Using Screw Anchors with a 5-Line Array
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I was having a discussion recently with @john_lovett in Massachusetts, who has been considering using helical (screw) anchors on his 5-line array system to reduce costs. @azure_cygler I believe you also used screw anchors on your 5-line arrays last season. I wondered if you might be able to comment on the pros and cons of this configuration - working with divers for installation, servicing anchors in the off season, etc. I know @michael_doall has had good luck with this anchoring system in shallow waters on Long Island, but curious if others have found it to be a good long-term solution for a deeper water system. (I believe at low tide John's site is appx 9 feet.)
@lindsay_olsen - Thanks for bringing this up. I am curious to know how other farmers have used helical anchors for securing their gear. I used Danforth anchors for my inaugural season and they proved to be unworthy as the kelp got heavy towards the end of the growing season. I had arrays blow out and massive tangles. So looking for alternatives that are cost effective for setting up 30+ anchors on either end of my arrays.
Here's an image of one of the anchors, that gives you an idea of the force upon them. This one was actually still holding on somehow despite the pretzeled shape!
I'd love to hear how others are using helical anchors and what techniques work for installing from a boat (without diving). Any advice would be appreciated!
John, that's an impressive failure of that Danforth and a nice demonstration of the forces these systems can impose on anchoring gear. Pretension is important in these multi-line systems and not only does it control vertical sag allowing greater spacing between support buoys, but it also controls sideways sag, reducing the risks of neighboring lines getting tangled.
I never been able to justify the installation costs of helical anchors in deep water, so I have no first-hand insight to offer on their user. However, while a vertical installation seems appropriate for an omnidirectional boat mooring, I worry about the implications of large and persistent horizontal loads in one direction on such anchors. It would seem that the shaft would get pulled sideways and either bend or rotate the helical plates reducing their effectiveness.
I’m hoping that mechanized helical anchor installers emerge that can screw them in at specified angles and aimed according to the planed direction of pull.
We just got back from Puerto Rico where we installed a good-sized catenary module using eight 400-pound dual-shank drag embedment anchors. We are real careful to get them aligned with the intended pull as they are being lowered using a slip rope.
@john_lovett - A pretzel indeed! Would you be willing to share what type of array(s) you were using (single, 5-line, catenary, etc), bottom type, whether your lines were oriented parallel or perpendicular to current, and the average and/or peak current velocity on your site?
Hi Cliff! I love your stuff and the aluminum spreaders bars you made for me last year held up amazingly.
I need to rethink my farm layout to consider embedment anchors. What lengths (and setups) are you typically running with your different sized OceanTend anchors?
Kendall, I was using 5 line arrays in 100’ and 150’ lengths. So at full growth I was calculating for between 1250 and 2000lbs of weight on each array. My site has a sandy bottom with current reaching 8kts. I ran the lines parallel to the current, which definitely made tending and maintenance easier.
John, you asked "What lengths (and setups) are you typically running with your different sized Tend Oceananchors?"
Much depends on the situation, including depth, tide rise & fall, bottom, type, etc. Scope is always good if you have the room within your lease. If not you can make up for it with some heavier chain at the anchor and larger end buoys. More scope means more consistent pretension to reduce growline sag at all tides. 3:1 at MHW with some chain seems to work. 4:1 or 5:1 without chain.
Cliff
John, that's an impressive failure of that Danforth and a nice demonstration of the forces these systems can impose on anchoring gear. Pretension is important in these multi-line systems and not only does it control vertical sag allowing greater spacing between support buoys, but it also controls sideways sag, reducing the risks of neighboring lines getting tangled.
I never been able to justify the installation costs of helical anchors in deep water, so I have no first-hand insight to offer on their user. However, while a vertical installation seems appropriate for an omnidirectional boat mooring, I worry about the implications of large and persistent horizontal loads in one direction on such anchors. It would seem that the shaft would get pulled sideways and either bend or rotate the helical plates reducing their effectiveness.
I’m hoping that mechanized helical anchor installers emerge that can screw them in at specified angles and aimed according to the planed direction of pull.
We just got back from Puerto Rico where we installed a good-sized catenary module using eight 400-pound dual-shank drag embedment anchors. We are real careful to get them aligned with the intended pull as they are being lowered using a slip rope.
@john_lovett - A pretzel indeed! Would you be willing to share what type of array(s) you were using (single, 5-line, catenary, etc), bottom type, whether your lines were oriented parallel or perpendicular to current, and the average and/or peak current velocity on your site?
Hi Cliff! I love your stuff and the aluminum spreaders bars you made for me last year held up amazingly.
I need to rethink my farm layout to consider embedment anchors. What lengths (and setups) are you typically running with your different sized OceanTend anchors?
Kendall, I was using 5 line arrays in 100’ and 150’ lengths. So at full growth I was calculating for between 1250 and 2000lbs of weight on each array. My site has a sandy bottom with current reaching 8kts. I ran the lines parallel to the current, which definitely made tending and maintenance easier.
John, you asked "What lengths (and setups) are you typically running with your different sized Tend Oceananchors?"
Much depends on the situation, including depth, tide rise & fall, bottom, type, etc. Scope is always good if you have the room within your lease. If not you can make up for it with some heavier chain at the anchor and larger end buoys. More scope means more consistent pretension to reduce growline sag at all tides. 3:1 at MHW with some chain seems to work. 4:1 or 5:1 without chain.
Cliff
@lindsay_olsen I use helical anchors for my oysters and kelp and swear by them. In my experience, when they are properly installed, there is nothing else that can provide the same reliable holding power in all directions. I have tested this through the past 7 years of farming and have only had one helical anchor pull out. That anchor failed due to ice, very difficult to overcome the force of an ice sheet moving through the farm. Helicals can easily be installed by a diver and don't require a large boat and heavy gear to transport and install.