Services

Services

■■ Liquefied■Natural■Gas

■■ Gas■Compressor■Stations

■■ Off■Loading■Gas■and■LNG

■■ Normal■and■Floating■LNG■Terminals

Gas and LNGConsulting Services

8

2 Gas and LNG

3Gas and LNG

Natural Gas

Natural gas is a hydrocarbon gas mixture,

which consists primary of methane and a

fraction heavier hydrocarbons as well as im-

purities like water and carbon dioxide. Natu-

ral gas is widely used as an important energy

source for many applications including hea-

ting, electricity generation, industrial power

generation and as a fuel for vehicles. Gas is

often found in the near vicinity of oil, but also

together with oil in deep underground natu-

ral rock formations.

In order to use natural gas as a power source,

it is processed to clean the gas and remove

impurities to meet the requirements of the

end user.

Dynaflow Research Group (DRG) has perfor-

med a large number of static and dynamic

calculations related to the structural integrity

of gas pipeline systems, and their supporting

installations and equipment. Dynamic flow

calculations play a major role in solving vibra-

tion and pulsation problems.

Liquefied Natural Gas (LNG)

On account of its low density, it is not straight-

forward to store natural gas or transport the

gas by road. Additionally, transporting natu-

ral gas across the oceans is highly impracti-

cal. Furthermore, the existing gas pipeline

network is already close to capacity such that

a significant number of new pipelines are re-

quired to fulfill the need for gas in the near

future.

“LNG has become an impor-tant part of modern energy transportation”

Cooling natural gas to about -160 degrees

Celsius at atmospheric pressure results in

the condensation of the gas into liquid form,

DRG■provides■consulting■services■and■engineering■solutions■for■Liquefied■Natural■Gas■(LNG)■systems

Liquefied Natural Gas

4 Gas and LNG

known as Liquefied Natural Gas (LNG). LNG

is natural gas that has been temporarily con-

verted into a liquid for the ease of storage

and transport. The volume of LNG is about

1/600th of the equivalent volume of natural

gas in the gaseous state.

While LNG is reasonably costly to produce,

recent technologies are considerably redu-

cing the costs to liquify the natural gas and

regasify the LNG. Natural gas is converted

into a liquid at a liquefaction plant (LNG ex-

port terminal), after which it is used for trans-

porting the natural gas to the markets. Here,

the LNG is regasified (LNG import terminal)

and distributed as pipeline natural gas.

Rapid phase transition explosion

The very low operating temperature around

-160 degrees Celsius means that it is extre-

mely important to remove water and car-

bon dioxide and other components that will

freeze under the low temperature necessary

for LNG storage and transport. One of the

major risks of LNG is a rapid phase transition

explosion (RPT), which occurs when cold LNG

comes into contact with water.

Transport and regasification of LNG

LNG production and transportation requi-

res an important infrastructure consisting of

one or more LNG trains, each of which is an

independent unit for gas liquefaction. Sub-

sequently, the LNG is loaded onto ships and

delivered to a regasification terminal, where

the LNG is reconverted into gas. The regasi-

fication terminals are usually connected to

a storage and pipeline distribution network

to distribute the natural gas to the local dis-

tribution companies or independent power

plants (IPPs).

LNG terminals

Liquefied natural gas is used to transport na-

tural gas over long distances, often by sea. In

most cases, LNG terminals are purpose-built

ports used exclusively to export or import

LNG, an example of which is the Gate termi-

nal in Rotterdam harbour.

The LNG is stored in large insulated tanks.

Although very efficient insulation is applied,

heat does inevitably leak into the LNG. In-

evitably, heat leakage will warm and vapou-

rise the LNG. LNG boils at -160°C when at

atmospheric pressure. By boiling the liquid

natural gas evaporates and becomes natural

gas. The process of evaporation (phase chan-

5Gas and LNG

ge) takes a large amount of energy from the

liquid. This amount is called the heat of eva-

poration and makes evaporation an efficient

cooling mechanism. By letting gas escape the

LNG-tank is kept at atmospheric pressure

and therefore the liquid in the tank is always

kept at -160°C. Any heat that leaks in causes

evaporation of the liquid which cools the

remaining liquid. The combination of high

quality insulation and cooling by evaporation

causes only a relatively small amount of boil-

off is necessary to maintain the temperature,

called auto-refrigeration. The boil-off gas re-

sulting from on-shore LNG storage tanks is

usually compressed and fed to natural gas

pipeline networks. Some LNG carriers use

boil-off gas for fuel.

Dynaflow Research Group

Dynaflow Research Group (DRG) has a broad

experience in providing assistance to the de-

sign and verification of natural gas and LNG

terminals, supporting equipment and corres-

ponding transportation lines. The considered

systems include:

■ Design of LNG and Gas storage tanks,

■ Gas compressor stations, including coo-

ler banks, filters and compressors,

■ Coolwater and firewater systems of Ter-

minals, primarily GRE piping,

■ LNG terminals and floating platforms,

■ Off-loading of natural gas (jetty),

■ High and low pressure vessels contai-

ning gas, LNG and other type of fluids.

Practical engineering solutions are provided

to these complex piping systems and the at-

tached equipment. Examples of these analy-

sis types are pulsation or acoustic analyses,

mechanical response studies, structural

(thermal and stress) analysis (FEA) and detai-

led flow calculations (CFD).

Transient flow software packages are often

used to simulate and analyse surge, water

hammer, pulsations and transient accous-

tical behaviour of liquid and gas piping sys-

tems. As a result of a pulsation analysis, the

magnitude of the unbalanced forces are cal-

culated for each pipe section, and these can

be used in a mechanical response analysis.

Such a mechanical response analysis will be

performed by means of a pipe stress soft-

ware package with dynamic capabilities, such

as CAESAR II, PipePlus or FE/Pipe.

DRG provides solutions which are able to

comply with a range of industry standard Co-

des such as ASME, DIN, NEN, AD Markblat-

ter, API 618, API 674 codes and VDI 3842.

6 Gas and LNG

7Gas and LNG

Gas compressor stations

A gas compressor station enables the trans-

portation process of natural gas from one lo-

cation to another. While transporting natural

gas through a gas pipeline, the gas needs to

be constantly re-pressurized at certain dis-

tance intervals.

The location of the compressor station heavi-

ly depends on the type of terrain but also on

the number of gas wells in the vicinity of the

compressor station. A large numer of gas

wells and frequent elevation changes will re-

quire more compressor stations.

The gas in compressor stations is normally

pressurized by special turbines, motors and

engines. As the name implies, the compres-

sor station compresses the natural gas, this is

needed for the gas to be transported through

the pipeline.

Additional compressor stations are needed

Design■and■installation■of■gas■compressor■stati-ons■require■complex■engineering■solutions

Gas Compressor Stations

due to the pressure loss that the moving gas

experiences along a pipeline route, typically

every 70 to 150 kilometers. The size of the

station and the number of compressors va-

ries, based on the diameter of the pipe and

the volume of gas to be moved. Nevertheless,

the basic components of each compressor

station are similar.

Centrifugal and reciprocating com-pressors

When the natural gas has reached the com-

pressor station, it is compressed by a com-

pressor powered by either a turbine, electric

motor or internal combustion engine. Tur-

bine compressors are fueled by using a small

portion of the energy from the gas they com-

press. The turbine itself serves to operate

a centrifugal compressor, which contains a

type of fan that compresses and pumps the

natural gas through the pipeline. Some com-

8 Gas and LNG

pressor stations are operated by using an

electric motor to power the centrifugal com-

pressor. This type of compression does not

require the use of any of the natural gas from

the pipe, however it does require a reliable

source of electricity.

Reciprocating natural gas engines are also

used to power some compressor stations.

The advantage of reciprocating compressors

is that the volume of gas pushed through the

pipeline can be adjusted incrementally to

meet small changes in customer demand.

Mechanical Integrity Analysis

Any structure has a number of mechanical

resonance (natural) frequencies. If these

frequencies coincide with those of external

excitations, for example those due to pumps

or the fluid flow within a pipe, then any small

pipe deflection caused by the excitation me-

chanism at these frequencies, could be am-

plified and result in resonant vibrations in the

mechanical structure.

These mechanical vibrations, if persistent,

could result in problems due to Low Cycle

Fatigue or High Cycle Fatigue. The fatigue ca-

pabilities of your piping structure or pressure

vessel can be assessed by means of dedica-

ted Finite Element Analysis (FEA) software,

keeping in mind Code compliance with rele-

vant Codes such as ASME, DIN, NEN and EN.

Pulsations and Mechanical Res-ponse

Reciprocating compressors produce pulsati-

ons in the suction and discharge piping that

can be damaging to the piping and to the

equipment itself. The pulsations can lead to

potential fatigue failure, undesirable vibrati-

ons, reduced efficiency or errors in flow mea-

surement results.

“By optimizing gas compres-sor stations gas transporta-tion benefits are increased”

Also, pulsations in the piping system might

result in cyclic stresses and fatigue problems.

A pulsation analysis is most often performed

either in the design phase or as a result of

a failure in the field. Field problems usually

require inspecting and numerous measure-

ments taken by an expert to help identify the

exact nature of the pulsation.

A “Design Phase” analysis is typically compli-

cated since it requires that the analyst makes

sure that all the possible and relevant sce-

narios are defined and that the worst-case

9Gas and LNG

scenario is simulated. All potential excitation

frequencies and gas conditions must be in-

vestigated, and all the applicable rules of API

618 must be satisfied. Furthermore, climate

and local ambient air changes may result in

variations in the speed of sound of up to 15%.

Typically, a variety of gas and ambient condi-

tions as well as critical load cases will be ana-

lyzed. The worst of these cases will then be

used for the mechanical response analysis.

A pulsation analysis is often performed in

conjunction with a mechanical response ana-

lysis using dedicated pipe stress software.

Here both stress analysis software and BOS-

pulse can be used to check the mechanical

shaking forces and the API 618 allowables to

ensure smooth compressor operation.

The API allowable acoustic level is specified

on a per frequency basis and so each fre-

quency contribution to the pulsation must

be evaluated separately. BOSpulse applies

a time history approach and automatically

decomposes the calculated pressure pul-

sations to produce an API 618 pulsation

compliance report for all sections of the pi-

ping system. Alternatively, a separate har-

monic analysis for each compressor loading

component can be performed if desired.

As a result of the pulsation analysis, for each

pipe section, the magnitude of the unbalan-

ced forces are calculated and used in the

mechanical response analysis. This mecha-

nical analysis is performed by means of a

pipe stress software package with dynamic

capabilities, such as CAESAR II, PipePlus or

FE/Pipe.

Gas Liquid Separation

As the pipeline enters the compressor sta-

tion, the natural gas passes through scrub-

bers, strainers filters or separators. These

different types of equipment are all designed

to remove any free liquids or dirt particles

from the gas before it enters the compressor.

In order to study and optimize the liquid se-

paration from the gas full three-dimensional

multi-phase flow fields can be obtained. For

complex and sensitive systems, it can be ne-

cessary to investigate the three-dimensional

flow field. This can be obtained by perfor-

ming a full Computational Fluid Dynamics

(CFD) analysis.

10 Gas and LNG

11Gas and LNG

Pressure surges in off-loading

At LNG terminals, the liquefied gas is off-

loaded from the LNG ship by means of jetty

constructions. The off-loading lines running

from these jetties up to the storage tanks can

be up to several kilometers in length.

An example of a steel off-loading line is a

system with a diameter of 1000mm which

connected a storage tank with a loading arm

upon the jetty. The design flow rate reached

a maximum of 2000 ton/hr. The analysis of a

number of anticipated transient update sce-

narios was required.

Three transient upset scenarios were simula-

ted and investigated in depth:

■ The rapid closure of the control valve in

the loading arm. The valve was an emer-

gency valve which closed to prevent the

spoil of LNG if the ship, from which LNG,

is being unloaded, moves too far from

DRG■provides■consulting■services■and■engineering■solutions■for■off-loading■gas■and■LNG

Off Loading Gas & LNG

the jetty

■ The emergency trip of the pump on-

board of the ship

■ The closure of the valve, located just

upstream of the storage tank, when the

LNG in the tank reaches its maximum

liquid level

A transient one-dimensional fluid model of

the pipeline was produced in BOSfluids to si-

mulate and analyze the transient upset con-

ditions. From the transient flow model, un-

balanced forces were extracted, and applied

in a dynamic mechanical time-history ana-

lysis using CAESAR II. The dynamic stresses,

displacements and support reactions caused

by the various upset conditions were calcula-

ted and assessed. Subsequently, the dynamic

pipe stresses were assessed according to the

ASME B31.3 code.

In steady operation (no transient valve or

pump actions), it was observed that vapor

12 Gas and LNG

pressure level was reached at the top of the

siphon immed-iately upstream of the LNG

storage tank for low liquid levels in the tank.

Due to the low pressures at the top of the

siphon, a large vapor filled sect ion was crea-

ted at the top of the siphon. During transient

conditions the cavitation caused by the vapor

condensing downstream of the siphon led to

significant unbalanced forces in the piping.

The results indicated that the occurence of

the use of a low tank nozzle would prevent

this cavitation and also eliminate the associa-

ted unbalanced forces.

Cavitation and column separation

Closure of a safety valve or the tripping of the

pump will stop the LNG flow in a relatively

short period. As a result, low pressures can

be created in the loading arms and immedi-

ately downstream of the loading arms and in

pipe bridges.

These low pressures can reach vapor pres-

sure levels and as result column separation

would occur. Under most circumstances the

flow decelerates and reverses. This couses

the cavity to collapse which is accompanied

with large pressure spikes.

The transient flow simulations performed

using BOSfluids® for the off-loading line, sho-

wed that indeed large pressure waves were

generated. These had a peak value of up to

30 barg, and travelled through the line some

time (about 30 seconds) after the start of the

most critical transient scenario. The pressure

wave was seen to be caused by the collapse

cavity in between the liquid columns.

The dynamic mechanical analysis showed

that the pressure wave generated large un-

balanced forces for a very short duration of

time, which would causes pipe stresses im-

mediately downstream of the loading arm

that were in excess of those permitted under

the ASME B31.3 code for occasional loading.

Large displacements were also predicted in

these critical locations. Therefore, additional

pipe supports were recommended in the cri-

tical parts of the line.

Dynamic pipe stresses

Due to the large displacements and stresses

seen on the off loading line, a dynamic pipe

stress analysis was also required. However,

for the critical locations, it was not feasible

to reduce the displacements through the in-

troduction of further restraints on account of

the flexibility and strength of the pipe brid-

ges.

13Gas and LNG

The mechanical response modes were identi-

fied and were excited as a result of the unba-

lanced loads obtained.

The calculated dynamic stress levels were

used to determine which sections of the off-

loading line were likely to suffer fatigue is-

sues.

Through modifying the pipe supporting wit-

hin the system, it was possible to change the

frequencies of the mechanical response, and

thereby eliminate the fatigue problem.

BOSfluids®

For years, piping engineers have laboured

with simple hand calculations, or user-un-

friendly software products when in need of

a simple tool to analyse the impact of pulsati-

ons upon their piping system.

BOSfluids® has been built specifically to ad-

dress the needs of the piping engineer, who

requires details of the dynamic forces acting

on a piping system.

BOSfluids® is an interactive computer simula-

tion package that is able to model the steady

state and transient flow in liquid or gas car-

rying piping systems. It has been developed

in-house by Dynaflow Research Group and

has been extensively used on projects for

our clients. BOSfluids® has been commerci-

ally available since 1998.

“By reducing pressure surges, the system integrity and safety can be drastically improved”

14 Gas and LNG

15Gas and LNG

LNG Terminals

Liquefied natural gas is used to transport na-

tural gas over long distances, often by sea. In

most cases, LNG terminals are purpose-built

ports used exclusively to export or import

LNG, an example of which is the Gate termi-

nal in Rotterdam harbour.

Before or after liquefied natural gas (LNG) is

transported over long distances, the LNG is

stored in large insulated tanks. Although very

efficient insulation is applied, heat does inevi-

tably leak into the LNG.

LNG terminals involve large installations, of-

ten forming purpose-built ports to exclusi-

vely export or import LNG, such as the Gate

terminal in Rotterdam harbour. Consequent-

ly, the design and verification of gas and LNG

terminals requires the assessment of the

mechanical integrity of a wide variety of sup-

porting equipment and connected transpor-

A■recent■trend■in■the■design■of■LNG■terminals■are■floating■LNG■terminals

tation lines. The considered systems include

but are not limited to:

■ Design of LNG and Gas storage tanks

■ Gas compressor stations, including coo-

ler banks, filters and compressors

■ Coolwater and firewater systems of Ter-

minals, primarily GRE piping

■ LNG terminals and floating platforms

■ Off-loading of natural gas (jetty)

■ High and low pressure vessels contai-

ning gas, LNG and other type of fluids

“Structural integrity of LNG terminals is important for safety and the environment”

Engineering solutions are provided to these

complex piping systems and the attached

equipment. Examples of these analysis types

are pulsation or acoustic analyses, mechani-

cal response studies, structural (thermal and

stress) analysis (FEA) and detailed flow calcu-

Normal and FloatingLNG Terminals

16 Gas and LNG

lations (CFD).

Case: LNG terminal design calcula-tions

For a new regasification LNG plant with tre-

atment facilities design verifications have

been performed. Sea water required for the

LNG vaporizing duty of the terminal was to

be shared with the sea water requirements

of the neighbouring power station located

in the same industrial area. Existing sea wa-

ter intakes, facilities (sea water filtration and

pumping) were used to supply sea water to

both the power station and the LNG terminal.

A dedicated line was to be routed to the LNG

terminal to supply sea water to the booster

pumps. From the LNG vaporizers, sea wa-

ter is to be fed back to the main sea water

lines to the power station. As cold seawater

was beneficial for operating the power plant

condensers, the seawater requirements of

the power plant and the LNG terminal could

be adequately integrated as seawater used

for vaporizing LNG is actually cooled. DRG

has performed an extensive mechanical res-

ponse analysis for this LNG terminal.

Structural integrity of lines

Static and dynamic stress analysis of suppor-

ting systems at LNG terminals, for instance in

plant piping, or the cooling, fire and dump

lines is critical. Often, the lines are fabricated

of steel or using Glassfiber Reinforced Epoxy

(GRE) to deal with corrosive and erosive en-

vironments.

The piping of the considered systems can be

above ground or buried and includes several

connections to above ground equipment.

System routing and pipe properties used for

the analysis are based on data provided by

the suppliers. The resulting pipe stresses are

assessed for their conformance with the ISO

14692 code for GRE lines or the applicable

ASME code, such as B31.3, for steel lines.

Surge analysis

The time-dependent unbalanced loads

caused by the transient flow of a pump failu-

re, pump start-up and subsequent failure can

be applied on a dynamic time-history stress

model are calculated.

With these unbalanced loads a mechanical

response analysis over a wide range of ope-

rating conditions can be executed. The re-

sulting maximum dynamic stresses are com-

bined with the static operational stresses and

assessed according to the applicable design

code.

17Gas and LNG

FLNG compressor modules

A floating production, storage and off-

loading(FPSO) unit is a floating vessel used

by the offshore oil and gas industry for the

processing of hydrocarbons and the storage

of oil. An FPSO vessel is designed to receive

hydrocarbons produced from nearby plat-

forms or subsea production facilities, process

them, and store oil until it can be offloaded

onto a tanker or, less frequently, transported

through a pipeline.

The relatively large bore suction and dischar-

ge piping connected to the natural gas com-

pressor onboard a floating LNG platform

may be exposed to extreme load conditions.

In addition to the normal thermal and pres-

sure design loads also loads due to large

wind velocities as a result of storm fields pas-

sing by having to be accommodated by the

pipework this may result in large compressor

nozzle loads. As a result of the ocean waves

the FLNG compressor module is also subject

to rocking motions.

The rocking motions impose accelerations

on the piping. For this type of piping the pipe

material stress is hardly ever governing for

the design. The allowable compressor nozzle

loads (API 617) are in general ruling. A piping

support arrangement will be designed to ac-

commodate the external loads due to wind

and barge movements and minimize the

resulting compressor nozzle loads. Gener-

ally this requirement conflicts with a piping

support arrangement designed to minimize

nozzle loads resulting from thermal expan-

sion.

DRG has been involved with the analysis of

various compressor piping layouts for the

feasibility of reconciliation of these con-

flicting requirements on a FLNG Terminal.

The target for such an analysis is to keep the

nozzle loads under all load conditions within

a safety margin that conforms with API 617.

Source: Hoegh

18 Gas and LNG

19Gas and LNG

DRG■provides■consulting■services■and■engineering■solutions■for■flow■and■mechanical■problems■rela-ted■to■Gas■and■LNG■systems

What can we do for you?

What can we do for you?

The engineers at Dynaflow Research Group

(DRG) have a thorough understanding of the

necessary fundamental physics related to

natural gas and LNG. We have a broad expe-

rience in providing assistance to the design

and verification of gas and LNG terminals,

supporting equipment and corresponding

transportation lines. The considered systems

include:

■ Design of LNG and Gas storage tanks

■ Gas compressor stations, including coo-

ler banks, filters and compressors

■ Coolwater and firewater systems of Ter-

minals, primarily GRE piping

■ LNG terminals and floating platforms

■ Off-loading of natural gas (jetty)

■ High and low pressure vessels contai-

ning gas, LNG and other type of fluids

A thorough understanding of the problem is

of crucial importance in order to assist you

in optimizing your process, to increase your

profit and the safety of your system. There-

fore, we believe we are well-positioned to

tackle your challenges related to your LNG

terminal or floating production platform.

Our consulting services are related to pulsati-

on or acoustic analysis, mechanical response

studies, structural (thermal and stress) analy-

sis (FEA) and detailed flow calculations (CFD).

Communication

To us communication with our clients during

a project is of upmost importance. For each

project the client is updated regularly with

the progress of our work, and we liaise with

the client to ensure the best information is

available with which to conduct the analyses.

20 Gas and LNG

“Dynaflow■Research■Group■(DRG)■is■a■world■wide■well■respected■consultant.■We■help■our■clients■to■solve■their■most■complex■and■critical■technical■issues”■

Dynaflow Reseach Group

Consulting services

We provide engineering consulting services in all

aspects of design and analysis for the Petro- che-

mical industry. Our work often requires a multi-

disciplinary approach where we combine exper-

tise in fluid flow behaviour, dynamic oscillations,

FEM and stress analysis with sophisticated analy-

sis software to predict system performances.

Training

DRG offers a wide range of training courses such

as software training, fiberglass training, dyna-

mics and stress training. Most of these training

courses are offered on a regular basis during

the year. We also develop customised training

programs with our customers fit to their specific

needs.

Products

DRG has been developing software for many

years, which has resulted in several commercially

available software packages such as BOSfluids®,

BOSpulse®, Jive and Hades. We also provide tech-

nical consulting services, and develop numerical

software that can be used in computer simulati-

ons and other types of scientific computations.

Research

DRG conducts research on different aspects of

pipe-system design and pressure vessels. Most

of this research is done in close collaboration

with Paulin Research Group and their Houston

test facilities (www.paulin.com). Dynaflow Re-

search Group provides support to clients with

their R&D to help them continuously improve

their products.

21Gas and LNG

Topic specific brochures:

• Consulting Service Series• Software Product Series• Training Series

Visit our website www.dynaflow.com or send an e-mail to info@dynaflow.com

Houtsingel 95 2719 EB Zoetermeer The NetherlandsReg nr. 27320315

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+31 79 361 5150+31 79 361 5149info@dynaflow.comwww.dynaflow.com