The Spar Platform: A Design That Transformed Deepwater Development
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The spar is the only successful dry-tree solution for deepwater production that can operate successfully in the deepest fields and the most severe environments. Its deep draft results in natural periods outside the range of waves, which has led to its wide acceptance for different field scenarios. The complete paper is an extensive review of the evolution of spar designs, focusing on the progression of work that ultimately led to the application of a transformative concept to the oil industry.
The spar can support a drilling rig as well as top-tensioned production risers in water depths thousands of feet greater than the water depth limit for a tension-leg platform (TLP). It is especially well equipped to support steel catenary risers (SCRs) using the pull-tube option, which allows the SCR to serve as a continuous welded steel containment for hydrocarbons from the seafloor to the topsides and protects the riser from vortex-induced vibration in the fastest part of the current profile. Broadly speaking, there are three configurations of spars: classic, truss, and cell, with the common feature being that the center of buoyancy is higher than center of gravity. Table 1 of the complete paper lists all oil and gas spar production platforms that have been installed at the time of writing, in chronological order of installation.
The complete paper devotes several pages to the spar’s initial development, including the crucial role of Edward Horton, the inventor and designer behind the spar production and storage concept and the TLP, and some of his colleagues. Years of development and navigation of various design challenges culminated in the installation of the Neptune spar in 1996 on time and budget. After the installation and success of Neptune, several other classic-design spars were implemented.
The riser system on the Neptune spar had two unique features: buoyancy cans provided the tension, and the riser passed through a point of high bending and potential wear at the keel. The keel joint was a straightforward design; a sleeve around the main riser pipe provided wear protection and distributed the bending in the riser to two endpoints rather than at a single contact point.
Another important innovation in the spar design is the mooring system. Because of the minimal motion, at the mooring connection points near the center of spar-pitch motions, the mooring legs may be designed as semitaut members with uplift on the anchors.
The truss spar replaces the steel shell midsection of the classic spar with a truss structure. This saves steel weight and drag and was thought capable of reducing the issue of vortex-induced motion (VIM).
In February 1995, Horton built and tested a truss spar specifically sized for a Chevron field. No strakes existed on this model because it was thought there would be enough damping from the truss to mitigate the VIM without strakes. However, this turned out not to be the case, so strakes were added to the hard tank of the truss spar.
The truss spar includes heave plates between the bays. These plates have the primary benefit of trapping added mass to increase the heave period. The heave plates also add a considerable amount of damping in heave, making it possible to design spars that were shorter than the classic spars. This was a major construction advantage because the hulls could now be built in one piece in Finland and dry transported to the US. The truss-spar design quickly increased in popularity, and several contracts were signed through the late 1990s and early 2000s.
The initial classic spars and the truss spars had provision for a drilling rig and a workover rig, or only a workover rig, on the spar platforms with top-tensioned risers as the primary production risers producing from the wells underneath the platform. The favorable spar motions even enabled the spar to be offset laterally for a drilling strategy in which the spar was displaced on its moorings and a mobile offshore drilling unit was positioned over the well pattern to drill new development wells. The smaller heave motions of the spar made feasible the use of the top-tensioned risers.
In 2001, Horton developed the idea of the cell spar for smaller and mid-sized fields. This design consisted of tubes, each approximately 20 ft in diameter, assembled to form a ring to provide buoyancy and support for a deck. The motivation was to reduce fabrication costs and schedule. The cell spar’s construction contrasted with the complicated and multifaceted process required for construction of classic and truss spars. The tubes (cells) of the cell spar could be rolled and ring-stiffened using methods familiar to Gulf of Mexico fabrication yards. Longitudinal stiffening was not required. The hull could be constructed in the Gulf, and the long transportation time of the hull from Finland could be avoided.
Kerr McGee’s Red Hawk spar (Fig. 1) was fabricated completely in Ingleside, Texas, on the Gulf Coast.
Wet Tree Spars
Initial spars had been used as a dry-tree concept with top tensioned risers. The production came from the wells located under the spar platform and exported through SCRs or flexible risers, which are easier to install and cheaper compared with other riser options. Because of smaller heave and surge motions, a spar can support SCRs with sufficient strength and fatigue performance. This made possible the option of subsea tie-back import risers using SCRs from wells located away from the platform.
Use of pull tubes for supporting SCRs is unique to truss- and cell-spar platforms because the long hull length allows gradual transition of the riser in the hull from vertical to the hangoff angle required at the exit point. The open structure of the truss section also helps with flexibility to route the pull tubes through the hull structure from the hard tank center well to the exit point on the edge of the soft tank and at the desired hangoff angle at the soft tank exit point from the hull.
The pull-tube option does not require underwater mechanical connection in the risers and, therefore, eliminates any potential leak path. It also removes the need for saturation diving or remotely operated vehicle jumper installation. All wet-tree spars have been truss spars or cell spars except for one.
Topsides installation and Floatover
The first spars had to be towed to deep water and upended before the topsides could be installed. Generally, a temporary deck was used for mooring and riser installation before the topsides came out. Small topsides could be installed by a derrick barge single lift, but larger topsides required multiple lifts and long offshore hookup times. Semisubmersibles and TLPs could have their decks installed and commissioned quayside. For spars to be handled in the same manner, however, the large commissioned topsides would have to be floated over the spar. Several research and development efforts, including numerous model tests since the 1990s, have been focused on developing topsides installation for the spar using the floatover method.
There have been several large topsides deck floatover installations on gravity-based structures, TLPs, and semi-submersibles. These were mostly in calm water in Brazil and in the fjords near Stavanger or Haugesund in west Norway. Offshore floatovers had been performed on several fixed platforms, but on only one floating platform, the Auger TLP in the Gulf of Mexico.
The floatover technology was promoted for spars in the early 2000s. In 2005, Murphy Oil agreed to use the floatover method for the Kikeh spar in Malaysia. Topsides deck installation by catamaran floatover was also performed later on the Aasta Hansteen spar topsides weighing approximately 24,000 t in the fjord near Digernessundet in west Norway, where sheltered deep water is available to perform installation by floatover. In areas around the world where sheltered deepwater areas are available, such as in west Norway and east Canada, floatover topsides installation is recommended.
Future of Spar Platforms
A number of promising future potential applications exist for spars. First, the emerging lower Tertiary play in the Gulf of Mexico requires solutions that address the challenges of 20-ksi wellhead pressures. A dry-tree solution could be instrumental in development of these fields, and the spar is the preferred dry-tree solution. The industry is also trending toward smaller, more-efficient production platforms. Spar applications may provide optimal solutions for different field-development and regional host options.
Future development or utility-support platforms may require a much smaller deepwater platform. They are likely to be unmanned in order to be economically competitive with other energy investments. The payloads for these unmanned platforms will generally be much lower than those of all previously installed spars. The installation cost of these platforms is significantly lower than that of the existing manned spar platforms.
The Spar Platform: A Design That Transformed Deepwater Development
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