mwa report

STUDENT INDUSTRIAL PROJECT (SIP) REPORT

JANUARY 2015-AUGUST 2015

TECHNIP GEOPRODUCTION (M) SDN. BHD

Student’s Name : KEE RI HONG

Matric No : 16259

Programme : Civil Engineering

Host Company : TECHNIP (M) SDN. BHD. (K. L)

Company Supervisor : Anthony Musto

UTP Supervisor : Nurul Izma Mohammed-Dr

ACKNOWLEDGEMENT

First and foremost, I would like to thank Technip Geo production (M) Sdn. Bhd from the bottom

of my heart for providing me this amazing and wonderful opportunity to have my industrial

training experience with a company as prominent as this. Besides, I would like express my

sincere appreciated to Mr Anthony Musto, the Discipline Leader of Subsea Structural

Engineering Division, my host company supervisor and Dr Nurul Izma Bt Mohammed, my UTP

Supervisor for their willingness to share their knowledge and expertise to make my internship

experience become more valuable and wonderful.

Furthermore, I would like to extend my utmost gratitude to my host company supervisor, Mr

Anthony Musto and Mr Muhammad Khafif Zol Azlan, senior structural engineer for being my

mentor, teaching and guiding me throughout my internship period. The knowledge that I have

gained throughout my internship period is essential and significant in moulding me into

becoming an experienced structural engineer. Apart of that, I would like to appreciate their

patience in teaching and guiding me toward achieving the goals that have been set throughout

my internship period.

Last but not least, I would like to appreciate all the staff at Technip Geo production (M ) Sdn.

Bhd. for becoming the important people in helping me make this industrial training enjoyable

and memorable. I would like to thank you to everyone here for being part of my industrial

training journey.


ABSTRACT

A Mid Water Arch (MWA) is one of the buoyant subsea structures that tethered on the mean sea

level by certain buoyancy forces. Hence, understanding the responses and stresses of the MWA

due to the effect of pressure that acting on the surface of the tank and the existing uplift force

while the buoyancy tank is submerged into the seawater is important. In this study, these areas

have been investigated through both numerical simulations by Finite Element Analysis (FEA)

software, ABAQUS 6.14 and mathematical calculations based on ASME Section VIII Division 1

& 2. In order to carry out the FEA analysis testing, it is necessary to perform the preliminary size

of buoyancy tank calculation based on ASME Section VIII Division 1. After obtained the

geometry of the buoyancy tank, the MWA model will be modelling in ABAQUS. There are three

types of simulations have been performed by FEA analysis in order to perform the unity checks

(UC): axial compressive stress testing, bending stress testing and combined stresses testing. The

FEA analysis testing results are use for comparison and validation of the mathematical

calculations based on ASME Section VIII Division 1 & 2. The findings from this study have

shown that the results obtained from numerical simulations aligned with the results obtained

from mathematical calculations.

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Chapter 1

1. Introduction

1.1 Introduction of Student Industrial Internship Program (SIIP)

Student Industrial Internship Program (SIIP) is a compulsory program for every

student who is undertaking the Bachelor’s Degree at Universiti Teknologi

PETRONAS (UTP). Duration of this Student Industrial Internship Program is

approximately 28 weeks which is 7 months. This program is carried out within two

semesters. Students are required to take part in Student Industrial Training (SIT) for

the first semester, roughly around 14 weeks while for the remaining 14 weeks,

students are required to undergo Student Industrial Project (SIP). This is the full cycle

of the Student Industrial Internship Program (SIIP). The main objective of this SIIP is

to get to know the real working environment. Throughout this exploration, students

are able to relate their theoretical knowledge to practical application in the real

industry. This program will be able to develop skills of students in various areas,

which include safety practice, work ethics, communication, teamwork, management

and even technical expertise. Students will be able to obtain working experiences and

skills while performing their tasks during their internship period.

There are several objectives of the industrial project program according to the Student

Industrial Project Guidelines for Students are as follows:

To apply and integrate the knowledge especially theoretical knowledge in the

industry. The objective of the industrial project is to provide a chance for

students to apply theoretical knowledge learned in the university into the real-

world working environment. Therefore, the student will have a better

understanding of the knowledge learned by having hands-on experience

working with the industrial practitioners.

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To enhance the analyzing skills especially in the complex engineering and

technical projects or problems. There will be an attached project assigned to

the students during their internship period. Within the internship period,

students will be exposed to the real working environment. Hence, students

will be able to learn to work in a team and independently to solve and

manipulate the assigned project and tasks given by the host company.

To enhance the soft skills of students in term of evaluating and proposing

solutions for given tasks and projects. In this case, students will have the

chance to work among engineers from difference background. Students will

have the chance to join with the professional to overcome engineering-based

problems arise throughout the project. Throughout the project, students will be

able to gain valuable experience and experience while working. Hence,

students will be able to enhance their confidence level and communication

skills throughout the journey.

While performing the project assigned by the supervisor of the host company,

students will have the opportunity to apply their communication, practical and

technical skills gained to potential employers. Students will be able to practice

their engineering activities with engineers.

During the internship period within 28 weeks, students are required to complete

the logbook report assigned by UTP. The written logbook report will be checked

and assessed by the host company supervisor. The last few weeks before the

second semester of the internship, every student will be assigned a UTP

supervisor. Furthermore, students will be required to compile the important

information into a Student Industrial Project Report. Students are required to

present the project to the UTP supervisor and Host Company supervisor during

the SIP Assessment Visit.

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1.2 Introduction of Student Industrial Project Report

Student Industrial Project Report is the summary of all the relevant information of the

task and project that has been assigned to the student within the second semester of

the Student Industrial Internship Training. This SIP report will consist of 6 parts. The

contents of this report will be totally depending on the project that the student has

assigned.

The author was handling the design of Mid Water Arch (MWA) Tank and Structure

system. The Mid Water Arch system includes buoyancy tank, gutter, bulkhead and

gravity base. While designing a Mid Water Arch Tank and Structure system, all the

design parameters were considered and the structural integrity check has to be

performed. All selected equations and figures were taken from references mentioned

at the end of this report.

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1.3 Objectives

Objective of this study is to investigate and analyze Finite Element Analysis (FEA)

ABAQUS when it is used to evaluate maximum allowable applied stresses of a Mid

Water Arch (MWA) system as a test specimen and to investigate the induced stresses

while various types of loadings applied on the system. The results obtained from FEA

ABAQUS will be compared with the results obtained by hand calculations based on

ASME Section VIII Division 1 &2. A Mid Water Arch system will be modeled in

FEA ABAQUS. As it was being loaded by various types of loadings in order to

determine the induced stresses and maximum allowable applied loadings. Multiple

analyzes will be investigated are listed as follows:

1. Mid Water Arch (MWA) system under internal pressure.

2. Mid Water Arch (MWA) system under external pressure.

3. Mid Water Arch (MWA) system under design pressure.

4. Mid Water Arch (MWA) system under buoyancy force/uplift force.

5. Mid Water Arch (MWA) system under design pressure and uplift force.

The objective of these analyzes is to obtain various types of unity checks (UC) based

on the induced stresses on a MWA system under various types of conditions by using

FEA ABAQUS. By placing different types of loadings on a MWA system, it is easier

to obtain the induced stresses created by the applied loading and maximum induced

stresses created by the maximum allowable applied loadings. Hence, the unity check

(UC) based on FEA ABAQUS analysis can be calculated.

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1.4 Scope of Study

This study focuses on Finite Element Analysis (FEA) calculation for the design of

Mid Water Arch (MWA) tank and structure. In normal cases, ASME analytical

calculation is used for the design of Mid Water Arch. Therefore, short section on

ASME analytical calculation is included.

A comparison between ASME analytical calculation and FEA calculation for design

of Mid Water Arch tank and structure and structural integrity is carried out in this

study.

1.5 Problem Statement

As global crude oil price continues to drop, the world is in need of low cost

harnessing alternatives to supplement a tremendous dropping price of crude oil price.

In recent times, many developed countries have pushed for low cost harnessing

alternatives which can reduce the production cost of fossil fuels especially crude oil.

The marine structures are commonly used in harnessing the fossil fuels especially

crude oil. Such marine structures technologies are still at their infancies and present a

huge engineering challenge. Therefore, there are some guidelines, design codes,

research and engineering practice have been developed in recent year in order to

improve the structural integrity of the marine structures and yet reduce the production

cost. The Finite Element Analysis (FEA) has been famous for structural integrity

analysis especially on strength and fatigue analysis of marine structures. Therefore, in

order to optimize the design of MWA system especially on structure integrity, Finite

Element Analysis (FEA) is chosen to perform the structure integrity check of MWA

system.

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1.6 The Relevancy of the Project

Mid Water Arch (MWA) system is commonly used as a subsea buoyant to support

the flexible risers or umbilical in a designed riser’s configuration. A comparison

between the ASME Analytical Calculation and FEA Calculation for the design of

MWA Tank and Structure is performed in this study. This study will always be

relevant to what the author has previously studied because MWA system is a marine

structure that is used as a structure that can transmit loads from the riser to the gravity

base. This loads transmission is relevant to the subject that the author studied before,

which is: Structural Analysis.

1.7 Feasibility of Project

The duration of Student Industrial Project (SIP) is 14 weeks. The training schedule of

the author for the entire SIP period was constructed based on this project.

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Chapter 2

2.0 Background and Literature Review

2.1 MWA SYSTEM DESCRIPTION AND DEFINITIONS

A Mid Water Arch (MWA) is one of the buoyant subsea structures that are tethered

on the mean sea level by certain amount of buoyancy force. MWA is commonly used

to support an array of the flexible riser and umbilical to perform a designed riser

configuration. Thus, the cumulative riser’s tension can be reduced at the both end of

the riser and umbilical. This is able to protect the integrity of the supported flexible

riser and umbilical. Besides that, MWA is designed to maintain the allowable riser’s

curvature and avoid the clashing or entanglement of riser without affecting the

touchdown point of the riser. The MWA is also used to maintain a constant

touchdown point. Hence, the flexible riser and umbilical system will be able to

achieve a designed configuration such as: lazy-S or steep–S riser configuration. Both

types of the riser’s configurations are commonly used in conjunction of Floating

Production Storage and Offloading (FPSO) facilities.

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2.2 MWA Riser Configurations

Flexible riser is designed with certain flexibility to cope with floater motions. The

flexible riser will be designed with certain riser configuration such as Lazy-S, Plaint-

S or Lazy-wave configuration to absorb floater motions. The flexible riser is designed

to be having high dynamic resistance. Therefore, it can be operated under deep and

harsh environment.

Lazy-wave Riser Configuration

Lazy-wave riser configuration is commonly used in place with many FPSOs around

the world. In this configuration, the flexible riser will be supported with multiple

small buoys. The lazy-wave configuration is used to minimize riser movement from

wave and current action. Normally, setting lazy-wave riser buoyancy at fifty percent

more than the total depth as the depth increase beyond 1000 meter will leave

significant weight applied to the turret.

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Figure 1: Lazy Wave Configuration

Lazy-S and Steep-S Configuration

Lazy-S configuration is created by a tethered buoy by separating flexible riser into

lower J-catenary and upper U-catenary. In this type of configuration, the tethered

buoy can used to solve the problem encountered with the touchdown point. The

subsea buoy can be used to absorb the tension induced by the floater. Hence, the

touchdown point will be eventually experienced only a little or no tension variations.

A lazy-S might be facing compression problems at the riser touchdown if large vessel

motions happened.

A lazy-S configuration required a complex installation procedure, therefore lazy-S

configuration will be only considered if catenary and wave configuration is not

suitable for that particular field. A Mid Water Arch system which consists of mid-

water arch, tether, and tether base is required for this type of configuration.

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Figure 2: Steep S configuration

Figure 3: Lazy S configuration

Pliant Wave Configuration

Pliant Wave configuration is almost similar as steep wave configuration where a

subsea anchor is applied in this configuration. Subsea anchor is used to controlling

the touchdown point. In this configuration, tension in the riser will be transferred to

the anchor and not to the touchdown point. Hence, this configuration is able to

accommodate a wide range of bore content densities.

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Figure 4: Pliant Wave Configuration

Pliant S Configuration

Pliant S configuration is normally applied for shallow water applications where large

vessel offsets are difficult to accommodate. Plaint S configuration consists of a lower

tether and clamp attached to the riser just above the touchdown point. The dynamic loads

in the riser at touchdown are able to be reduced by lowering the tether. Bend restrictors

will be used in this configuration to prevent bending.

The MWA divides the risers into upper and lower catenary section. Besides, MWA is

able to provide the riser system with sufficient compliance to accommodate surface

vessel movements. The MWA is mainly designed to provide sufficient net buoyancy to

support the risers and umbilical in their designed configuration. The spacing between the

individual supported riser and umbilical can be maintaining by providing the lateral

guidance. The clamp at the top of the MWA is used to fix the riser on top of it. The

gutters are used to ensure the Minimum Bending Radius (MBR) is achieved for each

riser.

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Figure 5: Pliant S Configuration

2.3 MWA, Tether, and Tether Base General Arrangement

A MWA system is combined a MWA, tether, and a tether base. The buoyant MWA is

tethered to a tether base at the seabed. A MWA system will be discussed in this

section. A MWA system arrangement is shown in Figure XXX below.

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Figure 6 : General Arrangement of a Typical MWA System

2.4 MWA Description

A Mid Water Arch (MWA) is designed to support flexible risers at the seabed.

Therefore, the designed mid-water arch system needs to have sufficient net buoyancy

to support the flexible risers at their designed configuration. Each MWA made up of

two buoyancy tanks. The buoyancy tanks are used to support gutters with a clamp

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Lateral Deflectors


housing that the riser is locked into. The key features of the MWA are shown in

Figure XXX. The main components of the MWA are described in the following

section.

Figure 7: Mid Water Arch (MWA) Description

Buoyancy Tanks

The tanks are made up of a cylindrical body and two ellipsoidal heads. They

provide sufficient net buoyancy to ensure a restoring force. Hence, the MWA

response can be satisfied all the riser performance requirements and avoid

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Lateral Deflectors


any slack in the tether.

Support Gutter

The support gutter is a series of the curved plate. The curved plates are

designed for the riser or umbilical to rest. The curvature is designed to ensure

that no riser will be subjected to a curvature below the applicable operational

Minimum Bending Radius (MBR). Separate arch plates are normally located

on each side of Mid Water Arch. A gap at the 12 o’clock position at the

support gutter will be provided for flexible product clamps.

Bulkhead

Bulkhead is a vertical plate which welded to the buoyancy tanks. The

bulkhead is used to support the gutter at a regular interval.

Guide System

The guide system is a system where located on top of MWA and is used to

aid the installation of the risers on the MWA. The guide system is used to

maintain clearances between adjacent risers and is used to transfer lateral

loads applied on the risers to the MWA during operation.

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Tethering Point

Each MWA is held at its given position by two mooring chains that are set up

in a ‘Y’ configuration. The tethers are connected to MWA at the pad eyes

location. Normally, the pad eyes are designed to have a swivel function.

Lifting Pad Eyes

Lifting Pad Eyes is used for lifting the MWA during transportation, load out,

and installation. Normally, the lifting pad eyes will be connected to the upper

part of the gutter.

Anodes

Sacrificial anodes are used in the MWA system to provide catholic protection

over the design life of the system.

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2.5 Geometry and Layout

General

A MWA system has to be designed in a correct geometry to provide the

sufficient net buoyancy to support flexible risers in a correct manner. A

correct geometry for a MWA is able to ensure that the designed curvature is

always equal to or greater than the permitted operation Minimum Bending

Radius (MBR) to ensure that the flexible risers will work for all the possible

rotations and rotations during operation.

The parameters have to be taking care while designing the MWA are shown

follow:

Buoyancy Requirements

Operational MBR of the Supported Flexible

Design Radius of Support Gutter and Lateral Deflector

Spacing of Flexible Products across the Width of

Extent of Lateral Deflectors

Clamp Housing Allowance

Buoyancy Requirements

Dynamic analysis of a complete MWA system has to be performed to obtain

the net buoyancy requirement for a MWA system. There are two main

constraints have to be focus on while performing dynamic analysis, which

are avoidance of slack mooring tether condition and the buoyancy provided

have to be sufficient to restore the forces required while the flexible riser is

operating. The net buoyancy force can be calculated from the equation

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provided as below.

The net buoyancy provided by the MWA will be vary over time due to

several factors such as marine growth, anode depletion and water absorption

by buoyant materials, such as syntactic foam.

These factors have to be accounted for in establishing the generated MWA

Net Buoyancy at the early stage of installation and at the end of the specified

design service life. The value of the MWA net buoyancy has to be at least the

minimum required MWA net buoyancy at the end of the service life to

prevent failure occurred while servicing.

The manufacturing tolerances on mass have to be clearly understood while

designing the MWA system. During the manufacturing process, the

manufacturing tolerances on mass have to be diligently monitored. The

manufacturing tolerances on mass are one of the critical factors to both the

installation process as well as the operating performance of the MWA.

Hence, upper and lower bound of the MWA system mass have to be

estimated and should be maintained throughout the manufacturing process to

ensure the minimum required buoyancy is achieved.

Operational MBR of the Supported Flexible

According API Specification for Unbonded Flexible Pipe 17J, the behavior

of the flexible will become less dynamic if the flexible is in contact with the

MWA support gutter compared to the unsupported catenary. Therefore, the

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MWA Net Buoyancy (Te) = MWA Displacement (Te) – MWA Self Weight (Te)


operational MBR of the flexible riser can be reduced at the supported region.

The current edition of API Specification for Flexible Pipe Ancillary

Equipment 17L1, section 9.3.3 states that the “3D bending radius of the

supported flexible pipes on the gutters have to be at least 1.25 times the

largest storage MBR of the supported flexible pipes”. In this context, the

gutter will be equal to the support gutter. Correspondingly, according to the

current edition of API Specification 17J table 9 states the dynamic-supported

operational MBR is 1.1 times the storage MBR.

However, both values are only recommended minimums. Therefore, a larger

value may be required to be set for the specific riser or umbilical depending

on the environmental exposure and flexible construction. In order to increase

the required crush resistance of the flexible over the MWA, it may require

increasing the radius. However, a tight radius will negatively impact on the

service life and the fatigue resistance of the flexible riser.

Design Radius of Support Gutter and Lateral Deflector

The design radius of support gutter and a lateral deflector is depending on the

MBR. The design radius shall not be less than the permitted operational

MBR. The design radius of the support gutter plate will be depending on the

largest value of permitted operational MBR substrate the outer radius of the

flexible product.

The design lateral deflector radius is depending on the permitted operational

MBR of the flexible riser. According to API Specification 17J, the minimum

design radius of support gutter and lateral deflector is 1.5 times the storage

MBR. A larger value might be required if the environment is harsh.

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Spacing of Flexible Products across the Width of Support Gutter

The width of support gutter is provided to ensure support to the furthest most

flexible product in their worst off lead conditions. In order to minimize the

MWA plan area, width and radius of support gutter should be minimized.

Therefore, the hydrodynamic forces can be reduced during installation and in

service. The mass to lift the MWA and space required during installation of

MWA can be reduced as well. In order to minimize the effect of different

sizes and tension variation, the spacing and arrangement of the flexible

products over the MWA should be chosen.

Extent of Lateral Deflectors

The arrangement of the lateral deflector is mainly used to manipulate and

control the lateral movements of the flexible products and the MWA.

Dynamic analysis has to be performed as part of the MWA system design

process to calculate the extent of the combined movement of the MWA

system.

Clamp Housing Allowance

At the initial stage of the design, finalized flexible product clamp dimensions

are not available. Therefore, the mid-water arch structure is initially sized an

allowance for the clamps. There is a rule and guideline to follow and apply

the early design phase for the allowance for the clamps. Since the allowance

of the clamps will have a direct effect on the plan area of the mid-water arch

structure, therefore a more accurate value for the length of the clamp is

known, the required gap should be redefined.

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2.6 Component Design

General

The design of the following components will be described in detail:

Types of Buoyancy

Buoyancy Tanks

Lift Points

Support Gutter

Tethers

2.6..1 Types of Buoyancy

Steel Tank Buoyancy

The buoyancy tank is made up of ring stiffened cylinders with tori

spherical dome ends. MWA has operated in shallow water depth. The

hydrostatic pressure will be generated at the shallow water where were

low enough to allow a lightweight tank structure to generate the required

buoyancy force sufficiently. Nitrogen will be pre-charged into the

buoyancy tank to prevent buckling occurred where the pressure different

between the tank and its internal pressure can be reduced.

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2.6..2 Buoyancy Tanks

Steel buoyancy tanks are designed according to the ASME Section VIII. They

are generally made up by a ring stiffened cylinder and torispgerical dome

ends. There are several considerations needed to be addressed in buoyancy

tank structural design which are as follow:

Tank Outer Diameter and Tank Length

The outer diameter of the tank will influences displacement and

buoyancy of the tank. However, this parameter is the most expensive

parameter since increasing of outer diameter will eventually increases

the following parameters:

Shell plate area

Girth weld lengths

Dome end sizes

Ring frame lengths

Ring frame number sizes to resist buckling collapse

Conventional manufacturing limits on torispherical dome ends will be

chosen if the outer diameter less than 7.0m. Diameter beyond this range, a

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special manufacturing method is required.

The minimum length of the tank is depends on the spacing of the risers

across the MWA. However, the maximum length of the buoyancy tank

limited by practicalities of installation.

Design Pressure Selection

Design pressure will influences requirement of tank shell thickness and

stiffening requirements. Nitrogen will be pressurized into the buoyancy

tank in order to reduce the differential external pressure at depth.

Dynamic pressure component due to wave passage must be considered

while designing the design pressure.

2.6..3 Lift Point

Lift point design for MWA should be designed in accordance with DNV OS

H205.

2.6..4 Support Gutter

Support Gutter Webs

Deep plate webs are generally used as support gutter plates and are

connected to the buoyancy tanks. The deep plate webs are spread

longitudinally to coincide with the tank ring frames.

Support Gutter Transverse Stiffening

Transverse stiffening is designed to support the arch plate between the

main support webs. The design of the transverse stiffening is used to

resist the radial line load created by riser tension acting over the MWA.

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2.6..5 Tethers

Weight, fatigue performance, creep, wear resistance, handling and stiffness of

tether must be considered properly while designing tether for MWA system.

Section 10 of API 17L1 [C3] and API RP17L2 [C4] are the design and

material guidance for tethers. In normal cases, traditionally steel chain is

selected for tether design on MWA system.

2.7 MWA System Design

2.7..1 Design Process

Flexible, Structural and Geotechnical disciplines are involved while designing

the MWA system.

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Finalize Installation Method and Procedure

Geotechnical Design-Detailed geotechnical design-Finalize dimensions and masses of foundation components-Installation assessment-Establish permitted scour depth

Detailed Structural Design-MWA Base & Tether Design-Strength and Fatigue Design-Installation Analysis-Load out analysis-Seafastening Design-Corrosion Protection

Flexible Pipe Service Life Analysis

Dynamic Analysis of catenary system

Select Options

Preliminary MWA DesignPreliminary Tether Base Design

Selection of Riser Configuration and Pipe Structure

Geotechnical DesignStructural DesignFlexible Design


Figure 8: MWA System Design Process

The activities outlined the design process of MWA system. The design process will

be explained in the following sections.

2.7..2 Flexible Product Design

A complete design of the flexible products will be presented by the flexible

product designer. The permitted operational MBR and net buoyancy

requirement to be used in MWA will be presented at this stage.

2.7..3 Dynamic Analysis

A complete dynamic analysis for the proposed riser configuration will be

performed. The flexible design process is very essential because this process

will output many important design requirements of the MWA system.

Sensitivity studies shall be considered to quantify the effect of assumptions

such as: Marine growth and hydrodynamic factors on MWA design.

All the in-place operational load conditions must be defined while performing

the dynamic analysis.

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2.8 Structural Assessment

Key considerations involved in the structural assessment of MWA system will be

outlined in this section. The components of MWA system require to be assessed

based on standard design codes.

2.8..1 Strength Assessment

Buoyancy Tank Design Assessment

A typical steel buoyancy tank shall be designed based on ASME Section VIII

Division 1&2. ASME Section VIII Division 1&2 provides both an analytical

calculation approach to verify the buoyancy tank design and its structural

integrity. Finite Element Methods (FEM) shall be performed to verify the

structural integrity of the buoyancy tank design.

2.8..2 Design Load Cases

Structural integrity of Mid Water Arch (MWA) system has to be checked for

the load conditions such as pressure testing, onshore lifting and load-out, sea-

fastening and transportation. The in-place conditions should be taking into

account as well due to the effects of buoyancy and riser system load.

2.9 Finite Element Analysis (FEA)

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2.9..1 Introduction

Finite Element Analysis (FEA) is a computerized method that used to predict

how a product reacts while differences types of load conditions applied on the

model. Finite element analysis shows the responses of the product while the

existing loadings applied on it. There are two general types of FEA analysis

which are commonly used in industry: 2-D modeling and 3-D modeling.

While 2-D modeling is a simplified model that allows the analysis to run on a

normal computer. The result obtained from a 2-D modeling is normally less

than the accurate result. However, 3-D modeling provides a more accurate

result.

Finite Element Analysis works by breaking down a real object into thousands

to hundreds of thousands of finite elements. Mathematical equations will be

used in this FEA analysis to predict the behavior of each element. All the

individual behaviors will be added up by the computer system to predict the

actual behavior of the object.

Finite Element Analysis (FEA) will be able to predict the behavior of products

affected by various types of physical effects, which are:

Mechanical stress

Mechanical vibration

Fatigue

Motion

Heat transfer

Fluid Flow

Electrostatics

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Plastic injection molding

Chapter 3

3.0 References

3.1 General

The following section outlines the reference documents for this study.

3.2 Project References

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No Document Title

1 MWA SYSTEMS AND GRAVITY BASE DESIGN REPORT


3.3 Codes and Standards

The following is a list of design codes and standards used for this study.

3.4 Software

No Software Application

1 ABAQUS 6.14 Finite Element Analysis

2 MathCAD 14.0 Analytical Calculations

Chapter 4

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No Description Code No

1 Boiler & Pressure Vessel Code, Section VIII-

Rules for Construction of Pressure Vessels

ASME VIII

2 ASME Boiler and Pressure Vessel Code –

Materials

ASME II, Part D


4.0 Methodology and Approach

Overview

This section outlines the overview of this study: “Comparison between ASME

Analytical Calculation and FEA Calculation for the Design of Mid Water Arch Tank

and Structure”. A brief introduction on Mid Water Arch Tank and Structure, ASME

Analytical Calculation and FEA Calculation will be presented in this section.

In addition, the calculation steps of preliminary design of Mid Water Arch system

based on ASME Section VIII Division 1 & 2 will be discussed in this section. The

modelling steps and structural integrity check of MWA system performed in FEA

ABAQUS will be outlined in this section.

4.1 Introduction

The title of this study is “Comparison between ASME Analytical Calculation and

FEA Calculation for the Design of Mid Water Arch Tank and Structure”. Throughout

this study, two types of calculations will be shown, which are ASME Analytical

Calculation and FEA Calculation. ASME Analytical Calculation is a more

conservative method for MWA system design compared to FEA Calculation because

ASME Analytical Calculation is only focus on 2 dimensional (2D) whereas FEA

Calculation is focus on 3 dimensional (3D). In a 2D model, the loads only can be

transmitted in two directions with a maximum of 4 degrees of freedom whereas in a

3D model, the loads will have more pathways to transmit because in a 3D model, the

loads can be transmitted in three directions with a maximum of 6 degrees of freedom.

Therefore, in this study, the ASME Analytical Calculation for the design of Mid

Water Arch Tank and Structure will be performed. The aim of this ASME analytical

calculation is to obtain the preliminary design of buoyancy tank of MWA system. In

order to ensure that the designed buoyancy tank has the ability to perform at the

optimum level throughout the life service, a series of unity checks will be performed

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in this analytical calculation based on ASME Section VIII Division 1 & 2.

In addition, in this study, a dimensional MWA system based on ASME Section VIII

Division 1 & 2 will be modeled in the Finite Element Analysis (FEA) software,

ABAQUS. Various types of load conditions will be tested on the MWA system. The

outputs of the finite element analysis will be used for strength and fatigue analysis of

MWA system. A structural integrity check will be performed by using the outputs

obtained from FEA analysis.

Lastly, a series of outputs from ASME Analytical Calculation and FEA Calculation

will be tabulated into a table. The comparison between both calculation methods will

be discussed in detail.

4.2 Steps of ASME Analytical Calculation for the Design of MWA Tank and

Structure

Introduction

ASME Section VIII Division 1&2 are used for the design of MWA tank and

structure. The general steps for the design of MWA system will be outlined below.

Step 1 Determine the required volume of the buoyancy tank based on the required buoyancy data and environmental data

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Density= massvolume

[Equation 1]

Where

density is the density of the submerged medium (seawater)mass is the gross buoyancy forces requiredvolume is the volume of the buoyancy tank


Step 2 Determine the required length and external diameter of buoyancy tank based on the required volume obtained from equation 1

Figure 1 Geometry of the buoyancy tankStep 3 Selection of buoyancy tank thickness*Note: The process of selection has to take fabrication wise into consideration

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VolumeN

=πD2

4L [Equation 2]

Where

volume is the required volume of the buoyancy tankN is the number of the buoyancy tankD is the external diameter of the tankL is the length of the buoyancy tank


Step 4 Determine the geometry of ellipsoidal based on the thickness of the tank

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Note: The calculation is performed based on the equations stated above


Step 5 Determine the allowable external pressure of the buoyancy tank based on the thickness of the tank

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Step 1

Determine the elastic buckling stress based on ASME Section VIII DIvision 2 equation 4.4. 19

Step 2

Determine the predicted buckling stress based on ASME Section VIII DIvision 2 equation 4.4. 25 - 4.4.27

Step 3

Determine the design factor based on ASME Section VIII DIvision 2 equation 4.4. 1 - 4.4.3

Step 4

Determine the allowable external pressure Determine the design factor based on ASME Section VIII DIvision 2 equation 4.4. 28


Step 5 Determine the allowable internal pressure of the buoyancy tank based on the thickness of the tank

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P=SE x ln( 2tD

+1) [ASME SECTION VIII DIVISION 2 Equation 4.3.1]

where

t is the required thicknessD is the internal diameter P is the allowable internal pressureE is the joint efficiency based on TABLE 7.2 from ASME VIII Div II

S is the Tensile strength (at design temperature)


Step 6 Determine the design pressure of the buoyancy tank based on the environmental data given

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Step 1

Determine the minimum allowable internal pressure

Step 2

Calculate the external pressure based on the environmental data given

Step 3

Determine the design pressure by using the equation belowDesign Pressure = External Pressure - Internal Pressure


Step 7 Perform an Unity Check (UC) based on Axial Compressive Stress Acting Alone

Step 8 Perform an Unity Check (UC) based on Compressive Bending Stress

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Assumptions to be considered: 1. Lateral unbraced length of cylindrical member is assumed to be subjected to buckling.2. Both pad eyes locations are assumed to be pinned.

UC (compressive stress )= Applied Compressive StressAllowable Compressive Stress


Step 9 Perform an Unity Check (UC) based on Shear Stress

Step 10 Perform an Unity Check (UC) based on Axial Compressive Stress and Hoop Compression

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Assumptions to be considered: 1. Both pad eyes locations are assumed to be fixed

UC (bending stress )= Applied bending StressAllowable Bending Stress

UC (shear stress )= Applied shear StressAllowable shear Stress

UC (compressive axial design Stress )= Applied compressive axialdesign StressAllowable compressive axial design Stress


Step 11 Perform an Unity Check (UC) based on Compressive Bending Stress and Hoop Compression

Step 12 Perform an Unity Check (UC) based on Shear Stress and Hoop Compression

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UC (shear hoop Stress )= Applied shear hoop StressAllowable shear hoop Stress


Step 12 Perform an Unity Check (UC) based on Axial Compressive Stress, Compressive Bending Stress, Shear Stress and Hoop Compression

Important note: If one of the UC FAILED to achieve less than 1, step 3 to step 12 have to repeat by using a higher value of thickness.

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UC (all stress Stresses )= Applied all StressesAllowable all Stresses


4.3 Steps of FEA Calculation for the Design of MWA Tank and Structure

Approach

In this section, the step by step guideline to model a MWA system in FEA ABAQUS

will be outlined. The investigation of various loading conditions applied on the MWA

system will be studied in detail. The fully elastic steel material properties are loaded

into FEA ABAQUS. The end results of this study will investigate how a MWA

system in FEA ABAQUS reacts to various types of loading conditions in fully elastic

condition until it reached the yield point.

Modeling:4.3..1 Created Mid Water Arch System in FEA

Mid Water Arch system is a system made up by various components such as: bulkhead, gutter and buoyancy tank. Therefore, several steps needed to be performed in order to model a full MWA system in FEA Analysis ABAQUS. All the modeling steps of a MWA system are described in details below.

4.3..2 A MWA system is made up of:a. Buoyancy tankb. Stiffenerc. Separatord. Main & Intermediate Bulkheade. Center Bulkheadf. Gutter

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The modeling steps of each component of MWA system is described as follow:

BUOYANCY TANKThe dimensions from Figure 1 were then modeled into FEA ABAQUS. Since the buoyancy tank is in cylindrical shape, the revolve function was used in FEA. Half of the longitudinal cross section was modeled as shown in Figure 2 and revolved around the y-axis (centerline).

Figure 2: Sketch of Buoyancy tank (without stiffeners and separators)

With the sketch complete, the part is revolved around the centerline to develop the buoyancy tank (without stiffeners and separators). The stiffeners and separators will be inserted into the buoyancy tank in order to enhance the stiffness of the buoyancy tank. The revolved buoyancy tank is shown in the Figure 3.

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Figure 3: Created part revolved around center line

StiffenerThe dimension of the stiffener is obtained from Figure 1 and then modeled into FEA ABAQUS as shown in Figure 4. Since it is a circular stiffener, the revolve function was used in FEA. The cross section of stiffener was modeled as shown in Figure 4 and revolved around the y-axis (centerline).

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Figure 4: Sketch of stiffener

With the sketch complete, the part is revolved around the centerline to develop the stiffener. A modeled stiffener in FEA ABAQUS is shown in Figure 5.

Figure 6: Stiffener

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SeparatorThe dimension of the separator is obtained from Figure 7 and the separator was modeled into FEA ABAQUS. Since, the separator is a made up of a circular plane and stiffeners. Therefore, the planar function was used FEA in order to generate the separator whereas, extrude and revolve function were used in FEA to generate the stiffeners on the separator. A separator model in FEA is shown in Figure 8.

Figure 8: Separator

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Buoyancy tank with separator and stiffenerBuoyancy tank with separator and stiffener are modeled in the Assembly mode. First and Foremost, the stiffeners and separator are arranged according to the calculated spacing based on ASME Section VIII Division 1&2 in the assembly mode. Lastly, the

buoyancy tank, separator and stiffener are merged in the Assembly mode. The full model of the buoyancy tank is shown in the Figure 9 & Figure 10.

Figure 9:Buoyancy tank with separator and stiffeners

Figure 10:Half buoyancy tank

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Main, Intermediate & Center BulkheadThe dimension of main, intermediate and center bulkhead is shown on the Appendix and the main, intermediate and center bulkhead are modeled according to the given dimension. Since the bulkheads are in plane shape, therefore the planar function is selected while modeling the bulkhead. The sketches of bulkheads and the bulkheads model in FEA ABAQUS are shown in Figure 11 & 12.

Figure 11: Main & Intermediate Bulkhead

Figure 12: Center Bulkhead

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GutterThe dimension gutter is shown on the Appendix and the gutter is modeled according to the given dimension. Since the gutter has a length of 200 000 mm, therefore the extrusion function is selected while modeling the gutter. The sketch of gutter and the gutter model in FEA ABAQUS is shown in Figure 11 & 12.

Figure 11: Gutter

Mid Water ArchComponents of the Mid Water Arch (gutter, bulkhead and buoyancy tank) are arranged according to the Appendix in the Assembly mode. The components of the Mid Water Arch are merged in order to create a perfect welding to the MWA system. The full model of the Mid Water Arch System modeled in FEA ABAQUS is show in Figure 12.

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Figure 12: Mid Water Arch

MaterialMaterials used for a MWA system are steel materials which is ASTM A537 Class 2 and S355J2. Table 1 presented the steel material specifications for MWA system.

Design Steel Grade Specification

Item Steel Type Steel Grade SpecificationBuoyancy tanks, Tank

Separator Plate Plate ASTM A537 Class 2

Horizontal Gutters and clamp slots, Vertical gutters, bulkheads,

stiffeners and gussets including tank separator

stiffeners

Plate S355J2

Table 1: MWA Material Specification

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Material PropertiesMaterial properties of the steels used in MWA system are shown on the Table 2.

Properties Unit Value Remarks

Specified Minimum Yield Strength

MPa355 S355J2415 ASTM 537 Class 2

Steel Density Kg/m3 7850 -Young’s Modulus Mpa 200, 000 -Shear Modulus Mpa 80000 -Poisson’s ratio - 0.30 -

Table 2: Material Properties of steels used in MWA

In order to perform the fully elastic MWA system loading analysis, two materials had to be created. The material properties of the two materials are shown on Table 2. The physical properties that needed to be applied to the MWA system are Young’s Modulus, Density and Poisson’s ratio. The input data of material properties are shown in Figure 13&14.

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Figure 13: Material Properties of ASTM A537 Class 2

Figure 14: Material Properties of S355J2

To perform the MWA loading analysis, the elastic mechanical properties are required because this will provide the stress-applied loading relationship up to the yield point. The stress-applied loading relationship is used to obtain the maximum allowable

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applied loads and the maximum applied load before the steel material yielded. Therefore, the elastic mechanical properties are required to create a linear stress-strain relationship of the steel material.

Mesh

The mesh of the MWA system is an essential element that needed to be studied in detail. In order to obtain the stresses that are realistic, the meshed component cannot be in poor condition. A correct number of seed size needed to be assigned to the MWA system in order to obtain the reasonable output stresses. If the assigned mesh elements are too large then the stresses between each element can be magnified. On the other hand, if the mesh is too fine, the part might consume too much memory space within the computer and it will prolong the analysis. Hence, selection of the mesh density is very important for FEA analysis. Therefore, a correct mesh density is very important to obtain an accurate result. In this MWA loading analysis, a seed size of 50 is used throughout the whole system. The meshed MWA system in FEA ABAQUS is shown in the Figure 15.

Figure 16: Meshed MWA

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Boundary Conditions and Loading

Mid Water Arch (MWA) system is used to support the flexible risers or umbilical under the seabed and tethered by a gravity base while operating. Therefore, the translation of MWA system is fixed in all the three direction and the rotation of the upward direction, z- direction is fixed while operating. Moreover, the MWA system will be subjected by external pressure on the external buoyancy tank surface and internal pressure on the internal buoyancy tank surface uniformly. Besides, the existing buoyancy force in the vertical direction will created an uplift force to the buoyancy tank itself. In order to simulate the real operating environment for MWA system in FEA ABAQUS, the boundary conditions of the MWA system has to be fixed in all the three translation direction which is U1, U2 and U3. For rotational of the MWA system in FEA ABAQUS, the UR3 has to be fixed. The applied boundary conditions of the MWA system are shown in the Figure 17.

Figure 17: Boundary Condition of MWA

Various types of loading are then applied to the MWA system separately in FEA ABAQUS. 5 types of analyses were performed separately in FEA ABAQUS. The loadings applied on 5 types of analyses in FEA ABAQUS were shown on the Table 3.

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Type of Analysis Applied Loading1. Mid Water Arch (MWA) system under internal pressure.

Internal Pressure

2. Mid Water Arch (MWA) system under external pressure.

External Pressure

3. Mid Water Arch (MWA) system under design pressure.

Design Pressure

4. Mid Water Arch (MWA) system under buoyancy force/uplift force.

Body force (Uplift force/volume of tanks)

5. Mid Water Arch (MWA) system under design pressure and uplift force.

Design Pressure & Body force

Table 3: Types of Applied Loading

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Mid Water Arch (MWA) system under internal pressureIn order to perform the first type of analysis (Mid Water Arch (MWA) system under internal pressure), the internal pressure has to be in the form of linear perturbation. In this case, there are 10 steps of internal pressure in linear perturbation form are created. The amount of internal pressure applied for each step is shown in the Table 4 and Figure 18 show the location where the internal pressure applied on the MWA system in FEA ABAQUS.

Steps (linear perturbation) Applied Internal Pressure (MPa)

Applied Internal Pressure 1 0.2Applied Internal Pressure 2 0.4Applied Internal Pressure 3 0.6Applied Internal Pressure 4 0.8Applied Internal Pressure 5 1.0Applied Internal Pressure 6 1.2Applied Internal Pressure 7 1.4Applied Internal Pressure 8 1.6Applied Internal Pressure 9 1.8Applied Internal Pressure 10 2.0

Table 4: Applied Internal Pressure

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Figure 18: Applied Internal Pressure on MWA system

The applied internal pressure as shown in Table 4 for each step has the increment of 0.1 MPa from the previous step. This increment is used to obtain the linear stress-applied internal pressure relationship. For this linear stress-applied internal pressure relationship, the maximum allowable applied internal pressure and the maximum applied internal pressure before the MWA buoyancy tank yielded can be obtained.

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Mid Water Arch (MWA) system under external pressureIn addition, the second type of analysis (Mid Water Arch (MWA) system under external pressure) was performed by create the step named external pressure. The step has to be in the form of linear perturbation. In this case, there are 10 steps of external pressure in linear perturbation form are created. The amount of external pressure applied for each step is shown in the Table 5 and Figure 19 show the location where the external pressure applied on the MWA system in FEA ABAQUS.

Steps (linear perturbation) Applied External Pressure (MPa)

Applied External Pressure 1 0.2Applied External Pressure 2 0.4Applied External Pressure 3 0.6Applied External Pressure 4 0.8Applied External Pressure 5 1.0Applied External Pressure 6 1.2Applied External Pressure 7 1.4Applied External Pressure 8 1.6Applied External Pressure 9 1.8Applied External Pressure 10 2.0

Table 5: Applied External Pressure

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Figure 19: Applied External Pressure on MWA system

The applied external pressure as shown in Table 5 for each step has the increment of 0.1 MPa from the previous step. This increment is used to obtain the linear stress-applied external pressure relationship. For this linear stress-applied external pressure relationship, the maximum allowable applied external pressure and the maximum applied external pressure before the MWA buoyancy tank yielded can be obtained.

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Mid Water Arch (MWA) system under design pressureFurthermore, the third type of analysis (Mid Water Arch (MWA) system under design pressure), the design pressure has to be in the form of linear perturbation and the design pressure is calculated based on the equation: Design Pressure = External Pressure – Internal Pressure. In this case, there are 10 steps of design pressure in linear perturbation form are created. The amount of design pressure applied for each step is shown in the Table 6 and Figure 19 show the location where the external pressure applied on the MWA system in FEA ABAQUS.

Steps (linear perturbation) Applied Design Pressure (MPa)

Applied Design Pressure 1 0.1Applied Design Pressure 2 0.2Applied Design Pressure 3 0.3Applied Design Pressure 4 0.4Applied Design Pressure 5 0.5Applied Design Pressure 6 0.6Applied Design Pressure 7 0.7Applied Design Pressure 8 0.8Applied Design Pressure 9 0.9Applied Design Pressure 10 1.0

Table 6: Applied Design Pressure

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Figure 20: Applied Design Pressure on MWA system

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Mid Water Arch (MWA) system under buoyancy force/ uplift force

Moreover, the forth type of analysis (Mid Water Arch (MWA) system under buoyancy force/ uplift force), the applied body force has to be in the form of linear perturbation and the applied body force is calculated based on the equation: Body force = Uplift force/ Volume of the buoyancy tanks, where the volume of the tank is 6071858176 mm3 (in my this project). In this case, there are 10 steps of uplift force in linear perturbation form are created. The amount of body force applied for each step is shown in the Table 7 and Figure 20 show the location where the body force applied on the MWA system in FEA ABAQUS.

Steps (linear perturbation) Applied Body force(N/mm3)

Applied Uplift force (N)

Uplift force 1 1E-4 6.07E+05Uplift force 2 2E-4 1.21E+06Uplift force 3 3E-4 1.82E+06Uplift force 4 4E-4 2.43E+06Uplift force 5 5E-4 3.04E+06Uplift force 6 6E-4 3.64E+06Uplift force 7 7E-4 4.25E+06Uplift force 8 8E-4 4.86E+06Uplift force 9 9E-4 5.46E+06Uplift force 10 10E-4 6.07E+06

Table 7: Applied Uplift force

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Figure 21: Applied body force on MWA system

The applied body force as shown in Table 7 for each step has the increment of 1E-4 N/mm3 from the previous step. This increment is used to obtain the linear stress-applied body force relationship. For this linear stress-applied body force relationship, the maximum allowable applied body force and the maximum applied body force before the MWA buoyancy tank yielded can be obtained.

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Mid Water Arch (MWA) system under design pressure and buoyancy force/ uplift force

Last but not least, the fifth type of analysis (Mid Water Arch (MWA) system under design pressure and buoyancy force/ uplift force), the applied combined forces have to be in the form of linear perturbation. In this case, the combined forces included design pressure and uplift force. There are three studies were studied in this analysis. The first study: the design pressure has a constant increment of 0.1 MPa and the body force has a constant increment of 1E-4 N/mm3. The second study: the design pressure is fixed to be 0.1439 MPa and the body force has a constant increment of 1E-4. The third study: the body force is fixed to be 5.392E-4 N/mm3 and the design pressure has a constant increment of 0.1 MPa. The input loadings for the three studies are shown in Table 8, 9 & 10 and Figure 22 show the region where the body force and design pressure applied.

Steps (linear perturbation)

Applied Design

Pressure (MPa)

Applied Body force(N/mm3)


Combined force 1 0.1 1E-4 6.07E+05Combined force 2 0.2 2E-4 1.21E+06Combined force 3 0.3 3E-4 1.82E+06Combined force 4 0.4 4E-4 2.43E+06Combined force 5 0.5 5E-4 3.04E+06Combined force 6 0.6 6E-4 3.64E+06Combined force 7 0.7 7E-4 4.25E+06Combined force 8 0.8 8E-4 4.86E+06Combined force 9 0.9 9E-4 5.46E+06Combined force 10 1.0 10E-4 6.07E+06

Table 9: Combined forces (First Study)

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Second study


Applied Design

Pressure (MPa)



Combined force 1 0.1439 1E-4 6.07E+05Combined force 2 0.1439 2E-4 1.21E+06Combined force 3 0.1439 3E-4 1.82E+06Combined force 4 0.1439 4E-4 2.43E+06Combined force 5 0.1439 5E-4 3.04E+06Combined force 6 0.1439 6E-4 3.64E+06Combined force 7 0.1439 7E-4 4.25E+06Combined force 8 0.1439 8E-4 4.86E+06Combined force 9 0.1439 9E-4 5.46E+06Combined force 10 0.1439 10E-4 6.07E+06

Table 10: Combined forces (Second Study)

Third study


Applied Design

Pressure (MPa)



Combined force 1 0.1 5.329E-4 6.07E+05Combined force 2 0.2 5.329E-4 1.21E+06Combined force 3 0.3 5.329E-4 1.82E+06Combined force 4 0.4 5.329E-4 2.43E+06Combined force 5 0.5 5.329E-4 3.04E+06Combined force 6 0.6 5.329E-4 3.64E+06Combined force 7 0.7 5.329E-4 4.25E+06Combined force 8 0.8 5.329E-4 4.86E+06Combined force 9 0.9 5.329E-4 5.46E+06Combined force 10 1.0 5.329E-4 6.07E+06

Table 11: Combined forces (Third Study)

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Figure 22: Combined Forces on MWA system

These three studies were studied in detail in this project in order to obtain the maximum allowable combined forces and the maximum applied combined forces before the MWA system yielded.

4.0 Results, Discussions, Conclusion

4.1 FEA Results

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The FEA results include Von Mises stress and principal stresses from the FEA ABAQUS program. The results will include analysis for the MWA system under the various loading conditions.

4.2 Mid Water Arch (MWA) system under internal pressure

Induced Von Mises stress is shown in the Table 12 while various amounts of internal pressure are applied on the MWA system. An induced Von Mises stress and applied internal pressure relationship is plotted and shown in Figure 23. From the Figure 23, it shown that the induced Von Mises stress and applied internal pressure is in linear relationship.

Internal Pressure (MPa)

Von Mises Stress (MPa)

0.2 73.1

0.4 146.2

0.6 219.3

0.8 292.4

1.0 365.5

1.2 438.6

1.4 511.7

1.6 584.8

1.8 657.9

2 731.0

Table 12: Induced Von Mises stress (Internal Pressure)

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0 0.5 1 1.5 2 2.50

100

200

300

400

500

600

700

800

73.1

146.2

219.3

292.4

365.5

438.6

511.7

584.8

657.9

731f(x) = 365.5 x

Induced Von Mises Stress Against Applied Internal Pressure

Induced Von Mises Stress Against Applied Internal PressureLinear (Induced Von Mises Stress Against Applied Internal Pressure)

Figure 23: Induced Von Mises Stress against Applied Internal Pressure

Figure 23 shows that the relationship between the induced Von Mises stress and applied internal pressure in linear relationship. Therefore, the maximum applied internal pressure before the buoyancy yielded and the maximum allowable internal pressure can be obtained from the equation of this linear relationship. The calculation performed in MATHCAD as shown below.

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From the calculation above it shown the maximum applied internal pressure before the buoyancy tank yielded is 1.135 MPa and the maximum allowable applied internal pressure is 0.761 MPa. The validity check of these two values performed in FEA ABAQUS and the result is shown in Figure 24 & 25.

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Figure 24: Maximum Allowable Internal Pressure

Figure 25: Maximum Internal Pressure before the tank yielded

Based on the result obtained in FEA ABAQUS, it shows that the induced Von Mises stress against applied internal pressure is in linear relationship. The induced Von Mises stress of MWA system when the maximum allowable internal pressure, 0.761 MPa applied on it is 278.10 MPa in FEA ABAQUS software. Whereas, while 1.135 MPa of internal pressure applied on it, the Von Mises stress induced in 414.8 MPa.In order to perform the unity check in this study, the principal stresses of buoyancy tank of the MWA system needed to be abstracted while the applied internal pressure (0.50 MPa) and the maximum allowable internal pressure (0.761MPa) applied on it. The principal stresses of buoyancy tank are shown in Table 13.

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Type of Principal Stresses (MPa)

Applied Internal Pressure

Maximum Allowable Internal Pressure

Unity Check

Principle Stress(MPa)


S11 146.7 223.3 0.656963726S22 143.3 218.1 0.657038056S33 0 0S12 102.3 155.6 0.657455013

4.3 Mid Water Arch (MWA) system under External pressure

Induced Von Mises stress is shown in the Table 13 while various amounts of external pressure are applied on the MWA system. An induced Von Mises stress and applied external pressure relationship is plotted and shown in Figure 26. From the Figure 26, it shown that the induced Von Mises stress and applied external pressure is in linear relationship.

External Pressure (MPa)


0.2 73.10.4 146.20.6 219.30.8 292.41.0 365.51.2 438.61.4 511.71.6 584.81.8 657.92.0 731.0

Table 13: Induced Von Mises stress (External Pressure)

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0 0.5 1 1.5 2 2.50

100

200

300

400

500

600

700

800

f(x) = 365.5 x

Von Mises Stress against Applied External Pressure

Von Mises Stress against Applied External Pressure Linear (Von Mises Stress against Applied External Pressure )

Figure 26: Induced Von Mises Stress against Applied External Pressure

Figure 26 shows that the relationship between the induced Von Mises stress and applied external pressure in linear relationship. Therefore, the maximum applied external pressure before the buoyancy yielded and the maximum allowable external pressure can be obtained from the equation of this linear relationship. The calculation performed in MATHCAD as shown below.

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From the calculation above it shown the maximum applied external pressure before the buoyancy tank yielded is 1.135 MPa and the maximum allowable applied external pressure is 0.761 MPa. The validity check of these two values performed in FEA ABAQUS and the result is shown in Figure 27 & 28.

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Figure 27: Maximum Allowable External Pressure

Figure 28: Maximum External Pressure before the tank yielded

Based on the result obtained in FEA ABAQUS, it shows that the induced Von Mises stress against applied external pressure is in linear relationship. The induced Von Mises stress of MWA system when the maximum allowable external pressure, 0.761 MPa applied on it is 278.10 MPa in FEA ABAQUS software. Whereas, while 1.135 MPa of external pressure applied on it, the Von Mises stress induced in 414.8 MPa.In order to perform the unity check in this study, the principal stresses of buoyancy tank of the MWA system needed to be abstracted while the applied external pressure (0.6439 MPa) and the maximum allowable external pressure (0.761MPa) applied on it. The principal stresses of buoyancy tank are shown in Table 13.

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Applied External Pressure

Maximum Allowable External Pressure

Unity Check



S11 -188.9 -223.3 0.845947156S22 -184.6 -218.1 0.846400734S33 0 0S12 -131.7 -155.6 0.846401028

4.4 Mid Water Arch (MWA) system under Design pressure

Induced Von Mises stress is shown in the Table 14 while various amounts of design pressure are applied on the MWA system externally. An induced Von Mises stress and applied design pressure relationship is plotted and shown in Figure 27. From the Figure 27, it shown that the induced Von Mises stress and applied design pressure is in linear relationship.

Design Pressure (MPa)


0.2 73.10.4 146.20.6 219.30.8 292.41.0 365.51.2 438.61.4 511.71.6 584.81.8 657.92.0 731.0

Table 14: Induced Von Mises stress (Design Pressure)

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0 0.5 1 1.5 2 2.50

100

200

300

400

500

600

700

800

f(x) = 365.5 x

Von Mises Stress against Applied Design Pressure

Figure 26: Induced Von Mises Stress against Applied Design Pressure

Figure 26 shows that the relationship between the induced Von Mises stress and applied external pressure in linear relationship. Therefore, the maximum applied external pressure before the buoyancy yielded and the maximum allowable external pressure can be obtained from the equation of this linear relationship. The calculation performed in MATCAD as shown below.

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From the calculation above it shown the maximum applied design pressure before the buoyancy tank yielded is 1.135 MPa and the maximum allowable applied design pressure is 0.761 MPa. The validity check of these two values performed in FEA ABAQUS and the results shown in Figure 29 & 30.

Figure 27: Maximum Allowable Design Pressure

Figure 28: Maximum Design Pressure before the tank yielded

Based on the result obtained in FEA ABAQUS, it shows that the induced Von Mises stress against applied external pressure is in linear relationship. The induced Von Mises stress of MWA system when the maximum allowable design pressure, 0.761 MPa applied on it is 278.10 MPa in FEA ABAQUS software. Whereas, while 1.135 MPa of design pressure applied on it, the Von Mises stress induced in 414.8 MPa.In order to perform the unity check in this study, the principal stresses of buoyancy tank of the MWA system needed to be abstracted while the applied design pressure (0.1439 MPa) and the maximum allowable design pressure (0.761MPa) applied on it. The principal stresses of buoyancy tank are shown in Table 13.

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Applied Design Pressure

Maximum Allowable Design Pressure

Unity Check



S11 -42.22 -223.3 0.189072996S22 -41.25 -218.1 0.189133425S33 0 0S12 -29.43 -155.6 0.189138817

4. Mid Water Arch (MWA) system under buoyancy force/uplift force

Induced Von Mises stress is shown in the Table 14 while various amounts of body force are applied on the MWA system externally around the buoyancy tanks. An induced Von Mises stress and applied body force relationship is plotted and shown in Figure 27. From the Figure 27, it shown that the induced Von Mises stress and applied body force is in linear relationship.

Body force (N/mm3)

Volume of tanks (mm3)

Uplift force (N) Von Mises Stress (MPa)

1.00E-04 6071858176 6.07E+05 12.472.00E-04 6071858176 1.21E+06 24.933.00E-04 6071858176 1.82E+06 37.44.00E-04 6071858176 2.43E+06 49.865.00E-04 6071858176 3.04E+06 62.336.00E-04 6071858176 3.64E+06 74.87.00E-04 6071858176 4.25E+06 87.268.00E-04 6071858176 4.86E+14 99.739.00E-04 6071858176 5.46E+06 112.21.00E-03 6071858176 6.07E+06 124.7

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0.00E+00 2.00E-04 4.00E-04 6.00E-04 8.00E-04 1.00E-03 1.20E-030

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120

140

f(x) = 124683.636363636 x − 0.00799999999996715

Von Mises Stress against body force

Von Mises Stress against body forceLinear (Von Mises Stress against body force)

Figure 26 shows that the relationship between the induced Von Mises stress and body force in linear relationship. Therefore, the maximum body force before the buoyancy tank yielded and the maximum allowable body force can be obtained from the equation of this linear relationship. The calculation performed in MATHCAD as shown below.

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From the calculation above it shown the maximum body force can be applied before the buoyancy tank yielded is 7.909E-4 N/mm3 and the maximum allowable body force is 5.299E-4. The validity check of these two values performed in FEA ABAQUS and the result is shown in Figure 29 & 30.

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Figure 29: Maximum Allowable Body force Figure 30: Maximum Body force before the tank yielded

Based on the result obtained in FEA ABAQUS, it shows that the induced Von Mises stress against applied external pressure is in linear relationship. The induced Von Mises stress of MWA system when the maximum allowable design pressure, 0.761 MPa applied on it is 278.10 MPa in FEA ABAQUS software. Whereas, while 1.135 MPa of design pressure applied on it, the Von Mises stress induced in 414.8 MPa.In order to perform the unity check in this study, the principal stresses of buoyancy tank of the MWA system needed to be abstracted while the applied design pressure (0.1439 MPa) and the maximum allowable design pressure (0.761MPa) applied on it. The principal stresses of buoyancy tank are shown in Table 13.

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Applied Body force Maximum Allowable Body force

Unity Check



S11 -25.65 -107.4 0.238827S22 98.22 411 0.238978S33 0 0S12 20.98 87.79 0.23898

3.1.5 Mid Water Arch (MWA) system under design pressure and buoyancy force/uplift force

There are three difference studies performed in Mid Water Arch (MWA) system under design pressure and buoyancy force analysis. The studies that will be discussed including:

1. Design pressure has a constant increment of 0.1 MPa and the body force has a constant increment of 1E-4 N/mm3.2. Design pressure is fixed to be 0.1439 MPa and the body force has a constant increment of 1E-4.3. Body force is fixed to be 5.392E-4 N/mm3 and the design pressure has a constant increment of 0.1 MPa.

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Body force Applied Pressure Von Mises Stress1.00E-04 0.1 58.932.00E-04 0.2 117.93.00E-04 0.3 176.84.00E-04 0.4 235.75.00E-04 0.5 294.66.00E-04 0.6 353.67.00E-04 0.7 412.58.00E-04 0.8 471.49.00E-04 0.9 530.31.00E-03 1.0 589.3

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0.00E+00 5.00E-04 1.00E-03 1.50E-030

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400

f(x) = 369303.636363636 x − 0.00799999999998136

Von Mises Stress Against Body Force

Von Mises Stress Against Body ForceLinear (Von Mises Stress Against Body Force)

0 0.2 0.4 0.6 0.8 1 1.20

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f(x) = 369.303636363636 x − 0.0080000000000382

Von Mises Stress against Applied Design Pressure

Von Mises Stress against Applied Design PressureLinear (Von Mises Stress against Applied Design Pressure)

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MPa


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mwa report

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