Study Examines Causes of Upheaval Buckling in Shallow Subsea PIP Lines - Oil & Gas Journal

7/27/2019 Study Examines Causes of Upheaval Buckling in Shallow Subsea PIP Lines - Oil & Gas Journal

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Home More Transportation Study examines causes of upheaval buckling in shallow subs ea PIP lines

Study examines causes of upheavalbuckling in shallow subsea PIP lines

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8/18/13 Study examines causes of upheaval buckling in shallow subsea PIP lines - Oil & Gas Journal

www.ogj.com/articles/print/volume-106/issue-6/transportation/study-examines-causes-of-upheaval-buckling-in-shallow-subsea-pip-lines.html

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Palmer et al. derived the downward restraints to prevent upheaval buckling by substituting Equation 2 into Equation

1.3

An idealized curve for the initial deformation of the pipe provides the bas is for developing Equation 3. In practice,

initial deformation of an offshore pipeline differs somewhat from the known deformation of the seabed. Finite

element models developed for a pipeline buried along an idealized hill on the seabed attempted to use the shape of

the seabed as the initial imperfection of the pipeline, evaluating the differences i n the shapes of the pipeline and

seabed as well as their effects on buckling potential.


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The pipeline used is 4-km long with both ends fixed in the axial direction. It has a 406.4-mm OD, 7.1-mm WT, and

submerged weight of 1,778.1 Newtons/m. The Youngs m odulus of the pipe is 2.06 1011 pascals and the friction

coefficient between the pipe and surrounding s oil is 0.5. Equation 2 defines the initial imperfection of the seabed,

shown in Fig. 1 with a length of 70 m and heights of 0.2 m, 0.5 m and 0.8 m, respectively.


Fig. 2 shows the critical temperature vs. the imperfection height of the seabed. For a small height, the numerical

solution lies very close to the analytical solution. As the hill height increases, the difference between the two

solutions increases. The increasing embedm ent of the pipe around the top as the height of imperfection increases

and the difference of peak height between the pipeline and s eabed cause this. Us ing the seabed curve as the initial

imperfection of the pipeline leads to a conservative critical temperature. Such an approach, however, is still feasible

in engineering as the difference is less than 10 C.

Pipe-in-pipe

In a pipe-in-pipe system, thermal insulation m aterials fill the annulus between the inner pipe and outer pipe. The

outer pipe normally maintains ambient temperature, while the inner pipe assum es the sam e temperature as the

medium. Temperature changes in the m edium caus e axial deformations of the PIP system around the pipe ends or

around the bend where a pipeline changes direction.

In addition to relative deformation between the outer pipe and the seabed, relative deformation also occurs between

the inner and outer pipes, the compressive load of the inner pipe being partially transferred to the outer pipe in

tension. Axial compres sive forces decrease towards the pipe end or bend as axial deformations develop.


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Locations away from the end or bend where there are no relative displacem ents among the seabed, inner, and

outer pipes s how the systems highest s ystemic axial compress ive loads for a given temperature rise. These

locations are m ore sus ceptible to upheaval buckling on an uneven seabed. The outer pipe does not take any axial

thermal loads and only leads to increases in bending stiffness.

Equations 4 and 5 express the forces affecting a pipe-in-pipe system.

Numerically modeling the two sets of pipe-in-pipe systems lis ted in Table 1, including weights per unit length, tests

these equations. The weight of the inner pipe includes the weight of medium (oil) and the weight of the outer pipe is

the submerged weight (subtracting buoyancy).


Both models use a 4,000-m pipeline with both ends fixed in the axial direction. The friction coefficient between the

inner pipe and outer pipe is 0.2, and the friction coefficient between the outer pipe and s eabed is 0.5.

Fig. 3 shows the critical temperature used in the first model for the three heights in Fig. 1, preserving the

temperature difference of 10 C. or less referred to in the analytical results of Equations 4 and 5.


The second model considers a 100-m imperfection caused by the high bending s tiffness of the pipes, using the

same imperfection heights us ed in the first model. Fig. 4 s hows the critical temperatures from both num erical and

analytical approaches. As s hown in Figs. 3 and 4, the critical temperature increased due to the high bending

stiffness of the PIP system in comparis on with the single pipe, and the analytical solutions match reasonably well

with the numerical solutions.

Limitations

Equation 4 requires an initial deformation of the pipeline in a PIP system comparable to the initial im perfection of the

seabed. A short imperfection combined with bending stiffness, however, causes s uspension of the pipeline around

its toes to deviate from the s hape of the seabed.

A given lifting height off a flat seabed requires the pipeline be s uspended under its s ubmerged weight. Equation 6

estimates the sus pended pipe length corresponding to height, .


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Numerical models developed for an imperfection height of 0.8 m verified the minimum length of the initial

imperfection applicable for Equation 4. Both the numerical model and analytical help calculate the critical

temperature under the submerged weight of pipes. Figs. 5a and b s how the results for the first and second m odels,

respectively.


The minimum length from Equation 6 can act as a cut-off point for Equation 4, below which the shape of pipe is

dominated by its bending s tiffness and the critical temperatures are m ore or less the same. Equation 7 calculates

the critical compres sive load leading to upheaval buckling under a given initial im perfection in these circumstances.

Equations 6 and 7 apply to partially or fully buried offshore pipelines. They also apply to a s ingle pipe in which only

the bending s tiffness of the pipe is considered. Pipelines l aid on seabed, however, are likely to experience lateral

buckling, making these equations not applicable.

Intermediate spools

Expansion loops (i.e., intermediate spools) m ay reduce the axial thermal s tresses in the inner pipe of a PIP system

with high design temperatures. Expansion loops place the outer casing pipe s egments between the loops under

tension during operation. These tension forces tend to increase the critical temperature and reduce the bucklingpotential of the PIP s ystem. Equation 8 provides the maximum compressive load of a PIP system wi th intermediate

spools.

A bulkhead typically instal led in the straight segment near the spool connects the inner and outer pipes . Equation 9

provides the bulkheads equilibrium.

Soil restraints in the axial direction also control the outer pipes end dis placement at the spool (Equation 10).

The effective length, LE, in Equation 10 represents no more than the half spacing of intermediate spools (Equation

11).

A PIP system des igned for high temperatures functions best when the outer pipe is put in tension (i.e., P2 > 0) by

choosing a relatively short interval for intermediate spools (Le = 0.5Ls).

Pipeline engineering casts the axial resis tance from the expansion loop, Rspool, as negligible, simpl ifying Equation


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8 as Equation 12.


Equation 12 calculates the m aximum compressive force, Pmax, for a s elected spacing, Ls, in the PIP system. A

maximum force larger than the critical load, Pcr, from Equation 7, requires reduction of the spacing between the

spools . Fig. 6 shows the maximum compress ive force in the PIP system from the sim plified and numerical

approaches. Neglecting the resistance from the spool makes the maximum compress ive force from the sim plified

approach slightly less than from the num erical.

Equations 8-12 also apply to a PIP system with a dogleg; spacing Ls acting as the dis tance between the dogleg and

adjacent expansion loop or another dogleg in a s hort PIP system without an intermediate spool.

Design e xample

Engineering offshore pipelines without intermediate spools uses a given temperature of the medium and a seabed

profile derived from a site survey. Equation 5 can calculate the maximum compressive force, while Equation 6

calculates the minimum length for the pipeline in conformance with the hill-type seabed. Equation 7 calculates the

downward restraints required for the system to avoid upheaval buckling.

Partially exposed pipelines on the seabed use the submerged weight of the pipe and medium as the downward

restraint. Buried pipelines us e the submerged weight of the pipe and medium as well as the soil over the pipe.


Model 1 in Table 1 represents pipelines crossing a 70-m imperfection on the seabed with 0.5 m and 1.0 m s oil

covers, respectively. Equation 7 calculates the critical temperatures, shown in Fig. 7 for three heights of imperfection

along with results from the finite-element approach. The results show the PIP system can transport oil at a

temperature up to 120 C., a typical burial depth of 1.0 m, and a corresponding slope of imperfection is 0.011 (0.38-

m high and 35-m long).

If the sl ope of the imperfection is more than 0.011, burying the pipeline deep into the seabed around the hill top

(overbend of the pipeline) provides the most effective means of preventing bucking. Doing so not only reduces the

height of the initial im perfection, but also increases the downward restraints.

Concrete mats, sand bags, or cobbles placed on top of the pipeline at the hilltop can also act as restraining means.

A very hard seabed where hydraulic jetting is not poss ible m ight use pre-plough or trenching equipments to reduce

the imperfection.

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Equation 7 verifies the effectiveness of the chosen m eans.

References

1. Hobbs , R.E., In-Service Buckling of Heated Pipelines, ASCE Journal of Transportation Engineering, Vol.

110, pp. 175-189, 1984.

2. Taylor, N., and Gan, A., Submarine Pipeli ne Buckling Imperfection Studies, Thin-Walled Structures, Vol.4,

pp. 295-323, 1986.

3. Palmer, A.C., Ellinas , C.P., Richards, D.M., and Guijt, J., Design of Submarine Pipelines Against Upheaval

Buckling, OTC 6335, Offshore Technology Conference, Hous ton, May 7-10, 1990.

4. Raoof, M., and Maschner, E., Thermal Buckl ing of Subsea Pipeli nes, OMAE-ASME Pipeline Technology, Vol.

5, 1993.

5. Sriskandarajah, T., Anurudran, G., Ragupathy, P., and Wilkins , R., Design Considerations in the Use of Pipe-

In-Pipe Systems for HP/HT Subsea Pipelines, International Society of Offshore and Polar Engineers

conference, Bres t, France, May 30-June 4, 1999

The authors

Jack X. Liu ([email protected]) is a visiting professor at China University of Petroleum, Beijing,

and president of Liu Advanced Engineering LLC, Houston. He has als o served as a principal

engineer at Technip USA. He holds a PhD (1996) from Rensselaer Polytechnic Institute, Troy, NY,

and a BS (1987) from Huazhong University of Science and Technology, China. He is a registered

professional engineer in Texas.

H.M. Zhang ([email protected]) is a chief engineer in the sales department at Shengli oil field.

He has als o severed as a m anager of the transporting oil company of Shengli. He holds a BS from

HuaDong Petroleum Institute (1985) and an MS from China University of Petroleum (2003).

Q.R. Meng ([email protected]) is an engineer at Rong-Sheng Machinery Manufacture Ltd. of

Huabei oil field, RenQiu. She holds an MS (2006) from China University of Petroleum (Beijing).

Hong Zhang ([email protected]) is a professor at China University of Petroleum,

Beijing. He holds a PhD (2003) from China University of Petroleum, Beijing. He is a

member of the American Society of Mechanical Engineers.

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Study Examines Causes of Upheaval Buckling in Shallow Subsea PIP Lines - Oil & Gas Journal