Offshore pipelines are constructed by welding together straight pipe joints, and in an ideal world, the pipeline would stay completely straight, never moving. In reality, all pipelines contain bends. These may be manufactured bends to accommodate changes in route near platforms, gentle curves to follow the seabed profile or avoid obstacles, designed-in curvature to allow flexibility for thermal expansion, bends caused by operation or the environment (e.g. seabed movement).
In offshore pipelines these curves or bends that were not manufactured are known as areas of out-of-straightness. Out-of-straightness is tolerable as long as the causes are understood and engineering assessments show that there is no threat to pipeline integrity. However, they can be a real concern and lead to loss of serviceability by cross-sectional collapse and even failure if the bend becomes too tight and the bending strains exceed critical levels. The environmental damage, disruption and cost can then be substantial.
This article will provide an understanding of why pipeline out-of-straightness occurs and how a pipeline operator may identify, measure and monitor out-of-straightness in operational pipelines as part of their integrity management strategy.
Understand the Crooked
Offshore pipelines are usually subjected to very detailed engineering analyses during design and installation due to the cost of construction and the complexity of repair. Hence, any out-of-straightness caused by design features, installation circumstances and operational conditions should be well understood at the time of commissioning. With the useful lifespan of pipelines being decades, circumstances change, and unexpected events happen during their life.
The main reasons for out-of-straightness can be categorized into design- and installation-related origins:
- Directional changes due to pipeline routing and installation conditions
- Seabed topography and features (e.g. potholes, boulders and coral outcrops)
- Gravity pull where pipeline lacks support (e.g. freespans and catenary risers)
- Crossings of existing infrastructure, such as pipelines and cables
- Crossings of buckle trigger structures
- Tie-in to pipeline in-line or end termination structures
And pipe movements due to operational and environmental loadings:
- On-bottom instability due to current and wave action
- Soil movements (e.g. by soil transportation, scouring, slope instabilities and seismic activities)
- Impact by trawl boards, anchors and icebergs
- Lateral or upheaval buckles caused by thermal expansion
- Pipe walking (also referred to as pipe ratcheting) due to cyclic pressure, and temperature loading and gravitational pull or axial tension from, e.g., catenary risers
- “Non-designed” freespans and sagging
As pipeline movements during operations are a known common phenomenon, an operator’s integrity management program should include measures for identifying, measuring and monitoring pipeline out-of-straightness where such a threat exists.
It is therefore essential that a baseline curvature or bending strain profile is established after commissioning that can be used as a reference for future inspections. This is so that bending strain levels can be monitored and any movement identified, so that timely intervention can take place if required.
Identifying, Measuring and Monitoring Out-Of-Straightness
After offshore pipeline installation, it is common practice to perform a detailed survey to confirm the position, check for any unsupported sections (freespans) that may need support and identify any out-of-straightness (lateral or vertical). For pipelines that will operate at high temperatures, it is then common to complete a detailed expansion and buckling analysis to identify locations where it may be necessary to place rock over the pipe to control movement. Remotely Operated Vehicles (ROVs), towed sonar systems and, more recently, Autonomous Underwater Vehicles (AUVs), traditionally complete these surveys. These inspections may involve cameras, single-beam, multi-beam and side-scan sonar systems, pipe-tracker, cross-profiler, sub-bottom profiling and many others. During service, similar systems are deployed on a regular basis (for example annually) to identify changes or new features.
No single tool is perfect when it comes to measuring pipeline out-of-straightness, but one cost-effective method offers the prospect of providing accurate measurements for most offshore pipeline applications: deploying an inertial measurement unit (IMU) as part of an in-line inspection (ILI). IMU inspection will give the out-of-straightness for the full length of the pipeline from launcher to receiver. Data is collected for many locations where traditional methods are limited, for example topside piping, buried sections, tunnels, shallow water, offshore to onshore transitions, and areas of bad visibility. In addition, weather conditions and currents do not limit deployment.
A data collection frequency of up to 800 Hz (equivalent to approximately one data point for every one centimeter) and the ease of performing repeated runs make the IMU method ideal for identifying, measuring and monitoring out-of-straightness in offshore pipelines.
What is an IMU, and How Does it Work?
An IMU, as used for pipeline in-line inspections, is an electronic device that measures and reports linear accelerations and rotational rates, using one accelerometer and one gyroscope per axis for each of the three axes: pitch, roll and yaw. As the IMU only detects and records linear accelerations and rotational rates (changes in speed and direction). Features such as bends, changes in wall thickness, and even girth weld penetrations are easily detected. Vibration of the ILI tool can cause “noise” in the data. Post-processing is required to remove “noise” and compute curvature, bending strain and the XYZ data points providing the pipeline position.
A limitation of using IMU measurements for global pipeline position mapping is that the post-processing of the data introduces errors. Any measurement errors, however small, are accumulated over time, leading to a “drift”: an ever-increasing difference between the calculated location and the actual location. Current high precisions IMUs can, from a known location, provide accurate XYZ mapping over a distance of up to two kilometers after which the accumulated drift makes the mapping accuracy beyond acceptable standards. Generally, over a 2-km section with known end coordinates, the achievable position accuracy will be within 0.7 m at any location.
For onshore pipelines, reference points, e.g. established by GPS measurements, are used to correct for drift errors at regular intervals. For offshore pipelines, this is more difficult, as it often requires ROV or diver support to establish the reference points. As such, using an IMU for overall position mapping is normally not considered a cost-effective option for offshore pipelines, but monitoring and assessment of both local movement and bending strains are fully feasible.
Deployment of IMU Tools
The first deployment of IMUs for in-line inspections goes back to the 1990s. Since the first deployment, the traditional use of IMU inspection technology has been for onshore pipelines, and the primary use has been to make it easier to locate where to dig when doing field verifications of features such as corrosion and dents. The IMU is normally deployed as part of an ILI combo tool that includes, e.g., magnetic flux leakage (MFL) and geometry sensors.
IMU Data Analyses as Part of Pipeline Integrity Management
Any pipeline assessment based on IMU data starts with converting the accelerometer and gyro data into curvature measurements and then XYZ data points referenced to the launch point. This process includes a review of the “raw” IMU data and identification of sections where the data quality may impact the accuracy of further assessments. If the “noise” is limited to narrow frequency bands, data smoothing/filtering may be applied to remove/minimize the impact on accuracy.
If two or more IMU runs have been performed, the bending strain patterns may be compared to identify areas of pipe movement. Using pipeline curvature data generated by different inspection vendors is generally acceptable for this type of assessment providing data of sufficiently high resolution are available. For pipe sections with established pipe movements, the pattern of bending strain change can be assessed for characteristics that may reveal the cause of movement, e.g. pipe walking, anchor drag or freespan developments and provide risk indicators for further pipeline movement. If the pipeline movement can be determined to be predominantly displacement-controlled, the bending strains can be checked against appropriate strain limits to prevent failure by local buckling or fracture.
As the IMU tool is normally deployed as a part of an ILI combo tool that includes, e.g., MFL and geometry sensors, a significant amount of additional data will be available from the ILI inspection. The out-of-straightness data provides significant benefits for pipeline defect assessments and diagnosis, as the risk of failure can be assessed based on the bending strain or movement coinciding with identified geometric features, metal loss and cracks.
Assessment assumptions often applied in strain assessment are:
- Bending dominantly displacement-controlled (strain acceptance criteria are applicable). Note that this would not be applicable for bending strain related to spanning, which is load controlled.
- Longitudinal loading is an insignificant contributor to max strain level (bending capacity not significantly impacted)
- Past and future fatigue damage is negligible (strength not impacted by development of fatigue cracks)
- Ductility and yield strength of girth weld overmatch parent pipe metal (welds are as strong or stronger than adjacent linepipe)
For bending strain locations where "basic" root cause and criticality assessments fail to provide sufficient granularity, collected ILI geometric data will allow for advanced finite element assessments of identified features under realistic bend loading.
In-line Inspection Limitations
Current ILI technology can only “see” pipe wall and cross-sectional features, and, as such, other inspection methods are required to identify and monitor external features and threats to the pipeline, e.g. freespan start, end and gap height, cathodic protection, debris, seabed condition, surface damage to coatings, etc. While in-line inspections may be indispensable for the integrity management of offshore pipelines, they have to be seen as part of a bigger all-encompassing inspection program.
Summary: The value of ILI with IMU in the Integrity Management of Offshore Pipelines
Bending caused by ground movement and thermal expansion has resulted in failures of offshore pipelines. As such, it is important that pipeline out-of-straightness is identified, measured and monitored from the day of installation to the end of useful life so that timely intervention actions may be taken. One method of achieving this goal is by applying in-line inspection with IMU as part of the integrity management plan.
Figure 1: Vertical strain peaks at freespan location
Achievable benefits from a single IMU inspection:
- Curvature measurements that, if assessed together with external inspection data of e.g. freespan locations, provide superior input to ultimate strength checks and span modal analysis. (see figure 1)
- Bending strain profile, providing insights into unintended forces acting on the pipeline.
- Detect global geometric anomalies, for example anchor drags (see figures 2 and 3).
- Combining IMU with Geometry and MFL or UT inspection technology, bending strains can be accurately correlated to other features (corrosion/dents/gouges/wrinkles), allowing code compliance checks and detailed finite element analysis.
Figure 2: Curvature and horizontal bending strain (plus local deformation) indicate anchor drag
Achievable benefits from multiple IMU inspection runs include:
- Monitoring changes to the pipeline profile and bend strain levels.
- Early detection of upheaval buckles (not easily detectable by ROV for buried pipelines).
- Measure effect of thermal expansion on end terminations, tie-in spools and pipeline buckles.
- Monitor pipe walking.
- Early detection and monitoring of pipeline on bottom instability
- Monitor for changes to span profiles and thereby optimize the need for costly external inspections.
Historically, IMU has only been deployed with occasional internal inspections for corrosion or other damage. For managing high-temperature pipeline behavior, there would be significant benefits in more frequent inspections for different pressures and temperatures, and to track incremental lateral or upheaval buckling. The rapid development of IMU technology now means that small, high-accuracy, energy-efficient units are available that can be deployed on low-cost cleaning tools. Thus, IMU units can be launched at a significantly higher frequency than seen today, providing real benefits in terms of pipeline integrity monitoring.
In-line inspection with IMU is not a standalone integrity management solution, but it is an invaluable and cost-efficient partner to other inspection methods such as ROV/AUV.
Figure 3: Curvature and bending strain indicate displacement concluded to be anchor drag