By Will Brindley, Research Engineer – Electrical Infrastructure
I’ve spent most of the last decade thinking about how to prevent mooring failure. But failures will continue to happen. According to data from the UK P&I Club, around 1 in 10 personal injuries in harbours are caused by mooring failures, most of which are from parted ropes and wires. Although less frequent than other marine incidents, parting ropes are extremely hazardous: 1 in 7 of these incidents are reported to result in a fatality.
Ropes or wires under high tension are unavoidable throughout our offshore wind industry; offshore installation vessels, crew transfer vessels, test equipment and floating wind turbines all rely on moorings in harbours and offshore. To protect people in our industry, we need a way to predict what happens if a rope breaks. A first-of-its kind test has recently been performed by Holmes solutions on a mooring rope snapback event. The test demonstrates the raw power of a mooring failure.
I wondered if a snap-back event could be replicated using a computer simulation. The video below shows a comparison between the test and a simulation using OrcaFlex software. OrcaFlex is normally used to model the response of large floating structures in water, so this is an unusual application. The software produces a surprisingly realistic response of a high energy snap-back event. The rope tip is travelling at 230 m/s (515 mph) on contact, which severely deforms the 50mm steel scaffold poles. This is a fatal impact.
Video: Holmes Physical Test (left) and ORE Catapult OrcaFlex simulation (right)
The proof-of-concept results demonstrate that snap-back can be simulated to understand and visualise risk. The higher the ‘stretch’ of the rope, the higher the snap-back risk. The simulation models stretchy Nylon rope, whereas using steel wire releases much less energy during failure.
Properly validated simulations can be used to quantify the benefit of using lower elongation ropes. However, simulations should only be used to extrapolate from a good test data set; there is no replacement for physical tests. Speaking to Ben Poulter at Holmes Solutions, there is a clear need for further tests to better understand the problem, and calibrate predictive simulations. More industry attention is needed to develop an improved understanding of rope snap-back behaviour, which we can use to build better training and technology to reduce risk to marine personnel.
Whilst this issue applies for all berthed vessels, the offshore energy industry is one worthy of specific attention due to the size and complexity of the structures involved. It is of course unfair to say nothing is being done about this issue. Solutions have been developed for the market to reduce the risk of snapback – for example snap-back arresting ropes, where a non-load bearing element is used to restrain the rope after failure. Serious action is being taken to reduce the frequency of mooring snapback through better mooring procedures and rope integrity management practices. For example, a crew member was seriously injured in the Zarga snapback incident in a UK-port in 2015, which led to the release of the latest edition of the Mooring Equipment Guidelines (MEG4) in 2018 to reduce the incidence of mooring failure.
However; the author is aware of multiple quayside mooring failures and near-misses on large offshore structures, which are not as well shared and learned from. Large offshore structures such as floating wind turbines and heavy lift vessels can fall in a higher risk category when moored in harbors, because port equipment was not originally designed to moor these structures. Specifically the bollards and terminations are high risk components, with several incidents in recent years of mooring bollards breaking free. We are likely to see over a 1000 floating wind structures moored alongside ports for the ScotWind and Celtic Sea floating wind delivery – it is imperative that mooring safety is integral to the planning of offshore construction and operations.
Those required to work in proximity to tensioned ropes are a fraction of a second away from a 100m whip travelling at the speed of sound.