Tow out on crane vessel, lift off and lowering - NTNU · Figure 26: Velocity as a function of time...

Tow out on crane vessel, lift off and

lowering

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1. Preface This report is written in the course TMR2386098,8 Marine Operations at NTNU. The project count 30% on the final grade in the course. Each group member has contributed equally to the project writing the different chapters. The nature of the project enforced the group to continuously work together towards the final product. The different topics to be answered in the project demanded great logistic cooperation and planning skills within the group. It was agreed upon in the group that the hours spent in represented the final product. Trondheim 10 March 2006 __________________ __________________ _________________ Eskil V. Kjemperud Ole A. Hansen Roy A. Erland


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2. Table of Content 1. PREFACE __________________________________________________________________________ - 2 - 2. TABLE OF CONTENT _______________________________________________________________ - 3 - 3. FIGURES AND TABLES______________________________________________________________ - 4 - 4. ABSTRACT_________________________________________________________________________ - 5 - 5. INTRODUCTION____________________________________________________________________ - 6 - 6. MAIN PHASES OF OPERATION ______________________________________________________ - 7 -

6.1 FACTS ON THIALF AND TEMPLATE _____________________________________________________ - 7 - 6.1.1. Thialf _______________________________________________________________________ - 7 - 6.1.2. Template ____________________________________________________________________ - 8 -

6.2. MAPPING AND EVALUATION OPERATION ________________________________________________ - 9 - 6.3. LOAD-OUT ______________________________________________________________________ - 11 - 6.4. TOWING ________________________________________________________________________ - 11 -

6.4.1. Towing configurations, directional stability of the towed structure ______________________ - 12 - 6.4.2. Choice of towing lines _________________________________________________________ - 14 - 6.4.3. Towing velocity ______________________________________________________________ - 14 - 6.4.4. Effect of propeller race ________________________________________________________ - 14 - 6.4.5. Static and dynamic loads in towline ______________________________________________ - 15 -

6.5. LIFTING ________________________________________________________________________ - 16 - 7. METOCEAN CONDITIONS IN THE BARENTS SEA ____________________________________ - 16 -

7.1. THE BARENTS SEA ____________________________________________________________ - 17 - 7.1.1. Wind ____________________________________________________________________ - 17 - 7.1.2 Waves ___________________________________________________________________ - 17 - 7.1.3 Temperatures______________________________________________________________ - 18 - 7.1.4 Currents _________________________________________________________________ - 18 -

7.2 CONCLUSIVE REMARKS – METOCEAN CONDITIONS _______________________________________ - 20 - 8. WEATHER WINDOW ______________________________________________________________ - 21 -

8.1. DIFFERENCES IN THE OPERATIONS AT ORMEN LANGE FIELD VERSUS THE BARENTS SEA ___________ - 21 - 8.2. ESTABLISHING THE WEATHER WINDOW NEEDED FOR OUR OPERATION________________________ - 23 -

9. ANALYSIS - LIFTING ______________________________________________________________ - 27 - 9.1 CHOICE OF EQUIPMENT _____________________________________________________________ - 27 -

9.1.1 Hoisting line _________________________________________________________________ - 27 - 9.1.2. Tugger Line _________________________________________________________________ - 28 - 9.1.3. ROVs ______________________________________________________________________ - 29 - 9.1.4. Visualisation, Stability and Synchronization aids ____________________________________ - 29 -

9.2. MAIN PHASES OF LIFTING OPERATION_________________________________________________ - 31 - 9.2.1. Lift off _____________________________________________________________________ - 31 - 9.2.2. In Air ______________________________________________________________________ - 33 - 9.2.3. Crossing splash zone __________________________________________________________ - 33 - 9.2.4. Deeply submerged ____________________________________________________________ - 33 - 9.2.5. Landing ____________________________________________________________________ - 34 -

9.3. THEORY NEEDED FOR ANALYSIS _____________________________________________________ - 34 - 9.3.1. Response in regular waves _____________________________________________________ - 35 - 9.3.2. Establishing the response statistics _______________________________________________ - 35 - 9.3.3. Frequency domain and time domain ______________________________________________ - 36 - 9.3.4. Coupled dynamic model for barge and load ________________________________________ - 36 -

9.4. ANALYSIS_______________________________________________________________________ - 38 - 9.4.1. Influence of added mass near bottom _____________________________________________ - 38 - 9.4.2. Vertical oscillation of wire including mass _________________________________________ - 41 -

10. FURTHER WORK _________________________________________________________________ - 44 - 11. REFERENCES ____________________________________________________________________ - 45 -


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3. Figures and Tables Figure 1: The Thialf .............................................................................................................. - 8 - Figure 2: The Template......................................................................................................... - 8 - Figure 3: The red line shows the first AUV run, and the black line shows the “lawnmower” pattern of the second AUV run. ............................................................................................. - 9 - Figure 4: 3D-view. Made using multibeam bathymetry. .................................................... - 10 - Figure 5: This image, created from sub bottom sonar, shows layers of sediments. ........... - 10 - Figure 6: Towing configuration inshore. ............................................................................ - 13 - Figure 7: Towing configuration offshore............................................................................ - 14 - Figure 8: The Barents Sea division. Mironov 1996 ............................................................ - 16 - Figure 9: Average Wind Speed and Direction .................................................................... - 17 - Figure 10: Variation in the 100-year Extreme Wind Speed................................................ - 17 - Figure 11: Average significant wave height ....................................................................... - 18 - Figure 12: Variation in the 100-year significant wave height............................................ - 18 - Figure 13: Ocean Currents in the Barents Sea................................................................... - 19 - Figure 14: Geographical overview. Murmansk and Shtokman, including inshore transit (www.googleearth.com) ...................................................................................................... - 22 - Figure 15: Lifting Operation - main phases ....................................................................... - 23 - Figure 16: example on significant wave height plotted against time. Measurements every three hours........................................................................................................................... - 24 - Figure 17: Illustration of Lifting ......................................................................................... - 27 - Figure 18: Configuration of Lift. Tandem lift ..................................................................... - 27 - Figure 19: tugger line configuration .................................................................................. - 28 - Figure 20: Illustration of tugger lines................................................................................. - 28 - Figure 21: Shows the wideband telemetry. There will be communication between both the template and Thialf, the landing zone and Thialf and the ROV and Thialf. The ROV will also support with live video update of the whole operation. ...................................................... - 30 - Figure 22: Illustration of Degrees Of Freedom (Nielsen2, 2006)....................................... - 32 - Figure 23: The two different parts of the analysis of the landing: a. the heave motion of template due to crane motion; b. the effect of added mass near the seafloor. .................... - 38 - Figure 24: Estimated added mass when approaching the seafloor .................................... - 39 - Figure 25: Height over seafloor as a function of time [s] .................................................. - 40 - Figure 26: Velocity as a function of time [s] ...................................................................... - 40 - Figure 27: Acceleration as a function of time [s] ............................................................... - 41 - Figure 28: Shows vertical motion of the template due to a sinusoidal wave with amplitude of 1 m, 2.5 m, 5 m and 7 m. They are all showed on the same scale so the differences in the motion of the template are more visible. ......................................................................................... - 43 - Table 1: Dimensions of the Thialf ......................................................................................... - 8 - Table 2: Crane capacity on the Thialf. ................................................................................. - 8 - Table 3: Dimensions on the Template................................................................................... - 8 - Table 4: Towing, NORSOK STANDARD J-003.................................................................. - 12 - Table 5: Special towing operation, NORSOK STANDARD J-003...................................... - 12 - Table 6: Minimum breaking strength of towlines in tonnes................................................ - 13 - Table 7: Weather window requirements for each sub-phase .............................................. - 25 - Table 8: Types of ROVs (Berg, 2006) ................................................................................. - 29 -


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4. Abstract In this report we have presented the main phases of a marine operation defined as installing an 1150 tonne well-template on the seafloor at the future gas field Shtokman in the eastern Barents Sea. These phases are Mapping, Load-Out, Towing and Lifting. We have evaluated the most critical part of each of the above mentioned operational phases. The report also identifies differences in operation from an Ormen Lange operation vs. a Barents Sea operation. This difference pertains to geographical location and environmental conditions. The geographical location of the Shtokman field introduces the difference in accessibility. The time to reach Shtokman field is five times the time to reach the Ormen Lange field. A towing operation executed at 6 knots speed will be estimated to have a duration of over 2 days (50 hours). The environmental conditions present the other difference. The report presents Meteorological and Oceanographic data over the Barents Sea; data such as Wind, Waves, Temperatures and Currents. The report concludes that the wind and waves in the Barents Sea are milder than in the North Sea. The temperatures are lower while the currents remain relatively unchanged. Two other phenomenons are also presented, one negative and one positive. The different currents that meet produce at times thick fog. However from late April the area experience the midnight sun with daylight twenty four hours a day. The report has presented a method of estimating weather windows needed based on probabilistic methods and statistics. The report has also estimated a weather window based on previous projects and examples. For the analysis part we have as mentioned analyzed the landing phase. Our results were that added mass reaches infinity when the template approaches the bottom. Added mass has been calculated in two different ways with the results within the same magnitude. We also revealed that the heave motion of the crane tip will not have any significant importance of the motion of the template during the landing phase.


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5. Introduction Last summer the first of two subsea templates were installed at the Ormen Lange field west of Kristiansund. The marine operation was executed during one weekend in late August. This report is a feasibility study of the marine operation “Tow out on deck of crane vessel, lift off and lowering” in the problem text. This problem consisted of installing the template from Ormen Lange at the planned Shtokman field located in the eastern Barents Sea. The topics that were given in the problem text:

- Describe the main phases of the operation - Identify critical operational phases - Identify differences in an Ormen Lange vs. Barents Sea operation - Identify information about vessel etc for analysis - Describe how we would perform the analysis - State how we would estimate duration of each phase and weather limitations - Perform a careful evaluation of a chosen operational phase

We felt it natural to construct our own disposition of this report in order to more naturally answer all the issues given. We have as an introduction discussed the main phases of this kind of marine operation. We have introduced the meteorological and oceanographic conditions that control the area. Then we have illustrated the differences in operation and the critical phases of each operational phase. Finally we have presented a more careful analysis of the final phase - Landing. We would take the opportunity to emphasize that each of the seven topics in the problem-text has been equally prioritized. This was done due to the problem-text provided little information on what was the essential part of the project given.


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6. Main Phases of Operation The general phases for an operation similar to the one we are performing are defined below:

• Mapping • Load-Out (template on Thialf in Murmansk) • Towing

- In sheltered waters/Inshore - Offshore

• Lifting - Lift Off - In Air - Splash Zone - Deeply Submerged (under wave zone) - Landing

The topics above are identified as the general steps in our kind of operation (Nielsen1, 2006). The different topics will be discussed in the following sub-chapters. The mapping and evaluation and load-out steps will only briefly be discussed, while the towing and Lifting phase will be more thoroughly analysed. We will first present the relevant facts and data on Thialf and the template used in the project.

6.1 Facts on Thialf and Template

6.1.1. Thialf The data on the Thialf is gathered from the web side of the owners, the Dutch company Heerema Marine Contractors. (www.heerema.com) Thialf is the largest Deepwater Construction Vessel operated by Heerema Marine Contractors, is capable of a tandem lift of 14,200 mT. The dual cranes provide for depth reach lowering capability as well as heavy lift capacity to set topsides. This multifunctional dynamic positioned DCV is tailored for the installation of foundations, moorings, SPAR's, TLP's, templates and integrated topsides, as well as pipelines and flowlines. We will in this chapter only present the direct facts about the Thialf and the Template. More facts are listed in the Appendix A for further interest. The Appendix is also referred to in the project as a whole.


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Dimensions

Length overall 201.6 m

Length of vessel 165.3 m Breadth 88.4 m

Depth to work deck 49.5 m Draught 11.8-31.6 m

GRT 136,709 t NRT 41,012 t Mass 10.000 t

Transit speed; 1 tug 7.0 knots Table 1: Dimensions of the Thialf

Figure 1: The Thialf Portside and starboard crane

Main hoist revolving 7,826 sht up to 31.2 m Auxiliary hoist 1,000 sht 36.0 - 79.2 m Whip hoist 220 sht 41.0 - 129.5 m

Table 2: Crane capacity on the Thialf.

The short ton (sht) is a unit of mass equal to exactly 907.18474 kg.

6.1.2. Template

Table 3: Dimensions on the Template

Length 44 m

Breadth 33 m

Height 15 m

Mass 1150 mT

Figure 2: The Template


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6.2. Mapping and evaluation operation The mapping and evaluation should result in finding the optimal place for the template given some boundary conditions as approx. location. The mapping will most efficiently be done with the use of a mapping-AUV. These AUV`s have three sonar mapping devices on board. Each of the three sonar devices on the AUV measures a distinctive aspect of the sea floor. Multibeam sonar collects bathymetry (topography) information for creating detailed maps. Sidescan sonar produces 3D views of the bottom by bouncing sound waves off the bottom and measuring the strength of the reflected signals. As motioned on the Monterey Bay Aquarium Research Institute homepage:

“..these signals can detect subtle topographic features only a few centimeters high, and also indicate whether the seafloor is soft or hard, rocky, muddy, or silty. Sub-bottom sonar can show boundaries between layers of mud and rock below the seabed and therefore tell us about past geological events.”

This kind of information is essential for our subsea operation. There are some advantages using an AUV for mapping instead of traditional soars. The AUV can go with constant distance to the bottom i.e. 30 m at the lowest over the seabed. Distance to the bottom is important when it comes to quality of the results. In addition, an AUV can map an area three times faster than a ship towing sonar equipment. The AUV will first map the area as shown as a red line in figure 1. The next step is to find the best location for the template. The AUV is programmed to fly in a “lawnmower” pattern, making a series of side-by-side passes to completely map the whole chosen area.

Figure 3: The red line shows the first AUV run, and the black line shows the “lawnmower” pattern of the second AUV run.

On the following page some data such as 3D mapping from an AUV is illustrated.


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Figure 4: 3D-view. Made using multibeam bathymetry.

Figure 5: This image, created from sub bottom sonar, shows layers of sediments.


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6.3. Load-Out The load out phase is assumed to be carried out in Murmansk, which would be the closest Russian harbour to Shtokman. The load out phase is defined according to NORSOK standard (www.standard.no) as the horizontal transfer of the heavy object, i.e. the template from land onto the transport vessel, being Thialf. The requirements to the load out phase are the same for operation in the Shtokman case as they were in the Ormen Lange case. The requirements for load out are described in DNV Rules for Planning and Execution of Marine Operations (1996) Pt.2 Ch.1 Sec.2 and pertains to aspects such as

- Planning - Loads - Loadcases and analysis of forces - Structures and soil - System and equipment - Load out vessel - Operational aspects - Special cases

This part of the operation will be a controlled lifting operation at the harbour. There will be no problems with stability. The planning mentioned above is quality insurance for the entire operation. It will be difficult to stop or halt the operation when it has been started.

6.4. Towing To analyse the towing operation we would start by planning the operation. We need information of the towed object, the towlines and the tugs. There are checklists for towing operations given in Norsok Standard J-003 as shown on the next page. Also some information concerning different towing configurations is showed in Appendix B.


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Table 4: Towing, NORSOK STANDARD J-003

Table 5: Special towing operation, NORSOK STANDARD J-003

6.4.1. Towing configurations, directional stability of the towed structure Resistance and stability of the towed object has to be calculated to find the configuration and number of tugs needed. DNV (1996) give the following formula for estimating the wave drift

component of the resistance for box shaped barges. [ ]2 0.52 12WD SBF H LL

− .

Where WDF

SH B, L and T is the breath, length and draft respectively.


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The expression should be applied for barges with 3.0LB> and 6.0B

T> and for towing speed

close to zero (Low Froude numbers). The number of tugs will also be affected by the chosen towing velocity. The route of transportation (e.g. in sheltered waters or offshore) influences the number of needed tugs need for manoeuvring, and the towline layout. In sheltered waters manoeuvrability is the most important consideration. For that reason the towlines should be short, and the tugs located so that forces may easily be applied in any direction. During the offshore part of the tow, towing velocity and loads in the towlines are important considerations. Longer towlines should therefore be applied. The layout of the towing system will influence the course stability of the towed object. For unrestricted towing DNV (1996) requires the following minimum breaking strength of the towline in tonnes:

3BP for BP<40 (3.64-0.8BP/50)BP for 40<BP<90 2.2BP for BP>90

Table 6: Minimum breaking strength of towlines in tonnes

BP is the continuous static bollard pull in tonnes. For offshore towing DNV also requires a

minimum length of the towline given by: 2000m

BPLMBP

= , where MBP is the certified

minimum breaking load of the towline. Thialf can ensure 6 knots speed with a deck loading of 2000 tonnes with her own propulsion. Since towing was the criteria in the exercise, this possibility is eliminated. Two tugs will tow Thialf the entire distance. The configurations of the towing are shown in figure 6 and 7. As explained over the configuration of the tug is important with respect to direction stability and towing speed. With regards to towing configuration; in inshore the two tugs will operate in opposite direction. This is to ensure good manoeuvrability and no collision with ships, land or other obstacles. In open sea both tugs will work in the same direction.

Figure 6: Towing configuration inshore.


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Figure 7: Towing configuration offshore.

6.4.2. Choice of towing lines The bollard pull of the tugs has to be checked to choose the right towline layout. Towline material, diameter and length must be specified in the towline layout. The key parameters to be considered in the choice of towline are: Breaking load, stiffness, weight, wear and corrosion characteristics, damping and handling. The stiffness is essential to the dynamic forces of the towline.

6.4.3. Towing velocity The towing velocity depends on the number of tugs, towing configuration and overall resistance. If the towing requires a small weather window, the towing velocity will be of great importance. For our operation we will assume a towing velocity offshore to be approximately 6 knots. This towing velocity is assumed on the basis of the velocity that Thialf can produce by own engine. (www.heerema.com)

6.4.4. Effect of propeller race In cases where the towline is short, the tug propeller may induce flow velocities at the towed structure which increases the towing resistance significantly. The force on the structure decreases as the distance from the propeller increases. The total force due to propeller race depends on how the flow is deflected by the structure. The only practical way of estimating the force is by model testing. However, DNV (1996) recommend the use of an efficiency factor to account for the effect of the propeller race:

expint

0.0151

towline

Aa

L

η−⎡ ⎤

= +⎢ ⎥⎣ ⎦

(6.1)

- expA is the projected area of the cross sectional area of the towed object in 2m - towlineL is the length of the towline in meters. - 2.1η for barges.

The formula is recommended for towlines longer than 30 m.


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6.4.5. Static and dynamic loads in towline Breakage of the towline can be caused by local chafing/wear, kinking/crushing, cut by propeller, cut due to bottom contact, fatigue or overloading. Static and dynamic behaviour of the towline is needed to solve these problems properly. Assuming that the towline is supported at the same vertical level at both ends, and constant mechanical properties along the whole length of the towline we can find the horizontal and vertical coordinates for a static analysis (Nielsen1, 2006):

(6.2)

(6.3) The sag of the cable, mz , has a correction due to the elastic elongation of the cable.

20 1

8mw L Hz

H EA⎛ ⎞= − +⎜ ⎟⎝ ⎠

(6.4)

The tension in the towline can be found by:

cosh wxT HH

⎛ ⎞= ⎜ ⎟⎝ ⎠

(6.5)

The stiffness is important when considering the dynamic behaviour of the towline. The total stiffness is given by the sum of the elastic and the geometric contribution:

( )20

3

1 1 112E G

w L LLk k k EA H= + = + (6.6)

If the towline is subjected to transverse forces due to viscous drag, the apparent stiffness will increase. When calculating the dynamic loads we assume the towed structure to be at rest or a constant low forward velocity. We can model the towline by two strings in series, one due to the geometric stiffness and one due to the elastic stiffness. In parallel with the elastic stiffness we can model a damper due to viscous forces on the towline. For reducing extreme loads in the towline we may use a rendering winch. The winch should be modelled as a spring – mass damper system. The tension due to drag forces may be written as:

2 3

2

111920

HD D

S S

k LwLT C d c cH H

ρ (6.7)

SH is the static mean tension, w is the weight per unit length in stretched condition.

230

20

116

1 12m

H wx s sEA H

w s Hz zH EA

⎛ ⎞ ⎛ ⎞+ −⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

⎛ ⎞+ +⎜ ⎟⎝ ⎠


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The total dynamic load is approximately given by:

( )22dyn D GT T k c+ Δ (6.8)

6.5. Lifting The main operational phases of the lifting operation will be discussed more in detail in chapter 9.

7. MetOcean Conditions in the Barents Sea In the following chapter we will introduce the general Meteorological and Oceanographic conditions of the Barents Sea (divisions 1-5 +7) around the Shtokman area (south in division 5) towards the Pechora Sea (division 6) shown in figure 8 by Mironov (1996).

Figure 8: The Barents Sea division. Mironov 1996 The different conditions that will be briefly discussed are wind, waves, temperatures and currents. We will use these conditions as a basis of comparison for identifying differences in operation at Ormen Lange field versus the Barents Sea. We will present the Metocean conditions in detail only for the Barents Sea. The reason for this is the importance of knowledge about the weather statistics to determine the weather windows.


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7.1. The Barents Sea All figures from Torsethaugen 1989.

7.1.1. Wind The highest wind speeds in the western Barents Sea (division 1-2) occurs around Bjørnøya (division 2) where the average speed lies around 9 m/s (fig 9) and the 100-year extreme wind speeds is observed around 36 m/s (fig 10). The wind speed decreases towards the east and north and is on average 8 m/s around the Shtokman area with a 100-year reoccurrence period of 30 m/s. This implies that we could experience quite significance wind forces during operations. However the prevailing depends on the season. During winter, south-western winds dominate whereas north-western winds dominate the summer season (Torsethaugen, 1989).

Figure 9: Average Wind Speed and Direction

Figure 10: Variation in the 100-year Extreme Wind Speed

7.1.2 Waves The average significant wave height in the Barents Sea is largest around Bjørnøya and is about 2.3 m (figure 11, Torsethaugen, 1989). The waves enter from the South-West and decreases somewhat towards the east and the Pechora sea (region 5 and 7). The wave climate in the Barents Sea tends to be milder than in the North Sea. The significant wave height with return period of 100-years is about 13.5 m. This however will be in the winter season. If we observe figure 12 we see that the 100-year significant wave height in the summer season is about 7.5 m and around 2.0 m west of the Shtokman area. The tendency of the average wave height is declining towards the east.


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Figure 11: Average significant wave height

Figure 12: Variation in the 100-year significant wave height

7.1.3 Temperatures Surface temperature in the western Barents Sea has the tendency to decrease from the south to north and from west to east. The average temperature in the Barents Sea varies from below 0 degrees Celsius to around 10 degrees during a whole year. However observed minimum temperatures vary from -15 in south west to around -38 in the north-northeast. The worst temperatures are in February with an average of -18.3 degrees. The extreme temperatures are close to -40 degrees Celsius.

7.1.4 Currents In the area around Bjørnøya different water masses are meeting. The mild northbound Norwegian coastal and Atlantic waters meet the cold arctic currents from Franz Josef Land (fig 13). The Norwegian coastals and Norwegian Atlantic currents enter from the south and southwest. The currents have been measured at surface speeds of 0.75-0.80 m/s.


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Figure 13: Ocean Currents in the Barents Sea


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7.2 Conclusive remarks – MetOcean conditions From the chapter above we can conclude with the following: Due to the landfast ice in the winter season our operation will be carried out in the summer season. As discussed before the wind conditions in the Barents Sea are much smaller/weaker than in the North Sea and decays towards the east. The same tendency is noticed in the wave conditions where they are remarkably smaller in the Barents Sea than in the North Sea. The temperatures however are considerably lower in the Barents Sea than in the North Sea. There is little or no discrepancy with regards to the current speed conditions. There is however one other consideration to make in connection with the currents. The northbound gulfstream transports relatively hot streams of water into the Shtokman area, where also the gulfstream meets the southbound arctic currents. This produces an oxidation process which creates very thick fog. Fog hinders the visibility quite significantly. One of the major advantages by moving the operation into the Barents Sea is that from late April there is midnight. This provides daylight twenty four hours a day which eases the working conditions. Icebergs and ridges are neglected in our operational considerations due to the operation is being carried out in the summer time. 90% of icebergs in the Barents Sea are breaking off the glaciers in Franz Josef’s Land. The probability of icebergs drifting into the Shtokman area is low due to the current pattern, the long drift time and the mild temperatures contributing to the melting process. (Løseth, 2005)


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8. Weather Window In order to predict the required weather window for our operation we need information on weather statistics. The extreme weather conditions in The Barents Sea are already described in chapter 7. MetOcean Conditions in the Barents Sea. We will first identify the differences in an Ormen Lange operation versus the Barents Sea.

8.1. Differences in the operations at Ormen Lange field versus the Barents Sea The first of the 1150 tonne well templates were installed at Ormen Lange field during a weekend in August 2005. The installation at 850 meter water depth broke a Norwegian water depth record. In order to install the templates the operation had to be precisely planned. The planning process was detailed for almost two years in advance. Every single phase of the operation had been made accounted for in detail. Since the vessel and the template are the same; the only differences in operation will be the environmental conditions. We will first discuss the differences for each phase of the marine operation as listed in chapter 6 and then identify critical aspects of the environment causes. Load Out The load out phase is assumed to be carried out in Murmansk, which would be the closest Russian harbour to Shtokman. In chapter 6.1.1. we identified the vessel and template to be the same as used in the Ormen Lange operation. Therefore we assume that we will have the same method of load out as in Ormen Lange. The requirements to the load out phase are the same for operation in the Shtokman case as they were in the Ormen Lange case. Towing The towing operation is defined into two stages.

1) Inshore 2) Offshore

The difference of these two towing phases is the speed and the towing configuration. This is mainly due to the increased demand for manoeuvrability in sheltered waters. However in order to show the differences in the time window required in the two different operations, the speed is assumed to be equivalent to 6 knots (www.heerema.com) for the entire towing phase. No safety margin is put into the time estimate. It is emphasized that the following estimate is strongly simplified to illustrate the difference in operational time.


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Figure 14: Geographical overview. Murmansk and Shtokman, including inshore transit (www.googleearth.com) The Ormen Lange field is located approximately 120 km off the coast of Kristiansund. The time window needed for this towing phase in the Ormen Lange was approximately

120 120000 1*( ) 10.810 60 *606*0,5144

km mT hoursmknots s ss= = =

As in comparison to our operation where the total distance from Murmansk to the Shtokman field is 550 km, the time window needed is:

550 550000 1*( ) 49.510 60 *6010*0,5144

km mT hoursmknots s ss= = =

We observe that the operation will take almost 5 times as much time in the Shtokman case as in the Ormen Lange. The distance and the time it takes to transport the template is then identified as the only difference in the towing phase of the operation. This demands a wider weather window for this phase. Positioning at Location As approaching the exact latitude and longitude for the Shtokman area the Thialf would be expected to take control of its own manoeuvrability and release from the tugs. The Thialf has then two choices for positioning, 1. Dynamic Positioning (DP) and 2. Mooring lines. The preferred system in this case is the dynamic positioning system which enables more manoeuvrability. If we were to use the mooring system the difference in the two operations would be the depth which is 850 [m] for Ormen Lange and about 300 [m] for the Shtokman area. Lifting Operation The lifting operation will commence after the positioning is completed. The template is to be lowered to a final depth of approximately 300 [m]. The weather window needed for this operation is smaller than the one needed for the installation at Ormen Lange as Ormen Lange


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is situated 850 [m] below the surface. As already identified, the wave- and wind-statistics at Shtokman field are milder than at Ormen Lange. The weather window requirements for phase [1], [2] and [3] in figure 15 (Nielsen2, 2006), regard the wave and wind statistics. Based upon the discussion, the metocean conditions for the Barents Sea; the weather window required for commencing and completing phase [1], [2] and [3] in the figure is probable to occur more frequent in the Barents Sea than at Ormen Lange. Phase [4] will demand less time due to the smaller depth. Phase [5] will demand the same amount of time as at Ormen Lange.

Figure 15: Lifting Operation - main phases

8.2. Establishing the Weather Window needed for our Operation To establish a required weather window, proper weather statistics is crucial. The extreme value statistics are the most important. This is either based upon the short time or the long time approach. The 3-hour stationary sea state is assumed as the dominating parameter. In our case we apply the long term approach. The demand for our operation is such that when each phase has started, the weather window must be guaranteed for the entire phase. This demands as small time interval as possible for each phase of our operation (Nielsen1, 2006). Our operation as mentioned in chapter 7.2 Conclusive remarks – MetOcean conditions our operation are not to be planned for extreme weather conditions, so we need to establish estimates on the expected duration of each phase of the operation. These enables the operation to be executed a certain period in the mildest season of the year, being the summer season. Event though the chapter on MetOcean Conditions in The Barents Sea gave us information needed on wave height, wind, temperatures and current, it gave us little information on the persistence of the weather, i.e. the ‘duration’ and the ’occurrence’. This has to be estimated using probabilistic methods on statistics. In order to estimate an approved calm period (when the different phases are acceptably executed) we could assume a similar table as the one below. The table gives us significant wave heights and its duration. The wave height is assumed to be measured every 3-hour.


- 24 -

Figure 16: example on significant wave height plotted against time. Measurements every three hours The demand is here that the operation is to be carried out when the measured Hs is below the critical Hs. Critical Hs is set to be 4 m on the basis of table 2.1 from Nielsen1 (2006). The calm periods is defined as the time between one negative level crossing and the following positive level crossing as

, ,calm neg c pos ct tτ = − (8.1) Similarly we define the storm periods as

, ,storm pos c neg ct tτ = − (8.2) From the two equations above it is acceptable to perform our operations in the time interval which equation (8.1) predicts. Consequently no operations can be performed in the time interval predicted by equation (8.2). Based on empirical data we could estimate the cumulative probability of duration of each calm period by a two parameter Weibull distribution:

( )

( ) 1 c

ttP t e

β−

= − (8.3) Where ct and β are unknown. They depend on the significant wave height. Where the mean duration of each calm period is

( ) * totcalm s

c

TP HN

τ = (8.4)

Significant wave height as a function of time. measured every 3 hours

0,00 1,00 2,00 3,00 4,00 5,00 6,00 7,00

3 9 15 21 27 33 39 45 51 57 63time [hours]

Hs [m]

Measured Hs Critical Hs


- 25 -

Where totT is the total duration of the time series considered and cN is the number of calm periods. To further establish the weather window needed for our operation we will follow the example in chapter 2.4. in Nielsen1 (2006). This example deals with heavy lift operation performed at the Troll field. The template installed in the example was 1400 tonnes, our template is 1150 tonne. The water depth is assumed to be the same; 300 m. The lifting configuration is also the same as presented in chapter 9.1.1 of this project. We identify this example relevant to compare with our operation. We divide our operation into the following. The load-out phase and towing phase is not considered here.

Operation

No Description

Max sign wave height

[m]

Pre-install time

[hours]

Duration [hours]

Safety margin additional time

[hours] 1 Hold vessel in position 3 1 2 1 2 Release tugs 3 1 1 1 3 Attach Transponders 3 1 1 1 4 Attach weights 3 2 2 1 5 Deploy ROV 3 1 3 1 6 Prep for lift off - 0 7 - 7 Cut final sea fastening 4 1 4 12 8 Lift off – in air 4 1 6 8 9 Through splash zone 2,5 2 3 8 10 Lowering 4 3 3 12 11 Positioning of template 4 - 17 24 12 Landing – suction anchors 4 1 6-24 12-48 13 Prep on vessel - 0 2 - 14 Template levelling 4 1 24 12 15 Swaging and cutting 4 16 Template levelling 4 17 Swaging and cutting 4 1 24 12 18 Installed survey 3,5 1 6-32 1

Table 7: Weather window requirements for each sub-phase

The Description of each sub-phase is listed in table 7. Maximum significant wave height allowable for each phase is listed. The Pre-instalment time is the preparation time needed prior to each operation, and the Duration is the estimated duration if no unforeseen incidents occur. Depending on how critical the operation is we have added some extra Safety margin due to the importance of finishing the phase once started. Remarks to the Operational phases We assume that due to the crew capacity we have the ability to perform the phases’ pre-instalment in parallel with each on-going phase. The lift of from deck (phase 8) is the first critical phase, but due to the template is located on the deck of the crane vessel this can be done in 4 m high waves.


- 26 -

The lowering through splash zone (phase 9) will demand maximum wave height of 2,5 m due to the slamming effects. Due to the demand for precision of the orientation of the template we demand a larger weather window in the final positioning of the template (phase 11). Phase 12 could be performed in 6 hours if the possibility of running all four suction anchor pumps simultaneously. If not, these could be run in a 2-by-2 parallel or simply one by one changing between the four in intervals until all are finished. Phase 18; installed survey could be carried out in intervals of 6 hours, 32 hours if performed continuously. The absolute critical phases in operation are the phases from cutting the sea fastening of the template until the template is in position on the sea floor. The entire time of these phases are approximately 33 hours. The operation can not be interrupted. The total time of the operation is approximately 155 hours without the safety margins in the forecast (Nielsen1 2006).


- 27 -

9. Analysis - Lifting In this chapter we will make a more carefully evaluation of one of the topics we have previously presented in the report. The topic we have decided for further analysis is; the landing on seafloor phase, or Landing phase. The analysis will contain detailed discussion of the following points in the landing phase:

a. Choice of equipment b. Analyse and estimate relevant static and dynamic forces and responses c. Discuss relevant modifications of the equipment and methods

9.1 Choice of Equipment The choice of equipment will relate to operational assistance such as

- Hoisting wire - Tugger line - ROVs - Visibility, Stability and Synchronization aids

9.1.1 Hoisting line The Hoisting line is defined as the line attached to the heavy lift object which transfers the load from the object to the lift vessel. In our kind of operation we handle the template in a tandem lift. The configuration of the hoisting lines is then as illustrated in figure 17 and 18.

Figure 17: Illustration of Lifting

Figure 18: Configuration of Lift. Tandem lift The load for the hoisting line to carry is the total weight of the template. We know from previous chapters that the mass of the template is 1150 tonnes. The total mass of the template in water under constant load neglecting impulse loads is then defined as the mass plus the added mass.


- 28 -

This is the load which the hoisting line will have to carry, disregarding the possible impulse loads. The impulse loads which may occur could be due to large horizontal displacement near the seabed and sudden impact with the sea floor. If these impacts occur we will have possible snatch loads which would be disastrous for the operation. The choice of hoisting line will be on the basis of the topics discussed above. There are generally six types of lines to use in this type of operation: chain, wire rope, metal mesh, natural fibre rope, synthetic fibre rope, or synthetic web. The hoisting lines tend to be placed into three groups:

- Chain - Wire rope and mesh - Fibre rope web

Each type has its own particular advantages and disadvantages. Factors to consider when choosing the best lines for the job include size, weight, shape, temperature, and sensitivity of the material being moved, and the environmental conditions under which the hoisting wire will be used. The choice of type of line in our case will be steel wire rope. Wire rope is composed of individual wires that have been twisted to form strands. Strands are then twisted to form a wire rope. When wire rope has a fibre core, it is usually more flexible but less resistant to environmental damage. Conversely, wire rope with a core that is made of a wire rope strand, which we plan to use, tends to have greater strength and is more resistant to heat damage. When selecting a wire rope line to give the best service, there are four characteristics to consider: strength, ability to withstand fatigue (e.g., to bend without distortion), ability to withstand abrasive wear, and ability to withstand abuse. Strength of wire rope is a function of its size (e.g., diameter of the rope), grade, and construction. (Princeton, 2005)

9.1.2. Tugger Line A tugger line is a line which is also attached from the crane vessel onto the template via a deadman anchor. The deadman anchor is lowered down almost horizontal aligned with the template, and the line is run through a sheave block as indicated in figure 19. However in the lifting phase the tugger line has no load carrying objective, illustrated in figure 20 as the lines which lie slack from the template.

Figure 19: tugger line configuration

Figure 20: Illustration of tugger lines


- 29 -

We will have reciprocal interaction between the crane vessel and the load system where we in principal will have a 12 degree of freedom (DOF) dynamic system with both hydrodynamic and structural interaction.

9.1.3. ROVs The demand for ROV use in our type of operation is basically split into two. First we use it as a part of the requirement for accurate acoustic positioning. The second task of the ROV is what actually decides which type of ROV we will use. This task is to unhook the hoisting wires in the four locations these are attached to the template as shown in figure 18. Based on this information we could choose a ROV for our operation. From the ROC committee the different ROV systems are categorized as

Class Capability Power (hp) LCROV (electric) Observation

(<100 metres)

<5 Small &#9 (electric) Observation

(<300 metres)

<10 Large (electric) Observation/Light Work

(<3000 metres)

<20 Ultra Deep (electric) Observation/Data collection

(>3000 metres)

<25 Medium (electric/Hyd) Light/Med Heavy Work

(<2000 metres)

<100 Large (electric/Hyd) Heavy Work/Large Payload

(<3000 metres)

<300 Ultra Deep (electric/Hyd) Heavy Work/Large Payload

(>3000 metres)

<120

Table 8: Types of ROVs (Berg, 2006) We know from previous chapters that the depth around Shtokman is <300 metres and the work to be done could be categorized as Medium Heavy work. We then choose a class Medium (electric/Hyd) ROV for our operation (Berg, 2006).

9.1.4. Visualisation, Stability and Synchronization aids The landing operation contains many critical situations. The most important element is to ensure exact position with acceptable errors. GPS will have to short range in water (1-4m approximately). Due to salinity, temperature and pressure variations in the sea layers, the echo sounder will have problems creating good acoustic data. Deflection increases large variation in the over motioned water qualities. Rapid internal waves can also occur and result in significant changes in sound speed over periods of just a few minutes. The determination of the speed of sound in water is a key requirement for accurate acoustic positioning. An operation like this demands a high accuracy and position solution with fast positional update rate. Thialf operates without the requirement for cable connection to sensors on the


- 30 -

template and all data must be transferred on an acoustic line. Thialf uses high speed wideband telemetry instead of long base line (LBL) acoustic transmission. The reliability of this system is better than the conventional LBL. This operation can not be stopped when it has been started therefore it is crucial that the reliability within the weather window is 100%. The templates are required to be installed within ±2.5m of design location and within ±2.5° of design heading. For complete redundancy, all corners of the Thialf will be equipped with speed sensors; the template can be equipped with sub-sea gyro (laser ring gyro), compass and Doppler measurement tool. An ultra short base line (USBL) system will be positioned on board Thialf to provide an acoustic reference for the barge's dynamic positioning system and to track the two work class ROVs. There will also be transponders on the template and the bottom fundament (www.oilonline.com). The system is illustrated in figure 22.

Figure 21: Shows the wideband telemetry. There will be communication between both the template and Thialf, the landing zone and Thialf and the ROV and Thialf. The ROV will also support with live video update of the

whole operation.


- 31 -

9.2. Main Phases of Lifting Operation

9.2.1. Lift off Thialf will start the lifting procedure with finding its right position using the DP system. A DP system consists of three parts: sensors, GPS, a computer system and thrusters to make adjustments. The accuracy of the DP system is within 1 2m . A lifting operation consists of five main parts. These parts can be divided into motion of the (lifted) structure due to lifting and impacts due to motion.

• Motion: Horizontal sliding • Impact: Impulse loads

This lift operation is defined as a heavy lift operation whereas the load is more than 1-2 % of the crane vessel displacement (>1000 tonne). The template weights 1150 ton and is more than 10% of the weight of the lifting vessel. (Nielsen1, 2006) First the crane vessel is located at stand-off position with the use of DP system. Since Thialf is a barge with two pontoons the situation of the vessel is critical. The DP will at all times find the most suited direction to perform the lift operation. The vessel is equipped with a computer controlled ballast system with capacity of 10000 tonnes of water/half hour. During heavy lift these tanks ballast tanks are filled to increase the restoring moment. The rotation angles will be smaller due to the relation:

(9.1) (9.2) If the GM is increased the frequency will increase and the rotation angle in roll will be reduced. Hoisting lines are attached to the template and pre-tensioned before the lift. The template can slide horizontally and vital parts can be damaged. During the lift off phase the valves to the ballast tanks are opened and corrections are med due to position of the lifted template. The lifting speed can be around 5 cm/s. In our Heavy lift operation we would in principle have 12 DOFs. The tugger line reduces this number by removing the 3 DOF that control the rotations of the template. This simplifies the system into a 9 DOF as illustrated in figure.

44

44

33n

C gV GM

CM A

ρ

ω

=

=+


- 32 -

Figure 22: Illustration of Degrees Of Freedom (Nielsen2, 2006) If the template collides into other objects it can be devastating both for the template or other objects. Generally the motion characteristics for the vessel and the template in air must be known. This is explained in the chapter 9.3. Theory needed for analysis. It might be useful to calculate maximum tolerable impulse load (worst case scenario) if something goes wrong. Impulse loads or slamming forces are forces with high pressure peaks during impact between a body and water. Identification of some impulse load parameters will be important:

1. Chose between impulse duration:

• Long impulse, 1 0.5t T⟩ • Short impulse, 1 0.2t T⟨

2. Assume geometrical description of the force load

Triangular impulse shape: 11

1 ,2 o

tI P t DT

π= = (9.3)

Rectangular impulse shape: 11, 2o

tI P t DT

π= = (9.4)

Sinusoidal impulse shape: 1 10

1

sin , 4t

ottI P dt D

t Tπ= =∫ (9.5)

(Bergan et. al. 1986) I is the impulse load, oP is the maximal intensity of the load and D is the load amplification factor. The impulse is calculated and the material properties must be known to calculate stresses and then check for failure. Equation (9.6) shows the maximum dynamical deflection.

maxtot

Ic m

ε =i

(9.6)

Stability criteria must also be taken into consideration when the position of the load. The lifting vessel must at all time ensure balance her self with the use of the ballast tanks.


- 33 -

9.2.2. In Air

• Motion: Pendulum • Impact: Collision, impulse loads on structure

The template is raised to satisfied height, and then moved horizontally. Collision with other object can be a risk. This is often ensured by cleaning the deck of the lifting barge for obstacles, or make sure the latitude of the template is large enough. The template is slowly lowered towards the sea surface. The same collision problems as in the lift off phase must be considered.

9.2.3. Crossing splash zone

• Motion: 6 degrees of motion • Impact: Dynamic loads, snatch loads

It is important to choose the best weather window to perform an operation like this. The wave loads on the structure can produce a tremendous force. When the structure is lowered into the water, the buoyancy will reduce the force on the lifting wire. This means that ballast water is pumped out. The largest environmental forces are found in the wave zone. Particle acceleration, currents and wind are all at maximum in the splash zone. A heave compensator is not used in heavy lift. The DP system control-personnel play an important role. They have to be able to control every situation and make the right decisions at every time. It is often used a graphical hydrodynamic calculation program, so the whole process can be executed many times before the actual installation. The coupled motions combined with the environmental loads are at the largest in this phase. The impulse loads from the waves must be taken into consideration. The wave loads are found as explained in chapter 9.3. The impulse loads from waves will be found as explained in lifting from deck, only now the deflection can not be calculated linearly. The differential equation must be solved in the time domain due to dynamic response.

9.2.4. Deeply submerged Current induced resonance on the hoisting wire will be the critical load in this phase.

• Motion: Vertical motion • Impact: Current induced resonance on wire

Finally below the wave zone, the easiest part of the operational phase commences. The template is lowered down to its final position. The added mass value will be influenced when a structure comes close to the free-surface or another body (the bottom). In chapter 9.4 Analysis the results are shown for a circular cylinder when there are long wave lengths (the


- 34 -

frequency goes to infinity). Closer to the wall the greater the added mass will be. This is the same effect the template will experience when it is lowered toward the bottom. The increased added mass value will have an influence at 20% of added mass in infinite water. This will happen at H=15m (ref chapter 9.4).

9.2.5. Landing

• Motion: Vertical motion-Horizontal offset • Impact: Impulse loads

This is further discussed in chapter 9.4. Analysis.

9.3. Theory needed for Analysis The stresses, displacements and other structural response qualities is the vital outcome of a loading calculation. These qualities are used as the actual dimensioning criteria. Torgeir Moan explains in “Design of Offshore Structures” that the design codes contain extensive information about the capacities of structural components, but there are generally few recommendations about the response analysis. His conclusion is that the complexity of offshore structures calls for an elaborate load effect analysis. When the structure is not exposed to time varying loads, or dynamic loads, we use static analysis, In connection with marine operations, wave-, wind-, damping- and inertia-loads will act on the structure together with the excitation and these are all time varying loads. To achieve good results we need to do the calculations in the time domain. The lifting operation is divided into five phases. For rough estimate it is possible to do a static linear structural response calculation. This will probably be sufficient in some of the phases. These rough estimates can be used as a control system when dynamic analysis results are calculated. The stresses on the structure due to environmental loads, is the goal with operational calculations. First we need knowledge about the environmental loads on the structure and then how the structures will response to these loads. This chapter will deal first introduce some underlying assumptions (wave theory and coupled motion) valid for all lifting phases, and then each lifting phase is introduced containing special information needed to analyze that operation phase.


- 35 -

9.3.1. Response in regular waves The following part is mostly taken from Faltinsen (1993). An irregular sea can be formed from results from regular wave components. The hydrodynamic problem can be divided into two main sub problems, A and B:

A: “The forces and moments on the body when the structure is restrained from oscillating and there are incident waves. The hydrodynamic loads are called wave excitation loads and composed of so called Froude Kriloff diffraction forces and moments.”

B: “The forces and moments on the body when the structure is forced to oscillate with the same wave excitation frequency in any ridged-motion mode. There are no incident waves. The hydrodynamic loads are identified as added mass, damping and restoring terms.”

There is a linear connection between A and B, so added up it will give the total hydrodynamic forces. Integration of the fluid pressure forces over the body surface gives resulting forces and moments on the body. Equation (9.7) shows the hydrodynamic added mass and damping loads due to harmonic motion.

2

2j j

k kj kj

d dF A B

dt dtη η

= − − (9.7)

The added mass and damping terms coefficients are dependent on the motion mode. That means that added mass in heave for a body is not necessarily the same as added mass in other motion modes.

9.3.2. Establishing the response statistics For any given wave frequency the wave force can be described as a linear function of the wave height, and the frequency of the load will be the same as the frequency of the wave. For any given load frequency the response of the structure is a linear function of the amplitude of the load and will have the same frequency as the load. From Newland (1993) we have:

(9.8)

(9.9)

(9.10)

(9.11)

0 0

0 0

0 0

( ) ( ) ( )

( ) ( )

( ) ( ) ( ) ( ) ( )

( ) ( ) ( )

H

H

M H X

X M H

F H

x H F

x H H H

H H H

ω ω ς ω

ω ω

ω ω ς ω ω ς ω

ω ω ω

=

=

= =

=

i

i

i i i

i


- 36 -

This is the transfer functions we need to be able to calculate the forces on the structure from the waves. To discover the transfer functions, a wave spectrum is needed.

• Type of spectrum, (chosen) The spectrum gives you all quantities about the waves at all times. Through the response transfer functions the loads on the structure is found.

2( ) ( ) ( )X XS H Sςω ω ω= i (9.12)

- ( )XS ω is the response on the structure - 2 ( )XH ω is the squared transfer function of the product of the mechanical- and

hydrodynamic transfer function and - ( )Sς ω is the wave spectrum

The results often show peaks both at the maximum 2 ( )XH ω value and ( )Sς ω value.

9.3.3. Frequency domain and time domain Calculations must be done for all frequencies both for the wave spectrum and the transfer function ( )XH ω must be known. When we only uses the particular solution (a solution which does not remember the initial conditions) we operate in the frequency domain and if the homogeny solution is included we can bring the solution into the time domain. The process goes from a completely description to a partly realization.

1

( ) sin( )i i ii

x t x tω ε∞

=

= +∑ (9.13)

For each time t the phase angle iε will be randomly statistical independent. This means that two processes will be different, and for each time t we choose, a new calculation must be done. This is a CPU consuming process, but must be done to ensure good results in dynamic analysis.

9.3.4. Coupled dynamic model for barge and load The following part is mostly taken from Nielsen1 (2006). The coupled dynamic for a vessel must be considerate as there is a mutual interaction. The different matrixes must first be established.

• Establish mass matrix • Establish total stiffness matrix (3 main contributions)

i. Establish restoring (stiffness) matrix ii. Establish restoring contribution for the hydrostatic effects (use of DP

system) iii. Establish restoring matrix due to vessel-load coupling effects


- 37 -

Then we calculate the undamped eigenvalues and eigenmodes for the coupled system through the following equation:

2( )ω− + =M C x 0 9.14) Here the eigenvalues and eigenmodes of the system are given from:

1−=λx M Cx (9.15) To each eigenvalue 2

0i iλ ω= there is a corresponding eigenvector which defines the contribution from each of the different degrees of freedom to this specific resonant mode of motion. Since this is a coupled system the eigenmodes will have contributions from several of the usual degrees of freedom. When the hydrodynamic forces have been found it is easy to set up the equations of rigid body motions. The equation of linear and angular momentum is presented by:

(9.16) (9.17) (9.18)

- M is the generalized mass matrix - C the total stiffness matrix - B the total damping matrix - jF is the complex amplitudes of the exciting force and moment-components with the

force and moments-components given by the real part of ei tjF e ω− .

12 .. .

1

44

44

33

[( ) ei tjk jk k jk k jk k j

i

n

M A B C F e

C gV GM

CM A

ωη η η

ρ

ω

−

=

+ + + =

=

=+

∑


- 38 -

9.4. Analysis

Figure 23: The two different parts of the analysis of the landing: a. the heave motion of template due to crane motion; b. the effect of added mass near the seafloor.

We have analyzed two critical/important effects connected to the landing phase as illustrated in figure 23. The first is the added mass effect close to a wall (bottom). The template will be dropped from a certain height and the added mass will be calculated when the template close up to the bottom. The second effect is the crane tip motions effects on the template. Here we have simulated higher waves than the operation can allow. We have only considered pure heave motion on the crane tip and not included the pitch motion of Thialf.

9.4.1. Influence of added mass near bottom We want to analyse the landing of the template on the seafloor. One important effect in the analysis is the effect of added mass in heave near the bottom and the velocity. As a basis for solving this problem, we used exercise 4 in this course from 2003. We had to idealize the problem and make some assumptions to fit the problem to our specific case. First we assumed the template hanging perfectly still a distance h over the seafloor. At this position we dropped the template leaving it in a free fall. The template has the dimensions 44 m, 33 m, and 15 m for length, breath and depth respectively. We knew the mass, 1150 tons, and made a estimate on the buoyancy to find the submerged weight. To be able to do the calculations we had to transform the rectangular form into a circular form. We based the transformation on the area of the rectangular shape and computed a radius for a circle with the same area.

[ ]44 33 21.5Ar mπ π

= = =i (9.1)

Then we did some further simplifications assuming the template being totally flat. We also neglected viscosity. Added mass in heave for this horizontal circular disk for h >> r is given by:


- 39 -

3

338 2.7159e+007[ ]3

A r kgρ= = (9.2)

When the template approaches the seafloor, we could find an estimate for the added mass in heave changing as a function of the distance z from the bottom:

4 3

338 5 3( ) log

8 3 2r r rA zz z

πρ π ρ⎛ ⎞⎛ ⎞= + −⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠ for 1z

r<< (9.3)

From this expression we see that when 0z → then 33A →∞ . The drop of the template was done from half the distance of the calculated radius over the seafloor, with initial condition of zero speed. The result for the variation of added mass is shown in figure 23. The graph of the estimated added mass was calculated using Matlab, see appendix C for the script.

Figure 24: Estimated added mass when approaching the seafloor We wanted to find the velocity as the template hit the seafloor. To do this we could look at our simplified circular disk and use conservation of energy as criteria:

( ) ( )kin pot kin potstart stopE E E E+ = + (9.4)

The kinetic energy will be zero at the start of the drop with the initial condition being zero speed. The potential energy at the start will be equal to the submerged weight, w, multiplied


- 40 -

with the distance h over the seafloor. At impact with the bottom the potential energy will be zero, i.e.

( ) 233

1 ( 0) ( 0)2

m A z V z wh+ = = = (9.5)

33

2( 0)( 0)

whV zm A z

⇒ = =+ =

(9.6)

For our special case we have that the added mass will go towards infinity when the distance to the seafloor is very small. This means that the velocity, V, will go towards zero. The effect of this is that we will have a soft landing. This is of course a very simplified case, and just a little tilt of the template will result in a finite velocity during impact. We will also get a horizontal component to the force and the template will slide horizontally. The results for height, velocity and acceleration are illustrated in figure 24, 25 and 26. To find the velocity as a function of z, we used forward Euler integration. See appendix C for Matlab script.

Figure 25: Height over seafloor as a function of time [s]

Figure 26: Velocity as a function of time [s]


- 41 -

Figure 27: Acceleration as a function of time [s]

9.4.2. Vertical oscillation of wire including mass The horizontal offset and the effect of hydrostatic pressure are ignored for all calculations. We have seen the effect on coupled motion between the template and Thialf. Form a first prediction we can assume small motions on Thialf, because of the natural frequency is out of bound of the first order wave forces.

(9.7) (9.8) (9.9) The added mass is will be frequency dependent and with a length-breadth ratio of 2 (33/15 ≈ 2) the added mass could be calculated from equation (9.10) found in Pettersen (2004)

2d 2

2d

A33 =1.36A33 *L=A33=3.8574e+007

aπρ , a is the breadth of the template. (9.10)

From Larsen (2005), nT over 20 sec in heave for an semi-sub will remove the risk of resonance with most of the first order wave forces, but resonance with second order wave forces (and also drift forces) can still come to pass. This added mass value is calculated with a different formula than in equation 9.2, which gave the similar result:

333

8 2.7159e+007[ ]3

A r kgρ= =

,

33

2 25.75[sec]

+A 3.0418e+008 [kg]

Aw 18099450 [N/m]

eqn Thialf

eq

eq

eq

MT

K

M V

K g

π

ρ

ρ

= =

= =

= =


- 42 -

Both the values is in the same magnitude, but not identical. This shows that added mass is difficult to calculate and numerical methods should be used to achieve a better estimates (model testing can also be done). With these assumptions mentioned over, we chose to create a vertical motion of the crane tip. The dynamic displacement can be calculated with the following formula taken from marine Operations by Finn Gunnar Nielsen:

(9.11) (9.12) The x coordinate is set to 300m (depth of the position of the template). ( , )x tη is the vertical motion of the template. aη is the amplitude of the wave on the top end of the hoisting line. m is the weight of the hoisting line. M is the weight of the template. L is the length of the hoisting line. The AE is calculated on the basis of some simple assumptions:

• 8 hoisting lines with a radius of 4cm each • They consist of pure steel, just to make the calculations easier

(It was hard to find information about this subject) When wave height is 2 meters, aη =1m, which is the critical limit, set by the weather window, the motion of the template is less than 5 cm. With 7m motion of the crane tip we get a little bit under 0.5 m motion of the template (NOT realistic, but shows how small the motions on the template will be). The conclusion is that the motion of the crane tip will have small deflection because of the large weight of the template, and can therefore be neglected in further calculations. The results are shown in figure 28 and the Matlab script is in appendix D.

2

1 tan( )( , ) [ sin( )]cos( )

tan( )a

m kLkMx t kx tm kL

kM

mk wavenumberAE

η η ω

ω

+=

−

= =


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Figure 28: Shows vertical motion of the template due to a sinusoidal wave with amplitude of 1 m, 2.5 m, 5 m and 7 m. They are all showed on the same scale so the differences in the motion of the template are more visible.


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10. Further Work This project shows only the main topic in a marine sub sea installation operation. All the main points mentioned in this report must be further elaborated and more careful analysis must be executed. First more careful calculations of weather windows must be carried out. This is the dimensioning parameter for the time allowed to be used for each operational phase. This project only cover more detailed analysis of the landing phase in the lifting operation. All the different phases in both the towing and the lifting operation must be further analyzed. The Ormen Lange installation planning took two years, this project had duration of 3 weeks so it is obvious some work left to do before the installation of the template can begin. During next summer we will have the first proposition ready.


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11. References Nielsen, F. G. 1; Lecture Notes in Marine Operations, 2006. Nielsen, F.G.2; ‘Slides from lectures’, 2006. Mironov, E.; ‘Uniform Ice Regions of the Barents Sea Ice Regime’, Proceedings of the 13th International Symposium on Ice (IAHR), 1996. Torsethaugen, K; ‘Wind and Waves in the Barents Sea’, Naturdatakonferanse, SINTEF report STF60, 1989. Løseth, S; ‘Lecture Notes in AT-327 – Arctic Offshore Engineering’, 2005 Berg, T.E.; ‘Marine Operations – Submarines, AUVs, UUVs and ROVs’, 2006 Bergan, P.G, Larsen, P.K, Mollestad, E.; ’Svingning av Konstruksjoner’, 1986 Faltinsen, O.M.; ‘Sea loads on ship and offshore structures’, 1990 Newland, D.E.; ‘An Introduction to Random Vibrations, Spectral and Wavelet Analysis’, 1993 Pettersen, B.; ’Marin Hydrodynamikk og Konstruksjonsteknikk GK1’, 2004 Larsen, C.M.; ’Marin Dynamikk’ 2005 NORSOK; ‘J-003 Marine Operations’, 2006. Det Norske Veritas; ‘Rules for Planning and Execution of Marine Operations’, 1996. Heerema Marine Contractors; ‘http://www.heerema.com/content/hmc/equipment/thialf/default.htm’, 2006. Monterey Bay Aquarium Research Institute; ‘www.mbari.org’, 2006.


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Princeton University; ‘http://web.princeton.edu’, 2005 The Original Online Source for the Oil Industry; ‘www.oilonline.com’, 2006

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Appendix A - Thialf - Mooring system 12 Delta Flipper anchors of 22.5 mT each, on 3 1/8" wire ropes of 2,400 metres long. Minimum breaking strength 480 mT. Kongsberg Albatross ADP 503 and ADP 311 automatic and dynamic positioning and mooring assistance. Main hoist lifting height 95 m above work deck for each crane. Lowering depth of auxiliary hoists 460 m below work deck at minimum radius. Main hoist plumbing depth Lowering depth of main hoist at minimum radius with 3,500 mT is 307 meters below heel point and with 2,990 mT 351 metres below heel point. Heel point is 24.4 metres above work deck. Tandem lift Main hoist 14,200 mT at 31.2 metres radius (subject to stability calculations). Ballast system Ballast pump capacity 20,800 cubic metres/hour. Dynamic position system The Thialf is equipped with a Class III Dynamic Positioning system with the following characteristics: Thrusters 6 x 5,500 kW - 360 degrees azimuth, total thrust 400 mT Modes of Operation

• Manual • Joy-stick • Auto-pilot • Full DP mode • Position mooring

Special DP functions

• Track follow • Heavy lift • Follow floating object • External force compensation

Position reference systems

• 2 x satellite DGPS • 1 x mechanical taut wire (300m) • 1 x Artemis • 2 x acoustic SSBL/LBL • 1 x Fan-beam laser

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Deck load / Transit speed Deck load capacity 15 mT/square metre Total deck load capacity 12,000 mT Transit speed with 12,000 tons deck load 6 knots at 12.5 metres (43.6 ft) draft. Thialf lifting capacities / weight

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Towline: Examples of towline layouts. (Fylling 1979)

Characteristics of some ocean going tugs, anchor handling and multipurpose vessels.

Sea Husky Lenght 27.5m Breadht 8.5m Depth 4.7m Gross tonnage 255 tons Speed 13.3 knots Bollard pull 45.2 tonsf

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Appendix C -Matlab script for ’Influence of Added Mass near Sea Bottom’ %Landing the Ormen Lange Template %Script originally by S. Fouques 23/03/03, edited --/03/2006 by the undersigned. close all; clear all; clc; g=9.81; %accelleration of gravity [m/s^2] rho=1025; %desity of water %Dimensions of the template geometry: l=44; %lenght [m] b=33; %breath [m] h=15; %heigth [m] tm=1.15e6; %total mass [kg] %We spilt the geometry and look at the upper rectangular part and the %suction anchors seperatly: %Converting geometry %Circular disk (upper rectangular part) rt= sqrt(l*b/pi); % radius of a circle with the same area as the template md=tm; %mass of the disk hu=h/3; %height of upper rectangular part wd= -md*g + rho*g*l*b*hu/7; %submerged weight of the disk [N]. We use the scaling factor of .6 because the volume is not a solid filling the whole volume. zdp=0.05:0.002:.5*rt; %disk deltaz=1e-8; %to estimate the derivative of the added mass dt=0.005; t=0:dt:120; %Creating empty tables Vddot=zeros(1,length(t)); Vd=zeros(1,length(t)); zd=zeros(1,length(t)); V0=0; %initial conditions zd(1)=.5*rt; %drop distance over the seafloor %Template fall (circular disk fall) i=1; while (i<length(t))&(zd(i)>0) dA33ddz=-(A33d(zd(i),rt)-A33d(zd(i)+deltaz,rt))/deltaz; Vddot(i)=(wd-dA33ddz/2*Vd(i)^2)/(md+A33d(zd(i),rt)); Vd(i+1)=Vd(i)+Vddot(i)*dt; zd(i+1)=zd(i)+Vd(i)*dt; i=i+1; end imaxd=i-1; %Plot the results hold on plot(zdp,A33d(zdp,rt),'r-.') xlabel('z [m]') ylabel('Added mass') hold off legend('Ormen Lange template, as a flat disk') figure(2) subplot(3,1,1) plot(t(1:imaxd),Vddot(1:imaxd)) ylabel('dV/dt [m/s^2]') title('Disk fall') subplot(3,1,2) plot(t(1:imaxd),Vd(1:imaxd)) ylabel('V [m/s]') subplot(3,1,3) plot(t(1:imaxd),zd(1:imaxd)) xlabel('t [s]') ylabel('z [m]') function ret=A33d(z,a) rhow=1025; ret=max(8/3*rhow*a^3,pi/8*rhow*a^4./z+(log(8*pi*a./z)-5/3)*rhow*a^3*3/2);

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Appendix D -Matlab script for vertical motion for template- close all; clear; clc; g=9.81; rho=1025; % Natural period %constant %pontoon pl=168.3; pb=88.4; pd=7; antp=2; %slice sl=15; sb=15; sh=10; ants=8; Aw=sl*sb*ants; %calculations vp=pl*pb*pd*antp; vs=sl*sb*sh*ants; M=(vp+vs)*rho; C33=rho*g*Aw %table gk1 A332d=1.36*pi*rho*pd^2; A33=A332d*pl*2; %eq mass i hiv Meq=M+A33; Tn=2*pi*sqrt(Meq/C33); w0=sqrt(C33/Meq); demp33=0; %calculation of DLF and w/w0 FREC=[]; DLF=[]; w1=[0:.01:10]; for (i=1:999) DLF1=0; DLF1=1/sqrt((1-(w1(i)^2/w0^2))^2+(w1(i)^2)*(demp33^2/C33^2)); DLF(i)=DLF1; FREC(i)=w1(i)/w0; end % % plot % figure1 = figure(... % 'FileName','F:\Skole\4klasse vår\Marine operations\Prosjekt\fig1.fig',... % 'PaperPosition',[0.6345 6.345 20.3 15.23],... % 'PaperSize',[20.98 29.68]); % % % Create axes % axes1 = axes('Parent',figure1); % axis(axes1,[0 5 0 35]); % xlabel(axes1,'w/w0'); % ylabel(axes1,'DLF'); % hold(axes1,'all'); % % % Create plot % plot1 = plot(FREC,DLF); % HEAVE motion of the template %template l=44; b=33; h=15; w=1150000; corr.f=1/18; V=l*b*h*corr.f; A332d=pi*(b/2)^2*rho; A33=A332d*l %hoisting line L=300; emod=2e11;


%calculated stiffness rad=4/100; area=2*pi*rad^2*8; AE=emod*area; %AE=1e11; %Marine Operations c2=AE/L; wireperm=20; wirew=L*wireperm; M=w*g-rho*g*V; elongation=L/AE*(wirew*L/2+M); eta=1; w0=0.2439; k=sqrt(abs((w0^2)*wirew/AE)); wavelength=2*pi/k for i=1:1000 t(i)=pi/100*i; B(i)=(1+wirew/k/M*tan(k*L))/(wirew/k/M)-tan(k*L); disp1(i)=eta*(cos(k*L)+B(i))*(sin(k*L))*cos(w0*t(i)); F1(i)=eta*sin(w0*t(i)); end eta=2.5; for i=1:1000 t(i)=pi/100*i; B(i)=(1+wirew/k/M*tan(k*L))/(wirew/k/M)-tan(k*L); disp2(i)=eta*(cos(k*L)+B(i))*(sin(k*L))*cos(w0*t(i)); F2(i)=eta*sin(w0*t(i)); end eta=5; for i=1:1000 t(i)=pi/100*i; B(i)=(1+wirew/k/M*tan(k*L))/(wirew/k/M)-tan(k*L); disp3(i)=eta*(cos(k*L)+B(i))*(sin(k*L))*cos(w0*t(i)); F3(i)=eta*sin(w0*t(i)); end eta=7; for i=1:1000 t(i)=pi/100*i; B(i)=(1+wirew/k/M*tan(k*L))/(wirew/k/M)-tan(k*L); disp4(i)=eta*(cos(k*L)+B(i))*(sin(k*L))*cos(w0*t(i)); F4(i)=eta*sin(w0*t(i)); end


%PLOTTING THE FIGURES createfigure(F1, disp1, F2, disp2, F3, disp3, F4, disp4)

function createfigure(y1, y2, y3, y4, y5, y6, y7, y8) %CREATEFIGURE(Y1,Y2,Y3,Y4,Y5,Y6,Y7,Y8) % Y1: vector of y data % Y2: vector of y data % Y3: vector of y data % Y4: vector of y data % Y5: vector of y data % Y6: vector of y data % Y7: vector of y data % Y8: vector of y data % Auto-generated by MATLAB on 21-Mar-2006 18:09:33 %% Create figure figure1 = figure(... 'FileName','F:\Skole\4klasse vår\Marine operations\Prosjekt\untitled.fig',... 'Name','Displacement of template due to sinusodial wave with different amplitude',... 'NumberTitle','off',... 'PaperPosition',[0.6345 6.345 20.3 15.23],... 'PaperSize',[20.98 29.68],... 'PaperType','a4letter'); %% Create axes axes1 = axes('OuterPosition',[0 0.4985 0.4823 0.5015],'Parent',figure1); ylim(axes1,[-1 1]); title(axes1,'Amplitude of incoming wave = 1m'); xlabel(axes1,'time[s]'); ylabel(axes1,'displacement[m]'); hold(axes1,'all'); %% Create plot plot1 = plot(y1,... 'Parent',axes1,... 'DisplayName','Displacement of template'); %% Create plot plot2 = plot(y2,... 'Parent',axes1,... 'DisplayName','Sinus wave',... 'YDataSource','F1'); %% Create axes axes2 = axes('OuterPosition',[0.4823 0.4985 0.5177 0.5015],'Parent',figure1); ylim(axes2,[-1 1]); title(axes2,'Amplitude of incoming wave = 2.5m'); xlabel(axes2,'time[s]'); ylabel(axes2,'displacement[m]'); hold(axes2,'all'); %% Create plot plot3 = plot(y3,... 'Parent',axes2,... 'DisplayName','F2',... 'YDataSource','F2'); %% Create plot plot4 = plot(y4,... 'Parent',axes2,... 'DisplayName','disp2',... 'YDataSource','disp2'); %% Create axes axes3 = axes('OuterPosition',[0 0 0.4823 0.4985],'Parent',figure1); ylim(axes3,[-1 1]); title(axes3,'Amplitude of incoming wave = 5m'); xlabel(axes3,'time[s]'); ylabel(axes3,'displacement[m]'); hold(axes3,'all'); %% Create plot plot5 = plot(y5,... 'Parent',axes3,... 'DisplayName','F3',... 'YDataSource','F3'); %% Create plot plot6 = plot(y6,... 'Parent',axes3,... 'DisplayName','disp3',... 'YDataSource','disp3'); %% Create axes


axes4 = axes('OuterPosition',[0.4823 0 0.5177 0.4985],'Parent',figure1); ylim(axes4,[-1 1]); title(axes4,'Amplitude of incoming wave = 7m'); xlabel(axes4,'time[s]'); ylabel(axes4,'displacement[m]'); hold(axes4,'all'); %% Create plot plot7 = plot(y7,... 'Parent',axes4,... 'DisplayName','Template motion',... 'YDataSource','F4'); %% Create plot plot8 = plot(y8,... 'Parent',axes4,... 'DisplayName','Wave motion',... 'YDataSource','disp4'); %% Create legend legend1 = legend(axes4,{'Wave motion','Template motion'},'Position',[0.4066 0.4655 0.1674 0.05951]);

Tow out on crane vessel, lift off and lowering - NTNU · Figure 26: Velocity as a function of time...

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Transcript of Tow out on crane vessel, lift off and lowering - NTNU · Figure 26: Velocity as a function of time...