Offshore Pipework Survey Identifies Fatigue Risk

Working to Achieve

Minimising the risk of Vibration Induced Fatigue in Process Piping for an offshore oil and gas company by following guidelines set out by the Energy Institute.

The Challenge

An oil and gas company experienced failure of some pressure gauge connections due to pipework vibration on a platform off the coast of the Netherlands. AVT Reliability (AVTR) were asked to mobilise to the platform to complete a platform wide Pipework Vibration survey on all Small-Bore Connections (SBC’s) to determine which locations had vibration levels that could cause fatigue
failure.

If the reading was found to be over concern an additional reading taken on the Mainline to confirm if the high reading was local to the SBC or a mainline vibration issue forcing the SBC to also vibrate. Some of the lines in the scope were attached to the export compressor on the rig which had a variable speed range. For these locations, a speed sweep of the compressor was carried out to determine which operating conditions caused higher vibration levels.

Bump tests were also carried out at these locations to calculate the Natural Frequency of the pipework or SBC to allow a comparison with the frequency at which high vibration levels were found.

Our Approach

AVTR’s Asset Integrity Division (AID) made an initial trip to the platform to assess the pipework and find any locations classified as Concern or Problem as detailed in the Energy Institute Guidelines for the Avoidance of Vibration Induced Fatigue Failure in Process Pipework.

AVTR had found 15 locations which were either over the Concern or Problem line as detailed in the EI Guidelines. AVTR used our own Machine Sentry® database and sensor to create a survey route for vibration locations and complete a vibration survey of the pipework at each Small-Bore Connection identified.

One of the vibration templates within Machine Sentry® calculates a Concern Factor (CF) defined by AVTR to correspond to the Concern and Problem lines as detailed in the Energy Institute guidelines. A CF of 1 equates to the Concern line and a CF of 5 equates to the Problem line.

Our Findings

During the survey AVTR found 15 locations which were over Concern as detailed in the Energy Institute Guidelines.

One location was found to have a Concern Factor of 14, considerably above the EI Guidelines Problem Line. It was determined that this location experienced resonance at 13Hz. Temporary strapping was used to restrain this location which brought the Concern Factor down to 2.7 and proved that a brace would be a suitable solution for this problem.

The Solution

Due to the large number of issues picked up by AVTR several different solutions were recommended to solve each problem on a case-by-case basis. The client fabricated their own solutions based on AVTR’s recommendations. Examples of the solutions used are:

Bracing- Some locations which suffered from resonant vibration were solved by increasing the stiffness, the simplest way to do this in the case of SBC’s is to brace it back to the mainline.

Removal of Weight At some locations heavy double block and bleed valves were installed which were not required for the process. In these cases, reducing the weight of the SBC was effective at increasing the natural frequency enough to get it outside of the region of excitation, an example of before and after can be seen below:

Our Findings

A second trip to the platform was carried out after many of the recommendations were actioned to assess how effective they had been. A repeat the survey was completed to assess how successful the corrections were and if any additional measures were required.

AVTR published a report after each visit to the platform highlighting where concern vibration levels had been found and whether the issues had been rectified or whether additional work needed to be carried out to get the vibration levels below those recommended in the EI Guidelines.