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JOURNAL OF SUBSEAAND OFFSHORE

SC I ENCE AND ENG INEER ING

VOLUME . 1

MARCH 2 0 , 2 0 1 5

I S S N : 2 4 4 2 6 4 1 5

P U B L I S H E D BY I S OMA s e

Journal of Subsea and Offshore -Science and Engineering-

Vol.1: March 20, 2015

ISOMAse

International Society of Ocean, Mechanical and Aerospace -Scientists and Engineers-

Contents

About JSOse

Scope of JSOse

Editors

Title and Authors PagesA Study of The Dynamic Longitudinal Hull Structural Responses and Ultimate Strength of Drillship

I Dewa G.A.S. Yuda, Eko B. Djatmiko, Daniel M. Rosyid

1 - 8

Wave Induce Motion of Round Shaped FPSO C. L. Siow, J. Koto, H. Yasukawa, A. Matsuda,

D. Terada, C. Guedes Soares, Muhamad Zameri bin Mat Samad, A.Priyanto

9 - 17

Appropriate Model for Mooring Pattern of a Semi-Submersible Platform Hadi Sabziyan, Hassan Ghassemi,

Farhood Azarsina, Saeid Kazem

18 - 25


Vol.1: March 20, 2015

ISOMAse


About JSOse

The Journal of Subsea and Offshore -science and engineering- (JSOse), ISSN’s registration no: 2442-6415 is an online professional journal which is published by the International Society of Ocean, Mechanical and Aerospace -scientists and engineers- (ISOMAse), Insya Allah, four volumes in a year which are March, June, September and December.

The mission of the JSOse is to foster free and extremely rapid scientific communication across the world wide community. The JSOse is an original and peer review article that advance the understanding of both science and engineering and its application to the solution of challenges and complex problems in subsea science, engineering and technology.

The JSOse is particularly concerned with the demonstration of applied science and innovative engineering solutions to solve specific subsea and offshore industrial problems. Original contributions providing insight into the use of computational fluid dynamic, heat transfer, thermodynamics, experimental and analytical, application of finite element on offshore and subsea, offshore structural and impact mechanics, stress and strain localization and globalization, metal forming, behaviour and application of advanced materials in shallow and deepwater, shallow and deepwater installation challenges, vortex shedding, vortex induced vibration and motion, flow assurance, ultra-deepwater drilling riser, wellhead integrity and soon from the core of the journal contents are encouraged.

Articles preferably should focus on the following aspects: new methods or theory or philosophy innovative practices, critical survey or analysis of a subject or topic, new or latest research findings and critical review or evaluation of new discoveries.

The authors are required to confirm that their paper has not been submitted to any other journal in English or any other languages.


Vol.1: March 20, 2015

ISOMAse


Scope of JSOse

JSOse welcomes manuscript submissions from academicians, scholars, and practitioners for possible publication from all over the world that meets the general criteria of significance and educational excellence. The scope of the journal is as follows:

• Shallow, Deep and Ultra-deep Water and Arctic Pipelines and Offshore Structure • Shallow, Deep, Ultra-deep Water Installation Challenges • Subsea and Offshore Challenges in Pipe Materials, Flow Assurance, Multi Phases Flow,

Equipment and Hardware • Flexible Pipe and Umbilical • Riser, Mooring Lines Design and Mechanics System • Advanced Engineering, Lateral and Upheaval Buckling, Pipeline Soil Interactions • Challenges of High Pressure - High Temperature (HPHT) in Ultra-deep Water • Vortex Shedding and Vibration Suppression (VIV & VIM) • Project Experiences, Case Study and Lessons Learned on Subsea and Offshore • Integration of Management, Materials, Safety and Reliability • Certified Verification Analysis (CVA) • Subsea and Offshore Structures Construction.

The International Society of Ocean, Mechanical and Aerospace –science and engineering is inviting you to submit your manuscript(s) to [email protected] for publication. Our objective is to inform the authors of the decision on their manuscript(s) within 2 weeks of submission. Following acceptance, a paper will normally be published in the next online issue.

ISOMAs

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Jaswar

Associate

Ab. SamAdhy PAdi MaiAgoes PAhmad Ahmad BambaCarlos GHarifudHassanMohamMoh HaMohd NMohd YMohd ZMusa MPriyonoRadzuaRudi WSergey SunarySutopo Tay ChoWan Ro

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Journal of Subsea and Offshore -Science and Engineering-, Vol.1

March 20 , 2015

1 Published by International Society of Ocean, Mechanical and Aerospace Scientists and Engineers

A Study of The Dynamic Longitudinal Hull Structural Responses and Ultimate Strength of Drillship

I Dewa G.A.S. Yuda,a,* Eko B. Djatmiko,b and Daniel M. Rosyid,b

a)Master Degree Student,Marine Technology Post-Graduate Program, Institut Teknologi Sepuluh Nopember(ITS), Surabaya, Indonesia. b)Dept. Of Ocean Engineering, Institut Teknologi Sepuluh Nopember (ITS), Surabaya, Indonesia. *Corresponding author: [email protected] Paper History Received: 29-January-2015 Received in revised form: 4-March-2015 Accepted: 19-March-2015 ABSTRACT Sustainability studies will support the development of drillship design. One primary aspect to be explored during the design stageis the global structural responses due to the excitation of head sea waves. This paper presents the global structural response analysis by implementing the so called quasi-static approach and the effect for ultimate strength. In this respect, a motion analysis should be first carried out to obtain the coupled heave and pitch motion data for a range of regular wave frequency.The next step is performing the quasi-static computation for a number of wave frequencies, where the magnitudes of heave and pitch motions are considered important. After accomplishing RAO of shear force and bending moment calculation, further analysis is obtained the extreme global structural responses by implementing the spectral approach. It will be inputed to Global Finite Element Model (FEM) to get the ultimate condition of drillship material. Final results of the global analysis indicates the ultimate condition is exceeded on 12 meter wave height by the Structural Stress output of 585 Mpa. KEY WORDS: Drillship; Dynamic Wave Load; Structural Spectral Response; Finite Element Analysis; Ultimate strength. NOMENCLATURE

EPI Eastern Part of Indonesia

MBPOD Million Barrels of Oil Per Day ITS Institut Teknologi Sepuluh Nopember RAO Response Amplitude Operator Lpp Length between Perpendicular B Breadth H Height T Draught LCB Longitudinal Center of Bouyancy LCF Longitudinal Center of Floatation KMT Transversal Keel to Metacenter KML Longitudinal Keel to Metacenter BMT Transversal Bouyancy to Metacenter BML Longitudinal Bouyancy to Metacenter Hs Significant Wave Height FEM Finite Element Method SF Shear Force BM Bending Moment LWT Lightweight DWT Deadweight Mjk Matrixof ship mass and mass moment of inertia Ajk Matrix of hydrodynamic added mass coefficient Bjk Matrix of hydrodynamic damping coefficient Kjk Matrix of hydrostatic stiffness, Fj Matrix of excitation Forces (F1, F2, F3) and Moments (F4, F5, F6) w(x) Weight distribution along the ship hull Δ(x) Buoyancy distribution along the ship hull ITTC International Towing Tank Conference ISSC International Ship Structure Congress m0 The area under the structural response spectra m2 The second moment of the area under structural

response spetra Ts Storm duration (3.0 hours)


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α The probability of exceedance The level of confidence that the extreme response

σ Stress matrix σ σ σ σ σ σ Stiffness matrix of model Deformation matrix

fL Longitudinal stress of component fT Vertical stress of component fLT Vertical plane stress fUL Ultimate strength of material Sm Strength reduction factor (1 for ordinary steel) 1.0 INTRODUCTION Indonesia requires an oil supply of approximately 1,450 MBPOD, while the national production could only reach the range of 860 MBPOD. In future the shortage is planned to be tackled by exploiting the resources from the EPI, where a reserve in the quantity of 3.5 billion barrels is predicted to be available, with a further potential up to 50 billion barrels. However it should be realized that oil reserves in EPI mostly located in the deepsea and harsher environment. Deepsea oil and gas exploration and exploitation such as in EPI necessitate the utilization of floating structures to support the operation. Therefore national capacity building in the design and engineering of floating offshore structures should be greatly encouraged. One type of the floating offshore structures of interest is the drillship.

An intensive study on drillship has been commenced at ITS since 2013, covering the basic design, motion analysis and followed by the operability analysis [1]. The preliminary evaluation of the global structural responses of the drillship induced by wave excitations has been concluded that moonpool is a critical area of structure [2]. Results of evaluation will be in the form SF andBM to be further implemented in the structural design of the drillship. In the current study, Dynamic hull structural response analysis is conducted by implementing the designated quasi-static approach, which is basically the enhancement to the classical static wave approach long practiced in the monohull ship design [3]. While in the classical approach the wave considered in the analysis is characterized with length equal to ship length and height equal to 1/20 of the wave length, the quasi-static will consider the effect of waves in a number of frequencies. Realizing that one of the most critical design condition is the longitudinal strength, then the current study is chiefly concentrated to observe the effect of head waves, with dominant motion modes to be tackled are heave and pitch.

In general the quasi-static analysis will be conducted in a number of regular wave frequencies where the heave and pitch motions are regarded to have important effect on the global structure responses. The heave and pitch motion

data is taken from executing a hydrodynamic mathematical model based on the 3-dimensional diffraction theory. The regular wave height accounted for has a small amplitude and hence the heave and pitch motions are also relatively small so that assumption of straight down weight vector of every hull section applies. For each wave frequency selected the computation of global structural responses is conducted for one wave cycle, divided in 11 time steps. Therefore information on the changing of SF and BM distribution along the hull in each time step could be recorded. This could be further evaluated to see any of those that may instigate sizable stress or deflection on the hull structure.

After accomplishing computation on all of the wave frequencies observation is then made to pick up the maximum SF and BM in each frequency, at any position of interest on the drillship hull. For the global structural analysis input, all maximum SF and BM in each drillship position (section 1-40) should be computed. By the variation of frequency, it will be arranged as SF and BM RAO for each station. Such RAO will be incorporated in the spectral analysis to derive the extreme values of SF and BM. It will necessary for evaluation of global ultimate strength of the structure. 2.0 METHODOLOGY 2.1 Data Accumulation The first primary data for this study is the reference ship, namely the drillship Oribis One.as made avaliable by Fossli and Hendriks [4]. Based on this data, New drillship was designed as reported in ref [1]. The general arrangement is exhibited in Figure 1 with principal particulars as presented in Table 1.

Figure 1: General Arrangement of Drillship [1].

The peculiar feature of a drillship is the arrangement of

moonpool to accommodate the extension of drill pipe and riser from the drilling rig down to the seabed. As a moonpool basically is a large opening, then hull structural strength in this location is


March 20 , 2015


reduced substantially. Therefore certain strengthening should be established in this respect.

Table 1: Principal parameter of drillship [2].

Parameter Design Hydro Check

Difference (%)

Displacement (ton) 35,193.0 35,421.7 0.65 Lpp (m) 156.0 156.0 0.00 B (m) 29.9 29.9 0.00 H (m) 15.6 15.6 0.00T (m) 9.0 9.0 0.00 LCB to Midship (m) 3.265 3.270 0.15 LCF to Midship (m) -7.203 -7.164 0.54 KMT (m) 13.29 13.33 0.30 KML (m) 222.82 223.21 0.17 BMT (m) 8.64 8.68 0.41 BML (m) 218.17 218.55 0.18

The next data needed in the study is related to the environment

regarded as the primary source of excitation. For this the wave distribution data has been obtained from ABS in 2010 [5], related to the world wave scatter diagram, as contained in table 2. Based on this data, wave spectral analysis is calculated as the increasing of Hs. In this study,depend on the summary over all periods of the worldwide wave data, Distribution of Hs is arranged from 1 up to 14m. In this range, The Significant wave height will occured as the probability value in the table 2. All probability will cause the structural effect for the drillship. So, this range will be very important to be analyzed.

Table 2: Unrestricted worldwide wave data [5].

2.2 Modeling the Drillship Hull. Following the design and development of the general arrangement, further step is aimed at the establishment of hull model. There are 2 model that will be established for this analysis. The first one is related to general design, hydrostatic and hydrodynamic analysis as depicted in figure 2, and the second is required for Finite Element analysis as shown in figure 3. Model of FEM is the result of mesh sensitivity stage. For global structural analysis, The number of meshes are 2,114,808 units.

The hull model for general design is related to the lines plan and Bonjean curve of drillship as shown in figure 4 and 5. This model design and computation is helped by Hidrostar Software. Finite element model of drillship is made by using the construction design of drillship as shown in figure 6 up to 10. By the rule of drillship construction[6], It will arranged in the 3 dimensional desain that contained with thousand of elements. Number of element is dependent on the mesh size definition in

FEM All The design is combined with the vessel weight distribution to obtain SF and BM distribution, then the stress of structure.

Figure 2: Hull model for general design with Hidrostar Software.

Figure 3: Hull structure for Finite Element Model.

Figure 4: Lines plan of drillship [1].

Figure 5: Bonjean curve of drillship [2].

Figure 6: Center frame design of drillship.

Figure 7: Longitudinal Stiffener design of drillship


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Figure 8: Typical frame design of drillship.

Figure 9: Bulkhead design of drillship.

Figure 10: Moonpool frame design of drillship.

2.3 Weight Distribution Ship weight comprises of LWT and DWT. In addition to common ship components, the LWT of a drillship also covers the drilling equipments, drilling rigs, and specific topside facilities. In the case of DWT, specific components for a drillship include drill water, liquid mud, brine, base oil, barite and/or bentonite, cement, and bulks. For the current study the weight distribution of the drillship is shown in Figure 11. By the 40 It is noticeable from the graph that for station 16 – 19 the weight distribution is low, as

this is the position of moonpool. By this Dristribution in each frequency, RAO of each potition (station) in drillship as a load in regular wave will be obtained by quasi static approach.

Figure 11: Total weight distribution of drillship

2.4 Procedure of Computations The first stage of computation is directed towards the generation of drillship motion RAO. The numerical model is developed on the basis of the 3-dimensional diffraction theory. Computation is executed for the drillship in free floating stationary condition induced by the regular head waves in the frequency range from 0.25 up to 2.0 rad/s. The general motion equation is expressed as follows [7,8]: ∑ ζ ζ ζ ; , 1 … 6 (1)

The second stage of the computation is aimed at generating the shear force and bending moment. The shear force is basically obtained by integration of the difference between the ship weight and buoyancy distribution. Whereas the bending moment is obtained from integrating the shear force distribution, as described by the equation 2 and 3 [3].

∆ (2)

(3)

After completing the SF and BM computation in regular waves

up to the composition of their RAOs, the next stage of analysis is dedicated to attain the characteristics of global responses in random waves. For this case the spectral form to be applied is the ITTC/ISSC spectrum [7] which is suitable in common world waters.

Following the global responses in random waves, the most probable extreme value of the structural response brought about a random wave may be found by applying the equation 4 [7].

2 2 ln (4)

To be more conservative in the design so that a certain level of

structural response would not be exceeded, or in other words the


March 20 , 2015


level of confidence is to be enhanced, then equation 4 should be modified into equation 5. Usually � is taken as a small value, for instance 1% or 0.01. Reversely the level of confidence that the extreme response would not be exceeded is calculated as 1-0.01, that is 0.99 or 99%.

2 2ln (5)

By increasing Hs, structural response of SF and BM on each component (station) will be obtained in computation. This will be inputed to FEM of drillship. Stress on elements (component) will be computed by Finite Element method as results. The results depend on the number of mesh of the model. Therefore, mesh sensitivity step should be done before computation. Equation 6 shows the method to obtain result stress in FEM [9].

σ (6)

Ultimate strength analysis will be obtained by applying the

ultimate limit strength of the component material. Structure of drillship will be failure when the limit of material ultimate strength is exceeded, as shown in equation 7 and 8 [9].

(7)

(8)

3.0 RESULT AND DISCUSSIONS 3.1 Drillship Motions The hydrodynamic computation for the stationary and free floating drillship in the propagation of regular head waves yields the heave and pitch RAOs as depicted in figure 12. RAO’s computation of this study is used hidrostar software. The heave motion characteristic of the drillship is amenable, with largest RAO value approaching 1.0 m/m at very low frequency. The pitch motion has pronounced resonance with the peak RAO of some 1.36 deg/m at the wave frequency of about 0.45 rad/s. The heave and pitch data contained in figure 7 will then be used in the quasi-static analysis as explained in the sub-section.

Figure 12: Heave and pitch RAOs of the drillship

3.2 Quasi-Static Analysis For the quasi-static analysis the heave and pitch motions related to totally eleven frequencies are selected to cover the appropriate range of the motion effects. Referring to figure 12, the range accordingly is between 0.25 rad/s up to 1.25 rad/s, with an interval of 0.1 rad/s. In relation to each frequency, the magnitudes of heave and pitch motion are then measured. Combined with the corresponding phase angles, heave and pitch motion elevations are then established together with the wave elevation. The computation of these elevations for one cycle is listed in table 3 and the plots are presented in figure 13. The elevations of motion are indicated by Zw, Zz and Zθ, respectively, for this particular example, computation and plots are made for the case of wave and motion having a frequency of ω = 0.65 rad/s and the associated period for one cycles is T=9.67 sec, with a time interval of about 0.967 sec. The elevations on each time will be further applied in the computation of the global structural responses, namely SF and BM.

In each time step the ship buoyancy of every station along the hull due to wave and motion elevations is computed. The buoyancy distribution so obtained is then correlated to the weight distribution, as shown in Figure 11, by means of equation (2) to derive the shear force distribution. Subsequently, referring to equation (3), the integration on the shear force is carried out to generate the bending moment distribution. The computation is conducted for all 11 time steps, and the results are plotted as exemplified in Figure 14. This figure essentially exhibits the wave elevation along the hull, hullposition brought about the heave and pitch motions, shear force distribution as well as bending moment distribution for each time step in one cycle.

Table 3: Computation of wave load motion elevation at ω=0.65rad/s Time step Wave Heave motion Pitch motion

(sec) Zw (meter) Zz (meter) Zθ (deg) 0.000 1.000 -0.346 -0.490 0.967 0.809 -0.077 -0.163 1.933 0.309 0.223 0.226 2.900 -0.309 0.437 0.528 3.867 -0.809 0.484 0.629 4.833 -1.000 0.346 0.490 5.800 -0.809 0.077 0.163 6.767 -0.309 -0.223 -0.226 7.733 0.309 -0.437 -0.528 8.700 0.809 -0.484 -0.629 9.670 1.000 -0.346 -0.490

Figure13: Plots of wave and motion elevation at ϖ=0.65rad/s

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March 20 , 2015


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Figure 23: FEM Results of drillship (a)Hs=4m (b)Hs=6m (c)Hs=8m (d)Hs=10m (e)Hs=12m (f)Hs=14m

4.0 CONCLUSION A study has been carried out to investigate the global structural responses of a 35,000 ton class drillship due to the head waves excitations by applying the quasi-static approach of dynamic loads. Based on the results of computation and analyses, a number of conclusions may be put forward, as follows: • The regular wave excitation instigate a moderate motion

characteristic in the heave mode, with maximum RAO value approaching unity or 1.0 m/m at very low frequency. For the pitch mode, the RAO curve show a pronounced hump at the frequency of 0.60 rad/s, having a magnitude of some 1.36 deg/m. The pitch motion is considered to have a significant effect on the shear force and bending moment which also have the magnified curves at frequency of about 0.60 rad/s.

• Making use of the quasi static approach it is possible to generate the global response distributions which explicitely indicate the effect of vessel motions in elapsed times. This represent an enhancement to the classical static wave approach analysis.

• Computation by adopting the quasi-static approach yields the maximum shear force on the drillship in the extent of 76.6 MN to occur at section of moonpool (station 16) when induced by a regular head wave with frequency of 0.65 rad/s. Further, the computation gives a maximum bending moment of 1096.49 MNm with peak frequency of 0.65 rad/s.

• The spectral analysis shows that the region of drillship moonpool (station 16) has the biggest extreme value on the longitudinal dimension of drillship as increasing Hs (from 1 up to 14 m).

• The FEM computation and analysis predict that drillship will get ultimate failure in about 12 meter Hs with 585 Mpa stressby inputing spectral responses of SF and BM. The critical stress will mostly happened on the moonpool region.

ACKNOWLEDGEMENTS The authors would like to convey a great appreciation to : • Fresh Graduate Scholarship Program of DIKTI & ITS that

supported this study. • Bureau Veritas as ITS’s partner that provides hidrostar

licensed Software to assist this research. REFERENCE 1. Yuda, I.D.G.A.S., Djatmiko, E.B. and Wardhana, W. (2013).

Evaluation on the Motion and Operability Aspects in the Design of a 35,000 ton Displacement Drillship. Proc. of Seminar on the Theory and Application in Marine Technology, SENTA 2013, Surabaya, Indonesia.

2. Ariyanto, S., Djatmiko, E.B., Murtedjo, M., and Yuda, I.D.G.A.S. (2014). A Study of The Longitudinal Hull Structural Responses on a 35,000 Ton Class Drillship due to Wave Load by the Quasi-Static Approach. Proc. The 9th International Conference on Marine Technology, MARTEC 2014, Surabaya, Indonesia.

3. Rawson, K.J. dan Tupper, E.C.(2001), Basic Ship Theoryvol. 1, Butterworth-Heinemann, Oxford, UK

4. Fossli, B. And Hendriks, S.,PRD12,000 Drill Ship; increasing Efficiency in Deep Water Operations,Proc. of IADC/SPE Drilling Conference, Orlando, Florida, USA.

5. ABS (2010), Guide for Spectral-Based Fatigue Analysis for Floating Production, Storage and Offloading (FPSO) Installation, American Bureau of Shipping, USA, May

6. ABS (2011), Drillship : Hull Structural Design and Analysis, American Bureau of Shipping, USA.

7. Djatmiko, E.B. (2012), Behavior and Operability of Ocean Structure on Random Waves, ITS Press, Surabaya, Indonesia

8. Chakrabarti, S.K. (1987), Hydrodynamics of Offshore Structures, Computational Mechanics Publications Southampton Boston, Springer-Verlag, Berlin

9. Paik JK, Thayaballi AK. 2003. Ultimate Limit State Design Of Steel Plated Structures, John Wiley & Sons.

10. Burke, R.J. (1982), The Consequences of Extreme Loadings on Ships Structures, Proc. of Extreme Loads Response Symposium, SSC/SNAME, Arlington, VA, USA, Oct.

11. Djatmiko, E.B. (1995), Identification of SWATH Ship Global Structural Responses Utilizing a Physical Model, Research Project Report, LPPM-ITS, Surabaya

12. DNV (2011),Modelling and Analysis of Marine Operations, DNV-RP-H103, Norway


March 20, 2015


Wave Induce Motion of Round Shaped FPSO

C. L. Siow,a, J. Koto,a,b,*, H. Yasukawa,c A. Matsuda,d D. Terada,d, C. Guedes Soares,e Muhamad Zameri bin Mat Samad,f and A.Priyanto,a

a)Department of Aeronautics, Automotive and Ocean Engineering, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia b)Ocean and Aerospace Research Institute, Indonesia c)Department of Transportation and Environmental Systems, Hiroshima University, Japan d)National Research Institute of Fisheries Engineering (NRIFE), Japan e)Centre for Marine Technology and Engineering (CENTEC), Instituto Superior Técnico, Universidade de Lisboa, Portugal f)Department of Materials, Manufacturing and Industrial Engineering, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia *Corresponding author: [email protected] and [email protected] Paper History Received: 15-February-2015 Received in revised form: 17-March-2015 Accepted: 19-March-2015 ABSTRACT

Wave response motion and dynamic stability of the new generation Round Shaped FPSO structure in ocean environments is required to be investigated properly to ensure the safety and the operability of this new proposed model. The main objective of this research is to predict the wave induced motion response of the designed Round Shaped FPSO. In this study, the motion response of the Round Shaped FPSO is simulated by diffraction potential theory with Morison heave damping correction method. To ensure the validity of the simulated results, wave tank experiment in the model scale 1:110 was conducted. Upon completed the experiment, the time series data are converted by fast Fourier Transformation method to obtain the response amplitude, RAO of Round Shaped FPSO in 6 degree of freedom measured in the experiment. In the comparison, both the experimental result and numerical result are agreed between each other in this research. Based on the simulation results, it is observed that wave response characteristic and the dynamic stability of the Round Shaped FPSO is good in most of the ocean environment. KEYWORDS: Round Shaped FPSO; Wave Response; Diffraction Potential; Damping Correction; Motion Test.

1.0 INTRODUCTION

Deep water oil and gas exploration is a costly industry activity due to the required of high technology level during exploration. In deep water region, floating structures is more comparative compare to fix structure. Typically, the Floating Production Storage and Offloading, FPSO structure is converted from an old tanker ship but some of the FPSO owner is interest for new constructed hull. In term of system requirement, the FPSO is required better performance in stability either static or dynamic condition compare to resistance performance. The different of operation requirement causes related industry to develop new generation FPSO with more practical hull form to the offshore desire.

In year 2008, Lamport and Josefsson, (2008) carried a research to study the advantage of Round Shaped FPSO over the traditional ship-shaped FPSO [1]. The comparisons were made to compare motion response, mooring system design, constructability and fabrication, operability, safety and costing between both the structures. One of the finding on their study is their designed structures motions are similar for any direction of incident wave with little yaw excitation due to mooring and riser asymmetry. Next, Arslan, Pettersen, and Andersson (2011) are also performed a study on fluid flow around the Round Shaped FPSO in side-by-side offloading condition. FLUENT software was used to simulate three dimensional (3D) unsteady cross flow pass a pair of ship sections in close proximity and the behavior of the vortex-shedding around the two bluff bodies [2].Besides, simulation of fluid flow Characteristic around Round Shaped FPSO by self-develop programming code based on RANs method also conducted by A. Efi et al.[3].

To predict the characteristic of the new Round FPSO concept,


March 20, 2015


numerical simulation is an effective method to apply in initial study. To study the wave motion response of FPSO, diffraction potential method is frequently used and the accuracy of this method to predict the structures response was also detailed studied. The diffraction potential theory estimates wave exciting forces on the floating body based on the frequency domain and this method can be considered as an efficient one to study the motion of large size floating structure with acceptable accuracy. The good accuracy of this diffraction theory applied to large structures is due to the significant diffraction effect that exists in the large size structure in wave [4].

In this study, the motion response of a Round shaped FPSO is simulated by self-developed programming code based on diffraction potential theory with Morison damping correction method. The accuracy of this programming code is checked with the previous semi-submersible experiment result which carried out at the towing tank belong to Marine Technology Centre, Universiti Teknologi Malaysia [5].

Besides, this research also predicts the wave motion response of this Round shaped FPSO by experimental method. A series of wave tank experiment is conducted at National Research Institute of Fisheries Engineering (NRIFE), Japan. The series of regular wave experimental are conducted with the designed Round FPSO model in scale 1:110. The model also fixed in the wave tank with model scale mooring lines so the experiment can capture the wave frequency motion and slow drifts motion [6]. The mooring design is conducted before the experiment so the suitable mooring line is applied to achieve the experiment target [7]. In this paper, the discussion is focused on the data analysis process and the wave motion response tendency of the designed Round Shaped FPSO. 2.0 NUMERICAL CALCULATION

2.1 Diffraction Potential In this study, the diffraction potential method was used to obtain the wave force act on the Round Shaped FPSO also the added mass and damping for all six directions of motions. The regular wave acting on floating bodies can be described by velocity potential. The velocity potential normally written in respective to the flow direction and time as below: Φ , , , , (1)

, ,, , , , ∑ , , (2)

where, g : Gravity acceleration

: Incident wave amplitude : Motions amplitude : Incident wave potential : Scattering wave potential : Radiation wave potential due to motions

: Direction of motion

From the above equation, it is shown that total wave potential in the system is contributed by the potential of the incident wave,

scattering wave and radiation wave. In addition, the phase and amplitude for both the incident wave and scattering wave is assumed to be the same. However, radiation wave potentials are affected by each type of motions of each single floating body inside system, where the total radiation wave potential from the single body is the summation of the radiation wave generates by each type of body motions such as surge, sway, heave, roll, pitch and yaw,

Also, the wave potential must be satisfied with boundary conditions as below:

0 0 (3)

0 (4)

0 (5)

~√

0 ∞ (6)

(7) 2.2 Wave Potential By considering the wave potential only affected by model surface, SH, the wave potential at any point can be presented by the following equation:

; ; (8)

Where P =(x, y, z) represents fluid flow pointed at any coordinate and , , represent any coordinate, (x, y, z) on model surface, SH. The green function can be applied here to estimate the strength of the wave flow potential. The green function in eq. (8) can be summarized as follow:

;1

4

, , (9) Where , , in eq. (9) represent the effect of free surface and can be solved by second kind of Bessel function. 2.3 Wave Force, Added Mass and Damping The wave force or moment act on the model to cause the motions of structure can be obtained by integral the diffraction wave potential along the structure surface.

, , (10) where, is diffraction potential,

Also, the added mass, Aij and damping, Bij for each motion can be obtained by integral the radiation wave due to each motion along the structure surface.

, , (11)


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, , (12)

Where in eq. (10) to eq. (12) is the normal vector for each direction of motion, i = 1~ 6 represent the direction of motion and j = 1~6 represent the six type of motions 2.4 Drag Term of Morison Equation The linear drag term due to the wave effect on submerge model is calculated using Drag force equation as given by Morison equation:

(13) Where is fluid density, is projected area in Z direction,

is drag coefficient in wave particular motion direction, is velocity of particle motion at Z-direction in complex form and is structure velocity at Z-direction

In order to simplify the calculation, the calculation is carried out based on the absolute velocity approach. The floating model dominates term is ignored in the calculation because it is assumed that the fluid particular velocity is much higher compared to structure velocity. Expansion of the equation (13) is shown as follows:

(14)

By ignoring all the term consist of , equation (14) can be reduced into following format.

(15)

The above equation (15) is still highly nonlinear and this is

impossible to combine with the linear analysis based on diffraction potential theory. To able the drag force to join with the diffraction force calculated with diffraction potential theory, the nonlinear drag term is then expanded in Fourier series. By using the Fourier series linearization method, equation (15) can be written in the linear form as follow:

(16) Where, in equation (16) is the magnitude of complex fluid particle velocity in Z direction. From the equation (16), it can summarize that the first term is linearize drag force due to wave and the second term is the viscous damping force due to the drag effect.

According to Christina Sjöbris, the linearize term in the equation (16) is the standard result which can be obtained if the work of floating structure performance at resonance is assumed equal between nonlinear and linearized damping term [8].

The linearize drag equation as shown in equation (16) now can be combined with the diffraction term which calculated by

diffraction potential theory. The modified motion equation is shown as follows:

(17) Where is mass, is restoring force, , , is heave added mass, heave diffraction damping coefficient and heave diffraction force calculated from diffraction potential method respectively.

is the viscous damping and

is the drag force based on drag term of Morison equation. 2.5 Differentiation of Wave Potential for Morison Drag Force To obtain the drag force contributed to heave motion, the wave particle velocity at heave direction must be obtained first. This water particle motion is proposed to obtain from the linear wave potential equation. From the theoretical, differential of the wave potential motion in Z-direction will give the water particle motion in the Z-direction.

As mentioned, the drag force in Morison equation is in the function of time; therefore, the time and space dependent wave potential in the complex should be used here. The wave potential in Euler form as follows:

, , 18

The expending for the equation (18) obtained that , , · cos sin

· cos sin 19 Rearrange the equation (19), the simplify equation as follows

, , · cos sin 20

Differentiate the equation (20) to the Z-direction, the water particle velocity at Z-direction is shown as follows:

, ,

· cos sin 21

Since this numerical model is built for deep water condition, hence it can replace the equation by and the equation (21) is becoming as follow:

, , · cos sin 22

In the equations (18) to (22), is the wave amplitude, is the

gravity acceleration, is the wave speed, is wave number, is the horizontal distance referring to zero coordinate, is the time dependent variable.

The horizontal distance, and the time dependent variable, can be calculated by the following equation


March 20, 2015


cos sin 23

24 In equation (23) and equation (24), the variable is wave heading angle, is the leading phase of the wave particle velocity at the Z-direction and is time.

To calculate the drag forces by using the Morison equation, equation (22) can be modified by following the assumptions below.

First, since the Morison equation is a two dimensional method, therefore the projected area of the Z-direction is all projected at the bottom of structure.

Second, as mentioned in the previous part, this method applies the absolute velocity method and the heave motion of model is considered very small and can be neglected; therefore, the change of displacement in Z-direction is neglected.

From the first and second assumption, the variable at equation (22) is no effected by time and it is a constant and equal to the draught of the structure. By ignore the time series term, and then the equation (22) can be become as follow:

, , · Cos Sin 24

2.6 Determination of Drag Coefficient Typically the drag coefficient can be identified from experimental results for the more accurate study. In this study, the drag coefficient is determined based on empirical data calculated based on section 2.6. In order to be able to calculate the empirical data, the Round Shaped FPSO is assumed as a vertical cylinder. Secondly, the laminar flow condition is applied to calculate the drag damping and drag force based on section 2.3, so it is match with the assumption applied in diffraction potential theory. 3.0 WAVE TANK EXPERIMENT

3.1 Experimental Setup This experiment is conducted in wave dynamic tank with the length, wide and depth of 60m, 25m and 3.2m respectively. Before the experiment start, the Round FPSO model was fixed in the middle of tank by four mooring lines which connected between the fairleads located in bottom of FPSO with the anchors which sink into the bottom of tank. Each anchor used in the experiment has the weight of 20kg in air. The view of FPSO model inside wave dynamic tank after installed with mooring lines is showed in Figure 1.

The Round Shaped FPSO model is experienced six degrees of freedom during the experiment. The linear DOF motions of the FPSO models on model size mooring are measured by theodolite camera system. The theodolite camera is able to capture the positions of the reflective optical tracking markers placed on the FPSO model automatically. In this setup, the height of the reflective optical tracking markers is 0.547m above the vertical center of gravity of the Round Shaped FPSO model [6]. The Rotational DOF motions of the FPSO models are measured by gyroscope installed in the center of gravity of the FPSO.

Figure 1: FPSO model fixed with mooring lines in wave basin in static condition.

A servo-type wave height measurement device attached to the carriage which located at the position between FPSO model and wave generator to record the wave height generate by the wave generator. To ensure the wave height measure by wave measuring device is not influence by the existing of Round FPSO, the carriage installed by the servo-type wave height measurement device is moved to the location where the distance between the FPSO to wave measuring device and the distance between wave generator and wave measuring device are 15m both.

All the measurement devices are linked to separate computer to maximize the consistency of the measuring speed. To synchronize the devices and ensure all devices start and stop measure the data without delay, a wireless remove is used to given the order to start and stop all measurement devices [6].

3.2 Linear Motion Data Transformation As mentioned, the height of the reflective optical tracking markers is 0.547m above the vertical center of gravity of the Round Shaped FPSO model. This also means that the position of the FPSO in wave tank measured by the theodolite camera is not located in the center of gravity of the model. To obtain the exact position of the model referred to model’s the center of gravity, the linear motion data must be transferred to the center of gravity of the model. To transfer the data, respective roll, pitch and yaw motion of the model occurred at the same time must be considered in the calculation. The relationships between the positions of the reflective optical tracking markers with the position of center of gravity of model by consider the roll, pitch and yaw motions are showed in Figure 2.

Reflective optical tracking markers


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Figure 2: The relations between the positions of reflective optical tracking markers with position of center of gravity of model.

From the Figure 2, , , represent the x, y and z position of the reflective optical tracking markers while , , are the x, y and z position of the center of gravity of model. The relationship between both the position is in the function of length of rod, R, roll angle( ), pitch angle( ), yaw angle ( ) and model initial heading angle ( ). Therefore, the position information at model center of gravity can be calculated as follow:

25

26

27

Where, , and can be calculated by following

equation

1

/

28

/ 29

30

31

32

After the position of model referred to its center of gravity is

obtained the for entire time series, the information can be used to calculate all 6 degree of motions of model. In this experiment setup, the Rotational motions of the FPSO models are measured by gyroscope installed in the models’ center of gravity, hence, the measured roll, pitch and yaw motion by the gyroscope can be

directly used for the model rotational motions data. However, extra treatment is needed for the linear motions which measured by theodolite camera because the time domain position data obtained from the theodolite camera is the model position in the wave tank without consider it direction. By consider the model initial position and initial heading direction, the position data returned from theodolite camera can be used to obtain the model surge, sway and heave motion. In Figure 3, the plan drawing showed the different of global coordinate where these data are measured by theodolite camera and the local coordinate system which required in calculating the linear motion of the FPSO due to the wave.

Figure 3: Plan view of coordinate system.

In Figure 3, X and Y represent the global direction use in the

experiment setup while x and y are the local direction where the zero position of local coordinate system is located in the model center of gravity before the wave arrived. The model initial heading angle ( ) is measured from wave progress direction and positive follow clock direction. By reset the zero global coordinates to the model center of gravity at calm sea condition, the 6 DOF motions of the model can be calculated as follow,

/ 33

34 And the six degree freedom of motion for the Round Shaped

FPSO can be calculated from the equation (35) to equation (40).

, 180 35

, 180 36

, 37 , 38

, 39


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, 40

3.3 Fourier Series Transformation The experiment data collected in time series provide the information of wave frequency motion for all 6 degree of motion and slow drift motion for horizontal plan motion. To split the different motion data, the analysis can be conducted in frequency domain where the amplitude of the different types of motion can be extracted from the motion amplitude occur at respective frequency.

According to sampling theorem, discretely frequency (Fs) of signal data must be at least twice to the highest continuous signal frequency (F). The continuous signal frequency should discrete by the rate follow the sampling frequency, 1/Fs. Let the discrete sample of the continuous signal have the magnitude of x(k), k=1,2,3,…,n and period between the sample is 1/Fs than a function of a continuous signal, f(t) can be reconstructed back from the discrete sample by the equation below:

41

Where,

sin 42

To convert the data in time domain to the frequency domain,

Fast Fourier Transform method can be applied. The relationship between function in the time domain, f(t) and frequency domain F(f) can be related by the equation below:

43

Also, for the variable j, it represents the square root of (-1) in

the natural exponential function.

cos sin 44

Therefore, the discrete data can be written in complex number form as follows:

45

And,

cos2

46

sin2

47

And, i = 2b is the number of data require by Fast Fourier Transform method where b can be any integer number larger than or equal to 1.

Finally, the magnitude, phase and frequency of the signal can

be calculated by following equations:

2 48

49

50

4.0 MODEL PARTICULARS

The objective of this research is predicting the wave motion response of new designed Round Shaped FPSO. The designed Round Shaped FPSO has the diameter at the draft equal to 111.98meter and draft of 31.91meter. The model was constructed from wood following the scale of 1:110 (Table 1).

Upon the model complete constructed inclining test, and roll decay test were conducted to identify the hydrostatic particular of the Round Shaped FPSO model. The dimension and measured data of the model was summarized as in Table 1.

Table 1:Particular of Round Shaped FPSO Symbol Model Full Scale

Diameter (m) 1.018 111.98 Depth (m) 0.4401 48.41 Draught(m) 0.2901 31.91 Free board(m) 0.150 16.5 Displacement (m3) 0.2361 314249 Water Plan Area (m2) 0.8139 9848.5 KG (m 0.2992 32.9 GM (m) 0.069 7.6

In this study, the numerical method is applied to execute the

wave motion response of Round Shaped FPSO. The panel method developed based on diffraction potential theory with Morison damping correction as presented at part 2 of this paper required to generate a number of meshes on the model surface in order to predict the distribution of wave force act on this Round FPSO model. To reduce the execution time, symmetry theory is applied in the calculation and total number of panels generated for execution in each symmetry side is525 (1050 for whole model) for immerse part. The sample of mesh of Round Shaped model used in the numerical calculation is shown in Figure 4.


15 P

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March 20, 20

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March 20, 2015


Comparing the heave response tendency predicted by both the methods, the Figure 7 also shown the numerical result (blue line) is fixed quite well to the heave response data collected from experiment (red dot). This shown that the developed numerical method which combined the Morison drag term with diffraction potential theory can be applied to predict the heave response of this Round Shaped FPSO even in damping dominate region and obtained reasonable accuracy result. In overall, it can observed that the numerical result is able to predict the 6 DOF wave motion response of the Round Shaped FPSO in good accuracy if compare to experiment result. Since this designed FPSO model is 4 side symmetry (bow and stern, port-side and starboard side), then the study conducted in the head sea condition is enough to present the motion response characteristic of this model.

Figure 7: Heave motion responses of Round Shaped FPSO

5.2 Round Shaped FPSO Wave Response The predicted Round Shaped FPSO wave response characteristic for surge, pitch and heave motion by numerical and experiment method is showed in Figure 5 to Figure 7. In the head sea condition, the surge motion of the Round Shaped FPSO is low for short wavelength region. From the Figure 5, it is showed that the surge RAO which correspond to wave height is below 1.0 for the wavelength less than 980 meters. The relative low surge RAO is important for the offloading operation. This is because lower motion in horizontal plan can help in reducingpossibility of crash between FPSO and ship during offloading operation. Besides that, lower horizontal plan motion is important to avoid damage to the riser.

The more important in FPSO design is reduce the motion in the vertical plan motion. The Round Shaped FPSO also showed relative good wave response characteristic in heave and pitch response in short wavelength region. The FPSO design normally is depend on it operating environment condition. As example, let consider the normal operational of the FPSO in wind seas is

between 2 to 7 seconds (wavelength 6 ~76 meter) and ocean swells condition of 12 to 18 seconds (wavelength 225 ~506 meter) (Example of North West Australia sea condition) [9]. The Round Shaped FPSO has very low pitch and heave motion response in the wind sea condition. However, the designed FPSO is not favor to operate in the swell condition due to the natural heave period is located in between the swell sea wave period for the selected operating environment.

From the study, it is observed that the Round Shaped FPSO model have very low pitch or roll motion even though in swell sea environment. The pitch or roll RAO of the FPSO at wavelength less than 506 meter is below 0.5 Rad.m/m (

⁄ ). To able the FPSO operate normally, the roll angle must as low as possible especially for FPSO installed with fractionating columns such as LGP and LNG FPSO. The maximum allowable roll amplitude for this type of FPSO is typically below 2 degrees) [10]. Hence, the low pitch or roll motion of Round Shaped FPSO could be help to reduce the down time in most of the wave condition.

From the simulation and experiment test, it is proved that the Round Shaped FPSO have a good dynamic stability in wave condition. The designed FPSO have the natural heave and pitch period occur at 16.1 seconds and 25.7 seconds respectively. By compared to the environment condition, the natural heave period and natural roll/pitch period of this FPSO success avoided the region where most ocean wave take place. However, the heave motion response of this FPSO should be proper modified for it operating environment so the large heave motion in swell condition can be avoided. In opposite, the natural roll/pitch period is located outside the ocean swells and wind sea periods. This advantage also help the FPSO remain stable in most of wave condition. 6.0 CONCLUSION

The study the wave frequency motion of Round Shaped FPSO is conducted by numerical method and experimental method in this research. Both the numerical and experimental result is agreed between each other for surge, pitch and heave motion of FPSO. The simulation results showed that the Round Shaped FPSO has good wave frequency response and the motion response is kept in low amplitude in most of ocean environment. From the research, it is obtained that the designed Round Shaped FPSO has good dynamic stability where this is an important factor to reduce the down time of FPSO in normal operation. Besides, this FPSO also suitable to operate in more stringent requirement, such as design for a LPG or LNG FPSO installed with the fractionating columns. It is believed that maximum roll amplitude of the Round Shaped FPSO can be remain below 2 degrees for most of the wave condition due to low roll or pitch RAO in wind sea and swells sea condition and the roll/pitch natural period is designed far away from the most frequent occur ocean condition. In future, the Round Shaped FPSO design should be focused in heave motion response due to the heave natural period is still possible to fall in the ocean swells condition at some of the FPSO operating environment. From the research, the Round Shaped FPSO can be considered as an alternative to replace ship hull FPSO due to the good wave motion response characteristic and good stability in wave.

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ACKNOWLEGMENT The authors are very grateful to Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, Ocean and Aerospace Research Institute, Indonesia, Department of Transportation and Environmental Systems, Hiroshima University, Japan, National Research Institute of Fisheries Engineering (NRIFE), Japan and Centre for Marine Technology and Engineering (CENTEC), Instituto Superior Técnico, Universidade de Lisboa, Portugal for supporting this research. REFERENCE 1. Lamport, W. B. and Josefsson, P.M.(2008). The Next

Generation Of Round Fit-For-Purpose Hull Form FPSOS Offers Advantages Over Traditional Ship-Shaped Hull Forms,2008 Deep Gulf Conference, December 9-11, New Orleans, Louisiana, USA.

2. Arslan, T., Pettersen, B. and Andersson, H.I. (2011). Calculation of the Flow Around Two Interacting Ships, Computational Methods in Marine Engineering IV, L.Eça, E. Oñate, J. García, T. Kvamsdal and P. Bergan (Eds.), pp. 254-265.

3. Afrizal, E., Mufti, F.M., Siow, C.L. &Jaswar. (2013). Study of Fluid Flow Characteristic around Rounded-Shape FPSO Using RANS Method. The 8th International Conference on Numerical Analysis in Engineering: 46 – 56. Pekanbaru, Indonesia.

4. Kvittem, M.I., Bachynski, E.E. & Moan, T. (2012). Effect of Hydrodynamic Modelling in Fully Coupled Simulations of a Semi-Submersible Wind Turbine. Energy Procedia 24.

5. Koto, J., Siow, C.L., Khairuddin, Afrizal, N.M., Abyn, H., Soares, C.G., (2014). Comparison of Floating Structures Motion Prediction between Diffraction, diffraction-viscous and diffraction-Morison methods. The 2nd International Conference on Maritime Technology and Engineering. Lisboa, Portugal.

6. Siow, C.L., Koto, J, Yasukawa, H., Matsuda, A., Terada, D., Soares, C.G., Zameri, M. (2014). Experiment Study on Hydrodynamics Characteristic of Rounded- Shape FPSO, The 1st Conference on Ocean, Mechanical and Aerospace-Science and Engineering-.Pekanbaru, Indonesia.

7. Siow, C. L., Koto, J, and Khairuddin, N.M. (2014). Study on Model Scale Rounded-Shape FPSO’s Mooring Lines, Journal of Ocean, Mechanical and Aerospace Science and Engineering, Vol. 12.

8. Christina Sjöbris, (2012). Decommissioning of SPM buoy, Master of Science Thesis, Chalmers University of Technology, Gothenburg, Sweden.

9. Jinzhu Xia. (2012). FPSO Design to Minimise Operational Downtime due to Adverse Metocean Conditions off North West Australia. Deep Offshore Technology.Perth, Australia

10. Khaw T., Rawstron P. and Lagstrom K., (2005), A New Approach to the Design of Monohull FPSOs, 24th International Conference on Offshore Mechanics and arctic Engineering, Halkidiki, Greece.


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Appropriate Model for Mooring Pattern of a Semi-Submersible Platform

Hadi Sabziyan,b Hassan Ghassemi,a,*, Farhood Azarsina,b and Saeid Kazem,c

a)Department of Ocean Engineering, Amirkabir University of Technology, Tehran, Iran. b)Department of Marine Sciences and Technology, Science and Research Branch, Islamic Azad University, Tehran, Iran. c)Department of Ocean Engineering, Khalij-e Fars University, Bushehr, Iran. *Corresponding author: [email protected]

Paper History Received: 5-March-2015 Received in revised form: 14-March-2015 Accepted: 19-March-2015 ABSTRACT

Exposure to environmental conditions at sea for a floating structure is inevitable. Environmental conditions that wave forces are the most important of them have a big impact on floating structures as well. Due to the nature of semi-submersible Platforms that they are exposed to the wave forces, therefore minimizing of tension force in mooring lines and choosing an appropriate mooring system always has been discussed. This article investigate the tension force on mooring lines of a semi-submersible platform when that it has been exposure to 0, 45 and 90 degrees of sea wave direction with the environmental conditions of the Caspian Sea with using Flow-3d (version10.0.1) software. Also the seven symmetric mooring systems in the form of 4 and 8 numbers of mooring lines’ systems have been used to choosing the best modes.

KEYWORDS: Semi-Submersible Platform; Mooring Lines; Tension Force; Flow-3D. 1.0 INTRODUCTION

Today with the increasing human demand on energy resources, especially deep water oil resources, using many kinds of offshore platforms in coastal states to exploit the resources of the continental shelf more and more attention has been. On the other hand, since the search for oil and gas in the deep waters (over 600 meters) is advanced, therefore, it is impossible to use a fixed

platform in such depths. Fixed platforms due to heavy construction and installation

costs are commonly used up to limited depths of 360 m to 450. Hence the idea of using floating platforms in deep water that has the ability to use in deep water more and more attention has been. Those Semi- submersible platforms are one of them. In deep water, semi-submersible platforms due to the floating nature of them are being exposed to external dynamic loads resulting from the wind and waves.

In fact, for controlling of vertical responses in floating structure, the decisive factor is the size and shape of the floating platform and to reduce the response must be paid to the optimization platform. Therefore, according to aim of this study, the horizontal responses of a floating platform for discussion and analysis of tension force in mooring lines has been studied.

In the past, much research has been done on the mooring lines, such as the research cited Fylling & Lie, studied on design of mooring systems, riser systems and anchors in floating platforms both individually and together, and have provided a calculation method for layout optimization of Anchors [2]. Ferrari & Morooka found an optimum design method for mooring of semi-submersible platforms [3]. Daghigh et al introduced an optimum design for mooring pattern and Floating bridge sizes of Urmia Lake and sizes using disjoint elements in parametric solutions and optimization of anchor’s weight in mooring issue has been be discussed [4].

Maffra et al investigate feasibility of optimizing mooring for a semi-submersible platform with using Genetic Algorithm and Finally, found it possible and useful [5]. Mazaheri & Mesbahi obtained to maximum displacement of the structural that is very essential to arrangement of mooring lines with design of a


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network model of artificial intelligence, For a period of N years. Comparison of artificial intelligence network results shows that this network can well be used to predict the structural response due to arbitrary loads [6]. Mazaheri & Incesik used reply axis method for predicting the mooring tension force [7]. Jordan & Beltran-Aguedo made possible estimation of physical parameters of a cable line in viscous environment with introducing an adaptive algorithm that is sensitive to the depth, by inputs of this algorithm that are forces and positions that determined by equipment on the vessel itself [8].

Rezvani & Shafieefar investigate the optimization of floating platform mooring system with aim of reducing horizontal platform responses by using package SESAM that has considered the Mimosa software [9]. Garrett by studying a moored semi-submersible platform in the Gulf of Mexico at depth of 1800m was performed concurrent dynamic analysis using both time domain and frequency domain [10]. Davies et al investigated the effect of fiber stiffness on the response of mooring lines in deep water [11]. Stansberg studied Current effects alone and in combination with the effects of waves on a moored floating structure in different sea conditions [12]. Huilong et al studied effect of mooring nonlinear stiffness in the hydrodynamic response of floating structures and was provided a new formula to the nonlinear stiffness matrix for chain mooring system [13]. Waals investigated the effect of wave direction on the low-frequency wave motions of floating structures and the tension forces in mooring lines [14].

Ma et al analyzed mooring system of drilling ship that was designed for the South China Sea in depth of 1500 meters [15]. Lassen et al studied the behavior of chain mooring components in both laboratory and numerical models under the pretension and outside of the plate stress condition [16]. Su-xia et al are examined slack Problems due to the tension reduction mooring lines [17]. Zhu & Ou studied the hydrodynamic behavior of a semi-submersible platform with sea waves and wind forces [18]. Model scale rounded-shape FPSO’s mooring lines studied by Siow et al [19]. and Studied the effect of mooring lines pattern in a semi-submersible platform at Surge and sway movements by sabziyan et al [20]. 2.0 SEMI-SUBMERSIBLE PLATFORMS

Before drilling in waters up to a depth of 18 meters from the ship consists of one or more Pantones (cubic large volumes), the deck was done to keep the columns. Such as floating platforms transferred to the operating area and then enter the water into the Pantones then platform sit on sea floor and began drilling.

It observed that over floating time of these structures in response to stimulation of the sea waves, they display slightly small movements. Later, this property was used in the design of semi-submersible platforms.

Of course, today in this kind of platforms while still drilling in deepwater, semi-submersible platform remains in floating situation and just used some mooring lines to keep of the platform in the desired position.

Figure 1 is a semi-submersible platform that has two pantones and four vertical supplier columns that connect pantones to deck. This platform can be moored by anchors, cable or chain in the desired position.

For depths greater than 475 meters, position and stability of semi-submersible platform has been controlled dynamically. The Dynamic Positioning System (DPS) has several small thrusters that quickly change direction or turn on and turn off.

The resultant path of forces generated by thrusters is in the opposite direction of the waves and external forces such that maintain the platform's initial position.

Figure 1: A semi-submersible platform

3.0 CHARACTERISTICS OF THE SEMI-SUBMERSIBLE PLATFORM

The characteristics of semi-submersible rigs can be mentioned the following: A) Because of the low draft to the platform displacement, have minimum dynamic response to waves and the natural frequency of them for Roll, Pitch and Heave movement is large and for Surge, Sway and Yaw movement is small and in contrast to the 100-year wave with period of 14-17 seconds will be resonance. B) Have a moveable displacement speed of 8-10 m / s. C) This platform has a large deck for drilling operations performance. D) Mooring system must satisfy with appropriate conditions for drilling operations that accuracy required for the maintenance of the platform in a position with a maximum error of 1% of the water to have a designated location. E) To a depth of over 475 meters using from catenary mooring lines is impractical and in these cases, vessels that their positions dynamically stabilize and control are used. F) In semi-submersible platforms that equipped with dynamic positioning system, riser connection to the platform to be cut in bad condition and in calm sea established. G) Maximum water depth and weather conditions where drilling by these platforms is possible is a function of mooring system,


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portable riser and drilling pipe by platform and capability of drilling equipment’s vertical movements. 4.0 DISADVANTAGES OF SEMI-SUBMERSIBLE PLATFORMS

These platforms have the disadvantage that, in general, the following cases can be cited: A) The high cost of initial construction and operating costs per day. B) Deck limited for storage (oil or gas) cause low static equilibrium and reserve buoyancy. C) Structural constraints due to the trend toward structural fatigue. D) Limitations and the high cost of the cross from Suez Canal and Panama. E) Restrictions on the dry dock for repair and service. G) Stiffness of mooring system in raging Sea.

It should be noted that the variable deck loads have tripled in the last 20 years, especially in the case of semi-submersible platforms with displacement of 150,000 tons that have cranes which can transport up to 10,000 tons structures. For these platforms also has created problems for which they have been led to the overturning or sinking.

So the rules for constructing of these platforms have changed, and higher safety factor and additional measures such as increasing the number of columns, bumps on the joints between the columns and other Pantones, increase in the volume of displacement of Pantones and divide the deck into the water impermeable units, have been applied.

The transverse elements such as horizontal bracings, connect bodies to each other; in the absence of their, large forces at the junction between the columns and the deck will be done and causes great stress and structural damage caused by fatigue. 5.0 MOORING OF SEMI-SUBMERSIBLE PLATFORM A floating structure, experience sustained loads and instability loads in the own lifetime which causes a change displacements in relation to the position of the platform. Floating structures usually using are mooring, tendons stretched vertically, dynamic positioning, or a combination of these are kept in good condition.

Maintain the integrity of such systems like riser and bridges depends on the capabilities of positioning systems and restrictions the using of floating platforms to be determined by the constraints of retaining the position.

Therefore integration of positioning systems is a major factor in the successful design of a semi-submersible structure that taken into account. The positioning system of a floating structure can be a single point mooring system (SPM) or spread mooring system. Single point mooring system is more used in ship shape structures and spread mooring system is more used in semi-submersible structures. The third type is a dynamic positioning system. In the symmetrical pattern system, all the lines of the characteristics and composition of the materials (chain / cable) are identical.

Basically symmetrical pattern systems are used for the vessels

that has been operate in one place for a long time. An important characteristic of this pattern is that the direction of environmental forces, waves, currents and wind, has no important effect on its function.

In other words, symmetric systems using symmetric and homogeneous distribution lines in different directions can be mentioned platform position against the environmental forces that are imported from different directions.

In the Semi-submersible Platform, for proper operation and preventing damage on systems and equipment of excavation units it required that horizontal motion of structures be limited to less than 1% of water depth [21], that is defined in Table1.

Table1) Limits of semi-submersible platform movement Max surge/sway

amplitude (% of water depth)

Duration (%)

Operation

5 43.4 Drilling

3 12.5 Running easting

- 11 Cementing and well testing

1 9.9 Blow out preventer and riser handling 6.0 HYDRODYNAMIC ANALYSIS OF SEMI-SUBMERSIBLE PLATFORM

Since in deep water, large floating structures with heavy volume displacement are used, Diffraction theory applied to calculate these forces. However, structures have slender members; Morrison equation in the hydrodynamic analysis of semi-submersible platform is used, also.

Analyzing a floating platform consists of solving the differential equation of motion in 6 degrees of freedom with regard to environmental forces. Degrees of freedom consist of three translational degrees (Surge, Sway and Heave) and three rotational degrees (Pitch, Roll and Yaw). The structure motions in these degrees of freedom depends on various factors such as mooring stiffness, environmental loads, geometry of structure, damping rate, and so on. However, mooring systems of semi-submersible platform creates restorative forces in the horizontal plane and therefore has been controlled the motions in 3 degree of freedom (Surge, Sway and Heave).

A moored semi-submersible structure placed against to the 2 types of hydrodynamic wave drift forces. A mean wave drift force that causes uniform structure displacements and another low frequency hydrodynamic thrust force that creates Surge, Sway and Yaw motions about uniform displacement of structure [22]. 7.0 MODELING

Intended in simulate a case study, using Flow 3 D hydrodynamic software that gives the time domain analysis for study the movements of the structure. In this simulation used a platform with specifications of GVA 4000 platform like Iran ALBORZ


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platform and Extreme environmental conditions of the Caspian Sea Published by SADRA Company that Identified significant wave height (Hs) for 100 -year wave as 9.5 M and sever condition wave period (Tp) for 100 -year wave as 12.8S. Figure 2 shown the actual model in FAVOR1 with 4 mooring lines and without determine the boundaries of water.

Figure 2: Actual model in FAVOR without determine the boundaries of water

In this simulation used a platform with specifications of GVA

4000 platform like Iran ALBORZ platform. Table 2 defined the characteristics of this platform.

Table 2: characteristics of Iran ALBORZ platform 12.9 m Columns diameter 98.6 m Overall length. 73.4 m The distance between

the columns in the longitudinal direction

78.8 m Overall Width

54.7 m The distance between the columns in the transverse direction

73.4 m

Pontoon length from outside to outside

20665 ton

Tonnage moved in the transportation draft (7.2 m)

80.5 m

Length of Pontoons

26525 ton

Tonnage moved in the survival draft (16.2 m)

18.5 m

Width of Pontoons

28621 ton

Tonnage moved in the operational draft (19.5 m)

7.5 m height of Pontoons

At this research, modeling is done by taking a survival

1 Fractional Area Volume Obstacle Representation

situation. Mooring lines Components are presented in Table 3. These lines have different sections with different lengths and materials.

Table 3: Mooring lines Components Water depth (m)

Component Dimension (mm)

Length (m)

Breaking strength

(ton)

Weight in air

(kg/m)100 Chain 76 1400 545.4 129 500 Chain 76 1800 545.4 129

1000Chain 76 1000 545.4 129 Wire 86 1000 510.1 30.7

Also, for analyze the variable patterns of mooring lines

geometric arrangement from 7 types of layout that those are, 3 kinds with 4 lines in the geometrical arrangement (figure 3) and also with 4 kinds with 8 lines in the geometrical arrangement (figure 4) (Angles of 30, 45, 60 and 90 between adjacent mooring lines) lines and by applying of wave forces in the 0, 45 and 90 degree of direction has been studied.

Figure 3: Type 3 of mooring system arrangement with 4 mooring lines


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Figure 4: 4 kind of mooring system arrangement with 8 mooring lines

To evaluate the impact of environmental conditions on the structural motions, also we use the Caspian Sea environmental information with water depth under 1000 Meters that SADRA Company Published and apply the final threshold conditions in order to assess the maximum possible displacement structures are used that these data are presented in (Table 4).

Table 4) Caspian Sea environmental threshold conditions data 100 Year- wave 1 Year- wave Environmental

conditions

(m) 9.5 (m) 5.6 Hs

(s) 12.8 (s) 10.3 Tp

Also to apply the environmental wave forces on the semi-

submersible structure for large members used from Diffraction theory (Equation 1) and for thin members used of Morrison equation (Equation 2) that these equations are as following:

(1)

| | (2) where In equation 3, ρ is fluid density, CI is inertia coefficient, D is pile diameter, ω is angular velocity of fluid particle, H is wave height, k is wave number, y is depth from sea base, h is wave height adjacent pile and ε is angle of arrears.

And In equation 4, Cm is inertia coefficient, ρ is fluid density, D is pile diameter, is horizontal accelerate of fluid particle in pile axis line, Cd is drag coefficient and u is horizontal velocity of fluid particle in pile axis line.

In Figure 5 the results of the maximum forces acting on mooring lines in different patterns under different angles of incident wave has been determined.

Geometry of the flume created for semi-submersible platform in a water depth of 500 m and also meshing of model has been selected as integrated single block. The dimensions of these cells should be considered that the FAVOR conditions can be satisfied. Validation for grid cell size done with 4 meshing networks with 106, 1.2*106, 1.5*106 and 2*106 cells was in kind 6 of mooring systems. And more converged results for the grid cells with 1.2*106 are valid. It is used fine mesh in the near range of semi-submersible platform to enhance network quality, and coarse mesh far filed.

In flow3d software both to Cartesian coordinates and cylindrical coordinates there are 6 surfaces to define the boundary conditions. Surge movements analysis of semi-submersible platform specified in Figure 5 for 7 kind mooring systems and 3 incoming wave with angles of 30, 45 and 90 degree. Also, Sway movements under different angles of the incident wave are defined in Figure 6 for 7 kind mooring systems and 3 incoming wave with angles of 30, 45 and 90 degree. And tension forces in mooring lines under different angles of the incident wave are defined in Figure 7 for 7 kind mooring systems and 3 incoming wave with angles of 30, 45 and 90 degree.


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Figure 5: Semi-submersible platform Surge movements.

Figure 6: Semi-submersible platform Sway movements.


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Figure 7: Maximum tension forces exerted in the mooring lines 8.0 CONCLUSIONS

We applied the Flow3d software to analyze seven various mooring system types of a semi-submersible platform. According to our numerical results following conclusions can be drawn: 1. With investigation on analysis results, in mooring lines

pattern with 4 mooring lines, According to distribution of Hydro- dynamic forces in fewer lines compared with 8 mooring lines it has be seen that in general, the tension force in the mooring system lines is more and the use of this type of mooring system is not recommended.

2. According to the results of analysis, most appropriate mooring system is Pattern Type 6 (60 degree angle between the two adjacent mooring lines) that causes the lowest range of horizontal structure motions.

3. Also with study the tension forces that acted on the mooring lines with 8 lines systems, it has been seen that the minimum mode is happened in kind 4 arrangements (mooring lines with angle of 30 between the adjacent lines). Although kinds 5 and 6 with respect to the balance of tension forces acted on the mooring lines in different angles of applied hydrodynamic forces and the low tension forces in the mooring lines can be use.

4. In general, use the 8 mooring lines significantly reduced the average amount of semi-submersible platform displacement and overall amount of tension force acting on the lines so it can be reduced between 25 to 60 percent but 3 ways of mooring system with angles of 30, 45 and 60 ° between the adjacent mooring lines responses better than a angle of 90 ° between the adjacent mooring lines.

REFERENCES

1. Barltrop, N. D. P., “Floating Structures: a guide for design and analysis”, Volume one, Oilfield Publications Limited (OPL), Ledbury, England, 2003.

2. Fylling I.J. & Lie H., 1986, “Mooring System Design Aspects of Environmental Loading and Mooring Systems Optimization Potential”, International Conference on Offshore Mechanics and Arctic Engineering, USA.

3. Ferrari J.A. & Morooka C.K., 1994, “Optimization and Automation of the Semi-Submersible Platforms Mooring Design”, International Conference on Offshore Mechanics and Arctic Engineering, Houston, Texas.


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4. Daghigh M., Paein Loulaei R.T. & Seif M.S., 2002, “Mooring System Design and Analysis for the Flooding Bridge of Urmia Lake”, 12th International Conference on Offshore Mechanics and Arctic Engineering, Oslo, Norway.

5. Maffra S., Pacheo C. & Menezez M., 2003, “Genetic Algorithm Optimization for Mooring System”, Rio de Janeiro, Brazil.

6. Mazaheri S. & Mesbahi E., 2003, “Sea Keeping Analysis of a Turret-Moored FPSO by Using Artificial Neural Networks”, 22nd International Conference on Offshore Mechanics and Arctic Engineering, OMAE 2003-37148, Cuncun, Mexico.

7. Mazaheri S. & Incecik A., 2004, “Predicting the Maximum Mooring Force of a Moored Floating Offshore Structure”, 23rd International Conference on Offshore Mechanics and Arctic Engineering, OMAE 2004-51245, Vancouver, British Columbia, Canada.

8. Jordan M.A., Beltran-Aguedo R., 2004, “Nonlinear Identification of Mooring Lines in Dynamic Operation of Floating Structures”, Journal of Ocean Engineering 31, pages 455–482.

9. Rezvani A. & Shafieefar M., 2007, “Mooring Optimization of Floating Platforms Using a Genetic Algorithm”, Ocean Engineering 34, pages 1413–1421.

10. Garrett D.L., 2005, “Coupled Analysis of Floating Production Systems”, Journal of Ocean Engineering 32, pages 802–816.

11. Davies P., Baron P., Salomon K., Bideaud C., Labbe J.P., Toumit S., Francois M., Grosjean F., Bunsell T. & Moysan A.G., 2008, “Influence of Fiber Stiffness on Deepwater Mooring Line Response”, 27th International Conference on Offshore Mechanics and Arctic Engineering, OMAE 2008-57147, Estoril, Portugal.

12. Stansberg C.T., 2008, “Current Effects ON a Moored Floating Platform in a Sea State”, 27th International Conference on Offshore Mechanics and Arctic Engineering, OMAE 2008-57621, Estoril, Portugal.

13. Huilong R., Jian Z., Guoqing F., Hui L. & Chenfeng L., 2009, “Influence of Nonlinear Mooring Stiffness on Hydrodynamic Performance of Floating Bodies”, 28th

International Conference on Offshore Mechanics and Arctic Engineering, OMAE 2009-79697.

14. Waals O.J., 2009, “The Effect of Wave Directionality on Low Frequency Motions and Mooring Forces”, 28th International Conference on Offshore Mechanics and Arctic Engineering, OMAE 2009-79412, Honolulu, Hawaii, USA.

15. Ma G., Sun L. & Wang H., 2009, 28th International Conference on Offshore Mechanics and Arctic Engineering, OMAE 2009-79320, Honolulu, Hawaii, USA.

16. Lassen T., Storvoll E. & Bech A., “Fatigue Life Predictio drilling ship n of Mooring Chains Subjected to Tension and out of Plane Bending”, 2009, 28th International Conference on Offshore Mechanics and Arctic Engineering, OMAE 2009-79253, Honolulu, Hawaii, USA

17. Su-xia Z., You-gang T. & Hai-xiao L., 2009, “Study on Snap Tension in Mooring Lines of Deepwater Platform”, 28th International Conference on Offshore Mechanics and Arctic Engineering, OMAE 2009-79881, Honolulu, Hawaii, USA.

18. Zhu, h. and Ou, Jinping. (2011), “Dynamic Performance of a Semi-Submersible Platform Subject to Wind and Waves”, Journal of Ocean Univ, China, vol. 10, No. 2, p.127-134.

19. Siow, C.L., Koto J., Khairuddin N.M, (2014), “Study on Model Scale Rounded-Shape FPSO’s Mooring Lines, Journal of Ocean, Mechanical and Aerospace-Science and Engineering-, Vol.12, 2014.

20. Sabziyan H., Ghassemi H, Azarsina F., and Kazemi S., 2014, “Effect of Mooring Lines Pattern in a Semi-submersible Platform at Surge and Sway Movements.” Journal of Ocean Research, vol. 2, no. 1: 17-22. doi: 10.12691/jor-2-1-4.

21. API Recommended Practice (RP2nP), “Analysis of Spread Mooring Systems for Floating Drilling Units”, Second Edition, DC 2005.

22. GVA industrial consulting group, "Whata contractor learned in north sea Drilling", Ocean Industry Dec. 1972- p.10-1.

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