Recommendations for LPA Mooring Systems and Designs
Casco Marine, LLC
When assessing your farm’s location there are a lot of things to consider; water temp, tidal flushing and currents, nutrient levels, salinity, navigation, water depth, tidal range, bottom type, exposure to weather, etc. Once you find the right location, decide what kind of gear and growing method you want to implement you still have to figure out how to ensure that your mooring system is robust enough to maintain the constant tension of a longline while handling the dynamic forces of storms, currents, and wind, and that it’s individual components are capable of standing up to the marine environment. There are so many different types of moorings and anchors to choose from, how do you know which one is right for your location and gear configuration?
Aquaculture mooring systems are different from boat moorings for several reasons. In most cases sea farms with gear on site, be it on the surface, mid-water column or on the bottom have the gear attached to a longline that is pulled tight between two moorings. The forces applied to the longline are transferred to the mooring in a nearly constant direction with some slight changes depending on the tides and other conditions. Boat moorings are designed to swing with the wind, tide and current meaning that the direction of loading is constantly changing. On a tensioned longline with consistent directional forces applied to the moorings there is a tendency for many types of moorings to “creep” or drag causing the longline to go slack potentially causing fouling, chafe, or other problems.
We are also trying to maintain a taught longline at low tide without having our buoys and gear submerged at high tide. One way to achieve this is by having at least 2:1 scope between your end buoy and mooring. If you use chain for the connection, the weight and the catenary (parabolic arc) in the chain pulls the longline taught even at low water, But make sure your buoys have enough buoyancy to stay afloat at high water while under tension. More buoyancy will be required with a constant-tension longline than with a vertical chain such as with a boat mooring. Yellow, LD-3 (Low Drag) buoys work well in many cases.
Concrete and Granite Blocks will work well especially when you have hard/rocky bottom when other types of moorings and anchors are not able to “bite” or dig into the bottom. Concrete blocks are not overly expensive but are heavy and difficult to move around without the right equipment. Granite is more expensive but has a longer service life. Both will require a marine contractor or additional equipment to install or remove them. They can drag if overloaded or “creep” from the constant directional loading of a tensioned longline, but they will move slowly because of their mass.
Mushroom Anchors are small and mid-weight, they have a lower price point for holding-power when compared to blocks and can be set/moved by yourself from a moderately sized boat, but they need to be properly sized (weighted), dug-in or “set” properly to hold. They work especially well in soft and mud bottom, but are just a weight on hard or rock bottom if they cannot be dug-in properly. If the direction of pull from your surface gear changes too much the anchor can “trip” and disengage from the bottom but will likely catch again when loaded in a new direction.
Danforth or Plough Anchors are relatively light, inexpensive and easy to install but will have to be large enough to hold all of your gear in a winter storm. They work well in sand, shell and mud bottom, but not as well on really hard or rocky bottom if they cannot be dug-in, and in soft mud can drag when there is a large loading from a storm-surge, waves, wind, etc. Like mushrooms they can be “tripped” if the direction of pull changes causing your site to move off-station, and may not catch again if fouled.
Helical Moorings are a large helical plate on a shaft, like a big screw. They come in various diameters and lengths. They are relatively inexpensive with regard to holding power, but must installed by a SCUBA diver or specialized drilling equipment which can increase the total cost of installation if you hire a marine contractor to install them. The “round-shaft” helix can be installed manually in soft bottom and the larger “square-shaft” helix require a specialized hydraulic unit and experienced diver. They perform exceptionally well in soft, sand, shell and mud bottom, but cannot be installed in ledge or very hard/rocky bottom. They will not trip, or drag if sized and installed properly making them ideal for a constantly tensioned longline at the surface. They can also be used to “back-up” a mooring block to keep them from dragging/creeping. Removal is done by unscrewing them from the bottom.
Once you select your mooring type based on your bottom characteristics, surface gear, loading, budget, etc. you need to decide how to connect your surface gear to your moorings. Use appropriately sized chain or rope, shackles and thimbles to bring it all together. In most cases 3/8” galvanized chain and/or ½” rope with eye-splices and galvanized thimbles is adequate for an LPA. You will need to size your shackles to your chain/rope, mooring and your surface buoy. The catenary in chain will help redirect the loading on the surface more parallel to the bottom and reduce shock loading but rope is much cheaper and most types will stretch to absorb shock loading. Make sure you do not use dis-similar metals in your assembly or you will learn about galvanic corrosion quicker than you may think, stick with all galvanized hardware and use anti-seize or “Lanocote” on threads. Don’t forget a zip-tie mousing for cheap added security.
When you’re doing your planning, consider your mooring gear inspection and replacement intervals. Check for chafe and wear on a regular basis to prevent a potentially costly failure, identify your weakest link and and know what to watch for, plan your time and budget to include inspection and replacement intervals of mooring system components. For example ⅜” galvanized chain should last around 5 years in this kind of system where as ½” chain should last about 7 years before wear and wastage warrants replacement. Do some math and decide what makes the most sense for you.
It’s easy for your mooring system to be out-of-sight and out-of-mind, but it is like the tires on your car. It’s what keeps everything where you want it. So take the time to think about all of your options and conditions to figure out what is the right choice for you. If you aren’t sure which direction to go reach out to a marine contractor for a consultation.
Below are Capt. Eric Oransky's recommendations for LPA mooring systems:
Case studies are authored by industry members at the request of The Maine Aquaculturist. Authors are selected based on experience and expertise in a key business aspect of aquaculture. See our About page for more information.
Casco Marine, LLC
The Maine Aquaculturist