Project (Ozorishin, A.)

8/2/2019 Project (Ozorishin, A.)

1/39

Arctic offshore engineering

Course paper AT - 327 Arctic Technology

Studies program/specialty

Master degree/Marine technology

Autumn, 2011

Title of project:

" Simplified calculations and analysis of FPSO mooring

systems in the Kara Sea"

Author:

Alexey Ozorishin .................................(signature of author)

Supervisor:

Sveinung Lset ..............................(signature of supervisor)


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Abstract

In recent years the interest in the Arctic Ocean is increasing because of the huge

reserves of hydrocarbons which were identified in this region. A striking example is the

Kara Sea. There are 2 unique fields Leningradskoe and Rusanovskoe. But in addition to

prospecting new deposits, we are faced with new challenges, such as cold climate,

darkness (more than 3-4 months), fog, polar low pressures (the weather may change less

than 1 one hour). The arctic nature is at different sensitivity to pollution of any kind,

especially with respect to hydrocarbons, because of cleaning ice from hydrocarbons is

much harder than collecting oil spills from the water. Also the Arctic shelf is very

isolated area, and in case of accidents, aid to await nowhere. Therefore, the safety and

reliability must be much higher. The errors are not allowed

In developing Arctic offshore fields, using FPSOs is the most attractive options

because of lack of oil infrastructure. In intermediate waters (sat at 100 meters) using

mooring system is the most interesting option as compared to use of DP. This option

creates additional problems, however, due to heavy ice conditions, ridges, icebergs, sea

water, currents, waves, wind, etc.

In this paper will be presented and analyzed mooring systems for shallow-water

conditions in the Kara Sea.


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Abstract........................................................................................................................ I

List of Contents.......................................................................................................... II

Introduction .................................................................................................................. 1

1 The ice and the weather conditions in the Kara Sea................................................... 2

1.1 Description of the region........................................................................................ 2

1.2 Climate..................................................................................................................... 3

1.3 Hydrology................................................................................................................ 3

1.4 Sea ice...................................................................................................................... 4

1.5 Hummocks and ridges. ........................................................................................... 4

1.6 Bottom contour........................................................................................................ 5

1.7 Summary and particular conditions for oilfields .................................................... 5

2 Interactions between sea ice and mooring system...................................................... 6

3 Ice force...................................................................................................................... 7

3.1 Breaking Force........................................................................................................ 8

3.2 Rotation Force......................................................................................................... 9

3.3 Sliding Force............................................................................................................ 10

4 Mooring system.......................................................................................................... 11

4.1 Catenary and Taut Leg Moorings............................................................................ 11

4.2 System design.......................................................................................................... 13

4.3 Mooring force.......................................................................................................... 14

4.4 Types of mooring lines........................................................................................... 17

5 Anchor system............................................................................................................ 19

6 Methods for reducing heavy ice loads....................................................................... 21

6.1 Vessel and mooring features.................................................................................... 21

6.2 Ice management....................................................................................................... 22

Conclusions.................................................................................................................. 27

References..................................................................................................................... 28

Appendix A.................................................................................................................... A

Appendix B.................................................................................................................... B

Appendix C.................................................................................................................... C


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Introduction

Moored vessel is the most attractive solution in the Arctic conditions. A lot of

operations can be performed such as drilling, exploration, production, processing,

storage, offloading, etc. But ice conditions creates additional challenges for mooring

system compared to open water. This may lead to disconnection, and thus stop

production, and a colossal waste of money.

In this project, we have considered one of the pressing problems of the Arctic seas,

namely, ice conditions and the associated impact on the mooring system of an FPSO.

The first section provides characteristics of weather, ice conditions and soil properties.

Fuser, considering a simplified model of the FPSO mooring system and the ice cover, the

static interaction with the FPSO is shown. Thereafter, the required parameters of the

mooring system, such as vertical and horizontal distance from the connection point

between the mooring system and the submerged turret to the touch down point, mooring

line geometry, etc. are analyzed and calculated. Then, the anchor selection is described.

The next part describes the ice management as the main event of reduction of ice loads.

The last part of the necessary calculations are presented for discussion


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1 The ice and the weather conditions in the Kara Sea

1.1 Description of the region

The Kara Sea is a marginal sea bordering on the Arctic Basin in the north, the

Barents Sea in the west and the Laptev Sea in the east. The coastline is strongly irregular

with large bays (Baidaratskaya, Gydanskaya and Ob Bays and the Yenisey Gulf) being

deeply entrenched to the mainland shore. The Kara Sea is usually subdivided by

oceanographic conditions into two regions - southwestern and northeastern with the

boundary passing along the line from Cape Zhelaniya to Dikson Island [3]. Two unique

gas fields have been explored in the Kara Sea: Leningradskoe and Rusanovskoe. Total

resources of these fields and other small fields in the Kara sea are more than 40 BTOE

(Figure 1).

Figure 1 - Map of Kara sea [3].

Leningradskoe Field

Rusanovskoe Field


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The main morphometric characteristics of the Kara Sea are as follows [3]:

Total area: 883 000 km2

Water volume: 98 000 km3

Mean depth: 111 m

Mean depth of oilfields 111 m

Maximum depth: 600 m

1.2 Climate

The air temperature in the Kara Sea is consistently below zero for 8 months from

October to May. The coldest period is from December to March when the mean monthly

air temperatures are between -14 to -26 . The summer period lasts only for 4

months: from June to September. The mean monthly summer air temperature is not more

than 7 [3].

1.3 Hydrology

In winter, the water masses in the shallow sea regions become almost uniform

from the surface to the bottom and have a temperature of approximately -1,8 . The

main mass of water (the heat sink of the Siberian Rivers), arrives at the sea during the

spring when it is still ice-covered. Usually in the beginning of July, the water temperature

increases first slowly and then with the sea becoming ice cleared more actively achieving

its maximum by the end of August. In September to October, the surface water

temperature decreases to the freezing temperature. In summer in the southwestern Kara

Sea the salinity in the surface layer decreases, as a result of ice melting and the inflow of

flood water, achieving its minimum in August-September. The surface water salinity is

predominantly 30 to 32 . The most brackish water is in the south near the mouths of

large rivers, where the surface water salinity decreases to 10 .


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In the shallow Kara Sea area, wind-driven currents prevail but have variable directions

and speeds. The tidal sea level oscillations are not greater than 0,5 m while the wind-

driven surge water rise in the coastal areas can be 2 m to 3 m [3].

1.4 Sea ice

From October to May/June, the East Siberian Sea is entirely covered by sea ice.

Ice growth lasts until the end of May. The ice cover at the beginning of melting consists

of thick first-year ice (maximum 180 cm) and occupies about 80 % of the sea surface in

the western part and about 65 % in the eastern one. From historical databases, on

average, old ice (multi-year and second-year) from the Arctic Basin occupies 12 % of the

sea surface in the western part of the sea and 30 % in the eastern one. A minor part of the

ice cover consists of younger ice: thin first-year ice (30 cm to 70 cm) and medium first-

year ice (70 cm to 120 cm) [3].

An ice concentration of 10r10 exists during the winter and the beginning of spring

(Riska, 1995). The predominant ice drift is from the south to the north. The maximum ice

drift speed is about 0.5 m/s. The drift direction and speed can vary during the short time

period depending on the wind direction [4].

1.5 Hummocks and ridges

Only first-year ridges have been observed in the southwest region of the Kara Sea.

Generally, the ridges consist of blocks 0.20.6 m thick. The average hummocking is 23

balls _approximately 40% to 60% of the ice cover is hummocked.. The number of ridges

per kilometre is less than 4 in 88% of the occasions. The average sail height is 2 m and

the maximum one is 5 m (Romanov, 1991). The last value was observed in the northern

regions and is overestimated for perspective hydrocarbon fields. With a keel to sail ratio

of 4.5, a keel draught of 1820 m is possible [4].


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1.6 Bottom contour

The sea lies almost entirely on the shelf with a depth of 100 meters. Two trenches

- St. Anne's with a maximum depth of 620 meters (80 26 '. W. 71 18' east. D. (G) (O))

and Voronin with a depth of 420 meters - cut the shelf from north to south. The East

Novaya Zemlya trough, with depths of 200-400 m runs along the eastern coast of Novaya

Zemlya. The Shallow (50 meters) Central Kara plateau is located between the trenches.

The bottom of the shallow water and hills is covered with sand and sandy silt. Troughs

and basins are covered with gray, blue and brown silt [7].

1.7 Summary and particular conditions for oilfields.

The Kara Sea the most prominent representative of the Russian Arctic Shelf. Ice

conditions are severe for 7-8 months. There may be a multi-year ice. The thickness of

multi-year ice achieves 4 m. The medium ice thickness is more than 1 m and maximum

1.8 m. Salinity is high (30 ). This information can be no exact due to 2 biggest rivers

(Ob and Yenisey) flow into the Kara sea. But the south part of the sea is hydrologically

closed area and salinity increases (regard to Rusanovskoe field). Most important data for

calculation are shown in the table 1.

Table 1 - Ice parameters

Ice properties, units Value

Thickness, m 0,7 - 1,2 m. We take max 1.8 m

Salinity, p.p.t. 30

Density, kg/m3

900 (average value)

Temperature, C -20 (average for the Kara sea)

Young Modulus, Nm-2 9,33109 (for isotropic ice at -16 C)

Poisson's ratio 0.325 (for isotropic ice at -16 C)


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2 Interactions between sea ice and mooring system

The mathematical model for motions:

+

+

=

where, M is the mass of a vessel X is the displacement of the vessel, =( + ) is the hydrodynamic force, = is the mooring force, is the iceloads which we describe later.

Suppose that we consider a two-dimensional model of the vessel with one degree

of freedom (surge motion) [1]. The width of the ice being equal to the width of the vessel.

The ice uniformly affects the entire width of the vessel. During loading, the ice shifts

and the vessel mooring system is at a peak value. We consider the static model. It is the1st step of the mooring system analysis. The continue of this paper will be presented in

the new papers. Since the system is static, we can neglect the hydrodynamic and mass

forces: =


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3 Ice force

The ice sheet is bent until flexural failure, then the broken ice piece is rotated and,

finally positioned parallel to the hull. Thereafter the vessel meets the new portion of ice

and a new breaking cycle starts (figure 2). The ice force is decomposed into 3 phase: ice

breaking, ice rotation and ice sliding . An elastic bending model was described by Nevel

(1992). Rotation and friction forces were modeled by Lindqvist. We consider the static

contribution: simultaneously we have all phase of force to show the maximum ice load:

= + +

Figure 2 - The idealized penetration dependent part of the ice force. Contributions from

the breaking force, rotation force and friction force are indicated [1].


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3.1 Breaking Force

We assume that the ice sheet is a semi-infinite elastic beam on an elastic

foundation. Breaking force equals [2]:

Vertical breaking force

= 0,68 5

0.25

Relation between vertical and horizontal forces is shown In IS0 19906: = As result, the horizontal breaking force equals:

= 0,68 5

0.25

where

beam of vessel and ice flexural strength of ice, depends on brine volume

= 1,76

5,88

and

brain porosity = 49,18 + 0,53

where

salinity

temperature (-20 < T < -2)

density of water the thickness of the ice gravity accekiration Young's modulus depends on brine volume as well = 0(1 )

0 elasticity modulus of fresh water


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= + friction coefficient the stem angle of the vessel

3.2 Rotation Force

The rotation force is based on the difference in potential energy for a floating floe

and a floe parallel to bow. Difference in potential energy [1]:

= 12

( )()2 where

density of ice breaking length

we can define the breaking length from the maximal flexural strength [2]:

= 6 2

= 2

6 We assume = 1,76 5,88

The horizontal rotation force includes a relation between the vertical and

horizontal forces: [1]:

= =12

( )()2

where

a - penetration into the ice sheet at failure


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3.3 Sliding Force

We assume this force in a simplified case like the difference in density between

water and ice. We decompose the force in 2 parts: load on the sloping part of the vessel

1 and load on the bottom of the vessel 2

1 = ( ) 2 = ( )

where

length of vessel draught of vessel

= 1 + 2

These calculations are presented in Appendix A.


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Figure 3 - Typical Single Point Moored Catenary System [6]

A taut-leg system makes use of the material properties of the mooring line,

namely its elasticity. A typical taut-leg arrangement is shown in Figure 4 . Taut-leg

moorings are relatively new and are typically used in deep water to limit FPSO offsets

[6].

Figure 4 - Typical Single Point Moored Taut-Leg System [6]

In our case we a choose catenary mooring system, because the depth is only 100m,

the soil conditions prevailing is sand and silty sand and the cost of a taut mooring system

is more expensive due to special light lines. Furthermore, a taut system might not be

flexible in case of large ice forces, allowing only very small vessel motions.


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4.2 System design

In the picture we see the different types of the mooring design (Figure 5 ). Since

conditions in the Kara Sea are extremely severe, we may choose the most safety design

from 3rd, 5th and 6th options, because they have equal number of lines. But the anglebetween lines in 6th option is small and it increases the probability of interaction

between risers and lines. In the third option, the angle between the beams lines is large

and the load will be treated unequally. The best choice is number 3. In the discussions in

the next chapters we analyze one line to simplify the calculations, but in Appendix A we

take into account this.

Figure 5 -The design options of the mooring line [6]

1 2 3

456


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4.3 Mooring force

In the previous chapter we assume = (Figure 2) and we defined .We now consider parameters and loads in the mooring line (Figure 6)

Figure 6 - Offset position due to loads [6]

We know that = , also we know the depth of area (h) and assumeparameters of the chain (e.g. R4 see Appendix B). We assume that the mooring system

has an ideal catenary geometry. In this case we can define the horizontal tension on the

bottom (T0), the length of the mooring line (S) and necessary horizontal length from the

connection point between the mooring system and the submerged turret (A) to the touch

down point (B), axial tension (T) (Figure 7).


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Figure 7 - Catenary geometry of the mooring line.

If we consider a piece of the mooring line [5] we can determine the connection

between vertical tension and horizontal (figure 8)

=

Figure 8 - Element of mooring line

From this equation =

and the catenary equation = cosh 1

we can find all necessary parameters:

The horizontal distance from the connection point between the mooring system

and the submerged turret (A) to the touch down point (B):


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= arccosh + 1

length of the mooring line (S):

= sinh

Vertical Tension (V):

= sinh

Axial tension (T)

= 2 + 2

Now we can check the chain:

< where

safety factor breaking load

Horizontal Tension on the bottom (B):

2 =

02 + (

)2

From this we obtain

0 = 2 ()2 =

Calculations are shown in Appendix B


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We can now use this value to calculate the necessary parameters of the anchor. But

first we select the type of mooring line and describe our selection.

4.4 Types of mooring lines

Steel chain

Steel chain is the commonly used type of mooring line. There are two types of

chain, the studless and studded chain (Figure 9). The studless chain is mostly used for

permanently moored platforms, while the studded chain is frequently used by drilling

platforms. The steel chain is the heaviest mooring line, this weight makes it less suitable

for deep water but it also gives the system more capacity to withstand forces. Any weak

link of the chain and local wear are main disadvantages.

Figure 9 - Studless and studded chains respectively [10].

Steel wire

Further to the steel chain, the steel wire is used a lot in the mooring systems of

floating platforms. The wire has the advantages of a higher elasticity (spring effect) and a

lower weight. But it is sensitive to abrasion and corrosion. Unlike the steel in the circuit it

has a parallel coupling.


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Fiber rope

The latest development is the application of fiber ropes for mooring systems.

Polyester and Aramid are the mostly used fibers. These fibers have a lower elasticity and

break strength then the iron chain and wire, and they have a lower weight. Main lack is

very sensitive to mechanical impact (for example cutting).

Fiber rope is not suitable in our situation, because the depth is shallow and we

don't need special lightness. Also catenary system involves the interaction of the mooring

lines from the seabed, which is detrimental to the rope.

We can compare the effect of the weight on the geometry of the mooring lines of

the system (Figure 10). Hang off length of wire has twice bigger size comparing with

chain. This increases the likelihood of damage to lines of external factors. Another

deciding factor in choosing the line is the cost, the price of chains below the wire ropes.

Figure 10 - Steel wire and chain mooring line geometry

(calculations are presented in Appendix C).

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200

y

x

Steel wire Steel chain


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5 Anchor system

Types of anchor system:

Dead weight

The dead weight is the oldest anchor. The holding capacity is generated by the

gravity force and by the friction between the anchor and the seabed. Common materials

in use today for dead weights are steel and concrete.

Pile

The pile is a steel pipe that is installed into the seabed. The holding capacity of

the pile is generated by the friction of the soil along the pile. The pile has to be installed

to a great depth below the seabed to obtain the required holding capacity. The pile is

capable of resisting both horizontal and vertical loads.

Drag embedment anchor

This is the most popular type of anchoring point today. The drag embedment

anchor has been designed to penetrate into the seabed. The holding capacity is generated

by the resistance of the soil in front of the anchor. The drag embedment anchor is suited

for resisting horizontal loads, but not for large vertical loads.

Suction anchor

Like the pile, the suction anchor is a hollow steel pipe, although the diameter of

the pipe is much larger than that of the pile. The suction anchor is forced into the seabed

by means of a pump connected to the top of the pipe, creating a pressure difference.

When the pressure inside the pipe is lower than at the outside, the pipe is sucked into the

seabed. After installation the pump is removed. The holding capacity of the suction

anchor is generated by the friction of the soil along the suction anchor and lateral soil

resistance. The suction anchor is capable of withstanding both horizontal and vertical

loads [8].


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We have chosen the catenary mooring design in the shallow water. Therefore the

horizontal loads are dominating. To provide necessary resistance using drag embedment

anchor will be enough (Figure 11).

Figure 11 - drag embedment anchor [8]

Unfortunately we can't show calculation of anchor holding capacity as it is

complex and we don't have enough data about properties of soil conditions.


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6 Methods for reducing heavy ice loads.

6.1 Vessel and mooring features.

Previously, we described that after the ice is broken and rotating, the ice slides

over the vessel bottom creating a risk of interaction between the ice and the mooring lines

and, the ice and riser.Another challenge may be due to the presence of hummocks and

ridges. They can impact the riser and mooring lines. Therefore the design of the

submerged turret shall provide safety clearance for mooring lines and risers. Also the

turret construction has to include external protective casing to prevent damage from ice

loads.

In general ice drift depends on the current direction. It is a very unstable and

unpredictable phenomenon. This proves the analysis of drift buoy data (Figure 12). We

see that 30 minutes is enough to change direction in the opposite way. Accordingly the

vessel design shall be adapted to this. The bow is the most robust part of the vessel. The

bow has to heading into the ice drift. This can be achieved by placing the turret in the

bow. In this case, to change the direction of the bow takes minimum time.

Figure 12 - Modeled movement of the ice drift. Dots every 10 minutes [2]


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6.2 Ice management

It is known that high ice concentration (over 8/10th) increases the ice loads on the

vessel . That's why one of the major and more effective ways to reduce the ice loads is ice

management. Typically we can distinguish between several phases of ice management:

1) Analysis of ice conditions and definition of a class of ice loads; to define the

ice concentration and to determine the age of the ice (first year or multiyear), the

thickness of the ice, the drift speed and the common drift direction.

2) Selection of icebreaker class (strength and thickness of ice), and their number.

The number of the icebreakers depends on the severity of ice conditions and type of

offshore operation. Table 2 shows the main classes of breakers and their approximate

equivalents of the various regulatory organizations

Table - 2. Approximate Equivalents for Vessel Classed as Icebreakers [9]


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3) Methods of ice management. Commonly they depends on drift speed, thickness

and concentration of ice. Here are some of the methods:

Linear

The linear technique is an icebreaking pattern used by a support vessel to break

pack ice up-drift of a floating platform in straight lines, parallel to the direction of the ice

drift. This icebreaking pattern is typically used when the ice drift speed is fast and the ice

drift direction remains reasonably constant [9].

Figure 13 - Linear technique [9]

Sector

The sector technique is an icebreaking pattern that provides wide managed pack

ice coverage around the approaching ice drift direction. This technique requires the

support vessel to steam back and forth across the drift-line between 2 designated bearings

that create the sector. This pattern is typically used when the ice drift speed is slow and/or

when the drift direction is variable or changing rapidly [9].


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Figure 14 - Sector technique [9]

Circular

The circular icebreaking technique is a procedure that requires the support vessel

to steam in a circular pattern up-drift of the platform location. The diameter of the circles

will vary with the speed of the ice drift, and the maneuverability and speed of the support

vessel. This pattern is typically used in high concentrations of thin ice or small diameter

thick ice floes and when the ice drift direction is variable. A circular pattern is also made

completely around a platform as an effective method to relieve ice pressure [9].

Figure 15 - Circular technique [9]


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Pushing ice

Pushing ice is an effective way of removing medium and large ice floes from the

drift line. The pushing direction is usually at right angles to the approaching ice. The

benefit of pushing a large floe instead of breaking it is that the threat to the platform is

removed from the drift-line whereas if the ice is broken up-drift, the broken remnants

may still pose a threat. Care must be taken to properly forecast any change to the ice drift

to ensure that the ice will not become a threat at a later time. To allow full power

pushing, the bow strength of the vessel(s) used must be appropriate. At least two vessels

are often used to prevent floe rotations [9].

Figure 16 - Ice pushing technique [9]

Propeller Wash

Propeller washing of small pieces of thick ice, even if present in high

concentrations, can be very effective to reduce or prevent ice accumulation against the

platform. This technique is particularly effective when used by vessels fitted with

azimuth main propulsion. Such a system allows the support vessel to remain almost

stationary up-drift of the platform on the drift line, with the propellers angled outwards

and using high power to wash ice to each side of the platform. There are sometimes

restrictions to the use of this approach, for example, if there is only one vessel and poor

visibility which prevents knowing what ice may be coming from further up drift [9].


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Figure 17 - Propeller technique [9]

According to the Kara' ice conditions: the low drifting speed (5 m/s), the ice

thickness (from 70 to 120 cm) and the location of hydrocarbon fields (between 2 opposite

current. Systems, Figure 1), the sector technique is the most fitting method. There has to

be minimum 2 icebreakers to provide ice management and 1 icebreaker to provide towingof ridges, hummocks or small icebergs. Another phase of the ice management is the ice

dispersal to decrease ice accumulation on the bow. The wash propellers of the FPSO help

to achieve this goal.


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Conclusions

It is presented and analyzed FPSO mooring system for arctic conditions in shallow

water, on the example of the Kara Sea in this paper. The paper considers a simplified

static model, the interaction of the ice with the mooring system. The main load is the ice

impact due to the severe ice conditions in the Kara Sea.

The optimal design of the mooring system is catenary mooring system. Its

advantages over the taut-leg system are lower cost and flexibility. Important part of the

analysis is to design the location of the mooring lines. The most appropriate design is

four bundles of three chains in each, because this design approach reduces possibility of

damage of risers by interacting with mooring system and is highly reliable. Furthermore,

ice load 606,7 kN, which is found after mathematical calculation, have been used to

select a steel chain grade R4. The following parameters of the chain were calculated:

required horizontal distance from the connection point between the mooringsystem and the submerged turret to the touch down point =559,2 m,

length of the mooring line =570 m, axial tension (322,5 kN), comparing the minimum breaking load (9864 kN) with

axial tension, taking into account margin of safety i.e. three (3).

The geometry of steel chain was compared with the geometry of steel wire.

Analysis of the graph shows that the steel chain has smaller offset (as a flexible riser has

limited motion) and less hanging-off area. Then different types of anchor were

considered. The most attractive anchor is the drag embedment anchor, because of the

predominance of horizontal loads. At last but not the least ice-load reducing methods are

described, mainly Ice Management. The different techniques of Ice Management have

been analyzed and the Sector technique was chosen due to its suitability to slow drift of

thick ice with possible rapid change of direction.

The paper deals, as already mentioned, mainly about influence of ice on FPSO mooring

systems. Since the focus on the Arctic Shelf is growing, the subject of the paper is worthy

of considering though it definitely requires comprehensive studies thoroughly.


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References

1. Aksnes, V. (2011), Experimental and Numerical Studies of Moored Ships in Level

Ice, PhD thesis, NTNU, Trondheim

http://ntnu.diva-portal.org/smash/get/diva2:404324/FULLTEXT02

2. Lset, S. (2011): Lecture notes for the course Arctic Technology (AT-327), University

Studies at Svalbard, October 2011

3. ISO 19906, Arctic Offshore Structures, Int Standardization Organization, Geneva

4.Lset, S., Shkhinek K., Gudmestad O.T. (1999), Comparison of the physical

environment of some Arctic seas, Cold Regions Science and Technology, Volume: 29,

Issue: 3, Pages: 201-214

5. Gudmestad O.T. (2011), Lecture notes for the course Marine Operations (MOM-490),

University of Stavanger, fall 2011

6.http://www.offshoremoorings.org/moorings/2008/group%20a/systemdesign.html

7.http://ru.wikipedia.org/wiki/%CA%E0%F0%F1%EA%EE%E5_%EC%EE%F0%E5

8. Anchor manual 2005

http://www.offshoremoorings.org/moorings/

9. Peter Dunderdale, Noble Denton Canada Ltd (2005), Pack ice management on the

Southern Grand Banks Offshore Newfoundland

ftp://ftp2.chc.nrc.ca/CRTreports/Pack_Ice_Management_05.pdf

10.http://www.offshoremoorings.org/moorings/2008/group%20b/site/line.html
http://ntnu.diva-portal.org/smash/get/diva2:404324/FULLTEXT02http://ntnu.diva-portal.org/smash/get/diva2:404324/FULLTEXT02http://www.offshoremoorings.org/moorings/2008/group%20a/systemdesign.htmlhttp://www.offshoremoorings.org/moorings/2008/group%20a/systemdesign.htmlhttp://www.offshoremoorings.org/moorings/2008/group%20a/systemdesign.htmlhttp://ru.wikipedia.org/wiki/%CA%E0%F0%F1%EA%EE%E5_%EC%EE%F0%E5http://ru.wikipedia.org/wiki/%CA%E0%F0%F1%EA%EE%E5_%EC%EE%F0%E5http://ru.wikipedia.org/wiki/%CA%E0%F0%F1%EA%EE%E5_%EC%EE%F0%E5http://www.offshoremoorings.org/moorings/http://www.offshoremoorings.org/moorings/ftp://ftp2.chc.nrc.ca/CRTreports/Pack_Ice_Management_05.pdfftp://ftp2.chc.nrc.ca/CRTreports/Pack_Ice_Management_05.pdfhttp://www.offshoremoorings.org/moorings/2008/group%20b/site/line.htmlhttp://www.offshoremoorings.org/moorings/2008/group%20b/site/line.htmlhttp://www.offshoremoorings.org/moorings/2008/group%20b/site/line.htmlhttp://www.offshoremoorings.org/moorings/2008/group%20b/site/line.htmlftp://ftp2.chc.nrc.ca/CRTreports/Pack_Ice_Management_05.pdfhttp://www.offshoremoorings.org/moorings/http://ru.wikipedia.org/wiki/%CA%E0%F0%F1%EA%EE%E5_%EC%EE%F0%E5http://www.offshoremoorings.org/moorings/2008/group%20a/systemdesign.htmlhttp://ntnu.diva-portal.org/smash/get/diva2:404324/FULLTEXT02


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29

11. http://www.viking-moorings.com/Portals/96/dokumenter/chain/chain.pdf

12 http://www.bridon.com/x/downloads/oilandgas/Diamon%20Blue.pdf


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A - 1

Appendix A

Ice Force calculations

Ice properties, units Value

Height, m 0,7 - 1,2 m. We take max 1.8 m

Salinity, p.p.t. 30

Density, kg/m3 917 (theoretical density)

Temperature, C -10 (average for the Kara sea)

Young Modulus, Nm-2

9,33109(for isotropic ice at -16)

Poisson's ratio 0.325 (for isotropic ice at -16)

Vessel parameters (average parameters of big FPSO (100 000 - 120 000 BOPD)

Parameter, units Value

Stem angle, 25

Length between perpendiculars, m 100

Draught, m 10

Beam, m 25

Hull-ice friction 0.1

Other Parameters Value

Water density, kg/m3 1025

Breaking Force

brain porosity = 0,03 49,1810 + 0,53=0,16

flexural strength depends on brine volume = 1,76 5,880,16 = 0,168 MPa


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A - 2

Young's modulus = 0(1 0.16) = 7,83 1092

=sin(25) + 0,1

cos (25)

cos(25) 0,1 sin (25) = 0,594

Breaking Force = 0,68 25 0,168 106 1025 9,81 1,8

5

7,83 109 0.25

0,594 = 119

Rotation Force

breaking length -

= 0,168 106 25 1,82

6 119 103 = 19

rotation force - assume that penetration into the ice sheet at failure a = 0

=12

(1025 917)9,81(19)225 1,8 sin (25)19

0,594 = 113,7

Sliding Force

load to the sloping part of the vessel - 1

1 = 0,1(1025 917)9,81 25 1,8 1025 0,594 = 60

2 = 0,1(1025 917)9,81 25 1,8 100 10

25 = 374


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A - 3

The sliding force to bow is negligible, we can overlook this.

Total Force

= 119 + 113,7 + 374 = 606.7


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B - 1

Appendix B

Mooring force calculations

Chain parameters [11]

Initial Data

Horizontal tension - = , kN 606,7Weight of the chain in the air, kg/m 200

Submerged weight of the chain, kg/m 192 (approximately)

Breaking load, kN 9864

Depth, m 100 (average)


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B - 2

We choose this design of mooring system:

The ice can influence the following ways:

In this case, 3 lines will be under load

and


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B - 3

In this situation, The load will be distributed to 4-6 lines (depending on the

stiffness of the system) but the main burden will fall on the next line (marked in yellow).

Let's assume, that all force is resisted 2 lines. It means, that horizontal tension of 1 line =

2

=606,7

2= 303,35

Horizontal distance from the connection point between the mooring system and

the submerged turret (A) to the touch down point (B):

= 303,35 103192

arccosh 100 192303,35 103 + 1 = 559,2

length of the mooring line (S):

= 303,35 103192

sinh 192303,35 103 559,2 =570

Vertical Tension (V):

= 303,35 103 sinh 192303,35

103

559 = 109,6

Axial tension (T)

= 109,62 + 303,352 = 322,5

If compare axial tension multiplied by safety factor (say 3) with breaking load (BL) we

see:

3 = 967,8 < = 9864 is okNevertheless, we use static model, which doesn't include dynamic loads from wind,

current and waves.

Horizontal Tension on the bottom (B):

0 = = 303,35 Anchor has to resist this tension multiplied by safety factor.


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Appendix C

Data for Steel wire and chain mooring line geometry by catenary equation

=

cosh

1

Steel wire parameters [12]:

Steel wire data to graph

X 0 100 200 300 400 500 600 700 800 900 1000 1050

y 0 0,9 3,6 8,1 14,4 22,6 32,6 44,4 58,0 73,5 90,7 100,0

Steel chain data to graph

X 0 50 100 150 200 250 300 350 400 450 500 559

y 0,0 0,8 3,2 7,1 12,7 19,8 28,6 38,9 50,9 64,5 79,8 100,0

Project (Ozorishin, A.)

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