Subsea Systems

Services

Services

■■ Subsea■Production■Systems

■■ Subsea■Mechanical■Integrity■

■■ Flow■Assurance■for■Subsea■Production■Systems

■■ Pipelines,■Flowlines■and■Risers

Subsea SystemsConsulting Services

2

2 Subsea Systems

Source: FMC Technologies

3Subsea Systems

Subsea Production Systems

With the depletion of onshore and offshore

shallow water hydrocarbon reservoirs, the

exploration and production of oil in deep

water has become a challenge to the offshore

industry. Since these subsea developments

are moving further offshore and into deeper

waters, the technical challenges of such pro-

jects are continuously increasing.

For deep water developments a wide range

of subsea layouts and production systems,

greatly differing in complexity, are utilized.

A subsea production system consists of a

subsea completed well, subsea Christmas

trees and wellhead systems, subsea tie-in to

flowline system, jumpers, umbilical and riser

system and subsea equipment to operate

the well. The single or clustered well can be

connected through the flowline to a fixed

platform, FPSO (Floating Production, Storage

and Offloading) or onshore facilities. Alterna-

tively, an existing subsea production system

can be connected to a newly operated pro-

duction well by means of a subsea tie-back,

that have become popular in the develop-

ment of new oil and gas reserves.

Due to the high pressures, potentially large

temperature gradients and the harsh envi-

ronment in deep-water, the subsea systems

and equipment are subjected to complex and

critical load cases. Therefore in all offshore

pipeline systems the transportation of fluids

including the flow of oil, gas, water and mix-

tures thereoff should be analyzed to optimize

performance and minimize the operational

risks.

Subsea Flow Assurance

The flow in subsea transportation pipelines

and tie-backs are often governed by risers.

Both risers and flowlines are located on the

DRG■provides■consulting■services■for■Subsea■Engineering■problems■related■to■Subsea■Flow■Assurance■and■Subsea■Structures,■Equipment,■Risers■and■Flowlines

Subsea Engineering

4 Subsea Systems

seabed, such that these typical components

may be subject to thermal and mechanical

problems. In particular, transient heat and

mass transfer phenomena are often present

in specific components and need to be ana-

lyzed and/or simulated in order to achieve a

proper system design.

The deliverability of hydrocarbon products

from the reservoir to the end user is essential

to the success of oil and gas developments,

but often turns out to be the bottleneck of

the system. Pipelines, either onshore or off-

shore, play a vital role in ensuring a reliable

production process, which provides a mana-

geable and profitable flow of fluids from the

subsea wellhead to the fixed platform, FPSO

or onshore facilities.

Hydrocarbons are rarely found in single-

phase flow during transport from a reservoir

to production platform. It is these multiphase

flows that form the most serious hazard in

the production process. In subsea operati-

ons, controlling multiphase flows is of the up-

most importance, as these can have adverse

effects on the often long riser lines that are

used to surface hydrocarbon products from

the subsea production system.

Multiphase Flows

There are several critical problems that can

be attributed to the occurrence of multi-

phase flows, which typically consists of wa-

ter, oil and gas. For instance, a mixture of

water and hydrocarbons can form hydrates

that block pipelines obstructing the flow.

Additionally, slugs may be formed within the

pipelines, which can cause severe damage to

downstream processing facilities. It is there-

fore of high importance to be able to predict

and control the slugging in the subsea pro-

duction system.

OLGA®, a widely used software tool to simula-

te multiphase transport of oil, water and gas-

es is often used for multiphase flow analysis,

together with PVTsim®. When necessary, full

CFD (Computational Fluid Dynamics) simula-

tions are performed to make detailed flow

assessments of all kinds of equipment like

separators, slug catchers etc. and connecting

“DRG assists the client to in-crease their perfomance and reduce the risks of subsea production”

5Subsea Systems

flowlines between the subsea well head and

fixed platform, FPSO or onshore facilities.

Using the broad knowledge and wide ranging

expertise in multiphase flows, DRG is able to

interprete the complex multiphase flow re-

sults in order to assist the client to increase

the performance and reducing the risks of

subsea production.

Subsea Structures and Pipelines

Since subsea flowlines are subjected to high

external pressure, collapse and buckling cri-

teria are very important. These criteria de-

pend on the diameter to wall thickness ratio,

pipe imperfections and load conditions such

as axial tension, bending stress and external

pressure. A good knowledge of these criteria

is required to come to a sufficiently robust

flowline design. Using a fully validated sui-

te of design tools DRG is capable of solving

some of the oil industry’s most difficult pro-

blems such as detailed lateral buckling and

pipeline walking issues.

The increased activities in subsea processing,

such as separation or compression, requi-

res more advanced structures. The nozzles

of subsea vessels are differently loaded in

the subsea regime, on account of the high

pressure and low temperature, than their

counterparts in onshore installations. Dyna-

flow Research Group has broad experience

in performing FEA (Finite Element Analyses)

simulations of various types of equipment,

which might be loaded due large pressures

and critical thermal effects in accordance

with many international codes such as ASME,

EN, BS, etc.

Dynaflow Research Group

We believe it is important not only to have

a sound grasp of the physics behind these

phenomena but also to have a very hands-

on and pragmatic approach to your real life

problems.

Due to our highly qualified staff and expe-

rience in this field, DRG is often required to

solve problems involving subsea production

systems, subsea flowlines from wellhead to

fixed platform, FPSO or onshore facilities,

and attached equipment like separators and

slugcatchers.

DRG provides engineering services in all pha-

ses of a subsea system design.


6 Subsea Systems

Source: Aker Kvaerner

7Subsea Systems

Manifold Design and Analysis

Subsea manifolds have been used in the de-

velopment of oil and gas fields to simplify the

subsea system, minimize the use of subsea

pipelines and risers and optimize the fluid

flow production sytem. The manifold con-

sists of an arrangement of piping and valves

designed to combine, distribute, control and

often monitor fluid flow. Subsea manifolds

are installed on the seabed connected to se-

veral wells to collect the product or to inject

gas or water into the well. The most common

subsea manifold is a PLEM (PipeLine End

Manifold) which is directly connected to the

pipelines.

The subsea manifold should be designed to

provide sufficient piping, valves and control

equipment to collect all produced oil and gas

or to inject fluids like gas, water or chemicals.

Dynaflow Research Group assists its clients

DRG■provides■consulting■services■and■engineering■solutions■to■improve■and■verify■the■mechanical■integrity■of■subsea■systems

Subsea Structures and Equipment

to improve their manifold design and to ve-

rify its structural integrity. Manifold modules

belong to subsea structures and therefore,

their structural integrity should follow rele-

vant standards for subsea structures such

as ISO 13819-1 and ISO 13819-2. The piping

of the subsea manifold should be designed

to comply with the ASME code. DRG has an

indepth knowledge of these codes.

Mechanical Response Analysis

Any structure will have a number of mechani-

cal resonance (natural) frequencies. If these

frequencies coincide with those of external


8 Subsea Systems

excitatons, for example those due to produc-

tion pumps or the fluid flow within a pipe,

then any small pipe deflection caused by the

excitation mechanism at these frequencies,

could be amplified and may result in vibrati-

ons in the mechanical structure.

These mechanical vibrations, if persistent,

could result in problems due to LCF (Low

Cycle Fatigue) or HCF (High Cycle Fatigue).

As a result of a surge analysis or multiphase

flow analysis (using OLGA® or BOSfluids®),

for each pipe section, the magnitude of the

unbalanced forces are calculated and used in

the mechanical response analysis. This me-

chanical analysis is performed by means of

a pipe stress software package with dynamic

harmonic capabilities. In addition to the flow

transients inside the pipe, the effects of tidal

waves and currents are incorporated.

Subsea separator vessel design

Performing a part of the required separa-

tion process at the seabed, enables a more

effective production. Additionally, the need

for processing facilities on the fixed platform,

FPSO or onshore facility can be significantly

reduced. Dynaflow Research Group assists

its clients in optimizing their in-line subsea

separator technology.

Multiphase (subsea) flow separator vessels

often encounter slugs. The parts of the se-

parator that is most vulnerable to slug loads

are the internals to promote gravity separa-

tion, sand removal, demisting internals and

cyclone scrubbers.

The effects of slug forces working on the in-

ternals can be analyzed using dedicated CFD

(Computational Fluids Dynamics) and FEA

(Finite Element Analysis) techniques. The

numerical results should be combined and

compared with analytical multiphase flow

calculation. It is also possible to assess the

flow (detailed structure and medium proper-

ties) for a three-phase flow, which is difficult

to predict. Common analytical theories can

be used to predict the flow regimes based

upon the process conditions supplied by the

client.

When the kind of flow structure is known and

what the average flow velocities are for the

different flow phases, the slug loads can be

determined. Based on the determined slug

loads and the geometry and configuration of

the internals, a detailed FEA (Finite Element

Analysis) analysis can be performed to assess

the fatigue performance.


9Subsea Systems

Subsea wellheads and trees

In the subea production system the wellhead

and Christmas trees are the most vital pieces

of equipment. The subsea wellhead system

has the same functions as a conventional

surface wellhead. It needs to seal the casing

strings in the well, supports the BOP (Blow

Out Preventer) during drilling and supports

the tree during production. The primary

function of the subsea Christmas tree is to

ensure a structural and pressure containing

anchoring point on the seabed for drilling

and production. All internal components of

the subsea tree to support the casing strings

and provide guidance and mechanical sup-

port need to be designed conform the rele-

vant code and regulations.

Depending on the project and field develop-

ment, DRG assists its clients designing their

subsea tree to comply with codes like API

6A, API 17D, API RP 17H, ASME B31.3, ASME

B31.8, ASME BPVC VIII, among others. Detai-

led dedicated FEA (Finite Element Analysis)

techniques will be employed to satisfy the

main subsea tree requirements like sealing

of the casing strings, interface between tree

system and BOP and to accept all loads on

the subsea wellhead system from drilling and

production (including thermal expansion).

Dynaflow Research Group is often involved

with the design and verification of a subsea

structure, including:

■ Analysis of the mechanical integrity of

the subsea manifold, including detailed

mechanical response analysis and mul-

tiphase flow behavior,

■ Proper design of subsea equipment like

separators, subsea wellheads and trees

to increase drilling and production effi-

ciency.


10 Subsea Systems

Source: Emerson

11Subsea Systems

The deliverability of hydrocarbon products

from the reservoir to the end user is essential

to the success of oil and gas developments,

but often turns out to be the bottleneck of

the system. Especially in subsea systems, ri-

sers and flowlines play a vital role to ensure

a production that generates a reliable, mana-

geable and profitable flow of fluids from the

subsea well and tree up to the fixed platform,

FPSO or onshore facilities. The most signifi-

cant challenge of subsea flow assurance is

prevention and control of solid deposits that

could potentially block the flow of product.

Steady state hydraulic and thermal performance analysis

Using software like OLGA® or BOSfluids® a

steady state flow model can be generated

which is basically the first step in the analy-

sis of the subsea flowline performance. The

relationship between flow rate and pressure

DRG■provides■consulting■services■and■engineering■solutions■of■Flow■Assurance■of■Subsea■Systems■in-cluding■steady■state,■transient■multiphase■flows,■heat■transfer■and■erosion■modelling

Subsea Flow Assurance

drop along the subsea flowline, tie-back and

riser is determined, leading to maximum and

minimum allowable flow rates.

Subsequently, the temperature and pressure

distributions along the subsea flowlines are

obtained to ensure that the conditions in

the flowline are such to avoid the formation

of hydrates during steady-state conditions.

When the temperature distibution is ob-

tained, an insulation combination is chosen

that prevents the temperature at the riser

base of a tie-back subsea system from fal-

ling below the minimum value for cooldown

at the maximum range of production rates.

Using dedicated software solutions, the

maximum flow rate in the system can be ob-

tained to ensure that arrival temperatures do

not exceed any upper limits defined by the

separation or dehydration processes or by

equipment design.

12 Subsea Systems

Transient flow behavior and ther-mal performance analysis

Water temperatures at the sea bottom can

be very low, almost at freezing point. When

the flow in a pipeline on the seabed comes

to a standstill, the temperature of the hydro-

carbon pipe content can quickly decrease.

Low temperature in a pipeline can lead to

the formation of paraffin, asphaltenes and

hydrates, which can cause significant, even

catastrophic, operational issues.

The transient subsea flowline analysis typi-

cally include start-up, shutdown and blow

down scenarios as well as pigging and slug-

ging occurances. During these scenarios, it is

important to maintain the fluid temperature

above the hydrate dissociation temperature

corresponding to the pressure at every loca-

tion. To meet this important requirement, it

may be necessary to incorporate a combina-

tion of an insulated pipeline and the injection

of chemical inhibitors into the transient simu-

lation to prevent hydrate formation.

BOSfluids® and OLGA®

Successful design and operation of multi-

phase production systems rely on detailed

understanding of the fluid flow behaviour.

BOSfluids® is the engineering software packa-

ge that analyzes fluid transients in piping sys-

tems and relates this information back to the

mechanical system transferring the fluid.

Additionally, Dynaflow Research Group is

experienced in the use of OLGA®, a widely

used software tool to simulate multip-

hase transport of oil, water and gases.

For complex and sensitive systems, it

can necessary to investigate the three-

dimensional flow field. This can be obtained

by performing a full Computational Fluid Dy-

namics (CFD) analysis.

Flow regime prediction

Multiphase flow can take many different

forms. The most severe multiphase flow

regime is slug flow, subsea flowline or riser

sections completely filled up with liquid and

holding up the gas flow. Slugging can cause

large pressure transients or potentially lead

to flooding of liquid at the receiving end at

the platform. Slugs can also result in an in-

creased volume of solid deposits, corrosion

and erosion.

“DRG assists in improving thermal and pressure control to increase subsea produc-tion rates”

13Subsea Systems

It is, however, extremely difficult to predict

the detailed structure of a multiphase flow

and therefore to determine the medium

properties. Several theories are often used

to predict the type of flow regime for a mul-

tiphase flow, sometimes even supported by

measurements. With these theories it beco-

mes possible to estimate what flow structure

is likely to occur and what the actual average

velocities are for the different phases.

Erosion assessment of subsea flowlines and structures

The integrity of the subsea system and

structures is of upmost importance during

the production life of a field. Sand particles

in the produced hyrdrocarbon can lead to

erosion as it is transported from the well to

the platform or onshore facilities through

various subsea structures. Therefore it is

necessary to obtain good knowledge about

the particle travels, which can be a good

indication where erosion might occur over

time. Prior understanding of erosion in

a subsea system can help assure system

integrity by taking measures as cladding

of the subsea structures or incorporate

predictive devices to monitor erosion rates.

Dynaflow Research Group uses advanced

CFD (Computational Fluid Dynamic)

techniques in combination with several

recent erosion correlations, to reliably predict

erosion rates in subsea systems. The regions

where maximum erosion (“hot spots”) is

likely to occur under complex flow behavior,

i.e. several multiphase flow regimes can be

highlighted.

CFD is often used as well for various multip-

hase flow problems related to subsea pro-

duction systems like slug catchers, separa-

tors, mixing vessels among others.

DRG is able to assist you using all of the above

mentioned techniques to verify and improve

the performance of the subsea production

system related to flow assurance issues.

14 Subsea Systems

15Subsea Systems

Pipelines, flowlines and risers are crucial in

production of oil and natural gas. Unreliable

fluid flow can affect the productivity of the

whole production process. Particularly since

flowlines and risers often deal with complica-

ted multiphase flow conditions, a sound ana-

lysis and thorough understanding of the riser

design is of vital importance to keep produc-

tion high and to minimize losses.

Structural integrity of rigid and flexible risers

A subsea production system consists of con-

ductor pipes connected to floaters on the

surface on one end and to the wellhead at

the seabed. There are essentially two types of

subsea risers: rigid risers and flexible risers.

Alternatively, an hybrid riser is constructed

by combining the two.

The rigid risers forms an extension from the

flowline that is hung from the platform in a

Using■a■broad■Subsea■Engineering■experience,■DRG■provides■high■quality■support■for■the■design■verification■of■subsea■risers■and■flowlines

simple catenary, SCR (Steel Catenary Riser).

The rotational movement between riser and

the platform is equiped with flexible joints to

cope with the movement. The second type of

rigid riser is called TTR (Top Tensioned Riser)

and consists of a long circular cylinders con-

necting the seabed to the platform. These

risers are subject to steady currents with va-

rying intensities and oscillating wave flows. At

the top, these TTRs are equiped with tensio-

ners to maintain the angles of the riser pipes.

Flexible risers are constructed using multiple

layer composite pipes with relative bending

stiffness. Layers of different types of steel

(stainless and carbon) are used to provide

sufficient strength, while adding enough

flexibility. Flexible risers are successfully ap-

plied in deep and shallow water flowline sys-

tems.

For the preparation of FEED (Front End En-

gineering Design) documentation, detailed

Subsea Risers and Flowlines

16 Subsea Systems

strength and fatigue assessments of both

rigid and flexible risers are performed to ve-

rify:

■ Extreme response satisfying API 2RD

and extreme rotation for the flexible

joints,

■ Fatigue life related to Vortex Induced

Vibrations and oscillating Wave loading,

■ Interference with the floater constructi-

ons.

Prevent risers slugging

The very nature of risers, vertical lines with

multiphase flow conditions, makes riser lines

and the connecting flowlines highly suscepti-

ble for slugging. The slugs in a riser or vertical

part of a pipeline are often created near the

lowest point of the line when the liquid in a

gas-liquid flowline fills the complete cross-

section of the pipe. Consequently, the gas

pressure will drastically increase behind the

liquid, such that the liquid slug is blown out

of the riser.

The slugs, developed in a riser, have the ten-

dency to grow in size when travelling up the

riser. These slugs can grow as long as many

hundreds of meters with periods in the or-

der of hours. The slugs can grow even longer

than the riser itself, a condition called severe

slugging. Liquid carryover or problems rela-

ted to liquid- and pressure-control problems

might occur when long liquid slugs and gas

surges are processed.

DRG assists its clients to avoid or minimize

slug problems by accommodating your riser

design, for example by applying choked flow,

or with the design verification of separator

vessels.

Critical transient flow scenarios

Considering a riser line landing on a platform

with the riser exposed to slug flow. A valve

protects the platform from pressures excee-

ding a designed maximum. Different critical

scenarios need to be defined of to pressure

surges due to sudden valve closure related to

the characteristic time and thus the length of

the slug and hence the slug volume.

For valve closure times longer than the cri-

tical closure time the valve is called “slow

closing” and the reflected pressure wave

reaches the valve before it is fully closed. For

times shorter than the critical closure time

the valve is “fast closing”. The resulting pres-

sure surges can be calculated by considering

the deceleration of the fluid. Additionally,

Source: Total

17Subsea Systems

very high pressures are able to develop when

an amount of gas is trapped and compressed

in between the slug and the valve.

DRG assists its clients by performing compre-

hensive riser studies. Often corrugated pipes

are susceptible for excitation of the longitu-

dinal acoustic modes of the pipe due to the

multiphase flow, so called ‘singing risers’.

Sometimes, a full CFD analysis is required to

study vortex shedding and slug build-up in

flexible risers and transportation lines.

Collapse, buckling and fatigue of pipelines and risers

Buckling is a failure mode characterized by

structural instability in a compressive stress

field. Since subsea flowlines are subjected

to high external pressure, special attention

should be given to collapse and buckling cri-

teria.

These criteria depend on the diameter to wall

thickness ratio, pipe imperfections and load

conditions such as axial tension, bending

stress and external pressure. A good know-

ledge of these criteria is required to arrive at

a robust pipeline design, which exhibits suf-

ficient safety against buckling.

Flowline stability design from sub-sea tree to processing facility

In subsea production it is vital to ensure that

hydrocarbons are transported in stable pipe-

lines. Here vertical and lateral stability may

be critical. Regarding vertical stability, not

only floatation problems of the pipeline are

to be considered, but also sinking problems.

When a pipeline is running over a sandy se-

afloor, liquefaction of the soil might result in

too deep settling of the pipeline in the seaf-

loor sediment.

A lateral stability analysis is performed in all

waters subjected to large current and waves,

DRG perfoms these analyses often in compli-

ance with the appropriate codes (i.e. DNV-

RP-F109) to perform an absolute lateral stabi-

lity analysis, a the generalized lateral stability

analysis or a full dynamic lateral stability ana-

lysis.


18 Subsea Systems

19Subsea Systems

DRG■provides■consulting■services■and■engineering■solutions■to■improve■Subsea■Production■Systems

What can we do for you?

What can we do for you?

Due to our highly qualified staff and expe-

rience in this field, DRG is often required to

solve problems involving subsea production

systems, subsea flowlines from wellhead to

fixed platform, FPSO or onshore facilities,

and attached equipment like separators and

slugcatchers.

Dynaflow Research Group can assist you with

the following engineering assignments:

1. Analytical multiphase flow regime and

heat transfer assessment,

2. Pressure drop calculations,

3. Transient multiphase and thermal flow

analysis of subsea flowlines and risers,

4. Predicting hydrodynamic and terrain/

riser induced slugs,

5. Development of the operating envelope

cold and hot start-up, cooldown blow-

down and warm-up,

6. Detailed flow studies using Computati-

onal Fluids Dynamics of slug catchers,

separators and multiphase subsea flow-

lines and equipment,

7. Analysis of the mechanical integrity of

the subsea manifold, including detailed

mechanical response analysis and mul-

tiphase flow behavior,

8. Proper design of subsea equipment like

separators, subsea wellheads and trees

to increase drilling and production effi-

ciency.

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. We liaise with the

client to ensure we have the most accurate

information to conduct our analyses and to

ensure the clients remain closely involved.

20 Dynaflow Research Group

At■Dynaflow■Research■Group■(DRG)■we■support■our■clients,■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- chemical industry. Our work often re-

quires a multi-disciplinary approach where

we combine expertise in fluid flow behaviour,

dynamic oscillations, FEM and stress analysis

with sophisticated analysis software to pre-

dict system performances.

Training

DRG offers a wide range of training courses

such as software training, fiberglass training,

dynamics and stress training. Most of these

training courses are offered on a regular ba-

sis during the year. We also develop custo-

mised training programs with our customers

fit to their specific needs.

Products

DRG has been developing software for many

years, which has resulted in several commer-

cially available software packages such as

BOSfluids®, BOSpulse®, Jive and Hades. We

also provide technical consulting services,

and develop numerical software that can be

used in computer simulations and other ty-

pes 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 collabo-

ration with Paulin Research Group and their

Houston test facilities (www.paulin.com).

Dynaflow Research Group provides support

to clients with their R&D to help them conti-

nuously improve their products.

21Dynaflow Research Group

Topic specific brochures:

• Consulting Service Series• Software Product Series• Training Series

Visit our website www.dynaflow.com or send an e-mail to [email protected]

Houtsingel 95 2719 EB Zoetermeer The NetherlandsReg nr. 27320315

T F E W

+31 79 361 5150+31 79 361 [email protected]

Subsea Systems

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