.2006
6,1
71850 05.2006
6,6
71850 06.2006
4,4
71850 07.2006
5,2
71850 08.2006
5,4
71850 09.2006
6,3
71850 10.2006
8,3
71850 11.2006
10,7
71850 12.2006
11,3
71850 01.2007
10,1
71850 02.2007
9,2
71850 03.2007
9,0
Lowest
Date
4,4
06.2006
Highest
Date
11,3
12.2006
Table 1: Wind data from Haltenbanken.1
Figure 4: Wind velocities in July and December. 2
1
2
Data from e-mail from the Norwegian meteorological institute
http://planverk.nofo.no/web_AP3_3.asp?NAVN=Njord+A&Month=Juli
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2.3 Waves
Below follow some wave data from the area. Not surprisingly, also the wave conditions are
much more suitable in the summertime. The monthly mean value for Hs in July is approximately
1.5 meter.
Figure 5: Monthly mean values of Hs for the Haltenbanken site 3
In Figure 6 the cumulative probability for significant wave height is shown. The dotted line
represents data for an average year. For instance the probability of Hs < 2 meter in summertime
is 70%. The solid-drawn lines correspond to Hs level of given return periods. Starting with Tp=3
years up to Tp=25 years.
From Error! Reference source not found. the wave statistics for Haltenbanken between June
and August for all directions are shown. The numbers in the diagram are probability per
thousand to get a wave condition with a certain significant wave height Hs and zero crossing
periods T0. For instance the probability is 1/1000 to get a wave condition with Hs=6-7 meter and
T0=8-9 seconds.
Figure 6: Cumulative probability of Hs, P(Hs) in summer season.4 And wave statistics for Haltenbanken
between June and August. 5
3
http://icoads.noaa.gov/jcomm_tr13.pdf
Vik and Kleiven 1985.
5
(Global Wave Statistics. N. Hogben 1986)
4
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2.4 Current
In general, current is not a big problem for this area, compared with for instance south of
English sector, where the tidal range and currents are large. The maximum current is about 1
knot, and diminishes linearly to zero at sea bed.
As shown in Figure 6 the current direction changes 90 degrees from water depth of 3 meter to
100 meter. This phenomenon is very important to take into account during the water entry and
landing of the spool piece. The reason for this current twisting is caused by the Coriolis Effect,
and is called Ekmans spiral.
Figure 6: Current direction at 3 and 100 meter water depth 6
2.5 Weight of spool piece
According to the “Tioga Pipe dimensions and weight”7 the typical weight of a 30’ with 0.625’
wall thickness is:
kg
0.453
lbs
lbs = 291.22 kg
⋅
196.08
foot 0.305 m
m
foot
kg
The total weight of the spool piece: 107 m ⋅ 291.2
= 31161.3kg ≈ 31ton
m
6
(http://www.npd.no/NR/rdonlyres/FAF926A2-5137-4717-9778-79971872DCB4/0/Rapport43.pdf ).
7
(http://www.tiogapipe.com/TiogaChart.pdf)
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3. Operational phases
3.1 Load-out and preparations
The spool piece has been assembled on the pier in Kristiansund and will be loaded on to a barge
here. We will use the same method to load the spool onto and off the barge – several wires are
attached along the spool and assembled together above the center of gravity of the spool. One
crane is used to lift the spool and tugger lines are applied at the ends to control the yaw motion
of the spool. A crane on the pier is used for on-loading and the main crane on the offshore vessel
“Normand Mermaid” is used for off-loading. The spool will be fastened to the barge by
welding. If there is calm water and the calculations are correct, there should not be any critical
issues in this phase of the operation.
Figure 7:Lifting arrangement
3.2 Towing to site
During towing it is important to keep both static and dynamic stability of the barge. After an
analysis of the directional stability of the towing system with one tug (see below), we found the
need to use two tugs to tow the barge, one in front and one behind. This also helps to maintain
good maneuvering of the barge in sheltered waters and to better keep the barge steady when
lifting the spool off the barge at the site.
3.2.1 Directional stability
We first looked at the possibility to tow the barge with one tug. If the barge is not static and
dynamical stable during tow out, it might get unwanted motions in sway and yaw. This is not
preferred when maneuvering in narrow waters, and it could cause snatch loads on the towline
and in worst case the towline could break.
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3.2.2 Static stability
When exposed to a small external force in either sway or yaw, the restoring forces will oppose
the external force if the barge is static stable. We assume sway and yaw motions to be
uncoupled and the static stability criterion is given by:
δ Fy
δM
+a
<0
δψ
δψ
Where
Fy (ψ ) : Force acting in sway direction
due to current on barge when
M (ψ ) : Moment acting in yaw direction
it is exposed to a yaw angle ψ .
and
δ Fy
δψ
ψ =0
δM
δψ
= −U 2 mT
ψ =0
= U 2 (m22 + xT mT )
m22 : mass of barge and added mass in sway
mT : 2-D added mass coefficient for the trailing edge
xT : distance from center of gravity to trailing edge (always negative)
U: speed of current on barge (speed of barge)
a: the distance from the center of gravity of the barge to the mooring point for the
towing line
Normally
δ Fy
δM
< 0 and
> 0 . This indicates that the sway force will act stabilizing on the
δψ
δψ
vessel while the yaw moment will act destabilizing.
To make the barge as hydrodynamically efficient as possible is has a stern which allows the
water to “slip” easily away. In other words; it has no transom stern and the area perpendicular to
the flow of water is very small. Because the added mass in the stern is directly proportional to
the area at the stern we may assume that the barge does not have any contribution to added mass
in the stern and set mT = 0 . We get that:
δ Fy
δψ
ψ =0
=0
and
δM
δψ
ψ =0
= U 2 m22
Unfortunately, we see that the static stability criterion becomes:
U 2 m22 < 0
This expression is false, because both m22 and U 2 are always positive.
There are a lot of assumptions involved in the preceding paragraph. One of them is the fact that
δ Fy
δM
the formulas for
and
are based on slender body theory, which is more suitable for a
δψ
δψ
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ship than a barge, because of the L/B ratio of the barge. In addition to this, the slender body
theory implicates non-viscous flow, but in reality there will be viscous flow.
δF
Positively, the effect of viscous flow will ensure a negative value of y , which will give a
δψ
contribution to the restoring forces, but as the criterion for static stability is not fulfilled, we
should do a closer analysis. The problem is to determine the magnitude of the restoring forces
δF
on the barge due to viscous effects (which will contribute to the term a y in the static
δψ
stability criterion), when the barge is given an angleψ in yaw.
Since the towing system is not statically stable it will neither be dynamically stable, since it
doesn’t have enough restoring forces to oppose the external forces it is exposed to.
To increase the static stability one would normally increase the distance between the center of
gravity of the vessel and the mooring point for the towing line. To see the effect of this, we need
to do a further analysis on the effect of viscous forces on the barge. Instead of doing this we plan
to use two tugs, one in front of the barge and one behind.
This is frequently done in similar operations, and in addition to assist in towing out, two tugs
will better keep the barge in position at the site. A brief calculation of the needed bollard pull for
the second tug to maintain static stability is done in chapter 4.6 “Stabilizing tug”.
Figure 8:Tow out
3.3 Installation
We will use the main crane on “Normand Mermaid” to lift the spool off the barge and lower it
down to the seabed. This operation is divided into five phases, each phase containing critical
issues that need attention. The whole operation must be done when the weather and sea state
allows for it.
1. The spool is lifted off the barge in the same manner as we did at the pier in Kristiansund.
After the spool piece has been lifted off, “Norman Mermaid” will move sideways to get
clear of the barge. When considering this operation, we have a coupled system with 18
degrees of freedom – barge, spool and vessel, each with 6 degrees of freedom. Limiting
parameters would be possible snatch loads in the crane wire and damage to the spool if
impact with barge should occur. This is directly related to the wave conditions. Also, the
operation cannot be executed if there is too much wind, because this could result in
unwanted drift of barge/vessel during lift off. This could result in horizontal sliding of
spool on the deck and cause damages to spool, equipment or even personnel.
2. Tugger lines are applied to better control the motion of the spool when it is in the air. An
important issue will be to avoid collision with the ship side. This can occur if the waves
are too big resulting in large motions of the spool, if there is too much wind accelerating
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the spool, or if an instance of Mathieu instability emerges and we get large pendulum
motion.
3.
The crane on “Normand Mermaid” is heave compensated and even though there are
wave forces acting on the vessel, the spool can be lowered at even speed. Given this, we
must estimate the vertical hydrodynamic and static forces on the spool as it enters the
water. Again, possible snatch loads on the wire must be considered. We must also avoid
forces that can unbalance the spool. If the spool piece has any kind of bended shapes, it
is important to think of rotational stability as the pipe enters the water. It has happened
before that curved spool pieces have twisted when entering the water due to change in
buoyancy.
4. As the spool descends towards the seabed, we must keep an eye on possible current
forces acting on the spool. These forces can push the spool away from its landing site on
the seabed, and it might be necessary to move the vessel to counteract this. In general
one must be aware of vertical resonance as the spool is lowered down, but the heave
compensated crane should help us here. To avoid any snatch loads, it is important that
the crane cable is tight at any time.
5. Finally, the spool will arrive at the landing site on the seabed. Before the spool piece can
be lowered the last part, it has to be rotated exactly over the second PLEM. This is
simply done by moving the crane vessel in the right position. In advance the ROV
connection tools are already lowered down, and prepared at the PLEMs, and the pipe can
be connected once it has touched the sea bed. “Normand Mermaid” is equipped with two
ROVs - one fairly large work class ROV of about 5 tonnes and one observing ROV. The
work class ROV will then connect with the tools, and couple the spool piece to the
PLEMs. We must avoid large impacts between the spool and the seabed and the existing
subsea installations.
Figure 9: Landing
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4. Vessels
As presented before, we need four vessels. Two tugs, one crane vessel and one barge. The
choice of tugs depends on the specifications of the barge. The towing tug must be able to tow
the barge at requested speed. The other tug may be smaller and will operate behind the barge.
This barge will apply directional stability as well as navigation in narrow fjords. The crane
vessel should be dimensioned to lift the spool from the barge. All calculations and numbers are
presented in (Calculations, Appendix A).
4.1 Barge
An appropriate barge for transporting the spool is the (Viking barge 1) from Viking supply ships
AS. The most critical criteria for transporting the spool is its length, 107 meters. Viking barge 1
is 91 meters, and will therefore fit almost all the length of the spool, except 8 meters on both
sides. Because of this we had to take some precautions avoiding destructive slamming on the
spool. Some wave limitations are applied to this, but because of the barges design, the waves are
to be extreme if they could harm the spool.
Figure 10: Viking barge 1
4.2 Towing tug
To find an appropriate tug, we had to find the total towing force of the barge. According to
(Taubåtkompaniet), the pulling force of the tug should be 60 tons with the selected draught and
velocity. We will do some calculations to see if the estimate is right.
4.2.1 Towing force calculations
The data we have collected from the barge are taken from the web page (Viking barge 1), as
well as from (Taubåtkompaniet). The most relevant values are presented as follows.
Input data
Lenght over all (LOA):
Breadth
Depht:
Light weight:
Towing velocity
Towing velocity
91,44[m]
27,4[m]
6,1[m]
1850000[kg]
3,5[knots]
1,8[m/s]
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Load
Cb at 1,8 meter waterline
minimal draught bow
minimal draught stern
Table 2: Barge data
Barge on spool
31000,0[kg]
0,8735[-]
1,8[m]
2,3[m]
Because the spool is light compared to the barge, we have to add ballast to fulfill the minimum
requirements for towing of the barge. The minimal required draught respectively in the bow and
the stern are 1,8m and 2,3m. The barge will then have a slight aft trim. This is to avoid
slamming forces under the barge in heavy weather. The block coefficient presented is correct at
that specific draught. Calculations are shown below.
Calculations
Displacement:
Total weight
Needed ballast:
Table 3: Barge calculations
4486,457[m3]
4621,1,[tons]
2740,1[tons]
According to (Noble Denton) minimum towline pull required (TPR) shall be computed for zero
forward speed against a 20m/s wind, 5,0m significant sea state and 0,5m/s current, acting
simultaneously.
The wave resistance is calculated by the formula (Faltinsen 90):
B
FW = ρ ⋅ R 2ξ 2 ⋅
2
Where R is the shape factor, and ξ is the wave amplitude.
The effects needed because of the propeller race are also included in respect to the extra velocity
the propeller induces on the barge. The towing cable length is assumed to be 50m. (Haneferd
07). Calculated values and coefficients are presented in the Calculations in appendix B. The
formulas and values are taken from tug specifications (Tug Pawlina) and (Nielsen 03). The extra
induced velocity according to these assumptions is 1,2m/s. This extra velocity is added to the
towing velocity used to calculate the drag force.
The drag force of the barge is calculated by Morrison’s drag equation:
1
FD = ρ ⋅ v 2CD AS
2
The drag force coefficient CD is given by (Taubåtkompaniet), and consists of drag forces and
viscous forces on the hull. The velocity induced by the propeller race as well as the current
according to (Noble Denton) are also added to the towing velocity. The wind resistance is
calculated with Morrisons formula, where Cd is estimated to be 1. This gives the following
values:
Forces on barge
Wave drift force, Fwa
Drag force, Fd
Wind resistance Fw
Towline pull required (TPR)
Table 4: Towing force on barge
17,9[kN]
318,1[kN]
31,6[kN]
367,5[kN]
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4.2.2 Tug efficiency factor
The tug efficiency factor has to be applied to the TPR according to (Noble Denton). This is due
to decreased performance on tug under harsh weather conditions as well as necessary safety
factors. For Hs = 5m, BP can be related to TPR by:
Table 5: Towline pull required related to bollard pull
According to the table above, the results are given in tons:
Towline pull required and continuous bollard pull
Towline pull required (TPR)
37,5[tons]
Continuous bollard pull (BP)
65,9[tons]
Table 6: Towline pull required and continuous bollard pull
The calculations of the continuous bollard pull, 65,9 tons is close to the suggested BP from the
(Taubåtkompaniet) and must be regarded as reliable. This bollard pull is calculated regarding
the worst possible excepted weather conditions. More calm weather as well as longer towline
will decrease the pollard pull considerably. However, if these towing terms should occur, the tug
will need this bollard pull to reach the operation area in time.
4.2.3 Strength of towline and towline connections
The minimum breaking loads (MBL) of the towline and the bridle legs shall be related to the
bollard pull (Noble Denton). Following rules are applied in our calculations:
Table 7:Minimal break loads of towline
Because of our offshore operation location at Haltenbanken, we have to use the “other areas”
rules. The ultimate load capacity (ULC) of towline connections including bridle legs have
following restrictions:
ULC = 1, 25 ⋅ MBL
ULC = MBL + 40
Whichever is less to be utilized.
Towline and towline connection strength
Minimum breaking loads (MBL)
163,5[tons]
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Ultimate load capacity (ULC)
203,5[tons]
Table 8:Towline and towline connection strength
Now we have calculated the total bollard pull of the tug. The criteria of the tug is to be
functionally offshore, and to have a bollard bull of at least 65,9 tons. (Tug Pawlina) with its
bollard pull of 67,1 tons should be appropriate to tow the barge.
Figure 11: Tug Pawlina
4.6 Stabilizing tug
The second tug is used to improve maneuvering, directional stability and to keep the barge in
position on the site. It will be moored at the stern of the barge. During tow out we want to take a
closer look at what the bollard pull of this tug must be to improve the directional stability of the
barge. This will be useful in deciding the size of the tug.
To estimate the second tug’s bollard pull we must look back at the static stability criterion and
δM
and compare it to the restoring moment
look at the derivative of the destabilizing moment
δψ
provided by the second tug.
Figure 12: stabilizing tug
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Here we assume that the barge will rotate around the point P, to simplify the calculations.
The calculations we did for the one tug system to check if the system was static stable or not,
told us that the destabilizing moment, M (ψ ) needs to be balanced to obtain stability. We can
δM
2
compare
ψ =0 = U m22 with the derivate of the moment from the second tug acting on the
δψ
δ M tug
. The contribution from the tug should at least equal this. Viscous effects will also
δψ
δF
provide restoring forces (we get a negative contribution from the term a y in the static
δψ
barge
stability criteria), so if
δM
δ M TUG
= U 2 m22 ≤
δψ
δψ
it should be ok.
If we assume the yaw angle ψ to be small, we can write M TUG = FB ⋅ L ⋅ψ .
This gives us
δ M tug
= FB ⋅ L , which again gives us the force on the barge from the second tug
δψ
FB ≥
U 2 m22
L
Calculations give these results
U: 3 knots = 1,54 m/s
L: length of barge = 90m
m22 : mass and added mass of barge, M + A22
M: mass of barge = 4600 e3 kg
A22 : added mass of barge:
A222 D
= 2,7 (Faltinsen 90)
ρπ a 2
A22 = A222 D ⋅ L = 2,7 ⋅ ρ ⋅ π ⋅ a 2 ⋅ L = 782492 kg
And finally
FB ≥
U 2m22 1,542 ⋅ (4600e3 + 780e3)
=
= 141769 N
L
90
This corresponds to a pull of about 15 tons.
The tug operating behind the barge is not required to be as powerful as the barge in front. An
example could be (Tug Lieni) with its bollard pull of 35 tons. The estimate of the pull is rather
rough, and the parameters can be discussed. Among them are the effects of propeller race and
15
TMR 4225 Project work
Barge on spool
current on the barge, and how much this affects the directional stability and thereby the need for
pull from the second tug. We have not included the pull of the second tug in the calculations of
the bollard pull of the towing tug. This is because the calculation of the pull of the second tug
only is rough estimates, and questions of uncertainty are involved.
Figure 13: Tug Lieni
4.7 Crane vessel
The crane vessel requires a capacity of at least 31 tons at sufficient distance from the crane
foundation. It must also have a heave compensator to make the operation easier and to minimize
the risk. The vessel (Normand Mermaid) is recommended by (Haneferd 03). This vessel meets
the requirements of crane capacity of 100 tons. Normand Mermaid is also fit to use the heave
compensator because the load does not exceed 1-2% of the vessel weight (Nielsen 03).
Figure 14: Normand Mermaid
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TMR 4225 Project work
Barge on spool
5. Gantt chart
5.1 About8
The Gantt chart has in the recent decades become one of the most popular project scheduling
aides. It is a relatively simple chart which, with the help of bars and time indicators, gives a
wide overview of a projects subtasks and the planned duration of those.
ID
Mon 16 Apr
0
6 12
18
Tue 17 Apr
0
6 12
18
Wed 18 Apr
0
6 12
18
Thu 19 Apr
0
6 12
18
Fri 20 Apr
0
6 12
18
Sat 21 Apr
0
6 12
18
Sun 22 Apr
0
6 12
18
Mon 23 Apr
0
6 12
18
Tue 24 Apr
0
6 12
18
Wed 25 Apr
0
6 12
18
1
2
3
4
5
Figure 15: Gantt chart example
One is able to define simple constraints such as
• start to start; meaning that two subtasks need to start simultaneously
• start to finish; meaning that a task may start when the preceding task is finished
• finish to finish; meaning that two or more tasks should end at the same time
• work time; how many hours a day work should be executed, weekends etc.
•
While the project is being executed one is also able to define things such as progress and
completion of tasks and it is relatively simple to assess if one is on schedule or not. One of the
drawbacks with a gantt chart is that one is not able to see how labor intensive, weather restricted
or expensive etc. the various tasks are. In addition it may be hard to keep it organized and tidy
when the number of sub tasks is large.
But as a simple tool for scheduling and overview purposes, the gantt chart is perfect.
5.2 Gantt chart for our project.
We have decided to divide the project into the following tasks and subtasks
• Planning of marine operation
• Preparation of harbour
• Harbour Operation
o Preparation of tug
o Preparations of spools
o Lifting of spools onto barge and sea fastening
o Connect tug and barge
• Offshore preparations
o Transit of barge
o Transit and preparation of crane vessel
o Connection of crane vessel and barge
o Deployment of ROV
• Lifting operation
o lift off barge
o pass splash zone
o lowering to bottom
8
http://en.wikipedia.org/wiki/Gantt
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TMR 4225 Project work
Barge on spool
o landing of spool and preliminary connection
In our case we have decided that the work time is 24 hours a day. This is made possible by two
facts:
• The operation will take place during the summertime, relatively far north, meaning that
the lighting conditions will be sufficient throughout the night. Still, very critical
operations like entering the splash zone should be avoided in the middle of the night
• The crew is most probably working in a 6 hours on 6 hours off schedule, meaning that
they already have an artificial diurnal rhythm and they will have the same performance
capabilities regardless of day and night.
The times which are indicated in the Gantt chart are approximate values which either are
“guessed” qualitatively or estimated based on assumptions which we think are reasonable. Some
examples are given below:
• The transit distance is 130 km and average speed is 3 knots [1.54 m/s]; this gives us a
time of 23.5 hours, this number is rounded up to 24 hours.
• The same thing can be done about the lowering time. 330 m / 0.5 m/s= 660 s=11
minutes. This would be a very small post on the Gantt diagram such that it is rounded
up to one hour.
• Most other times are assumptions.
This gives us the following Gantt chart (can be seen in a larger version in the appendix) :
ID
Task Name
1
planning of marine operation
preparation of harbour
Marine Operation
Harbour Operation
Preparation of tug
Preparations of spools
Lifting of spools onto barge and seafastening
Connect tug and barge
Offshore preparations
Transit of barge
Transit and preparation of crane vessel
Connection of crane vessel and barge
Deployment of ROV
Lifting operation
lift off barge
pass splash zone
lowering to bottom
landing of spool and preliminary connection
18
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
20
22
Mon 18 Jun
0
2
4
6
8
10
12
14
16
18
20
22
Tue 19 Jun
0
2
4
6
8
10
12
14
16
18
20
22
Wed 20 Jun
0
2
4
6
8
10
12
14
16
18
Figure 16: Gantt chart
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TMR 4225 Project work
Barge on spool
6. Offshore Standard DNV OS H101
The offshore standard DNV OS H101 (OS hereafter) is a relatively extensive standard written
for industrial and commercial marine operations. The scope of our project is not as large as such
projects, so we have assumed that most of the rules are satisfied and are just going to comment
one the rules we found to be most important.
Seen from a weather point of view there are two kinds of operations considering weather and
time restrictions: unrestricted operations (lasting for more than 72 hours) and restricted
operation (lasting for less than 72 hours).(see section OS Section 4 B 500) The largest difference
between those two is that one is allowed to introduce restrictions on the operations lasting for
less than 72 hours, which are stricter than the official OS states. This means that a structure
which is used in an operation which lasts for less than 72 hours may be constructed weaker than
a construction which is designed for an operation lasting for more than 3 days. The reason for
this is that the accuracy of weather forecasts only is within an acceptable limit for 3 days.
As can be seen from the Gantt chart (Figure 16: Gantt ) our total operation time is approx. 2.5
days (60 hours). According to the OS Section 4 B 500 our operation may thereby be
characterized as a weather restricted operation, as it has duration of less than 72 hours.
Moreover we have three sub operations which all have a duration of less than one day. Those
sub operations have different weather restrictions and are relatively independent of each other.
The harbor operations are of course taking place in the harbor. The harbor of Kristiansund is
situated in relatively sheltered waters such that there probably won’t be any waves larger than 1
meter in the area.
The transit operation includes two tugs towing a relatively light barge. The barge has a large
freeboard and the spool only extends 5 meters on either side of the barge. This makes it possible
for the towing operation to take place in relatively high waves. Haneferd 07 referenced that
Acergy uses a maximum significant wave height of 5 meters in towing operations. This wave
height is only exceeded by 1,4% of the sea states during June- August in that region, according
to “global wave statistics” (Hogben 86). Moreover the transit only takes about 24 hours, which
allows for quite accurate weather and wave predictions.
It is the lifting operation which is of highest importance considering the wave restrictions. The
“bottleneck” of the operation is the relative motion of crane vessel and barge. Here Haneferd
recommended a maximum significant wave height of 1,5-2 meters. Again referring to Hogben
we can see that this wave height is exceeded by 41,7% of the sea states in the region within the
period of June-August.
From this one sees that the event which is the most dependent on the weather is the event which
is furthest away in time, meaning that the weather predictions will have a certain uncertainty.
Let’s investigate what weather predictions must be satisfied, to give the order for mobilization:
The spools may be loaded on to the barge almost independently of weather. One restriction
might be rain or heavy winds, considering that the spools will be connected to the barge by
welding. After the spools have been attached and the tug connected to the barges one needs 24
hours of waves below 5 meters, followed by at least a 10 hour period of less than 2 meters wave.
Preferably this period should be extended to 14-16 hours to include unforeseen happenings.
The operational criterion is defined by C0=α*OPLim where OPLim is the operational limit: 5
meters for the first 24 hours and 2 meters for the following 16 hours. The α symbolizes an
uncertainty factor in the forecasts. This uncertainty factor is unique for different areas and is
found from tables. We us table A-3 in appendix B of OS and find assume that we have a
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TMR 4225 Project work
Barge on spool
forecast level B and monitoring on board. Monitoring means that all design criteria such as
wave height and wind etc are monitored within reasonable accuracy and update rate. Any
abnormalities should be reported and investigated and variation borders should be defined.
This gives us the following α-values for the initial forecast:
Figure 17: α-factors
With time and monitoring those factors will of course increase because the uncertainty is
constantly decreasing. The weather forecasts should be updated no fewer than at least once
every 12 hours.
When the operation is starting at the pier the forecast should thus not have a larger forecasted
value for the wave height than C0=α*OPLim=0.62*2=1.24 meters.
In this particular operation wind and current velocities are of small importance. The ship is
equipped with a good DP system which enables it to counteract the currents. When the spool is
drifting off course the crane vessel will change its position to counteract the drift off.
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Barge on spool
7. Forces acting on spool while deploying
7.1 Water entry forces, calm water
The water entry is a critical phase, and it is necessary to find the vertical hydrodynamic forces.
We assume calm water, and use this formula to find the water entry forces:
i
F3 = ρ gV + A33 ⋅U z +
dA33 2
⋅U z
dh
Assumptions:
• Ideal, non-viscous, non-compressible fluid
• No viscous effects
• Body dimensions << wave length
The first part in the equation is hydrostatic force, the second added mass force and the last is
“slamming” force. The hydrostatic force is a function of the volume V of the submerged spool.
The acceleration in the added mass force, is found under the assumption that the lowering
velocity before entering the water is U=1m/s, the retardation time is 0.5 seconds and the sinking
velocity is Uz=0.5 m/s.
i
Uz =
U −U z
m
= 0.25 2
t
s
The slamming force can according to equation (9.39) in (Faltinsen 90) be written as:
dA33 2 ρ
⋅U z = Cs 2 RU z2
dh
2
Cs =
5.15
1 + 8.5
h
R
+ 0.275
Where Cs is:
h
R
The next problem is to find the volume of the submerged spool as it is sinking into the water.
First we found the area of a circle segment:
A=
R2
(θ − sin θ )
2
Below follows an Excel spreadsheet with calculations of the vertical hydrodynamic force. The
force is also plotted as a function of submerged spool, h. This calculation is as mentioned only
for calm water, and the added mass- and slamming forces would in real life be larger. From the
spread sheet underneath, we se that the hydrostatic force is the main contribution to the total
force. We also notice that the slamming force is largest the moment it hits the water, witch is
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TMR 4225 Project work
Barge on spool
reasonable. In this case we have used a constant negative acceleration for the added mass force.
This is not quite correct, as the acceleration will change during the water entry. As mentioned
above, this analysis is for calm water only. With waves in addition, we can expect a larger
contribution from the slamming- and added mass force.
L
D
R
lowering vel, U
sinking vel, Uz
time of retardation
negative accel.
rho
g
107m
0,762m
0,381m
1m/s
0,5m/s
0,5s
1m/s^2
1025
9,81
submerged spool, h Area of circular
[m]
segment
Cs
0,00
0,00 5,15
0,10
0,04 1,67
0,20
0,10 1,09
0,30
0,17 0,89
0,40
0,24 0,81
0,50
0,32 0,78
0,60
0,39 0,79
0,70
0,44 0,82
0,76
0,46 0,84
Hydrostatic
Added
Slamming
force
mass force
force
0,00
38002,21
102723,55
179423,91
260898,23
341278,85
414427,34
471802,09
490543,36
0,00
3873,82
10471,31
18289,90
26595,13
34788,87
42245,40
48093,99
50004,42
1005,60
325,33
212,30
173,00
157,71
153,20
154,46
159,17
163,12
Total
vertical
force [kN]
1,01
42,20
113,41
197,89
287,65
376,22
456,83
520,06
540,71
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Barge on spool
7.2 Vortex induced oscillations
When the spool is lowered towards the bottom, vortex shedding may occur. It is important that
the vortex shedding frequency is not in the area of the resonance frequency of the spool. A
simple analysis will be conducted here.
1 D
The vortex shedding period is according to (Faltinsen 90) found by TV = i
where St is the
St U C
Strouhals number which is Reynolds number dependent, D is the diameter of the spool and Uc
is the sinking velocity.
Figure 18: Strouhals number vs. Reynolds number
First the Reynolds number Rn =
Uc D
needs to be found. ν is the cinematic viscosity of
Uc D
=
ν
0.5m / s *0.762m
= 3.2*105 . According to Figure 18:
6
2
ν
1.2*10 m / s
Strouhals number vs. Reynolds number this is exactly in the critical Strouhal region, meaning
the Strouhals number is somewhere between 0.2 and 0.5. This gives us a eigenperiod range of
7.62[s] < T < 3.05[s] . It is critical that the eigenperiods of the spool are not in that region. But
there are several ways of preventing resonance oscillations. By either increasing or decreasing
the sinkage speed one will get a more disambiguously defined Strouhals number. At the same
time one can adjust the effective distance between the crane wires which are attached to the
spool. By changing the distance between those wires one will change the length of the
oscillating element and thereby also change the eigenperiod.
saltwater. Rn in this case is Rn =
The eigenperiod/frequency of a spool may be found by literature such as (Bergan 86). The
EI
where ωn can be found by determining
formula for the frequency is given by ωn = ωn
ml 4
different border conditions. We will assume it is fixed in either end due to symmetry effects.
Then ωn =22.37 for the first eigenvalue. E = 2,1*109 . I is given by
result I y =
π
4
(R4 − r 4 ) =
π
4
where r is the inner and R the outer diameter. This gives us the
(0.3814 − (0.381 − 0.016)4 ) = 0, 0026 m is approx 290kg/m and the
23
TMR 4225 Project work
Barge on spool
length can be varied. In this exemplified calculation we will use 10 meters. This gives us the
EI
2.1*1011 *0.0026
=
22.37
*
= 22.37 * 4.33 = 97 Hz → T = 0.01s .
ml 4
290*104
Here we can clearly see that resonance oscillations are of no importance when the crane wires
are situated so close to each other and the descending velocity is low.
following result: ωn = ωn
24
TMR 4225 Project work
Barge on spool
8. Use of Remotely operated vehicle ROV
In almost any marine operation the use of ROV is important. After the structure has entered the
water, the ROV becomes eyes and hands of the engineers. For our operation we need both
observation ROV and work class ROV.
8.1 Observation ROV
After the spool piece has been launched into the water, the Observation ROV is in use until the
operation is done. In the lowering–phase it is important to check that the spool piece is stabile
and that the crane cable is tight at any time. During the landing phase it is crucial to have good
and clear images of the spool and the PLEM (Pipe Line End Manifold) at all time. On the Njord
site the water depth is too large to use divers (max depth usually 100 meter), so the only way to
get visualization is by ROV. The Observation ROV is also very useful for the pilot of the Work
class ROV, to get a kind “bird's-eye” view of what he is doing. Below is typical vehicle data for
an Observation ROV:
•
•
•
•
•
•
•
•
•
Maximum working depth: 1500 m
Length: 1260 mm
Height: 625 mm
Width: 825 mm
Forward thrust: 66 kg
Lateral thrust: 47 kg
Vertical thrust: 43 kg
Launch weight (basic vehicle): 210 kg
Payload: 40 kg
Figure 19: Observation ROV, Seaeye Lynx9
8.2 Work ROV
The Work ROV is a much more robust and heavy vehicle than the Observation ROV, that can
be equipped with heavy duty manipulators and hydraulic tooling. The Work class ROVs can
either be permanently fitted with different tools, or it is set up with tools dependant on the task
to be carried out.
In our case the connection tools are so heavy that it has to be placed at the sea bed by guiding
wires. The tools are located in connection with the PLEM, and the ROV is free to land on and
hook up with the tool. The connection tools are hydraulically powered, and flow of hydraulic oil
is controlled by the ROV. Below is an illustration of how the connection could take place:
9
(http://www.seabed.pl/firm/seaeye/seaeye.html )
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TMR 4225 Project work
Barge on spool
Figure 20: Connection of spool piece with hydraulically powered tool and Work class ROV 10
The Work ROV we are going to use is called Hercules.
Length: 2400 mm
Width: 1850 mm
Height: 2050 mm
Weight (in air): 2750 kg
Through frame lift: 3000 kg
Payload: 150 kg
Maximum working depth: 3000 m
Shaft power 120 hp
Hydraulic power 103 hp
Figure 21: Hercules, Work class ROV
10
(http://www.verderg.com ).
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TMR 4225 Project work
Barge on spool
9. Feasibility study of the operation
The operation we have studied in this project work has been to transport a 107 meter long spool
piece on a barge from Kritsiansund to the Njord field, and install it at the sea bed.
The first phase of the operation is Load out and preparation of the spool piece. The spool is
lifted onto the barge with a large crane located on the pier, and fastened to the barge by welding.
This part of the operation is quite straight forward, since the harbour in Kristiansund is well
protected from waves and weather.
During towing of the barge we will use two tugs, one in front and one behind. This phase is
quite weather dependant, and the maximum significant wave height we can handle during the
tow out is 5 meters.
There are a couple of other ways to handle the tow out, and also installation of the spool piece.
One way is to tow the spool piece submerged between two tugs. Another method is to tow the
spool underneath the crane vessel (Acergy). In addition to the tugs and the barge, we are going
to use a multi-purpose Inspection, Maintenance and Repair (IMR) and construction ship called
“Norman Mermaid”. By towing the spool piece through the moon pool of the ship, both tugs
and barge can be skipped. Another great benefit with the subsurface towing is that the critical
water entry phase can be neglected. An operation like this requires a lot more preparation at the
harbour before towing. In our case spool piece is a bit to long to be transported like this. It
would be a great challenge to get long pipe stabile as it hangs under the ship.
After the towing to the Njord field, the spool piece has to be lifted off the barge. This is the most
weather critical phases during the operation, and the maximum significant wave height Hs is put
to be 1.5-2 meter due to relative motions between the barge and the crane vessel ( Haneferd 07).
If it is possible to carry the spool piece onboard the crane vessel, the whole problem with
relative motions between the vessels could be disregarded.
After the spool piece has been lifted of the barge, “Norman Mermaid” will move sideways to
get clear off the barge. Another option could be to move the barge after the Lift off, but this
would probably be more troublesome. When free off the barge, the crane operator can start to
lower the spool piece towards the water. The water entry phase is also a very critical, as the pipe
goes from one medium to another; air to water. The first thing to consider is the water entry
forces. In our report we have carried out some calculations of these forces, but only for calm
water. To get a real picture of especially the slamming forces, the calculation should have
included waves. If the spool piece got any kind of bended shapes, it is important to think of
rotational stability as the pipe enters the water. It has happened before that curved spool pieces
have twisted and rolled when entering the water due to change in buoyancy. As mentioned
above the whole water entry phase can be disregarded by subsurface towing.
The journey towards the sea bed is more comfortable stage. The spool piece will be connected
to a guide line, and continuously lowered until it is some meters over the first PLEM. To avoid
any snatch loads, it is important that the crane cable is tight at any time. Before the spool piece
can be lowered the last part, it has to be rotated exactly over the second PLEM. This is simply
done by moving the crane vessel in the right position.
27
TMR 4225 Project work
Barge on spool
In advance the ROV connection tools are already lowered down, and prepared at the PLEMs,
and the pipe can be connected once it has touched the sea bed. The work class ROV will then
connect with the tools, and couple the spool piece to the PLEMs.
This method as it is described above is actually being used in the industry. Companies like for
example Acergy AS have used this kind of method and are going to use it in future projects.
(Haneferd 07) They have proven it to be a feasible and accepted method of installing spools.
The feasibility analysis and use of well known procedures is a very important part of a marine
operation. DNV have included two topics in their rules about the use of new technology:
And we seem to comply with those two rules. Therefore this operation is feasible.
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TMR 4225 Project work
Barge on spool
10. Conclusion
We were able to do a satisfactory analysis of many factors involved in the load-out, tow-out and
installation of a spool on a barge.
Our insight into the planning of a marine operation was greatly expanded by investigating such
things as vessels, stability, directional stability, lifting forces; vortex induced motions, weather
windows and forecast accuracies.
Several of the group members will have summer internships in companies like Acergy and Aker
Marine Contractors, which have such kind of operations as main business. This project has
given us a good introduction into the potential work tasks we will be given, and we are looking
forward to experience the difference between our planning and how the planning is conducted in
the industry.
If the work on this project should be continued it would be natural to investigate safety factors
and weather windows even further. At the same time economical aspects should be considered.
Most of all; a variety of different solutions should be investigated to find the best combination
of safety, feasibility and economy.
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TMR 4225 Project work
Barge on spool
References
•
•
•
•
•
•
•
•
•
•
•
•
•
Nielsen, F.G.; “Lecture notes in Marine Operations”; Tapir trykk. Trondheim 2003
Faltinsen, O:”Sea Loads on Ships and Offshore Structures”; Cambridge University
Press; Cambridge 1990
Hogben,N ; N.M.C.Dacunha; G.F. Olliver, “Global wave statistics”, Unwin brothers
limited, 1986
Haneferd, Rasmus; Acergy AS: E-mail correspondence April 2007
Bergan, P.G., P.K. Larsen og E. Mollestad (1986): Svingning av konstruksjoner. Tapir,
Trondheim
DNV: Offshore standard DNV OS H101:
http://www.ivt.ntnu.no/imt/courses/tmr4225/exercises/teamwork/2007/DNV-OSH101.pdf
http://www.offshore-technology.com/projects/njord/ (accessed on April 17th, 2007)
Information about Viking barge 1 are collected through information from Robert
Indegård, employee at Taubåtkompaniet, whom has commercial management for barges
within Viking Barges KS.
http://www.boa.no/
Noble Denton – General guidelines for marine transportations
http://www.nobledenton.com/guidelines/0030-2.pdf
Viking barge 1, barge within Viking Barges KS
http://vikingsupply.egroup.no/barges.asp
Tug Pawlina, Utilized towing tug for operation
http://www.tugmalta.com/pawlina.html
Tug Lieni, Utilized stabilization tug for operation
http://www.tugmalta.com/lieni.html
Normand Mermaid, Crane vessel
http://www.acergygroup.com/publicroot/webresources/6Q5ANARDUN/$file/Normand%20Mermaid.pdf
Pictures
•
Sources found in footnotes for each picture
30
planning of marine operation
preparation of harbour
Marine Operation
Harbour Operation
Preparation of tug
Preparations of spools
Lifting of spools onto barge and seafastening
Connect tug and barge
Offshore preparations
Transit of barge
Transit and preparation of crane vessel
Connection of crane vessel and barge
Deployment of ROV
Lifting operation
lift off barge
pass splash zone
lowering to bottom
landing of spool and preliminary connection
18
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
20
22
Mon 18 Jun
0
2
4
6
8
10
12
14
16
18
20
22
Tue 19 Jun
0
2
4
6
8
10
12
14
16
18
20
22
Wed 20 Jun
0
2
4
6
8
10
12
14
16
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TMR 4225 Project work
Task Name
1
Appendix
ID
Barge on spool
31