The MacCulloch oil field lies 250 km north-east of Aberdeen on the United Kingdom continental shelf (UKCS) block 15/24b. The field was discovered in 1990 and came on-stream in 1997 using the North Sea Producer FPSO. The field has five active wells. MacCulloch was operated by ConocoPhillips when the subsea inspection was performed.
Many offshore pipelines are required to operate at high temperatures and pressures. A temperature rise results in thermal expansion of the pipeline. Friction forces between the pipe and the seabed restrains the pipeline along its route, which in turn increase axial stress, with the result that buckling may occur. Buckling may have serious consequences for the integrity of the pipeline if not properly taken into account.
A pipeline will tend to buckle in the direction of smallest resistance. The direction of the buckle therefore depends upon the installation and pipe configuration:
- In a free span the pipe will buckle downwards
- If it has been installed on the seabed, the pipe will buckle sideways
- A buried pipe will tend to buckle upwards
Vertical buckling of a pipeline is called upheaval buckling, see Figure 1.
Offshore pipelines are buried for various reasons; for example, in order to:
- eliminate the risk of trawl gear snagging the pipeline and pulling it out of position.
- avoid anchor snagging
- protect the pipeline from dropped objects near installations
- protect it from iceberg scouring
- stabilize pipelines with low submerged weight.
The flowline at the MacCulloch field was buried. Upheaval buckling was observed during a subsea inspection in 2010.
The calculated radius of curvature based on the data provided by the ROV was estimated to approximately half of the minimum bend radius (MBR) allowed for storage defined by the manufacturer of the pipeline, i.e. the bend was more severe than the pipeline design allowed.
In order to evaluate its operability, the operator requested a detailed study of the flowline. The study had two objectives:
- Global analysis: to understand the mechanism of the observed upheaval buckling and suggest potential remedies to stabilize the flowline
- Local analysis: Detailed analysis of the cross section to investigate the possible effect of extreme bending beyond minimum bending radius.
The global analysis was performed in SIMLA, which is MARINTEK’s modelling software package for numerical analysis for offshore pipelines in deep waters and hostile environments.
The local model was analysed in MSC.Marc, which is a powerful, general-purpose, nonlinear finite element analysis software package that accurately simulates the response under static, dynamic and multi-physics loading scenarios.
A flexible pipe or flowline usually consist of several layers.
The most usual layers, and their roles, are:
- Carcass: Provides the pipe with radial support to resist external loads (crushing and hydrostatic pressure)
- Pressure sheath: Leak-tight barrier
- Pressure armour: Provides the required radial strength to resist internal pressure
- Tensile armour: Provides the required axial tensile strength capacity
- External plastic sheath/Outer sheath: Prevents the steel wires from coming into direct contact with seawater and provides mechanical protection for the outer tensile armour wire layer.
The local model of the MacCulloch flowline was built according to the specification of the flowline cross-section.
The integrity of the flowline was assumed to be critical with respect to the condition of the carcass. For this reason, the carcass was modelled in detail(Figures 6 and 7).
The flowline consists of two helically wound layers of tensile armour wires. Both layers are critical and were also modelled in detail in the FE-model (Figure 8).
The finite element model was built on the basis of methods developed by 4Subsea AS.
The results of the FM analysis showed that the carcass did not collapse at the observed radius of curvature (Figure 9). However, the analysis indicated that the carcass would collapse at a radius of curvature slightly lower (i.e. slightly tighter bending) than the observed radius of curvature, (Figure 10).
The analysis showed that the stresses in the carcass and tensile armour layers were high at the observed radius of curvature. The margin to failure was small; the carcass material was in the plastic region, i.e. stresses beyond the yield limit, the strain in the pressure and outer sheaths was significant, there was high stress in the tensile armour wires, etc. If a probabilistic analysis had been applied to this model, the probability of failure would therefore have been high.
The analysis of the local model employed a one-off loading, and cycling loads were not used. The integrity of the pipe would probably have deteriorated in the course of time due to repeated start-up and shutdowns.
On the basis of the survey data provided by the ROV, the results of the analysis and the probability of failure, MARINTEK recommended replacing the flowline, although it was made clear that any changes to the survey data would affect the results and the conclusions drawn.
Because of the importance of the observed radius of curvature, in early 2012, a more detailed and accurate survey was performed on behalf of ConocoPhillips. The primary objective of this inspection was to accurately determine the radius of curvature of the flow line at the site of the anomaly. The new survey showed that the radius of curvature was much larger than estimated in the first survey (i.e. less critical). It was even above the defined MBR for this flowline. The new observations showed that the requirements specified by the flowline manufacturer were not violated, and replacement of the flowline was not necessary. The anomaly will be closely monitored for any developments in the future.
To see the figures in this article, please open the pdf version of the article in MARINTEK Review No. 1-13