(Em)Bracing innovation

Installed in 65m water depth in 1998, the Siri platform was an innovation. Now, it has taken another innovation – taking onshore cable stay engineering design concepts for use subsea – to allow the platform to continue operation. Elaine Maslin reports.

Artist’s illustration of the cable stay layout on Siri. Images from Subsea 7.

The Siri area is close to the Norwegian Border, roughly 220km off the west coast of Denmark. The area contains five fields, Siri, Nini, Nini East, Cecilie and Stine, all producing from Paleocene sandstone reservoirs between 1800m and 2200m below the sea level.

Built at the Kvaerner Rosenberg yard in Stavanger, the Siri platform, 100% owned and operated by Denmark’s DONG Energy, was the first application of GustoMSC’s innovative MOPUstor design, comprising a three-legged jackup with process facilities.

The structure slotted into a support structure based on a 300,000 boe capacity steel storage tank placed on the seabed, with the wellhead caisson supported from a sponson attached to the side of the jackup’s tank.

In 2009, during a routine inspection, cracks were discovered in the sponson structure. To arrest further damage while a permanent repair was conducted, a temporary solution was installed, which involved positioning a beam with hydraulic jacks to support the sponson/caisson.

A number of options for a permanent repair solution were evaluated before DONG Energy chose to mature an idea put forward by engineering consultancy Atkins, which was then developed in partnership with Subsea 7.

The idea, thought to be a first for the offshore oil and gas industry, was to install cable stays between the platform legs to stiffen the three-legged platform structure, with the aim to reduce the natural period of the platform from around 6.5 seconds to approximately 3 seconds; this would reduce the platform’s response to environmental loads thereby reduce movement and loading in the sponson/caisson.

DONG Siri Project Manager for Subsea 7, Alan Cassie, says the project scope presented unique challenges, not least the innovative thinking around taking a product from a different industry and using it in the oil industry.

“Taking cable stays used for bridges and using them for an offshore application is quite innovative,” he says. “There were a series of options that could have been chosen; however they also presented a number of challenges, but ultimately, in the end, we believe the option we chose was the right decision.”

Braced and ready for action – DONG’s Siri platform, complete with cable stays.

The design is based around clamps on each of Siri’s legs, top (just beneath the “hull” of the platform) and bottom (on top of the oil storage tank), between which the cable stays would be diagonally installed as braces. In the final design, the clamps weighed more than 150-tonne and the lower clamps stand 6.4m from base to nose. Fabricated in two pieces, including cast sections (the clamp nose section which supports the cable anchorage), the clamps are then bolted into place using 56 M90 bolts.

The 60m-long cable stays have a 355mm outer diameter (including sheath), and 169 stranded cable, with each strand comprising seven wires.

However, unlike standard cable stays, the stays for Siri had to be designed to be water tight; this added significantly to the complexity of the assembly operation. A bespoke facility was set up in Dundee, Scotland, to assemble the stays, this included a set of articulated towers to allow the cable ends to be held vertically whilst the end anchorages were sealed with epoxy resin and wax to make them water tight.

“Once the design was over the line the next challenge was to install it,” Cassie says.

The dive support vessel Seven Falcon, with a 250-tonne crane, meant Subsea 7 had the capability to install the clamps. Offshore installation involved complex simultaneous operations between the dive support vessel and platform based operators, including rope access technicians.

Taking the load – one of the top clamps.

For the installation of the clamps, pre-installed guide clamps were used. Winch platforms, each weighing 150-tonne a piece and measuring 15m sq, also had to be installed at the top of each platform leg. The winches, three per winch platform, the largest with 120-tonne capacity, would be used to bring the components in to place, working with the construction vessel. A subsea winch was also used to provide supplementary hold back where needed to control the components.

Each of the lower clamps, lined with a specially selected low yield strength material, was then lowered onto guide clamps in two pieces. Divers/RAT’s installed the upper and lower bolts and then an automated bolt rack was used to locate and tighten the remaining 46 bolts (each bolt 90mm-diameter, 2m-long and weighing 300kg), creating a metal to metal grip between the clamp and the leg tubular.

Next, the upper clamps were installed. “The subsea installation tolerances were very tight. The upper clamps required a tolerance of 0.15°,” Cassie says.

The cables were also installed from the Seven Falcon. Using specially designed installation aids, the operation involved complex simultaneous operations from the vessel and platform based winch operators. Key to the operation was to avoid damage to the cables, particularly the water tight duct and the threaded upper end of the anchorage.

Once in place, the tensioning was performed by a jacking system designed by Subsea 7. “Conventionally when tensioning stays, you’d use a weight jack,” Cassie says. “We saw this wasn’t going to work, it would take too long and we felt the risk of thread damage was too high. We decided to design our own tensioning system, which allowed the cable catenary to be removed in one stroke; avoiding possible damage as well as speeding up the operation.”

The system was built into the ends of the cables. Four hydraulic cylinder rams (500-tonne each with 8/900mm stroke) extended and pulled the cable end pieces through the bearing plate – all without damaging the thread which would hold the cable in place. They were powered using a hydraulic power unit designed to be able to make sure each of the four cylinders progressed in tandem while the catenary was being pulled out.

Each lower clamp stands some 6.4m from base to “nose”.

First the stays were each pre-tensioned to 500-ton, to take the catenary out of the cable, before being tensioned to 1250-tonne. In 2012, one clamp was installed, followed by a further two in 2013 and the three upper clamps and the cables in 2014. The project, carried out over 350 vessel days using the Seven Falcon, completed in July 2014.

Reductions in the platform motion were achieved and, as a result, DONG was in a position to remove the restrictions on manning the 65-man platform and return to normal platform operations.

“We were very pleased with the way the procedures worked on the offshore campaign,” Cassie says. “There was a lot of sensitivity around lifting these large sections of the storage tank. We had almost no shutdown period on the platform, only about 15 days to put in the clamps on the north leg because of access challenges due to the position of the riser.”

Reflecting on the project, Cassie says lessons learned were: “Don’t underestimate the challenge of delivering something innovative and new and don’t under estimate the ability of good engineers to resolve those problems.”

“Are we going to use cables in other parts of the North Sea,” he asks. “Possibly not, but it is part of our tool kit now and reflects our general principle around engineering innovative solutions for mature assets, something we are very proud of.”