The initial concept, by Allseas technical director W.P. Kaldenbach, was of a vessel capable of lifting entire platforms, and in 1987 Allseas declared its intention to build it. The initial idea featured two rigidly connected, self-propelled supertankers, with a large slot at the bows enabling it to install platform topsides in one piece. Early designs featured a flotation and ballasting system and active motion compensation system to facilitate a controlled transfer of a topsides’ weight from the vessel to a platform substructure. Allseas developed the original idea to include steel jacket installation, jackets and topsides removal and rigid pipelay capabilities. The emphasis switched from the conversion of two existing tankers to a new-build hull in 2004, though retaining the catamaran concept. The decision was prompted by a lack of suitable vessels to convert, the lower costs associated with a new-build, and the need to house sophisticated equipment – such as a dynamic positioning system – in the hull. In 2007, twenty years after the idea was first conceived, Allseas announced plans to build the Pieter Schelte, a twin-hulled platform installation / decommissioning and pipelay vessel. Named after the offshore pioneer Pieter Schelte (father of Heerema), the design featured a lifting system at its bows for lifting platform topsides up to 48,000 tonnes (53,000 short tons) and a lifting system at its stern for lifting steel jackets up to 25,000 tonnes (28,000 short tons). The design also included pipelay equipment to handle pipe diameters ranging from 15–175 cm (6–68 inches) at water depths exceeding 4,000 m (13,000 ft).
Pioneering Spirit is the world’s largest vessel, in terms of her gross tonnage (403,342 gt), breadth (123.75 ;m or 406 ft), and displacement (1,000,000 tonnes or 980,000 long tons). The maximum 48,000-tonne (53,000-short-ton) topside lift capacity is achieved by operating as a semi-submersible. For removal of topsides, the vessel straddles the intended payload with the slot formed by the twin bows. The slot measures 122 m × 59 m (400 ft × 194 ft) (L×W). After straddling the payload, Pioneering Spirit takes on ballast to lower, and two sets of eight (one set per bow) retractable motion-compensated horizontal lifting beams are slid under the payload. Once the load is secure, the vessel offloads the ballast, rising in the water and partially transferring the load to the beams. In the final stage a fast lift system is used that lifts the payload up to 2.5 m (8.2 ft) in 15 s. Two tilting lift beams for the installation or removal of steel jackets, up to 25,000 tonnes (28,000 short tons) in weight, will be located at the vessel’s stern. A 5,000 tonnes (5,500 short tons) special purpose crane built by Huisman is scheduled for delivery in the second half of 2018. The tub mounted crane will be available for additional lifts for jacket and topsides installation such as pile handling and bridge installation.
As per update on July 2019, Allseas is planning to upgrade the lift capacity from 48,000 tonnes to 60,000 tonnes. This reason for the same is to enable the removal of Statfjord A platform in the North Sea. The details of the upgrade are currently unknown.
Allseas also constructed two barges to assist Pioneering Spirit. If the water depth is not sufficient to allow the vessel to approach the dock, Pioneering Spirit can unload structures to Iron Lady, a 200 m × 57 m (656 ft × 187 ft) (L×W) barge with a shallower draft. Bumblebee was built specifically to store the Stinger and STF when it is not in use.
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