As interest in deepwater applications continues to grow, so too does the need for effective and long-lived solutions to meet the particular challenges and demands of deep and ultra-deep moorings. And with the limitations of traditional catenary moorings in terms of performance and station keeping becoming more pronounced with depth and steeper cable angling, the industry has increasingly looked to technological innovation – principally in modelling and materials – to provide the necessary answers.
The optimal design for any mooring must reflect the interplay of a variety of functional and external factors, such as anticipated traffic and usage, operational demands, water depth, local currents, tidal flows and predicted wave size. In recent years, the need for a perfect compromise between these often conflicting influences and desired performance characteristics for deepwater systems has begun to push the envelope in terms of weight reduction, stiffness and strength.
The natural consequence has been to force a paradigm shift away from the universal dominance of chain and wire-rope catenary installations. While this has, unsurprisingly, added impetus to the development of fibre rope alternatives, it has also prioritised big improvements in the available models for system design.
One particularly promising move in this direction came in April 2009, when Cambridge University in collaboration with BP Research began a three-year project to improve and extend present modelling methodologies for ultra-deepwater applications.
Funded by the engineering company Noble Denton, the project is specifically intended to examine ways to remove the present levels of uncertainty that surround some aspects of developing projects beyond 1,500m. The amount of data capture arising from computational modelling methods for ultra-deepwater construction imposes a large time element on design assessment, while the physical constraints of testing tank size imposes unavoidable limits on the efficacy of using scale models.
The research project sets out to circumvent the problems inherent in both, principally by attempting to improve understanding of the dynamics of deepwater moorings and couplings. In the words of Noble Denton's assurance and consulting managing director RV Ahilan – who holds a doctorate in engineering fluid mechanics from Cambridge – "If you can turn a 3,000m-depth problem into a 1,000m water-depth problem, then you can do a model test of a 1,000m-depth problem. But understanding the physics for that transformation will be our biggest challenge."
If the team satisfactorily overcomes this hurdle the industry will gain a potentially powerful new tool to explore deepwater design with a level of confidence previously only possible from truly representative scale modelling.
Lighter, stronger and stiffer
Traditional, all-chain catenary moorings approach the limit of their capability at a depth of about 450m, while combined chain/wire-rope systems remain effective to about double this depth before the weight of the mooring begins to sag excessively, and performance and station keeping deteriorate. To go deeper, the need for stiffer and stronger systems to improve mooring function while reducing weight has led to an upsurge of interest in lightweight fibre cables – and has driven a considerable amount of development in the surrounding technology.
Moreover, in today’s post-credit crunch world, the significant savings in terms of installation and operational costs that such systems could potentially offer has not gone unnoticed.
The concept is scarcely a new one; the Brazilian national oil company Petrobras effectively pioneered the use of deepwater polyester tethers in the mid-'90s and the first entirely polyester rope deepwater mooring system was installed in 1997 in 1,400m of water. Since then, more than 50 temporary and permanent polyester deepwater mooring systems have been installed – amounting to a total length of more than 800km.
As the depths involved have become greater, other synthetic fibre materials have attracted interest as potentially stronger and stiffer, low-weight rope materials, with the promise of even better behavioural characteristics for deep and ultradeep water moorings. The roll-call is impressive, comprising aramid (aromatic polyamides, such as Kevlar, Twaron and Technora), high-modulus polyethylenes (the likes of Spectra and Dynema) and the liquid crystal polyester Vectran – the same high-tech material used to make the landing air-bags for Nasa’s Mars rover probes.
The additional relative strength of these kinds of fibre compared with conventional alternatives enables them to be used in smaller diameters, which in turn means shipping and handling is significantly reduced, and makes for another clear bottom-line benefit.
However, arguably the biggest advantage of synthetic fibres lies in their longevity. Tension cyclic load testing of polyester rope has consistently indicated that performance is not significantly affected, even after the equivalent of hundreds of years of operational cycling, even at tensions exceeding those typically encountered in deepwater applications. With the consideration of fatigue life an essential factor in mooring design, the future is increasingly synthetic.
Simpler and easier
Present approaches to deepwater mooring have their roots in systems originally designed for much shallower environments but this is set to change, as several marine and offshore companies look to develop novel technologies specifically intended for deepwater projects. In February Offspring International and First Subsea announced a joint programme to define a new state-of-the-art innovation in subsea mooring design and, when a big name in mooring systems teams up with a chief connector specialist, the collaboration is likely to achieve something significant.
Their main goal is to produce an enhanced, purpose-built technology that will enable deepwater connections to be made more easily than existing approaches allow. As Brian Green, GM at First Subsea, explains: "There is a real need for mooring systems that better reflect the rigours of installing in deep water. They need to be designed specifically for this environment, lighter and less complex and, as a result, delivering higher performance than existing systems. Working with Offspring International we are able to bring together the complementary skills and technologies in mooring and connector technology that will allow us to develop the next generation of deepwater mooring systems."
A range of moves that may also ultimately simplify installations is underway elsewhere in the industry, perhaps none so unusual as work being done by the Fraunhofer Institute and the German Research Centre for Artificial Intelligence (DFKI Bremen), aimed at providing remotely operated vehicles (ROVs) with a sense of touch.
Despite "ROV-friendly" rigging, robots working in deepwater – and their operators – face enormous challenges including navigating and holding station against moving currents in the pitch black. However, all that is about to change with the development of printed-on strain gauges, produced by atomising a nano-particle containing solution, which is then sprayed into position as an aerosol, controlled by a sophisticated software guidance system and precisely contained by a shroud of focusing gas.
Being printed rather than glued on means that the sensors can easily be located on curved surfaces and, with each one only ten microns wide, it becomes possible to apply multiple strips close to one another, offering unprecedented touch sensitivity.
The project is in its early days – the team presented its sensor-equipped "octopus" model robot at the Sensor and Test Trade Show at Nuremberg in May – but there is clearly much potential in an autonomous or semi-autonomous robot that is able to accurately sense its surroundings. For the remainder of the sector, while the future may look less sci-fi, there is little doubt that the move to ever greater depths will continue to encourage new collaborations and drive further innovation in deepwater mooring technologies.