Fatigue Analysis of Offshore Drilling Unit · 2018-12-06 · 1 Fatigue Analysis of Offshore...

1

Fatigue Analysis of Offshore Drilling Unit

Md Rezaul Karim

Master Thesis

presented in partial fulfillment of the requirements for the double degree:

“Advanced Master in Naval Architecture” conferred by University of Liege "Master of Sciences in Applied Mechanics, specialization in Hydrodynamics,

Energetics and Propulsion” conferred by Ecole Centrale de Nantes

developed at West Pomeranian University of Technology, Szczecin in the framework of the

“EMSHIP” Erasmus Mundus Master Course

in “Integrated Advanced Ship Design”

Ref. 159652-1-2009-1-BE-ERA MUNDUS-EMMC

Supervisor:

Prof. Maciej Taczala, West Pomeranian University of Technology, Szczecin

Reviewer: Prof. Hervé Le Sourne, ICAM

Szczecin, February 2015

P 2 Md Rezaul Karim

Master Thesis developed at West Pomeranian University of Technology, Szczecin

Contents DECLARATION OF AUTHORSHIP ....................................................................................... 6

ABBREVIATIONS .................................................................................................................... 7

ABSTRACT ............................................................................................................................... 8

SHORT DESCRIPTION ............................................................................................................ 9

1. INTRODUCTION ................................................................................................................ 10

1.1 Background ................................................................................................................ 10

1.2 Objective .................................................................................................................... 10

1.3 Methodology .............................................................................................................. 10

1.4 Schedule..................................................................................................................... 12

1.5 Types of Fatigue Failure: ........................................................................................... 12

1.6 Sources of Fatigue: .................................................................................................... 13

2. OFFSHORE DRILLING PLATFORMS: ........................................................................ 14

2.1 Introduction: .............................................................................................................. 14

2.2 Components of Offshore Rigs: .................................................................................. 14

2.3 Category: ................................................................................................................... 15

3. STUCTURE ANALYZED: SEMISUMMERSIBLE UNIT ............................................ 22

3.1 Introduction: .............................................................................................................. 22

3.2 Classification: ............................................................................................................ 22

3.3 Example of similar model: ........................................................................................ 23

4. SOFTWARE PROCEDURE: .......................................................................................... 25

4.1 Introduction: .............................................................................................................. 25

4.2 Sesam Genie: ............................................................................................................. 25

4.3 HydroD-Wadam: ....................................................................................................... 26

4.4 Sestra: ........................................................................................................................ 27

4.5 Xtact: ......................................................................................................................... 28

4.6 Postresp:..................................................................................................................... 29

4.7 Stofat:......................................................................................................................... 29

5. STRUCTURAL MODELLING ....................................................................................... 30

5.1 Modelling Set-Up ...................................................................................................... 30

5.2 Pontoon: ..................................................................................................................... 32

5.3 Column: ..................................................................................................................... 32

5.5 Deck: .......................................................................................................................... 33

5.5 Derrick: ...................................................................................................................... 34

5.6 Boundary Conditions: ................................................................................................ 35

5.7 Panel Model: .............................................................................................................. 36

Fatigue Analysis of Offshore Drilling Unit 3

“EMSHIP” Erasmus Mundus Master Course, period of study September 2013 – February 2015

5.8 Morison Model: ......................................................................................................... 38

5.9 Structural Model: ....................................................................................................... 39

6. HYDRODYNAMIC ANALYSIS: ................................................................................... 41

6.1 Analysis Setup ........................................................................................................... 41

6.2 Global Motion Response Analysis: ........................................................................... 47

7. GLOBAL STRUCTURAL STRENGTH ANALYSIS .................................................... 54

8. GLOBAL FATIGUE ANALYSIS AND RESULT: ........................................................ 56

9. CONCLUSIONS AND RECOMMENDATIONS: ......................................................... 61

REFERENCES ......................................................................................................................... 63

APPENDIX .............................................................................................................................. 64

Appendix A: Summary of Model Properties ........................................................................ 64

Appendix B: Element Fatigue Check Result ........................................................................ 68

ACKNOWLEDGEMENTS ..................................................................................................... 84

P 4 Md Rezaul Karim


List Of Figures

Figure 1 Flowchart of Methodology ........................................................................................ 11 Figure 2 Detail Schedule with Gantt chart ............................................................................... 12 Figure 3 Component of offshore rigs (19) ............................................................................... 15

Figure 4 Common Types of Drilling Rigs (BOEMRE, 2010c) ............................................... 16 Figure 5 Fixed Platform Rig (Left) and Gravity-Based Structure (Right) (Source: Prof:

Tadeusz Graczyk Lectures at ZUT) ......................................................................................... 17 Figure 6 Varieties of mobile offshore drilling units (MODUs). Drill Barge (TODCO via

NETL, 2011), Jack-Up Rig (Transocean, 2011), Semi-submersible Rig (Eni, 2008), Drill Ship

(BP p.l.c., 2011). ...................................................................................................................... 19 Figure 7 Tension Leg Platform (Magnolia TLP, Source: Prof: Tadeusz Graczyk Lectures at

ZUT) ......................................................................................................................................... 21 Figure 8 Column Stabilized Semisubmersibles (Left: Ring Pontoon Design, Right: Twin

Pontoon Design, Source: Petrowiki) ........................................................................................ 23

Figure 9 Similar Drilling Platform model (Maersk Drilling deep-water semi-submersible rig

Source: Prof: Tadeusz Graczyk Lectures at ZUT) ................................................................... 23

Figure 10 Semi-submersible platform - Maersk Drilling deep-water semi-submersible rig

(Source: Prof: Tadeusz Graczyk Lectures at ZUT) .................................................................. 24 Figure 11 Flowchart of Software Procedure ............................................................................ 25 Figure 12 Schematic illustration of the capabilities of Sestra (12) .......................................... 28

Figure 13 Color coding of structural members ........................................................................ 31 Figure 14 Color coding of thickness ........................................................................................ 31

Figure 15 Pontoon .................................................................................................................... 32 Figure 16 Column ..................................................................................................................... 33 Figure 17 Deck ......................................................................................................................... 33

Figure 18 Deck Framing System .............................................................................................. 34 Figure 19 Derrick ..................................................................................................................... 34

Figure 20 Boundary conditions (Source: DNV-RP-C103_2012-04) ....................................... 35

Figure 21 Boundary conditions applied on pontoons ............................................................... 36

Figure 22 Panel Model ............................................................................................................. 37 Figure 23 Wet Surface for Hydrodynamic Analysis ................................................................ 37 Figure 24 Meshed Panel Model ............................................................................................... 38

Figure 25 Morison Meshed Model ........................................................................................... 38 Figure 26 Structural Mesh Model ............................................................................................ 39

Figure 27 Problematic mesh elements. .................................................................................... 39 Figure 28 Final Structural Mesh Model ................................................................................... 40 Figure 29 Hydro Model of Semisubmersible Drilling Platform .............................................. 41

Figure 30 Four compartments inside two pontoons ................................................................. 43 Figure 31 Off body points to define sea state grid .................................................................. 43 Figure 32 Mass model of the Drilling Unit .............................................................................. 44 Figure 33 RAO for different wave directions at relative points (0, 0, and 35) ........................ 47 Figure 34 RAO of Heave ......................................................................................................... 48

Figure 35 RAO of Roll ............................................................................................................. 48 Figure 36 RAO of Pitch ........................................................................................................... 49

Figure 37 Damping Matrix ....................................................................................................... 49 Figure 38 RAO at relative point (0, 0, and 35) after addition of damping ............................... 50

Figure 39 Surface wave loads at 180 degree heading and 0.053 Hz frequency (Pitch RAO) . 51 Figure 40 Surface wave loads at 270 degree heading and 0.045 Hz frequency (Heave RAO) 51 Figure 41Surface wave loads at 270 degree heading and 0.042 Hz frequency (Roll RAO) .... 52 Figure 42 Surface wave loads at 180 degree heading and 0.033 Hz frequency (Surge RAO) 52 Figure 43 Surface wave loads at 270 degree heading and 0.033 Hz frequency (Sway RAO) . 53



Figure 44 Surface wave loads at 315 degree heading and 0.166 Hz frequency (YAW RAO) 53 Figure 45 Global structural model ........................................................................................... 54

Figure 46 Von-Misses Stress at 45 degree wave heading and 0.1111 Hz excitation frequency

.................................................................................................................................................. 55

Figure 47 Von-Misses Stress on Column ................................................................................. 55 Figure 48 Scatter Diagram for the North Atlantic (Source: DNV-RP-C205) .......................... 56 Figure 49 DNV-SN Curves (Stofat_UM) ................................................................................ 57 Figure 50 DNVC-I SN curve plotted from STOFAT .............................................................. 57 Figure 51 Wave Spreading function for short crested Sea ....................................................... 58

Figure 52 Definition of the wave direction (heading angle) in this investigation. ................... 59 Figure 53 Maximum Usage Factor of the Structure ................................................................. 59 Figure 54 Fatigue Life of Global Structure .............................................................................. 60

List of Tables Table 1 Main Dimensional Parameter for Semisubmersible Analyzed ................................... 24

Table 2 Material Properties of the Structural Model (St52) ..................................................... 30 Table 3 Sea State Direction Set ............................................................................................... 42 Table 4 Mass Model Properties of the Structure ...................................................................... 44

Table 5 Load cases for Heading and Wave periods ................................................................. 45 Table 6 Long term response from Postresp .............................................................................. 50 Table 7 Fatigue life in Critical connections ............................................................................. 60

P 6 Md Rezaul Karim


DECLARATION OF AUTHORSHIP

I declare that this thesis and the work presented in it are my own and have been generated by

me as the result of my own original research.

Where I have consulted the published work of others, this is always clearly attributed.

Where I have quoted from the work of others, the source is always given. With the exception

of such quotations, this thesis is entirely my own work.

I have acknowledged all main sources of help.

Where the thesis is based on work done by myself jointly with others, I have made clear exactly

what was done by others and what I have contributed myself.

This thesis contains no material that has been submitted previously, in whole or in part, for the

award of any other academic degree or diploma.

I cede copyright of the thesis in favour of the West Pomeranian University of Technology

Szczecin, Poland.

Date: Signature



ABBREVIATIONS

CN

COG

CT

DNV

GL

Hs

FE

FP

FEA

JP

L-File

MODU

NA

OS

RAO

RP

Tp

TLP

UM

Classification Notes

Centre of Gravity

Compliant Tower Platforms

Det Norske Veritas

Germanischer Lloyd

Significant Height

Finite Element

Fixed Platform

Finite Element Analysis

Jack-up Platform

Load Interface File

Mobile offshore drilling units

North Atlantic

Offshore Standard

Response Amplitude Operator

Recommended Practice

Peak Period

Tension Leg Platform

User Manual

P 8 Md Rezaul Karim


ABSTRACT

Drilling operation in deep water, harsh environment and remote locations becomes a key trend

for the offshore industry to fulfil increasing demand for energy. For operational conditions wave

induced loads are more significant for the offshore installations. Therefore, to ensure integrity

and structural safety, the wave induced loads have to be taken into account. One of the

approaches to accomplish this task is to perform a fatigue analysis with the extreme

environmental loading on the offshore platform using rules and practices recommended by

classification societies.

The main objective of the thesis has been to present a case study of a semisubmersible drilling

unit regarding the fatigue analysis. The applied approach consisted in finite element modelling

of the global structure, applying hydrodynamic loads using recommended offshore design

codes, transferring wave loads from hydrodynamic model to structural model and perform the

fatigue analysis with most unfavorable combination of environmental conditions. Among

different methods of fatigue analysis, the spectral method is considered as most suitable in

which long term distribution of stresses is calculated using wave scatter data.

For finite element modelling SESAM Genie was used while HydroD Wadam was used to

analyze hydrodynamic loads and also transfer the loads to the finite element model for

subsequent structural analysis. These hydrodynamic loads were applied for a number of wave

directions and for a range of wave frequencies covering the necessary sea states and the results

in form of stresses were obtained. These results were then used to calculate fatigue damages at

given points in the structural model using another software – Stofat.



SHORT DESCRIPTION

A short description of this whole paper is given below:

This paper contains nine chapters in total. First chapter described the introduction of the project

including what is to be achieved, why this project needs to be done, methodology of the project

and detail time allocation with chart.

Second chapter contains a brief overview of the offshore drilling structures, different types of

offshore structures, their basis of applications and operation

In third chapter, the description of Semisubmersible platform which is used as a case study for

the current master thesis has been given with brief overview, types and example.

Fourth chapter includes a brief description of software tool used for this thesis and flowchart of

methodology.

Fifth chapter presents the global structural modeling of the drilling unit studied in detail. It

describes the 3d modelling for FE analysis with material properties, meshing and boundary

conditions to perform subsequent analysis

Sixth chapter contains Hydrodynamic Analysis of the structure including detailed overview of

analysis setup, mass model properties and subsequent global motion response for the given set

of extreme environmental loading conditions.

Global structural strength is presented on Seventh chapter with detailed load case and von-

misses stress

Finally global fatigue analysis result is listed in eighth chapter with SN-curve and scatter data

of the desired location.

Ninth is a conclusion chapter, which deals with the initial aim and objectives, achievement and

some suggestions for the future development of the work.

References and some important appendices are attached at the end.

P 10 Md Rezaul Karim


1. INTRODUCTION

1.1 Background

As drilling extended further offshore into deeper water to access additional energy resources,

the structures are also largely exposed to stresses which induced by time variation. These type

of stress pattern are the forces generated principally by the sea waves. These loads are repeated

for thousands of cycles through the lifetime of the structure. After many cycles the accumulated

damage reduces the ability of the structural member to withstand loading. Global Fatigue

analysis is one of the approach to quantify wave induced load effects to ensure integrity and

structural safety of the offshore platform. A methodology has been developed for global fatigue

analysis of an offshore drilling unit with extreme environmental loading condition.

1.2 Objective

The main objective is to determine screening fatigue lives based upon DNV S-N curve and

wave scatter data. The calculated fatigue life indicate the distribution of fatigue sensitive area.

In order to meet the objective, the following sub-targets are to be fulfilled for the Global Model

analysis:

Make a 3D-model of the structure for FE analysis

Calculate the hydrodynamic loads in the frequency domain.

Identify critical locations with respect to von Mises stress for different wave direction.

Identify fatigue critical locations using linear FE-analysis. Perform the analysis for

different wave directions (heading angles).

1.3 Methodology

The objective with this study is to simulate numerically and analyze the structure response

followed by the fatigue life of an offshore drilling unit considering wave direction, magnitude

of wave loads and the location of interest. Following numerical analysis have been performed:

Hydrodynamic Analysis

Structural Response Analysis

Fatigue Analysis

Figure 1 depicts the methodology of the thesis and all the steps has been described briefly on

later sections.



Figure 1 Flowchart of Methodology

For the FE modeling and analysis of the drilling unit, DNV software Sesam GeniE has been

used. The hydrodynamic simulation is carried out in the DNV software HydroD Wadam. The

hydrodynamic simulations are performed in the frequency domain for the structure operating

in north Atlantic. For the frequency domain, a Bretschneider spectrum with 26 frequencies and

8 wave directions are chosen. The global motion response of the structure was analyzed using

a post-processing software named POSTRESP. The FE-simulations are carried out in the DNV

software SESTRA for linear structural FE-analysis. The fatigue analysis was done in STOFAT

and results are graphically presented with XTRACT. A brief description of software used for

analysis is given in Chapter four.

3D-Modelling

(Sesam-GeniE)

Hydrodynamic Analysis

(HydroD-Wadam)

Structural Analysis

(Sestra)

Global Response

(Xtract)

Fatigue Analysis

(Stofat)



1.4 Schedule

Detail schedule of the thesis is presented below with Microsoft Project Gantt chart.

Figure 2 Detail Schedule with Gantt chart

1.5 Types of Fatigue Failure:

Two categories of fatigue damage are generally recognized and they are termed high frequency

and low frequency fatigue. In high frequency fatigue, failure is initiated in the form of small

cracks, which grow slowly and which may often be detected and repaired before the structure

is endangered. High frequency fatigue involves several millions of cycles of relatively low

stress (less than yield) and is typically encountered in machine parts rotating at high speed or

in structural components exposed to severe and prolonged vibration. Low frequency fatigue

involves higher stress levels, up to and beyond yield, which may result in cracks being initiated

after several thousand cycles.



1.6 Sources of Fatigue:

Cyclic Load

Whipping

Springing

Engines and propeller

1.6.1 Cyclic Loads:

Offshore structures of all types are generally subjected to cyclic loading from wind, current and

waves. Dynamic wave produce stress fluctuations in the structural members and joints and are

the primary cause of fatigue damages. In deep water environments wind loads represent a

contribution of about 5 % to the environmental loading. Current loads are mostly considered to

be unimportant in the dynamic analysis of offshore structures, because their frequencies are not

sufficient to excite the structures. Wave loads are considered as the main source of excitation

for current piece of work.

1.6.2 Whipping:

Shocks between wave and ship bow is known as slamming, this shocks generates vibration

which is known as Whipping. So it is induced by wave impacts under the ship’s flared bow, the

overhanging stern, or the bottom, leads to transient, decaying hull girder vibrations which

typically occur in moderate or harsh seaways. For the current thesis work whipping is not taken

into account because of deep-water consideration and non-flared bow.

1.6.3 Springing:

Springing is caused by regular, periodic wave trains that excite resonant hull girder vibrations

occurring in low to moderate seaways. If excitation of waves equal to 1st beam or 2nd beam

natural frequency of structure then resonant occur and passenger moves/ jumps with structure.

As springing is negligible in deep-water, it is not considered for current work.

1.6.4 Engine Excitation Frequency:

High frequency response coming from engine or propeller creates severe vibration and initiate

fatigue cracks on nearby parts.



2. OFFSHORE DRILLING PLATFORMS:

2.1 Introduction:

One of the remarkable accomplishments of the petroleum industry has been the development

of technology that allows for drilling wells offshore to access additional energy resources. The

basic offshore wellbore construction process is not significantly different than the rotary drilling

process used for land based drilling. The main differences are the type drilling rig and modified

methods used to carry out the operations in a more complex situation. Depending on the

circumstances, the platform may be fixed to the ocean floor, may consist of an artificial island,

or may float.

For offshore drilling a mechanically stable offshore platform or floating vessel from which to

drill must be provided. These range from permanent offshore fixed or floating platforms to

temporary bottom-supported or floating drilling vessels.

Despite an increase in complexity, improvements in drilling technology have allowed more

complex well patterns to be drilled to a greater depth such that additional hydrocarbon resources

can be developed at a greater distance from the drilling or production structure, allowing more

energy to be produced with less environmental impact.

2.2 Components of Offshore Rigs:

Following are the major components of the offshore rigs as presented on figure 3 as well.

Hull – initially rigs were built out of tanker hulls, so the terminology remains same

Power Module – converts available fuel into power for the station

Process Module – onboarding and offloading of supplies and products

Drilling Module – the traditional drilling rig apparatus

Quarters Module – where the crew sleeps and eats

Wellbay Module – access to the well and other equipment

Derrick – the oil derrick



Figure 3 Component of offshore rigs (19)

2.3 Category:

There are two basic categories of offshore drilling rigs those that can be moved from place to

place, allowing for drilling in multiple locations, and those rigs that are temporarily or

permanently placed on a fixed-location platform (platform rigs). Jack-ups, semisubmersibles

and drill-ships make up the majority of the offshore rig fleet and all are used worldwide. Other

rig types such as platform rigs, inland barges and tender-assisted rigs are used as well, but they

are fewer in number and are generally used in specific geographic areas. Common types of

offshore platforms are listed below:

Jack-up drilling rig

Fixed platform

Gravity-based structure

Compliant Tower

Tension-leg platform

Spar platform

Semi-submersible platform

Drillship



Figure 4 Common Types of Drilling Rigs (BOEMRE, 2010c)

Common types of drilling rigs are presented with figure 4 with a brief description below:

Fixed platforms are built on concrete or steel legs, or both, anchored directly onto the

seabed, supporting a deck with space for drilling rigs, production facilities and crew

quarters. Such platforms are, by virtue of their immobility, designed for very long term

use (for instance the Hibernia platform). Various types of structure are used, steel jacket,

concrete caisson, floating steel and even floating concrete. Steel jackets are vertical

sections made of tubular steel members, and are usually piled into the seabed. Concrete

caisson structures, pioneered by the Condeep concept, often have in-built oil storage in

tanks below the sea surface and these tanks were often used as a flotation capability,

allowing them to be built close to shore and then floated to their final position where

they are sunk to the seabed. Fixed platforms are economically feasible for installation

in water depths up to about 1,700 ft (520 m)



Figure 5 Fixed Platform Rig (Left) and Gravity-Based Structure (Right) (Source: Prof: Tadeusz Graczyk

Lectures at ZUT)

A Gravity Based Structure can either be steel or concrete and is usually anchored

directly onto the seabed. Steel GBS are predominantly used when there is no or limited

availability of crane barges to install a conventional fixed offshore platform, for

example in the Caspian Sea. There are several steel GBS in the world today (e.g.

offshore Turkmenistan Waters (Caspian Sea) and offshore New Zealand). Steel GBS do

not usually provide hydrocarbon storage capability. These structures are generally

feasible in shallow water depth till 100 m although the deepest GBS being used at Troll

field in Norway at water depth of 303 m.

A compliant tower (CT) is a fixed rig structure normally used for the offshore

production of oil or gas. The rig consists of narrow, flexible (compliant) towers and a

piled foundation supporting a conventional deck for drilling and production operations.

Compliant towers are designed to sustain significant lateral deflections and forces, and

are typically used in water depths ranging from 1,500 and 3,000 feet (450 and 900 m).



At present the deepest is Petronius in 535 m of water (the tallest freestanding structure

in the world.), in operation since 1998. With the use of flex elements such as flex legs

or axial tubes, resonance is reduced and wave forces are de-amplified. This type of rig

structure can be configured to adapt to existing fabrication and installation equipment.

Compared with floating systems, such as Tension-leg platforms and SPARs, the

production risers are conventional and are subjected to less structural demands and

flexing. This flexibility allows it to operate in much deeper water, as it can 'absorb' much

of the pressure exerted on it by the wind and sea. It can deflect (sway) in excess of 2%

of height. Despite its flexibility, the compliant tower system is strong enough to

withstand hurricane conditions.

Mobile Offshore Drilling Unit (MODU) are drilling rigs that are used exclusively to

drill offshore and that float either while drilling or when being moved from location to

another. They fall into two general types: bottom-supported and floating drilling rigs.

Bottom-supported drilling rigs are barges or jack-ups. Floating drill rigs include

submersible and semi-submersible units and drill ships. Various MODUs are presented

on Figure 6.

A drilling barge consists of a barge with a complete drilling rig and ancillary equipment

constructed on it. Drilling barges are suitable for calm shallow waters (mostly inland

applications) and are not able to withstand the water movement experienced in deeper,

open water situations. When a drilling barge is moved from one location to another, the

barge floats on the water and is pulled by tugs. When a drilling barge is stationed on the

drill site, the barge can be anchored in the floating mode or in some way supported on

the bottom. The bottom-support barges may be submerged to rest on the bottom or they

may be raised on posts or jacked-up on legs above the water. The most common drilling

barges are inland water barge drilling rigs that are used to drill wells in lakes, rivers,

canals, swamps, marshes, shallow inland bays, and areas where the water covering the

drill site in not too deep.



Figure 6 Varieties of mobile offshore drilling units (MODUs). Drill Barge (TODCO via NETL, 2011),

Jack-Up Rig (Transocean, 2011), Semi-submersible Rig (Eni, 2008), Drill Ship (BP p.l.c., 2011).

Submersible drilling rigs are similar to barge rigs but suitable for open ocean waters of

relative shallow depth. The drilling structure is supported by large submerged pontoons

that are flooded and rest on the seafloor when drilling. After the well is completed, the

water is pumped out of the tanks to restore buoyancy and the vessel is towed to the next

location.

Jack-up drilling rigs are similar to a drilling barge because the complete drilling rig is

built on a floating hull that must be moved between locations with tug boats. Jack-ups

are the most common offshore bottom-supported type of drilling rig. There are two jack-

up types; independent-leg jack-ups make up the majority of the existing fleet. They have

legs that penetrate into the seafloor and the hull jacks up and down the legs. Mat-

supported jack-ups are as the name implies, the mat rests on the seafloor during drilling



operations. Cantilever jack-ups are able to skid out over the platform or well location,

while slot units have a slot that fits around a platform when drilling development wells.

Once on location, a jack-up rig is raised above the water on legs that extend to the

seafloor for support. Jack-ups can operate in open water or can be designed to move

over and drill though conductor pipes in a production platform. These MODU's-Mobile

Offshore Drilling Units are typically used in water depths up to 400 feet (120 m),

although some designs can go to 550 ft (170 m) depth. They are designed to move from

place to place, and then anchor themselves by deploying the legs to the ocean bottom

using a rack and pinion gear system on each leg.

Semisubmersibles are a common type of floating structure used in the exploration and

production of offshore hydrocarbons. These platforms have hulls of sufficient buoyancy

to cause the structure to float, but the structural/equipment weight of the platform and

the mooring system keeps the structure upright. Typically, four to eight vertical, surface

piercing columns are connected to these pontoons. The columns themselves may have

cross and horizontal bracing to provide structural strength and triangulated rigidity for

the platform. The minimal water plane area contributed by the vertical columns results

in long heave, pitch and roll natural periods and the hydrodynamic loading can be

minimized at the dominant wave period by careful selection of pontoon volume and

water plane area. A more detailed description of this type of offshore platforms has been

discussed in the Chapter 3 of this thesis.

A drillship is a maritime vessel that has been fitted with drilling apparatus. It is most

often used for exploratory offshore drilling of new oil or gas wells in deep water or for

scientific drilling. The drillship can also be used as a platform to carry out well

maintenance or completion work such as casing and tubing installation or subsea tree

installations. It is often built to the design specification of the oil Production Company

and/or investors, but can also be a modified tanker hull outfitted with a dynamic

positioning system to maintain its position over the well. The greatest advantages these

modern drill ships have is their ability to drill in water depths of more than 2500 meters

and the time saved sailing between oilfields worldwide. Drill ships are completely

independent, in contrast to semi-submersibles and jack up barges. In order to drill, a



marine riser is lowered from the drillship to the seabed with a blowout preventer (BOP)

at the bottom that connects to the wellhead.

TLPs are floating platforms tethered to the seabed in a manner that eliminates most

vertical movement of the structure. TLPs are used in water depths up to about 6,000 feet

(2,000 m). The "conventional" TLP is a 4-column design which looks similar to a

semisubmersible. Proprietary versions include the Seastar and MOSES mini TLPs; they

are relatively low cost, used in water depths between 600 and 4,300 feet (180 and 1,300

m). Mini TLPs can also be used as utility, satellite or early production platforms for

larger deep-water discoveries. An example of TLP is given below.

Figure 7 Tension Leg Platform (Magnolia TLP, Source: Prof: Tadeusz Graczyk Lectures at ZUT)

Spars are moored to the seabed like TLPs, but whereas a TLP has vertical tension

tethers, a spar has more conventional mooring lines. The spar has more inherent stability

than a TLP since it has a large counterweight at the bottom and does not depend on the

mooring to hold it upright. It also has the ability, by adjusting the mooring line tensions

(using chain-jacks attached to the mooring lines), to move horizontally and to position

itself over wells at some distance from the main platform location. The first production

spar was Kerr-McGee's Neptune, anchored in 1,930 ft (590 m) in the Gulf of Mexico;

however, spars (such as Brent Spar) were previously used as FSOs.



3. STUCTURE ANALYZED: SEMISUMMERSIBLE UNIT

3.1 Introduction:

For current work Semi-submersible drilling unit is used which are the most common type of

offshore floating drilling rigs and can operate in deep water and usually move from location to

location under their own power. These platforms have hulls (columns and pontoons) of

sufficient buoyancy to cause the structure to float, but of weight sufficient to keep the structure

upright; can be ballasted up or down by altering the amount of flooding in buoyancy tanks.

“Semis” as they are called as the have columns that are ballasted to remain on location either

by mooring lines attached to seafloor anchors or may be held in place by adjustable thrusters

which are rotated to hold the vessel over the desired location known as dynamically positioned.

Semi-submersibles can be used in water depths from 200 to 10,000 feet (60 to 3,000 m).

3.2 Classification:

Most common design of semisubmersible rigs are the column-stabilized semisubmersible unit

where two horizontal pontoons are connected via cylindrical or rectangular columns to the

drilling deck above the water. Column stabilized semisubmersible units design can be classified

as follows (figure 8)

Ring Pontoon Semisubmersibles: Ring pontoon designs normally have one continuous

lower hull (pontoons and nodes) supporting 4-8 vertical columns. The vertical columns are

supporting the upper deck.

Twin Pontoon Semisubmersibles: Twin pontoon designs normally have two lower

pontoons, each supporting 2-4 vertical columns. The 4-8 vertical columns are supporting

the upper deck. In addition it may be strengthened with diagonal braces supporting the

deck and horizontal braces connecting the pontoons or columns.



Figure 8 Column Stabilized Semisubmersibles (Left: Ring Pontoon Design, Right: Twin Pontoon Design,

Source: Petrowiki)

3.3 Example of similar model:

For the current thesis, simplified model of a twin pontoon column stabilized semisubmersible

unit is considered which consist of 4 sets of columns legs, 2 horizontal pontoons, 2 bracings

and a drilling derrick that has been chosen to perform fatigue analysis. In Figure below a similar

existing model of the semisubmersible drilling unit has been shown

Figure 9 Similar Drilling Platform model (Maersk Drilling deep-water semi-submersible rig Source: Prof:

Tadeusz Graczyk Lectures at ZUT)



Figure 9 and 10 represent Column stabilized dynamically positioned semisubmersible drilling

rigs with capability to attach to an 8-point pre-installed mooring system; provisions to attach to

a 12- point pre-installed mooring system.

Figure 10 Semi-submersible platform - Maersk Drilling deep-water semi-submersible rig (Source: Prof:

Tadeusz Graczyk Lectures at ZUT)

Main technical dimensions of analyzed structure of current thesis are listed below in Table 1

Table 1 Main Dimensional Parameter for Semisubmersible Analyzed

Parameters Technical Data

Characteristic Length= Length of Pontoon 80.6 m

Height of Pontoon 7.5 m

Width of Pontoon 16 m

Height of Column 33.5 m

Diameter of Column 12.9 m

Height of Deck 8 m

Spacing of Columns, center to center 54.72 m



4. SOFTWARE PROCEDURE:

4.1 Introduction:

SESAM software package developed by DNV is used for modelling and analysis of the

structure. The DNV software package SESAM consists of different modules which depend on

the simulation that is supposed to be carried out. The following SESAM-software is used;

GeniE, HydroD-WADAM, SESTRA, POSTRESP, STOFAT and XTRACT. Flowchart and

brief description of software procedure is given below:

Figure 11 Flowchart of Software Procedure

4.2 Sesam Genie:

The Sesam GeniE software is a software tool for designing and analyzing offshore and maritime

structures made of beams and plates. Modelling, analysis and results processing are performed

in the same graphic user interface. The use of concept technology makes the Sesam

GeniE software highly efficient for integrating stability, loading, strength assessment and CAD

exchange. All data are persistent enabling the engineers to do efficient iterative re-design of a

structure.

Global MOdel-GeniE

(T1.FEM)

Wadam

(G1.SIF, L1.FEM)

Sestra

(R1.SIN)

Stofat

.Vtf

Postresp

Xtract

Xtract



For floating structures, the Sesam GeniE software can perform static and dynamic linear

analysis for structures subjected to wave, wind, current, and ballast and equipment layout. The

loads and accelerations from the waves and compartment content are defined by the Sesam

HydroD software and they are automatically applied to the structure model independent of the

hydrodynamic panel model.

The wave loads create input to fatigue assessments of both beams and plates using a stochastic

approach. By using the sub-modelling techniques it is very easy to perform a global fatigue

analysis to scan for critical areas

GeniE may be used as a stand-alone tool using a direct analysis approach which also include:

Finite element mesh generation

Finite element analysis

Finite element results visualization

Environmental loads calculation

Code checking and rule based design

Openness towards leading CAD vendors

4.3 HydroD-Wadam:

The Sesam HydroD software is a tool for hydrostatic and hydrodynamic analysis. For the

hydrodynamic part of analysis a sub module included in the HydroD package named as Wadam

has been used. Wadam is a general analysis program for calculation of wave-structure

interaction for fixed and floating structures of arbitrary shape, structures and ship hulls.

The Wadam software is based on widely accepted linear methods for marine hydrodynamics,

the 3-D radiation-diffraction theory employing a panel model and Morison equation in

linearized form employing a beam model. These analyses are normally performed in the

frequency domain, but it is also possible to do it in time domain (Linear as well as non-

linear).The loads are automatically used by the structural analysis. The response and loads may

be graphically assessed in animations. The analysis capabilities in Wadam comprise:

Calculation of hydrostatic data and inertia properties

Calculation of global responses

Calculation of selected global responses of a multi-body system

Automatic load transfer to a finite element model for subsequent structural analysis

http://www.dnv.com/services/software/products/sesam/sesam_hydrod/index.asp

http://www.dnv.com/services/software/products/sesam/sesam_hydrod/index.asp



First and second order 3D potential theory for large volume structures

Morison’s equation and potential theory when the structure comprises of both slender

and large volume parts. The forces at the slender part may optionally be calculated using

the diffracted wave kinematics calculated from the presence of the large volume part of

the structure.

The Wadam results may be presented directly as complex transfer functions or converted to

time domain results for a specified sequence of phase angles of the incident wave. For fixed

structures Morison’s equation may also be used with a time domain output option to calculate

drag forces due to time independent current.

The same analysis model may be applied to both the calculation of global responses in Wadam

and the subsequent structural analysis. For shell and solid element models Wadam also provides

automatic mapping of pressure loads from a panel model to a differently meshed structural

finite element model.

4.4 Sestra:

Sestra is the program for linear static and dynamic structural analysis within the SESAM

program system. It uses a displacement based finite element method. Sestra is computing the

local element matrices and load vectors, assembling them into global matrices and load vectors.

The global matrices are used by algebraic numerical algorithms to do the requested static,

dynamic or linearized buckling analysis. It is interfaced with other program modules of SESAM

for:

Finite element model generation — performed by the preprocessors

Load calculation — performed by the hydrodynamic analysis programs

Results evaluation and presentation — performed by the postprocessors

The analysis capabilities of Sestra are schematically illustrated in Figure 12



Figure 12 Schematic illustration of the capabilities of Sestra (12)

Structural response due to dynamic loading can be analyzed by a quasi-static method, i.e. a

static analysis in Sestra with ‘quasi-static’ loads. The method involves neglecting dynamic

effects of the structure. A quasi-static analysis is often used when the frequency or time-

variation of the load is much lower than the lowest Eigen frequency of the structure. This is

also called stiffness controlled dynamics because the mass and damping forces in the structure

are small compared to the forces resulting from elastic and possible inelastic strains.

4.5 Xtact:

Xtract software is a FE results presentation postprocessor – a high-performance general purpose

model and results visualization program. Xtract presents structural analysis results in alternative

ways: deformed model, contour (iso-) curves, and numeric data on model display, X-Y graphs

and tabulated data. Based on stresses computed by the analysis program Xtract computes and

presents derived stresses: stresses decomposed into membrane and bending parts, principal

stresses and von Mises stress. In addition to its general presentation features the animation

feature of Xtract is especially useful for presenting results from hydrodynamic analyses. The

motion of a vessel in waves may be animated with the resulting stresses in the hull. Interactive

zooming, rotating, panning and cutting allows to achieve the best view of your model.



4.6 Postresp:

The Postresp software have been used to do statistical post-processing of general responses

given as transfer functions in the frequency domain analysis performed for global response of

the platform. The transfer functions have been generated by the hydrodynamic program HydroD

Wadam.

4.7 Stofat:

The Stofat software is a postprocessor for fatigue design. The fatigue calculations are based on

responses given as stress transfer functions. The stresses are generated by hydrodynamic

pressure loads acting on the model. These loads are applied for a number of wave directions

and for a range of wave frequencies covering the necessary sea states. The loads are applied to

a finite element model of the structure whereupon the finite element calculation produces results

as stresses in the elements. Stofat uses these results to calculate fatigue damages at given points

in the structural model. The program also calculates usage factors representing the amount of

fatigue damage that the structure has suffered during a specific design life



5. STRUCTURAL MODELLING

5.1 Modelling Set-Up

The objective of the global structural modeling is to create Panel model, Morison and global

structural finite element model of the drilling platform for use in hydrodynamic and subsequent

structural analysis.

The SI-units has been used to for modelling:

Mass = [kg]

Length = [m]

Time = [s]

Applying these units in the analysis the output will then have the following units:

Force = [N] = [kgm/s2]

Stress = [N/m2] = [Pa]

St52 is used as a material which has following properties:

Table 2 Material Properties of the Structural Model (St52)

Material Property Value Unit

Yield Stress 2.35x 108 Pa

Density 7850 Kg/m3

Young’s Modulus 2.1x1011 Pa

Poison’s Ratio 0.3

Thermal co-efficient 1.2x10-5 delC-1

Damping co-efficient 0.03 N.s/m

Modelling has been done with Sesam GeniE as shown below with color-coding:



Figure 13 Color coding of structural members

Connections between deck and column, column and pontoon are modelled with thicker plate

which is also presented with color coding below:

Figure 14 Color coding of thickness



Global model includes following:

the longitudinal stiffness of the pontoons,

the axial and bending stiffness of the braces,

the axial and bending stiffness of the columns,

the in plane and vertical bending stiffness of the deck

5.2 Pontoon:

The pontoon has been created with shell elements where upper parts are as flat plates and lower

parts are as cylindrical and spherical as shown in Figure 15. Both top and bottom of the pontoon

are flat and rectangular. Longitudinal bulkhead is used which runs along the longitudinal central

axis of each of the pontoon. Longitudinal bulkhead and pontoon shell are key components of

the pontoon. To get geometric stiffness between pontoon shell and longitudinal bulkhead

couple of transvers watertight bulkheads are modelled. Combined framing is used. Local

reinforcements and minor reinforcement has been omitted in the global analysis model.

Figure 15 Pontoon

5.3 Column:

Four Vertical circular columns are modelled to withstand global stiffness as shown below on

Figure 16. Both longitudinal and transverse bulkheads are included with vertical shell plates.



Pontoon and deck connections are possibly the critical region of stress concentration and are

also modelled.

Figure 16 Column

5.5 Deck:

The transversal and longitudinal bulkheads are modeled along with the outer deck shell member

(Figure 17). The girders and the framing system to the upper deck shell were modelled using

T-section and stiffeners are modelled with L-Section bar. Local details i.e. brackets, buckling

stiffeners, etc. has been neglected as they don’t contribute significantly to the global strength.

Figure 17 Deck



Figure 18 Deck Framing System

5.5 Derrick:

Derrick that holds the drilling apparatus has been created with two different pipes (diameter)

as shown on Fig. 19 that contains four vertical legs continued by the sloping legs to the top.

Outer frames are created with the pipe diameter 1m and thickness 0.05m. Outer pipes are

connected with crossbar pipes of 0.8 m diameter and 0.04 m thickness.

Figure 19 Derrick

El. 38.5

El. 46 m

El. 55 m

El. 65 m



5.6 Boundary Conditions:

To avoid rigid body motion of a global structural model at least 6 degrees of freedom have to

be fixed. Three vertical supports should be defined by springs representing the total water

plan stiffness of the structure:

𝑘 = 𝜌𝑤.𝑔. 𝐴𝑤.

Where

Aw = is the total water plan area of the unit (m2).

ρ = density of water = 1025 Kg/m3

g= gravity of 9.81m/s2

k = 1025 kg/m3 x 9.81 m/sec2 x Aw = 10 055x Aw [N/m]

A set of boundary conditions is illustrated in Figure, with the following restraints:

3 vertical restraints (Z)

2 transversal horizontal restraints (Y)

1 longitudinal horizontal restraint (X).

In the figure the two points with fixation in Y have the same Y-coordinate and all three points

have the same Z-coordinate.

Figure 20 Boundary conditions (Source: DNV-RP-C103_2012-04)



Figure 21 Boundary conditions applied on pontoons

The spring stiffness below each corner column is then kspring = k/3 = kz 1, 2, 3. In addition,

horizontal supporting, in transverse and longitudinal direction, is represented by springs equal

0.1 (10% of vertical stiffness is applied in the horizontal direction) of the total vertical spring

stiffness. The transverse horizontal stiffness is applied in two (2) support points, y-direction,

and one (1) spring element is applied in the longitudinal, x direction. kx1 = ky1, 2.

5.7 Panel Model:

A panel model is used to calculate hydrodynamic forces from potential theory. It is the part of

structural model that subjected to the water. Panel model modelling a dummy hydrodynamic

pressure load is applied to the wet surface. Only outer surface of pontoons and columns are

taken into account as a panel model and no internal structural components and bulkheads were

considered as they are not exposed to the hydrodynamic pressure. Wet surface of the structure

that has defined as a panel model is shown below in Figure 22 and Figure 23



Figure 22 Panel Model

Simple meshing techniques have been used to create the panel model. Since the model is double

symmetric only one quarter of the panel model is modelled as shown in Figure 24. The

remaining parts of the model are generated in Software Wadam by the yz-xz symmetry option.

Figure 23 Wet Surface for Hydrodynamic Analysis



Figure 24 Meshed Panel Model

5.8 Morison Model:

The Morison model consists of beam elements representing the transverse bracing. It is used to

viscous damping and drag forces (Morrison forces) of the unit using Morison theory. The

buoyancy and mass forces will be calculated by the panel (radiation and diffraction) model. The

slender pipe sections of Morison model connects the pontoon-column assembly of the structure.

The drag coefficients were assumed as a uniform numerical value of 0.7(Cd) in the horizontal

and vertical axes of the semisubmersible platform for the hydrodynamic analysis. The added

mass coefficient, Ca is set to 0.0 (The added mass is defined as Cm = 1+ Ca in HydroD).A default

value of meshing element length was used to create the mesh model of the Morison elements

as shown below in Figure 25.

Figure 25 Morison Meshed Model



5.9 Structural Model:

It consists of all the structural components including the bulkheads, the girders, bracings and

other key structural connections of the drilling platform. This structural model included the

panel and the Morison model along with the finite element assembly of the deck structure as

illustrated in Figure 26.

Figure 26 Structural Mesh Model

During analysis, problems are detected on some local area elements because of inappropriate

meshing (Figure 27).After checking the Sestra.lis file, bad elements shapes are identified and

fine mesh is used only on the particular elements to get the appropriate result (Figure 28)

Figure 27 Problematic mesh elements.



Figure 28 Final Structural Mesh Model

In the modelling of structural model a combination of dummy hydro pressure load, equipment’s

load and self-weight load is applied. However during the hydrodynamic analysis elements

below still water lines is separated from dry elements with defined hydrodynamic pressure.

Fine mesh



6. HYDRODYNAMIC ANALYSIS:

Wave loads are computed by HydroD Wadam using Morison’s equation and potential theory.

Wadam is an integrated part of the SESAM suite of programs which is tailored to calculate

wave loads on models created by the SESAM Genie. The results from the Wadam global

response analysis stored on a Hydrodynamic Results Interface File (G-file) for statistical post

processing in Postresp. The loads mapped to structural finite elements stored on the Loads

Interface File (L-file) for a subsequent structural analysis in Sestra.

A Hydro model was created using HydroD software to perform hydrodynamic analysis on the

global structure of the drilling unit. Hydro model is shown in Figure 29 below which is

consisted of finite element model of the structure.

Figure 29 Hydro Model of Semisubmersible Drilling Platform

6.1 Analysis Setup

Analysis was set up to get motion response and transferring the wave loads to structural model.

All models including Panel, Morison and Structural model was translated by -13.5m in z

directions which is considered as operating draft of the unit to ensure that mean sea level is

around z= 0 as per recommendation of the software manuals. This operating draft of 13.5 m



allows an acceptable balance of net buoyancy and static forces acting on the semisubmersible

model analyzed with minimal trim and heel condition.

For analysis setup following steps has been defined in HydroD Wadam package:

Analysis was done with frequency domain and the drag effects of the incident waves

were defined as linearization by stochastic method.

First direction defined with 0 degree and last direction 315 degree with respect to the

platform longitudinal axis to the sea state with the step value 45 degree

Table 3 Sea State Direction Set

No. of Direction Direction(deg)

1 0

2 45

3 90

4 135

5 180

6 225

7 270

8 315

Period is set between 0.5 to 25 sec with the step value 1 sec

Spectrum is setup to Bretshneider spectrum which is also known as a 2 parameter

Pierson-Moskowitz spectrum where only Hs (significant wave height) and Tp (Peak

period) need to be defined. Design wave was selected based on most extreme

environmental conditions of North Sea with 100 year return period. Significant Wave

height Hs=13.6m and Peak Period (Tp) =16s was found for the current operation of

mobile offshore units.

Spreading function of exponent 2 is selected to define short crested sea where main

heading is assumed to be 45 degree with respect to longitudinal axis. In short crested

sea other wave directions are taken into account than the current main wave direction

Hydro model is defined as floating

The water depth of the location was assumed to be uniform 300 m in the central North

Sea region and typical water depth for operation of drilling unit in that region. Water

density, Kinematic viscosity and Gravity is also defined with the value 1025Kg/m3,

1.19e-006m2/s and 9.80665 m/s2 respectively.



Sea state duration is selected as 3 hours which has been introduced as a standard time

between registrations of sea states when measuring waves. In connection with stochastic

response analysis, linear (Airy) theory is used

Four compartments are created inside two pontoons are shown in Figure 30, which

contains fluid with 900 Kg/m3 density. Same permeability is assigned for all

compartments

Figure 30 Four compartments inside two pontoons

To calculate wave pressure on defined point and to define grid to represent sea surface

off body points are used which was then post processed in Postresp is show in Figure

31 below. The range for the off-body points was taken as a grid system between (300m,

200m) to (-300m, -200m) with an interval of 25m in x axis and 20m in y axis.

Figure 31 Off body points to define sea state grid

Drift forces are calculated with far field integration (horizontal) method



Higher frequency limit in HydroD is around 2, too fine mesh create too much nodes

which may make size problem in Stofat so rather coarse mesh is used for global

analysis

Mass Model properties for the Hydrodynamic Analysis is given below in Table 4

Table 4 Mass Model Properties of the Structure

Property Value Unit

Mass of structural Model 23.6 x 106 Kg

Buoyancy Volume 25.1x103 m3

Centre of Buoyancy in coordinate(x,y,z) (0, 0, -13.5) m

Centre of Gravity in coordinate (x,y,z) (0, 0, -10) m

Radius of Gyration (x,y,z) 30.9, 29.7,38.6 m

Roll-Pitch Centrifugal Moment (XYRAD) -5.67 x10-14 m2

Roll-YAW Centrifugal Moment (XZRAD) 0 m2

Pitch-YAW Centrifugal Moment(YZRAD) -4.63 x10-15 m2

The mass model of the structure as show belong in Figure 32 was created by HydroD using

“user defined” option of homogenous density panel model with input co-ordinate system and

“fill from buoyancy” tab

Figure 32 Mass model of the Drilling Unit



The load-cases based on combination of wave direction and wave period analyzed for different

loading conditions has been mentioned as follows

Table 5 Load cases for Heading and Wave periods

Load

Case Heading Period

Load

Case Heading Period

Load

Case Heading Period

Load

Case Heading Period

2_01 0 0.5 3_01 45 0.5 4_01 90 0.5 5_01 135 0.5

2_02 0 1.5 3_02 45 1.5 4_02 90 1.5 5_02 135 1.5

2_03 0 2.5 3_03 45 2.5 4_03 90 2.5 5_03 135 2.5

2_04 0 3.5 3_04 45 3.5 4_04 90 3.5 5_04 135 3.5

2_05 0 4.5 3_05 45 4.5 4_05 90 4.5 5_05 135 4.5

2_06 0 5.5 3_06 45 5.5 4_06 90 5.5 5_06 135 5.5

2_07 0 6.5 3_07 45 6.5 4_07 90 6.5 5_07 135 6.5

2_08 0 7.5 3_08 45 7.5 4_08 90 7.5 5_08 135 7.5

2_09 0 8.5 3_09 45 8.5 4_09 90 8.5 5_09 135 8.5

2_10 0 9.5 3_10 45 9.5 4_10 90 9.5 5_10 135 9.5

2_11 0 10.5 3_11 45 10.5 4_11 90 10.5 5_11 135 10.5

2_12 0 11.5 3_12 45 11.5 4_12 90 11.5 5_12 135 11.5

2_13 0 12.5 3_13 45 12.5 4_13 90 12.5 5_13 135 12.5

2_14 0 13.5 3_14 45 13.5 4_14 90 13.5 5_14 135 13.5

2_15 0 14.5 3_15 45 14.5 4_15 90 14.5 5_15 135 14.5

2_16 0 15.5 3_16 45 15.5 4_16 90 15.5 5_16 135 15.5

2_17 0 16.5 3_17 45 16.5 4_17 90 16.5 5_17 135 16.5

2_18 0 17.5 3_18 45 17.5 4_18 90 17.5 5_18 135 17.5

2_19 0 18.5 3_19 45 18.5 4_19 90 18.5 5_19 135 18.5

2_20 0 19.5 3_20 45 19.5 4_20 90 19.5 5_20 135 19.5

2_21 0 20.5 3_21 45 20.5 4_21 90 20.5 5_21 135 20.5

2_22 0 21.5 3_22 45 21.5 4_22 90 21.5 5_22 135 21.5

2_23 0 22.5 3_23 45 22.5 4_23 90 22.5 5_23 135 22.5

2_24 0 23.5 3_24 45 23.5 4_24 90 23.5 5_24 135 23.5

2_25 0 24.5 3_25 45 24.5 4_25 90 24.5 5_25 135 24.5

2_26 0 25 3_26 45 25 4_26 90 25 5_26 135 25



Load Case Heading Period

Load Case

Heading Period Load Case

Heading Period Load Case

Heading Period

6_01 180 0.5 7_01 225 0.5 8_01 270 0.5 9_01 315 0.5

6_02 180 1.5 7_02 225 1.5 8_02 270 1.5 9_02 315 1.5

6_03 180 2.5 7_03 225 2.5 8_03 270 2.5 9_03 315 2.5

6_04 180 3.5 7_04 225 3.5 8_04 270 3.5 9_04 315 3.5

6_05 180 4.5 7_05 225 4.5 8_05 270 4.5 9_05 315 4.5

6_06 180 5.5 7_06 225 5.5 8_06 270 5.5 9_06 315 5.5

6_07 180 6.5 7_07 225 6.5 8_07 270 6.5 9_07 315 6.5

6_08 180 7.5 7_08 225 7.5 8_08 270 7.5 9_08 315 7.5

6_09 180 8.5 7_09 225 8.5 8_09 270 8.5 9_09 315 8.5

6_10 180 9.5 7_10 225 9.5 8_10 270 9.5 9_10 315 9.5

6_11 180 10.5 7_11 225 10.5 8_11 270 10.5 9_11 315 10.5

6_12 180 11.5 7_12 225 11.5 8_12 270 11.5 9_12 315 11.5

6_13 180 12.5 7_13 225 12.5 8_13 270 12.5 9_13 315 12.5

6_14 180 13.5 7_14 225 13.5 8_14 270 13.5 9_14 315 13.5

6_15 180 14.5 7_15 225 14.5 8_15 270 14.5 9_15 315 14.5

6_16 180 15.5 7_16 225 15.5 8_16 270 15.5 9_16 315 15.5

6_17 180 16.5 7_17 225 16.5 8_17 270 16.5 9_17 315 16.5

6_18 180 17.5 7_18 225 17.5 8_18 270 17.5 9_18 315 17.5

6_19 180 18.5 7_19 225 18.5 8_19 270 18.5 9_19 315 18.5

6_20 180 19.5 7_20 225 19.5 8_20 270 19.5 9_20 315 19.5

6_21 180 20.5 7_21 225 20.5 8_21 270 20.5 9_21 315 20.5

6_22 180 21.5 7_22 225 21.5 8_22 270 21.5 9_22 315 21.5

6_23 180 22.5 7_23 225 22.5 8_23 270 22.5 9_23 315 22.5

6_24 180 23.5 7_24 225 23.5 8_24 270 23.5 9_24 315 23.5

6_25 180 24.5 7_25 225 24.5 8_25 270 24.5 9_25 315 24.5

6_26 180 25 7_26 225 25 8_26 270 25 9_26 315 25



6.2 Global Motion Response Analysis:

The response of the structure was measured in terms of its Response Amplitude Operators

(RAOs) for the 6 degree of freedom which is presented with POSTRESP software below:

Figure 33 RAO for different wave directions at relative points (0, 0, and 35)

The RAO is shown above for a relative point (0, 0, and 35). There are two peaks, one at 21s

(amplitude 8.1) which comes from the heave resonance mainly and one at 24s (amplitude 2.4)

which comes from the roll resonance mainly. The worst wave direction is 270 and 45 degrees

for both peaks. RAO for Heave, Roll and Pitch are given below in Figure 34, Figure 35 and

Figure 36:



Figure 34 RAO of Heave

Figure 35 RAO of Roll



Figure 36 RAO of Pitch

Since the critical wave periods are related to natural periods and viscous damping is important

for the vertical motions of a semi-submersible. So Wadam was rerun after introducing some

damping in heave, roll and pitch. 5% damping is given in all modes as shown in below Figure.

.

Figure 37 Damping Matrix

The new RAO is shown in Figure 38. There are still two peaks, one at 14s (amplitude 0.6)

which comes from the roll resonance and one at 24s (amplitude 1.6) which comes from the

heave resonance. But now the first peak is significantly larger.



Figure 38 RAO at relative point (0, 0, and 35) after addition of damping

The worst wave direction is 180 degrees for the largest peak and 270 degree for highest peak.

DNV-NA scatter diagram is used and short crested sea states is assumed.

From Postresp long term response is calculated for 270 degree heading as given in Table 6

Table 6 Long term response from Postresp

Heading 270 Year=1 Year=5 Year=10 Year=50 Year=100

RAO 7.95 8.93 9.36 10.4 10.8

Output from the HydroD Wadam was represented as a global motion response and

hydrodynamic loading on the drilling unit. Surface wave loads on the platform for various load-

cases based on excitation frequencies for different motions has shown below in Figure 39 to

Figure 44 with contour plots. From the figures it is clear that heave, pith and roll motions are

more dominant where other motions like Sway, yaw or surge motions are almost negligible.

Sway or yaw motions are negligible because of symmetry of the structure and also in deep

water sway motion is negligible.



Figure 39 Surface wave loads at 180 degree heading and 0.053 Hz frequency (Pitch RAO)

Figure 40 Surface wave loads at 270 degree heading and 0.045 Hz frequency (Heave RAO)



Figure 41Surface wave loads at 270 degree heading and 0.042 Hz frequency (Roll RAO)

Figure 42 Surface wave loads at 180 degree heading and 0.033 Hz frequency (Surge RAO)



Figure 43 Surface wave loads at 270 degree heading and 0.033 Hz frequency (Sway RAO)

Figure 44 Surface wave loads at 315 degree heading and 0.166 Hz frequency (YAW RAO)



7. GLOBAL STRUCTURAL STRENGTH ANALYSIS

The hydrodynamic load is transferred to structural model (Figure 45) for subsequent quasi-

static analysis of the structure. The rigid body motions of the model were restrained by means

of applying spring elements to provide the required balance of forces to the structural loading.

The load cases of the structure are listed below:

Self-weight of the structure

Equipment’s which are positioned symmetrically in four positons of the structure, each

of them are 15000Kg, 4m height, 3m length and 5m width

Hydrodynamic loads from WADAM analysis

Four mass points at top of the derrick where each mass point has a mass of 2.0E5 Kg

The local effect of the wind and current loads on the structure is considered negligible

compared to extreme wave loading of 100 year return period

Figure 45 Global structural model

The quasi- static structural analysis were done for different wave frequencies and headings.

Some von-misses stresses are plotted below with XTRACT software tool in Figure 46 and

Figure 47. The von Mises stresses are low, since the structure response from only one frequency

can be considered in a frequency domain analysis but the wave load is a sum of frequencies

that are in different phases relative to each other. Combined effects are considered in later

section during fatigue analysis.



Figure 46 Von-Misses Stress at 45 degree wave heading and 0.1111 Hz excitation frequency

Figure 47 Von-Misses Stress on Column



8. GLOBAL FATIGUE ANALYSIS AND RESULT:

Stofat software tool is used for spectral fatigue analysis which is followed by frequency domain

hydrodynamic analysis from HydeoD Wadam and quasi-static structural analysis from Sestra

that was executed earlier. Harmonic waves of unit amplitude at different frequencies and

directions are passed through the structure and generate a set of stress transfer functions which

are read into Stofat through the Result Interface File and used in the long term stochastic fatigue

calculations. In spectral method, long term fatigue calculation is based directly on a scatter

diagram, response spectrum and SN-curves as input. SN curve is used to define the fatigue

characteristics of a material subjected to repeated cycle of stress of constant magnitude. The

wave climate is presented by a scatter diagram representing North Atlantic which provides the

frequency of occurrence of a given parameter pair (e.g. (HS, Tz)) as shown in Figure 48 below:

Figure 48 Scatter Diagram for the North Atlantic (Source: DNV-RP-C205)



The SN-curve delivers the number of cycles required to produce failure for a given magnitude

of stress (Figure 49). DNVC-I SN-curve is used which is default for Stofat (Figure 50)

Figure 49 DNV-SN Curves (Stofat_UM)

Figure 50 DNVC-I SN curve plotted from STOFAT



A Bretschneider wave spectrum is used. Priority is given to worst usage factor. Usage factor is

above 0.80 and design fatigue life is set to 20 years. Cos2 is the wave-spreading function to

define short crested sea for fatigue analysis which is plotted from STOFAT below:

Figure 51 Wave Spreading function for short crested Sea

STOFAT obtains the principal stresses from SESTRA and calculates the accumulated partial

damage .The accumulated partial damage is weighted over sea states and 8 wave directions.

The wave directions that are considered are 0, 45 up to 360 degrees. The definition of the wave

direction, or heading angle, is presented in Fig. 52



The locations of the most critical elements in terms of usage factors are presented in Figure 53.

The usage factor is defined as the design life - life in service - divided by the calculated fatigue

life. For example, if the usage factor is 1.0, it will result in failure after 20 years, or if the usage

factor is 0.5, it will result in failure after 40 years.

Figure 53 Maximum Usage Factor of the Structure

Beam 90°

Bow 135°

Following 0°

Quarter 45°

Quarter 315°

Beam 270°

Bow 225°

Structure

Figure 52 Definition of the wave direction (heading angle) in this investigation.

Head 180°



Figure 54 Fatigue Life of Global Structure

Fatigue life is shown in above Figure 54 based on element of the global structure. Fatigue life

for critical connections are presented below which are the average value of adjacent elements

of the connections.

Table 7 Fatigue life in Critical connections

Connections Fatigue Life (Years)

Deck to Column Above 50

Column to Pontoon Around 30

Column to Brace Above 50

Deck to Derrick Around 40 The fatigue-critical locations are presented in Table 7 for a fatigue analysis with equal

probability for all wave directions. It seems reasonable that the elements that have the maximum

usage factors and minimum fatigue life are located in the column to pontoon region for

combined loading conditions; column to pontoon connections is also the maximum stressed

region of the unit. The Appendix B present the elements with the maximum usage factors for

different wave directions and the calculated fatigue life for these elements.



9. CONCLUSIONS AND RECOMMENDATIONS:

Global fatigue analysis of an offshore drilling unit with extreme climatic conditions was

presented in this study. First, a global model of the platform was done followed by

hydrodynamic analysis. A linear structural FE-analysis and fatigue analysis was performed for

the global model in order to localize the critical locations. The analyses showed that the critical

regions are located in the column to pontoon connections, column to brace connections and

deck to derrick connections. Among these locations, column to pontoon connections showed

the worst fatigue life.

A study on the influence of different wave directions was performed in order to find the most

critical wave direction. The worst wave direction is found at 270 and 180 degrees. The

maximum stress level due to wave induced loading were found to be occurring at around similar

wave frequency range (f= 0.041-0.047 Hz or T= 21-24 sec).

The motion response is one of the critical factor during drilling operation of the platform. Heave

is one of the most significant motion response for operations of drilling equipment’s. Heave

response was found maximum at 270 and 180 degree wave heading and peak response is at

around 8 seconds.

There are more analyses that could have been performed if there had been more time reserved

for the project. Here is some recommendations for future work:

Local sub-models can be created and analysis for local models can be performed as

stress ranges could locally become significant

More detailed local non- linear finite element analysis and consideration of mooring

lines & riser system can be done for more precise information regarding structural

strength.

Other sources of excitation for example; wind, current, engine and propeller response,

whipping, springing etc could be taken into account to get more accurate result.



The effect of the weld can be considered since offshore structures are commonly built

with welded plating. Tensile residual stresses from welding will emphasize the crack-

growth, which will decrease the fatigue life.



REFERENCES

1. Guidelines to Assess High-Frequency Hull Girder Response of Container Ships by

DNV-GL, 2014

2. “Analysis and Design of Ship Structure”, Chapter 18, Philippe Rigo and Enrico

Rizzuto

3. “Probabilistic Fatigue of offshore structures”, G. Sigurdsson, University of Aalborg,

Sohngaardsholmsvej 51, DK-9000 Aalborg, Denmark

4. Global Response Analysis for Semisubmersible Offshore Platform, Niraj Kumar Singh,

Thesis, EMSHIP-2013

5. Global and Detailed Local Fatigue Assessment of a Container Vessel, Camilla Knifsund

& Andrea Tesanovic, Chalmers University of Technology, 2012

6. DNV-RP-C206, Fatigue Methodology of Offshore Ships, 2012

7. DNV-RP-C103, Column-Stabilized Units, April 2012

8. DNV-CN-30.7, Fatigue Assessment of Ship Structures APRIL 2014

9. DNVGL-RP-C203:Fatigue design of offshore steel structures, 2014

10. DNV Sesam GeniE user manual

11. DNV Sesam HydroD user manual

12. DNV Sesam Sestra user manual

13. DNV Sesam Xtract user manual

14. DNV Sesam Postresp user manual

15. DNV Sesam Stofat user manual

16. DNV-Mesh Guidance-GeniEv5.1, February 2010

17. DNV GeniE Tutorial

18. DNV HydroD Tutorial

19. http://www.boem.gov/2012-2017-Lease-Sale-Schedule/ -Picture for Components of

Offshore Rigs

20. http://www.brighthubengineering.com/marine-engines-machinery/30775-different-

types-of-offshore-production-platforms-for-oil-extraction/#- Picture of Fixed Platform

http://www.boem.gov/2012-2017-Lease-Sale-Schedule/

http://www.brighthubengineering.com/marine-engines-machinery/30775-different-types-of-offshore-production-platforms-for-oil-extraction/#-

http://www.brighthubengineering.com/marine-engines-machinery/30775-different-types-of-offshore-production-platforms-for-oil-extraction/#-



APPENDIX

Appendix A: Summary of Model Properties -------------------------------

ALL COORDINATES ARE GIVEN IN THE INPUT COORDINATE SYSTEM

THE RADII OF GYRATION AND CENTRIFUGAL MOMENTS OF THE MASS MATRIX

AND THE RESTORING COEFFICIENTS ARE GIVEN RELATIV TO THE MOTION

REFERENCE POINT

(ORIGIN OF THE GLOBAL COORDINATE SYSTEM).

UNITS DATA:

-----------

ACCELERATION OF GRAVITY G = 9.80665E+00 [L/T**2]

WATER DENSITY RHO= 1.02500E+03 [M/L**3]

GEOMETRY DATA:

--------------

CHARACTERISTIC LENGTH L = 8.06000E+01 [L]

VERTICAL COORDINATE OF STILL WATER LINE -ZLOC = 0.00000E+00 [L]

NUMBER OF NODES IN THE MORISON MODEL NMNOD = 45

NUMBER OF MORISON ELEMENTS NMELM = 47

NUMBER OF BASIC PANELS = 740

NUMBER OF SYMMETRY PLANES IN

THE PANEL MODEL = 2

TOTAL NUMBER OF PANELS = 2960

DISPLACED VOLUMES OF THE PANEL MODEL

VOL 1 = 2.48355E+04 [L**3]

VOL 2 = 2.48363E+04

VOL 3 = 2.48398E+04



MASS PROPERTIES AND STRUCTURAL DATA:

------------------------------------

MASS OF THE STRUCTURE M = 2.57415E+07 [M]

WEIGHT OF THE STRUCTURE M*G = 2.52438E+08 [M*L/T**2]

CENTRE OF GRAVITY

XG = 8.32135E-16 [L]

YG = 1.44719E-16 [L]

ZG =-1.05366E+01 [L]

ROLL RADIUS OF GYRATION XRAD = 3.13273E+01 [L]

PITCH RADIUS OF GYRATION YRAD = 2.88244E+01 [L]

YAW RADIUS OF GYRATION ZRAD = 3.80055E+01 [L]

ROLL-PITCH CENTRIFUGAL MOMENT XYRAD =-5.78876E-14 [L**2]

ROLL-YAW CENTRIFUGAL MOMENT XZRAD = 0.00000E+00 [L**2]

PITCH-YAW CENTRIFUGAL MOMENT YZRAD = 2.31550E-15 [L**2]

HYDROSTATIC DATA:

-----------------

DISPLACED VOLUME VOL = 2.50991E+04 [L**3]

MASS OF DISPLACED VOLUME RHO*VOL = 2.57266E+07 [M]

WATER PLANE AREA WPLA = 5.16672E+02 [L**2]

CENTRE OF BUOYANCY

XCB = 3.64765E-09 [L]

YCB =-4.55957E-09 [L]

ZCB =-1.35398E+01 [L]

TRANSVERSE METACENTRIC HEIGHT GM4= 1.26292E+01 [L]

LONGITUDINAL METACENTRIC HEIGHT GM5= 1.26270E+01 [L]

HEAVE-HEAVE RESTORING COEFFICIENT C33= 5.19349E+06 [M/T**2]

HEAVE-ROLL RESTORING COEFFICIENT C34= 0.00000E+00 [M*L/T**2]

HEAVE-PITCH RESTORING COEFFICIENT C35= 0.00000E+00[M*L/T**2]

ROLL-ROLL RESTORING COEFFICIENT C44= 3.18623E+09 [M*L**2/T**2]

PITCH-PITCH RESTORING COEFFICIENT C55= 3.18569E+09 [M*L**2/T**2]

ROLL-PITCH RESTORING COEFFICIENT C45 = 0.00000E+00 [M*L**2/T**2



2.8 ENVIRONMENTAL DATA:

-----------------------

WATER DEPTH = 3.00000E+02 [L]

NUMBER OF WAVE LENGTHS = 30

NUMBER OF HEADING ANGLES = 8

WAVE DESCRIPTION:

WAVE WAVE WAVE WAVE ANG.

LENGTH NUMBER PERIOD FREQUENCY

1 1.56078E+00 4.02568E+00 1.00000E+00 6.28319E+00

2 6.24311E+00 1.00642E+00 2.00000E+00 3.14159E+00

3 1.40470E+01 4.47298E-01 3.00000E+00 2.09440E+00

4 2.49724E+01 2.51605E-01 4.00000E+00 1.57080E+00

5 3.90194E+01 1.61027E-01 5.00000E+00 1.25664E+00

6 5.61880E+01 1.11824E-01 6.00000E+00 1.04720E+00

7 7.64781E+01 8.21567E-02 7.00000E+00 8.97598E-01

8 9.98897E+01 6.29012E-02 8.00000E+00 7.85398E-01

9 1.26423E+02 4.96997E-02 9.00000E+00 6.98132E-01

10 1.56078E+02 4.02568E-02 1.00000E+01 6.28319E-01

11 1.88854E+02 3.32701E-02 1.10000E+01 5.71199E-01

12 2.24752E+02 2.79561E-02 1.20000E+01 5.23599E-01

13 2.63771E+02 2.38206E-02 1.30000E+01 4.83322E-01

14 3.05910E+02 2.05394E-02 1.40000E+01 4.48799E-01

15 3.51159E+02 1.78927E-02 1.50000E+01 4.18879E-01

16 3.99495E+02 1.57278E-02 1.60000E+01 3.92699E-01

17 4.50854E+02 1.39362E-02 1.70000E+01 3.69599E-01

18 5.05112E+02 1.24392E-02 1.80000E+01 3.49066E-01

19 5.62065E+02 1.11788E-02 1.90000E+01 3.30694E-01

20 6.21422E+02 1.01110E-02 2.00000E+01 3.14159E-01

21 6.82816E+02 9.20187E-03 2.10000E+01 2.99199E-01

22 7.45838E+02 8.42433E-03 2.20000E+01 2.85599E-01



23 8.10070E+02 7.75635E-03 2.30000E+01 2.73182E-01

24 8.75123E+02 7.17977E-03 2.40000E+01 2.61799E-01

25 9.40660E+02 6.67955E-03 2.50000E+01 2.51327E-01

26 1.00641E+03 6.24320E-03 2.60000E+01 2.41661E-01

27 1.07215E+03 5.86038E-03 2.70000E+01 2.32711E-01

28 1.13773E+03 5.52258E-03 2.80000E+01 2.24399E-01

29 1.20304E+03 5.22277E-03 2.90000E+01 2.16662E-01

30 1.26801E+03 4.95517E-03 3.00000E+01 2.09440E-01

HEADING ANGLES (ANGLE BETWEEN POS. X-AXIS AND DIRECTION

OF WAVE PROPAGATION):

IN DEGREES IN RADIANS

1 0.00000E+00 0.00000E+00

2 4.50000E+01 7.85398E-01

3 9.00000E+01 1.57080E+00

4 1.35000E+02 2.35619E+00

5 1.80000E+02 3.14159E+00

6 2.25000E+02 3.92699E+00

7 2.70000E+02 4.71239E+00

8 3.15000E+02 5.49779E+00



Appendix B: Element Fatigue Check Result

NOMENCLATURE:

Element Name of element

Stat = PASS or FAIL: *FAIL = UsageFactor > 1.0

UsageFact Accumulated damage (Usage factor)

ChkPnt Fatigue check point number of element

ChkPlc Check at: Stress/surface/corner/mid-plane/or membrane points

AccFatLif Design fatigue life/usage factor (year)

StrsCycle Total number of stress cycles

SNCurve SN curve name.

atSide -z side or +z side of shell element

ElType Element type

X-coord. X coordinate of fatigue check point

Y-coord. Y coordinate of fatigue check point

Z-coord. Z coordinate of fatigue check point

ElThck Element thickness

AxialScf Resulting Axial stress K-factor (SCF factor)

BendScf Resulting Bending stress K-factor (SCF factor)

ShearScf Resulting Shear stress K-factor (SCF factor)

WeibScale Scale parameter of Weibull distribution

WeibShape Shape parameter of Weibull distribution

StressRange Maximum Stress Range of principal stress

Coordinate reference system : Current superelement

Status on failure : *FAIL when UsageFactor > 1.0

Design fatigue life : 20.0 years

Fatigue calculation based on : Spectral moments of maximum principal stresses

STOCHASTIC ELEMENT fatigue check results

Run: FR1 Super element MODEL

Priority.....: Worst Usage Factor

Usage factor: Above 0.80

Element Stat Usage Fact ChkPnt ChkPlc AccFatLif StrsCycle SNCurve



E1 Type X-coord Y-coord. Z-coord.

E1Thck AxialScf BendScf ShearScf

WeibScale WeibShape StressRange

1768 *FAIL 6.62E+00 10(+z) SurfPt 3.02E+00 7.01E+07 DNVC-I

FQUS24 -2.73E+01 -2.09E+01 -6.00E+00

0.04 1.5 1.5 1.5

3.90E+07 9.74E-01 6.84E+08

8916 *FAIL 6.48E+00 4 (-z) SurfPt 3.09E+00 7.06E+07 DNVC-I

FQUS24 2.74E+01 -2.09E+01 -6.00E+00

0.04 1.5 1.5 1.5

3.86E+07 9.72E-01 6.83E+08

1924 *FAIL 6.37E+00 9 (+z) SurfPt 3.14E+00 7.03E+07 DNVC-I

FQUS24 -2.73E+01 2.09E+01 -6.00E+00

0.04 1.5 1.5 1.5

3.85E+07 9.76E-01 6.72E+08


FQUS24 2.73E+01 2.09E+01 -6.00E+00

0.04 1.5 1.5 1.5

3.78E+07 9.74E-01 6.62E+08


FQUS24 2.73E+01 2.09E+01 -6.00E+00

0.04 1.5 1.5 1.5

3.77E+07 9.76E-01 6.54E+08


FQUS24 -2.73E+01 -2.09E+01 -6.00E+00

0.04 1.5 1.5 1.5

3.65E+07 9.73E-01 6.43E+08


FQUS24 -2.73E+01 2.09E+01 -6.00E+00

0.04 1.5 1.5 1.5



3.66E+07 9.79E-01 6.34E+08


FQUS24 2.74E+01 -2.09E+01 -6.00E+00

0.04 1.5 1.5 1.5

3.66E+07 9.75E-01 6.40E+08


FQUS24 -2.74E+01 -2.09E+01 -6.02E+00

0.04 1.5 1.5 1.5

3.60E+07 9.74E-01 6.31E+08


FQUS24 -2.74E+01 -2.09E+01 -6.02E+00

0.04 1.5 1.5 1.5

3.57E+07 9.75E-01 6.27E+08


FQUS24 2.74E+01 2.09E+01 -6.02E+00

0.04 1.5 1.5 1.5

3.57E+07 9.73E-01 6.28E+08






FQUS24 -3.43E+01 -2.54E+01 -1.30E+01

0.047 1.5 1.5 1.5

1.95E+07 7.67E-01 7.12E+08


FQUS24 2.04E+01 2.54E+01 -1.30E+01

0.047 1.5 1.5 1.5

2.18E+07 8.10E-01 6.56E+08


FQUS24 2.25E+01 -2.07E+01 -1.14E+01



0.047 1.5 1.5 1.5

2.97E+07 1.02E+00 4.34E+08


FQUS24 -2.25E+01 2.07E+01 -1.14E+01

0.047 1.5 1.5 1.5

2.76E+07 9.79E-01 4.58E+08


FQUS24 -2.04E+01 2.34E+01 -1.30E+01

0.047 1.5 1.5 1.5

2.06E+07 8.00E-01 6.40E+08


FQUS24 2.25E+01 -2.07E+01 -1.14E+01

0.047 1.5 1.5 1.5

2.89E+07 1.02E+00 4.22E+08


FQUS24 2.04E+01 -2.54E+01 -1.30E+01

0.047 1.5 1.5 1.5

2.27E+07 8.59E-01 5.73E+08


FQUS24 -2.04E+01 -2.34E+01 -1.30E+01

0.047 1.5 1.5 1.5

2.45E+07 9.08E-01 4.96E+08


FQUS24 -2.25E+01 2.07E+01 -1.14E+01

0.047 1.5 1.5 1.5

2.58E+07 9.65E-01 4.51E+08


FQUS24 -3.43E+01 2.94E+01 -1.30E+01

0.047 1.5 1.5 1.5

1.93E+07 7.94E-01 6.18E+08


FQUS24 -2.25E+01 -2.07E+01 -1.14E+01

0.047 1.5 1.5 1.5

2.81E+07 1.02E+00 4.14E+08








FQUS24 -2.73E+01 -2.09E+01 -6.00E+00

0.04 1.5 1.5 1.5

3.90E+07 9.74E-01 6.84E+08


FQUS24 2.74E+01 -2.09E+01 -6.00E+00

0.04 1.5 1.5 1.5

3.86E+07 9.72E-01 6.83E+08


FQUS24 -2.73E+01 2.09E+01 -6.00E+00

0.04 1.5 1.5 1.5

3.85E+07 9.76E-01 6.72E+08


FQUS24 2.73E+01 2.09E+01 -6.00E+00

0.04 1.5 1.5 1.5

3.78E+07 9.74E-01 6.62E+08


FQUS24 2.73E+01 2.09E+01 -6.00E+00

0.04 1.5 1.5 1.5

3.77E+07 9.76E-01 6.54E+08


FQUS24 -2.73E+01 -2.09E+01 -6.00E+00

0.04 1.5 1.5 1.5

3.65E+07 9.73E-01 6.43E+08


FQUS24 -2.73E+01 2.09E+01 -6.00E+00

0.04 1.5 1.5 1.5

3.66E+07 9.79E-01 6.34E+08


FQUS24 2.74E+01 -2.09E+01 -6.00E+00

0.04 1.5 1.5 1.5

3.66E+07 9.75E-01 6.40E+08




FQUS24 -2.74E+01 -2.09E+01 -6.02E+00

0.04 1.5 1.5 1.5

3.60E+07 9.74E-01 6.31E+08


FQUS24 -2.74E+01 -2.09E+01 -6.02E+00

0.04 1.5 1.5 1.5

3.57E+07 9.75E-01 6.27E+08


FQUS24 2.74E+01 2.09E+01 -6.02E+00

0.04 1.5 1.5 1.5

3.57E+07 9.73E-01 6.28E+08






FQUS24 -3.43E+01 -2.54E+01 -1.30E+01

0.047 1.5 1.5 1.5

1.95E+07 7.67E-01 7.12E+08


FQUS24 2.04E+01 2.54E+01 -1.30E+01

0.047 1.5 1.5 1.5

2.18E+07 8.10E-01 6.56E+08


FQUS24 2.25E+01 -2.07E+01 -1.14E+01

0.047 1.5 1.5 1.5

2.97E+07 1.02E+00 4.34E+08


FQUS24 -2.25E+01 2.07E+01 -1.14E+01

0.047 1.5 1.5 1.5

2.76E+07 9.79E-01 4.58E+08


FQUS24 -2.04E+01 2.34E+01 -1.30E+01

0.047 1.5 1.5 1.5



2.06E+07 8.00E-01 6.40E+08


FQUS24 2.25E+01 -2.07E+01 -1.14E+01

0.047 1.5 1.5 1.5

2.89E+07 1.02E+00 4.22E+08


FQUS24 2.04E+01 -2.54E+01 -1.30E+01

0.047 1.5 1.5 1.5

2.27E+07 8.59E-01 5.73E+08


FQUS24 -2.04E+01 -2.34E+01 -1.30E+01

0.047 1.5 1.5 1.5

2.45E+07 9.08E-01 4.96E+08


FQUS24 -2.25E+01 2.07E+01 -1.14E+01

0.047 1.5 1.5 1.5

2.58E+07 9.65E-01 4.51E+08


FQUS24 -3.43E+01 2.94E+01 -1.30E+01

0.047 1.5 1.5 1.5

1.93E+07 7.94E-01 6.18E+08


FQUS24 -2.25E+01 -2.07E+01 -1.14E+01

0.047 1.5 1.5 1.5

2.81E+07 1.02E+00 4.14E+08






FQUS24 3.43E+01 -2.54E+01 -1.30E+01

0.047 1.5 1.5 1.5

2.40E+07 9.01E-01 5.19E+08


FQUS24 2.04E+01 2.34E+01 -1.30E+01



0.047 1.5 1.5 1.5

2.20E+07 8.55E-01 5.61E+08


FQUS24 -3.42E+01 -2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.77E+07 7.62E-01 6.58E+08


FQUS24 2.25E+01 2.07E+01 -1.14E+01

0.047 1.5 1.5 1.5

2.77E+07 1.01E+00 4.18E+08


FQUS24 -3.42E+01 2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.88E+07 7.84E-01 6.28E+08


FQUS24 -3.43E+01 2.54E+01 -1.30E+01

0.047 1.5 1.5 1.5

1.85E+07 7.84E-01 6.13E+08


FQUS24 3.43E+01 2.94E+01 -1.30E+01

0.047 1.5 1.5 1.5

1.95E+07 8.35E-01 5.34E+08


FQUS24 -3.42E+01 2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.82E+07 7.79E-01 6.15E+08


FQUS24 -2.90E+01 2.07E+01 -9.48E+00

0.047 1.5 1.5 1.5

1.84E+07 7.86E-01 6.03E+08


FQUS24 -3.43E+01 -2.94E+01 -1.30E+01

0.047 1.5 1.5 1.5

1.71E+07 7.80E-01 5.89E+08


FQUS24 -3.42E+01 -2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.69E+07 7.67E-01 6.12E+08








FQUS24 -3.42E+01 -2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.68E+07 7.61E-01 6.25E+08


FQUS24 -2.06E+01 2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.79E+07 7.77E-01 6.02E+08


FQUS24 -2.04E+01 2.94E+01 -1.30E+01

0.047 1.5 1.5 1.5

1.72E+07 7.63E-01 6.06E+08


FQUS24 3.42E+01 2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.86E+07 8.23E-01 5.33E+08


FQUS24 -2.25E+01 2.07E+01 -1.14E+01

0.047 1.5 1.5 1.5

2.19E+07 9.13E-01 4.62E+08


FQUS24 -3.42E+01 2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.76E+07 7.83E-01 5.89E+08


FQUS24 3.42E+01 -2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

2.17E+07 8.94E-01 4.78E+08


FQUS24 2.04E+01 -2.34E+01 -1.30E+01

0.047 1.5 1.5 1.5

2.15E+07 8.92E-01 4.86E+08


FQUS24 -3.42E+01 2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.74E+07 7.79E-01 5.89E+08


FQUS24 3.43E+01 2.54E+01 -1.30E+01

0.047 1.5 1.5 1.5

1.78E+07 8.14E-01 5.31E+08


FQUS24 2.25E+01 -2.07E+01 -1.14E+01

0.047 1.5 1.5 1.5

2.54E+07 1.02E+00 3.77E+08








FQUS24 -3.42E+01 -2.42E+01 -1.16E+01

0.047 1.5 1.5 1.5

1.61E+07 7.62E-01 5.96E+08


FQUS24 -2.06E+01 -2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.72E+07 7.91E-01 5.53E+08


FQUS24 3.42E+01 2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.76E+07 8.12E-01 5.28E+08


FQUS24 -3.44E+01 -2.34E+01 -1.30E+01

0.047 1.5 1.5 1.5

1.65E+07 7.76E-01 5.82E+08


FQUS24 3.43E+01 -2.94E+01 -1.30E+01

0.047 1.5 1.5 1.5

2.20E+07 9.24E-01 4.34E+08


FQUS24 -2.57E+01 2.05E+01 -1.12E+01

0.047 1.5 1.5 1.5

1.77E+07 8.00E-01 5.47E+08


FQUS24 -3.42E+01 -2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.59E+07 7.66E-01 5.80E+08


FQUS24 3.42E+01 -2.74E+01 -1.13E+01



0.047 1.5 1.5 1.5

2.12E+07 9.04E-01 4.51E+08


FQUS24 -2.90E+01 -3.41E+01 -9.48E+00

0.047 1.5 1.5 1.5

1.56E+07 7.61E-01 5.80E+08


FQUS24 -2.90E+01 2.05E+01 -1.12E+01

0.047 1.5 1.5 1.5

1.69E+07 7.69E-01 5.91E+08


FQUS24 -2.04E+01 -2.94E+01 -1.30E+01

0.047 1.5 1.5 1.5

1.51E+07 7.41E-01 6.13E+08






FQUS24 -2.06E+01 2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.69E+07 7.76E-01 5.67E+08


FQUS24 3.42E+01 -2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

2.05E+07 8.94E-01 4.52E+08


FQUS24 3.42E+01 2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.73E+07 8.22E-01 5.00E+08


FQUS24 -2.90E+01 -3.42E+01 -1.12E+01

0.047 1.5 1.5 1.5

1.47E+07 7.43E-01 5.90E+08




FQUS24 3.42E+01 2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.69E+07 8.12E-01 5.06E+08


FQUS24 -2.90E+01 2.05E+01 -1.12E+01

0.047 1.5 1.5 1.5

1.66E+07 7.87E-01 5.42E+08


FQUS24 -2.26E+01 -2.13E+01 -9.75E+00

0.047 1.5 1.5 1.5

2.40E+07 1.02E+00 3.60E+08


FQUS24 -2.06E+01 2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.60E+07 7.67E-01 5.57E+08


FQUS24 -3.42E+01 3.06E+01 -1.16E+01

0.047 1.5 1.5 1.5

1.63E+07 7.86E-01 5.38E+08


FQUS24 -2.06E+01 -2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.60E+07 7.84E-01 5.31E+08


FQUS24 -3.42E+01 2.42E+01 -1.16E+01

0.047 1.5 1.5 1.5

1.61E+07 7.78E-01 5.48E+08






FQUS24 2.57E+01 -2.09E+01 -7.73E+00

0.047 1.5 1.5 1.5



2.34E+07 1.02E+00 3.56E+08


FQUS24 -2.05E+01 -2.42E+01 -1.16E+01

0.047 1.5 1.5 1.5

1.67E+07 8.08E-01 4.98E+08


FQUS24 -2.05E+01 2.42E+01 -1.16E+01

0.047 1.5 1.5 1.5

1.63E+07 7.83E-01 5.32E+08

2951 PASS 9.94E-01 6 (+z) SurfPt 2.01E+01 6.32E+07 DNVC-I

FQUS24 -2.04E+01 2.34E+01 -1.30E+01

0.047 1.5 1.5 1.5

1.65E+07 8.00E-01 5.15E+08

9906 PASS 9.94E-01 1 (-z) SurfPt 2.01E+01 6.75E+07 DNVC-I

FQUS24 3.42E+01 -2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.99E+07 9.02E-01 4.26E+08


FQUS24 -5.61E+00 2.74E+01 -2.05E+01

0.038 1.5 1.5 1.5

2.33E+07 1.00E+00 3.70E+08

9964 PASS 9.75E-01 10(+z) SurfPt 2.05E+01 6.61E+07 DNVC-I

FQUS24 3.42E+01 -2.42E+01 -1.16E+01

0.047 1.5 1.5 1.5

1.94E+07 8.87E-01 4.42E+08


FQUS24 2.06E+01 2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.56E+07 7.75E-01 5.46E+08


FQUS24 2.26E+01 2.13E+01 -9.75E+00

0.047 1.5 1.5 1.5

2.34E+07 1.01E+00 3.52E+08


FQUS24 -2.06E+01 -2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.47E+07 7.55E-01 5.57E+08


FQUS24 5.61E+00 2.74E+01 -2.05E+01

0.038 1.5 1.5 1.5

2.32E+07 9.99E-01 3.69E+08








FQUS24 -5.61E+00 -2.74E+01 -2.05E+01

0.038 1.5 1.5 1.5

2.35E+07 1.00E+00 3.62E+08


FQUS24 -5.61E+00 2.74E+01 -2.05E+01

0.038 1.5 1.5 1.5

2.33E+07 1.00E+00 3.68E+08


FQUS24 -2.90E+01 2.05E+01 -1.12E+01

0.047 1.5 1.5 1.5

1.62E+07 7.89E-01 5.21E+08


FQUS24 5.61E+00 -2.74E+01 -2.05E+01

0.038 1.5 1.5 1.5

2.34E+07 1.00E+00 3.63E+08


FQUS24 5.61E+00 2.74E+01 -2.05E+01

0.038 1.5 1.5 1.5

2.31E+07 9.99E-01 3.68E+08


FQUS24 -2.90E+01 -3.39E+01 -7.73E+00

0.047 1.5 1.5 1.5

1.75E+07 8.48E-01 4.54E+08


FQUS24 -5.61E+00 -2.74E+01 -2.05E+01

0.038 1.5 1.5 1.5

2.34E+07 1.00E+00 3.61E+08


FQUS24 5.61E+00 -2.74E+01 -2.05E+01

0.038 1.5 1.5 1.5

2.33E+07 1.00E+00 3.61E+08




FQUS24 -3.44E+01 3.14E+01 -1.30E+01

0.047 1.5 1.5 1.5

1.65E+07 8.19E-01 4.79E+08


FQUS24 -2.04E+01 -2.34E+01 -1.30E+01

0.047 1.5 1.5 1.5

1.98E+07 9.06E-01 4.04E+08


FQUS24 3.44E+01 -2.34E+01 -1.30E+01

0.047 1.5 1.5 1.5

2.04E+07 9.22E-01 4.10E+08






FQUS24 -2.06E+01 2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.53E+07 7.68E-01 5.29E+08


FQUS24 2.04E+01 2.34E+01 -1.30E+01

0.047 1.5 1.5 1.5

1.78E+07 8.56E-01 4.54E+08


FQUS24 3.42E+01 3.06E+01 -1.16E+01

0.047 1.5 1.5 1.5

1.60E+07 8.24E-01 4.59E+08


FQUS24 2.06E+01 -2.74E+01 -1.13E+01

0.047 1.5 1.5 1.5

1.70E+07 8.29E-01 4.76E+08


FQUS24 2.90E+01 2.05E+01 -1.12E+01



0.047 1.5 1.5 1.5

1.77E+07 8.54E-01 4.55E+08


FQUS24 -3.41E+01 2.93E+01 -9.76E+00

0.047 1.5 1.5 1.5

1.57E+07 8.02E-01 4.90E+08


FQUS24 2.04E+01 2.94E+01 -1.30E+01

0.047 1.5 1.5 1.5

1.34E+07 7.29E-01 5.99E+08


FQUS24 -3.42E+01 -3.06E+01 -1.16E+01

0.047 1.5 1.5 1.5

1.43E+07 7.72E-01 5.07E+08


FQUS24 2.57E+01 -2.05E+01 -1.12E+01

0.047 1.5 1.5 1.5

1.89E+07 9.06E-01 4.08E+08

Number of elements printed: 108

Number of elements failed: 80



ACKNOWLEDGEMENTS

First of all I would like to thank Almighty Allah to give me strength and ability to finish the

thesis on time and for always being there for me.

Then I would like to express my heartfelt gratefulness to my supervisor Professor Maciej

Taczala from ZUT for giving me the chance to do the thesis on Fatigue Analysis of Offshore

Drilling Unit, especially for his confidence on me. He give me proper guidance and advice to

go through the work.

I would also like to thank Prof. Philippe Rigo for his excellent coordination of the EMSHIP

program and all of my Professors, faculty members and my friends from EMSHIP for their co-

operation throughout this 18 months period.

I am very grateful to all my colleagues from my internship company DNV-GL, Gdynia, Poland,

for their support, friendliness and hospitality during my three months stay. I would like to

thanks especially to Mr. Tomasz Msciwujewski, head of the section of Advisory Maritime and

Offshore for giving me this opportunity and access to all services that need to finish my thesis

work. Mr. Maciej, Ms Marzena, Ms Agnieszka and my friend Mr Tomek deserves my

wholehearted thanks for making my stay at DNV-GL Gdynia very special and memorable.

I would also like to thank my parents, their dreams made me bring here and give me strength

to finish my master’s program and also for their love, support and encouragement.

This thesis was developed in the frame of the European Master Course in “Integrated Advanced

Ship Design” named “EMSHIP” for “European Education in Advanced Ship Design”, Ref.:

159652-1-2009-1-BE-ERA MUNDUS-EMMC.

Fatigue Analysis of Offshore Drilling Unit · 2018-12-06 · 1 Fatigue Analysis of Offshore...

Documents

Transcript of Fatigue Analysis of Offshore Drilling Unit · 2018-12-06 · 1 Fatigue Analysis of Offshore...