May 2023

A flexible workaround the FOW vessel challenge

In this series, we promised to show you some of the most exciting concepts that we have developed in response to challenges presented to us by our clients: energy contractors - big and small - looking to find better approaches to traditional operations on and offshore.

But what about using proven approaches for non-traditional operations?

Let’s look at offshore wind, and specifically floating offshore wind (FOW).

As tangible wind power generation targets are being set, the global supply chain challenges become evident - particularly in terms of lack of specialist vessels for cable and mooring installations.

With this realisation, the discussion over applying oil & gas methodologies to enable green energy developments gathers momentum.

While O&G skeptics may say you cannot use old approaches for new energy generation, others will argue that it is important to leverage the knowledge and expertise gained from the veteran industry to accelerate green growth.

That’s also our line of thinking. We’re all for innovation - but more specifically, the practical type - the one that goes hand in hand with efficiency. That’s why we advocate for innovative thinking - for example, applying proven elements and techniques in a new way.

The mooring challenge

Most wind turbines today are fixed to the seabed, so-called bottom-fixed, in waters less than 60 metres deep. But almost 80% of the world’s offshore wind resource potential is in waters deeper than 60 metres.

The next generation of offshore wind turbines is designed to float further out to sea, where they will be kept in position using mooring systems – familiar sight around FPSOs and drilling rigs for decades.

One of the traditional approaches is with anchor handling vessels: equipped with daisy-chain winches, cranes, and other hydraulic systems, they are used to handle anchors and mooring lines. However, their back decks tend to be on the smaller end of the scale - which will become a problem the deeper and further offshore we go, as that will call for larger chain and longer polyester lengths.

Which also raises another question…

Steel or synthetic?

While conventional steel chains and wire for mooring systems have been the common theme in wind, developers are now looking to replace these with synthetic fibres such as polyester for some or all of the line.

There are several factors advocating for this preference: polyester mooring lines are lightweight, which makes them ideal for use in deepwater environments where traditional steel mooring lines would be too heavy.

Despite being lightweight, poly lines are also extremely strong, with a high tensile strength and excellent fatigue resistance. They also have a low stretch, which means that they can provide a stable and predictable platform for the wind turbine, even in rough seas.

Another important point to mention is that poly lines are easy to handle and install, which reduces the complexity and cost of the installation process.

Of course, the choice of mooring line material will depend on a range of factors, including water depth, wind and wave conditions, and the size and weight of the wind turbine.

However, the choice of anchor handling tug supply vessel (AHTS) may be more limiting. Fearnley Offshore Supply market analyst says that floating wind and offshore O&G markets in the North Sea, Brazil and deepwater West Africa require large AHTS vessels of more than 18,000 bhp, which makes up around 10% of the total AHTS fleet. The current competitive fleet of AHTS vessels with more than 220 tonnes of bollard pull (the expected capacity required for the floating market) is 92 vessels with utilisation of 70%.

Lessons to be learnt from GoM

So, how do you address the challenge of installing polyester mooring lines at greater distance and depth when experiencing a shortage of vessels with enough back-deck capacity?

Sounds like a challenge where innovative thinking could come in useful. And so, we have been testing the waters, so to speak.

A case in point is a recent deep-water mooring rope installation, where we worked with Jumbo Maritime’s heavy lift vessel (HLV) the Fairplayer at depths of 1,200m (4000ft).

Our portable flex-lay spread installed the mooring lines for a new floating production unit (FPU) in the Gulf of Mexico. The Fairplayer’s large deck capacity meant all the mooring rope reels could be carried onboard in a single trip - that’s 24 reels of poly rope carried and deployed with no port calls and using a non-traditional vessel.

Instrumental to this particular operation was MDL’s spread which let the polyester storage reels to be transpooled onto a larger deployment reel on board the HLV.

The more common approach would be to start by transpooling the polyester ropes from shore onto small drums located on the anchor-handling vessel, which would then take the anchor line to site, connect it to the anchor, lay it, and return to port to pick up the next reel. That means multiple port calls to execute the scope.


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Conceptual layout of MDL mooring spread
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MDL spread onboard the HLV


The MDL spread - which traditionally would be recognised as a "SURF spread" - deployed on the Fairplayer consisted of a Reel Drive System (RDS) and level winder to transpool a total of 24 transport reels onto a purpose-built, heavy-duty deployment reel, and a second RDS for the overside deployment. All offshore, while anchored next to the FPU.

The combination of a large back deck and our compact but highly capable equipment resulted in a fit-for-purpose solution that was much more efficient than using traditional approaches, such as deploying an anchor handler.

How about that wire?

Steel wire rope is a traditional mooring line material that is still commonly used in many applications. It is strong and durable - but also heavy and difficult to handle. Thankfully, we have experience in that also.

On this project off the coast of India an MDL flex-lay spread, with a tensioner as the master item, was used for transpooling and installation of a Spiral Strand Wire Rope (SSWR).

By applying MDL's tensioner - the gold standard for safety of handling on the portable rental market - the project team was able to maintain consistent line tension and squeeze on the SSWR, while MDL’s Failsafe Grip technology prevented unexpected loss of product. A safe and lean mooring solution, again the wire rope was installed in a single offshore trip, using a vessel of opportunity that was readily available in the region - a construction support vessel (CSV) in this instance.


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Tensioning equipment is required for both wire and synthetic mooring solutions, but the handling parameters of these will be very different. We are currently trialling some of our tensioner technology in handling of polyester rope in emulated offshore conditions - and we’re keen to test as many types of product as possible, so do get in touch if you want to get involved!

The cable challenge on the horizon…

Discussions on (and scheduling of) mooring installations for FOW go in parallel with dynamic cable plug-ins. Over the years in O&G MDL has had huge exposure to handling cables of all sizes and designs, including delicate fibre optic cables - and in that time we have innovated and optimised our flex-lay fleet to place ourselves in a state of readiness for FOW inter-array and power/composite cable installations.

But that is a whole topic in itself, which we will explore in a future blog.

Tapping into vessels of opportunity

The above are examples of how our so-called "SURF" technology can be readily applied to other products and markets to efficiently progress operations - using vessels of opportunity, or - in other words - bare-back vessels readily available in a given location or market.

The beauty of such case studies is that there is no difference in applying the same solutions to enable green energy development - while minimising risk of the "new" with proven systems and tested expertise.

Because - let's not forget - the equipment is only part of the story. What O&G track record brings to FOW is also experience of executing offshore projects. Our team has over 23 years of that: from managing all quayside services and logistics, to engineering and executing mobilisations on bare back vessels - an experience that may come in handy when the availability of specialist cable layers becomes a scheduling nightmare.

Our mission has always been to solve our clients’ challenges, and our innovation culture is here to help you gain the edge in your market - and thrive.