Site-specific investigation and anchor mooring design for a floating OTEC system offshore of Barranquilla, Colombia

Abstract

The main goal of this project is to determine the optimal location for an OTEC installation with a minimum lifespan of 30 years off the coast of Barranquilla and to make an anchor mooring design for the floater on which this installation is located. Bluerise has identified an area near the coast of Barranquilla for which OTEC can be applied. This area is situated within Colombia’s territorial waters (within 12 nautical miles, or 22.2 kilometres), where two locations have been identified by Bluerise: Location 1: 11.2028 latitude, -75.0003 longitude, Location 2: 11.2772 latitude, -74.9208 longitude.

Environmental conditions -
The daily wind direction is NE-ENE. There is no clear extreme wind direction. The daily waves have a dominant Northeast direction while the extreme waves have a dominant Northern direction. The extreme waves are generated far north of Barranquilla by very high wind speeds which explains the relatively high extreme significant wave heights in the area and the relatively low extreme wind speeds. The yearly average (nautical) surface current direction at the two possible floater locations is predominantly south or southeast. The top 20 strongest current speeds in the past few decades have come from the west or southwest however and therefore these are the normative current directions. The environmental conditions are equal for both possible floater locations. A temperature difference of 20_C is reached at warm water intake and cold
water intake depths of 30 and 763 meters, respectively. The depths at which a temperature
difference of 22_C is reached are 36 and 1023 meters (with temperatures of 27 and 5 degrees, respectively). The influence of the Magdalena river and upwelling is concluded to be negligible.

Marine traffic - The two locations with safety zones are located in a traffic-dense area. The area is getting more traffic intense in the upcoming years. However, it will not pose an immediate threat to the operation. As location 2 has slightly less traffic, it would be preferable from a safety point of view.

Seawater intake- and return pipes - Assuming a cold seawater intake temperature of 5C and a warm seawater intake temperature of 27C, the intake pipe lengths become 1023 and 36 meters, respectively. Based on the equation
of state, the mixed water return flow pipe length becomes 130 m. At this depth, the effects of a difference in density between the surrounding seawater and the mixed returned water are minimized. Also, the depth is outside of the euphotic zone which minimizes algae growth. If the intake water is higher than 27 degrees, the discharge temperature will have a higher temperature. Calculations with the Equation of State reveal that the warmer the discharge temperature,
the lower the density of the discharge water is. Whenever the discharge temperature is higher than the output temperatures, less depth is needed in order for the discharge water to be naturally buoyant. As the intake temperature fluctuates throughout the year, it is therefore advised to design the length of the discharge pipe at 120 meters.

Anchor mooring design - The proposed anchor mooring design consists of a spread-moored 4x3 taut mooring system. The lines are composed of three parts: a 50 meter chain connected to the ship, a 1290 meter fibre line part and another 150 meter chain at the end that is connected to the anchor. The floater is positioned in a 58,05 angle with respect to the north in a northeast direction. This ensures comfortable operation during daily conditions and will reduce fatigue build up. The hurricane conditions were found to be governing. The design complies with the basis of design stated in section 5.3 and with the DNV-OS-E301 code and the API Recommended Practice 2SK.