Application Note

1

Sunlight - temperature © Stéphane © Bommert

Bend stiffener - strain and fatigue

Shock/abrasion/ clashing - temperature Buoyancy modules - temperature © Øyvind Hagen

Weight - fatigue

Touch-down point - Geohazard - landslide, abrasion, friction - seismic - strain/ temperature temperature © Michael Grimes Subsea umbilicals and risers benefi t from continuous monitoring of fatigue and condition. SUBSEA UMBILICAL AND RISER INTEGRITY MONITORING Topside controlled asset integrity monitoring using optical fi bers for fully distributed strain and temperature sensing. > Temperature monitoring along the length of the asset provides not only incipient leak detection and location, but also continuous condition monitoring. > Static and dynamic strain monitoring provides fatigue and elongation detection for predictive maintenance/replacement decisions.

Why monitor subsea umbilicals and risers using fiber optic distributed sensing? Monitoring with distributed fi ber optic sensing provides detected. Details of the temperature or strain event’s size continuous real time information on temperature and/ and location are logged or sent as an alarm to the asset’s or strain events along the length of the asset, helping to control system, via SCADA, e-mail or SMS. Strain profi les detect incipient failures and thus avoid catastrophic loss. It are available to compute fatigue accumulation at every compliments in-line inspections. point along the structure, including known weak links such as under bend stiffeners. Using fi ber optic fi bers integrated into the umbilical or riser, Omnisens systems monitor strain and/or temperature along The frequency and type of information transmitted can be the length of that asset, continuously and in real time. Small altered remotely and post-installation. changes in temperature (± 0.1°C) or strain (0.002%) are 2 Umbilicals Umbilicals are increasing in length, weight, complexity and power transmission ability in response to deep- water production and subsea processing demand. Fiber optic sensing cable They are subject to: • Bending during reeling, transport, installation and operation due to internal and external forces • Damage from external forces (crushing or perforation) Conductor or fl uid line • Tension forces from the umbilical weight

• Material degradation in response to the hostile Theoretical sensing cable positions in an umbilical cable. This confi guration environment would measure bend and cyclical strain (both elongation and compression are measured by the Omnisens system.) • Severe load conditions Fiber optic distributed sensing offers valuable infor- Design validation mation to manufacturers from design to installation and to operators throughout the service life of the By providing temperature and strain information during umbilical cable. qualifi cation and performance testing, Omnisens systems help validate the umbilical design, delivering actual rather than ‘sum of the parts’ information on Bending strain sensing strain and temperature, guiding the design of the fi nal cable could be added product. here

Tensile strain and temperature sensing cable

Deepwater power umbilical showing detail of sensor bundle, see Reference (1).

Sensor integration into the umbilical The fi ber optic sensor, correctly integrated during manufacture, retains its accuracy, speed of response Qualifi cation testing of an instrumented umbilical and sensitivity not only during reeling and installation but also throughout the operational life of the umbilical. Monitoring umbilical cables in service Within the sensing cable the temperature sensors For umbilicals in service, continuous real-time condition are communication-grade single mode fi bers, while monitoring provides early detection of: the strain sensing cable is specially designed with a • Strain - which could lead to creep, cracks or tightly embedded, hermetically-sealed fi ber, delivering buckling, a leak, and/or a fast, linear, repeatable response. The sensing cables have a uniform diameter and are jacketed in a way to • Temperature events - hot or cold spots which may ensure a consistent friction fi t within the structure. indicate a leak, an insulation breakdown, friction between components or water ingress. Often a single fi ber selected from an existing optical fi ber cable serves as a sensor, if none is available, an An integrated distributed temperature sensor not only optical fi ber sensor (standard telecom single mode monitors the heat generated by the electrical load fi ber), can be integrated for monitoring. resistance in the conductors but also the heat from 3 61 Despite stringent testing on large umbilical designs to

60 ensure their long and safe operation, it may be diffi cult to reconcile operating conditions with calculations and 59 models. Installing optical fi ber sensors in the umbilical provides continuous online monitoring of strain. The 58 operator has the means to record the magnitude of each

57 strain cycle as well as the position and radius of bends. Knowing the actual strain experienced by the umbilical 56 enables prompt and appropriate action to be taken.

55

54 Static strain (elongation)

53 In operation the effects of gravity, bending and creep The thermal model shows the reason for including the temperature sensing may cause the umbilical to lengthen. An Omnisens in the center of the power umbilical, see Reference (1). system provides a continuous monitor as to whether and where elongation is taking place. The optical fi ber sensor has an amount of elasticity which friction between the components and, depending on allows it to detect both elongation and compression. its position, emanating from fl uids transported in the Elongation gives rise to positive fi ber strain whereas umbilical. compression introduces negative fi ber strain. The fi ber strain (elongation/compression = L (length of the umbilical)/L given in % or microstrain (10 ). Dynamic strain Once in operation, strain develops due to the action of waves, currents, hurricanes, vortex induced vibration (VIV) and, depending on the attachment, from the movements of the vessel or platform.

Strain events can be detected and located all along the umbilical. Umbilical elongation.

Dynamic strain

Time Depth

Dynamic strain

Time Sensing fi ber fi Sensing Strain profi le (%) Showing the dynamic strain experienced by an umbilical at different depths. 3 4 Temperature Temperature events along the length of an umbilical can indicate a change in the condition of the asset (material breakdown, a friction point, excessive electrical load, overheating of fl uids, eg: hydraulic fl uid), or a leak. For example, to ensure the temperature of an umbilical cable remains below its design specifi cation (say 65°C) during operation, (eg: following the installation of more powerful pumps at the wellhead), an umbilical can be monitored using an Omnisens system to provide a profi le of the temperature at each point along its length. The effects of various structures, eg: bend stiffener and buoyancy modules, on the umbilical system’s temperature can also be seen. Even with a high attenuation (14 dB) and Friction at the touch-down point can result in in temperature increases if substantial back refl ection from a fi ber optic rotary there is a bend restrictor or decreases if the insulation is damaged. joint (FORJ), temperature measurement along the power umbilical provides vital information on the temperature behavior of the cable.

Temperature profi le of a power umbilical showing the areas of highest temperature.

Despite high attenuation and back reflection “from FORJ and other connectors, temperature measurements can be made along the length of the umbilical, providing valuable information on its behavior under under load. ” 5 ROV umbilicals Risers The power umbilicals of ROVs (remotely operated The many types and configurations of risers are all vehicles) are at risk when on the winch onshore or vital to production. Their useful life can be reduced onboard ship during operation, especially in direct by excessive temperature or strain. Risks to risers, sunlight. Longer cables required for investigating and largely similar to those for umbilicals, include: repairing deep offshore assets are prone to kinking • external damage: eg: clashing and snagging once they are played out. • fatigue Using the integrated optical fiber cables to monitor the temperature of the ROV cable, both on the winch • corrosion and when unwound, provides condition monitoring • blockage from wax and hydrate formation and allows the cable to be optimized. Hotspots are detected and located within minutes and an alarm Damage or corrosion can lead to a crack, breakdown is raised. Since the operating temperature all along or delamination of materials, resulting in possible the cable is known, the cable load can be managed seawater ingress or leaks. Risers are also at risk within its recommended limits. from blockage when the temperature falls below a certain level, depending on the composition of the ROV umbilicals may kink, resulting in a temperature well stream. During shut-downs and as wells near or strain event which will immediately trigger an alarm. depletion (increasing water cut, decreasing pressure), Since the system can be set to alarm at given strain or the risks increase. temperature levels, operations can be halted before the cable’s critical limits are reached. The Omnisens Monitoring temperature in real time within 0.1°C with system can be used to provide a health-check of the meter accuracy along the length of the riser provides power umbilical following kinking or other trauma. the operator with an early warning of developing cold or hot spots, before the failure becomes catastrophic, since riser integrity may affect the platform integrity.

Sensing cable position in a riser. Sensor position(s) in a riser bundle

Outer sheath Foam

Water injection Annulus

Central support Armor wires Temperature/strain sensing optical fiber cable is located following agreement with manufacturer. Production bundle

Optical fibers

Bending and elongation lead to fatigue accumulation, a precursor for armor wire breaks. Capable of measuring strain with 1 Hz acquisition rate, fast enough to detect low frequency vibration, Omnisens systems can monitor fatigue accumulation. Knowing where failures are most likely to occur, the operator can act preemptively to avoid further damage. 6 Omnisens Service • DITEST interrogator measures strain and temperature along the umbilical or riser. Systems with a range of performances designed for Thanks to its distance range the interrogator umbilical and riser monitoring are supported with can be housed a distances of more than application engineering, commissioning and service 30 km from the asset being monitored. contracts. • Optical switch provides extra channels so that several risers or umbilicals can be monitored from the same interrogator, optimizing space and weight on the platform. Application engineering • Monitoring software (server-hosted) takes DITEST data to provide alarm Specialists are available to ensure that the Omnisens functions. Modbus, OPC and other communication protocols are available. system responds to client requirements, via: • Sensing system design, including modeling, Commissioning and installation dedicated sensing cable design and qualifi cation testing • System confi guration

• Sensing cable integration procedures, testing • On-site acceptance testing monitoring)en-01 riser & umbilical AN-005(subsea REF and validation • Optimizing alarms and zones • Confi guration optimization, including system • Remote access set-up redundancy • Operator training. • Design of junction boxes, cabinets, power supplies and other accessories • Alarm and communication options • System tests and validation.

Surveys Omnisens systems are available for monitoring an Service contracts umbilical or riser during testing, transportation, or A range of services ensures continuous availability during operation, helping: of the monitoring system. These include: • troubleshoot faulty umbilicals • Rapid ‘change-out’ in case of equipment failure • survey the temperature or strain along an umbilical or damage or riser, over a limited time. • On-site service preventative maintenance visit • Refresher training and system re-confi guration • Data interpretation.

Applications Benefits Actual temperature and strain condition is available to verify design elements Design validation and prototypes. Monitoring temperature and/or strain continuously helps detect incipient Condition monitoring in service thermal bottlenecks, leaks, elongation and bending enabling action to be taken before disaster strikes.

Reference: (1) Dana Dutoit, Marc Nikles, Etienne Rochat, Pete Willemoes, Heith Little, “Distributed Fiber Optic Strain and Temperature Sensor for Subsea Umbilical.” Proceedings of the Twenty-second (2012) International Offshore and Polar Engineering Conference, Rhodes, Greece, June 17–22, 2012 DITEST is a trademark of Omnisens. Omnisens SA Switzerland T: +41 21 510 2121 [email protected] www.omnisens.com