The Holstein truss spar, the largest ever built, was designed and constructed in 35 months, with the design starting in May 2001 and construction starting in April 2002. Due to its size, the hull could not be transported in a single piece; it was constructed in two yards, in Finland and the U.S. The dry transport of the hard tank from Finland was followed by a unique launch of the truss section and wet mating of the hull prior to wet tow to site. Innovative approaches were required to accomplish these project requirements, while satisfying BP's quality and safety requirements.
The Holstein field is located in an area of the Gulf of Mexico predominated by loop and eddy currents generated by the Gulf Stream. These currents resulted in: the largest mooring system ever installed for a spar, the heaviest and longest suction piles as well as considerable challenges over hull Vortex Induced Motions (VIM), and riser Vortex Induced Vibration (VIV).
The most significant innovation in the Holstein spar is the implementation of the Spar Supported Vertical Riser system, making Holstein the first spar to use a TLP type tensioning system with individual RAM style tensioners to support the Top Tensioned Risers (TTR) in lieu of the buoyancy cans used on all other spars to-date.
The paper describes those aspects of the project which made it unique and provides insight as to how challenges can be addressed through innovations or avoided in the future.
The production, drilling and export requirements for the Holstein field dictated a spar hull capable of supporting 47,000 short tons of payload. In addition, the centerwell is designed to accommodate either a buoyancy can riser support or a Spar Supported Vertical Riser (SSVR) option. The selection of the SSVR by the project team came after the centerwell dimensions had been set. The resulting Holstein hull displacement is 105,000 tons, the largest for any spar ever built. In comparison, the first production spar, Neptune, would fit inside the centerwell of Holstein. Its size puts it in between the Thunderhorse semi with a displacement of 130,000 tons and the Atlantis semi at 89,000 tons. See Table 1 for more details of the main weight characteristics of the Holstein Facilities. The Holstein field is located in Green Canyon 645.
Table 1. Weight Characteristics of the Holstein Facilities. (Available in full paper)
The purpose of this paper is to describe some of the challenges encountered during the design and construction of the Holstein truss spar and the innovations to overcome such challenges.
The paper addresses key challenges in order to give the reader an appreciation of the impact they can have on projects of this magnitude and highlights the fact that technology, if not available at the start of the project, could be developed and tailored to address the challenges. The impact of the challenges in the areas of: Hull configuration and global performance including VIM; hard tank design and fabrication; truss and soft tank design and fabrication; hard tank to truss mating; Top Tensioned Risers (TTR); and Steel Catenary Risers (SCRs) will be discussed.