Subsea operations

The offshore wind industry, it would appear, has been plagued by schedules and budgets that have far exceeded expectations, a trend that will inevitably become less of a headline-grabber as more farms come on line and experience is gained. The oil and gas industry, after all, went through similar teething issues in its heady early days. By their very nature the two industries rely heavily upon subsea operations for success, but they are inevitably better understood by the latter than they are by the former. Perhaps this could be a contributing factor to the schedule over-runs and spiraling budgets. Certainly, one particular wind farm subsea operation still in its infancy, and therefore lacking experience, is that of inter-array cable installations.

Offshore oil and gas operations vary in depth from transition zone projects to ever deeper waters. Conversely, the wind industry is characterized by operations still mainly in shallow water and close to shore. Whereas the oil and gas industry must contest with a myriad of operational considerations, the offshore wind industry exhibits characteristics to its subsea operations that are common wherever a proposed development is planned.

Experience speaks volumes

In October 2011, Reef Subsea Technocean successfully completed a campaign to install 41 inter-array cables on the Greater Gabbard Offshore Wind Farm. The project, Technocean’s first foray into such offshore operations as a service provider, gave the company a unique insight into an offshore wind farm’s development and how the simultaneous installation and burial of an inter-array cable using a plough can be greatly affected by the subsea conditions. The Greater Gabbard wind farm comprises of 140 turbines built on sandbanks approximately 23kms off the cost of Suffolk, England. The water depth is between 20m and 30m.

The installation of inter-array cables is a vital part of any wind farm development. Once a monopile foundation has been installed, surface operations take over and these form the bulk of any wind farm development. However, what happens beneath the surface is critical, often misunderstood, not always visible and, moreover, subject to the local environment. It is this latter characteristic that can produce unique situations during cable installation.

Understanding your environment

There must appear a false sense of security that because a wind farm is very often partially visible from shore, land operating methodology must apply. Subsea operations closer to the coast are fraught with far more dangers than those in the marginal waters of oil and gas, and most of them are environmental in origin. Shallow waters are the ground stone for near-shore wind operations; if the continental shelf commenced at the beach, offshore wind wouldn’t even be on the energy agenda.

Turbidity, currents and seabed geomorphology seem to be magnified exponentially when compared to offshore oil and gas operations in deeper waters. Throw in the sole reason offshore operations exist in this industry, wind, and you have yet another reason as to why subsea near-shore tasks can be so challenging. The ever-changeable subsea environment requires years of expertise to understand and in a new application such as inter-array cable installation, the dynamics of such an operation have revealed challenges that have hitherto not been previously understood.

The weather forecast, subsequently, is one of your closest allies offshore. Every contractor will rely heavily upon one, two or possibly three different weather forecasts to provide a window of operational opportunity. The predicted wave height, swell and wind speed will determine greatly whether an Remotely Operated Vehicle (ROV), plough or trencher will be launched or not. But for near-shore subsea operations, the subsea contractors put even more weight upon tidal predictions, not just the regularity but the direction and frequency of the resulting current.

A semi-diurnal tide may give you only two opportunities a day with which to safely carry out a specific subsea operation. At Greater Gabbard, depending on the phases of the moon, the current can run as fast as 4 knots. Spring and neap tides are significant factors for subsea operations and knowledge and careful observation of their characteristics will achieve success, reduce downtime and avoid failure.

Learning the hard way

The move away from barges to the use of DP vessels for cable installations has assisted subsea operations immeasurably but also presented new challenges. Such vessels exhibit deeper draughts and the waters of an offshore wind farm are, as previously mentioned, shallow in nature.

Whilst a Remotely Operated Vehicle (ROV) may be the mainstay of any offshore operation, regardless of industry, operations in shallow waters can be challenging. In 20m of water, for example, an ROV’s acoustic reference point on the vessel, the hydrophone, can be only 10m below the surface, providing an ROV pilot with a narrow navigational envelope in which to work. Careful umbilical management, not often cause for thought in deeper waters, is paramount and subsea ROV operations must consider every eventuality.

For example, should you lose your sense of direction in turbid, strong and shallow water, and then your position, as the acoustic signals fail to penetrate a vessel’s wash, you will see how quickly a routine subsea operation becomes alarming. In late 2009, due to inexperience in the inter-array installation market, Technocean, subcontracted to Subocean Group at the time, lost an ROV. Lessons were learned, changes were made and, to date, its replacement is still in use.

The soil conditions at Greater Gabbard warranted the use of a plough for the burial of each inter-array cable. When launching and recovering a 20t plough in close proximity to a fixed installation, numerous considerations must be made. A blow-on situation must be avoided at all costs. However, it is often the case that, due to the engineering design of the wind farm, the position of the vessel and deployment of the plough cannot avoid a potentially compromising position relative to the fixed structure.

A pre-engineered back-deck will determine the launch point for the plough and the cable, and this may not always be favourable in relation to the orientation of the cable exit from the fixed installation. Once deployed, a cable and plough will invariably enter in to a turn. The vessel, at this point, needs to maintain a particular direction and with a tide, swell and winds all affecting the vessel, timing and efficient operations will ensure that the product is deployed safely and without compromise.

Conclusion

The natural environment presents the biggest challenge to subsea operations within inter-array cable installations. Wind, waves, currents, visibility and shallow waters all combine to challenge the offshore contractor and these factors need to be understood during field operations.

New technology will inevitably assist in understanding and carrying out operations safer and more efficiently, but it will never completely remove what experience can teach a contractor.

Given the relative infancy of the industry it is understandable that not all aspects of offshore subsea operations are understood by everyone. Over time, this will change. It is critical for the success of any project that it has the buy-in of those who commission the development. The client needs to understand the reasons decisions are made by those managing offshore subsea operations. Working in shallow water is inherently hazardous and challenging.

The respect of those that have gained experience of working in similarly difficult conditions, either in oil, gas or renewables, must be taken on board. If a client is unable to understand the dynamics of the factors affecting a subsea operation, relations will be strained; this is inevitable. Only through further offshore wind farm developments will subsea operations be better understood and, ultimately, successful, on time and on budget.

With thanks to Michael Blease Shepley, Sales & Marketing at Reef Subsea UK Ltd, and Stuart Duncan, Project Manager at Reef Subsea Technocean