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Cranfield University

School of Energy, Environment and Agrifood

Offshore and Ocean Technology with Subsea Engineering

2015 - 2016

Offshore Inspection Assignment

Submitted By:

Nwakuna, Precious Chinonso

224***

Module Convenor: Dr. Mahmood Shafiee

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Topic:

Structural Condition

Monitoring of Subsea Risers

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ABSTRACT

The integrity of subsea drilling and production systems, including subsea risers, has

been a major concern to the offshore oil and gas operators as well as the

manufacturers. And to keep them operating in good and healthy condition even

beyond their expected end-of-life time with minimum risk to marine environment and

operation personnel has been a desire of all.

Different strategies have been employed and approved by different regulatory

agencies to ensuring healthy operations of subsea risers because of their exposure to

different harsh conditions, and condition monitoring is one of the approved approaches

under DNV-RP-F206, April 2008 (DET NORSKE VERITAS, 2008).

This paper looked at the need, practices and applications of structural condition

monitoring of subsea risers including the techniques involved. Structural condition

monitoring approach has been a useful tool in ensuring availability and reliability of

subsea risers from drilling through production life of an offshore oil and gas field for

maximum return on investment; it ensures proactive preventive maintenance.

INTRODUCTION

Structural condition monitoring is a process of observing and examining the health

conditions of a structure without affecting its operation, to determine when a

maintenance is necessary on the structure and cost effective. It is a proactive way of

detecting cause of failure before failure. Having a healthy subsea structure would be

a difficult one if its condition is not known to the owner or operator of the asset. The

key to a rewarding structural condition monitoring includes knowing (SKF Limited,

2011):

1. What to observe

2. Having the right tool for the observation

3. Knowing how to interpret it

4. When to act on any information gotten from the observation

Subsea risers are exceedingly important component of subsea hydrocarbon drilling

and production systems; they are used to convey hydrocarbons fluids from seabed to

surface facilities as well as operational fluids to seafloor. Regrettably, riser have high

risks of damage and failure due to their exposure to harsh environmental conditions;

like tide, vortex induced vibration (VIV), high tension, current, and extreme

temperature and pressure (Cheon-Hong, Hyung-Woo, Jong-Su, & Yeu, 2013). Thus,

there are needs to steadily monitor their conditions and keep records of the monitoring.

Successfully implementing structural condition monitoring in a subsea riser will

improve overall system performance despite the harsh conditions it is exposed to.

There are two kinds of subsea risers; risers used during drilling operations (subsea

drilling risers) and risers used for hydrocarbon production (subsea production risers).

And among production risers, there are flexible, rigid or bundle subsea risers.

Operational environment influences operators’ choice. Figure 1 below shows a sketch

of rigid subsea production riser in a subsea production system.

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Figure 1: Subsea Production System (Charles R. Orbell, Christian Leuchtenberg, Craig W. Godfrey, 2012)

OBJECTIVES OF STRUCTURAL CONDITION MONITORING ON SUBSEA RISERS

The main objective of structural condition monitoring of subsea risers is basically to

prevent potential risks from occurring on risers while in operation by scheduling

preventive maintenance when necessary. And this ensures excellent availability as

well as the reliability of the system in the face of harsh environmental conditions.

Availability is given as,

𝑨 = 𝑴𝑻𝑻𝑭

𝑴𝑻𝑻𝑭+𝑴𝑻𝑻𝑹 - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - -1

Where; 𝐴 = 𝑎𝑣𝑎𝑖𝑙𝑖𝑎𝑏𝑖𝑙𝑖𝑡𝑦

𝑴𝑻𝑻𝑭 = 𝑚𝑒𝑎𝑛 𝑡𝑖𝑚𝑒 𝑡𝑜 𝑓𝑎𝑖𝑙𝑢𝑟𝑒

𝑴𝑻𝑻𝑹 = 𝑚𝑒𝑎𝑛 𝑡𝑖𝑚𝑒 𝑡𝑜 𝑟𝑒𝑝𝑎𝑖𝑟

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For instance; a riser system with MTTF and MTTR of 364days and 7days (due to time

of getting spare and technicians for repair) respectively, will have availability of:

𝑨 = 𝟑𝟔𝟒

𝟑𝟔𝟒 + 𝟕

= 𝟑𝟔𝟒

𝟑𝟕𝟏= 𝟎. 𝟗𝟖𝟏𝟏

In this scenario if SCM is implemented, spare(s) needed for repair would be ordered

before failure, and technicians also would be informed beforehand, and this in no

doubt will reduce 𝑴𝑻𝑻𝑹 (for demonstration sake I would assume 𝑴𝑻𝑻𝑹𝑺𝑪𝑴 to be

3days). On the other hand, 𝑴𝑻𝑻𝑭𝑺𝑪𝑴 will significantly increase due to condition

monitoring, also for demonstration sake I would assume 𝑀𝑇𝑇𝐹𝑆𝐶𝑀 to have increased

by one-half of 364days.

𝟑𝟔𝟒 + (𝟏

𝟐 𝒐𝒇 𝟑𝟔𝟒) = 𝟓𝟒𝟔

Therefore;

𝑨𝑺𝑪𝑴 = 𝟓𝟒𝟔

𝟓𝟒𝟔 + 𝟑= 𝟎. 𝟗𝟗𝟒𝟓

Comparing availability when SCM is employed (𝐴𝑆𝐶𝑀) and availability with no SCM,

the availability, 𝑨, appreciated by 0.0134

FACTORS INFLUENCING SUBSEA RISER STRUCTURAL MONITORING

Integrity assessment of subsea risers has some contributing factors (Shafiee, 2015),

including:

1. Government Policy: failures of subsea structures (especially in the oil and gas

sector) are usually catastrophic and has caused the government to endorse

governing bodies to oversee the structures from design life through end of life.

2. Cost of Investment: investor’s desire their assets last the expected lifetime

(and even beyond) at optimal efficiency, to ensure determined return on

investment.

3. Cost of Accident: failure of subsea riser either during drilling or production

stage, more often than not result to loss of personnel or marine lives or both.

Including huge economic lost due to possible total loss of production, or

payment of fine and compensations.

POTENTIAL RISKS ASSOCIATED WITH SUBSEA RISERS

As stated before, subsea riser conveys hydrocarbon fluids from the sea bed to the top

side (production facility), hence it is found standing vertical or at an angle. This

exposes them well to more harsh conditions. The factors or conditions that are

monitored are usually:

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1. Corrosion: Corrosion is a process that leads to material deterioration due to

its interaction with the host community. It can occur both in the internal and

external surfaces of drilling and production subsea risers. And if not monitored

leads to system failure that could be catastrophic. Figure 2 below shows effect

of corrosion on subsea riser.

Figure 2: Subsea Riser Corrosion (Ecosse Global UK, 2013)

2. Sand Erosion: Usually, sand erosion are found at a point where there is

change in direction or diameter in production risers due to solids ingestion and

change in flowrate, and this is characterised by smooth surface with a sand

mound pattern This occurs on the internal surfaces of the riser (GE

Measurement & Control, 2015).

3. Strain and Cracks: Subsea risers are constantly faced with vortex induced

vibration due to the constant movement of the sea, and this causes a lot of

strain on the structure. Also, subsea riser are anchored at the seafloor as well

as at the top side which create strains at different point on structure of the riser.

And any form of strain can lead to fatigue crack which could result to a

catastrophic failure if not observed and controlled.

4. Marine Fouling: Subsea riser fouling refers to the growth of marine organisms

on structure of a subsea riser. It is also known as marine growth. They hardly

grow evenly on structure of any offshore structure but tend to agglomerate at a

particular spot. Due to their clustering behave they influence strain on a subsea

structure as a result of vortex induced vibration effect, this is because of

increase in surface area and unevenly distributed weight. In a non-vertical riser,

their agglomeration exert a concentrated force on the riser increasing tension.

In Figure 3 is a display of marine fouling on risers.

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Figure 3: Riser Marine Fouling (FoundOcean, 2011)

BENEFITS OF STRUCTURAL CONDITION MONITORING OF A SUBSEA RISER

Benefits of structural condition monitoring can never be over emphasised. And they

includes:

1. Safety of personnel on offshore platform

2. Safety of marine lives

3. Continuous production with maximum availability and reliability

4. It serves as a tool to implementing effective preventive maintenance

5. Extended useful life of the riser

6. It saves cost of unnecessary maintenance

7. Maximum return on investment

STRUCTURAL CONDITION MONITORING METHODOLOGY

Though there are two types of structural condition monitoring, the principle involve is

basically the same. The procedures used to carry out structural condition monitoring

of a subsea riser are shown schematically in Figure 4.

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Figure 4: Structural condition monitoring procedure

Firstly, the condition of the subsea riser is observed using a measuring instrument via a sensor. Through the sensor(s) data or a

datum is acquired (the data is stored if it is not a real-time monitoring), and sent to be processed so it could be analysed. This comes

in two folds; on one arm the information given is used to determine the present health condition of the riser (diagnosis) and on the

other arm the information is used to determine the deterioration trend and predicts future failure of the riser (prognosis). Usually, the

diagnosis and prognosis are performed using computer algorithms (Bencomo, 2012 ), though computer algorithms involved are not

discaused in this work. Finally, a maintenance action is recommended based on the outcome of the analysis. The recommendation

could be manually by an expert or automated using some computer models.

MEASUREMENT

DATA

ACQUISITION

AND

PROCESSING

DIAGNOSIS

PROGNOSIS

ADVISORY

GENERATION

SYSTEM DISPLAY

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TYPES OF STRUCTURAL CONDITION MONITORING OF A SUBSEA RISER

There are two types of structural condition monitoring (SCM), and both can be applied

to subsea riser risers – continuous structural condition monitoring and periodic

structural condition monitoring.

1. Continuous Structural Condition Monitoring: Continuous SCM steadily

monitors a subsea riser and warns whenever it detects any situation that could

lead to failure (Ferreira, Almedia, & Cavante, 2009). Data obtained are

processed using computer models on real time bases, and following the

outcome of the analysis, preventive maintenance decisions are taken.

Basically, continuous SCM are achieved by installation of different sensors at

points on the riser. The sensors are usually installed at the hotspots (that is,

areas highly susceptible to damage). Figure 5 below shows a flexible subsea

risers with sensors.

Figure 5: Subsea riser with sensors at hotspots (OMNISENS, 2013)

Most manufacturers integrate their sensors unto a clamping systems, this is to enable

easy installation. The installation could be carried out by a diver, using remotely

operated vehicle (ROV) or using rope access team (RAT). Comparing the three

deployment techniques; ROV can go to any depth which is limitation of using diving

techniques but divers can take manipulative tasks. On the other hand, RAT and diving

deployment techniques is effectively used at the splash zones (any section of subsea

structures on continuous wetting and drying that are highly susceptible to aggressive

external corrosion due to extreme environmental conditions). For the purpose of

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subsea riser continuous structural monitoring, different equipment manufacturers have

manufactured different sensor monitoring systems using different technologies. For

example; there is a trident monitoring system (Astro Technology) and a guided wave

ultrasonic system (from Subsea Integrity Group). According to Astro Technology, the

trident monitoring system was developed to effectively identify imminent failure due to

strain from vibration as a result of vortex induced vibration (VIV) and mechanical

stresses at the touchdown area of the subsea riser. It can monitor a riser even at the

depth of up to 1200ft, it uses a fibre optic sensing method which gives the system its

heightened reliability and accuracy, multiplexing capability, immunity to

electromagnetic interference. This is because signals can travel a far distance through

an optic fibre with no noticeable damping or influence of external signals. The Guided

wave ultrasonic system can be bounded on a structure of a riser prior to installation

(that is, permanently installed monitoring system – PIMS) or wrapped on already

installed subsea riser at the splash zone by rope access teams. It is capable of

monitoring corrosion and fatigue cracks beneath a protective wrapping (Astro

Technology, 2013). Thus, there is no need of removing existing protective coatings

before installation. Figure 6 below shows a rope access team deploying guided wave

ultrasonic monitoring system. Two major limitations of continuous structural condition

monitoring are: It is high CAPEX, and Noise could produce imprecise diagnosis.

Figure 6: RAT deploying GWU system (Booth, Pisarski, Nageswaran, & Mudge, 2011)

2. Periodic Structural Condition Monitoring: This involves monitoring or

inspecting the health condition of subsea risers at predetermined intervals

(Ferreira, Almedia, & Cavante, 2009). No permanent monitoring equipment is

installed. It is mostly used because it is more cost effective and capable of

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providing filtered data for prognosis and diagnosis with high accuracy.

However, there is possibility of failure occurring before the next monitoring

which defeats the objective of SCM. Periodic SCM could be carried out using

divers, ROV, Autonomous underwater vehicle (AUV) or a rope access team

deployment techniques. ROV and AUV can inspect at any depth where riser

installation is practicable, and can work at extended hours which are not

possible with divers. The main difference between ROV and AUV is that the

former is manned while the latter is unmanned. On the other hand, only RAT

and divers are effective in monitoring splash zones.

TECHNIQUES USED IN STRUCTURAL CONDITION MONITORING OF A SUBSEA

RISER

Structural condition monitoring of any structure is achieved by employing non-

destructive evaluation or testing (NDE or NDT). In this work, five techniques are

considered as listed below.

1. Acoustic Emission Technique: Acoustic emission testing is a non-destructive

way that can be used to obtain information regarding structure of a subsea riser by

detecting transient elastic waves produced due to sudden change in the condition of

the structure of the riser in response to external stimulus. This transient wave produced

could be as a result of cracks, melting, slip and dislocation movement or material phase

transformation. Thus, AE technique is capable of detecting crack initiation and crack

propagation. Using acoustic emission testing for subsea riser condition monitoring is

achieved simply by attaching a sensor on structure of the riser to monitor the riser and

picks up any acoustic wave that would be generated (NDT Resource Centre, 2001). Any elastic wave signal detected by the sensor is sent to the top sides via a

communication line for processing and analysis with some computer algorithm. With

this technique, strain and stresses due to vortex induced vibration would also be

effectively monitored and its possible failure prevented.

2. Visual Technique: This is also known as visual inspection, and it is the most

basic of all NDE techniques. Even when other structural condition monitoring

techniques is to be used, the inspector consciously or unconsciously carryout

visual inspection or testing first. It is done simply by looking at the subsea

structure to see if any surface imperfection or deterioration exist, it could be

corrosion, marine fouling or surface cracking due to fatigue. Visual inspection

can be enhanced using image magnifying devices.

3. Ultrasonic Technique: Ultrasonic testing is one of the commonly used non-

destructive evaluation (NDE) techniques which can effectively be used for

subsea riser structural condition monitoring, an example of ultrasonic detection

is shown in Figure 7. It utilises high frequency sound wave to detect flaws and

changes in material properties of the test object. The flaw detection is achieved

by causing sound wave of high frequency to travel through the structure of the

subsea riser, any flaw (example cracking) within the material causes echo

which the receiver picks before receiving echo from back surface of the material

and alerts the person(s) carrying out the monitoring of a flaw (David, 2015),

ultrasonic testing can be used for crack sizing. The probability of detecting a

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flaw is a function of wavelength of the sound wave used. It is certain to detect

a flaw if the length of the crack is larger than one half of its wavelength.

Figure 7: Ultrasonic Crack Detection (NDT Centre: The Hashemite University)

4. Radiographic Technique: This technique utilises the penetrating power of

some radioactive elements, usually gamma rays and X-rays, to obtain a

structural image of the riser. The image is examined and analysed and then

compared to the original structure, and any deviation calls for more action (this

technique has extensively been used in the medical line to detect internal

fractures). It involves using radioactive emitter and directing the radiation on the

material to be examined for defect(s) (NDT Resource Centre, 2001). Because

of its health risks to personnel and marine lives its use in the offshore

environments is limited though it has high tendency of flaws detection.

5. Eddy Current Technique: This is one of the methods in which the principle of

electromagnetism is utilised to conducting monitoring of subsea structures. In

Eddy current testing, a changing magnetic field is used to generate electrical

energy which cause current to flow in the test material (Subsea riser in this

case). Its operation is based on the principle that material defects interrupts

flow of current (NDT Resource Centre, 2001).This technique has capability of

detecting flaws through a protective coating (David, 2015).

DEPLOYMENT TECHNIQUES USED IN SCM OF SUBSEA RISERS

Four major deployment techniques used in structural condition monitoring (whether

continuous or periodic) employed by different asset integrity management companies

are as listed below. And, Table 1 below shows brief comparison of the different

techniques.

1. Rope Access Team (RAT): RAT technique is used to carryout visual and non-

visual monitoring and maintenance on subsea risers periodically, as well as

sensor installations for continuous monitoring. A member of the team is usually

abseiled from the platform to examine the condition of the structure. This

method is highly effective for monitoring from below the platform up to splash

zones because a member of RAT can quickly access and withdraw from the

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splash zone if the weather or sea condition is becoming unsafe which is not the

same if diving technique is used.

2. Diving: Divers can visually inspect a subsea riser, or use some NDE or NDT

equipment when needed to examine the condition of a riser (Herraez, Carlos,

2015).The measure limitations about diving is the maximum depth and time

they can work under the water because down the seabed less the amount of

oxygen that can support life despite a very high pressure. According to U.S.

Bureau of Reclamation, open circuit air SCUBA (self-contained underwater

breathing apparatus) diving shall not be conducted at depths greater than

100FSW - feet of seawater (Harris, 2006). In addition, it is mandatory for an

employer to provide health insurance before any diving operation. However, it

has high reliability for periodic monitoring if carried out within lay down

regulations.

3. Remotely Operated Vehicle (ROV): ROV is an underwater robot that can be

used for condition monitoring and inspection, maintenance and repair of subsea

risers, as well as installation of other subsea components. It is lowered into the

water and operated from the control room at the top side, the ROV pilot

observes and monitors the operation on a computer screen. ROV can monitor

subsea asset at any water depth and length of time (which is not practicable

with diving). It is not a continuous structural monitoring approach, but it gives a

real-time information about the riser while in operation. Also, it can be used to

install devices used for continuous monitoring. Figure 8 is a picture of an ROV

being lowered into the water from platform via a crane.

Figure 8: ROV deployment (NOAA, 2012)

Use of ROV has a lot of advantages and among them are:

It excellently safe, no human being is subjected to extreme harsh

conditions.

It is less expensive compare to diving.

It can work underwater for a long period with same efficiency.

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4. Autonomous Underwater Vehicle (AUV): AUV is similar to ROV only that

AUV is not manned. It is lunched and recovered from a marine vessel after each

operation. It has a sensor for scanning the structures which could be a sonar

or radiographic scanning. It stores the data obtained which is retrieve at the end

of the operation. Recently, AUV that are capable of communicating via a WIFI

for real time data delivery has been designed (Herraez, Carlos, 2015).

Table 1: SCM deployment techniques

Diving RAT ROV AUV

Cost Very high Low High High

Attention to weather High Very high Very low Very low

Risk to personnel life Very high High No risk No risk

water depth limitation Yes Not Applicable No+ No++

Splash zones Yes Yes No No

Need of control cord No Yes Yes No

+ ROV can go to any depth depending on the length of the control cord ++ AUV can go to any distance depending on the capacity of its battery

CONCLUSION

Structural condition monitoring (SCM) is a proven approach which can be used to

ensure excellent structural integrity of subsea risers. Installation of sensors and

gauges with full redundancy to continuously monitor the condition of subsea risers will

support excellent availability and reliability (the desire of every operator). And this will

help asset managers overcome the challenge of trying to know the appropriate time

to monitor subsea risers. Most times, different sensors are used to simultaneously

monitor the risers. Also, the effect of vortex induced vibration (VIV) can be minimised

by integrating VIV-suppressor system on subsea risers before installation.

Furthermore, the data and information obtained from SCM of subsea risers have been

helpful fine-tuning the design of subsea riser apart from suggesting the future

behaviour of the riser and when maintenance would be due. Finally, the benefit of

SCM of any critical component or system of subsea oil and gas production (like subsea

riser) can never be compared to its overall benefit.

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