A Technical Overview of the Proposed Rockabill Cable System

January 17th, 2018 by · 2 Comments

This Industry Viewpoint was contributed by Tom McMahon, co-founder of DeepSea Fibre Networks and managing director of McMahon Design & Management.

The Rockabill System, named after the Rockabill Lighthouse on the north Dublin coast, will run from Portrane situated just 15 km north of Dublin City to Lancaster in the  UK which is some 70 km north west of Manchester. The system will be routed to the north of other telecoms cables in the Irish Sea, as shown below, and will  provide both sub-sea and terrestrial diversity from existing systems.

The Rockabill System will provide connectivity from any Point of Presence (PoP) in Dublin to any PoP in Manchester, over privately-owned networks, with no reliance upon regulated or concessionary based assets. The system will also be managed under a single service level agreement from PoP to PoP with a dedicated in house network operations centre. The Ready for Service date for the system is Q2, 2019.

The Irish Sea – A Technical Overview

The principals of DeepSea Fibre Networks have over 23 years experience in the design, permitting and management of the installation of cable systems in the Irish Sea. Their in-depth knowledge will ensure that the Rockabill System is designed and installed to provide a secure and robust system over its lifetime.

Although crossings of the Irish Sea are typically short in length, the Irish Sea presents a challenging and aggressive environment for cable-lay with frequent damage occurring to systems historically. Furthermore the shortest crossing of the Irish Sea often presents the most difficulties in terms of sea-bed conditions and fishing activities.

For comparison purposes the Irish Sea can be divided into five sectors. These sectors are shown below and their general attributes, which influenced the route of the Rockabill System, are described.

Sector I - Northern Ireland to Scotland

This area is characterized by strong tidal currents in the North Channel, a narrow sea-way between Northern Ireland and Scotland. The strong tidal currents have resulted in the stripping of sediment leaving a rocky, gravelly substrate. This substrate provides little potential for cable burial and presents significant potential for bottom aggression in the form of cable-chafe, free-spans and fishing activities. This is also a busy shipping channel and areas such as the Burford Dyke were historically used for uncontrolled dumping of munitions. In addition to these sub-sea issues, the availability of terrestrial dark fibre backhaul is extremely limited on both sides. In summary, this is an unsuitable sector for cable lay.

Sector II – Dundalk Bay & South of Isle of Man

This area is characterized by relatively shallow water, slack currents and soft sandy and muddy sediments which provide good burial potential. However, this area is extremely heavily fished by  beam trawlers, bottom trawls for prawns and scallop dredgers. This intensive fishing activity greatly increases the risk of damage to cable systems despite the good burial potential and accordingly this is not an attractive  sector for cable lay.

Sector III – Upper Middle Irish Sea

This area is characterized by relatively shallow water,  slack currents, and relatively stable substrates. These substrates range from boulder clays to sands to gravels to mixtures of sands and gravels and muds. In general, the sector provides the potential for good burial. Even if the burial depth is shallow in the harder substrates, it still provides good protection to cables. The area is not heavily fished and shipping movements are not overly intensive. With careful route planning, a good level of  data capture and survey interpretation allied to specific  cable design and installation, this sector  can provide optimum routing conditions for robust, secure and reliable cable systems. Accordingly, this is  a good sector for cable lay and is the corridor where most of the middle Irish sea cable systems. are located.

Sector IV – Lower Mid Irish Sea

This area is characterized by relatively shallow water but with strong tidal currents. This sector comprises deep sandy sediments but these are in the form of mobile sand waves and are not conducive to robust and secure cable lay. The mobility of the sand waves is such that large free-spans will develop over time and leave any cable susceptible to inevitable damage, particularly from fishing activities. In summary, this sector is not suitable for cable lay.

Sector V – Southern Irish Sea/ St. Georges Channel / Celtic Sea

This area is characterized by the wash-out of the sand from the mobile sand wave fields  of the Lower Mid Irish Sea. This results in a stable sandy layer which provides good burial potential and good protection against the potential of damage from fishing activities, which are intensive in this area. Accordingly, this is a good sector for cable lay and this is the corridor used by many of the older systems as backhaul in the UK was restricted to Cornwall and Devon. A notable feature in this sector on the Irish terrestrial side is also the lack of availability of dark fibre backhaul. All of the systems built in this corridor availed of special conditions to allow the use of railway fibre for backhaul. This is a limiting factor in the use of this corridor.

Key Rockabill Design & Route Attributes

DeepSea Fibre Networks has taken into account these various technical restrictions in the Irish Sea and has carried out further detailed route engineering, marine survey interpretation and terrestrial backhaul investigations. The result is that the Rockabill System has been designed to be the most northerly  crossing of Sector 3, providing diversity from existing systems whilst remaining on a secure sub-sea route. The system has unique backhaul dark fibre routes in both Ireland and UK which do not rely on regulated assets or concessionary systems. This, coupled with the company’s in-depth experience of cable design and installation, will allow DeepSea Fibre Networks to provide dark fibre to the market at the most cost-effective rate.

In addition to the above, DeepSea Fibre Networks does not provide capacity services and is purely an owner, operator and manager of subsea fibre assets. In this regard DeepSea Fibre Networks does not compete with its customers and operates on a truly carrier neutral, open access basis. The company has its own in-house engineering team with specific expertise in the design, procurement, installation and operation & maintenance of cable systems. It also owns its own offices in Dublin where its NOC services are based. Consequently, DeepSea Fibre Networks can offer low cost, high quality in-house operations and maintenance services to its customers.

DeepSea Fibre Networks is a new Irish company founded and owned by Tom and Jim McMahon, both of whom have been involved in the sub-sea cable industry since 1997. Tom McMahon is a Civil Engineer and Jim McMahon is a Quantity Surveyor and Construction Manager. Together they have worked on the deployment  of eighteen sub-sea  fibre-optic cable systems including  nine systems in the Irish Sea, six trans-Atlantic systems and three sub-sea cables in the North Sea and the Western Approaches.  They also have extensive experience in the planning, design and project management of numerous terrestrial systems and cable landing stations.

Categories: Industry Viewpoint · Undersea cables

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2 Comments So Far


  • mhammett says:

    If terrestrial backhaul is the prevailing issue in some places, why doesn’t a fiber builder… build it?

  • Emerald says:

    Not sure what the commercial reason is for another system across to Ireland? Seems to be a real glut of capacity, so it’ll only increase price erosion.

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