Task Overview

Contacts:

Industry Challenge: There is little information regarding the reliability and extended survivability of turbines operating in the marine environment. Intervention costs for submerged equipment are very high and economics dictate that unplanned interventions be minimized over the operating life. Conversely, capital cost considerations dictate that structures not be over engineered.

Approach: Value engineering, balancing cost against durability, is required for composite turbine blades and critical elements of the support structure. Composite structural analysis will be performed. Consideration will be given to the potential of bio-fouling resistant polymers in the marine environment and low-cost manufacturing methods to produce large composite structures.

Outcomes and Impacts:

  • Research: Results from the composite design for turbines and bio-fouling study
  • Industry: Report describing opportunities for value engineering and fouling resistance
  • Regulatory: Implications of composite polymers in the marine environment

Structural Optimization of Composite Rotors

UW-NNMREC has developed an open source software, Co-Blade, which can be used for the structural analysis and design of composite blades for wind and hydrokinetic turbines. The objective of Co-Blade is to help designers accelerate the preliminary design phase by providing the capabilities to quickly analyze alternative composite layups and to study their effects on composite blade properties, deformations, and material stresses and strains.

Features of the Co-Blade software include:

  • Modeling of realistic composite blades
  • Computation of structural properties
  • Structural and modal analysis
  • Optimization of blade composite layup
  • Graphical post-processing capabilities

To obtain the Co-Blade code and documentation, please visit: https://code.google.com/p/co-blade/.

Co-Blade is able to model a large variety of composite layups and blade topologies.

This example from the Co-Blade structural model visualizes stresses in multiple layers of a composite blade for a hydrokinetic turbine, showing max stress failure criteria in: (a) the E-glass "blade-shell" material covering the exterior top surface of the blade, (b) the E-glass “root build-up” material, which lies directly under the "blade-shell" material, (c) the carbon fiber “spar cap” material, which lies directly under the "blade-root" material, and (d) the E-glass material on the exterior surfaces of the shear webs. Values greater than 1 for the max stress failure criteria indicate that the material has exceeded its maximum allowable stress.

Biofouling and Corrosion

From April 2009 to February 2010, coupons of materials which could be used in the rotor, drive train, or foundation of tidal energy devices were deployed in-situ on the seabed at a prospective tidal energy site to screen for biofouling and corrosion. Materials include glass and carbon fiber composites, stainless steel, aluminum, structural steel, and common steel. Several potential rotary bearing materials were also screened. Coatings, including high copper anti-fouling, low copper anti-fouling, and inert foul-release are also evaluated for their ability to control biological fouling. For smooth surfaces, limited biological fouling was observed on smooth surfaces. Stainless steel shows excellent corrosion resistance, while common and structural steels experience major surface oxidation after three months of exposure to the marine environment – even with sacrificial anodes.

Starfish growing in crevice space

Surface corrosion on structural steel after 3 months immersion with sacrificial anode in Admiralty Inlet

Carbon fiber composite after 3 months immersion in Admiralty Inlet
(no visible fouling)

However, interannual variability is high. During the May-August period in 2010 and 2011, pronounced biofouling has been observed on all uncoated surfaces. For this reason, a second round of static testing was conducted in partnership with Hempel, SA to evaluate the effectiveness of their foul-release coatings. Coated acryllic coupons were deployed, at depth, from February 2011 - November 2011. No biofuling was observed during the deployments, in sharp contrast to uncoated surfaces, as shown in the figure below.

Hempel, SA coatings and uncoated hydrophone pressure case after a 3-month deployment

Composite Aging

When composites are immersed in liquids, diffusion occurs over long time periods at the molecular level. While maximum moisture gains by weight are typically, at most, a few percent, the modification to composite properties can be much higher.

Leveraging Sea Spider deployments in Admiralty Inlet, four types of composites were screened for changes to the shear modulus after 9 months in-situ exposure to the marine environment:

  • Glass Fiber Reinforced Epoxy
  • Glass Fiber Reinforced Vinylester
  • Carbon Fiber Reinforced Epoxy
  • Carbon Fiber PrePreg

While the shear modulus for the prepreg carbon fiber remained stable, in other cases, significant degradation was observed (e.g., GFRE shear modulus reduced by 60%). Further study in this area appears warranted, using a combination of accelerated testing in the laboratory and opportunistic experiments in the field.

Publications: