GMC Mechanical Connector Breaks Through the Monopoly!

Moho Nord keeps the TLP flag flying
Upstream Technology
15 April 2016 00:00 GMT

With Total’s Moho Nord TLP now in place off Congo-Brazzaville, Adrian Cottrill looks at the project’s highlights, including the new players it has brought into the picture for the tendon system.

The latest example of that relatively rare breed of offshore facility — the tension-leg platform (TLP) — is now in place off the coast of West Africa.

The central highlight of installation work for Total’s Moho Nord TLP came on 17 February when the 12 vertical tendons connecting it to the seafloor were pulled in and locked off by contractor Heerema.

Next, the hull was de-ballasted to put those steel tube tendons in permanent tension, holding the structure some eight metres lower in the water than it would float naturally in normal service.

Now, for the rest of its life at this field off Congo-Brazzaville, the platform will simply sway like an inverted pendulum, with no inconvenient heave for its well system to have to deal with.

And that well system is a sizeable one. The unit’s purpose is to act as a dry wellhead platform for 27 wells. Next year, it will be joined by the newbuild floating barge Likouf, which will be moored 350 metres away and linked up to perform the production duties at Moho Nord and control the not-normally-manned TLP.

Before that, the TLP will spend time linked to a much closer companion for a long and challenging drilling operation. Over the next four years, development drilling for the first 17 wells is to be carried out from the platform with the help of a semi tender-assisted drilling (STAD) vessel, floating just 29 metres away.

This unit was due to be fully linked to the TLP in the early days of April. Given the problems that have arisen on a number of previous projects where a TLP and semi have been close-coupled like this, Total has put a great deal of effort into maximising the chance of trouble-free performance from the hawser system that keeps them close together.

Overall, as Total’s deputy TLP manager Stephane Le Guennec points out, the heart of a TLP is its tendon mooring system, and Moho Nord represents a number of firsts in that area. For a start, it is a breakthrough contract for the Techlam division of Hutchinson Oil & Gas as the first such system to be supplied by the French specialist company.

Until now, virtually all tendon systems have been provided by US company Oil States Industries. The contract is also a breakthrough on TLPs for sub-contractor GMC Deepwater, which has supplied the intermediate connectors to Hutchinson and executed the associated coupling operations at the field.

And a further first comes in the manufacturing of the tendon elements. This has been in the hands of German high-tech welding specialist EEW and has brought some useful innovations. Again, on virtually all previous TLP projects, leg element manufacture has been done by mainstream platform fabrication yards in the US.

Overall design and construction of the TLP has been in the hands of Hyundai Heavy Industries, teaming with Houston Offshore Engineering (HOE) for specialist expertise in the hull and tendons area, and with Doris Engineering on topsides design. Operator Total’s partners in Moho Nord are Chevron with 31.5% and SNPC with 15%.

As with all projects, not everything has gone completely smoothly for the Moho Nord platform and it has provided some useful lessons to add to the sum total of industry experience. It is also the first TLP to be placed since the Big Foot project in the Gulf of Mexico and the problems that arose there when it came time for that unit to be installed in May last year.

Installation is always the major challenge in such projects and Moho has fared well. Nevertheless, it also has provided lessons about tendon system installation and the possible need for operators to share their experiences, says Le Guennec.

He has been a central figure in the project right back to the days of concept development. With a background in naval architecture and hydrodynamics, he joined Total in 2003, coming from Technip, where he had been involved in new concepts for deepwater platforms. In fact, Moho Nord has turned out to be something of a family affair because his wife, Anne-Gaelle Moysan, is installation manager there.

By TLP standards the Moho platform is a good middle-ranking conventional structure. Like all sizeable TLPs, its hull takes the four-column approach. Here the hull measures 62 metres square and is held by three externally connected tendons at each of its corners.

Water depth is 780 metres, and down below, each of the dozen tendons connects to a foundation pile driven 110 metres into the seabed. In 100-year design conditions the platform will offset sideways by 40 metres, driven by current and squall.

The load, supported by the 9800-tonne hull, builds up in parts. First, there is a topsides operating weight of 7350 tonnes. Additional equipment needed during the drilling phase adds another 2500 tonnes.

Then there is the 4300-tonne total of the top-tension needed for the risers and the vertical component of the various lines that hang off and connect to the STAD and FPU. Including ballast, all this results in an overall hull displacement of 34,450 tonnes.

“The selection of a TLP at the conceptual stage was driven by our reservoir engineers,” says Le Guennec. “They wanted to access the wells and intervene frequently to maintain well productivity in the difficult Albian carbonate reservoir below. That meant we needed a dry tree unit, and a TLP is a perfect concept for that in West Africa.”

The story started in August 2011 with a five-month design competition between the Hyundai/Doris/HOE group and three rivals. These were Samsung with IntecSEA, Floatec with McDermott, and SBM with Technip and Daewoo. As winners, the Hyundai team then progressed to a “deep FEED” study and on to detailed engineering and fabrication following Total’s final investment decision in March 2013.

With its substantial topsides weight, the TLP was always going to be a standard hull type. All the competitors proposed basically similar four-column hulls, differing only in aspects such as column shape and slight variations in operating draft.

Tendon System

With tendon systems historically in the hands of the same supplier for 20 years or more, Total was keen to encourage competition in this area. As a result, in April 2014, the contract for the entire system was won by a new player, Hutchinson.

Based at Cernay in the east of France, its constituent Techlam was created in 1988. Early work involved high-tech applications such as cushioning inside France’s nuclear submarines, space rocket launchers and then in the oil industry.

Since 2000, it has done some 20 flexjoint projects, including at riser tower bases, gaining great expertise in these spherical sandwiches of metal and elastomer. For Moho, it used top connectors from GE company Vetco Gray.

Le Guennec is complimentary about Hutchinson’s performance. For example, “we wanted all connector parts to be interchangeable, to give flexibility during the installation phase. Even though it was not possible to do a full trial fit of all combinations, they achieved this through tight dimensional control.”

Another new player for this work, GMC, had the sub-contract to Hutchinson for just over 100 sets of pin-and-box connectors for the intermediate joints made at location between tendon elements. These 24-inch diameter couplings have a tension capacity of more than 2000 tonnes, providing high strength and fatigue resistance.

Like those used on earlier TLPs, the GMC design takes a variant of the concentric thread approach, using hydraulic pressure between pin and box to spread them and enable them to slip together.

“This is first time GMC has done a big project like this with their couplings,” says Le Guennec. “They did a very good job and the connector itself is very robust.”

Another change in tendon system tradition comes with Total’s choice of tendon element manufacturer, German specialist EEW, where the day-to-day business is high-tech welding. Those elements are 24-inch diameter steel tubes, generally of one-inch wall thickness.

On Moho, for the first time, the girth welds between the element body and its connectors are not ground completely flush, as is traditional, but instead left with their cap visible. Also, they are double-sided welds, made from both inside and outside, rather than just outside.

As well as eliminating unnecessary work, all this had useful implications for fatigue performance, reduced fabrication time, and yielded much improved repair rates. It also improved safety because grinders did not need to go into the confined space inside each tendon.

Installation Sequence

The work of securing the TLP at location divided into three phases — tendon pre-installation, TLP hook-up, and post-hook-up.

Pre-installation involved connecting all tendons at the seabed and leaving them held upright by temporary buoyancy modules (TBMs) — essentially large steel cans — so that they were ready and waiting when the platform arrived. Work started with the driving of large diameter seabed foundation piles, topped by a receptacle for tendon stabbing.

Then each full tendon was made up in an assembly frame on installation vessel Hermod. Moho’s tendons involved joining 10 individual elements, mostly 79-metres long. The slightly shorter lower and upper elements carry the necessary end connector assemblies.

With a tendon fully assembled and hanging off Hermod, its TBM was clamped on near the upper end. These cylindrical steel modules are now pretty standard in the industry. Moho’s are five metres in diameter and stand 25 metres high. They provide an uprighting force of 300 tonnes, enough to support a tendon even if it floods.

The crane vessel then moved over the foundation pile and stabbed the tendon into the one-way connector there. This is always a delicate operation, much influenced by current conditions, and it went well for Moho, typically taking four or five hours per tendon. With that done, the TBM could be blown full of air and the tendon released.

With all tendons in place, the final pre-installation act was to guard against clashing between the TBMs in each group by increasing their separation distance. This is done with spacer wires that pull the outer tendons sideways towards their opposite number in the next tendon group.

In the next phase, the draft of the TLP hull was reduced enough for it to be able to move into position above the tendon tops waiting 14.5 metres below water level. Lines were lowered through each porch point on the hull to grab the top of its appointed tendon. Then a constant tension of 10 tonnes was exerted to guide the tendon into its connector as the hull was gently ballasted down.

With the hull moving towards final operating draft, all was ready for the highlight of the whole sequence. The top connectors on the tendon porches were activated to lock onto the threaded section at the tendon tops so that the hull could no longer move upwards.

Then, as the hull moved gently in the swell above and continued to ballast down, the connectors ratcheted to target draft. With final length adjustments made, the hull could be de-ballasted to put full tension into the tendons and the platform finally became a TLP in the full sense.

The one remaining act was to remove the TBMs. These were flooded and their clamps and centraliser arrays then released so that the big steel cans could be gently pulled away.

As other projects have shown, TBMs are not to be trifled with, and their removal is tricky. At Moho, there were a couple of events with these industry-standard units, which with luck will prove of relatively minor significance.

At one point the field joint coating at a tendon’s intermediate connector was removed by part of a tank mechanism as it heeled during the removal process. Also, one TBM released prematurely, before it was completely flooded, and rose to contact the underside of the TLP hull just above.

Reflecting on the installation period, Le Guennec reckons “it could be good to have something like a joint industry project about TLP installation, where engineers can get round the table to share their experiences and learn from them”.

Tendon Handling

Installation of Moho’s tendons did not cause any real problems, though West Africa’s constant swell led to standby time now and then, despite the huge size of Hermod.
Upending of the 760-metre- long tendons called for particular care to avoid them clashing with surrounding structures. As Le Guennec points out: “You don’t want to have a scratch on those connectors that might affect fatigue life or sealability.”

One factor that slowed things down somewhat was the handling of the dozen cassettes in which tendon elements were delivered to location. This method has been used before — at Kizomba — but at Moho, because of competition for space on Hermod’s deck, not all cassettes and TBMs could be taken aboard.

So the transport barge had to stay alongside the crane vessel longer to allow for shuttling of full and empty cassettes, extended by the lengthy process of opening and then dismantling them on Hermod for return in pieces to the barge.

Hull Construction

Back in the earlier days of the project, hull construction proved straightforward at Hyundai’s Ulsan yard under the $700 million contract it won in March 2013. The hull was built on a quayside rather than in drydock, with the topsides alongside, convenient for later mating.

When that time came, Hyundai had originally planned to use its well-proven method of raising the 6000-tonne topsides using jacking towers and then move the hull underneath. As it turned out, the deck-raising operation became the first job for the company’s brand new floating sheerlegs barge, HD10,000.

The final big movement at Ulsan was loadout of the completed TLP onto the submersible transport barge Hysy 278 used by contractor Dockwise for the journey to West Africa. Moving this 16,000-tonne package “was quite some operation, pretty scary” says Le Guennec, “because it was on an air cushion and there was almost no friction at all as it moved on to a vessel subject to significant heel during load transfer”.

The TLP departed Ulsan on 30 November and 38 days later arrived at Gabon’s Port Gentil. It stayed there only long enough to be floated off its transport vessel and ballasted down to wet tow draft and then set off for the Moho Nord field.

Moho Nord’s other big floater, FPU Likouf, is currently scheduled to sail away from Ulsan in August. It will be moored 350 metres from the TLP and linked by six transfer lines. Then the $11 billion Moho Nord project is scheduled to be complete by the end of this year.

See the article in Upstream Technology Online