If debris is a threat to economic activity by clogging the trash rack, or to dam safety by impacting your gates or spillway, wouldn’t you want a solution that cost effectively mitigates that problem? If public safety is a concern due to unauthorised access in the tailrace, or distressed boaters in the headpond, wouldn’t you want a solution you can count on? And if you want bidding, procurement and acquisition costs to be lowered through the elimination of consumable expenses, US-based firm Pacific Netting Products says that its PNP MultiFunction Booms could be the solution to your long-term operational needs.

MultiFunction Booms

MultiFunction Booms are a patented modular boom system designed to be used in the head pond or tailrace for debris, ice, demarcation, fish guidance, public safety and security. With up to 1 million pounds of tensile strength and a variety of connection methods between boom sections, these fully customizable boom systems allow operators to specify the size, features, applications, results and performance that best meet their operational and financial objectives.

Designed to provide a 50 year service life, MultiFunction Booms are watertight, corrosion free, rot-resistant, UV-stable and capable of withstanding temperatures to 140 degrees below freezing. These booms offer the capacity for high deformation without fracture, an ideal characteristic for sites where ice, grounding, impact and heavy loads are expected.

Manufactured using custom extruded, colour coated high density polyethylene, their design allows individual boom sections to have lengths, diameters and wall thicknesses that meet site specific needs. Standard colours are orange, yellow or black.

Differing connections

Interconnections between boom sections reduce moving parts and wear items while providing attachments for anchors and boom articulation, says Pacific Netting Products. They accommodate different hydraulic conditions, pool fluctuation, debris passage or public safety needs. The differing connections are:

  • Flange bolted section: When boom sections can be flange bolted together, the entire boom becomes the structural load member. This methodology is ideal for applications where debris exclusion or guidance is necessary. Also for security and safety
  • Center Hole sections: This is an ideal solution for areas of high water velocity where strength and long product life are desired to meet security and demarcation applications. Sections are built with a center hole design using an HDPE welded thru-hull, fitted longitudinally within the boom. An internal steel sleeve with steel chain stopper plates allows open link chain to be used as the tension member.

To enhance safety, efficiency and reduce operations and maintenance, no wear or maintenance connections are located below the waterline. All connections between booms are designed and rated to match boom breaking strengths, except in the case of connections designed to fail at a predetermined load.

By reducing failure-prone hardware connections between independent sections, and eliminating wire or chain connections, MultiFunction Booms are both stronger and dependable, says the company, Because gaps between boom sections are eliminated, debris guidance and exclusion, demarcation, wave attenuation and security are dramatically improved.

Case study

The Lower Saint Anthony Falls (LSAF) Lock & Dam, owned by the US Army Corps of Engineers, is located on the Mississippi River in the heart of Minneapolis, Minnesota. It began operation in September 1956. The facility is home to a 9MW run-of-river hydroelectric project which is owned by SAF hydropower (a joint venture of Brookfield Renewable and Nelson Energy). The project features a control building, substation, transmission line, and ancillary facilities. The turbine-generator assembly is located downstream of an auxiliary lock tainter gate and consists of 16 matrix units in a stacked arrangement. The Matrix units are equipped with integral trash screens. Flow from the turbine-generator assemblies pass through concrete encased draft tubes that have downstream gates to regulate water flow. A 2.2m high Obermeyer crest gate is located on the top of the draft tubes.

The previous debris management system used at the project was ineffective in blocking floating debris from passing into the auxiliary lock and disrupting power generation. This debris was classified as large floating debris including logs, ice blocks or sheets. There was also smaller floating debris including leaves and smaller ice particles as well as large rolling debris such as sunken logs that roll along the bottoms during high flow/high velocity conditions.

Originally the debris management system included a floating boom located upstream of the auxiliary lock for boater safety, an ice breaker located at the entrance to the auxiliary lock chamber to keep larger floating ice blocks from entering the auxiliary lock and break the large blocks into smaller pieces which then pass into the auxiliary lock and the matrix unit trash screens, which mount to the front of the matrix units.

The original floating boom did not prevent debris and ice from becoming trapped on the matrix unit trash screens, pinned under the pressure and velocity of water. With debris accumulation, the screens would become partially blocked and the units would begin to vibrate, which would cause them to shut down. When a unit shut down, the trash would sometimes dislodge from the screen, float to adjacent operating units, and further disrupt power generation by shutting that unit down in domino effect. Positioned in the middle of the Mississippi River, during flood magnitude flows, large debris and ice caused additional impact and stress on the system.

Stanley Consultants was retained to complete a Debris Management Study for Brookfield Renewable in April 2015 and review potential alternatives for debris/ice management. Four alternatives were developed, costed and evaluated. All four alternatives included a series of booms and/or trash racks to block both floating debris and larger submerged debris from entering the auxiliary lock chamber. The substantial cost for materials, construction and engineering of the alternatives encouraged Brookfield to consider other alternatives and it requested Stanley to complete further studies of options for a boom that could be installed and provide effective debris and ice management without the need for a trash rack.

The study provided a review of multiple commercially available and individually fabricated trash boom systems for use in debris management. Design, construction and maintenance considerations were evaluated and a cost for procurement and installation was estimated.

Following extensive review, in June 2018 Brookfield contracted Pacific Netting Products to provide a MultiFunction Boom designed with flange bolted connections between sections and pad eye connections between boom ends at the dam and buoy.

Orientated in a triangular fashion as per the preferred requirements from the client, the upstream portion of the boom would be attached by a chain bridle through an impact absorbing, unsinkable, custom designed, foam filled composite anchor buoy to a caisson type anchor. On the lock side, the boom was built with custom features including a swimmer self-rescue ladder and steel grate safety walkway. The flange bolted connections and pad eye connections prevent typical boom failings. To assist with debris guidance the boom is fitted with a 1.2m deep debris skirt designed to guide ice and debris during normal conditions and fitted with debris overtopping guard to prevent waves and ice from easily overtopping the boom.

The key features of the boom are: 68.6m length overall; 0.9m diameter; 53mm wall thickness; SWL 143kg; boom weight 45kg/foot, weight bar 27kg and skirt weight 4.5kg/foot; nine sections, V shaped design; debris skirt, splashguard and custom swimmer’s safety ladder; designed operational flow for 10ft/sec flow.

Following installation – which was carried out by JF Brennan – Pacific Netting Products adjusted the Boom to reduce roll caused by hydrodynamic forces and to insure the skirt had the optimum perpendicular alignment to the water surface.

Now it is installed, on average the MultiFunction Boom is anticipated to increase the facility’s annual generation by p to 4600MWh, as unplanned shutdowns at the hydropower facility as a result of debris and ice are expected to be eliminated. Firm results for the economic benefits generated by its installation cannot be confirmed yet given that it has been in operation for just one year. However, all indications are that the boom is helping to deliver on its promises of increased generation, especially as the first year after its installation saw challenging ice conditions.