Completion times improve for drillship series

Moonpool size dictates newbuild

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The Discoverer Spirit, which took 19 months to construct in Ferrol, Spain, was delivered to the Gulf of Mexico for outfitting, in readiness for a long-term contract for Unocal.
Click here to enlarge image

Discoverer Deep Seas, the last in a trilogy of deepwater drillships being built by Astano for Transocean in northwest Spain, is due to sail shortly for the Gulf of Mexico. There, it will undergo further outfitting at Aker Gulf Marine, Ingleside, in readiness for a five-year charter to Chevron, starting in September.

This was the first instance of three oil company clients (the others being BP Amoco and Unocal) agreeing to a replica design for their newbuild drilling units. It was also the first such triplicate order placed with a single yard.

Delivery has not been problem-free. The first vessel in the series, the Discoverer Enterprise, encountered construction delays and mounting costs. These were due partly to damage incurred during a freak storm (when the moorings broke free), project re-engineering and also interface difficulties arising from the selection of multiple equipment vendors in the US. However, lessons have been applied to ensure that the later two vessels were closer to their time and cost targets.

Discoverer Spirit, the second vessel to be completed, was delivered last October to Aker Marine Contractors for installation of the pre-fabricated blowout preventer, mud and drilling modules. It should be ready for service to client Unocal within a few weeks. The Discoverer Enterprise, which arrived in Texas in June a year late, was then further delayed by an extended commissioning and test program. Final costs for this vessel are thought to be around US $430 million, compared with an estimated $ 345 million for each of the subsequent two. The original price quoted for the Discoverer Spirit by Transocean was $ 318 million.

Conversion discounted

The monohull Discoverer series is designed to work in up to 3,000 meters of water in winds of up to 70 km/hr and wave heights of 4.6 meters. Operational limits in dynamic positioning mode are winds of up to 150 km/hr and waves up to 12 meters.

Dimensions of the vessels include a 255-meter length and 38-meter breadth, with variable deck load of 20,000 tons for drilling equipment and topsides modules, and crude storage capacity of 144,000 bbl. Up to 200 people can be accommodated. Each drillship is equipped with twin derricks and two drillstrings to permit dual activity drilling, if required.

Early on in the project, Transocean had favored conversion of an Aframax tanker, but Astano had misgivings. Following a rapid in-house study, it persuaded the contractor and BP Amoco that a newbuild would match their requirements better. Astano felt that the proposed moonpool installations were too large for a tanker, and also doubted whether the tanker could cope with the increase in facilities demanded for dual activity drilling.

The sanctioned solution for the newbuildings includes a two-deck layout. At the rear on the upper level, the riser rack is situated above the engine and power generation rooms. Further forward, the pipe rack is located above the auxiliary machine space and storage areas. At the front of the upper level are is the accommodation and vessel control rooms, followed by the mud pump, the 25-meter long by 10-meter wide central moonpool area and the BOP (plus other modules, including crude storage). The lower level is used mainly to house water and ballast tanks.

Both fully-equipped drilling areas, featuring Transocean's patented Expedrill technology, are situated 40 ft apart on the 80 ft by 80 ft drill floor. Two drilling crews can be housed concurrently to perform different tasks. While one area is used for drilling, the other could be employed in cementing, logging or blowout preventer change-out. The DP system is driven by six 5.5 MW thrusters. Power is provided by six diesel generating sets, four outputting 7.5 MW and two 5 MW.

All the vessels are designed for at least 20 years' service. The interface between hull and topsides has been configured to provide maximum structural integrity. Extensive analysis was undertaken at the interface of the derrick substructure/vessel main deck, in view of the unusually heavy loads that would have to be accommodated. The midship section is also designed to minimize stress at the four corners of the rectangular moonpool.

To aid continuity of operations in remote, deepwater locations, the vessels can store up to 5,800 cu meters of diesel oil, which ensures 24 days uninterrupted service without the need to re-fuel.

Delaying factors

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The Discoverer Enterprise, the first in the Transocean "lookalike" drillship series, left the Astano yard in 1998.
Click here to enlarge image

Astano's scope of work for these vessels excluded the supply, installation, fabrication and hookup of drilling facilities. "Since most of the equipment is American, the client thought it made little sense to send it to Europe for assembly," says Juan Carlos Perez, Executive VP Commercial & Marketing for parent company Astilleros Espanoles. "So we had nothing to do with the topsides." He points out, therefore, that under the terms of Astano's contract, the two vessels completed so far have been delivered to Transocean within budget.

"Our contract variations did not exceed 2%. We delivered the Discoverer Enterprise 23 months after the contract award. The problems affecting the vessel after her arrival in the Gulf of Mexico relate mainly to the fact that the module fabricators there did not fulfill their sides of the bargain. Also, the topsides module integrator could not do the job on time and to budget.

"Construction of the Discovery Spirit, which is the second in the series, took 19 months, and it has just arrived in the Gulf of Mexico. No problems have been reported. Unocal praised the performance achieved with the vessel.

"We managed to reduce our own fabrication time for the Spirit by five months, compared with the Enterprise. That represents a 25% improvement. I would say there were three main factors behind this:

  • The learning curve, repetition effect and improvement on methods adopted for incorporating the large amounts of equipment associated with the drilling utilities (including better scheduling)
  • The commissioning procedures associated with the learning curve. You have to remember, these are ultra-deepwater units with new types of equipment being introduced for the first time.
  • The extraordinarily good relationship between the client and builder regarding the second unit (as opposed to the first). The first time you deal with a new client, it takes time for both parties to learn each others' ways."

Perez sees little point in debating the way forward for the next generation of ultra-deepwater vessels. "We are having ongoing cooperation with Transocean, because you can't improve designs without having sufficient and correct feedback from the existing ones. Maybe we can develop new ideas, once the current ships have been drilling for two or three years. So far, the information flow is good."

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