Pipelines up to about 5km in length can be transported and installed in very deep water by towing in a catenary between two tugs. The catenary tow is performed without additionalbuoyancy. The initial, shallow water section of the tow, from an onshore fabrication site to the point where the catenary is established, requires the use of additional temporary buoyancy, rated to much less than the full installation depth. The buoyancy can be removed economically in long strings if the buoys are constructed in such a way that they can be flooded to a condition of near neutral buoyancy. Detailed analysis of the effect of wave action and vortex induced vibration (VIV) shows that installation can be performed without incurring significant fatigue damage. The depth at which buoys are removed and catenary towing is initiated is based on an economic assessment intended to minimize the combined cost of the buoyancy and of the towing tugs. This technique is applicable for infield pipelines and risers.
Installation of pipeline systems by towing is a long established and proven technology. In the North Sea, nearly 60 pipeline bundles have been installed in water depths up to 450 metres using a technique called the 'Controlled Depth Tow Method' or CDTM. However 450 metres is close to the limit for North Sea practice because of the way in which buoyancy is provided. For West Africa, the Gulf of Mexico and Brazil, where many field developments are in water up to 2000m deep, a new methodology is required.
The proposed new methodology has its roots in existing technology and it will be described following a brief outline of the existing methodology.
In North Sea pipeline bundles practice, one or more pipelines are contained inside a carrier pipe, which serves both as a buoyancy element and as mechanical protection. The carrier pipe is pressurized with nitrogen, to approximately seabed ambient pressure at the installation depth, to enable it to resist external hydrostatic pressure. Pressurisation with nitrogen takes place onshore before the completed pipeline is launched, and the greater the installation water depth, the higher the nitrogen pressure must be. Higher pressures require thicker pipe, and eventually the carrier pipe becomes so heavy that it ceases to be an effective buoyancy element. The pipelines are contained within the carrier pipe and thus displace no water, so the carrier pipe must support the full dry weight of the pipelines rather than the submerged weight. The net result is that the practical limit for bundles, in which pipelines are contained within a carrier pipe, is about 500 metres.