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The move to exploration andproduction activities in deeperwaters is leading to larger and

stronger ships that are much more uele cient and saer to operate. WrtsilShip Designs successul VS491 AHS isat the oreront o these developments.Oil and gas reserves are becomingharder to fnd and exploit. Drillingurther out at sea and deeper into theocean oor - operations at depths greater than 1000eet (305 metres) are known as deepwater - used tobe prohibitively expensive, but higher oil prices andtechnological advances have made it easible. At theend o 2006, estimates indicated that some 10% oknown oil reserves, approximately 100 billion barrels,are in deepwater locations.

Support or drilling deepwater operations is providedby oshore supply vessels (OSVs). Te power required

to position and move oil and gas drilling and productionplatorms comes rom Anchor Handling ug Supply(AHS) vessels. Equipped with powerul engines and

winches, they can tow rigs rom one location to another,position and lit anchors, and also deploy items oequipment required or oil and gas production.

Rougher conditions and longer distancesEverything, including the AHS vessel itsel, has tobe bigger in deeper water, says Tor Vestbstad, SalesManager, Wrtsil Ship Design in Norway. Items

o equipment such as anchors,chain and wire are much heavierthan in normal oshore locations.Tis in turn means bigger vesselswith adequate stability and moreinstalled power. Te VS 491 is atypical example o an optimiseddesign which, in addition to its AHunctionality, has ull underdeckcapacity or items o cargo usually

handled by dedicated platorm supply vessels.As deepwater rigs are usually much urther rom the

shore, trips by support vessels can take hal a day oreven more each way, he says. Crewing and uel costsmake using one larger boat or such long trips a muchbetter option than employing several smaller vessels.

Also, uel consumption in this new generationo AHS vessel is only about hal that in vesselsdesigned beore higher oil prices and environmental

issues became part o the agenda. While much o thecurrent eet uses more than 25 tonnes/day at a speedo 12 knots, the VS 491 uses only 16 tonnes under thesame conditions, even though it is a signifcantly largervessel. Its optimised hull orm and propulsion solutionis a win-win solution that reduces running costs andimproves the environmental ootprint.

Equipped for multiple rolesAs well as providing anchor-handling and towingservices, AHS vessels can be equipped or fre-fghting

STRONGER, SAFER, ANDFAR MORE EFFICIENT

TEXT: R I C K M C A R T H U RPHOTOS: W R T S I L

Powerul andversatile members

o the ofshore oil

and gas industry,Anchor Handling

Tug Supply (AHTS)vessels come ina wide range o

shapes and sizes.

TRIPS BY

SUPPORT

VESSELS CAN

TAKE HALF A DAYOR EVEN MORE

EACH WAY.

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and rescue operations and oil recovery, andsometimes act as standby rescue vessels or

oilfelds in production. Tey have to beable to operate world-wide, have su cientpower to tow oshore structures andmaintain position while securing anchors,and also, when drilling or production workis completed, have winches powerul enoughto break out the anchors securing a platormto the ocean bed.

Fire-fghting capabilities require theability to pump and spray a minimumo several thousands o tons o water perhour, says Vestbstad. In addition to tankssuitable or use in oil recovery operations,

specialised cranes and alarge, open deck or storingand handling anchors,

AHS vessels have a large-diameter stern roller whichenables anchors, chainsand structures to be saelyreceived. Multiunctionalhigh-power winches allow

the chains and anchors to be handled in asae and controlled manner.

A series o 10 vessels to the VS491 designcurrently being built or Siem Oshore ASAat the Kleven shipyard in Norway is designedto have a bollard pull o 280-300 tonnes, hesays. (Bollard pull is the pulling orce a vessel

can exert under its own propulsion, and istested by arranging or the vessel to pull a

wire fxed to the shore and then measuringthe orce in the line.) Siem Sapphire, thethird in the series, was delivered in March thisyear and managed 301 tonnes. Just a coupleo years ago the biggest anchor handlershad a bollard pull o some 200 tonnes. Tisshows how things are changing as deepwateroperations become more common.

Bollard pull is a unction o engine

perormance, propulsion e ciency andhull shape. In the Siem Sapphire, the 301

tonnes is achieved by the two 8000 kW mainengines, a very good result in terms o theinstalled power, says Vestbstad. Te VS491is a state-o-the-art, high-end AHS vessel

with a wider, more beamy hull, shaped tomake the overall design as e cient as possiblein all its operational modes.

Safety is high on the agendaAs well as ensuring that the hull design andthe location o winching equipment allowsan AHS vessel to carry out its primarytowing and anchor-handling operationssaely, reducing the number o crew membersrequired on deck is another priority.

Dealing with a heavy anchor when theships stern is acing a 5-metre sea is verydangerous, says Vestbstad. In addition toother systems which promote sae operation,VS491 vessels in the Siem series eature atravelling crane equipped with robotic armsand controlled remotely rom the shipsbridge. As this crane can move up and downthe whole o the at deck, many di cult deckoperations can be handled without exposingpersonnel to danger.

Another design eature which makes asignifcant contribution to saety and comorton board is high-quality accommodation,

equivalent to that ound in good hotels.Modern acilities make it easier or shipownersto recruit good people and ensure they getproper rest between shits.

Te VS491 is one o those cases in whichall the parties involved are happy, saysVestbstad. Kleven Maritime, the shipyardbuilding the Siem series, is making a proft,and Siem Oshore, the company operatingthem, is pleased with the way they areperorming. Good design always pays o.

Unlike bulk carriers or tankers,whose hulls and propulsion systems

can be optimised or long voyages

at steady speeds, ofshore support

vessels have widely difering modes

o operation. Sometimes they are in

transit rom one location to another

at moderate or high speeds, and

very oten they are holding position

using dynamic positioning (DP)

technology in changing weather

and sea conditions.

Diesel-electric (DE) solutions are

common in the ofshore market.

The principle behind their operation

is to split the load between diferent

engines. The aim is to keep the

engines generating electrical power

running at 80-90% load, the point

in their operating range at which

they are most uel e cient.

One problem with normal DE

systems is that there are electrical

losses which can only be avoided by

using a mechanical solution, says

Vestbstad. In the VS491, a so-

called hybrid propulsion solution

provides most o the power or

larger loads mechanically while

covering lower levels o demandwith what could be called a

boosting electrical DE system. Its

an intelligent way o combining the

advantages o DE systems and a

mechanical solution.

Flexible and ef cientpower provision

DECK OPERATIONSCAN BE HANDLED

WITHOUT EXPOSING

PERSONNEL TODANGER.