[video]Trelleborg and SubC Partner help early risers
June 3, 2017
Trelleborg explains how a new, world-first approach enabled the replacement of riser buoyancy modules without any loss of production
In floating production operations, pipelines such as flexible risers, cables and umbilicals are often required to be held subsea in specific geometric configurations to prevent over utilisation of the system. Typically, this is achieved by attaching buoyancy modules to the outside of the pipeline, which provide uplift and maintain its location. These distributed buoyancy modules (DBMs) are comprised of a clamp and a buoyant jacket. The module is fitted to the desired location and locked with a fastening system, usually while the structure is topside, prior to the installation of the riser itself.
Previously it has not been possible to attach the buoyancy on a live riser; the riser would have to be disconnected to perform the operation, halting production and incurring significant costs as a result of the downtime.
However, in a collaborative project completed last year, Trelleborg and SubC Partner designed, manufactured and fitted a new form of DBM suitable for installation by an ROV. This was successfully fitted to a 12-inch (305-mm) production riser on a floating production storage and offload (FPSO) vessel in the UK Continental Shelf. Installation by ROV meant that the modules could be replaced on a live riser, in a steep wave configuration, without shutting oil production – a world-first achievement. According to the operator, this also represented an 80% cost saving compared with riser replacement, as well as additional savings thanks to avoided loss of production.
Trelleborg Offshore key account manager, Andy Hey, took InnovOil behind the scenes of the project.
C here As with a great many innovations, this project was less about a paradigm shift in subsea equipment, and more about the confidence and design capability to use existing equipment in a smarter way. “It was essentially a combination of clever design and the biggest ROV on the market,” Hey explained.
The operator had previously considered this type of replacement, but decided against it based on the amount of unknowns. Although several firms had tendered for the job of DBM provision, Hey believes that few had comparable engineering expertise, and he credits much of the success on the project to Trelleborg’s and Sub C Partner’s “very capable” design team. The approach to the project had two prongs: Trelleborg would design and build a new type of DBM, while Sub C Partner engineered a bespoke suite of hydraulic ROV tools capable of removing the old modules and deploying new ones.
This required a lot of investigation and testing from both parties. In the first phase, these tools had to be capable of positioning onto the existing DBMs, adjusting their ballast, cutting holding straps and then delivering the element to a crane for retrieval to the surface – with a similar process for the riser clamp. This would then be conducted in reverse to fit, secure and ballast a new clamp and module (see video).
It presented a number of difficult parameters. In addition to the design of the hydraulic tools for mounting, there were also issues of ballasting, and the amount of power to lift needed. Hey said the task of sourcing a suitable ROV which was capable of meeting these requirements was critical to the project success, especially due to the accuracy required in the removal and fitment of the Buoyancy modules From Trelleborg’s side, its internal clamp-to riser interface was based on a hinged version of its existing “Type 2” friction clamp, with nylon segments and rubber pads for added security.
This was particularly suitable because the clamp had no loose components, but the tightening mechanism was redesigned to work with the ROV interface. During testing, clamp performance was good, achieving a slip load in excess of 33 kN at ‘end of life’ conditions. In prototype testing the design’s hinged crossbars also allowed for fastener tightening despite a variance in the tightening sequence when performed by ROV.Customer Group Manager Andy Smith with Trelleborg’s offshore operation added that: “
A variety of installation features and precise datum points had to be built in to the buoyancy modules to provide a known and repeatable interface between ROV installation tooling and the DBM components. Dimensional tolerances on a buoyancy element shell are generally intrinsic to the roto-molding manufacturing process.”The buoyancy element fasteners – the mechanism that holds the two buoyancy halves together – also had to be captive. As a result, Trelleborg used a bolted flange tightening system instead of circumferential securing straps and tensioners.
However, this presented increased risk of module misalignment when the elements were paired. In response, engineered designed bespoke ‘floating’ nuts and bolts, which self-align upon engagement, meaning that every element would be aligned correctly and secured.Given the nature of the project, both parties had to provide a “definitive amount of information” to the operator to prove calculations and to understand how the ROV would perform. However, with the company’s backing, the project was cleared to proceed in January 2014.
Done in minutes
Installation began in August 2015. A joint team worked aboard an offshore service vessel for around three weeks, removing or adding one module per day. “Our goal was to install 10 and we did 9,” Hey said, although the one unsuccessful fitting was based on incorrect specifications – a problem which was easily rectified by obtaining the correct specifications once the error was recognized. In practical terms however, the changeover of individual modules was exceptionally fast compared to the traditional method. During the operation the team successfully reduced the average installation time per module, he explained and using a moon pool, that time has been reduced even further.
With all modules installed and secured as expected, the team continued to monitor the riser performance to ensure its safety. “As it moves, the umbilical fluctuates in pressures, and doesn’t remain in a static position. Once the full assembly was in place we monitored it via ROV and other equipment,” he explained. So far however, results have been extremely good, with the riser remaining in place as expected for well over a year now. Having been plagued by bad performance from a poorly configured riser for months, the operator impressed by the results.
Being a first-of-its-kind project, and given the size of ROV required, Hey acknowledged that the overall cost of design and installation was high. But with the system proven, and the partners comfortable with the system, future installations could possibly take even less time. In the case of this project, in addition to the savings identified compared with riser replacement, the savings achieved through continued production could dramatically offset the higher DBM installation costs.
With the system conceived as part of a partnership between the two companies, Hey confirmed that the two will be pursuing future work, marketing it both individually and collectively. Trelleborg itself has had several productive discussions, and a number of operators are now interested in retrofit projects, where the original riser clamp would be maintained and the buoyancy module replaced. With most risers requiring around 10-15 modules each, Hey said there is a strong argument for more operators to consider the method.
“This is one of the most complex and difficult retrofit applications to undertake, and its success through highly technical and complex engineered solutions is an indicator to the opportunities for further retrofit opportunities, enabling uninterrupted production whilst replacing or installing equipment in challenging environments,” Hey said.