AdvancePipeliner

Generally offshore pipeline and cable route selection is based on the following principles;

• Pipeline and cable should be routed to to minimize overall route length (or in general to minimize overall pipeline or cable laying cost)

To minimize costs of the project, the selected route shall be as short as possible (as far as possible, the selected route shall be straight line). Note that for a 16" offshore pipeline, reduction of every km in pipeline route length reduces 1 M$ of the total project costs

• Pipeline and cable should be routed in the proposed survey corridor
• Pipeline and cable should be routed away from any sea floor obstructions, hazards, or depressions which may create massive free spans
• Whenever practical the selected route should avoid anchorage areas, existing underwater objects such as sunken vessels and pilings, active faults, rock outcrops, and mud slide areas.
• Presence of other pipelines, installations, and wellheads should be considered
• Presence of regularly dredged areas and dumping grounds should be considered
• Instabilities of the soil beneath the pipeline or cable should be considered in pipeline routing
• Spanning analysis should be undertaken based on data from geotechnical survey to identify any locations where possible free spans will be longer than pipeline design span length
• Pipeline and cable should be routed to minimize number of crossings
• Whenever practical, areas with intensive fishing activities (regions with finishing grounds) should be avoided
• For trenched pipelines and cables, the sediment along the selected route should have sufficient thickness

References:

1. API RP-1111, Design, Construction, Operation and Maintenance of Offshore Hydrocarbon Pipelines, 1999.
2. BSI 8010, Part 3, Offshore Pipelines.
3. Guidance Nots on Geotechnical Investigations for Marine Pipelines, Society for Underwater Technology, 2004.

In designing pipelines for transmitting natural gas to long distance one of the first steps is estimating the internal diameter of the pipeline. Many factors should be considered in internal diameter estimation. These include specific gravity and viscosity of the gas, volume of the gas to be transmitted, and maximum elevation of the route.

Gas flow formulas which are used for pipeline diameter estimation can be categorized into three main groups;

1. Formulas in which the coefficient of friction is constant
2. Formulas in which the coefficient of friction is a function of diameter
3. Formulas in which the coefficient of friction is a function of Reynolds number

Generally the first group of these formulas leads to unacceptable estimation error and the formulas of third group need some calculation iterations due to their implicit nature.

One of the widely used formulas for estimating pipeline diameter is the Weymouth equation. In this equation it is assumed that the friction factor is just a function of pipeline diameter. The following equation represents the Weymouth equation in imperial units.

Where, D = Pipeline diameter, in. q = Gas flow rate, MMscfd (at 14.7 psia, 60 F) T = Average temperature, Rankin Tb = Base temperature, Rankin P1 = Inlet pressure, psia P2 = Outlet pressure, psia Pb = Base pressure, psia L = Pipeline length, miles z = Gas deviation factor at average temperature and pressure ? g = Gas gravity (air = 1) For a specific gas flow rate, the above equation can be solved to find minimum required pipeline diameter for gas transportation.

Following you can find link of a useful document which discusses type and accuracy all available formulas regarding this issue. The document has been issued by Pipeline Research Council International (PRCI).

Flow of Natural Gas Thru High Pressure Transmission Lines (Original Link) Flow of Natural Gas Thru High Pressure Transmission Lines (Mirror Link on This Site) Other Reference: Natural Gas Engineering Handbook, Dr. Guo, Gulf Publishing Company.