Model Test and Simulation of Mooring for Oceanic Seaweed Farming


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Model Test and Simulation of Mooring for Oceanic Seaweed Farming

  1. 1. International Conference on Marine Technology Kuala Terengganu, Malaysia, 20-22 October 2012 Paper Reference ID: UMT/FMSM/MARTEC/MTP-72 DETERMINATION OF HYDRODYNAMIC LOADS ON SEAWEED FOR USE IN DESIGN OF AQUACULTURE MOORING SYSTEM Amagee A.1*, Sulaiman O.O.2, Bahrain Z.2, A.M. Nurshiffa2 ,Kader A.S.A.3, Adi M.3 , Othman K.4, Pauzi M. A. G 2, Nasruddin I.2 and Ahmad M.F.1 1 Technip, JalanTunRazak, Kuala Lumpur 2 Universiti Malaysia Terengganu, 21030 UMT, Kuala Terengganu, Terengganu, Malaysia; 3 University Technology Malaysia, 21030 UTM, Skudai, Johor, Malaysia 4 Bureau Veritas, Jalan Sulatan Hishamuddin, Kula Lumpur *E-mail: amagee@technip.comABSTRACTSeaweed farming has potential economic benefits in Malaysia as a source of sustainable supplementary income forfishermen in remote areas. Prototype plantations have been deployed in coastal areas and therefore are competing forlimited space with other shoreside activities such as recreation and industry. If seaweed can be cultivated in relativelydeeper waters away from populated areas, production may be expanded using larger areas not presently exploited forother purposes. Prototype systems aquaculture plantations have so far relied on ad hoc, field developed mooringsystems, rather than engineering design principles. Such systems may not be sufficiently reliable for deepwaterdeployments, where repairs are more costly and resources are scarce. Nevertheless, considerable experience and hands-on knowledge has been obtained. So that the basic design concepts can be used as a basis for further improvements.In order to design the mooring system for a typical offshore installation, dynamic and hydrodynamic design loads areneeded. In the case of a jacket, the loading can readily be calculated using Morison’s formulations based on empiricaldata or for a floating structure like a Spar or FPSO existing software like WAMIT and dynamic simulation tools can beused. However, for a deep water aquaculture facility no similar tools are available to calculate the hydrodynamic forceson such flexible structures. It was decided that the quickest way to obtain design loadings for seaweed would be tocarry out model tests on some available seaweed samples. The main difficulty is that the flow kinematics (fluidvelocities/accelerations) that can be produced in the available model basin, at the Marine lab of the Universiti TeknologiMalaysia in Johor Bahru, where the tests are to be carried out, are suitable only at smaller scale (in a range typicallyaround 1/50), whereas the seaweed samples are actually fullscale (1/1) size and mass. So, there is a mismatch that has tobe overcome by suitable simplifying assumptions. This paper describes the model tests and analysis used to develop theloads needed so that the mooring system for the aquaculture system can be designed based on methods like those usedfor offshore structures.keywords: Offshore, Algae, Oceanic, Hydrodynamic, Coefficient1. INTRODUCTION OFFSHORE AQUACULTURE FOR OCEANIC SEAWEED MARINE ALAGE FARMINGThe ocean represent one big frontier for humanity to explore, much of it biological species remain untap. The case ofseaweed or marine algae stands significant because of its potential to that benefit solution to human activities that endwith unwanted effect. For example, the green algae (Chlamydomonas reinhardtii) seaweed is used as biomass,hydrogen fuel source, as it can produce superior amount of vegetable oil and heat compare with terrestrial crop. Marinealgae are used as food supplement that provide the body with additional fuel and immune body system regulatoryresponse. It contains extensive fatty acid profile including Omega 3 and Omega 6 and it contain abundant vitamins,mineral, and trace elements. Chondrus crispus know as carrageen is an excellent stabilizer in milk, gelatin and cosmeticproduct. Algae are used as fertiliser for livestock and soil; Algae are also used for waste water pollution control. Figure1 shows seaweed under test in towing tank. 1
  2. 2. Figure 1 Seaweed under testComponents of the Floating Structure is as followed (Figure 2) Frame Line: This line support the planting line for each block of the structure. It was made from synthetic fibre rope. Planting Line: Planting line is the main part of the structure which contains the planted seaweed. This line must be able to withstand the mature growth of seaweed weight. Separator Line: When the nature hit floating structure, the planting line tends to tangle. Therefore, this line will act as the separator between each planting line. Mooring Line: This line holds the whole structure at the surface to be in place with connection to the anchor. Since mooring system is a crucial part for floating structure. It must be designed to withstand the natural force and achieve stability through the use of mooring line tension. Buoy: Buoy is to provide a convenient means for connection of the floating structure on the surface to the mooring. Through the use of distributed buoyancy for each buoy, floating structures can achieve stability. The buoy also has to withstand the structural weight and additional load from the seaweed. Sinker: A sinker is made of concrete and placed on a catenary mooring line to ensure horizontal mooring at anchor, enhance mooring line energy absorption and affect mooring line pretension in a way that can be useful in controlling structural stability. Anchor Block: Anchor is designed in a large mass of concrete to keeps the floating structure at the place also to resists both horizontal and vertical movement. a. Physical system deployed at sea b. Mooring configuration Figure 2 Floating Structure for Ocean Farming System The study focuses to produce a design for offshore floating structure of seaweed farming. This design isrequired to 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 onthe importance of the structure and the return period of natural loads. Its service life is generally expected to be as longas 50 to 100 years with preferably a low maintenance cost. The structure will be operating 200 meters from the shore asa result; the structure is likely to experience more energizes wave action and stronger wind associated with deep waterregion. This design also considered 1-2 boat lanes within the structure blocks which is about 5 meters wide at theoriginal size. In the structural design of floating offshore structures, the external load and major load effect, such as 2
  3. 3. cross sectional forces, are determined from the rigid body motions. The dimensions of structural members andarrangement are subsequently determined so that the structure has sufficient strength and stiffness against the givenloads and loads effects. The hydrodynamic loads measured will be used to build approximate scale models of theseaweed. The model seaweed will mimic the Froude-scaled properties (mass, dimensions, added mass and damping) ofthe seaweed measured previously. Suitable material such as plastic ribbon, rubber tubing or even young seaweedseedlings will be used to build a sufficiently quantity of scaled seaweed.2. ENVIRONMENTAL LOAD CONSIDERATIONThe weather in Malaysia is mainly influenced by two monsoon regimes, namely, the Southwest Monsoon from lateMay to September, and the Northeast Monsoon from November to March. (K.C.Low, 2006). However the east coast ofpeninsular Malaysia is the area that exposed directly to the strong sea currents and periodic monsoon season which isprevalent off the east coast. Furthermore, with the existence of nature elements of the deep and open waterenvironment, seaweed farming is hard to be applied in this area. Regular waves were considered and generated by the wave maker for a few tests. Random waves spectrumwas based on the Jonswap spectrum ( for less than 1Yr or for 1Yr or greater, respectively.Froude scaling was applied to establish the relationship between full scale wave height (Hp) and period (Tp) and thecorresponding model scale wave height (Hm) and period (Tm), where Hm = Hp/50 and Tm = Tp/50 (Table 1). Incident waves will be measured and analyzed prior to the tests. Two wave probes will be installed forcalibrations: one in front of the carriage at the basin centreline and one to the side of the nominal position of the model.Wave force vector is generally expressed as the sum of linear wave force proportional to wave height and the slowlyvarying drift force proportional to the square of the wave height. Table 1 Full Scale Wave Return Period Full Scale Wave Height Full Scale Wave (Year) (m) Period (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 Mooring design for offshore platforms makes use of software tools which have been benchmarked againstmodel tests, computational data and full scale measurements for their given applications. Hydrodynamic loading on theplatform, risers and mooring system itself due to waves and currents are calculated using a variety of tools such aspotential flow, CFD and empirical data.3. MODEL TESTThe model test is required to determine the hydrodynamic loads due to waves and currents acting on seaweed and itsmooring system components. The total system loads must be suitable for use in designing a seaweed culturemooring system to avoid failure with potential loss of the valuable crop and possibly requiring costly repairs orreplacement of the mooring system components. A towing test conducted at UTM marine Lab involved (Figure 3) one method of research to determine thehydrodynamic loading coefficients (added mass and damping) in a few different configurations [6]. The samples whichare seaweed are dried, but will restore to nearly nominal properties when soaked in water for a period of time. In typicalpractice, the rows of seaweed are held using ropes separated by about 2.6 m between rows. For the drag measurement,the seaweed was attached to towing lines and the lines were towed from the moving carriage. A frame consisting ofaluminium channel sections attached to the towing carriage is used. The seaweed clumps then in turn attached to a ropeline. Tension load cells will be attached between the line and the frame and the measured forces recorded on the modelbasin’s data acquisition system (Figure 3a and b). 3
  4. 4. Frame to attach seaweed Figure 3 UTM Towing Tank and Carriagc Component of the full-scale platform and mooring cable characteristic first need to be clarified.A typicalblockhas an overall length of 100 meter and the breadth is 100 meter. One block of seaweed farming contains four mainropes for the frame, four main buoy and 30 load lines on which is the seaweed will be planted. Table 1shows thestructural properties of one block of seaweed farming (Table 2 and 3). Table 2 structural properties structural properties (one block) No/Quantity unit length overall 100 m breadth 100 m main buoy 4 main rope 4 load line rope 30 sea water density 1025 Kg/m^3 Table 3 Structural dimension Actual Item Model Structure Length Overall for 10 1000m 2m Blocks, L Breath, B 100m 2m Dimension for Each 100m x 100m 2m x 2m Block Mooring Depth, D 50m 2.5m The model test is desiged to investigate the modeling laws required for the system in question to be analyzed.The scaling parameters is very important in designing a model test and a few key areas of consideration in replicating aprototype structure for a physical model test. In order to achieve similitude between the model and the real structure,Froude’s law is introduce as the scaling method. Froude’s law is the most appropriate scaling law for the free andfloating structure tests (Chakrabarti, 1998). The Froude number has a dimension corresponding to the ratio of u2/gD where u is the fluid velocity, g is thegravitational acceleration and D is a characteristic dimension of the structure Figure 4 shows the scale modelconstructed by UMT students. The Froude number Fr is defined as Fr 2 gD/u The subscripts p and m stand for prototype and model respectively and λ is the scale factor. Assuming a modelscale of l and geometric similarity, the Froude model must satisfy the relationship: u 2p / gDp = u 2m / gDm 4
  5. 5. Important variable quantities of importance are derived from the equation and dimensional analysis as follows: Linear lp λ lm Speed up = √λ um Mass mp = λ 3 mm Force Fp =( λ3 / 0.975) *Fm Time tp = √λ tm Stress/Pressure Sp λ Sm Figure 4 Scale model of the physical system The model tests involved: • Measure drag loads of actual seaweed by towing • Dynamic tests to measure added mass and damping using PMM. Originally, it was planned to use the planar motion mechanism (PMM). • Hydrodynamic tests on component tests (Buoy, ropes, float, net,…) considered available from coefficient of mooring components. Industry data can be substituted for component loading during initial design work. • Complete system tests with scaled seaweed of 1/50 scaled model deployed at UMT The complete system tests will be used to confirm the adequacy of the preliminary design. Instrumentation required: 1. Load cells of suitable size and range will be attached to the seaweed lines 2. Wave probes 3. Native carriage speed record 4. Wave flap signal The load cells attached to the mooring spring are water-proof aluminium ring strain gauges that measureaxial tensile loads. The measured voltage outputs from the load cell strain gauges are connected by cables to thebasin’s native Dewetron Data Acquisition System (DAQ)) to be digitally sampled and stored. Software is used toconvert the measured voltage to tension readings. The load cells are appropriately-scaled and calibrated (100Nrange). Other instruments used in the tests are wave probes fixed at specific locations under the carriage andaccelerometers mounted on the model decks. Both are channeled to the DAQ to record measured data output. Avideo camera was positioned at strategic locations on the carriage for model motion recordings.3.1 Tests for Seaweed Hydrodynamic Coefficients Tests is carried out to identify hydrodynamic coefficients of an equivalent Morison model of the seaweedwhich will be suitable for use in typical mooring design and analysis package such as Arienne. Samples (clumps) ofdried seaweed, the dried will restore to nearly nominal properties when soaked in water for a period of time (Figure5). Typical size seaweed clump weigh up to 1.5kg in air, when fully grown. However, the natural buoyancy of theseaweed, make its weight in water almost insignificant. A sample clump of seaweed weight in air 4.1N and thecorresponding weight in water is 0.01N in UTM lab (Figure 2). 5
  6. 6. a. b. Figure 5 Seaweed A sample row of seaweed is attached to a frame and towed from the carriage. To determine thehydrodynamic coefficient, a series of tests including towing in calm water, towing in waves and wave-only tests willbe performed. As mentioned, originally, it was planned to use the PMM to perform forced oscillation tests. Theresulting loads can be analysed in a straightforward manner to determine the relevant coefficients which give similarhydrodynamic loads. However, the PMM system is not functioning at present. So, an alternative plan had to be developed. Themain difficulty is that the available seaweed samples represent full-scale (prototype) clumps, whereas, the modelbasin is equipped to generate wave and current kinematics (wave height <0.4m 0.5s<Period<2.5s, and speed<5 m/s)typical of model scale conditions. Therefore, a mismatch exists between the typical body (clump) dimensions andthe relative kinematics that can be generated. Some approximations and simplifying assumptions are needed toutilize the available facilities to determine the required coefficients. The tests of floating structure in regular and irregular waves will be carried out in the towing tank 120m x 4mx 2.5m of Marine Technology Laboratory UTM. This laboratory is equipped with the hydraulic driven and computercontrolled wave generator which is capable to generate regular and irregular waves over a period range of 0.5 to 2.5seconds. For this structural experiment, a model of 2m x 2m per block with 50 scale ration will be used (Figure 6). Figure 6 Seaweed test in towing tank for environmental sensitivity3.2 Environmental SensitivityFor this research, currents are considered because of the dominant contribution of load compared with wave and wind.In addition, it is expected that drag loads will account for a large portion of the wave-driven loads as well. So, bystudying the drag loads first, we hope to quickly arrive at an approximate model that is adequate for the design of majorsystem components. Static current loads are discussed in detail below[4]. Static loads due to current are separated intolongitudinal load, lateral load [5]. Flow mechanisms which influence these loads include main rope drag, main buoydrag, seaweed drag, and planting lines drag. The general equation used to determine lateral and longitudinal currentload areThus current loading on the system is considered at this stage, after the wave test, Morison equation will beincorporated acordingling. 6
  7. 7. Where: is the total inline force on the object, is the flow acceleration, i.e. the time derivative ofthe flow velocity the inertia force , is the sum of the Froude–Krylov force andthe hydrodynamic mass force the drag force , is theinertia coefficient, and the added mass coefficient, A is a reference area, e.g. the cross-sectional area of the bodyperpendicular to the flow direction, V is volume of the body. For instance for a circular cylinder of diameter D inoscillatory flow, the reference area per unit cylinder length is and the cylinder volume per unit cylinderlength is . As a result, is the total force per unit cylinder length. The solution chosen at present is to focus on certain key non-dimensional values and try to use the usedifferent scaling factors to apply the results at fullscale. In this way, the kinematics available in the towing tank canbe used. For example, it is believed that the physical behaviour of seaweed may be similar to that of cylinders inwaves and currents. Therefore, the approach closely follows that of the commentary section of the API RP-2A. For example, the effects on hydrodynamic loading of Keulegan-Carpenter number 2A KC  L Where: A= amplitude of wave partical motion, L= typical length of a seaweed clump Wave current flow reversal effects (r=ratio of current/wave orbital velocities) are expected to be similar tothose for cylinders, though perhaps somewhat more complex and with different regimes for seaweed. Table 1 (a, b)below shows the nondimensional parameters for a series of seastates, typical of the Southeast Asia metoceanclimate. Note, it is not envisaged to design the seaweed mooring for extreme environments such as rare 100-yearevents, 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 theseastates is KC>12, So, relatively large wave velocities are present. Also, the effect of flow reversal can bemaintained, at least in the near surface zone where the seaweed floats, by running the same waves at similar speeds.It is remarkable that the wave kinematics are very similar for all the different seastates. Therefore it is possible togreatly simplify the tests plan by appropriate choice of the scaling factors for each seastates. In fact it is sufficient totests a single wave to produce scaled wave kinematics for all the conditions, at least for the important parameter ofwave velocity at the surface. Then what varies between seastates is actually the current velocity, and resulting ratio r, of wave/currentvelocities. For the present, tests, it is decided to focus on deterministic, regular waves, representing the worst designwave for each seastate. These waves are characterized by Hmax and associated period Tasso. Therefore the tests can becarried out by using the same wave and changing the current velocity for each seastate (Table 4). Table 4a Fullscale Seaweed Parameters. Exceedence Prob/ Return Period 50% 90% 99% 1-year Hs m 0.9 1.8 2.8 5 Hmax m 1.9 3.6 5.4 9.6 Amax m 0.95 1.8 2.7 4.8 Tp s 4.6 6.4 8 10.7 Tasso s 4.3 6 7.4 10 Uc m/s 0.21 0.47 0.78 1.61 Mass g 1500 1500 1500 1500 Length, L cm 50 50 50 50 Amax/L 1.9 3.6 5.4 9.6 Diameter, D cm 1 1 1 1 Amax/D 95 180 270 480 Uw m/s 1.39 1.88 2.29 3.02 U m/s 1.60 2.35 3.07 4.63 Uc/Uw 0.15 0.25 0.34 0.53 r=Uw/Uc 6.61 4.01 2.94 1.87 KC No. = 2A/L 11.9 22.6 33.9 60.3 Re No. = UL/n 799,073 1,177,478 1,536,257 2,312,964 7
  8. 8. Table 4b Proposed Model scale Parameters. Model Scale 6 12 16 30 Exceedence Prob/ Return Period 50% 90% 99% 1-year Hs m 0.15 0.15 0.18 0.17 Hmax m 0.32 0.30 0.34 0.32 Amax m 0.16 0.15 0.17 0.16 Tp s 1.88 1.85 2.00 1.95 Tasso s 1.76 1.73 1.85 1.83 Uc m/s 0.09 0.14 0.20 0.29 Mass g 6.94 0.87 0.37 0.06 Length, L cm 8.00 8.00 8.00 8.00 Amax/L 1.98 1.88 2.11 2.00 Diameter, D cm 1.00 1.00 1.00 1.00 Amax/D 15.8 15.0 16.9 16.0 Uw m/s 0.57 0.54 0.57 0.55 U m/s 0.65 0.68 0.77 0.84 Uc/Uw 0.15 0.25 0.34 0.53 r=Uw/Uc 6.61 4.01 2.94 1.87 KC No. = 2A/L 12.4 11.8 13.3 12.6 Re No. = UL/n 52,195 54,385 61,450 67,566As these tests are currently ongoing, results will be presented as they become available in time for the conference.4. RESULTSThe test was carried out at different low direction, to simulate how the current environment will affect the seaweed.From Figure, it is clear that tow 1 line in transverse direction produce more drag. The maximum test speed at which thesystem get overload is 1.2m/s, at that point, the drag for 1 line is about 1.8, while for two tow line is about 0.2.Outrigger is also observed at the beginning of the speed, and nearly steady drag is observed for both cases at speed of0.5 to 0.8 (Figure 7). Tow Transversely 12 10 8 Tow 1 Trans Cd 6 Tow 2 Trans Power (Tow 1 Trans) Power (Tow 2 Trans) 4 2 y = 1.7693x-0.624 y = 0.3436x-0.624 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Speed Figure 7 Cd for drag in transverse direction Towing at diagonal shows similar trend withy much lower drag on the first line, however it is observed that thecurrent speed get the chance to impact internal lines that supposed to be protected by shielding effect. This revealed thatthe diagonal current direction can tend to impose more added mass on the system. Steady drag is observe between 0.5-1m/s (Figure 8). 8
  9. 9. Tow Diagonally 12 10 8 Tow 1 Diagonally Cd 6 Tow 2 Diagonally Power (Tow 1 Diagonally) y = 0.5368x-1.152 Power (Tow 2 Diagonally) 4 2 y = 0.6273x-0.727 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Speed Figure 8 Towing in Longitudinal direction Towing in the longitudinal direction revealed much less drag of about 0.3 at speed of 1m/s. Almost steadydrag is observed between 05-0.8 (Figure 9). Tow Longitudinally 1.2 1 0.8 Average Cd 0.6 Ave Speed y = 0.298x-0.496 Power (Ave Speed) 0.4 0.2 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Average Speed Figure 9 Figure 10 and 11 shows that strain increases with the increasing of current speed. The graph indicates that thesecond planting lines have a lower value compares to the first lines. This is due to the shielding affecting which theturbulent current acting on second line resulting the lower drag. 9
  10. 10. Figure 10 Drag for towing two lines in transverse direction Figure 11 Drag for towing two lines in diagonal direction5. CONCLUSIONAquaculture farming is widely being practice onshore or near shore, due to uncertainty about he force nature of nature,lack of established design methodology, rules and guideline. The case presented represents real life solution research tosolve problem facing aquaculture industry offshore. Extreme current speed is considered for the test. The resultobtained represent meaningful information for the design and simulation towards reliable deployment of very largefloating oceanic structure for seaweed farming and other aquaculture farming.REFERENCE[1]. American Petroleum Institute (1997). Recommended Practice for Design and Analysis of Station Keeping Systems for Floating Structures, Recommended Practice 2SK, 2nd Edition,[2]. Det Norske Veritas (2010). Offshore Standard for Position Mooring, Offshore guide,[3]. Buck, B. H. and Buchholz, C. M. (2004). The offshore-ring: A new system design for the open ocean aquaculture of macroalgae. Journal of Applied Phycology, 16, 355-368.[4]. North, W. J. (1987). Oceanic farming of Macrocystis, the problems and non-problems. In Seaweed Cultivation for Renewable Resources (ed. Benson, K. T. B. a. P. H.), pp. 3967.Elsevier, Amsterdam 10
  11. 11. [5]. Chakrabarti, S. (1987). Hydrodynamics of Offshore Structures. Plainfield, Illinois: WIT Press. Chakrabarti, S. (1998). Physical Model Testing of Floating Offshore Structures. Dynamic Positioning Coference.[6]. Huse, E. (1996). Workshop on Model Testing of Deep Sea Offshore Structures. ITTC1996, 21st International Towing Tank Conference (pp. 161-174). Trondheim, Norway,: NTNU, Norwegian University of Science and Technology, 1996.[7]. Moan, T. (2004). “Safety of floating offshore structures” Proc. 9th PRADS Conference, Keynote lecture, PRADS Conference, Luebeck-Travemuende, Germany, September 12- 17, 2004.[8]. O. O Sulaiman, et al (2012). Preliminary Design and Prototpe Scale Model of Offshore Aquaculture Floating Structure For Seaweed Ocean Farming. International Conference Ship and Offshore Technology, ICSOT 2012 (pp. 21-34). Busan, Korea: The Royal Institution of Naval Architects. 11