Review of Cabling Techniques and Environmental Effects Applicable

Review of Cabling Techniques and Environmental Effects Applicable to the Offshore Wind
Farm Industry – Technical Report
3.7.2 HISTORY
The history of safe installation and protection of subsea cables dates back to
the 19th Century when early telegraph cables across both the North Sea and
the Atlantic were subject to frequent damage from the local fishing activity and
other vessels dragging their anchors across the seabed. In this respect, little has
changed in over 250 years and all seabed cables and pipelines must be assessed
in terms of the risks of potential damage.
It was only in the early 1980s that effective cable burial became a primary
method for protecting subsea cables. The pioneers in this field were the owners
of the various offshore submarine telecommunication networks who realised
that the new fibre optic systems would require a robust protection system to
prevent costly interruptions to service. The new cables were being designed
to carry many thousand more circuits than their old analogue predecessors.
British Telecom conducted extensive research programmes through their
Martlesham Research Facility, where a number of onshore and offshore trials
were undertaken to establish the effectiveness of cable burial with varying
depths of burial and cover. The tests involved the use of different types of fishing
gear, commonly used both in shallow and deep water applications, to simulate
the real conditions of fishing gear passing over cables from different angles of
approach. At the same time, Shell UK Exploration and Production (Shell) were
conducting parallel tests to establish the potential effects of bottom trawl gear
on medium to small diameter pipelines in the northern North Sea. The Shell
study programme was subsequently supported by seven other North Sea oil and
gas operators as a joint venture.
3.7.3 SHELL STUDY PROGRAMME ON PIPELINES
The results of the pipeline research showed that the concrete coated pipelines
with diameters of 16 inches and above could be designed to tolerate trawl gear
loads without the need to lower or cover the pipelines. However for smaller
pipelines, or in the case where there was pipeline spanning, it was recognised
that pipeline protection requirements for individual pipelines needed to be
determined on a case by case basis. Subsequent to this research, a number of
specialist pipeline ploughs and large track burial vehicles were developed, built
and commissioned and put to effective use in the North Sea burying pipelines
and flowlines for the offshore oil and gas industry.
3.7.4 BRITISH TELECOM RESEARCH
The British Telecom research, having focused on the much smaller targets of
subsea cables, came to very positive conclusions that cables lowered into open
trenches stood a much improved likelihood of being protected from seabed
trawler gear. However, the best form of protection was afforded when a degree
of cover was placed back over the trenched cable. In this latter condition the otter
boards and trawl gear can easily pass over the cable without running the risk of
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