August 2014 Mission Statement Background
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- 2. Literature Review
- 2.1 Mooring Systems
- 2.1.1 Uplift-Resisting Anchors
- 2.1.2 Self-Embedding Anchor Concepts
- 2.1.3 Suction Caisson Anchors
Table of Contents2. Literature Review 8 2.1 Mooring Systems 8 2.1.1 Uplift-Resisting Anchors 9 2.1.2 Self-Embedding Anchor Concepts 10 2.1.3 Suction Caisson Anchors 14 2.2 Fluidised Beds 15 2.3 Sand Anchor Concept 17 2.3.1 Reasons for Developed Design 17 2.3.2 The Tripod 18 2.3.3 Forces on Tripod and Forces According to Scaling 20 3. Design Methodology 20 3.1 Designing the Experiment Layout 21 3.1.1 Experiment Diagram 21 3.1.2 Logistics of Assembling Certain Parts 23
3.3.1 Choice of Sand: Fluidising Properties, Availability and Health Risks 27 3.3.1 Investigational Experiments with Sharp Sand 29 3.3.2 Void Fraction Comparison 30 3.3.3 Sand Conclusions 31
3.6.1 Machining Parts 34 3.6.2 Alterations on the Model 37
4.2.1 Change of Grip during a One Hour Period 47 4.2.2 Change of Flow Rate during a One Hour Period 48 4.2.3 Further Testing 50 5. Discussion 54 5.1 Comments on the Experiments 54 5.2 Further Calculations from Experiment Results 55 5.2.1 Size Related Force 55 5.2.2 Creating Boundary Conditions for the Real-Life Model 56 5.2.3 Stokes Law 57 5.3 Error Analysis and Recommendations for Further Work 58 6. Conclusions 61 Acknowledgements 62 Bibliography 62 Appendices 65 Appendix A 66 Appendix B 67 Appendix C 70 Appendix D 72 Appendix E 73 Appendix F 74 Appendix G 76 Appendix H 77 Appendix I 80 Appendix J 81 Appendix K 82 Appendix L 83 Appendix M 84 The world depends on water, yet fresh water scarcity is already affecting every continent making it an increasing problem for many societies. It is crucial for healthy ecosystems and socio-economic development. By 2025 it has been estimated that 1.8 billion people will be in areas of absolute water scarcity and approximately two thirds of the world’s population could experience water stress according to UN’s World Water Day report [Uit07]2. Considering the world’s population is expected to reach 8.9 billion by 2050, water scarcity will become a harder problem to solve, where it is already unevenly divided leaving economically weaker countries suffering [Uni12]3. In California, USA, there has been a noticeable decline of water levels in ground water, especially since 2010. This has led to more water wells being drilled both during dry and wet periods to maintain water supplies [Bro14]4. With the increasing need of water for various purposes and evidently decreasing fresh water supply, providing water is becoming an economic and practical issue. Desalination of seawater could solve fresh water scarcity. Instead of having desalination plants driven by electrical power, S. Salter designed a Wave Energy Converter (WEC) in the 1970s that can directly use the wave energy for vapour-compression desalination process inside the device [Sal11]5. This creates a more sustainable desalination process, since source of energy is an important factor in today’s society, where energy demand is high and environmental impact must be considered. However, a common problem that WECs have to face is mooring of the device, since permanent foundations in seabed may reduce chance of project approval. This creates a difficult situation if there are not any rocks, cliffs or other seabed features that the WEC may be attached to instead. An anchor has therefore been designed, which could provide the solution for a sandy seabed. In short, it has been developed to float when filled with air and sink as water enters into it. When it approaches the seabed, it uses water to fluidise the bed to easily embed itself and finally, pull a partial vacuum to force the sand particles towards it due to pressure difference. This creates a strong grip. This project is aiming to test a simpler model of the design, to find if it is suitable to bring forward to next design stage. 2. Literature ReviewCreating a self-embedding and self-removing sand anchor for WECs is a new concept that lacks previous experience. Relevant research was therefore required to gain sufficient amount of knowledge needed to construct a successful initial model. This model would then lead to insight of the idea and contribute significantly to the final design. The requirements of the sand anchor are that it will be able to fluidise the seabed appropriately for both embedment and removing, create a strong grip during vacuum and finally be able to float when filled with air. 2.1 Mooring SystemsAnchoring techniques vary in a wide range of sizes and designs depending on their desired performance. The traditional drag-based anchor increases the drag the further it is moved along the seabed. Such anchor would not be practical for a WEC since it is not secured enough. It needs to be embedded firmly but temporarily. There are unique designs that may be learnt from, both old uplift-resisting concepts and self-embedding inventions. 2.1.1 Uplift-Resisting AnchorsUplift-resisting anchors were partly developed due to the increase of operations and constructions located at deeper waters, where there was a demand for a more advanced anchoring system. The requirements of these anchors were to have characteristics such as ‘highly efficient, reliable, and light weight’ as R. J. Taylor (1975) states, where major advantages compared with the drag anchor was that they used significantly less scopes of line and accessories [Tay75]6. The scope for an anchor is the ratio of length of anchor rode to the water depth, where rode is the anchor line [Pom14]7. It should be stressed that amount of line scope makes a great impact on costs in deeper waters; consequently a design using less rode is favoured. There are many categories for uplift–resisting anchors where a few will briefly be discussed[Tay75]6. One concept is assembled by anchor-projectile (including a piston and fluke), gun and reaction vessel working together to be propelled into the seabed at a high velocity. It can usually just be recovered at water depths less than 100 m, which is the main disadvantage. However, it is a compact and light weighted assembly making it easy to handle [Tay75]6. Vibrated direct-embedment anchors are slender designs. They embed themselves into the seabed through vibration. They can penetrate layered seafloors and have reasonable holding capacity, but they are difficult to handle and the seabed may not slope more than 10° [Tay75]6. The driven anchor is continually forced into the seabed through impulsive forces, where its specific design may vary. It has advantages in a sandy seabed and its movement is negligible compared to the maximum capacity reached. However, it is limited to roughly 300 m depth due to the surface hammers used to cause the impulsive forces and they require surface support [Tay75]6. Finally, deadweight anchors are ‘dense, heavy and resistant to deterioration in water’ as stated in [Tay75]6. These are simple designs that may alter depending on their specific operation at sea. They are economical, can be used for a wide range of seabeds and have a predictable uplift-resistance. However, they are not practical for anchoring anything beyond a few hundred kg [Tay75]6. Majority of these anchors are more ‘explosive’ and permanent. They are not temporarily lifted out from seabed easily, because they are designed to automatically resist it. It should however be noted that their characteristics such as efficient and reliable are qualities that are desired by this project’s sand anchor. Like the deadweight anchor, the tripod should use its own weight to sink into the seabed, but it also needs to be removed. 2.1.2 Self-Embedding Anchor ConceptsIt is known that self-embedding sand anchors have been developed historically. Although this is only one of the three requirements of this project’s anchor, a great deal may be learnt from these simpler designs. C. H. Howland speaks of a self-embedding anchoring system for his invention ‘temporary floating breakwater and causeway with simulated beach and kelp’ from 2011; which protects areas in remote locations in various actions such as remove oil from water during an oil spill [How11]8. He emphasizes that anchors should hold a mooring capacity of approximately fifteen times its weight according to navy ratings and that self-embedding as one of its features is not new engineering. The patent also mentions that jetted or screw anchor designs may result in reduced mass required and using a vacuum creates a high strength lightweight system [How11]8. Patent in [Tan77]9 by D. L. Tanner reviews an invention for a ‘mobile anchor and a method for embedding same’ from 1977. This hollow anchor has locking arms that are pivoted outwards when desired. The anchor may embed itself easily when arms are locked in by letting pressurised water enter in one end of the tube and jetting out at the other. D. L. Tanner discusses the displacement of the sand grains is what enables the embedment, which relates to this project’s fluidising feature [Tan77]9. See Figure 1 for a clearer view of the design. 
Figure 1 Schematic of the mobile anchor developed by Tanner, D L [Tan77]9 Furthermore, patent [Rho73]10 from 1973 consisted of an idea where a heavy block would be positioned on the seabed with a blade attached that could be pivoted along with an actuator. P. Rhodes, the inventor, designed it such that the actuator would enable the block to be swung to engage with the seabed and thereby bury itself deeper into the sand. Such motion meant that the model did not have to be dragged along the seabed to be embedded unlike most common anchors [Rho73]10, but instead influences the sand like this project’s anchor to sink in. Finally, the RoboClam, published April 2014, is one of the newest inventions that have shown impressive results found in [Win14]11. A. G. Winter discusses the inspiration of the design was the Atlantic razor clam which manipulates the seabed to burrow itself down as deep as 700 mm using approximately 10 N force. The soil manipulation reduces the energy required and burrowing drag to such extent that without it, the clam would only dig a depth up to 20 mm [Win14]11. In [Win14]11, the clam’s actions were studied to be imitated by the RoboClam, where test results showed the amount of energy required to locally fluidise the seabed was higher than pushing the design downwards. However, energy was more constant, and the higher energy was assumed to be due to tests carried out at shallow water where energy required opening and closing the valves at the end effector of the design would be larger than when in deeper water. The design was initially developed to burrow deep into the seabed to anchor miniature submarines, but may potentially have more uses [Win14]11. The comparison of digging cycles between the Atlantic Razor Clam and the RoboClam are shown in Figure 2 and Figure 3, where it is evident how the behaviour of the clam has been used as inspiration. Figure 3 illustrates how the RoboClam pumps out water to fluidise the seabed and dig itself as deep as the fluidised sand (light grey area) will allow. 
Figure 2 Atlantic Razor Clam digging cycle kinematics in [Win14]11 
Figure 3 RoboClam digging cycle kinematics from [Win14]11 2.1.3 Suction Caisson AnchorsThis anchoring system has increased its popularity in the oil and gas industry for deep-water operations due to the costs and challenges in installation equipment for driven piles that have previously been used. Suction caisson is simple, reliable and provides better control during installation as B. Sukumaran states in [Suk14]12. The anchoring system is first penetrated into the seabed through its own weight. This initial embedment is substantial enough to create a satisfactory seal to start the suction process. A submersible pump, located at the top of the sealed caisson, pumps water out from the inside of the caisson, creating a strong suction force due to a created pressure differential. This has clear similarities with the project’s anchor, proving that a strong grip can be achieved by creating a large enough pressure difference. The installation sequence is illustrated in Figure 4. 
Figure 4 Installation sequence of suction caisson from [Suk14]12 Investigating existing mooring systems showed that the concept of localised fluidisation to create quicksand-type behaviour has been used before to cause an anchor to sink easily using its weight. However, the tripod requires to be embedded for a few months at a time, to afterwards be removed unlike some uplift-resisting sand anchors. This complicates procedures, where the anchor legs must implant themselves deep enough to be secured for several months at minimum and additionally pull a partial vacuum successfully to create a strong grip. It therefore differs by combining many of the various features that past concepts have.
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