It is of paramount importance to ensure the structural integrity of offshore facilities. Unfortunately, this does not come easily. Deep water offshore developments are hugely complex and are often exposed to extreme environmental forces. Existing structures are subjected to ever-changing loading as well as severe ocean and environmental conditions. These pose engineering challenges and together with an intense focus on health, safety and environmental performance are putting ever greater pressure on operators to improve their structural integrity management capabilities. Dr Pei An, Consultant for Structural Monitoring, reports for Sonardyne.

Although the oil and gas industry has long appreciated the critical nature of their structural assets, it has been slow to embrace the concept of continuous structural asset monitoring subsea. In the last few years, structural condition monitoring has increasingly been recognised as a vital ingredient in integrity management, providing vital real-time in-situ data about the behaviour and performance of the structures from which their integrity can be inferred. The take up of condition monitoring has accelerated following several notable incidents, driving operators and contractors to seek a better understanding of risk.

Structural integrity monitoring has become increasingly  accepted after years of technology innovations in the field and successful real world deployments which have  demonstrated benefits to end users.

What needs asset monitoring?

This article will address four broad categories of subsea structural assets with integrity concerns:

  • Drilling risers and conductors
  • Production risers
  • Mooring lines and platforms
  • Subsea pipelines and infrastructure

Drilling risers can be subjected to accelerated fatigue damage when strong ocean currents are present, the current can cause a riser to vibrate laterally at its own natural frequency due to vortex shedding, an effect known as Vortex Induced Vibration (VIV). Once VIV has locked-in for a riser, the amplitude of vibration can increase dramatically and rate of fatigue damage accumulation increases substantially. Hence, the probability of a riser failure is increased and needs to be understood. Risers are connected to wellheads and conductors via the BOP and LMRP, and the vibration of risers will transfer to conductors so that they also bend from side to side experiencing accelerated fatigue damage. To compound this, the latest generation of BOPs and LMRPs are significantly larger in mass and size compared with their earlier counterparts. As such, the induced motion from the riser to the conductor may be amplified. It is obvious that the risk of fatigue damage to the conductor must be evaluated and carefully managed.

Production risers have a typical design life of 30+ years and are subjected to similar loadings to drilling risers. Given the long service life, it is once again important to understand  he accumulation of fatigue damage. One example is the hang-off and touch-down regions of Steel Catenary Risers (SCR) which experience the highest cyclic stresses due to dynamic bending of the riser. Another example is free standing hybrid risers. An air-filled and submerged buoyancy tank generates up-thrust tension to pull the riser upright. As a key indicator, this tension should be monitored continuously to detect any decrease in the tension due to buoyancy tank leakage. Tension monitoring inferring the integrity of buoyancy tanks.

In recent years, there has been a notable rise in the reporting of mooring line failures on moored production platforms such as FPSOs. Continual subsea monitoring can be used to analyse inclination and line tension and also immediately alert an operator should a failure occur.

Throughout their typical design life of 30+ years, production risers must be monitored for signs of fatigue damage. Hang-off and touchdown regions of Steel Catenary Risers (SCR) experience high levels of cyclic stress so are both particularly susceptible. This enables damaging operating conditions to be detected at an early stage and preventative action swiftly taken. 

For FPSOs, mooring line integrity is an area of increasing concern as more and more failures have been reported in the last few years. Subsea systems can monitor mooring lines continuously, detecting and reporting failures promptly to the operator, thereby allowing safe management of the asset. Such systems can also present mooring line inclination and the line tension in real-time in the marine control room.

There are many causes of concerns for operators of subsea pipelines, such as vibration at spans, spools, jumpers and sleeper regions due to VIV, Flow Induced Vibration (FIV), and slugging in multiphase flow all of which can cause accelerated damage. The sideways ‘walking’ of a subsea pipe due to slugging or thermal cycling and movement of subsea PipeLine End Terminations (PLET) also indicate pipeline integrity concerns. Pipeline wall thickness reduction due to corrosion and erosion is also of concern. In-situ monitoring can provide the data required for evaluating and mitigating these integrity issues.

Benefits of Asset monitoring

The purpose of monitoring is to ensure the safe operation of subsea assets. This is based on developing an understanding of how subsea structures and assets are responding to loadings and enabling faults to be detected at an early stage. In-situ real-time data measured by the asset monitoring systems allows structures to be analysed so as to determine if their integrity is jeopardised. This in turn helps to decide on the potential interventions which could be performed. Structural monitoring can also allow optimisation of production efficiency, savings on periodic inspection and facilitate proactive maintenance regimes, whilst extending service life safely. Major benefits of structure monitoring include:

  • Enhanced operational safety
  • Proactive integrity management
  • Enhanced operational efficiency
  • Asset life extension
  • Real-world design verification
  • Future improvements of structural designs

SMART TransponderSonardyne can provide the industry with a best-in-class structural monitoring tool. This is enabled by its portfolio of technologies including subsea communications, positioning and extensive experience in marine instrumentation. We have in excess of three decades of experience in supporting our customers to implement offshore structural monitoring systems providing reliable, field-proven, practical and useful structural monitoring solutions.

The Subsea Monitoring, Analysis and Reporting Transponder (SMART) is the latest product development from Sonardyne. The core of the system is a new Advanced Data Acquisition and Processing System (ADAPS) which is built around a highly capable micro-processor with the latest peripheral electronics. This battery-powered device brings together powerful subsea data processing capability, low power electronics, long duration logging, versatile sensor input, and acoustic/ optical telemetry into a single easy-to-deploy subsea instrument.

The SMART unit works seamlessly with existing Sonardyne technologies, including 6G acoustic telemetry systems, making use of associated subsea housings, batteries, low power electronics and sensors. In doing so, SMART leverages existing field-proven Sonardyne designs and technologies allowing low risk operational deployment.

Where higher data transfer rates are required, SMART can be connected with other wireless communication devices such as Sonardyne’s high speed optical modem, BlueComm. Here, data transfer rates from 5 to 500 Mbps can be achieved over distances up to 100 metres.

 SMART has multiple digital and analogue inputs which can be configured to connect a variety of sensors. Internal sensors are available for motion measurements including accelerometers, angular rate sensors and inclinometers, along with standard and high precision pressure and temperature sensors. The standard internal accelerometers and angular rate sensors exhibit a typical RMS noise level of 0.2 mg and 0.005 deg/s at a sampling frequency of 10 Hz, respectively. External sensor options include external force sensors such as strain sensors and shackle pins. Any external sensors are connected to the SMART unit via highly reliable external subsea connectors.

SMART softwareThe unit is fully programmable to set data logging frequencies, sample periods and sleep periods. During sleep, SMART has ultra-low power consumption to preserve battery power. Logged data acquired from the sensors is saved into two independent memory stores, fulfilling user requirements for redundancy. The on-board data processor can run sophisticated user specified algorithms such as spectrum analysis as well as simple data analyses such as Min/Max/Mean statistics, thresholding for alarms and critical event reporting. This allows data reduction subsea, creating information whilst retaining the raw data. This information can then be reliably transferred acoustically to the topside in a near real-time to enable true structural monitoring applications.