Read Dave Pye’s Recent Paper Presented at ICOE – Subsea Connectors in Marine Renewable Energy
At the present time the Marine Renewable Energy (MRE) sector, both Wave and Tidal, is moving towards the deployment of the first arrays or “farms” of generators having proven the basic design concepts with prototypes. Many of the problems of deployment will have been encountered during these initial installation processes but the way in which they were overcome will probably not be cost-effective for use on arrays. For instance in a lot of cases the export cable at the test site will have been “hard-wired” to the generator using a field-installable splice or a low-cost connector. This method is quite acceptable for deployments which are only intended to last for around 12 months (and no longer) and where no retrievals are planned for routine maintenance. It was thus the best way to proceed in this instance.
With longer-term deployments and multi-generator installations retrieval of the cable and splice to the surface for disconnection is not, in most cases, the best option for a number of reasons:
(a) Vessel time is very expensive and a specialist vessel may be required for this operation.
(b) The export cable would be disturbed with every retrieval and would have to be protected when returned to the seabed.
(c) Extra cable length would be required to facilitate its removal to the surface. This is normally estimated as being at least 2 x water depth, i.e. 100+ metres length in a depth of 50 metres. This extra cable length is also a significant additional cost.
(d) With the extended time required for this operation comes the additional problem of weather windows which, when taken in addition to the already restrictive slack-water periods, can greatly extend time on site.
From the above it can be seen that the costs of installation and maintenance necessitated by these early installation and retrieval methods are presently far too high and would limit the ability of MRE to begin bringing the cost of energy closer to that of other sectors. If MRE is to be financially viable a cost-effective solution to installation and maintenance by the introduction of new and innovative methods and products is essential.
SOME MISCONCEPTIONS REGARDING AVAILABLE CONNECTOR PRODUCTS
In the early days of Marine Renewable Energy it was often thought that there were already proven connector products available from the Oil & Gas industry. This was correct in that there were a number of companies producing products that had long and successful track-records of deployment in Oil & Gas and also in Military and Oceanographic applications. The truth of the matter is however very different in my opinion. Most recent Oil & Gas applications are in deep water, typically 500-3000 metres. In these environments the only real problem is the presence of high ambient pressures and these can be controlled by pressure compensation. The environment at these depths is somewhat lacking in oxygen and no really significant currents exist to refresh the supply so the impact of marine growth and corrosion is not too severe. Also, the temperature is very stable at around 4 deg C which also serves to reduce these problems.
The methods of physical connection and hook-up available in the Oil & Gas sector include both ROV intervention and the use of automatic connection via “stab-plates”. For the MRE sector it is virtually impossible to use ROV’s other than for observation due to the severe currents and short slack-water periods that exist in the areas of deployment. This means that if we are to discount retrieval of the cable to the surface for disconnection we are left with only one option – the incorporation of an automatic connection system.
Furthermore, the existing connector products were un-suited to the environment in shallow, warmer waters by virtue of their construction, materials in particular, and are expensive. Thus we have the situation where the ideal product for use in the MRE sector has to have superior long-term performance as well as being far cheaper – quite a tall order!
THE IDEAL PRODUCT
This does not, in my opinion, exist at present and some of the experiences with prototype generators have proved this to be the case. Consequently a wide variety of problems, major and minor, have been experienced with connection systems. The features that the ideal product should exhibit will have to include the following :
(a) Simplicity of design.
(b) Simplicity of operation.
(c) Long life without maintenance or deterioration of structure or reduction in electrical/optical properties.
(d) Protection of exposed components, particularly when un-mated.
(e) Field-installable at remote sites.
(f) Economically priced bearing in mind the above requirements.
(g) Good availability and after-sales support.
Most existing products will meet a number of the above requirements but by no means all. It is therefore the task of the subsea connector industry to come up with innovative designs that will come as near as possible to meeting these without compromising one of the most important, i.e. PRICE!
PRESENT METHODS OF CONNECTION
I have already indicated above that there are two main methods of connection, i.e. on the surface or underwater. These require two very different types of connector, dry- or wet-mateable, both of which have their own particular advantages and disadvantages.
A dry mate connector is by far the less complex of the two types and thus the cheapest by a factor of at least 3x when compared to a wet-mate. It would thus seem sensible to use this product where at all possible. However, as described above the need to raise the connector to the surface for mating or de-mating has many disadvantages in the time required and cable disturbance. Overall the reduced cost is far outweighed by these disadvantages and an evaluation of these will normally indicate that this is not the cheapest option overall. The main advantage of the use of dry-mates is that they are mated in a benign environment and seals can be tested before deployment. Wet mates are therefore far more expensive to purchase but will return significant cost-savings down-line. Wet-mates require connection to be undertaken subsea and thus the procedures for this have to be devised. Systems using stab plates (see above) are the most viable and a number of designs have been trialled already by the industry. The stab-plate is provided with features which allow certain misalignments to be accommodated during the mating phase.
The existing connector designs provide protection to the contacts when mated and some even provide this automatically when un-mated. In terms of meeting the requirements of the extreme conditions they are still lacking in some respects.
NORMAL CONNECTOR REQUIREMENTS FOR MRE
There are normally up to three separate connections to be made when hooking-up a generator – Medium Voltage / High Power (say 6.6kV and up to 150 amps), Low Voltage ( 24V 5-10 amps) and Fibre Optic (anything from 2 to 12 optics). At present there are connector options available for all of these requirements from other market sectors. The simultaneous mating of multiple connectors is provided by the incorporation of these into a stab-plate assembly but any reduction in the number of connectors will very significantly reduce the complexity of the design and hence overall cost of the assembly. It is therefore worth considering the production of Hybrid connectors which will contain more than one element in the one housing.
DESIGN FEATURES OF THE SEACON HYBRID
Any new design should incorporate as many proven features as possible whilst ensuring that the resulting product is innovative. To this end it is the intention to utilise electrical contacts from a long established range of connectors as well as an optical element that operates on proven principles but is reduced in size. To meet as many of the requirements in terms of electrical and optical capacity the operating voltage is set at 6.6kV(u), current at 250 amps (4 pins) and with 4 or 6 optical circuits. This will allow the product to be used on generators of up to 1 MW with ease or 2.5MW maximum. Low voltage electrical circuits can be added as the design is modular.
Great emphasis is placed on making the resulting connector as suitable as possible for long-term operation in shallow (up to 100M) water and the design will utilise the most appropriate materials for the application. Not only will this product reduce the size and complexity of any connection system it will also come at a significantly reduced cost.
REDUCTION OF MARINE GROWTH AND CORROSION
Marine growth and corrosion represent the most difficult problems to be encountered in this environment. The presence of a highly oxygenated water flow with somewhat higher temperatures than deep-water contributes to accelerated growth and corrosion. These must be addressed in a number of ways, primarily by material selection and protection. This does however contribute to a level of cost increase on the products.
TEST STANDARDS AND DESIGN REQUIREMENTS
At present there are many test standards existing both in the general electrical sector (IEC standards for instance) and for subsea connectors for Oil & Gas applications (STATOIL specifications). Whilst these are useful in establishing some test parameters they do not cover that sector-specific requirements of MRE. It would be very useful to set up a joint industry project to set out the basic requirements for test, even to the extent of producing an industry-accepted specification.
The connector design itself could also be considered for a degree of standardisation, not in terms of how it actually works as this would limit innovation but by specifying certain parameters such as depth rating, service life, maintenance periods, power capacity (typical voltages and currents), mating forces, misalignment tolerances and more. The establishment of these factors as well as test requirements would go a long way to helping the connector industry produce products that are fit for purpose at the best possible cost.
CONCLUSIONS AND A CAUTIONARY NOTE
In my opinion there are no connectors presently available that are especially suited to the MRE sector. Many manufacturers have offerings available which are well-proven in service but all are designed and tested for very different (and much less demanding) environments.
A decision has to be made by any developer as to the merits of increased capital expenditure weighed against reduced operating costs – this is common to most emerging industries. One shut-down caused by the failure of an unsuitable (cheap?) connector will more than wipe out any initial savings. However, increased capital cost would not be wisely spent on un-proven products. The device developers and the connector industry should therefore be seriously considering working closely together to ensure that any product offered is “fit for purpose”.
For a number of years now the Marine Renewable Energy sector has rightly been concentrating on getting prototype generators to a readiness level such that they can be deployed for extended periods. During this time the products and procedures used have of necessity been different from those which would be appropriate to multiple, long-term installations. Once developers become committed to array installation it will no longer be possible to use the existing connector products and there will be a need to move to what initially will be more expensive types. Cost reductions will come only with quantity. For connector manufacturers to become involved in developing products specifically suited to the MRE environment it is therefore necessary to have a better degree of certainty as to the future market – the continual “slipping to the right” of proposed installations does not assist in their making this commitment.
Dave Pye has 24 years of experience in the international subsea connector business. He holds a Higher National Diploma in Mechanical Engineering. His career includes: six years as a mechanical engineer (including a five-year engineering apprenticeship) at Rolls Royce Aero Engine Division, Derby, UK; 14 years as an engineer at Slingsby Engineering, the manufacturer of ROV’s and associated products; similar positions with various subsea connector manufacturers; and the last four years with SEACON working exclusively in the marine renewable energy field.