Introduction

The designer of any tidal stream device must first analyse the forces and moments, which have to be sustained by that device and next assess the effects of the local environment on the materials employed. From this data a suitable design solution must be produced within the overall constraint of economic viability. Tidal energy converters must be designed for ease and speed of installation with a high degree of modularity so that components can be serviced or changed out very quickly. There will be a requirement to repeatedly connect and disconnect flexible, high voltage submarine cables without risk of damage to cables during installation and operation of the device. In this respect, moorings are an important feature of the marine device. The methods of mooring and performing maintenance incorporated in this design can contribute significantly to the overall cost of the project.

Key Challenges

The key challenges for any such design are installability and operability:
Installability - The ability to develop and effectively deploy cost effective moorings and anchorages.
Operability - Understanding biofouling and interaction with the marine environment while keeping the level of maintenance required in operating the system to a minimum.

Both of these challenges have environmental operating conditions as their common denominator. The marine environment is an extremely challenging theatre of operation not least due to the following:
  • Salt water is very corrosive particularly in the splash zone (i.e. at water surface). Corrosion can affect the fatigue life of a component especially in the presence of stray electrical currents created by generation and transmission equipment, which is a distinct possibility in this application.
  • The materials will be subject to fluctuating forces so fatigue endurance is important, ultimately deciding their long-term survival.
  • Marine fouling has the ability to increase the mass of a device by as much as 50 kg/m2 every year thus increasing the structural loadings involved. (Shaw, 1982)

    • Structural Materials

    The two predominant options are concrete and steel, which have both been extensively used in many marine applications. Concrete has demonstrated durability and corrosion resistance and has the advantage that it can be easily formed into complicated shapes. It is also relatively cheap to construct and deploy however, concrete moorings in this context, due to the resistive forces required can become quite massive. This has the disadvantage of increased footprint with resultant environmental considerations. Steel is a well-proven material for use at sea, being widely used in the offshore oil and gas industry. However, steel is expensive, can develop fatigue problems and appropriate corrosion resistance must be provided in the form of epoxy resin paints, cathodic or galvanized protection.