Ultra-deepwater hydrocarbon pipelines face a number of design challenges in the form of demanding geotechnical characteristics of the selected route, extreme slope, construction loads arising from suspended catenary and loads introduced during conventional hydrotesting of the installed pipeline.
The paper details the design, manufacturing and construction approach adopted by the Turkstream project to eliminate hydrotest requirements. Following completion of pipelay, both pipelines were pre-commissioned with multiple cleaning pigs propelled by compressed Nitrogen and batches of mono-ethylene glycol (MEG) as desiccant for the moisture entrapped in the pipeline during construction.
South Stream Transport BV (SSTTBV) has constructed and commissioned an ultra-deepwater major gas transmission system in the Black Sea comprising of two (2) offshore pipelines installed in water depth up to 2200 m (Fig. 1). Each pipeline is designed for transportation of 15.75 billion cubic metres of natural gas per year over a seabed distance of some 940 km from Russia to Turkey at maximum operating pressure of 284 barg. Key parameters of the pipeline system are detailed in Table 1.
This project is widely considered as one of the most challenging pipeline projects to-date, stretching both pipe manufacturing and offshore construction capabilities of linepipe manufacturers and pipelay contractors.
Ultra-deepwater hydrocarbon pipelines generally span across continental shelf and abyssal plain of the intervening ocean. Selection of the shortest technically feasible pipeline route between the pre-determined landfall locations demand thorough assessment of technical limitations set by geotechnical characteristics of the planned route, pipeline material properties and construction methodology. In most cases of ultra- deepwater gas pipeline design, the loads encountered during the construction phase far exceed in-situ pipeline operating loads; tending to dominate pipeline design parameters. Where governing regulatory environment does permit elimination of in-situ hydrotest of the installed pipeline, design optimisation can yield significant economic and environmental benefit to the pipeline operator. Pipeline certification authorities, e.g. DNV-GL, have developed design code (e.g. OS-F101) to enable pipeline designer to secure "hydrotest waiver" via implementing stringent design verification, linepipe material selection, manufacturing quality control, advanced construction quality surveillance and "dry" pre-commissioning of the installed pipeline.