HIGH PERFORMANCE LAY VESSEL HEADS FOR COMPLETION THIS YEAR

Feb. 6, 1995
Construction of a new-generation pipelay vessel is nearing completion in Singapore ahead of commissioning and sea trials later this year. Allseas Marine Contractors S.A.'s Solitaire is being converted from a bulk carrier to the world's largest dynamically positioned lay vessel. It will incorporate a recently developed and tested automatic welding system. Allseas said during a presentation to the Journal at the Sembawang Shipyard Ltd. this past December that the vessel's features

Construction of a new-generation pipelay vessel is nearing completion in Singapore ahead of commissioning and sea trials later this year.

Allseas Marine Contractors S.A.'s Solitaire is being converted from a bulk carrier to the world's largest dynamically positioned lay vessel. It will incorporate a recently developed and tested automatic welding system.

Allseas said during a presentation to the Journal at the Sembawang Shipyard Ltd. this past December that the vessel's features will permit it nearly to double current lay rates of comparable lay vessels.

CONVERSION STRATEGY

Allseas undertook to build the vessel in August 1992, despite having no contract for its use. That situation has not changed, although the company believes long, large-diameter pipelines planned for Southeast Asia, among other areas, indicate the vessel will find work.

In September 1992, the company purchased Solitaire as a 125,000 dwt, 935 ft bulk carrier and in early 1993 let a $125-million contract to Sembawang for its conversion.

Solitaire's Specifications (13149 bytes)

Total cost of the vessel was projected by Allseas in 1992 at $300 million. Converting an existing vessel rather than building one from scratch promised accelerated delivery.

Choice of a ship rather than a semisubmersible lay barge affords the advantages of greater length for more welding stations, large pipe-carrying capacity (14,600 metric tons), and speed (15 knots).

Allseas says the vessel's length and high roll period give her a pitch and roll that permit work in waves up to 13-15 ft significant wave height. Her large buffer capacity makes her less dependent on pipe offloading from supply vessels.

The numerous welding stations, continuous vessel advance, and low percentage of mechanical downtime typical of dynamically positioned pipelaying promise that average pipelay speed on a large project will nearly double speeds of the world's current fleet of lay vessels, ship or barge, says Allseas.

Solitaire will be able to install a pipeline of approximately 1,000 km in one session.

The company says that the vessel is mainly an extrapolation of technology employed on Allsea's other lay vessel Lorelay and therefore has predictable performance.

Solitaire will also employ the Phoenix automatic welding system developed by Allseas in the past 3 years and successfully used aboard the Lorelay in 1993 in the North Sea (OGJ, Nov. 1, 1993, p. 77).

DESIGN CONSIDERATIONS

Allseas has designed Solitaire to meet the following requirements:

  • Sustained layrate of 6 km/day on large diameter pipelines.

  • Vessel workability at least equal to that of semisubmersible lay barge spreads.

  • Compliance with stringent requirements for allowable pipe stress and strain limits and possessing therefore a large installed tension capacity.

  • Compliance with the highest standards of operational safety.

Severe environmental conditions, typical for northern North Sea operations, have also been specified for the dynamic positioning system.

A traditional double-joint plant will be installed for prefabrication of double-joints which will be fed to the firing line.

Use of a dynamic positioning system leads to the following advantages, says Allseas, over use of anchors:

  • Continuous vessel motion forward

  • Strongly reduced mechanical downtime

  • No weather downtime that results from weather limitations of anchor-handling spread

  • In case of deteriorating weather, capability of a fast pipe-abandonment operation

  • No potential risk of damage to existing cables and pipelines

  • Less interference with other activities near platforms: possibility of working within anchor patterns of drill rigs and moored vessels; latitude in selection of start-up and lay-down positions; quick start-ups and lay-downs, resulting in short presence in platform areas.