Priliminary design and prototype scale model of offshore acqaculture flating structure for seaweed farming


Published on

  • Be the first to comment

  • Be the first to like this

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Priliminary design and prototype scale model of offshore acqaculture flating structure for seaweed farming

  1. 1. ICSOT: Developments In Fixed & Floating Offshore Structures, 23 – 24 May 2012, Busan, Korea PRELIMINARY DESIGN AND PROTOTYPE SCALE MODEL OF OFFSHORE ACQUACULTURE FLOATING STRUCTURE FOR SEAWEED OCEAN FARMING O. O Sulaiman, Allan Magee, M Ilham A Razak, W. Richard, W.B. Wan Nik, A.H.Saharuddin, A.S.A. Kader, Adi Maimun University Malaysia Terengganu, Malaysia, Technip, Geoproduction SDN, BhD, amagee@technip.comSUMMARYSeaweed farming has become one of the natural resources which are economically important. The existing cultivationsystem for seaweed is not suitable for deployment with most of deep and open water area. Moreover, the currentcultivation system are not environmental sustainable and economical unstable. This paper describes the design of theoffshore floating structures scientifically based on improvement of the Long Line System for commercializes scaleseaweed farming. Some key factors in the design, prototype and testing of floating offshore structures considered in thedevelopment of ocean farming technology system are discussed. coefficients are used to design the configuration and1. INTRODUCTION appropriately size components of the mooring system. Recent time shows trend where the concept of The system design employed industryVLFS is becoming increasingly popular all around the guidelines such as API RP 2SK, BV and DNVworld, especially in land-scarce island countries and in specification on how the data is to be used to design thecountries with long coastlines for a number of reasons. station keeping (mooring) system including anchor pilesVLFSs has been constructed and/or can be built to create and mooring components like shackles, chains, wires,floating airports, bridges, breakwaters, piers and docks, ropes, etc. that make up the system, to assure the systemstorage facilities (for instance for oil), wind and solar is strong enough to withstand the expected loads andpower plants, for military purposes, to create industrial maintain required factors of, emergency bases, entertainment facilities,recreation parks, sewerage treatment and waste disposal Design for ship and offshore performanceplants, nuclear power plants, mobile offshore structures depends on geometric shape / form factor, load on theand even for habitation. In fact, the last could become system and environmental load on the system factor data,reality sooner than one may expect: already different some of which are available from historical andconcepts have been proposed for building floating cities experimental data. However, in case of seaweed, theor huge living complexes. As a result, this sector has seaweed interacts directly with the environment, comparereceived a lot of research interests in recent years. to other marine structure where the structure has close interaction the the environment, also available data on Design of very large floating structure for the hydrodynamic loading is lacking. Neither are thereseaweed cultivation project is a socio-economic being any known computational tools which can readilycarried out as collaboration between UMT, Technip and simulate the loads on such flexible, buoyant structures.Bureau Veritas towards developing offshore aquaculture So, the best way to quickly obtain such data is throughmarine technology platform that will provide the use of model tests. For this purpose, samples ofsupplementary income to fishermen in rural areas by seaweed cultures is subjected to static and dynamic testsselling seaweed as a cash crop and enhanced effective for the purpose of determining the equivalent added massuse of ocean resources. Beside the benefits of developing and damping coefficients. The test result are then appliedlocal capabilities and hands-on learning, this study is in typical software tools to design a mooring system forpartly funded using funds for sustainable development. seaweed which has a good chance of surviving the expected metocean conditions for which it has been The project involves design of the mooring designed.system used to anchor and provide station keeping for theseaweed plantation platform to the seafloor in order toprevent tangling of the seaweed and excessive movement 2. POTENTIAL OF SEAWEEDof the platform. The project involve technology transferfrom offshore industry to design floating platforms for Seaweed farming has become an economicallyoffshore aquaculture, carry out model tests at UTM Lab important natural resource. It has wide applicationtowing tank facility with appropriate equipment and potentials similar to other commodities such as palm oilinstrument to determine the hydrodynamic coefficients of and cocoa. It application found uses product suchdifferent components, especially the seaweed, for use in cosmetic, medicine, gelatine, food, CO2 sink anddesign of the mooring system. The determined biomass energy source (Figure1). There is currently worldwide requirement to produce large amount. 1
  2. 2. ICSOT: Developments In Fixed & Floating Offshore Structures, 23 – 24 May 2012, Busan, KoreaHowever there is currently no proper system to deliver built for public use or for industrial facilities, is expectedthis demand (Kaur, 2009). to have a long service life (50-100 years) wit preferably low maintenance; and their safety, reliability and This paper discusses a solution to overcome the survivability are vital for their economic feasibility.problem of deep water and open water environment by VLFSs should also have good fatigue life, corrosion andproposing a design of offshore floating structure for fracture resistance as well as light structural weight toseaweed farming. This study introduces an appropriate ensure sustainability. (Chakrabarti, 1987).optimised design of the ‘Long-Line Cultivation Method’for seaweed cultivation that meets current demand. The Work on the design of hydroelastic analysis ofnew design of floating structure for seaweed ocean VLFSs may be found in the review papers mentioned atfarming will improve seaweed culture efficiency and its Kashiwagi, M. (2000a), Newman, J.N. (2005, Ohmatsu,adoptability and continuity of its operation. S. (2005), Suzuki, H. et al. (2006), Watanabe et al ( 2004b). Fujikubo et al worked on structural modelling global response analysis and collapse of VLFS. Inoue, K. (2003), work focuses on stress analysis of the system. Salama et al , focus on use of composite e material for VLFS. Mamidipudi and Webster (1994) undertook pioneering work on the hydroelastic analysis of a mat- like floating airport by combining the finite-difference method for plate problem and the Green’s function method for fluid problem. Wu et al. (1995) solved the a.CO2sink b.Biomass energy two dimensional (2-D) hydroelastic problems by the Figure 1: Seaweed application analytical method using eigenfunctions. Yago and Endo (1996) analyzed a zero-draft VLFS using the direct method and also compared this with their experimental3. CONCEPT OF VLFS results. Ohkusu and Nanba (1996) analyzed an infinite- length VLFS analytically. Kashiwagi (1998a) applied theThe Mobile Offshore Base (MOB) and Mega-Float, are B-spline panels for the analysis of a zero-draft VLFStypical Very Large Floating Structure (VLFS) with using the pressure distribution method. Nagata et al.unique concept of ocean structures that with different (1998) and Ohmatsu (1998) analyzed a rectangular VLFSbehaviour of from conventional ships and offshore by using a semi-analytical approach based onstructures. The engineering challenges are associated eigenfunction expansions in the depth direction. Che,with: Xiling (1993), work on the hydro elastic analysis of a i. Improvement of fatigue life: VLFS is subjected mat-like floating airport by combining the finite- to constant repetitive load, which lead to difference method for plate problem and the Green’s fatigues and cracks in interconnected module function method for fluid problem. and in connection points as well. ii. Detailed structural analysis: The structural In order to couple the problem between the structure designis required to fulfils serviceability and part and the fluid part, there exist two competing safety requirements in a cost-effective manner. approaches. These two approaches are sometimes For a novel structure like a Very Large referred to as the modal method and the direct method. In Composite Floating structure (VLCF), the most cases, the direct method is computationally more structural design needs a first-principle demanding than the modal method. Thus, the dry-mode approach that is based on a rational structural superposition method can use for the actual design of response analysis and explicit design criteria. VLFS. iii. Modification of station keeping system: Station keeping system involves numerical modelling. 4. MATERIAL AND METHODOLOGY iv. Parametric study: Parametric study is a also a key requirement to provide design guidelines for In the structural design of floating structures, the the construction of VLFC as well as to identify external load and major load effects, are determine from the improvements that can be made. hydro elastic body motions. The dimensions and arrangement are determined so that the structure has Behaviour, design procedure, environment, and the sufficient strength against the given loads and loadsstructural analysis of VLFS required adaptation of effects. Hydroelastic response analysis dealing withconventional ships and offshore structures design specifies design variable such as structural depth, length,knowledge. The design and analysis of VLFS are arrangement, and size are employed. The characteristiccharacterized with unprecedented length, displacement length and frequency derived by Suzuki and Yoshidaand associated hydroelastic, response. Lots of (1996) for VLFS are referenced during this process.improvements are currently required in this field to makethe application of VLFS more practical. VLFSs, whether © 2012: The Royal Institution of Naval Architects
  3. 3. ICSOT: Developments In Fixed & Floating Offshore Structures, 23 – 24 May 2012, Busan, Korea The details of the design consider deterministic The complete system tests will be used to confirm theanalysis of actual structural configurations that have adequacy of the preliminary design. Instrumentationvariable of structural characteristic. From the results of required:global response analysis of mooring components, thelocal stress response under combined load effects is 1. Load cells of suitable size and range will beevaluated. Through the evaluation of strength and attached to the seaweed linesserviceability limit states, both the arrangement of 2. Wave probesstructural members analysed Figure 2 showsmethodology adapted from International Ship Structure 3. Native carriage speed recordCongress. 4. Wave flap signal The load cells attached to the mooring spring are water-proof aluminium ring strain gauges that measure axial tensile loads. The measured voltage outputs from the load cell strain gauges are connected by cables to the basin’s native Dewetron Data Acquisition System (DAQ)) to be digitally sampled and stored. Software is used to convert the measured voltage to tension readings. The load cells are appropriately-scaled and calibrated (100N range). Other instruments used in the tests are wave probes fixed at specific locations under the carriage and accelerometers mounted on the model decks. Both are channeled to the DAQ to record measured data output. A video camera was positioned at strategic locations on the carriage for model motion recordings. 5. MODEL TEST The model test is required to determine the hydrodynamic loads due to waves and currents acting on seaweed and its mooring system components. The total system loads must be suitable for use in designing a seaweed culture mooring system to avoid failure with potential loss of the valuable crop andFigure 2: Typical Design Flowchart Modified from possibly requiring costly repairs or replacement of thethe Work of (ISSC 2006) mooring system components.Model tests involved:  Measure drag loads of actual seaweed by towing 5.1 Tests for Seaweed Hydrodynamic Coefficients  Dynamic tests to measure added mass and Tests is carried out to identify hydrodynamic damping using PMM. Originally, it was planned coefficients of an equivalent Morison model of the to use the Planar motion mechanism (PMM). seaweed which will be suitable for use in typical However, the equipment was not functioning. mooring design and analysis package such as So, this part of the test plan had to be modified. Arienne. Samples (clumps) of dried seaweed, the The revised test plan is described below. dried will restore to nearly nominal properties when  Hydrodynamic tests on component tests (Buoy, soaked in water for a period of time. Typical size ropes, float, net,…) considered available from seaweed clump weigh up to 1.5kg in air, when fully coefficient of mooring components. Industry grown. However, the natural buoyancy of the data can be substituted for component loading seaweed, make its weight in water almost during initial design work. insignificant. A sample clump of seaweed weight in air 4.1N and the corresponding weight in water is  Complete system tests with scaled seaweed of 0.01N in UTM lab, Figure 3. 1/50 scaled model deployed at UMT 3
  4. 4. ICSOT: Developments In Fixed & Floating Offshore Structures, 23 – 24 May 2012, Busan, Korea typical of the Southeast Asia metocean climate. Note, it is not envisaged to design the seaweed mooring for extreme environments such as rare 100-year events, typically used for offshore platforms, because the consequences of potential failure, while still undesirable, are considered much less severe. For the determination of coefficients, the KC number will be preserved. The KC number for all the a. b. seastates is KC>12, So, relatively large wave Figure 3: Seaweed velocities are present. Also, the effect of flow reversal A sample row of seaweed is attached to a frame can be maintained, at least in the near surface zoneand towed from the carriage. To determine the where the seaweed floats, by running the same waveshydrodynamic coefficient, a series of tests including at similar speeds. It is remarkable that the wavetowing in calm water, towing in waves and wave- kinematics are very similar for all the differentonly tests will be performed. As mentioned, seastates. Therefore it is possible to greatly simplifyoriginally, it was planned to use the PMM to perform the tests plan by appropriate choice of the scalingforced oscillation tests. The resulting loads can be factors for each seastates. In fact it is sufficient toanalysed in a straightforward manner to determine the tests a single wave to produce scaled wave kinematicsrelevant coefficients which give similar for all the conditions, at least for the importanthydrodynamic loads (Figure 4). parameter of wave velocity at the surface. However, the PMM system is not functioning at Then what varies between seastates is actuallypresent. So, an alternative plan had to be developed. the current velocity, and resulting ratio r, ofThe main difficulty is that the available seaweed wave/current velocities. For the present, tests, it issamples represent full-scale (prototype) clumps, decided to focus on deterministic, regular waves,whereas, the model basin is equipped to generate representing the worst design wave for each seastate.wave and current kinematics (wave height <0.4m These waves are characterized by Hmax and associated0.5s<Period<2.5s, and speed<5 m/s) typical of model period Tasso. Therefore the tests can be carried out byscale conditions. Therefore, a mismatch exists using the same wave and changing the currentbetween the typical body (clump) dimensions and the velocity for each seastate.relative kinematics that can be generated. Some Table 1a: Fullscale Seaweed Parameters.approximations and simplifying assumptions areneeded to utilize the available facilities to determine Exceedence Prob/ Return Period 50% 90% 99% 1-yearthe required coefficients. Hs m 0.9 1.8 2.8 5 The solution chosen at present is to focus on Hmax m 1.9 3.6 5.4 9.6 Amax m 0.95 1.8 2.7 4.8certain key non-dimensional values and try to use the Tp s 4.6 6.4 8 10.7use different scaling factors to apply the results at Tasso s 4.3 6 7.4 10fullscale. In this way, the kinematics available in the Uc m/s 0.21 0.47 0.78 1.61towing tank can be used. For example, it is believed Mass g 1500 1500 1500 1500 Length, L cm 50 50 50 50that the physical behaviour of seaweed may be Amax/L 1.9 3.6 5.4 9.6similar to that of cylinders in waves and currents. Diameter, D cm 1 1 1 1Therefore, the approach closely follows that of the Amax/D 95 180 270 480commentary section of the API RP-2A. Uw m/s 1.39 1.88 2.29 3.02 U m/s 1.60 2.35 3.07 4.63 For example, the effects on hydrodynamic Uc/Uw 0.15 0.25 0.34 0.53loading of Keulegan-Carpenter number r=Uw/Uc 6.61 4.01 2.94 1.87 KC No. = 2A/L 11.9 22.6 33.9 60.3 2A Re No. = UL/n 799,073 1,177,478 1,536,257 2,312,964 KC  L where, Table 1b: Proposed Model scale Parameters. A= amplitude of wave partical motion L= typical length of a seaweed clump wave current flow reversal effects (r=ratio ofcurrent/wave orbital velocities) are expected to besimilar to those for cylinders, though perhapssomewhat more complex and with different regimesfor seaweed. Table 1 (a, b) below shows thenondimensional parameters for a series of seastates, © 2012: The Royal Institution of Naval Architects
  5. 5. ICSOT: Developments In Fixed & Floating Offshore Structures, 23 – 24 May 2012, Busan, KoreaModel Scale 6 12 16 30 ration will be used. The floating structure dimension andExceedence Prob/ model are shown in Table1c, Figure 5.Return Period 50% 90% 99% 1-yearHs m 0.15 0.15 0.18 0.17Hmax m 0.32 0.30 0.34 0.32 Table 1c: Structural dimensionAmax m 0.16 0.15 0.17 0.16Tp s 1.88 1.85 2.00 1.95Tasso s 1.76 1.73 1.85 1.83Uc m/s 0.09 0.14 0.20 0.29 One of the important task in planning a modelMass g 6.94 0.87 0.37 0.06 test is to investigate the modeling laws required for theLength, L cm 8.00 8.00 8.00 8.00Amax/L 1.98 1.88 2.11 2.00 system in question to be analyzed. The scalingDiameter, D cm 1.00 1.00 1.00 1.00 parameters is very important in designing a model testAmax/D 15.8 15.0 16.9 16.0 and a few key areas of consideration in replicating aUw m/s 0.57 0.54 0.57 0.55 prototype structure for a physical model test. In order toU m/s 0.65 0.68 0.77 0.84 achieve similitude between the model and the realUc/Uw 0.15 0.25 0.34 0.53 structure, Froude’s law is introduce as the scalingr=Uw/Uc 6.61 4.01 2.94 1.87KC No. = 2A/L 12.4 11.8 13.3 12.6 method. Froude’s law is the most appropriate scaling lawRe No. = UL/n 52,195 54,385 61,450 67,566 for the free and floating structure tests (Chakrabarti, 1998).As these tests are currently ongoing, results will bepresented as they become available in time for the The Froude number has a dimensionconference. corresponding to the ratio of u2/gD where u is the fluid velocity, g is the gravitational acceleration and D is a characteristic dimension of the structure. The Froude number Fr is defined as Frame Fr 2 gD/u to attach seaweed The subscripts p and m stand for prototype and model respectively and λ is the scale factor. Assuming a model scale of l and geometric similarity, the Froude model must satisfy the relationship: Figure 4: UTM Towing Tank and Carriage u 2p / gDp = u 2m / gDm5.2 Complete System Tests Important variable quantities of importance are The hydrodynamic loads measured will be used derived from the equation and dimensional analysis asto build approximate scale models of the seaweed. follows:The model seaweed will mimic the Froude-scaledproperties (mass, dimensions, added mass and Linear lp λ lm Actual Speed up = √λ umItem Model Mass mp = λ 3 mm StructureLength Overall for 10 Force Fp =( λ3 / 0.975) *Fm 1000m 2m Time tp = √λ tmBlocks, LBreath, B 100m 2m Stress/Pressure Sp λ SmDimension for Each 100m x 100m 2m x 2mBlockMooring Depth, D 50m 2.5mdamping) of the seaweed measured previously.Suitable material such as plastic ribbon, rubber tubingor even young seaweed seedlings will be used tobuild a sufficiently quantity of scaled seaweed. The tests of floating structure in regular andirregular waves will be carried out in the towing tank120m x 4m x 2.5m of Marine Technology LaboratoryUTM. This laboratory is equipped with the hydraulicdriven and computer controlled wave generator which iscapable to generate regular and irregular waves over a Figure 5: Scale model of the physical systemperiod range of 0.5 to 2.5 seconds. For this structuralexperiment, a model of 2m x 2m per block with 50 scale 5
  6. 6. ICSOT: Developments In Fixed & Floating Offshore Structures, 23 – 24 May 2012, Busan, Korea6. ENVIRONMENTAL LOAD CONSIDERATION Components of the Floating Structure is as followed (Figure 6) The weather in Malaysia is mainly influencedby two monsoon regimes, namely, the Southwest  Frame Line: This line support the planting line forMonsoon from late May to September, and the Northeast each block of the structure. It was made fromMonsoon from November to March. (K.C.Low, 2006). synthetic fibre rope.However the east coast of peninsular Malaysia is the area  Planting Line: Planting line is the main part of thethat exposed directly to the strong sea currents and structure which contains the planted seaweed. Thisperiodic monsoon season which is prevalent off the east line must be able to withstand the mature growth ofcoast. Furthermore, with the existence of nature elements seaweed weight.of the deep and open water environment, seaweed  Separator Line: When the nature hit floatingfarming is hard to be applied in this area. structure, the planting line tends to tangle. Therefore, this line will act as the separator between each Regular waves were considered and generated by planting line.the wave maker for a few tests. Random waves  Mooring Line: This line holds the whole structurespectrum was based on the Jonswap spectrum at the surface to be in place with connection to the( for less than 1Yr or for 1Yr or greater, anchor. Since mooring system is a crucial part forrespectively. Froude scaling was applied to establish floating structure. It must be designed to withstandthe relationship between full scale wave height (Hp) the natural force and achieve stability through theand period (Tp) and the corresponding model scale use of mooring line tension.wave height (Hm) and period (Tm), where Hm =  Buoy: Buoy is to provide a convenient means forHp/50 and Tm = Tp/50. connection of the floating structure on the surface to the mooring. Through the use of distributed Incident waves will be measured and analyzed prior buoyancy for each buoy, floating structures canto the tests. Two wave probes will be installed for achieve stability. The buoy also has to withstand thecalibrations: one in front of the carriage at the basin structural weight and additional load from thecentreline and one to the side of the nominal position of seaweed.the model. Wave force vector is generally expressed as  Sinker: A sinker is made of concrete and placed onthe sum of linear wave force proportional to wave height a catenary mooring line to ensure horizontal mooringand the slowly varying drift force proportional to the at anchor, enhance mooring line energy absorptionsquare of the wave height. and affect mooring line pretension in a way that can be useful in controlling structural stability. Table 2: Full Scale Wave  Anchor Block: Anchor is designed in a large mass of concrete to keeps the floating structure at the Return place also to resists both horizontal and vertical Period Full Scale Full Scale movement. Wave Height Wave Period (Year) (m) (s) 90% 4.599 9.711 95% 4.850 10.25 1/12th 0.6 1.295 1-Yr 5.110 10.79 10-Yr 10.7 12.82 100-Yr 7.3-13.6 11.1-15.1 a. 3D View o first system Mooring design for offshore platforms makesuse of software tools which have been benchmarkedagainst model tests, computational data and full scalemeasurements for their given applications.Hydrodynamic loading on the platform, risers andmooring system itself due to waves and currents arecalculated using a variety of tools such as potential flow,CFD and empirical data. b. Physical system deployed at sea7. DESIGN OF OFFSHORE AQUACULTURESTRUCTURE FOR SEAWEED FARMING © 2012: The Royal Institution of Naval Architects
  7. 7. ICSOT: Developments In Fixed & Floating Offshore Structures, 23 – 24 May 2012, Busan, Korea understanding of systematic review on how to implement the structure in the real practice. The design of this system must determine the pros and cons of the design approaches in an attempt to reach the generic reliable system. Table 1 gives a list of structural design challenge parameters that would impact the performance of a floating structure system. Each design challenge is evaluated from the three main items of achieving stability using a method of the symbols. The symbols indicate ease with which each challenge might be overcome for each parts of the c. Mooring configuration system. As mention earlier, the structural design isFigure 6: Floating Structure for Ocean Farming impacted by the choice of each structure components.System Therefore, it must be included in the table of design challenge. The design is likely to provide the most The study focuses to produce a design for efficient floating structure that can stand the high impactoffshore floating structure of seaweed farming. This from the response of nature. A design such as buoy isdesign is required to meet the operating conditions, likely to be subjected to higher additional loading, whichstrength and serviceability requirements, safety will increase the systems response (motion) to waves.requirements, durability, visually pleasing to the Therefore, the design needs to be made to tolerate largerenvironment and cost-effective. An appropriate design motion. However, it shall increase the complexity of theservice life is prescribed depending on the importance of systems.the structure and the return period of natural loads. Its Table 3: Floating Structure Technicalservice life is generally expected to be as long as 50 to Challenges100 years with preferably a low maintenance cost. Main Structure Items On logistic, this structure will be operating 200meters from the shore as a result, the structure is likely to Structural Design Surface Mooring Anchorexperience more energizes wave action and stronger Complexity Components Linewind associated with deep water region. This design also Componentsconsidered 1-2 boat lanes within the structure blocks Buoyancy ● ● ○which is about 5 meters wide at the original size. Mooring System ○ ● ○ In the structural design of floating offshore Anchorsstructures, the external load and major load effect, suchas cross sectional forces, are determined from the rigid Cost/Complexity ○ ●body motions. The dimensions of structural members and Seaweed Load ○ ○arrangement are subsequently determined so that thestructure has sufficient strength and stiffness against the Installationgiven loads and loads effects. Simplicity ● ○ ○ Maintainability ● ○ ○8. DESIGN CHALLENGES Depth Innovative concept of the floating structure by Independence ● ○ ○definition has little or no history of past performance(ISSC 2006). The factors challenge are: (1) incompletely Seabed Condition ●defined structure, (2) new operating environment, (3) Wave Sensitivity ○ ●lack of verified design criteria, (4) new materials withdifferent properties of strength and fatigue, (5) new Maximum Currentstructure with different load control. Load ○ ○ ○ The design has to be reliable at any costs of the Structure Weight ○ ○environment and risk. It must have the ability to operate Structure Motion ● ● ○without failure in order to gain hundred percent benefitsfrom the operation. The design is required to come with Maximum Wave ○systematic analysis of each component and operational in Action Anglehydrodynamics condition. This will give the 7
  8. 8. ICSOT: Developments In Fixed & Floating Offshore Structures, 23 – 24 May 2012, Busan, KoreaKey: ● - Relative Advantage structural analysis (Elastic response analyses, Stress ○ - Relative Disadvantage analyses, collapse analyses, etc) should be carried out in this section to determine various ‘Design Limit states’ Blank - Natural Advantage for the proposed VLCFS. The design limit states during structural analysis for marine structures (i.e. VLFS) Table 3: Design Challenges include the ultimate limit state (ULS), the fatigue limit state (FLS), the serviceability limit state (SLS), and the The complexity of the system to create accurate progressive collapse limit state (PLS). ULS refers to thedesign that will increase the flexibility of the structure for ultimate event in which structural resistance has angreater responses and motions to wave loading. appropriate reserve. PLS refers to the progressive failurePredicting wave loads and dynamics for a stable structure of structures when subjected to accidental or abnormalrequire scientific analytical data that are more subject to load effects (DNV 1997). PLS for accidental load effectswave loading. Additional loads from the growth of is also called the accidental collapse limit state (ALS)seaweed along the planting lines are considered part of (Moan 2004). ULS corresponds to component designthe analysis in accession of the existing loads. Design of verification based on the elastic behaviour of thethe mooring system is also depends on the water depth. structure, and thus can be examined by the hydroelasticThe floating parts of the structure are likely to have response analysis. But PLS needs a progressive collapsecatenary mooring lines that are connected directly to the analysis that considers nonlinear structural behaviorsanchors. This system driven by the length of mooring such as buckling and yielding.lines which are needed to minimize vertical loading of Safety risk and reliability analysis to addressthe anchors. risk over the entire life of the complex system is being Drag embedded horizontally loaded anchors is applied to the design of the system. Qualitative riskassociated with a floating part of the structure. The assessment employed the following tools to identify riskprocess to identify suitable design for anchor consist of in the system. Hazard Identificationdetermining anchor holding capacity, burial depth, and  Checklistdrag distance. Moreover, seabed condition as the soiltype and soil depth also should also be considered. Once  Hazard and Operability Study (HAZOP)the anchor design has been selected, the anchor size is  Quantitative risk assessment and analysischosen to satisfy the required holding capacity. The method.required maximum holding capacity being discussed  Failure Modes and Effect Analysis (FMEA)further in (Headland, 1995)  Fault Tree Analysis (FTA) Extreme waves are important in designing most Quantitative scientific risk employed analysisof the offshore structures especially for structures that related to system failure and consequence related tooperates in the open sea. Generally, the submerge parts number of failure of mooring legs, cost, and causalof the structure can easily avoid extreme waves relative factors emanating from environmental the position at the surface. Compare to the floating Uncertainty analysis will be addressed during simulation,parts expose directly to the strong wind and rough waves. subsystem level analysis using FTA and ET as wellTherefore, a floating structure with high tolerant has to human reliability analysis. In analysing the failurebe described so it can be placed at a broader range of probability, it is important to bear in mind that a mooringsites. device is failed when the mooring reaction force W, due to oscillation of the floating structure, exceeds the yield Each design has a maximum depth that it can strength R. The floating structure drifts when all itsoperate efficiently. Therefore, the ability to install the mooring devices are failed. Failure of a mooring devicestructure over a broad range of depths increases the indicates presence of an event satisfying the followingnumber of sites suitable for that design. This design is condition:more flexible which experiences less force compared tofixed structure and also more economical viable, Pf (T )   dxi drktherefore it can go for greater water depth. Moreover,commonly the floating structure that depends on waterplane area can operate in both sites; shallow and deep m water sites (S. Butterfield, 2005). As this design Pr ob  Z K (t )  0,0  t  T X  xk , RK rk structure aims to operates at the deep water area more  k 1 than 50 m depth. Where: X is natural condition parameters, T duration of9. RISK BASED DESIGN REQUIREMENT the natural condition parameters, and the random variable for the final yield strength of mooring device k,Very large floating structure for fall under new system X and are independent of each other. The probabilitythat needs to be designed based formal safety system of a multi-point mooring system being failed by strongapproach. After modelling is finished, extensive © 2012: The Royal Institution of Naval Architects
  9. 9. ICSOT: Developments In Fixed & Floating Offshore Structures, 23 – 24 May 2012, Busan, Korea (m s 2  fs  k ) Y ( s )  F ( s )wind and waves induced current in specified service life Y (s) 1 TF  G ( s )  is given by the following equations (Figure 7) F ( s ) ms 2  f sk c. System control block diagram Figure 8: System description for deterministic analysisFigure 7: Reliability approach to number of mooring Using the extreme-value distribution of theannual maximum values as the distribution of natural The elements immersed weight, WI is defined BY:condition parameters, the annual reliability is given as: R (T) = 1 - Pf (T) WI = FB −WThe total reliability for years of service life isapproximated by the following equation: WI is positive, the element is positively buoyant (polypropylene subsurface floats), and if it is negative, RN (T) = (1 – Pf (T)) N the element is negatively buoyant (wire rope and10. MOORING FORCE ANALYSIS FROM FIRST shackles) (Randall, 1997).PRINCIPLE The drag Q in each direction acting on each mooringThe system analysis is best model from deterministic element is calculated using:approach and concludes with probabilistic and stochasticapproach. First principle system consider for the design QJ = ½ ΡW CDt AJUUJis shown in Figure 8. The governing equation foroscillation of the floating structure is defined as follows: Where Qj is the drag in [N] on element ‘i’ in water of density rw in the direction ‘j’ (x, y, or z), Uj is the velocity component at the present depth of theF= mooring element which has a drag coefficient CDi appropriate for the shape of the element, with surface F = Fenv(t)+Fmoor area Aj perpendicular to the direction j. At the depth of the element, the drag in all three directions [j=1(x), 2(y)where : displacement vector of horizontal plane and 3(z)] is estimated, including the vertical component,response of the floating structure; : inertia matrix of which in most flows is likely to be very small andthe floating structure; : added mass matrix at the negligible. Figure 9 shows system motion analysis.infinite frequency; : viscous damping coefficientvector; : Memory influence function; : Mooring Once the drag for each mooring element and each interpolated segment of mooring wire and chain havereaction force vector; Fenv : Environmental loading load been calculated, then the tension and the vertical anglesforce, Fmoor: mooring force. necessary to hold that element in place (in the current) can use the equation below to estimate the three [x,y,z] component of each element: Qxi Pf (T )   dxi drk a. Offshore aquaculture System b. System behavior (m s 2  fs  k ) Y ( s )  F ( s ) Figure 9: System motion analysis Y (s) 1 TF  G ( s )   F ( s ) ms 2  f sk 9
  10. 10. ICSOT: Developments In Fixed & Floating Offshore Structures, 23 – 24 May 2012, Busan, KoreaDrag coefficients are a function of Reynolds number, Re,as defined as: Re =UD/νWhere U is the velocity of the flow, D is thecharacteristic buoy dimension, and ν is the fluid Figure 9: Cable Subjected to concentrated Loadkinematic viscosity.By summing the forces at the attachment point, Where,component of the tension is given by Ray,Rby,Rcy,Rdy,Rey = Force acting at point A,B,C,D,E hb,hc,hd = Cable sag at point B,C,D Th = Fd L1,L2,L3,L4,L5 = Horizontal cable length. Tv = Fb −W Since, there are no horizontal loads, horizontal reactions, A and B should be the same. Taking moment about E,Where TH and TV are the horizontal and vertical Ray x (L1+L2+L3+L4) – Rby x (L2+L3+L4) – Rcy x (L3+L4)tensions in the submerged element; the forces, FD and – Rdy x (L4) = 0FB are the drag and buoyant forces respectively, and Wis the weight of the buoy. The resultant tension is Now horizontal reaction H may be evaluated taking moment about point C of all forces left of C. Total tension on the mooring lines is the otherimportant aspects that THV  to TH  considered. 2 2 have be TV Ray X ( L1  L2 )  Hxhc  Rby XL2  0Concentrated loads are spread cross the length of thecable and do not be equal. As they are flexible they do Ray .( L1  L2 )  Rby .L2not resist shear force and bending moment. It is subjected Hto axial tension only and it is always acting tangential to hcthe cable at any point along the length. If the weight ofthe cable is negligible as compared with the externally To determine the tension in the cable in the segment AB,applied loads then its self weight is neglected in the consider the equilibrium of joint Aanalysis. In the present analysis self weight is notconsidered (Figure 10).  Fx  0  T ab cos  ab  H H / L1 Tab  2 L1  hb 2 Considering equilibrium of joint B, C and D, one could calculate tension in different segments of the cable. The Total Length of the cable S is given then the required Figure 10: Cable analysis equation: Consider a cable ABCDE as loaded in Figure 9Let assume that the cable lengths, L1, L2, L3, L4 and sagat B, C, D (hb, hc, hd). The seven reaction components ateach point, cable tensions in each of the four segmentsand three sag values are to be determined. From the Alternatively, application of catenary curvegeometry, one could write two force equilibrium describes the shape the displacement cable takes whenequations ( ) at each of the point A, subjected to a uniform force such as gravity. TheB, C, D, and E. equation obtained by Leibniz and Bernoulli in 1691 in response to a challenge by Bernoulli and Jacob involve examining a very small part of a cable and all forces acting on it (Figure 11). © 2012: The Royal Institution of Naval Architects
  11. 11. ICSOT: Developments In Fixed & Floating Offshore Structures, 23 – 24 May 2012, Busan, Korea Cable sag (h) is value of cable form equation for point I/2, where l is the straight line distance between the position transducer and the application 1 h  y(1 / 2)  a(cosh( )  1) 2a The cable length is the length of the catenary curve from point -I/2 to point. Figure 11: Application of Catenary Curve (Forces Acting at Section 1-2) x S  S ( x)1 // 2  a sinh( )1 / 2 1 2 The sag (h) of the cable gets under the action of a 1/ 2gravitational force. The two points on the cable: points 1and 2 are further examined. The distance between point 1 1  2a sinh( )and 2 are considered so small, that cable segment 1-2 is 10. ENVIRONMENTAL LOADING AND STATION 2alinear. Dx and d will be the projections of section 1-2 KEEPINGlength to X and Y axes respectively. Mooring loads can be computed by means of A tightening force is acting at every point of static or dynamics analysis. Static analysis is appropriatecable. It is directed at a tangent to cable curve and when dynamic motions are not expected, dynamicdepends only on the coordinates of cable point. The analysis. Many successful mooring designs have beentightening force at point 1 be N and that at point 2 designed without a dynamic analysis. Dynamic load ISbe N+dN, where dN is a small addition due to difference likely when mooring is exposed to ocean wave attack,of coordinates. P is the weight of cable section 1-2. large winds, rapids wind shift, large currents and current-Weight is directed downwards, parallel to Y axis. induced eddies or macro vortices.Let α be the angle between the X axis and cable section1-2. For cable section 1-2 to be at rest and equilibrium Environmental loading additional loads onwith the rest of cable, forces acting on this section need moored floating structures are considered. Static loadsto balance each other. The sum of these forces needs to due to wind and current are separated into longitudinalequal to zero. load, lateral load, and yaw moment. Flow mechanismsProjections of sum of all forces acting at section 1-2 to X which influence these loads include friction drag, formand Y axes, give us the value for cable weight P. drag, circulation forces, and proximity effects. For the current system, wind induced and current induced will have moiré effect on the system. Fyc = ½ ρw Vc2 L T Cyc Sin (θc) Where: NX and NY are projections of tighting Where: Fyc = lateral current load, ρw = mass density offorce N to X an Y axes correspondingly. The ratio of water,Vc = current velocity, L = structure length, T =tightening force projections (N) represents the slope ratio draft, Cyc = lateral current-force drag coefficient, θc =of the force N. At the same time, cable weight P is cable current angleweight per unit length (q) multiplied by differential of arc(dS). Lateral current load is determined using the following P  qds equation:The first derivative of projecting of tightening force to Yaxis is the differential of arc. dN Y P ds dy  q  q 1  ( )2 Where: = lateral current load, in pounds, =mass dX dx dx dx density of water ,Vc= current velocity, in feet per second, Ac= projected area exposed to current, = lateral x  c1 y  a cosh( )  c2 current-force drag coefficient, c=current angle a C1 and C2 are coefficients that are defined by point of The lateral current-force drag coefficient is given by: origin in concerned system. We assume this point to be the lowest point of cable, then C1 = 0 and C2 = 1. 11
  12. 12. ICSOT: Developments In Fixed & Floating Offshore Structures, 23 – 24 May 2012, Busan, Korea = lateral current-force drag coefficient, = ratio of eccentricity of lateral current load = limiting value of lateral current-force drag measured along the longitudinal axis of the floatingcoefficient for large values of , = Limiting structure from amidships to waterline length = eccentricity of , =waterline length, in feetvalue of lateral current-force drag coefficient for = 1,e = 2.718, k = coefficient, T = draft, in feet For station keeping each mooring element has a time static vector force balance (in the x, y, and zLongitudinal current load is determined from directions), and that between time dependent solutions the mooring has time to adjust. The forces acting in the vertical direction are: (1) buoyancy (mass [kg]´g [acceleration due to gravity]) positive upwards (i.e. floatation), negative downwards (i.e. an anchor), (2) = Total longitudinal current load, = tension from above Newton], (3) tension from below,Longitudinal current load due to form drag = and (4) drag from any vertical current.Longitudinal current load due to skin friction drag,Form drag is given by the following equation: Figure 11: Cable analysisWhere: =longitudinal current load due to formdrag, =mass density of water = 2 slugs per cubic footfor sea water, Vc= Average current velocity, in feet persecond, B=Beam, in feet, T=Draft, in feet, =longitudinal current form-drag coefficient = 0.1 =current angle a. System to system interaction Position Before LoadingSkin friction drag is given by the following equation: Direction W ind Buoy wc Dir ec Current tio n Equilibrium Position X (After Loading) XWhere, = longitudinal current load due to skin C.G. Before Loadingfriction, = mass density of water = 2 slugs per cubic Y MxyT FyT Y Xfoot for sea water, Vc= Average current velocity, in feet Equiblirium C.G. Y (After Loading)per second, S=wetted surface area, in square feet, =longitudinal skin-friction coefficient Mooring Line = Reynolds number = VcLWL cos = kinematic viscosity of water (1.4 x 10-5 square feet FxT = Total Force Along X-Axis FyT = Total Force Along Y-Axis MxyT = Total Yaw Moment about Center of Gravity (C.G.)per second), = current angle X = Surge Displacement Y = Sway Displacement = Yaw RotationCurrent yaw moment is from: b. Whole system Figure 12: Multibody system = current yaw moment, in foot-pounds In each horizontal direction, the balances of =lateral current load, in pounds forces are: (1) angled tension from above, (2) angled tension from below, and (3) drag from the horizontal © 2012: The Royal Institution of Naval Architects
  13. 13. ICSOT: Developments In Fixed & Floating Offshore Structures, 23 – 24 May 2012, Busan, Koreavelocity. Buoyancy is determined by the mass and size cross sectional area and drag coefficient of thedisplacement of the device and is assumed to be a components. Drag force increase due to the increasing ofconstant (no compression effects and a constant sea current speed.water density). Other challenges in this design are systemmultibody analysis in respect to analysis and keeping the From Graph 17, it is important to note that goodwhole system together and system to system interaction current historical data for prediction of current direction(Figure 11). is useful for deployment of the system and mitigate possible drag; the maximum drag force for ultimate state11. PRELIMINARY RESULT AND DISCUSSION limit is for current 0-10 degree, while accident limit state design is expected to target current direction of about 90 Figure shows that there is potential for very degree.large drag force on the seaweed compare to mooringcomponents. The drag is minimal at current speedbetween 1-1,5, however at current speed2-3 m/s, theremuch more drag, the drag at speed above 3 is intolerablefor the mooring components and the seaweed. Figure 14: Drag Components Vs Current Direction at 1m/s Figure 13: Drag Force Vs Current Speed The growth rate represent dynamic situation for A total drag force estimation equally revealed the offshore seaweed plantation. Drag force as directthat there is potential spike for drag force at current relation with growth of the seaweed, and maximum dragspeed above 2m/s force that can leads to removal of the seaweed at full growth is 130 N. Figure 17: Drag Force with 5% Seaweed Growth Figure 15: Total Drag Force Vs Current Speed Figure 18 -20 shows some of the result obtained from model test. Figure 18 depict current speed and All the components on the floating structure acceleration based on Morison model. The testcontribute to the amount of drag force. This graph shows acceleration is maintained at less than 0.05the comparison of each component that will contribute tothe total force experience by the structure. Seaweed as 0.8 1.8the main contributor of the drag force followed byplanting line, frame rope, intermediate buoy and corner 0.6 1.6buoy. The main factors which contribute to this are the 1.4 0.4 1.2 0.2 13 1 m/s2 0 m/s 0 500 1000 1500 2000 2500 3000 3500 0.8 -0.2 0.6
  14. 14. ICSOT: Developments In Fixed & Floating Offshore Structures, 23 – 24 May 2012, Busan, Korea very large floating structure is challenges, with requirement that meet the operating conditions, strength and serviceability requirements, safety requirements, durability, visually pleasing to the environment and cost- effective. An appropriate design service life is prescribed depending on the importance of the structure and the return period of natural loads. Naval architecture of very large floating structure will grow for sustainable exploration of ocean resources. The novel design presented in this paper for the case f floating structure for offshore aquaculture structure, describe some of the generic challenges. Integrated approach that hybrid of intuitive deterministic, risk and reliability approach,Figure 18 shows strain condition at varied velocity,it simulation, experimental as well stochastic methods is aobserved for load balanced load cell of 50N, the system best approach for reliability assurance of the structure.get overloaded at 0.9. BIBLIOGRAPHY Strain Vs Current Velocity 80 Chakrabarti, S. (1987). Hydrodynamics of 70 Offshore Structures. Plainfield, Illinois: WIT Press. 60 Chakrabarti, S. (1998). Physical Model Testing 50 of Floating Offshore Structures. Dynamic Positioning Strain (N) 40 Conference. 30 Headland, J. (1995). Offshore Moorings. In G. 20 P. Tsinker, Marine Structures Engineering (pp. 311-365). 10 Chapman & Hall, International Thomson Publishing. 0 Che, Xiling (1993) -Techniques for hydroelastic 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Current Velocity (m/s) analysis of very large floating structures, (PhD thesis), University of Hawaii, 1993.Fig19 a-d shows drag and force relations. Ocean current Fujikubo,M., Nishimoto,M., and Tanabe, J.drag and Strain, system overloaded loaded current speed (2005) “Collapse analysis of very large floating structureof 1. Drag and drag force, drag force tend to increase at in waves using idealized structural unit method,”higher current speed Proceedings of the 18th Ocean Engineering Symposium, The Society of Naval Architects of Japan, CD-ROM, Tokyo, Japan. Kashiwagi, M. (2000a) “Research on hydroelastic responses of VLFS: Recent progress and future work,” International Journal of Offshore and Polar Engineering, 10, pp. 81–90. ISSC2006. (2006). ISSC Commitee VI.2 "Very large Floating Structures". 16th International Ship & Offshore Structures Congress 2, (pp. 391-442). Southampton, UK. Inoue, K. (2003) “Stress analysis of detailed structures of Mega-Float in irregular waves using entire and local structural models,” Proceedings of the 4th International Workshop on Very Large Floating Structures, Tokyo, Japan,219–228. K.C.Low (2006). Public Weather Services Workshop on Warning of Real-Time Hazards by Using Nowcasting Technology. Application of NowcastingFigure 17: Drag Force with 5% Seaweed Growth Techniques Towards Strengthening National Warning Capabilities on Hydrometeorological and LandslidesCONCLUSION Hazards . Ohmatsu, S. (2005) “Overview: Research on wave loading and responses of VLFS,”Marine The presented the deterministic approach to Structures, 18, pp. 149–168.structural design of very large floating structure for Newman, J.N. (2005) “Efficient hydrodynamicoffshore seaweed farming. Towing tank test analysis of very large floating structures,” Marineenvironmental loading coefficient will be used to check Structures, 18, pp. 169–180.the model from the mathematical model. The design of © 2012: The Royal Institution of Naval Architects
  15. 15. ICSOT: Developments In Fixed & Floating Offshore Structures, 23 – 24 May 2012, Busan, Korea Recommended Procedures and Guidelines.(2005). International Towing Tank Conference (ITTC).Testing and Exploration Methods Loads and Responses,Seakeeping Experiment 7.5-0207-02.1. S. Butterfield, W. M. (2005). EngineeringChallenges for Floating Offshore Wind Turbines.National Renewable Energy Laboratory. NREL.Copenhagen, Denmark. Sade, A. (2006). "Seaweed Industri in Sabah,East Malaysia". In A. T. Phang Siew-Moi, Advances InSeaweed Cultivation And Utilization In Asia (pp. 41-52).Kota Kinabalu, Sabah: University of Malaya MaritimeResearch Centre. Suzuki and Yoshida K (1996).. Design flow andstrategy for safety of VLFS, Proceeding of Int Workshopon Very Large Floating Structures, VLSF’96, Hayama,Japan, 21-27, 1996. Salama MM, Storhaug T, Spencer B. Recentdevelopments of composites in the oil/gas industry.SAMPE J 2002;38:30–8. Shaw J, Walsh T, Lundberg C, Reynolds H.Field experience in the application of spoolable carbonfiber pipe. In: Proceedings of3rd international conferenceon composite materials for offshore operations; 2000. 15