Mooring for LiDAR Buoys

Renewable energy sources are one of the ways to reduce CO₂ emissions. Offshore wind farms are indispensable in the transition from fossil fuels and achieving a climate-neutral energy supply by 2050. Before such wind farms are built, studies are performed on the designated location in order to map the conditions at the specific site. By using the acquired data, predictions about the potential wind energy yield and other meteorological and oceanographic parameters can be made.

For the purpose of acquiring these data, so called LiDAR buoys are deployed in measurement campaigns of several years. The key instrument of these buoys is a LiDAR wind sensor. The LiDAR technology makes use of an upward facing laser to create a profile of the wind speed up to considerable heights, which is of great importance for the predictions of the potential wind energy at the site.

Wind speed is not the only parameter measured using these buoys. These enormous self-contained LiDAR buoys are equipped with a range of different meteorological end oceanographic sensors. Data of these sensors is acquired, processed and sent by various telemetry options to shore, all of which require energy to operate. This energy is supplied by the solar panels surrounding the buoy structure and small windmills which, in combinations with large secondary batteries, add substantially to the size, weight and complexity of the LiDAR buoy.

All in all, a typical LiDAR buoy is an instrument of considerable size loaded with expensive and vulnerable equipment. Keeping it at its designated position, even in extreme offshore conditions, requires adequate mooring. Designing mooring for such a structure is quite a challenge. The mooring needs to keep the buoy at its position, while not hindering the wave measurements of the buoy. Measuring wave direction by means of a motion sensor requires the mooring not influencing the orbital following properties of the buoy, neither in the vertical direction, nor in the horizontal direction. The latter being the real challenge.

A flexible mooring designed to allow the buoy to follow the wave motion is just one of the challenges the mooring has to meet. Surviving offshore storms, preferably without damaging the sensitive equipment on board is another one. The mooring has to take care for a soft stop rather than a sudden stop when the mooring has reached its maximum length, resulting in a peak force on both the mooring line and the LiDAR buoy.

Datawell’s dedicated rubber cords, available in various degrees of hardness and diameters can be fitted with safety-lines of various material each with its specific purpose. For the presented LiDAR buoys a 50 mm diameter rubber cord equipped with a Nylon safety-line has been designed. Dimensions of the rubber cord and the safety-line match the characteristics of the LiDAR buoy. The rubber cord provides the required flexibility of the mooring in normal operation. The well-known elasticity of Nylon takes care of the soft stop of the buoy in extreme conditions.

A well-designed mooring based on the flexibility of a rubber cord allows for a taut mooring. Compared to a chain based mooring dragging over the seabed, a taut mooring has an obvious environmental advantage.
Long lasting deployment of the buoy makes wear between shackles and links in the chain a serious issue in traditional mooring design requiring inspection on regular intervals since this wear is rather unpredictable. An advantage of a mooring based on rubber cords is that this wear is negligible, the fatigue of rubber in a rubber cord-based mooring is relatively predictable.

While the mooring line layout that meets the various tasks poses a challenge, the engineering is no less of a puzzle. Obviously attaching a terminal to a rubber cord that becomes thinner with increases elongation of the rubber cord, up to five times the unstretched length, is a challenge in itself. The terminal design has to take care that fouling, sometimes razor sharp, on the terminal will under no circumstances damages the relatively vulnerable rubber.

Though a variety of rubber cords can be supplied with many options on the safety-line and terminal designs, new applications often require modified designs to meet the requirements of the specific application. Together with the customer, many challenges in the past have been met, each new challenge that is successfully overcome adds to the experience of our specialists, accumulating in expertise which can be used in future mooring applications.