Priliminary design and prototype scale model of offshore acqaculture flating structure for seaweed farming
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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, o.sulaiman@umt.edu.my 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 safety.space, 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
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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
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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. = 2A/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
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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
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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 sk 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 sk 9
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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
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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
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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
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