www.of fshore-engineer.com NOVEMBER 2005

PLUS: FLOATEC’S ERIC NAMTVEDT ON THE PATH TO TECHNOLOGY AGNOSTICISM

REQUIRED READING FOR THE GLOBAL OIL AND GAS INDUSTRY SINCE 1975

Barging inon Trent

Mexico’s deepdilemmaTiny Timorjoins the frayDouble firstfor Baobab

Mexico’s deepdilemmaTiny Timorjoins the frayDouble firstfor Baobab

Barging inon Trent

oe_novcv (EUROPE) 21/10/05 4:06 PM Page 1

As a new connecting bridge to thePerenco-operated Trent platformwas lowered into place in mid-

September, it marked the conclusion of animpressively compact 11-month design,build and installation programme for thenovel offshore structure at the other end ofthat bridge – a mobile offshore applicationbarge (MOAB).

The unmanned MOAB, elevated some20m above the sea surface alongside theexisting Trent fixed production platformin UK block 43/24, houses compression andprocess equipment. The new facility canhandle 3.4 million m3/d of gas and willallow independent Perenco to recover anadditional 1.4 billion m3 of gas from itsTrent and Tyne fields, in addition toproviding flexibility in accommodatingthird party gas streams in the ETS pipelinesystem linking Trent to the Bactonterminal. With an overall project price tagof £25 million to achieve these goals,MOAB sits at centre stage in demonstra-ting its cost effective credentials.

The MOAB concept was developed bynaval architect and marine engineeringconsultancy Overdick of Hamburg inGermany, targeted at fast track small- tomedium-sized offshore developments. Itcan serve as a standalone platform or as anannex as in the case of Trent, and canfulfil a variety of platform roles –

compression, production, wellhead,accommodation among them.

‘Whatever duty the MOAB performs,’explains Overdick partner AndreasRosponi, ‘the key factors are it allowsconcurrent engineering and fabrication, itis self-installing, simple to remove andreadily deployable. These combine to give

a solution that does not require heavy liftvessels and offers significant savings interms of schedule and capex.’

The Trent MOAB is the third of its kindto be designed by Overdick, with its twopredecessors having been contracted byoperator Talisman in Malaysia. In 2003, acompression and separation annexplatform was installed in Talisman’sBunga Raya field in the Malaysia/Vietnam joint development area in 56m ofwater, while earlier this year a standaloneintegrated wellhead and production MOABwent on location in 70m of water in theSouth Angsi field in Malaysia’s SouthChina Sea.

‘With deck payloads of 2500t and 4400trespectively, the first two were largerstructures than the latest one for Trent,which has a payload of 1200t,’ notesRosponi. ‘But the sizes and duties of thethree demonstrate the flexibility of theconcept.’

The concept he refers to might appear atfirst to be akin to using a conventionaljackup platform – the MOAB consists of amain hull supported by a steelsubstructure on the seabed and fourcorner legs extending above the seasurface. The ‘barge’ is wet-towed under itsown buoyancy to the offshore field whereits legs are lowered and its hull jacked upabove water. > page 26

w w w. o f f s h o r e - e n g i n e e r. c o m O F F S H O R E E N G I N E E R | n o v e m b e r 2 0 0 5 | 25

MOAB scores North Sea first

A neat engineering solution for building and installing fast track offshore facilities hasmade its debut in the southern North Sea. In this OE exclusive, Terry Knott talks to thedesigners of MOAB on the thinking behind the concept and describes its successfulinstallation for extending field life and enhancing recovery in the Trent gas field.

A neat engineering solution for building and installing fast track offshore facilities hasmade its debut in the southern North Sea. In this OE exclusive, Terry Knott talks to thedesigners of MOAB on the thinking behind the concept and describes its successfulinstallation for extending field life and enhancing recovery in the Trent gas field.

MOAB scores North Sea first

TOP: MOAB barge (left) installed and bridge-linked to Perenco’s Trent productionplatform in UK southern North Sea.

ABOVE: Developed by Germany’s Overdick,the MOAB concept was also used inSoutheast Asia.

installation

oe_p025 MOAB 21/10/05 10:42 AM Page 25

installation

26 | O F F S H O R E E N G I N E E R | n o v e m b e r 2 0 0 5 w w w. o f f s h o r e - e n g i n e e r. c o m

‘The main difference between MOABand a traditional jackup is that you don’thave to buy an integral jacking systemwhich then sits idle for 10-15 yearsoffshore, while still requiring mainte-nance,’ Rosponi points out. ‘The MOABdesign does not include permanentlyinstalled jacks – we use temporary strandjacks to lower the legs and raise the hull,and these are hired for the job and thenremoved. And unlike a jackup which isclassified as a seagoing vessel andtherefore must be flagged and have amaster and crew onboard, the MOAB isclassed as a fixed installation which onlyfloats temporarily, and hence does not

attract the costs of national flagging.’Indeed, says the company, when

Overdick was first approached in March2004 by Dixon Marine Consulting,representing Perenco, a jackup solution ofsome kind was being sought in therevamping of Trent to include acompression and process plant extension.MOAB was selected in August 2004 as a‘self installing’ solution for the 50m waterdepth, capable of 15 years of operation inexacting North Sea conditions.

Building the table topWork on detailed design and fabricationbegan in earnest two months later with

EPC contracts awarded by Perenco toKeppel Verolme in Rotterdam for the bargestructure and to Solar Turbines inHouston for the topsides modules –Overdick acted as ‘nominated subcontrac-tor’ for the engineering of the structureand transport and installation operations.

Keppel Verolme’s contract called fordelivery of the ‘table top’ platform – bare oftopsides – and the 30m bridge link. The550t hull, measuring 30m x 30m x 5.5m, wasprefabricated in five sections in theshipbuilder’s workshops and assembledinto the finished hull on the side of the drydock. To speed up the schedule, the cornersections of the hull, which guide and hold

Four 9m diametersuction cansprovide the seabed foundationfor the barge.

1500t bargesubstructurebeing builtaround the hull.

550t hullbuilt insections byKeppelVerolme,assembledon thedockside,and lifted intothe water fortow into drydock.

oe_p026 MOAB 21/10/05 10:43 AM Page 26

provides the seabed foundation for theinstalled platform – these were alsooutsourced by Keppel Verolme, as was thebridge, weighing 56t with allinterconnecting pipework installed.

The 75m long, 2m diameter legs,manufactured from plate up to 40mm thickand 80mm thick at the waterline to resistboat impact, were assembled onto thesubstructure by two Mammoet crawlercranes, which upended the completed legson the side of the dock and held them inplace for welding. The interconnectingbracings of the substructure were quicklyadded, followed by seafastenings betweenhull and substructure.

Next came the ‘hired ‘equipment’ whichwould make possible the installation of theMOAB in the field. For substructurelowering, four 500t strand jacks weremounted outside the hull on cantileveredplatforms, and were connected by steelcables to the substructure.

For deck lifting, eight 300t strand jackswere mounted inside the hull at thecorners. Steel cables passed through hullto anchor blocks at the top of each leg. Inaddition, a single 100t strand jack wasplaced at the top of an A-frame mountedbehind the hinged bridge, which wouldserve both as seafastening and to lower thebridge to the Trent platform. >page 28

installation

w w w. o f f s h o r e - e n g i n e e r. c o m O F F S H O R E E N G I N E E R | n o v e m b e r 2 0 0 5 | 27

the legs, were outsourced to a localsubcontractor. The hull was completed insix months in May this year and then liftedinto the water by sheerleg crane and towedinto dry-dock.

Fabrication of the 1500t substructureand legs progressed in parallel. Thesubstructure, which consists of fourpartially braced leg towers, was builtindoors out of prefabricated and pre-shaped tubulars – SIF supplied the bulk oftubulars with some from EEW. Thesubstructure elements were lifted into drydock and located around the hull. At thebase of each corner of the substructure isa 9m diameter, 5m high suction can which

CompetedMOABfloated andtowed out tothe field.

Hull,substructureand legscompleted,followed bytopsidesmodulesinstallation.

MOAB arrivesin field, thelegs arelowered anddeck jackedup. FARRIGHT: Leg inclamp guides.

oe_p027 MOAB 21/10/05 10:43 AM Page 27

‘The strand jacks are at heart of theMOAB concept,’ Rosponi explains. ‘Theyare readily available for hire, they arecompact, and they have very large liftcapacity.’

For the Trent MOAB, all jacks andassociated power packs and controls weresupplied by ALE Lastra.

Other hired equipment included fourcan-mounted suction pumps, plus onespare on deck, for pumping water from thesuction cans as they penetrated the seabed.The pumping system, capable of removing300m3/h at 500kPa, was delivered andoperated by Norway’s NGI and Framo.

Structural assembly of the barge wascompleted in June.

‘The topsides modules arrived in July,’says Rosponi. ‘The under-deck of the hullrequired additional reinforcement of 50t toaccommodate the point loads of thetopsides modules, which was done whilethe modules were en route from the USAby adding flanged webs and gussets tostiffen the deck. This activity was alwaysin the plan, as we had only specified toSolar a total weight not to be exceeded of900t, a horizontal and vertical centre ofgravity envelope, and footprint boundary.But addition of only 50t of steel to supporta 900t topsides is minimal, and the payoffwas in the fact that we did not have tomanage detailed design interfaces betweenhull and topsides.’

The topsides package included two

turbo-compressors, two turbo-generators,one slug-catcher for an incoming gaspipeline (with a riser in the north-east leg),a process module for handling liquids inthe gas, switchgear and control rooms.Perenco supervised installation of themodules on the deck over a seven weekperiod, preparing the MOAB for sailawayat the end of August.

All in oneThe completed MOAB was towed to theTrent field as a self-floating structure fromRotterdam by four Smit tugs over a periodof 65 hours in September.

‘The design allowed us to tow in seastates with up to 4m significant waveheights, and install with waves up to 1.5mhigh,’ notes Rosponi. ‘We were fortunateon the day of installation that the weatherwas very good and never exceeded 1mwaves. Plus the waves were of very shortperiod, which gave us basically no heave atall and very little roll.’

With seafastenings removed, the legsand substructure were lowered to theseabed using the four outboard jacks. Asthe horizontal bracing of the substructurebecame submerged, the upper clamp –each corner has a lower and upper clampfor final fixing of the structure’s legs – wasclosed around each leg to act as a guide.Rollers inside the clamps enabled thesmooth lowering of the leg. After touch-down, by using the four lifting jacks, the

deck was raised to leave around 0.5m ofhull remaining in the water, providing1000t of on-bottom weight. At the seabed,the suction cans self-penetrated throughthe first 2m of sand. A carefully controlledcombination of jacks and suction pumpsthen pushed the cans down to a depth of4.5m into the seabed, achieving a preloadof 12,000t, after which the deck waselevated to around 20m above water to belevel with the Trent platform.

Final fixing of the hull to the legs wasmade by closing both upper and lowerclamps at the four corners for horizontalloads, and installing two giant bolts thoughthe cable eyes in the anchor blocks on topof the legs – which were now down at decklevel – into the deck. The 3.5m long, 160mmdiameter bolts are manufactured bySuperBolt, says Rosponi, enabling them tobe tightened by normal torque wrench.The entire installation operation, plus thebridge link being established, took around72 hours, excluding standby time.

‘Installation of the Trent MOAB as acomplete unit was a first,’ he adds. ‘Thetwo Malaysian platforms were installed instages – substructure first, hull and legssecond, although they could have beencompeted in one if desired.

‘We believe the MOAB concept is veryrobust economically for payloads in the700t to 5000t range, and provides a veryattractive solution for fast track facilitiesinstallation.’

installation

28 | O F F S H O R E E N G I N E E R | n o v e m b e r 2 0 0 5 w w w. o f f s h o r e - e n g i n e e r. c o m

OE

56t bridge beinglowered into place tocomplete the 72 hourinstallation operation.

oe_p028 MOAB 21/10/05 10:44 AM Page 28