Various method exist for
joining ends of subsea pipelines. These methods include the following:
- · Flanged connection
- · Atmospheric welding
- · Hyperbaric Welding
- · Mechanical Connectors
Flanged Method
Flanges are pre-installed on each pipe end during laying. The
pipe ends are positioned approximately in line with the flanges 50-200 ft
apart. An adjustable fixture (template) is lowered to the seabed and temporary
attached to the flanges. The fixture is locked in position, released, and
raised to the surface. A rigid pipe spool is prepared to match the exact
dimensions of the fixture, lowered to the seabed, and bolted into place.
Figure 1. Flange Connection
Source :
A swivel-ring flange is
used on one spool end to facilitate alignment of the bolt holes in the flanges.
This method is generally limited to applications involving relatively small
diameters and shallow water, although flanges have been used to at least 36-in
diameter and in 500-ft water depths in the North Sea.
Flanges are low in cost,
but they can take a long time to install and may leak during pressure testing.
A leaking flange can be difficult to diagnose. In one installation, it took 2
weeks to locate the source of a small leak during a hydrotest, which was due to
a leaking flange. However, flanges are considered trouble-free once they have
been installed and tested. Flanges are sometimes used at the foot of risers to
facilitate replacement of a riser.
The process of
tightening large flanges has been made considerably easier and faster by the
use of a hydraulic bolt-tensioning tool. The Hydra-Tight tool, sold in the U.S.
by Flexatalic Gasket Co., has been used in the North Sea for several years. It
consists of a series of hydraulically operated tensioners which are attached to
protruding ends of the flange studs. Hydraulic power provided from the surface
causes the tensioners to tension each stud uniformly. The nuts may then be
tightened in as little as 3 hr using the Hydra-Tight tool. The primary
advantage, however, is uniform tensioning of the studs. This reduces the
likelihood of a leak, especially for large flanges.
A variation of the
flanged spool that is gaining wider acceptance is the use of a ball flange to
accommodate angular misalignment. Small diameter lines (10-12 in. or less) in
200-300 ft of water may often be lifted to the surface to make a connection
using a ball connector. The pipe is dewatered, if necessary, and one end is
raised by one or more lifting points. A ball-connector half is welded to this
first pipe end. A joint or two of pipe are first welded on to bridge any gap
between the two pipe ends.
The first pipe end is
lowered to the seabed so that it overlaps the second pipe end. A measurement is
taken on bottom, and the second pipe end is raised to the surface. The pipe is
cut, the second ball half is pipe end is welded on the pipe, and the pipe is
lowered to the seabed. The two pipe ends are then lifted slightly and the ball
halves are mated. The connected pipe is lowered to the seabed and the bolts are
tightened to lock and seal the ball joint.
If the pipes must be
dewatered for lifting, temporary end caps are attached to the ball halves
before the pipe ends are lowered. After the pipes are flooded, the temporary
caps are removed.
The ball connectors may
also be used in pairs at the ends of a rigid spool for new construction or for
a long spool repair when pipe ends can be lifted to the surface. Measurement of
the required spool length must be accurately made since the ball connectors
will provide only limited length adjustment. Moreover, an axial movement of
about one pipe diameter is needed to mate the halves of a ball joint.
Figure 2. Gas Pipeline Flange
Source:
Mouselli, A. H. Offshore Pipeline Design, Analysis, and Methods. USA:
PennWell Books. 1981.
Dega Damara Aditramulyadi
Student ID : 15512046
Course : KL4220 Subsea Pipeline
Lecturer : Prof. Ir. Ricky Lukman Tawekal, MSE, Ph. D.
Eko Charnius Ilman, ST, MT
Ocean Engineering Program, Institut Teknologi Bandung
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