This paper discusses the principal design considerations for a plastic pipeline to supply deep cold sea water to a shore based site A small prototype Ocean Thermal Energy Conversion (OTEC) project producing fresh water on the island of Tenenfe has formed the basis for this design work The design brief was to import deep cold sea water at low cost In common with other mid-ocean volcanic islands, the sea floor beyond the coastal shelf drops steeply reaching 1000m below sea level 4km from the shore At this depth the sea water temperature is approximately 8°C compared with 22°C at the surface OTEC projects use sea water as a renewable energy source for a variety of conversion processes, some to make electricity, some producing freshwater and others to use the nutrients in the deep ocean water for the culture of seafood crops The use of plastic pipe was proposed to reduce capital outlay
The laying of steel pipelines is expensive and complex particularly at depths greater than approximately 500m The preparation, laying and post-lay operations require significant amounts of raw materials, intervention, surface support craft, technical knowledge and experience and capital expenditure to be successful The aim in Tenenfe was to lay a pipeline for as little cost as possible to maintain overall project liability The project expenditure was to be reduced by using simple surveying techniques, using a cheaper pipe material than steel, optimising the size of the pipe for the application, simplifying the installation method, avoiding pipeline maintenance and minimizing the requirement for inspection of the pipeline Plastic pipe was chosen as the preferred material because it was relatively cheap, was readily available, it had suitable properties when used In a marine environment and, as there was negligible pressure containment required for this application, had adequate mechanical strength Laying plastic pipe has become more common in shallow water applications such as outfall and transport pipes there is, however, limited evidence of application in water depths of the order of that In Tenerife except In Hawaii and Taiwan [1,2,3,4,5,6,7,8,9]
This paper describes the conceptual method chosen for the installation of a pipeline at minimum cost Discussions primarily cover seabed survey, pipe design, installation method and an estimate of cost
Conclusions and recommendations emanating from the discussion are summarized at the end of the paper
In the oil/gas Industry it is normal practice when laying a new pipeline to carry out extensive surveying The surveys are carried out before, during and after laying the pipeline For this project we applied the surveying knowledge of our oil/gas Industry experience but at the same time ensuring we did not remain blinkered to the Ideas and methods that are only relevant to laying a pipeline for the oil/gas industry In the North Sea It was thus considered wise to commence this examination of survey techniques by as long the question ‘Why survey at all?’ It is suggested In this paper that all relevant factors such as pipeline size, material type, laying techniques and philosophies should be examined to determine the reasons for surveying to ensure minimal cost outlay
