0.8 knots. The vessel should be DP- 1 or bet-
ter; the sensor position shall be monitored by
a USBL (underwater positioning system). The
C W (clump weight) is a simple steel object which
can be easily manufactured onboard or on site
with a welder and scrap material.
ALD diver mode. This technology has been
designed to detect leak on flanges, valves, and
other critical components or short pipeline segments. The ALD acoustic sensor is hand held by
a diver while, on the support vessel, a specialized
operator monitors the acquired data.
ALD tow fish mode. The area between the land-
fall and the end of the section surveyed in vertical mode (estimated
KP 8.00, depth 40 m) was inspected by ALD tow fish technology. Here,
the fish are kept not more than 10 m above the pipeline. The tow fish
will not dive more than 10-15 m when operated from the small vessel.
For this reason, from depth 20-25 m, the shallow water procedures
shall be substituted by standard ALD tow fish procedures.
ROV acoustic sensor. The same sensor realized for ROV installation is applied to vertical mode inspection. The sensor is directive
and highly sensitive. In order to minimize electric interference, the
sensor includes the pre-processing electronic systems.
Dye detector. An optical sensor for dye (i.e. fluorescent) detection can
be connected and fixed to the acoustic sensor. Optical data are multi-
plexed with acoustic data and sent through the same transmission line
so that the sensor can be added without any additional requirement.
Data from the two sensors can be monitored simultaneously so
that optical and acoustic inspection can take place at the same time.
There are no standard criteria to evaluate and certificate the performances of an acoustic leak inspection system. The ability of any
technology to detect a leak depends upon a large number of parameters like: differential pressure, leak flow, leak size and shape, leak
position, pipeline content, ambient noise, leak to sensor range, etc.
The best parameter to keep into consideration for this purpose is the
dimension of the leaks detected on the field.
After the leak was located using ALD technology, the system was
shut down and the pipe was replaced in the damaged section. The
damaged specimen was sent to the laboratory, where a defect was
observed on the weld metals.
The main reason for this failure was the weld defect, which was
due to lack of fusion and excess penetration. So, the welding defect
led in turn to a failure during the hydrostatic test.
To determine the best methodology for leak detection and rectifica-
tion, it is recommended to collect all relevant data, including leak flowrate and pressure drop to determine the best rectification procedure.
In some cases, the pipeline is not capable of tolerating the pressure
during the hydrostatic test; and pressure drop will make it difficult to
increase the pipeline pressure in order to detect a leak by fluorescent
or acoustic leak detection. So, in many cases an ROV survey would
be the best option to check any rupture or separation for the subsea
In a given leak detection scenario, if the pressure drops fast and
the size of the leak is medium or large, there is more possibility to
find the leak, and a wider variety of solutions may be applicable. But
in situations where the pressure drop is less, acoustic leak detection
technologies may be the most effective option. With smaller leaks, it
may be difficult to find the leak by ROV or diver because of current
and minimum flow of the leakage.
Rectifying a leak
For the rectification of the leak, it may be best to concentrate on
local repair, and consider use of a smart clamp, which can be used as
a temporary or permanent repair on subsea pipeline.
Before installing a smart clamp (super fit) on leak point, it is mandatory to use the hydrostatic test to check the
reliability of connector. But if the hydrostatic
test is passed, it does not mean that the smart
clamp can control the leakage. For this reason,
stringent criteria should be used in the hydrostatic test, and the engineering team will define
this acceptance criteria to get good results. Since
the operation pressure of four-inch piggy-back
pipeline in this case study was 250 bar, a suitable
smart clamp (super fit) was installed to control
the leak. •
Acoustic leak detection—tow fish mode.
ROV acoustic sensor method.