PIPELINE RISERS

4.5.7 Pipeline Risers

4.5.7.1 General

An important consideration in the design of offshore pipelines is connection to surface facilities. Often, the pipeline on the seabed is connected to a riser, which extends to a surface producing facility.
Many types of pipeline risers have been used in the past, including risers that can be sat on site and pre-installed risers that can be connected to the pipe on the seabed by a subsea tie-in arrangement Selection of a particular installation method is influenced by several factors, including water depth, project schedule, economics and platform design. Specialised analysis of the pipeline and riser are needed to ensure flexibility of the connection and safety of the system.

Several methods exist for connecting a subsea pipeline to a pre-installed (existing) riser on a platform. They include the following:

4.5.7.2 Flanged Connections

Flanged connections are widely used for pipeline-riser tie-ins. Long pipe spools, fabricated in a jig aboard a work vessel, are usually used with flanges. Alternatively, swivels have been used to accommodate annular misalignments between the pipe and riser; the spools normally have right angle or Z-bends to provide flexibility in accommodating thermal and pressure expansion. In some cases, particularly in large diameter pipelines, rotating flanges are used to ease the installation.

Some operators favour flanges, while others favour hyperbaric welding. The advantage of flanges is that they permit easier repairs in the event of pipeline/riser damage or corrosion. There have been some reports of leaks and they can take a long time to locate. But this is not generally regarded as a major factor for eliminating flanges.

4.5.7.3 Hyperbaric Welding

Hyperbaric welding is conducted in an inlet atmosphere (nitrogen, argon or helium) at pressures relative to the depth of water. It has been used mostly for pipeline-riser tie-ins in the deep waters of the North Sea.

The hyperbaric work chamber and alignment frame are normally handled by a pipe-lay barge, a derrick barge or a large work vessel.

4.5.7.4 Subsea Atmospheric Welding

Subsea atmospheric welding is carried out inside a caisson, which is maintained at atmospheric pressure on the seabed. The gas within the caisson may be air, nitrogen, helium or argon, depending upon weld specification. Higher-quality welds can be obtained than those obtained under hyperbaric welding conditions.

An alternative system consists of a habitat chamber, which is a permanent part of the platform and into which pipe is pulled.

After pipe is pulled into the chamber, the chamber is sealed and pumped dry.

4.5.7.5 Mechanical Connectors

Mechanical connectors clamp two pipe ends together without the need for welding.

4.5.7.6 Surface Welding

The surface welding method is used for simultaneous installation of a pipeline and riser. It is most widely employed for pipelines up to about 30 in. diameter and in water depths to about 350 ft.

In this method, pipe is first laid on bottom near the platform. The lay barge lifts the pipe to the surface using davits, buoyancy devices, or both. A carefully planned pick-up procedure is used so that pipe is safely lifted without over-stressing. The riser is then set into position next to the platform leg and clamps are installed to fasten the riser to the platform legs.

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