Aegir Divers Repair Blairgowrie Sea Wall

1 June

Sea Wall Repairs

Southern Sails, Blairgowrie Yacht Club, Issue 140, June 2019
Article by John Coppock

In late 2017, a post-construction inspection was undertaken by Aegir Divers for BYS. It revealed a number of loose and broken bolts in the original section of the sea wall. Given the issues of the attenuator and finger damage during the previous winters, the other infrastructure elements pretty much flew under the radar until the inspection. The wall had been in service for approximately 15 years at this point.

Aegir Divers of Rosebud were engaged to check and repair the fixings. It was determined that we would replace all the bolts and bring the fixings back to the original specification. This required new packing plates, welded nuts and replacement of all bolts and washers.

For the most part the bolts were replaced quite quickly, and the divers worked as a three-man crew on the days that the weather permitted. However, 11 critical locations were found where bolts had broken and could not be removed. This was due to the bolts seizing or the ferrule sockets (wall fixing points) breaking. A couple of locations had two or more bolts (out of four) missing and the wall panels had to be secured with straps temporarily until the sockets could be cleared and new bolts inserted. The onset of winter and the northerly wind season also added to the complexity.

MAW Civil to the Rescue: Claw and Drill

John Plumridge, long-standing BYS member, had his workshop manufacture a custom-made 'claw' to support the wall panels where some serious underwater works were to take place.

The extraction of the broken bolts from the ferrule sockets on the walls required a custom-built drilling rig, also manufactured by the MAW Civil workshop. The concept was to create a three-dimensional drill small enough to fit in the 200mm gap between the piles and the wall section, drill a pilot hole and insert a screw extractor to remove the broken section.

The drill is unique. It is mounted to the pile by a 3D table that allows it to be adjusted to drill in three planes. The drill motor was mounted vertically and hydraulically powered from a power-pack unit on the surface. The remarkable thing was the small distance that the unit had to operate in. The drill bits were 50mm long and were required to drill holes in 30mm diameter bolts.

Most of the bolts were removed by screw extraction after that. A couple, however, had been welded into the socket at construction and had then sheared. These had to be burnt out with an underwater lance system to clear the ferrule hole. The divers videoed this and the clearance is like an underwater explosion in their faces.

Once the holes were cleared, a new stud screw was chemically glued into the ferrule socket.

SOME INTERESTING FACTS:

• Eventually 15 locations required some serious work.

• Each hole took at least one day to drill.

• Three divers were required each day.

• Each day the claw and the drill had to be dismantled and relocated to the next location.

• The claw weighs about 300kg clamped around a pile and supported two of the sea wall panels at a time.

• We broke a number of drills and extraction tools.

• If we had northerlies or westerlies, then we couldn't drill.

Going Forward

We have engaged a metallurgist to advise us about the expected life of the bolts. With this information we can progressively change out the bolts before they fail. We are also applying what we have learned to the through rods on the floating part of the marina.

We have a regular bi-annual underwater re-tension and inspection regime in place and will manufacture a spare lightweight claw that can be deployed quickly if required.

Many thanks must go to the MAW Civil group and Aegir Divers.