ClampOn Subsea CEM Paper World Pipeline March 2011

8/12/2019 ClampOn Subsea CEM Paper World Pipeline March 2011

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Subsea sensingSindre Halse Kristiansen, ClampOn AS, Norway, details the specifics of

non-intrusive corrosion-erosion monitoring for subsea applications.

Detection and monitoring of corrosion and erosion are essential prognostic

means in preserving material integrity and reducing the lifecycle cost of industrial

infrastructure, ships, aircrafts, ground vehicles, pipelines, oil installations, etc. Even

topside the conditions of operation can be extremely hostile, facing problems like

surface roughness, fluid loading issues, temperature variations, and a host of other factors

that make development of a robust wall thickness assessment tool a challenging task.

Deploying a monitoring system subsea makes the application even more demanding when

you have to take into account factors like high pressure and limited access. The ClampOnCorrosion-Erosion Monitor has already been installed successfully at several locations

topside and is now in the finishing stages of the subsea development. Non-invasiveness,

high repeatability and high coverage are amongst the selling points making it an excellent

candidate for subsea use. The technology is based on dispersion of ultrasonic guided wave

modes, and by using electromagnetism these waves can be transmitted through the pipe

wall without the sensor being in direct contact with the metallic surface. It is installed on the

outer pipe wall to produce real-time wall thickness information, not as a spot measurement,

but as a unique average path wall thickness.

IntroductionCorrosion and erosion in subsea installations can be detected by several methods. Pigable

pipelines are normally inspected at regular intervals and tracking of pipeline integrity isgenerally not a big problem. Some unpiggable pipelines can be inspected using cable-

operated tools, but such inspections are expensive and may require a shutdown of

production. However, subsea production templates, flow jumpers, manifolds and flowlines

can only be inspected by pre-installation of corrosion/erosion sensors or by use of ROV-

operated sensors. Current pre-installed sensor systems for monitoring pipeline integrity

have proven to be of limited value to the operators and ROV-operated sensors only provide

indicative and unreliable readings. A major challenge is that hot spots, i.e., areas particularly

susceptible to erosion/corrosion, are often detected after the template has been in operation

for a while. Accordingly, the ability to retrofit a corrosion-erosion monitor (CEM) on identified

hot spots subsea is crucial. Today, there are no such reliable systems commercially available.

The main objective of ClampOns subsea CEM project is to provide a reliable solution to

these problems.

ClampOns CEM utilises acoustic guided Lamb waves (AGLW), a technology that

gives an average wall thickness reading for larger pipe sections. The transducers are fixed

at pre-determined spots on the pipe to monitor the wall thickness loss in larger stretches

of the pipe, typically up to 2 m. The absence of any transducer movement or mechanical

motion adds a high degree of robustness to the instrument. As it is permanently installed

and needs no recalibration, it will be both more cost effective and reliable than other ROV-

controlled methods. A variety of system configurations are possible, ranging from stand-

alone monitoring stations with data logging to full real-time integration into existing data

infrastructure. This technique has been installed topside and is now ready to be taken

forward for subsea installation. Both a retrofitted ROV-solution and a solution where the

system is installed in advance of subsea deployment have been worked out.


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Working principleThe use of guided waves for defect detection in layered

structures and media have been investigated extensively, both

in academia and industry. However, effective use of these

complex Lamb waves for thickness assessment has not been

often exploited to provide a reliable quantitative estimate of wall

thickness loss in a structure. Guided wave inspection has the

potential to extend ultrasonic corrosion measurements in pipes

over long distances.1

The robust thickness assessment procedure involves

a comprehensive analysis of the phase and group velocity

dispersion characteristics of appropriate wave modes. The

choice of modes for the analysis constitutes an important

part of the design, as not all modes are equally sensitive to

variations in wall thickness. Also, complications arising owing

to mode overlapping and distortion have to be handled and

overcome. Long-term monitoring has to necessarily face the

fact that there might exist local thickness variations that are a

significant percentage of the average wall thickness, and mostguided wave modes do not display the robustness required

to smoothly integrate these changes into a quantitative (rather

than qualitative) thickness assessment. The CEM system

algorithm incorporates the use of modes (constant group

velocity or CGV modes) that provide maximum sensitivity to

changes in wall thickness within the constraints imposed by

the necessary robustness which the technique needs. In other

words, the presence of highly localised damage and defects will

be quantitatively incorporated into a robust average thickness

measurement, with the use of an effective spectral and temporal

dispersive analysis of the generated and received waveform.

Figure 1 shows the phase and group velocity dispersion curves

for Lamb waves in steel. The fundamental flexural mode A0

is especially well suited for wall thickness measurement in the

vicinity of its constant group velocity (CGV) point, as marked

in the figure. The position of this peak is, as we can see from

the figure, decided by the frequency x thickness product,

meaning the inspection frequency will be dependent on the wall

thickness of the inspected pipe. This CGV measurement is a

relative measurement, meaning that the system needs an initial

thickness value (measured during/prior to system installation),

which it uses as a baseline reading, and calculates changes in

average thickness from this initial value.

System operationThe CEM system consists of up to eight transducers and

an electronics unit that handles all signal acquisition and

processing. Two and two transducers operate consecutive

in a pitch catch mode of operation giving the average wall

thickness of the area between the transducers, which can be

up to 2 m in length. By choosing the transducer positions with

care, normally unavailable areas can be monitored, e.g. buried

parts of a pipeline. Requirements imposed by important factors

such as mode separation and spurious arrivals place limits on

the maximum and minimum distance between transducers.

These limits are functions of pipe thickness and diameter, and

need to be decided for each installation in order to be ableto maximise coverage area. In most cases, the transducers

will be placed on two rings around the pipe, as illustrated in

Figure 1.Normalised phase and group velocity curves forthe first acoustic guided Lamb waves.

Figure 2.An example of a transducer set up with six sensors.Usually the transducers are installed on two rings, and theareas between the transducers are monitored. The red areaindicates the size of the area monitored by one transducerpair.

Figure 3.Pre-installed solution with transducers mountedunderneath the coating.

MarchReprinted from World Pipelines


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Figure 2, and set up to monitor the area between the rings and,

if possible, the area along the ring. Because of wave diffraction,

the covered area stretches beyond the physical dimension of

the transducers, also indicated in Figure 2. Clearly, with the

transducers being permanently installed, the coverage area of

the system will be a certain fraction of the area over which the

system is deployed. Typically, this

fraction will be greater than 0.65, or

65%, and can reach almost 90%.

Wall thickness trends are

generated automatically and can

be observed in real-time on a

computer running ClampOn CEM

software, or logged internally in

a data logger. The topside of the

instrument is usually hard-wired to

a power supply and a computer

in a safe area, but it has also been

delivered with a battery pack andan industrial computer with internal

logging. At many subsea locations

it may not be applicable or possible

to hard-wire the instrument, and the

only option will be to use a battery

pack. ClampOn have developed a

battery powered solution for subsea

use with a battery design time of

five years. Replacement of the

battery pack can be done with an

ROV. Data will be saved to internal

memory and can be retrieved by an

acoustic modem, which allows for

two way communication. The system is designed to be easily

scalable for 8 24 in. pipe dimensions. Inherent limitations

imposed by transducer sensitivity limits, coupled with the CEM

ultrasonic technique, indicates that the system will perform

as expected on thicknesses varying from 5 - 35 mm, giving a

sensitivity of better than 1% of the wall thickness.

Subsea developmentEven though the AGLW technique used by ClampOns CEM

system makes it a good candidate for subsea use, there will

always be uncertainties when adapting a proven topside

solution to the subsea environment. As this is an instrument

based on ultrasound, we also have to take into account

changes in the acoustic premises, e.g. much better acoustic

coupling between pipe wall and water than pipe wall and air.

You will also have to face the fact that there will be limited

access to the instrument after installation and you will not have

the same opportunities when it comes to troubleshooting and

repair as when first installing an instrument topside. With this

in mind, many of the solutions selected for the subsea CEM

are inspired by other well-proven ClampOn products. The

electronics are based on the solution used for the topside

CEM version and the housing design are based on ClampOns

existing Deepwater model, rated to more than 3000 m.For existing non-piggable pipelines, only ultrasonic spot

measurement techniques are capable of ROV-installation.

Based on our knowledge and experience, the main reason for

the problems encountered with these retrofitted systems is the

piezoelectric transducers themselves. Such transducers are

highly affected by temperature, ageing effects and the acoustic

coupling between the ultrasonic sensor and the pipe wall. Lack

of long-term stability is the key issue. In order to maintain the

repeatability of the system, frequent

re-calibrations are needed. This is

not suitable for long term subsea

monitoring. In order to overcome

this stability issue, ClampOn has

developed electromagnetic acoustic

transducers (EMATs). EMATs allow

ultrasonic waves to be created and

picked up without being acoustically

coupled to the pipe wall, making

this a very stable solution that will

not change over time. The EMATs

have been designed with increasedsensitivity to make them less

vulnerable to installation deviation,

and to make it possible to place them

on the outside of the pipe coating.

The existing surface coating on the

pipe can remain in place and does

not have to be modified in any way.

ConclusionAs mentioned earlier, two different

solutions have been taken forward;

one ROV-installable and one pre-

installable solution. There will be

no difference in the way they operate and they will give the

same results when it comes to reliability and repeatability. The

pre-installable will have have the transducers fixed by more or

less the same mechanism as topside, and only the electronics

will be made ROV-retrievable. All transducers can be put

underneath the coating for better protection if desireable, as

indicated in Figure 3. Depending on the system set-up, the ROV

solution can consist of a main clamp containing the electronics

and one or two separate transducers clamps. The transducers

are based on a wet-mateable design and only need a clean

spot/location before being installe on a pipe structure. This is

achieved by running a cleaning tool to remove any fouling. The

ROV-installable system is flexible and installation both in bends

and straight sections are possible. A sketch of the ROV-clamp

for straight sections with electronic canisters and transducer

clamps are shown in Figure 4. As a part of the development

effort, it has been imperative to design and conduct

representative experimental work to verify design assumptions,

and to test the functionality of the ROV installable clamp, data

acquisition and acoustic properties. This development project is

now in its final stages and will shortly be deployed subsea for a

field trial.

References1. INSTANES, Geir; KRISTIANSEN, Sindre Halse; TOPPE, Mads and NAGY, Peter B.,The use of non-intrusive ultrasonic intelligent sensors for corrosion and erosion

monitoring, NACE 2010.

Figure 4.The ROV solution for ClampOns subseaCEM. The main clamp with the electronics chamberand a battery pack is in the middle and there isone transducer clamp on each side.

MarchReprinted from World Pipelines

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ClampOn Subsea CEM Paper World Pipeline March 2011