Anchor loads, chain & line tension : The problem

Over the past few years there have been a number of incidents on the Norwegian continental shelf (NCS) with failures in high-strength mooring chains. The chain failures occurred in mobile offshore units mooring systems even with R5 grade chains.

A few examples are Deepsea Nordkapp in 2020, Transocean Endurance & Enabler in 2019, Songa Enabler and Songa Endurance in 17/18 the latter one where the chain line fractured during anchor handling operation of anchor line. The chain fractured during pre-lay in good weather conditions and was tensioned to approximately 180 tons at the time of failure.

Many of these fractures have occurred both in normal operation and during the installation of anchor lines and for different levels of anchor line tension - also for levels that are significantly lower than the breaking limit.

We've also had incidents where vessel and crew have experienced the frightening and immense power of heavy equipment under heavy tension suddenly snapping. Back in 2011 the AHTS Skandi Vega (video) during hook up of the Songa Dee experienced just that and the CCTV recordings of it went viral and one could see the incredible lucky escape one of the crew managed to pull out of his hat.

Detecting defects in high strength mooring chains is notoriously difficult to identify. Even inspection might not uncover all and anchor handling operations are different in today's world than compared to just fairly recently.

• rigs moved around w all chains/wires onboard VS pre-laid systems
• Simple line assemblies VS very complex lines assemblies
• little handling of equipment VS equipment taken onshore/handled between each operation
• Low pre-setting anchor loads VS requirement significantly increased with "Ankringsforskriften 09"
• Larger AHTS + tandem/parallel/serial operations

Every anchor chain has a design MBL, known as "Minimum Breaking Load" and DNV has a set of rules in their DNV-RU-OU-0300 that cover operational limits in order to avoid load peaks above 65% of the MBL.

What will affect the MBL in a real offshore operation, let's say a retrieval operation for instance? Well quite a few factors actually, such as :

• Water depth (100m)
• Significant wave height (Hs) (1,5m -2m - 2,5m - 3m - 3,5m)
• Pay-out (1,3WD - 1,5WD - 1,7WD)
• Bullard Pull (200t)
• Type and size of chain (84mm chain R5)
• Direction of the external environmental forces acting upon the hull

Inputting the above into a typical chain load analysis tool one gets the following interesting calculations where the thick yellow line is the magic 65% of the MBL threshold :

First and foremost, one can see that the actual Hs plays an very important role. Just half a meter yields quite a remarkable uptick in tension load. Secondly one can see that vessel heading is also of importance, or at least understanding which direction the environmental forces are acting upon the vessel.

There is no question about it, stricter and better weather and environmental restrictions should be introduced. The below is the Delmar suggestions :

The above two measurements are challenging for the vessel to obtain correctly. Weather forecasts are inaccurate, human eyes and gut feeling is yet to be scientifically proven and closest wave buoy might be far far away.

Actual real-time and accurate measurements from a wave radar onboard the vessel itself is a surprisingly easy, cheap and simple fix. Having access to the below screen day and night would be critical in order to adhere to the restrictions mentioned above.

Another aspect is logging and monitoring of such operations. What the industry actually need is a live load monitoring solution. Showing actual line load, load history and peak loads correlated to actual weather conditions experienced and vessel movement.

An AHTS stern is uncompromising, it will rise with the ocean surface. Anchor chain already in tension would be significantly impacted by the heave of the vessel. This will create spikes in peak loads and could cause critical damage.

Logging of such is an important step going forward. However, it's the next step beyond that which has us Miros - Real-time Ocean Insights very very excited.

Wave & vessel motion prediction

On this particular subject matter our current offering provides useful data for the purpose of logging and monitoring. However it's obvious that accurate wave & vessel movement prediction would push it from being a reactive tool to a proactive tool as well.

Predicting the heave of the vessel a few minutes in advance would mean that the vessel would be able to take some of the tension of the line when there's an incoming wave of a certain size that will result in a heavy heave movement of the stern.

Follow Miros - Real-time Ocean Insights as stay updated on the progress of what we consider to be the holy grail within our field, accurate wave & vessel movement prediction.

Imagine other areas of use for this technology. Another interesting area would be with regards to DP operations and how to improve such. But that deserves an article on its own. Soon, if not before??