Analysis on the Effect of Subsea Buoy to the Tension of Spread

Mooring System

Murdjito, Madea Eka Silfiani and Eko Budi Djatmiko

Department of Ocean Engineering, Institut Teknologi Sepuluh Nopember, Surabaya, Indonesia

Keywords: FSO, Spread Mooring, Subsea Buoy, Tension.

Abstract: The problem that might occur in floating structures with mooring systems is clashing between mooring lines

with subsea equipment, for example pipelines. Addition of subsea buoys on the mooring line can lift the

mooring line so that it can avoid clashing. The addition of the subsea buoy can affect tension on the mooring

line. This research discusses the effects of subsea buoy to the tension of mooring line with a variation position

of subsea buoy. Variations on the position of one subsea buoy is arranged at the distance of 605 m, 577.5 m,

550 m, 522.5 m from anchor and two subsea buoys at the distance 605 m and 467.5 m from the anchor. The

analysis was carried out on stand alone and offloading conditions with wave directions 0°, 45°, 90°, 135°,

180°. The results after the addition of subsea buoys it has a smaller tension on the mooring line. The result of

the variation of subsea buoy position, the optimum position to get the smallest tension value is when the

variation of two subsea buoys with a distance of 605 m and 467.5 m from the anchor. From the results of the

analysis there is also no clashing between the mooring line and pipeline.

1 INTRODUCTION

Natuna is an area at the northern end of the Karimata

Strait. This area is one of the largest oil and gas

reserves in the world. Natuna is an area where there

are many offshore structure for oil and gas

exploration, either fix structure or floating structure.

At present, the development of offshore structure

design technology is continuing to explore oil and gas

in the Natuna area. One of them is the construction of

Floating Storage and Offloading (FSO).

Floating Storage Offloading (FSO) is a floating

structure in the form of a ship which serves to store

hydrocarbons and transfer to vessels or barges. In its

operation the FSO structure is movement caused by

environmental loads, such as waves, wind and

currents. So that the mooring system is needed on the

FSO structure. The purpose of this mooring system is

to limit movement and keep the FSO in place.

One type of mooring system that is usually used

is spread mooring. The mooring system consists of

several mooring lines that spread and are moored to

the seabed using anchors. This system does not allow

the ship to move or spin to reach a position where

environmental effects such as wind, current, and

waves are relatively small.

The construction of mooring systems there are

many factors that must be considered, one of the

distance between the mooring line and the mooring

line or with other subsea equipment. Clashing

between the mooring line and the pipe is one of the

problems that can be found. The addition of the

subsea buoy on the mooring line can avoid clashing

between the mooring line and the pipe, because the

subsea buoy can lift the mooring line so that clashing

does not occur. Addition of the subsea buoy can affect

the tension on the mooring line. This research

discusses the effect of adding subsea buoys to the

tension of mooring line with a variation of one subsea

buoy with four variations of position and two subsea

buoys.

2 LITERATURE REVIEW

Many studies and research analyze variations in

mooring system designs, such as subsea buoy

analysis on mooring systems. Examples of numerical

analysis on hybrid mooring systems with clump

weights and buoys by Yuan Z.M. et al. (2014) which

analyzed the type of new mooring line, hybrid

mooring system with clump weigth and buoys

(HMSWB). In this study Yuan Z.M only analyzed the

236

Murdjito, ., Silﬁani, M. and Djatmiko, E.

Analysis on the Effect of Subsea Buoy to the Tension of Spread Mooring System.

DOI: 10.5220/0010141902360244

In Proceedings of the 7th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management (ISOCEEN 2019), pages 236-244

ISBN: 978-989-758-516-6

effect of buoys on (HMSWB) because previously the

influence of the clump weight was discussed by Ji

C.Y. et al. (2011). This study concluded that installed

buoys can reduce tension on the mooring line.

Sundaravadivelu (1991) has a study that the

increase in submerged buoy net bouyancy can reduce

the excurce of the buoy. The buoys used in the study

were single point subsurface mooring. Fitria, Favi

Ainin (2018) conducted a research on adding clump

buoys to mooring systems which aimed to see the

effect of adding clump buoys to mooring line tension

and avoiding the potential for clashing between

mooring lines. The results of the study that the

addition of a clump buoy on the mooring line can

reduce tension and also clashing between the mooring

lines. Mavrakos (1997) analyzed the effect of adding

submerged buoys in the deep sea and has variations

in the number, size, and position of the submerged

buoy. The next analysis is the effect of adding

submerged buoy to the tension and dry length on a

single point mooring mooring system (Suseprasetyo,

2013). This analysis get the results of the submerged

buoy displacement in proportion to the amount of dry

length and the farther position of the submerged buoy

from fairlead has smaller tension.

3 OBJECTIVES AND SCOPE OF

STUDY

The objective of this research is to comprehend how

the subsea buoy in the mooring system affects the

tension of mooring line. The responses which to be

analysed are tension of mooring line, maximum

offset, and clashing between mooring line and

pipeline. The scope of study and boundaries of this

research are as follows.

- The mooring system used is spread mooring.

- FSO is analyzed in full load and ballast condition.

- Environmental data uses data in Natuna.

- Collinear environmental loading conditions.

- Variation of one subsea buoy at a distance of 605

m, 577.5 m, 550 m, 522.5 m from the anchor.

- Variation of two subsea buoys at a distance of 605

m and 467.5 m from the anchor.

- The size of the subsea buoy is fixed.

- Dynamic analysis using time domain method.

4 METHODOLOGY

The flow and procedure of this research was

conducted in stages as follows.

- Structural data uses Belida FSO data which is a

conversion from tankers, shuttle tanker data, &

environmental data using data in Natuna.

- Modelling FSO & shuttle tanker

- Model validation is to ensure the modeling is

accordance with the original structure. Validation

by comparing the hydrostatic data from the

software with the original hydrostatic data. Model

validation using reference from ABS (2018).

- Mooring system modeling, which uses a spread

mooring type with eight mooring lines and 45 ° &

60° angle configurations.

- The mooring line analysis conducted in this

study is the analysis of tension, offset, and

clereance between the mooring line and pipeline.

- Analyzes were performed without subsea buoys

and with subsea buoys.

5 RESULTS AND DISCUSSION

5.1 Structur Modeling

FSO modeling uses software by entering FSO

coordinates.

Table 1: FSO Data.

Parameter Unit

Value

Full load Ballast

LOA m 244.60 244.60

LPP m 233.00 233.00

Breadth m 42.20 42.20

Depth m 22.20 22.20

Dra

m 15.50 7.00

KG m 13.71 10.08

Displacemen

Ton 12588.60 58833.87

Table 2: Shuttle tanker Data.

Paramete

Uni

Value

LOA m 240.50

LPP m 230.00

Breadth m 42.00

Depth m 21.20

Dra

m 14.85

KG m 12.48

Displacemen

Ton 118643.87

Analysis on the Effect of Subsea Buoy to the Tension of Spread Mooring System

237

Table 3: Mooring Line Data.

Paramete

Uni

Value

pe -

Chain, R4

Studless

Len

th o

chain m 914

Size m

87 mm dia

MBL mT 783.35

Table 4: Mooring Hawser Data.

Paramete

Uni

Value

pe - Rope/N

lon

Size m

96 dia

MBL mT 154.076

Table 5: Subsea Buoy Data.

Paramete

Uni

Value

Wei

5600

Tin

i m 4.6

Diamete

m 2.8

Table 6: Environment Data.

Direction NE E SE S SW

Wind Speed

(m/s)

18 11 10 13 13

Wave Data

Hs 4.4 2.0 1.8 2.0 2.0

Tp 9.9 8.6 8.5 8.6 8.6

Current Speed (m/s)

Surface 0.89 0.80 0.62 0.62 0.76

30 m 0.69 0.62 0.48 0.48 0.67

3 m 0.45 0.41 0.35 0.35 0.43

This is modelling of FSO & shuttle tanker.

Figure 1: Modeling FSO with Maxsurf.

Figure 2: Side view FSO with Moses.

Figure 3: Front view FSO with Moses.

Figure 4: Modeling shuttle tanker with Maxsurf.

Figure 5: Side view shuttle tanker with Moses.

Figure 6: Front view shuttle tanker with Moses.

Mooring systems were modeling by software and

modeled in 6 variations in two conditions, stand alone

and offloading. The Variations were mooring line

without subsea buoy, mooring line with one of subsea

buoy at a distance from the anchor 605 m, 557.5 m,

550 m, 522.5 m, and mooring line with two subsea

buoy at a distance from the anchor 605 m and 476.5m.

Figure 7: Stand alone conditions.

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238

Figure 8: Offloading conditions.

Figure 9: Mooring Line with one Subsea Buoy.

Figure 10: Mooring Line with two Subsea Buoy.

5.2 Model Validation

The model was validated based on the ABS

(American Bureau of Shipping) MODU (Mobile

Offshore Drilling Unit) rules, the difference of

displacement modeling not exceed 2%.

Table 7: FSO Validation.

Loa

Data MOSES Validasi

ull Load 128588.6 128561 0.02%

Ballas

58796.11 57589.1 2.00%

Table 8: Shuttle Tanker Validation.

Loa

Data MOSES Validasi

Full Load 118644 118787 0.12%

5.3 Responses Amplitude Operator

Analysis

RAO (Response Amplitude Operator) analysis is

performed motion characteristics of FSO and shuttle

tanker. This analysis carried out when free floating

and moored condition in 6 degrees of freedom,

namely surge, sway, heave, roll, pitch and yaw. The

following is RAO of FSO and shuttle tanker during

full load and ballast conditions.

1. RAOs Free Floating Condition

Table 9: Max. RAO FSO Full Load Condition.

Motio

Unit

RAO Max.

0° 45° 90° 135° 180°

Surge m/m 0.97 0.69 0.00 0.69 0.97

Sway m/m 0.00 0.70 0.99 0.70 0.00

Heave m/m 1.00 1.00 1.45 1.00 1.00

Roll deg/m 0.01 1.60 2.21 1.59 0.01

Pitch deg/m 0.79 0.97 0.37 0.93 0.79

Yaw de

0.00 0.30 0.03 0.32 0.00

Table 10: Max. RAO FSO Ballast Condition.

Motion Unit

RAO Max.

0° 45° 90° 135°

180

Surge m/m 0.98 0.69 0.00 0.69 0.98

Sway m/m 0.00 0.70 0.99 0.70 0.00

Heave m/m 1.00 1.00 1.09 1.00 1.00

Roll deg/m 0.00 2.20 4.65 2.31 0.00

Pitch deg/m 0.73 0.78 0.12 0.77 0.73

Yaw de

0.00 0.32 0.04 0.33 0.00

Table 11: Max. RAO shuttle tanker Full Load Condition.

Motion Unit

RAO Max.

0° 45° 90° 135° 180°

Surge m/m 0.97 0.68 0.00 0.68 0.97

Sway m/m 0.00 0.70 0.99 0.70 0.00

Heave m/m 1.00 1.00 0.45 1.00 1.00

Roll deg/m 0.01 1.94 2.69 1.92 0.01

Pitch deg/m 0.85 1.01 0.36 0.88 0.88

Yaw de

0.00 0.31 0.05 0.32 0.00

5.4 Mooring Line Tension Analysis

Analysis of the mooring line tension was carried out

without subsea buoys and with subsea buoys in two

Analysis on the Effect of Subsea Buoy to the Tension of Spread Mooring System

239

conditions, namely stand alone conditions and

offloading conditions. Mooring line 6 has the largest

tension value compared to the mooring line 3, 4, 5

which has been added to the subsea buoy. So that a

comparative analysis of the position of the subsea

buoy is carried out on the mooring line 6.

1. Stand Alone Condition

For analysis of tension on the mooring line carried out

with conditions without subsea buoy, one subsea

buoy with four variations of position and two subsea

buoys in five of wave directions, namely, 0 °, 45 °, 90

°, 135 °, 180 °.

a. Wave Direction 0°

Figure 11: Max. Tension 0°.

b. Wave Direction 45°

Figure 12: Max. Tension 45°.

c. Wave Direction 90°

Figure 13: Max. Tension 90°.

d. Wave Direction 135°

Figure 14: Max. Tension 135°.

e. Wave Direction 180°

Figure 15: Max. Tension 180°.

From Figure 11 until Figure 15 indicates that the

largest mooring line tension on the mooring line

without subsea buoys. The mooring line with the

subsea buoy from the smallest to the largest is the

mooring line with two subsea buoys, the mooring line

with one subsea buoy with a distance of 605 m, 577.5

m, 550 m, 522.5 m from the anchor.

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2. Offloading Condition

For analysis of tension on the mooring line carried out

with conditions without subsea buoy, one subsea

buoy with four variations of position and two subsea

buoys in five of wave directions, namely, 0 °, 45 °, 90

°, 135 °, 180 °.

a. Wave Direction 0°

Figure 16: Max. Tension 0°.

b. Wave Direction 45°

Figure 17: Max. Tension 45°.

c. Wave Direction 90°

Figure 18: Max. Tension 90°.

d. Wave Direction 135°

Figure 19: Max. Tension 135°.

e. Wave Direction 180°

Figure 20: Max. Tension 180°.

From Figure 16 until Figure 20 indicates that the

largest mooring line tension on the mooring line

without subsea buoys. The mooring line with the

subsea buoy from the smallest to the largest is the

mooring line with two subsea buoys, the mooring line

with one subsea buoy with a distance of 605 m, 577.5

m, 550 m, 522.5 m from the anchor.

5.5 Offset Analysis

Stand alone and offloading condition are used in the

analysis.

1. Stand Alone Condition.

This analysis was carried out with 5 load directions

include 0

, 45

, 90

, 135

and 180

Analysis on the Effect of Subsea Buoy to the Tension of Spread Mooring System

241

Table 12: Max. Offset Stand Alone Condition.

Wave

Directions

Offset

x & y

Maximum Offset (m)

Tanpa

Buo

605

577.5

0°

x 0.80 1.67 1.05

y 0.00 0.00 0.00

45°

x 0.34 2.28 1.67

y 2.84 3.49 3.49

90°

x 0.41 2.34 1.72

y 2.69 2.90 2.82

135°

x 0.31 2.06 1.47

y 1.49 1.57 1.30

180°

x 1.43 2.89 2.24

y 0.14 0.08 0.08

Table 13: Max. Offset Stand Alone Condition.

Wave

Directions

Offset

x & y

Maximum Offset (m)

Tanpa

Buo

605 m 577.5 m

0°

x 0.50 0.47 2.53

y 0.00 0.00 0.00

45°

x 1.02 0.57 3.54

y 3.46 3.44 3.68

90°

x 1.09 0.46 3.53

y 2.70 2.62 3.45

135°

x 0.86 0.50 3.08

y 1.28 1.28 2.13

180°

x 1.58 0.90 4.54

y 0.08 0.08 0.08

2. Offloading Condition.

This analysis was carried out with 5 load directions

include 0

, 45

, 90

, 135

and 180

Table 14: Max. Offset Offloading condition.

Wave

Directions

Offset

x & y

Maximum Offset (m)

Tanpa

Buo

605 m 577.5 m

0°

x 0.81 1.67 1.56

y 0.00 0.00 0.00

45°

x 1.75 3.90 3.19

y 5.19 5.92 5.62

90°

x 2.41 4.78 3.92

y 6.72 7.75 7.28

135°

x 3.10 5.94 4.90

y 7.97 8.91 8.77

180°

x 1.44 3.55 2.67

y 0.14 0.00 0.00

Table 15: Max. Offset Offloading condition.

Wave

Directions

Offset

x & y

Maximum Offset (m)

Tanpa

Buo

605 m 577.5 m

0°

x 1.37 0.66 1.66

y 0.00 0.00 0.30

45°

x 2.42 1.61 5.18

y 5.34 5.15 6.40

90°

x 3.14 2.34 6.38

y 6.84 6.52 8.40

135°

x 4.02 3.19 7.87

y 8.26 7.85 9.11

180°

x 1.84 1.13 4.89

y 0.00 0.00 0.00

5.6 Clearance between Mooring Line

and Pipeline

Stand alone and offloading condition are used in the

analysis.

1. Stand Alone Condition.

Table 16: Clearance between Mooring Line and Pipeline.

Line

Clearance (m)

No Buo

605

577.5

3 0.00 6.51 10.10

4 0.19 17.42 14.41

5 0.19 17.42 14.41

6 0.00 6.51 10.10

Table 17: Clearance between Mooring Line and Pipeline.

Line

Clearance (m)

550 m 522.5 m

Double

Buo

3 12.45 4.77 25.73

4 10.84 3.20 29.90

5 10.84 3.20 29.90

6 12.45 4.77 25.73

From Table 16 and 17 shows that there is no

clashing between mooring line and pipeline after the

addition of the subsea buoy, but the clearance that

matches the criteria of DNV OS E301 which is in the

variation of one subsea buoy with a distance of 577.5

m from the anchor, one subsea buoy with a distance

of 577.5 m from anchor, and on the condition of two

subsea buoys. The biggest clearance occurred in the

condition of two subsea buoys, namely on lines 3 and

6 valued at 25.73 m and on lines 4 and 5 worth 29.9m.

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2. Offloading Condition.

Table 18: Clearance between Mooring Line and Pipeline.

Line

Clearance (m)

No Buo

605

577.5

3 0.00 6.13 10.04

4 0.19 15.92 12.74

5 0.19 15.92 12.74

6 0.00 6.13 10.04

Table 19: Clearance between Mooring Line and Pipeline.

Line

Clearance (m)

550 m 522.5 m

Double

Buo

3 10.58 5.22 24.88

4 10.11 4.39 27.55

5 10.11 4.39 27.55

6 10.58 5.22 24.88

From Table 18 and 19 shows that there is no

clashing between the mooring line and the pipe after

the addition of the subsea buoy, but the clearance that

matches the criteria of DNV OS E301 which is in the

variation of one subsea buoy with a distance of 577.5

m from the anchor, one subsea buoy at 577.5 m from

anchor, and on the condition of two subsea buoys.

The biggest clearance occurred in the condition of

two subsea buoys, namely on lines 3 and 6 valued at

24.88 m and on lines 4 and 5 worth 27.55 m.

6 CONCLUSIONS

The findings of the study could be revealed as

follows:

 Mooring line without subsea buoys at stand alone

conditions and in all loading directions has the

maximum tension. The largest tension from the

direction of 45° on line seven with a value of

1503.09 kN. The offloading condition, the

mooring line without the addition of subsea buoys

in all loading directions also has the maximum

tension value. In the direction of 135° on line

seven has the largest tension with a value of

1743.05 kN. For all tension it matches the criteria

of API RP 2SK, which is a safety factor less than

1.67.

 From the research it is known that the addition of

the subsea buoy reduces the tension on the

mooring line. On the mooring line with one

subsea buoy the further distance from the anchor

has smaller tension. At the mooring line with two

subsea buoys with a distance of 605 m and 467.5

m has the minimum value. This condition occurs

in all of wave directions when stand alone and

offloading.

 From the research it is known that the addition of

the subsea buoy can affect the offset of the FSO.

In stand alone and offloading conditions, the

highest offset is on the mooring line with two

subsea buoys and a mooring line with one subsea

buoy with a distance of 605 m from the anchor.

The value of all offsets that occur is in accordance

with the API RP 2P criteria.

 From the research it is known that the addition of

subsea buoys can avoid clashing between

mooring line and pipeline. However, for the

clearance between mooring line and pipeline that

matches the criteria of DNV OS E301 which is in

the variation of one subsea buoy with a distance

of 577.5 m from the anchor, one subsea buoy with

a distance of 550 m from the anchor, and in the

conditions of two subsea buoys. The biggest

clearance occurred on the mooring line with two

subsea buoys.

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