The Mooring System at Carradale North

In many ways Storm Ellen failed to live up to her promise. Wind strengths along the inner West coast proved less strong than had been predicted and the only major casualty was the disastrous failure of the mooring system at Carradale North, which resulted in this massive installation of ten large cages coming adrift.

The initial statement, issued by MOWI that day, suggested that the anchors had become detached from the seabed, see here:

‘On August 20, 2020, Mowi’s salmon farm at Carradale North, consisting of 10 circular net pens containing 550,000 salmon (@~4.2kgs), shifted position after its seabed anchors became dislodged during Storm Ellen that has hit the UK and Ireland. The company’s priority at this time is to secure the fish cages in place until Storm Ellen subsides, and to safeguard staff, contractors and fish stock.

‘The company has informed Marine Scotland of the event. A final update will be provided after recovery is complete’.

For the anchors to have become dislodged was perhaps a reasonable deduction in the immediate aftermath of the event, but would have raised questions about the adequacy of the design. Since the arrival of the North Sea oil sector considerable research has gone into the design of anchors and systems for the most testing of conditions. This has resulted in enormous spin-off for other sectors, from shipping to leisure boating. There is no publicly available information about the type or size of the anchors in use at Carradale North, nor any relative specifications or calculations, but for anchors simply to fail would raise considerable doubts about system design.

It turned out that this information was simply wrong, according to the statement issued by MOWI on 26 August. I have omitted the parts irrelevant to the mooring system.

‘Mowi Scotland’s salmon farm at Carradale North, consisting of 10 circular net pens containing 550,700 salmon (@~4.2kgs), shifted its position after becoming detached from its seabed anchors during Storm Ellen and strong tides in Kilbrannan Sound on August 20th, 2020.

….

‘Following thorough inspection by dive teams, the root cause of the incident appears to be breakage of mooring ropes that attach to the main system seabed anchors. The farm was installed five years ago according to the Marine Scotland A Technical Standard for Scottish Finfish Aquaculture, and the infrastructure inspected three months ago. The rope type is marine grade, 4.8cm in diameter with a break strength of 89.5 ton – roughly twice the maximum strength required for its application (ranging from 33t to 51t). The rope has been sent to third-party testing facilities in Aberdeen, with further investigation to follow….’

It would appear from the technical standard referred to in the second statement that in order to instal their ten cages at North Carradale MOWI would have required to commission, in sequence, a full site survey, covering the seabed and including analysis of tidal flow directions, strengths and heights, then a design based on the that data, followed by ordering and installing equipment, consisting of anchors and associated chains and/or ropes.

This release was accompanied by the image below, which is copied with the caption added by MOWI.

                                        

On the assumption that MOWI’s photograph is authentic one is tempted to suggest that the person who tested the system three months earlier should have gone to Specsavers!

As anyone with the slightest experience will confirm, all mooring systems degrade from the moment they are placed in the sea. Traditionally they have almost universally been constructed of galvanised steel, with a heavy anchor or anchors connected to a section of heavy chain to absorb the heave of the waves and lighter gauge riser chain to the surface. The links abrade against each other, removing the galvanising and allowing the metal to rust where sufficient oxygen is present. For this reason seasonal moorings are sunk when out of use.

It seems that until recently the aquaculture industry used very similar systems. Because of the year round use one imagines that inspections and replacements of degraded gear would have been ongoing. This presumably explains why in recent years the industry has moved to the use of synthetic rope systems, as can be seen from the technical standard.

It seems that the most common materials in use are nylon and polypropylene. Each has different properties, but the principal causes of degradation in use are likely to be:

Chemical -

Nylon readily absorbs water, to between 1.5% and 3.5%, polypropylene is much less liable to.

Ultra violet light degradation may have an impact

Anti fouling treatments ditto, depending on the chemicals used

Physical -

Impact from vessels, flowing debris etc is possible, but more likely is the wearing down of fibres as they constantly move against each other inside the strands.

Conclusions

I have asked the Scottish Government under FOI to share what information they are holding and will publish the results in due course.

Given the statement by MOWI that the ropes would have been subject to around half of their designed breaking strain when they actually broke something has gone very seriously wrong. Logically there can only be two explanations, either the ropes supplied have been woefully inadequate and there was nothing wrong with the inspection, or they were fine when supplied, have since degraded and it hasn’t been noticed.