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Optimising the efficiency of subsea installation maintenance

Published:  30 June, 2015

The inspection, repair and maintenance of subsea umbilicals can be a challenging process. Computer based simulations of flexible assets is critical in determining whether maintenance operations are required and to what extent. Ryan Diver*, a subsea engineer at Jee Ltd, discussed with ODEE, a recent comparative study of a control umbilical and the subsequent recommendations made to the client, following a subsea project located in the North Sea.

Ryan Diver, a subsea engineer at Jee Ltd told ODEE that the company secured a contract in 2015 with a major international operator to perform a study on a subsea main control umbilical (MCU), servicing two subsea control modules via a junction box located under one of their floating production vessels (FPV). The study was undertaken after a recent ROV survey of the MCU identified anomalies with the buoyancy modules.

A review of the footage revealed that travelling from the base of the umbilical the first module was missing; the second had split in half but remained tethered to the MCU, sliding up the line and stopping at the base of the third module (causing both to slide up towards the hog bend); while over the hog bend and into the sag bend the remaining modules had all relocated a few meters from their as-laid positions up towards the hog bend.

In order to reinstate buoyancy the client attached two new modules to the MCU. These new modules were positioned close to the as-laid locations of the detached and split modules and were of similar design. The split module, now redundant with the addition of two new modules, remained tethered to the MCU.

The combined effect of the new and relocated buoyancy modules, including the redundant split module, was a shift in the umbilical’s configuration, which now differed from as-laid. The client requested Jee Ltd to perform a comparative assessment between the current and as laid umbilical configurations to identify any integrity issues that might occur if the MCU was left untouched.

The objectives of the work were therefore to:

• Identify differences in the MCU’s minimum bend radius (MBR) – a critical measurement of umbilical and flexible integrity – and elevations above seabed between the as-laid and current buoyancy module arrangements; the latter with and without the split module attached

• Conclude whether any remedial work is required to relocate the buoyancy modules back to their as laid elevations or remove the redundant split module from the line

• Identify any additional concerns raised from the assessment

Ryan Diver described the challenges encountered while completing the assessment: “The MCU had been installed in the 1980s and most of its original installation and design documentation was either hard to locate or contradictory in its statements. Although some documents were found for the MCU and FPV the project had to be progressed without key data relating to the MCU’s bending stiffness, pay-out length and allowable service MBR; all critical parameters for umbilical and flexible analysis.”

Without the necessary data to hand Jee proceeded with the comparative study using assumptions in lieu of unknowns. These assumptions, although undesirable, were formulated based on a thorough review of all the available documentation and Jee’s extensive knowledge of flexible and umbilical analysis.

Diver continued: “Our experience sets us apart and our knowledge of similar systems allowed us to proceed with a sensitivity analysis covering a range of reasonable bending stiffnesses. The pay-out length was hand calculated based on a series of supplied documents and the allowable service MBR was estimated based on Jee’s knowledge of similar flexible systems.”

Jee went on to use OrcaFlex, a leading package for dynamic analysis of offshore marine systems, to complete the comparative assessment. The OrcaFlex model was built to comprise the full system which included: the subsea template, junction box, MCU, FPV and all necessary buoyancy modules.

The analysis demonstrated that the radius of the hog bend was less than 3 m for a range of bending stiffness values at the lower end of the assessed spectrum. This put the hog bend at risk, as typical service MBRs are between 1 and 3 m. If the MCU was to operate for long periods under in its current configuration there was a high probability that storm conditions and significant dynamic affects could cause a further reduction in the MBR, leading to failure of the MCU; a costly result from avoidable risk.

Upon completion of the analysis, Jee made the following recommendations:

• As a minimum the split module should be cut from the line to improve the MBR at the hog bend

• The buoyancy modules should be relocated to their as-laid positions to reduce the risk of compromising the hog bend MBR during service

• Regular general visual inspections of the umbilical should be undertaken as part of the on going integrity management of the assets. Future unplanned changes to the buoyancy configuration of the MCU should be recorded and reviewed for any potential impact on integrity.

Despite the incomplete data, Jee’s team said they were able to apply its in-depth subsea umbilical and flexible knowledge and project experience to draw practical conclusions and recommendations, supported by computer based analysis.

*Ryan Diver has worked on a wide range of projects including flexible installation analysis, pipeline freespan analysis and supporting targeted inspections of subsea pipeline. As a result Ryan has built a wealth of experience and in-depth knowledge of the subsea industry and in particular the analysis of subsea assets.

For further information please visit: www.jee.co.uk

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