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Evaluation Study of Free Spanning Subjected to

Hydrodynamic Loads.

Mohammed Jabbar Mawat

Engineering College, University of Basra.

mohammed.mawat@gmail.com

Ahmed Sagban Khudier, Sattar J. Hashim,

Engineering College, University of Basra. Engineering College, University of Basra.

Ah_civil1978@yahoo.com sattarhas@gmail.com

Abstract:

Suspended spans generally occur in subsea pipelines as a result of the irregularity of

seabed. Additionally the suspended spans mostly result from the scouring phenomena

around the installed non-buried pipeline. So as to discuss the hydrodynamic

surrounding the pipeline and determining the significant deflections and associated

stresses of the subsea pipeline in unsupported part, therefore, it’s very necessary to

study the hydrodynamic surrounding the pipeline in detail. A two main aims have

been done in this study, first assess the stresses at free span section and the second one

was the effect of soil characteristics in contact area between pipeline and the seabed

soil. A combined model of stresses/lateral displacement has been made. An ANSIS

model has been built on the offshore pipelines as a consequence of the combined

hydrodynamic loads such as wave/current effects. The calculations have been

computed by using the finite element method for the free span to describe the

surrounding environment in more accuracy. The pipeline stresses intensity increases

with closing to free span center. This is attributed to the fact that UY and UZ have

more maximum values at these region.

Keywords: Free span, subsea pipelines, Finite element method, Hydrodynamic load.

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1. Introduction

One of the serious problems, during the operational state, for the structural shelter of

pipelines is uneven areas in the seafloor, as they enhance the development of free

spans. The unsupported parts of pipeline that are touching the seabed at his ends, may

form because of the maladjustment of seabed or artificial supports like rock, beams,

another pipeline (DNV-RP-F105, 2006), or the scouring of underlying soil,

(Yaghoobi, 2012), in construction of on-bottom (unburied) method which presents a

common construction method in offshore pipelines systems, since this method results

to reduce of construction time and associated costs (Georgiadou, 2014). Consequently,

in the on-bottom offshore pipeline, single and/or multiple free spans (L) along its

length are formed (Fig. 1). When a pipeline is in a free span, fluid flow caused by

waves or currents or both will cause vortices to be formed and shed in the wake of the

flow which can lead to fatigue damage in the pipe (Carl M. Larsen, 2002).

Under this framework, numerous researchers have developed and utilized various

numerical models for the examining of different aspects in the dynamic behavior of

free span pipelines.

In (T. Elsayed, 2012) the outhor proposed an approach based on the developing of a

nonlinear finite element model for the viewing of subsea pipelines for free spanning.

Combined stresses/lateral displacement is functioning on subsea pipelines as a result

of combined hydrodynamic loads, particularly wave/current effects that are calculated

making use of the finite element model for free spans.

(Project Consulting Services INC.,1997) the study establishs a method to evaluate and

analyze the pipeline of free spans, based on the information generating from the

research.The information that concluded from the work is used to outline preventative

and steps which are corrective for the subsea pipeline free spans.

Figure 1. span performed on seabed

2. Numerical Model

The composite of hydrodynamic loads and pipe-soil interface considers most

challenges that have difficult in the submarine pipeline model (Kristian, 2008). A

nonlinear finite element model (FEM) is applied to model the hydrodynamic forces

and the interaction between pipeline and soil of seabed in free spanning analysis using

general package of ANSYS, Inc. Release 16.1 program. The analysis model includes

Friction forces and soil stiffness representation and it contains two elements, the first

is PIPE288 model a total length of the pipeline. PIPE288 has two-nodes with six

degrees of freedom at each node (displacement in the x, y, and z directions and

rotations about the x, y, and z directions), Fig. (2-a), and the second one is

COMBIN14 that used to represent pipe-soil interaction at side spans of pipeline (the

shoulders). The element COMBIN14 has two-nodes with three degrees of freedom at

each node: displacements in the x, y, and z directions of option longitudinal spring

damper. Fig. (2-b)(ANSYS Help). Figure 3 shows geometric configuration of pipeline

in ANSYS software with global coordinate system. Where mid span length was

divided into 50 element of PIPE288 with element length equals to 0.24m, and side

span was divided into 20 element of PIPE288 for each side (10 elements through 4.5

m starting from shoulder beginning and 10 elements through 1.5m at shoulder

ending). Same arrangements have been made for shoulder model with element

COMBIN14.

a- Pipe288 element b- Combin14 element

Figure 2: elements used in the Ansys Model

Figure 3. Modeling pipeline in ANSYS software

3. Boundary Conditions

The displacements in X, Y and Z directions for the node of the element combin14 that

symbolized by "α", see Figure 4, should be fixed. The nodes are located at ends of the

pipeline, "β", treated as no displacements and rotation in all directions. The other

nodes of the pipeline are leaved to be free and symbolized by "γ ".

Figure 4. Boundary conditions in the ANSYS Model

4. Loadings

Figure 5 explains submarine pipeline with proposal loads that exerted on the free span

part. The loads can be easily sorted into two groups: (a) static loads that is resulting

from weight, buoyancy, internal pressure and steady current, (b) dynamic loads that

appears from the motion of water around the pipeline free span which is generated by

current and waves as shown in table 1. The acting of hydrodynamic loads, on the free

span, which are divided into two groups: 1) drag, lift, and inertia forces, and 2) flow

induced vortex shedding on free span, the effect of VIV in the present paper is not

taken into consideration.

Figure 5: Typical exerted loads on the free-span of a submarine

Pipeline (Kristain, 2008).

Table 1: Loads Acting on the Offshore Pipeline Free Span

α

α

α

β

γ

γ

Load Analysis Type

Self Weight Static

Buoyancy Static

Internal pressure Static

Hydrostatic Pressure Static

Current Drag Force Static

Steady Lift Force Static

Wave Drag Force Dynamic

Inertia Force Dynamic

5. Analysis Procedure

The evaluation and assessment of free spans must consider the number of

variables that can be classified into the following categories:

Pipeline materials properties at the free span.

Pipeline contents properties at the free span.

Pipeline supports and the behavior of the pipeline free span geometrically on

the bed of sea.

Environmental properties around free span.

The data of these categories are listed in table 2.

First, a static analysis is achieved that includes the calculation of the static response of

the pipeline due to the static loads which is included in Table 1. Afterward the

dynamic analysis is conducted by applying wave action in normal direction on the

free span. The ocean loads are input globally by using ocean commands which is

involve the current and/or waves effect, drag, lift and buoyancy.

The following are the input groups of the ocean-loading which are available (ANSYS

help, 2016):

Basic (required for any ocean loading)

Current (optional, for applying drift current)

Wave (optional, for applying a wave state)

Zone (optional, for applying local ocean effects)

The wave is input along with Airy wave theory (often known as linear wave theory).

In the fluid dynamics, the Airy wave theory gives a description that is certainly

linearized for the propagation of gravity waves on the surface layer of a homogeneous

fluid. The theory supposes that the layer of the fluid has a uniform mean depth,

furthermore the fluid flow is inviscid, irrotational and incompressible.

Table 2: Properties of Free Span

Pipeline Outside Diameter(m) 0.3227

Pipe Wall Thickness(m) 0.0127

·Young’s Modulus(Pa) 21 * 1010

Poisson’s Ratio 0.3

Density of Steel(Kg/m3) 7850

Internal pressure(Pa) 21 * 105

Water depth(m) 37.0

Density of Sea Water(Kg/m3) 1025

Sea Current Velocity(m/s) 0.41

Boundary conditions Fixed-Fixed

Span Length(m) 12.0

Shoulder Length(m) 6.0

Wave high(m) 9.5

Wave period(s) 8.5

CDy, CDz, CM0.5, 0.5, 2

Sea bed soil type Loose sand

6. Results and Discussion

Figs. 6a-6b show the time series of displacements in y-direction (UY) and in z-

direction (UZ) respectively only at node 27, which is located at the center of the

unsupported pipeline. It must be mentioned that the effect of different load conditions

on the pipeline’s displacements is more obvious in the case of UZcompared to UY,

where the maximum absolute value of UZ is larger than value of UY

(a)

(b)

Figure 6. Time Domain of UY (a) and UZ (b) at node 27

Figs. 7a~7b illustrate the time domain of ϬbY and ϬbZ at node 27. The effect of weather

conditions is more significant in the case of ϬbZ (Fig. 7b), in which a larger increase of

the peak values and the amplitudes of ϬbZ is observed compared to ϬbY (Fig. 7a). It

could be seen the effects clearly in Figs. 8a~8b which shows the hydrodynamic force

in y-direction(FY) and in z-direction(FZ) respectively.

(a)

(b)

Figure 7. Time Domain of Bending stresses ϬbY (a) and ϬbZ (b) at element 26

(a)

(b)

Figure 8. Time Domain of hydrodynamic forcesFY (a) and FZ (b) at element26

Fig.9 and Fig.10, show the displacement and bending stress configurations in y-

direction and z-direction along the total length of the pipeline (free span (Lf) plus part

of pipeline's shoulders (Ls)) are shown respectively.

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Figure 9. Displacement (UY) and Bending stress (ϬbY) configurations along total

length of pipeline

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Figure 10. Displacement (UZ) and Bending stress (ϬbZ) configurations along total

length of pipeline

As in Figures 9 and 10 display the pipeline stresses intensity which is increases when

closing to the free span center. This result is attributed to the fact that UY andUZ have

more maximum values at these region.

7. Conclusions

The dynamic behavior of a single free span offshore pipeline is analyzed in this

work and the effect of various factors/parameters (different design conditions, wave

and current characteristics, soil characteristics and boundary conditions at the ends of

the pipeline) on its dynamic behavior is established.

It should be mentioned that the effect of different load conditions on the

pipeline’s displacements is more obvious in the case of UZ compared to UY,

where the maximum absolute value of UZ is larger than value of UY.

The effect of weather conditions is more significant in the case of ϬbZ (stresses

in z-direction). Where a larger increase of the peak values and the amplitudes

of ϬbZ are observed compared to ϬbY.

The pipeline stresses intensity increases with closing to free span center. This

is attributed to the fact that UY and UZ are affected more maximum values at

these region.

References:

ANSYS help of Release 16.1, 2016.

Carl M. Larsen, Kamran Koushan and Elizabeth Passano, 2002. "Frequency and

Time Domain Analysis Of Vortex Induced Vibrations For Free Span Pipelines".

The 21st International Conference on Offshore Mechanics and Arctic

Engineering, Oslo, Norway.

Kristian Ruby, 2008 ''Free-span Analyses of an Offshore Pipeline''. Department of

civil engineering, Aalborg University, master project

Project Consulting Services, INC. 1997. ''Analysis And Assessment Of Unsupported

Subsea Pipeline Spans''. United States Department Of The Interiorminerals

Managment Service.

Det Norske Veritas DNV-RP-F105, 2006. "Free Spanning Piplines". Recommended

Practice.

Sofia Georgiadou, Eva Loukogeorgaki and Demos C. Angelides, 2014 ''Dynamic

Analysis of a Free Span Offshore Pipeline.'' Proceedings of the Twenty-fourth

(2014) International Ocean and Polar Engineering ConferenceBusan, Korea.

T. Elsayed, M. Fahmy and R. Samir, 2012 '' A Finite Element Model for Subsea

Pipeline Stability andFree Span Screening.'' Canadian Journal on Mechanical

Sciences & Engineering Vol. 3 No. 1.

Yaghoobi, Mehdi; Mazaheri, Said; Jabbari, Ebrahim, 2012 '' Determining Natural

Frequency of Free Spanning Offshore Pipelines by Considering the Seabed Soil

Characteristics''. Journal of the Persian Gulf (Marine Science)/Vol. 3/No. 8/25-

34.