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Application of Pipeline Flexible Design in FPSO

In the design of ship pipeline, in order to prevent the influence of thermal expansion and cold contraction of the pipeline on the pipeline, the pipeline flexibility must be considered when designing the long pipeline; Expansion bend and expansion joint are two common design schemes for increasing pipeline flexibility. The influence of blind flanges force on pipeline layout shall be considered when selecting expansion joints.

In the design of ship pipeline, first of all, it is necessary to consider meeting the process requirements of pipeline, and also consider the stress conditions of equipment, pipeline and its components to ensure safe operation; The pipe stress is affected by the climate or the cold and hot changes of the medium on the pipe, and its change is mainly related to the flexibility of the pipe design; The purpose of pipeline flexibility design is to make the entire pipeline system flexible enough to prevent the following situations from occurring due to the temperature, dead weight, internal pressure and external load of the pipeline system or due to the restriction of the pipe support and the additional displacement of the pipe end: 1. The pipeline is damaged due to excessive stress or metal fatigue. 2. Leakage at pipe joints. 3. The thrust or torque of the pipeline is too large, which causes excessive stress or deformation of the equipment connected to the pipeline, affecting the normal operation of the equipment. 4. The pipe support is damaged due to excessive thrust or torque of the pipe.
In the XJ23-1 FPSO project undertaken by our factory, the flexible design of the pipeline has been fully considered; FPSO is a 100000 ton offshore floating production and storage unit built by our company for CNOOC. The length is 232.5 meters, the molded width is 46 meters, the molded depth is 24.1 meters, and the design draft is 16 meters; Due to the large size of the ship hull, in order to overcome the destructive impact of the sagging arch on the ship pipeline caused by the fluctuation of the ship in the sea with the waves, as well as the thermal expansion and cold contraction of the pipeline, it is necessary to consider selecting appropriate measures to increase the flexibility of the pipeline during the pipeline design. The flexible design of FPSO pipeline is mainly reflected in the main deck and the pipeline in the cargo tank and ballast tank. Due to the small size of the engine and pump cabin, the flexible design concept is naturally introduced into the pipeline.
Flexible design is usually divided into natural compensation and compensator; Natural compensation mainly includes L-type, Z-type, spatial type and rectangular compensation, as shown in the following figure:
20221021113408 72416 - Application of Pipeline Flexible Design in FPSO

Natural compensator

The compensator mainly includes various types of expansion joints: casing type compensator, wave type compensator, self commissioning casing type compensator, and filler culvert type compensator.
The above four types of natural compensation and filler culvert type compensators are mainly selected for the pipeline layout of FPSO. The design and selection of compensators are based on the characteristics and properties of various compensators, and different compensators are selected at different locations. In the pipeline design of engine and pump room, due to the characteristics of narrow space and dense pipelines, L-type and Z-type natural compensators are mainly used in the pipeline design layout; In the cargo oil tank and ballast tank, due to the complex structure of the bottom section, various longitudinal and transverse bones are staggered. In order to facilitate the installation of the pipeline and the pre opening of the structure, all pipelines requiring flexible design in the cargo tank area use filler culvert expansion joints; In the main deck pipeline design, in order to reduce the influence of blind plate force on the pipe support, the flexible design of all the main pipelines adopts rectangular compensator, except that the cargo oil transmission pipeline adopts filler culvert expansion joints; Because the expansion elbow is a rigid connection, all pipelines with expansion bends do not need to consider the blind plate force, and the stress it bears is borne by the pipeline elbow. For pipelines with expansion joints, because the pipelines at both ends of the expansion joints are disconnected, the pipeline stress cannot be overcome only by the friction force of the expansion joints, so at this time, a strong support is required at the other end of the pipe to bear the pipeline stress and blind plate force. The size of the blind plate force is:

F=PS/10000

In the formula:

  • F — Pipe blind plate force ton;
  • P — test pressure of pipeline system MPa;
  • S — sectional area of pipeline m2.

According to the above formula, the force of blind flange of different specifications can be calculated.

Serial No

Drift diameter

External diameter

Wall thickness

Test pressure/blind plate force ton

Test pressure/blind plate force ton

Test pressure/blind plate force ton

Test pressure/blind plate force ton

1

700

711

12.7

3

110.75

2.4

88.6

2

73.8

1

36.92

2

600

610

12.7

3

80.525

2.4

64.4

2

53.7

1

26.84

3

500

530

12.7

3

59.994

2.4

48

2

40

1

20

4

400

426

12.7

3

37.812

2.4

30.2

2

25.2

1

12.6

5

350

377

12.7

3

29.128

2.4

23.3

2

19.4

1

9.709

6

300

325

12.7

3

21.149

2.4

16.9

2

14.1

1

7.05

7

250

273

12.7

3

14.445

2.4

11.6

2

9.63

1

4.815

8

200

219

12.7

3

8.8312

2.4

7.06

2

5.89

1

2.944

9

150

168

9.5

3

5.231

2.4

4.18

2

3.49

1

1.744

10

100

114

8.6

3

2.2078

2.4

1.77

2

1.47

1

0.736

It can be seen from the above table that if expansion joints are used for pipelines with high pressure and large diameter, the blind plate force at the elbow of the pipeline is very large. To overcome such a large blind plate force, the fixed pipe support needs to be very strong.
The flexibility of the pipeline is not only related to the pipeline layout, but also closely related to the setting of the fixed pipe support. The position of pipe supports and hangers has a great influence on the force of the determined pipeline. The layout of pipelines and supports and hangers have influence on the stress distribution state, maximum stress value, end force and moment of the pipeline system. Whether the design of the pipeline is reasonable or not, and the proper setting of the support can improve the relationship stress distribution and the stress and torque conditions at the end points; In FPSO, the layout of the main pipeline and the setting of the pipe support shall be calculated with CAESARII pipeline stress calculation software; From the stress calculation process, the flexible design of the pipeline is closely related to the setting position of the fixed pipe support. In the same pipeline layout, the distribution and change of the pipeline stress state can be seen from the calculation results only by changing part of the pipeline direction; As shown in the figure:
20221021114354 50232 - Application of Pipeline Flexible Design in FPSO

In the original design, the secondary stress of 3090 pipe does not pass through, and the position is shown in the red part wind above:
The maximum point stress level of primary stress and secondary stress is:

Combination of working conditions

Node No

Calculated stress (kPa)

Allowable stress (kPa)

Calculated stress/allowable stress %

1 (SUS)

W+P1

540

13650

136000

10.0

2 (EXP)

L2-L1

3090

631973

337917

187

After adjusting the pipeline layout, the pipeline is as shown in the figure below:

20221021114657 64167 - Application of Pipeline Flexible Design in FPSO

After modification, the stress at 3097 point of tee passes through, and its stress level is as follows:

Combination of working conditions

Node No

Calculated stress(kPa)

Allowable stress(kPa)

Calculated stress/allowable stress %

1 (SUS)

W+P1

3097

4039

136000

3.0

2 (EXP)

L2-L1

3097

61785

335960

18.4

From the above examples, it can be seen that whether the flexible design of the pipeline is reasonable has an impact on the stress distribution of the pipeline, and the setting of the pipeline fixed support has a great impact on the pipeline. From the calculation process of the main cargo oil pipeline in FPSO, the impact of the fixed support on the stress is very obvious; The main oil pipe system of Party A is shown in the figure below.

20221021114952 57105 - Application of Pipeline Flexible Design in FPSO

The pipe system is arranged with filler culvert expansion joints. In order to ensure that the expansion and contraction of the pipeline do not produce displacement and stress accumulation, fixed supports are set between any two expansion joints of the pipeline. If there is a straight pipe section between the two expansion joints without any other accessories, it is most reasonable to set the fixed support between the two expansion joints; However, as there are tees and elbows in the pipeline, if the location of the fixed point is not properly selected, the stress of the tee may be too large, or the stress of the elbow may be too large due to the existence of blind plate force; In the stress analysis and calculation of FPSO cargo oil transmission pipeline, since the basic trend of the pipeline has been determined and cannot be changed any more, the pipeline stress can only be released by adjusting the position of the fixed pipe support, so that the stress of each part of the pipeline can reach the optimal state and meet the requirements of pipeline stress analysis. According to the stress calculation and analysis of the pipeline by CAESARII, we set corresponding fixed supports at the position close to the tee, the best position between the two expansion joints, and the elbow to effectively and reasonably distribute the stress of the entire pipeline. In order to reduce the stress effect of blind handling force on the elbow, we have set fixed supports at both ends of the elbow in the figure, as shown in the following figure:

20221021115440 63714 - Application of Pipeline Flexible Design in FPSO

As for the pipeline with expansion joint, the influence of blind plate force on the pipeline and the strength requirements for the pipe support shall be considered, so on the FPSO, except for the cargo oil pipe with too large pipe diameter, which uses expansion joints, the other pipelines use rectangular natural compensation expansion bends. Since the main deck pipeline is about 160m long, the thermal expansion and contraction of the pipeline have a great impact on the pipeline, and the thermal elongation of the pipeline is calculated according to the following formula:

△L =La(t2-t1) 

In the formula:

  • △ L – thermal elongation of pipeline cm;
  • L — total length of pipeline m
  • A – linear expansion coefficient of pipeline cm/(m · ℃)
  • T2 — temperature of medium in pipe ℃;
  • T1 – pipeline installation temperature ℃.

According to the expansion amount of the pipeline, we have set 3 to 4 expansion bends on each pipeline of the main A, as shown in the figure below. The main A of the whole ship has set up 90 rectangular expansion bends. The rectangular expansion bends effectively overcome the damage to the pipeline caused by the changes in pipeline stress due to sagging arch, thermal expansion and cold contraction, and extend the service life of the pipeline; The number of strong fixed supports is reduced.
20221021120559 34529 - Application of Pipeline Flexible Design in FPSO

From the analysis and calculation process of the main pipe support, the layout of flexible pipes should follow the following principles:

  • 1. The pipeline design of the pump room itself is flexible, but attention shall be paid to the seawater main pipe and ballast water main pipe, and the natural compensation bend shall be considered in the design;
  • 2. Flexible design shall be considered for straight pipelines with a length of more than 30m: natural compensation pipeline shall be used as far as possible for spacious main deck; For the bottom of the cargo tank area with complex structure, it is better to use expansion joints, and the fixed points of the pipeline are set on the watertight transverse bulkheads;
  • 3. The expansion bend shall be arranged orderly as far as possible in pipe rows, and the pipeline with large expansion shall be arranged outside the expansion bend;
  • 4. For the pipeline with expansion joint, try to avoid pipeline bending, so as to reduce the number of blind plate forces acting on the pipe support;
  • 5. Whether expansion joints or expansion bends are set, fixed supports shall be added to two expansion joints or expansion bends; However, a strong support shall be added to the fixed support of expansion joint with bent pipeline.

To sum up, the flexible design concept is very important in the design of ship pipeline. Whether flexible design can be properly applied and flexible design measures can be reasonably selected can not only reduce the stress distribution of the pipeline, but also extend the service life of the pipeline; Moreover, it can effectively reduce the construction cost.

Source: China Piping Solutions Provider – Yaang Pipe Industry Co., Limited (www.epowermetals.com)

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