Turkey-Cyprus submerged floating freshwater pipeline

Abstract

A contracting consortium, comprising the Turkish contractor Asim-Alarko and the Dutch contractor Van Oord ACZ, has submitted to the Turkish Water Authority a new concept for an offshore fresh water pipeline between Turkey and Cyprus. Where conventional offshore pipelines are laid on the seabed and are made from steel, this new pipeline concept incorporates a submerged floating pipeline made from High Density Polyethylene (HDPE) which is anchored to the seabed by vertical anchor lines. This new concept could simplify the installation procedure of pipelines since the submerged floating pipeline will be anchored at a relatively limited depth below the sea level. This avoids the high forces resulting from installing a pipeline at great water depths and buckling due to external pressures. After previous feasibility studies the consortium requested Aker Kvaerner to perform detailed engineering for above-mentioned new pipeline concept. As a result of this the present study has been initiated to start with the global design of the concept, which has finally resulted in an extensive study of the following aspects: • Study of the design parameters. • Study of the hydraulics in the pipeline • Study of the static behaviour of the proposed structure In the study of the design parameters it appears that especially some environmental data are difficult to quantify without doing an offshore survey; they are of great importance for the design. This concerns the marine growth at the relevant water depth, the magnitude and the direction of the deep-sea currents, the possible existence of internal waves and the soil properties. The properties of HDPE are also important for the design and therefore need accurate investigation, especially the stress-strain characteristics related to time and the creep behaviour of HDPE. Present material specifications appear to be not sufficient for final structural design. The pipeline must have the capacity to transport 75 million m3 (2.38 m3/s) of freshwater per year based on gravity flow. The hydraulic study showed that a pipeline with an internal diameter of 1.48 [m] is needed. Wall-friction is the dominant cause of hydraulic losses in the pipeline. Friction losses owing to bends, valves, contractions and enlargements appear to be almost negligible. For further hydraulic design there are two aspects that need special attention. The first aspect is the possible occurrence of under-pressures near the coast of Cyprus during steady state flow in the pipeline. It is preferred to avoid these under-pressures; consequently, an adequate pressure control system will have to be designed. The second aspect, which needs extra attention, is surge. Closing operations of valves will initiate pressure waves. These pressure waves will have an important influence on the structural behaviour of the system. Investigation will be necessary to establish whether HDPE is able to withstand the related short-term loads or if a pressure release system is necessary. From the static analysis it appears that the configuration, as proposed by the contracting consortium, with a distance of 500 [m] between the vertical anchor lines should be technically feasible. However, local stress problems, which arise near the connection points of the vertical anchor lines, still have to be solved. It could even be necessary that in order to solve these problems small improvements in the original concept will have to be made.