Open Water Testing
Underwater Soundscape
Purpose: To determine how many more decibels a marine energy device produces compared to pre-existing anthropogenic noise in the device’s deployment area. This is usually a knowledge requirement within the environmental permitting process for most marine energy devices.
Overview: Using acoustic velocity particle sensors, the location and intensity of the noise produced from the marine energy device and its external components can be determined. The sensors are placed within the watch circle of the device. Measurements are taken over the course of a couple days . This allows for other interactions with machinery that may be used in the installation or maintenance cycle of the marine energy device. For example, boats, pile drivers, helicopters, and other machinery’s noise can be documented. The data gathered can be used to determine how much more noise is added to the ambient soundscape as well as the marine energy device’s azimuthal anisotropy .
Instrumentation and Measurements: The NoiseSpotter© created by Integral Consulting is a multi DOF, compact array of acoustic particle velocity sensors that can measure natural and anthropogenic sounds in real time. This instrument has been used in multiple marine energy-specific projects so far such as CalWave’s deployment of their WEC off Scripps Pier in San Diego, California. DAISY can also be used.
Benthic Habitat Characterization
Purpose: Characterize the benthic habitat to use as a baseline to ensure the marine energy device minimizes imppacts to the benthic habitat throughput it's lifetime. Necessary for permitting purposes.
Overview: An area of interest is identified in the seabed or riverbed of where sediment profile imaging (SPI) cameras capture cross-sectional images while penetrating the seabed between 1-20cm below the sediment-water interface. Box core samples of sediment are also taken in various places within the area of interest which are then sieved through 1mm mesh to capture and identify any organisms that may live within the sediment. Plan view cameras take an image above the area that the SPI is taken.
Instrumentation and Measurements: Ocean Imaging System (OIS) Model 3731-D Sediment Profiling System camera for taking images and OIS Model 3831 remote head strobe for taking box core samples.
Mooring Stiffness
Purpose:The goal of this test is to measure the mooring stiffness of the MEC by displacing the buoy and measuring the mooring tension and buoy orientation and position.
The mooring system often has a substantial effect on the seakeeping and response of a MEC. It is often difficult to achieve the exact mooring configuration used in design because of errors in anchor placement, differences in mooring hardware, variations in mooring line length, among many factors. It is therefore useful to measure the “as deployed” mooring stiffness, which is defined as the relationship between a horizontal force applied to the WEC and the corresponding WEC displacement. The mooring stiffness is often asymmetric and will depend on the direction of pull. The mooring stiffness is typically non-linear as well. Measuring the mooring stiffness is important for simulation model validation and for determining the wave conditions needed for resonant device response.
The mooring stiffness test should be performed during calm conditions with and all moving parts locked down. The MEC should be ballasted to its working state. Prior to the test, the mooring configuration needs to be analyzed to determine the minimum combination of pull loads and range of pull loads, as well as the pull directions needed to fully characterize the mooring stiffness. For example, if a mooring system is symmetric across one plane, displacements only need to occur in two quadrants. If a mooring line is used that has a high degree of creep, the mooring stiffness test should be performed periodically for longer duration tests.
Testing
- Before the test starts, the static, unloaded position of the WEC must be known. This location can be measured during times of minimal metocean conditions (no current, wind and waves).
- If the pull vessel is to be anchored, anchor the vessel sufficiently far from the MEC so that the WEC – vessel – anchor line will be in the desired direction of pull. If the anchor winch is to be used to tension the mooring, then it is important to allow enough distance between the anchor and WEC so that all line pull loads can be achieved.
- Attach the pull line at a hardpoint on the MEC, ideally one that aligns with the direction of pull so that the mooring configuration is not distorted from the static configuration and so the device does not rotate. Then perform the stiffness testing as follows:
- If the vessel is at anchor, using either the anchor winch to pull the vessel and tension the pull line or use a winch/capstan or other method to tension the pull line.
- If the vessel is not at anchor, slowly increase the engine RPM until the desired pull line tension is reached.
- Record the position of the WEC, pull vessel, and corresponding line tensions.
- Repeat steps 3 and 4 for each pull line tension increment.
- Repeat steps 1-5 for each direction of pull.
During the testing, record the following data:
- Buoy position via a GPS with data rates of at least 1 Hz with a resolution of less than 1m.
- Vessel position via a GPS with data rates of at least 1 Hz with a resolution of less than 1m.
- Pull in tension via an in-line load cell. The load cell should have a range of at least 3 times the expected maximum line tension and a sample rate of at least 10 Hz.
This data can then be used to develop a tension versus displacement curve to determine the mooring stiffness coefficient.
Mooring Loads Monitoring
The goal of this test is to monitor and characterize the mooring loads of the MEC under normal and extreme metocean conditions, and in all MEC states (e.g. operation, maintenance, and protective).
As mentions above, the mooring system often has a substantial effect on the seakeeping, response, and energy production of a MEC. The mooring system can also impart significant structural loads and impulses during extreme weather events as MECs reach the limit of their travel and mooring stiffness greatly increases. Events such as snap loading can also occur when the mooring line goes slack and rapidly re-tensions. Therefore, monitoring the mooring loads can provide critical design information. For example, because moorings are critical components, they are often built with a large safety factor. Therefore, quantifying the real mooring loads can help reduce costs by potentially reducing these factors and safety factors of the MEC structure.
The mooring systems are also the station keeping component of the design and any failures due to mooring line parting, anchor movement or shackle/chain fatigue can dramatically affect the device performance, cause instabilities and even the loss of the device. Therefore, it is helpful to monitor the health of mooring system to reduce risk or mooring failure and the potential loss of the MEC.
Relevant standards and guidelines:
- IEC/TS 61600-10 Ed. 1.0 en:2015, Marine energy – Wave, tidal and other current converters – Part 10: Assessment of mooring system for marine energy converters (MECs)
- API RP 2I, In-service Inspection of Mooring Hardware for Floating Structures
- API RP 2SK, Design and Analysis of Station-keeping Systems for Floating Structures
- API RP 2SM, Recommended Practice for Design, Manufacture, Installation, and Maintenance of Synthetic Fiber Ropes for Offshore Mooring
- API RP 2T, Recommended Practice for Planning, Designing and Constructing Tension Leg Platforms
- API Spec 2F, Mooring Chain
- ABS, 8 Single Point Moorings
- ABS, 39 Certification of Offshore Mooring Chain
- ABS, 90 Application of Fiber Rope for Offshore Mooring
- BV NI432 Certification of fiber ropes for deep water offshore services
- DNV-OS-E301 Position Mooring
- DNV-OS-E302 Offshore Mooring Chain
- DNV-OS-E303 Offshore Mooring Fiber Ropes
- DNV-OS-E304 Offshore Mooring Steel Wire Ropes
- DNV-RP-E304 Damage Assessment of Fiber Ropes for Offshore Mooring
Monitoring
Each mooring line should have an in-load cell to monitor mooring line loads. If possible, the inclusion of inclinometers in the load cells is helpful to determine the angle of the mooring line. These measurements provide a direct assessment of the mooring line health and provide data to quantify loads during extreme events and identification of adverse conditions, such as snap loads. The mooring loads are simply a special case of the overall loads measurement testing and should follow the same method of bins analysis procedure to characterize the loading as a function of the sea state operating conditions.
- Mooring line tension via an inline load cell that is located near the WEC. Ideally the load cell should include an inclinometer to measure the rise angle of the line. Data rates between 10 and 100 Hz to capture the dynamic loads. A load cell must be selected very carefully to ensure it has the required range, resolution, the needed factor of safety (it should not be the weak link in the mooring), and that it is made of the correct material that is compatible with the rest of the mooring system. The connectors need to be rated for the application with the correct orientation. Most importantly, the connectors and loads cells must be designed to survive being submerged and subject to the motion and loading typically seen in this mooring system. Cheap load cells have a history of failure.
- Buoy position should be measured using a GPS with data rates of at least 1 Hz and a resolution of less than 1m.
- Water depth via a pressure sensor or acoustic altimeter to capture tidal and storm driven changes in water depth which can directly affect the lay of the mooring lines. Burst measurements should be made at least once an hour with the burst of long enough duration to average out wave effects.
- Buoy draft should be measured via a pressure sensor or altimeter located on the body of the WEC. Burst measurements should be made at least once an hour with the burst of long enough duration to average out wave and buoy motion effects.
- Buoy motion should be measured using a 6 DOF Motion sensor or Motion Reference Unit, of INS quality. While the motion measurement is not critical, it is recommended because WEC motion is affect by and affects the mooring line tensions. Data rates should be between 10 and 100 Hz connected with a GPS and compass/inclinometer to eliminate low frequency drift.
