Deepwater pile installation from the back of a boat - a complete success
Late last year, the driven-pile anchors for Murphy Oil’s Kikeh spar, now stationed 120 km off the coast of eastern Malaysia, were installed by Technip using a MENCK underwater hammer. Actually, Technip’s choice of equipment was reasonably straightforward because no other company in the world can supply hammer systems for use at depths like those found in Kikeh field – in excess of 1300 m.
As far as pile installation was concerned, depth was not the biggest issue at Kikeh – the current depth record for driven piles is held by Kerr McGee’s Constitution spar at 1564 m. What made this project different was that, for the first time, the whole operation was performed from a relatively simple, DP Class 2 monohull vessel, the Rockwater II, rather than from a more normal Class 3 semisubmersible crane barge or similar large – and expensive – vessel.
Lutz Micheel, MENCK project manager, explains: “The main challenge we faced was shortage of space. The deck area on, for example, the Saipem 7000 is around 9000 m2; the deck on the Rockwater II is 1150 m2 – and we were not allowed to monopolise it. We succeeded in getting the hammer and its underwater power pack, plus all our supporting equipment, into an area no greater than 400 m2. However, the confined conditions on the deck of the Rockwater II meant we did not have the luxury of a back-up hammer on the project, and so mechanical reliability was paramount.”
The other challenge for the hammer’s eight-man support team was connected with the lifting equipment available on the boat: a 150-t crane with a 13-m radius of operation – and only 475 m of wire. “Again, this was inconvenient but not something we could not deal with,” says Micheel. “Our MHU 500T hammer with its MUP-01 power pack weighs only 107 t in air, and the way we have designed the hammer’s trunnions meant that upending the unit to get it over the side of the boat was reasonably easy. To overcome the shortage of wire on the crane, Technip lowered the hammer to working depth using extension pennants. It was subsequently kept there for the 10 days required to complete the project. The fact that the piles had been previously stabbed into the seabed, and so did not need to be lowered with the hammer, was obviously also a factor in our favour.”
Using the MENCK hammer, Technip installed ten 84-indiameter mooring piles for the spar and four piles for a tenderassisted drilling unit, in four clusters roughly 3 km apart, in a total net driving time of just over 21 h.
“The project was a complete success from our standpoint,” says Micheel. “Most importantly, it was free of any safety or environmental incidents. It also proved that the hammer could be deployed from a less costly vessel, with generally better availability, than the large crane barges commonly used for this kind of work.”
MHU development
MENCK introduced its first MHU hydraulic hammers in 1980. The standard MHU hammer (designated the S series) was designed for use at the surface or in shallow water (down to 400 m). Models are currently available with energy ratings of between 150 and 3000 kJ.
Despite their advanced engineering, MHU S series hammers are limited in terms of the water depths at which they can operate. As the hammer is taken deeper, a point is reached when powering it from a diesel hydraulic unit at the surface becomes hazardous and impractical. In addition, the increasing hydrostatic head against which the hammer has to operate progressively reduces performance.
The answer that MENCK came up with in the late 1980s was an electrically driven hydraulic power system (designated MUP) that could be taken to depth with the hammer. The MUP is designed to fit around the hammer like a girdle. It is connected to surface by a single umbilical cable delivering electric power, compressed air and communications. MUPs are available for use with uprated MHU T series hammers (suitable for use down to 2000 m) with energy levels between 135 and 900 kJ.
MENCK is about to launch a U series of hammers that will be able to operate at depths as great as 3000 m. The existing MUP power pack is already rated for use at this depth.
Setting the pace – and the records
MENCK deepwater pile-driving hammers have been used by installation contractors on some of the industry’s defining projects over the past 10 years and have continually pushed the depth record for pile driving. So far, MHU hammers with MUP underwater power packs have contributed to 30 projects in water depths between 70 and 1564 m.
1998 Shell’s Ursa tension-leg platform (TLP), the largest TLP ever built, is held by 16 mooring piles in 1160 m of water. Each pile has a diameter of 96 in, is 135 m long and weighs 380 t.
1999 British-Borneo’s Allegheny ‘mini’ TLP is installed using driven piles (990 m).
2001 Kerr-McGee’s Boomvang and Nansen truss spars, the first structures of their kind, set the pile-driving depth record at 1186 m.
2002 Unocal’s West Seno A TLP (1025 m) becomes the first such platform to be installed in the Asia–Pacific region. A MENCK hammer is also used to install the 28 conductor piles on this project – the only deepwater conductor driving operations undertaken so far.
2003 Exxon’s Kizomba A TLP (1190 m) is the first TLP installed offshore West Africa.
2004 ConocoPhillips’s Magnolia TLP pushes the depth record to 1420 m.
2005 Kerr-McGee’s Constitution spar sets the current record of 1564 m.
2006 Murphy’s Kikeh spar (1350 m) becomes the first platform of this kind in the Asia–Pacific region. (All projects in the Gulf of Mexico unless otherwise stated.)