Locating subsea umbilical leaks
  from: Offshore Engineer
  by: Klaire Evans
  Wednesday, March 19, 2008

Dutch legend has it that Hans Brinker, the ‘Hero of Haarlem’, used his finger to plug a leaking dyke which threatened to engulf his town. More recently the Aberdeen University spinout company that borrowed his name was called in to locate a leak in a subsea umbilical on Norway’s Ormen Lange field. Brinker’s Klaire Evans explains.

Leaks in subsea umbilicals are problematic as they can be difficult to locate and subsequently seal. As umbilicals typically comprise several elements, including hydraulic lines, electrical conductors and filler material, surrounded by outer sheath(s), the eventual point of fluid egress may be some distance from the actual point of the internal line leak. Traditional leak location operations often use ROVs to conduct an external inspection of the umbilical to detect the point of fluid egress; if the umbilical is long then this can be a time consuming and costly process due to the amount of vessel time required. The practice of trenching and backfilling or rockdumping umbilicals adds an additional complication as inspection becomes more difficult.

Integrity management specialist Brinker Technology has developed a portfolio of products and services, including Platelets and Telepath, that allow operators to deal effectively with leakage situations in umbilical systems.

Different approach

Platelet technology takes a radically different approach to traditional integrity management methods in that it seals and locates leaks in a single integrated process. Discrete particles, known as Platelets, are deployed into a pipeline and use the fluid flow to carry them to the leak site. When they reach the region of modified flow around the leak site fluid forces entrain them into the leak and hold them against the pipe wall thus facilitating a seal. Each individual Platelet can be embedded with a traceable tracking device, typically a radioisotope, which enables the entrained platelet to be detected (and thus the leak site to be determined) either externally by ROV or internally using a sensory pig.

Crucially, Platelets are best deployed when there is flow in the line; which means they can be deployed as part of a routine pipeline operation causing minimal disturbance to production. An additional significant benefit is that Platelets are implemented remotely, removing the need for direct access to the leak. There are numerous applications for Platelet technology; from its first deployment it has continued to improve and evolve and is now being optimised for use in hydrocarbon lines, gas lines, flexible flowlines and downhole applications.

Umbilical leaks

Telepath addresses the need for remote and non-invasive leak location in umbilicals. It uses analytical and numerical modelling, backed up by expert analysis (gained from our previous experience of leakage situations) to provide this leak location service. This bespoke tool uses measurements of pressure and flow rate, alongside analysis of the pipeline and the fluid carried, to locate the leak quickly and accurately.

Whilst a leak locating technology may be traditionally viewed as reactive, there are significant benefits in the contingency approach, particularly when designing or installing a new umbilical. Telepath can be used in two modes: as an emergency response or as part of a contingency approach, including up to four stages. Not all of these stages are necessary for a leak to be located but an earlier entry will result in a more accurate location estimate due to Brinker’s prior knowledge of the pipeline infrastructure and it’s pre-leak behaviour.

Pre-leak activities allow Brinker engineers to become familiar with the pipeline and provide a basis for any leak location operations. Onshore preinstallation activities include analysis of the pipeline fluid’s hydraulic properties and familiarisation with the pipeline infrastructure to identify suitable access points. After the line has been laid then post-installation testing to obtain signature analysis data for the pipeline can be conducted.With the addition of a flow meter, it can be incorporated into post-lay testing as a post-installation confirmation of system integrity. Signature analysis testing allows Brinker to not only reduce the amount of testing required in the event of a leak but also to increase the accuracy of any leak location estimate.

On being alerted to an umbilical leak, Brinker requires data from the leaking line for analysis. Gathering this data does not require specialist equipment; readily available instruments such as standard pressure monitoring equipment and a flow meter would generally be adequate. If pre-leak testing has been undertaken, this data will be used as a baseline for comparison. If not, then the line can be compared to a standard flow regime. By studying the measurements of pressure and flow, the company uses its experience in leak location to provide an accurate location assessment.

As with all analytical methods, the accuracy of the location estimates using Telepath depends on the accuracy of the measurements made. In the recent application on StatoilHydrofs Ormen Lange field, Telepath successfully located a leak in a subsea umbilical to within 2% of the total line length (which was 119km).

Operational response

There are a number approaches to the use of Platelet technology and Telepath in umbilical systems. A typical procedure is given here.

If the pipeline in still in the design and commissioning phase then the recommended actions are:

• pre-installation Telepath activities to gain knowledge of key fluid properties and pipeline infrastructure;
• post-installation Telepath tests during routine pressure testing to get signature analysis information for the newly laid line; and
• analysis of the pipeline infrastructure to render it ePlatelet-friendlyf and ease any future Platelet deployment operations.

In the event of a leak occurring, the following actions are recommended:

• post-leak testing to determine changes in the line since previous testing;
• leak location using Telepath; and
• the use of traceable Platelets to refine the leak location (and potentially seal the leak, if required).

These recommendations will provide the operator with a rapid and accurate response in the event of a leak occurring.

In-service umbilicals can also benefit from a planned response. Brinker can undertake pre-leak testing and plan how leaks can be addressed. Such a contingency approach ensures a more effective response to any leaks.

Ormen Lange

In late summer 2006, Brinker was contracted by Norsk Hydro to locate a leak in a subsea umbilical on the Ormen Lange field, which lies in the More basin in the southern part of the Norwegian Sea in water depths of 800-1100m. Production from the field is routed to an onshore facility at Nyhamna before being transported through the Langeled gas export pipeline to the UK.

Ormen Lange has two main umbilicals, A and B, which are 119km and 123km long respectively. During post-installation tests, fluid was found to be leaking from umbilical A. No pre-leak signature analysis had been done in this case; however, Telepath could still be applied. In this scenario, some initial pressure and flow data, fluid properties and information related to the umbilical infrastructure were provided by the operator. After analysis it was found that the analytical model was sensitive to variations in a number of key parameters; thus an enhanced programme of testing was undertaken, together with a revised assessment of the environmental conditions incident to the umbilical.

From the Telepath analysis, an estimate was made of the leak location as being 30km offshore. An ROV deployed in this area later verified that fluid was exiting the line in the vicinity of this estimate. However, in umbilical bundles the eventual egress of fluid may be some distance from the actual line failure due to the presence of outer sheath(s). Brinker was therefore contracted to determine the exact leak location using Platelets.

In this case the client did not want to seal the leak and thus the Platelets were specifically engineered to pass through the leak when the line was overpressurised. Five sub-millimetre Platelets were irradiated and deployed into the line.

These Platelets then travelled with the flow 30km down the hydraulic line to the leak site where they were detected by external radiation detectors. OE

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