Spar, deep draft installations easier with heavy lift vessels

Sensing a growing market for Spar-type installations, Saipem will send its S-7000 vessel back into the Gulf of Mexico. The vessel is shown here lifting 10,400 tons for BP's Andrew project. (Photo by Alex Lanard, courtesy of Saipem) [66,452 bytes] With the rapid increase of deepwater field development in the Gulf of Mexico, a number of vessels, originally developed for the North Sea market, are finding new applications in the South Atlantic. The applications go beyond the original design

Saipem unit will enter US Gulf to pursue growing market

Eric Smith, Steven Davey
Saipem
With the rapid increase of deepwater field development in the Gulf of Mexico, a number of vessels, originally developed for the North Sea market, are finding new applications in the South Atlantic. The applications go beyond the original design basis. The heavy lift vessel Saipem 7000 is a case in point.

In fact, the consideration of this vessel, at the early conceptual design phase, can allow reservoir and field development engineers savings of both time and capital.

The Saipem 7000 will re-enter the Gulf of Mexico to install Exxon's Diana-Hoover Deep Draft Caisson Vessel (DDCV) in late 1999. In addition to installing the hull and deck, complete with facilities for accommodation, production operations and power generation, the vessel will also install the mooring system, the rigid steel catenary risers and the infield flowlines. Finally, the vessel will install the drilling package and handle accommodations support during initial hook-up, all in 4,800 ft of water while operating in dynamically positioned (DP) mode.

Vessel evolution

The DP semisubmersible heavy lift barges were originally designed to install large integrated decks on fixed structures in the North Sea. Critical design features were total lifting capacity, stationkeeping capability, accommodations capacity, and overall stability, under North Sea conditions.

In total, three such DP vessels were built, of which the Saipem 7000 is the largest and newest. Transit speed and variable deck loads were not issues originally, because mobilization distances were short and launch/transportation barges the norm. However, because of Italian maritime regulations, the S-7000 is also the fastest, at 9.5 knots. The S-7000 is 660 ft in length, has 100,000 sq ft of deck space, and can carry a load up to 16,800 short tons when working. The lifting capacity is 15,680 short tons, utilizing twin revolving cranes. The DP-3 system utilizes eight azimuthing thrusters.

The decline in use of large integrated decks in the North Sea is a direct function of the popularity of subsea field development and floating production systems. This shift in architecture convinced Saipem in 1994 to begin pursuing alternate applications for the vessel. For example, a 260 ft monopod platform, constructed in the vertical, was designed to be transported to the field on board the S-7000 and lifted into place. The approach was successfully used last year for an installation in the North Sea.

Spar installation

In order to take advantage of the DP lifting capacity of this barge, the Diana-Hoover topsides are being designed in the form of two "pancakes," rather than as a series of five or more smaller modules. The reason for constructing the deck in this way is to minimize the number of lifts, the effort expended in offshore hookup, and to allow for even weight distribution during the installation process.

Spar type systems, unlike other floating production systems, cannot have the topsides installed prior to tow out. The intrinsic, deep draft nature of the spar type hull requires installation of the topsides in deepwater, once the hull has been uprighted at the installation location. Moreover, the small footprint of the spar type hull, versus other floating systems, means that they are highly susceptible to heeling when asymetric loading occurs during the installation operation. In other words, placing a 5000-ton load off center means that the hull is no longer plumb when the second 5000-ton module is lifted into place.

Because of the reach and capacity of the S-7000, the asymetric loading problem disappears. By utilizing two, full span, pancake decks, each designed with symmetrical loads, the hull remains plumb during the installation process. Alternatively, depending on the topsides weight/capacity, a single fully integrated deck can be installed.

Because one or two large lifts occur, rather than a number of smaller lifts, proportionately more of the hookup activity can occur on dry land. The activity that remains, can be carried out in two phases, the first with the 800-bed, S-7000 present. Once basic power and housekeeping capabilities are in place, the spar type unit can serve as its' own accommodations unit. The only caveat is that the decks must be designed, from the outset, to take full advantage of the S-7000's DP lifting and accommodation capabilities.

J-Lay operations

In 1994, studies commenced concerning the conversion of the S-7000 into a combination derrick/pipelay barge through the addition of a deepwater J- lay pipelay system to the vessel. The original target was the Oman-to-lndia pipeline, a gas trunkline in up to 10,000 ft water depth.

While this particular project has not been installed, by Saipem or other bidders, Saipem continued to move forward and to market the vessel, in its' combo J-lay configuration, for work in the South Atlantic. The J-lay system pipe diameter-water depth capacities were cited previously. At great depths, the wall thickness of the pipe exceeds 50 mm and pipe weights exceed 1 ton/meter.

Much like the smaller DP monohull pipelayers, the idea was to develop market applications that utilized the vessel during the North Sea winters, normally a layup period for the larger North Sea semisubmersible construction barges, on projects in the South Atlantic.

One advantage of the monohulls - comparatively high transit speeds - typically 11-14 knots, means that the expense associated with mobilizing from the North Sea to the Gulf of Mexico, Brazil, or West Africa is not prohibitive in terms of time or cash. Most of the semisubmersible fleet is considerably slower, in the 4-6 knot range. The S-7000 is in the middle, at 9.5 knots.

The big advantage of the semisubmersibles, as pipelayers, is that they can carry virtually everything they need on board. When re-supply is required, their more robust DP systems and crane capacity mean that they can be re-supplied at the work location, rather than having to return to an onshore supply base or reeling base.

The historical trend then has been for the monohulls to cross the equator twice a year while the semisubmersibles lay idle in Rotterdam during the North Sea winter. However, when a longer pipeline is required, the multiple mobilizations required for the monohulls, to return to the reeling base, means that the semisubmersibles have an advantage. One rule of thumb says that if re-supply requires a single return to the spooling base, the monohulls can still compete; if two, then the semisubmersible comes out on top economically.

In offshore fabrication lexicon, the South Atlantic includes the Mediterranean as well as the Gulf of Mexico. The S-7000 has previously worked in Brazil for Petrobras, in the Gulf of Mexico for the installation of the foundation template for Conoco-Joliet, and also in South Africa in 1991.

A South Atlantic lifting project for the S-7000 has been completed and involved the installation off The Congo of the Kitina platform for Agip. The vessel is mobilizing to the Usari field for additional installation work before returning to the North Atlantic for the 1998 summer season.

Concurrent with these activities, design and construction of the new deepwater J-lay system is underway, with installation on the vessel scheduled for the early fall of 1999. The vessel will then mobilize to the Gulf of Mexico to begin installation of the Diana-Hoover DDCV. This will be followed by installation of the topsides and several days of hookup support. In December of 1999, the vessel's J-lay capabilities will be used to install flowlines and steel catenary risers for Diana-Hoover. This will be followed by the final lift, the installation of the onboard drilling rig. The S-7000 will then return briefly to the Northern Hemisphere for the 2000 summer season.

Economic rationale

The basic economic rationale for using the S-7000's lifting capacity, both in West Africa and in the installation of the Diana-Hoover deck, is the same as that which supported its' use in the North Sea over the last 10 years. Fewer, larger modules minimize offshore hookup time and consequently, the time and cost to first production. In addition, the remaining offshore scope of work can be accomplished in minimum time without major concerns about accommodation support.

The water depth of Diana-Hoover, at 4,800 ft, requires the use of DP. The S-7000 holds the world's record for lifting while on DP (11,600 short tons), with BP's Andrew project in the North Sea, accomplished in May of 1996. Prior to the arrival of a heavy lift vessel, the best alternate for spar type deck installations required multiple lifts of smaller modules, of about 4,500 short tons each, utilizing a large DP monohull crane barge.

The limits of the smaller single crane meant that deck loading on the spar was necessarily asymetric, requiring the installation of a costly ballasting system on the spar which counteracts the heel induced in the hull until all of the smaller modules were in place.

In the case of the Diana-Hoover DDCV, the lifting capacity, vertical clearance, and reach of the S-7000's twin cranes will allow for the installation of two large symetrically loaded decks without the need to install a ballasting system. As expected, the effort in scheduling the hookup activity during the initial phase, until the DDCV is self sustaining, will be significant.

Finally, the large deck area and capacity mean that virtually all of the materiel for a typical deepwater installation can be carried onboard, leading to another economic argument based on a single vessel mobilization.

Moorings, risers, pipelay

Because of J-lay capability, the S-7000 can install all risers, moorings, and infield pipelines up to 26 in. in 10,000 ft depths and 32 in. in 6000 ft. The vessel can carry 3,000 tons of pipe with individual joints of 160 ft. This quadruple joint allows for rapid laydown of the pipe, despite the fact that J-lay is dependent on two welding stations, once the pipe is in a vertical format, as opposed to conventional S-lay with multiple horizontal welding stations. Limitations on S-lay in deep water center on the stress induced in the pipe as it transitions from the horizontal to a near vertical configuration.

There are two limitations of the J-lay system:

  • Shallow water limitations for larger diameter pipe
  • The time required to complete the single vertical weld.
J-lay's shallow water limit for lines of 20 in. and greater is 650 ft. In shallower water, the bending stresses associated with J-lay limit the pipe diameter, so that 14-in. pipe is the maximum diameter for 325 ft depths. The ultimate shallow water depth for 4-in. pipe is 160 ft.

Saipem uses the proprietary Passo welding system on all of its' vessels, but has a new system under development that should be ready for commercial use within two years. The new system will be able to weld deepwater, large diameter pipe with wall thicknesses exceeding 50 mm in roughly 1/3 the time required by the Passo system. This will boost lay rates for all the Saipem vessels.

A further advantage of the ability to lay large diameter pipe in very deep waters is that field geometry can be changed to allow multiple hub-and-spoke development schemes with larger diameter tiebacks. This would be preferable to the current approach in which a series of remote satellites are all connected to a single tieback location, utilizing a multitude of small diameter lines. From a reservoir engineer's viewpoint, large diameter pipe capability allows for deepwater field designs that provide for higher depletion rates, due to lower variable deckload requirements for on board compression and power generation systems.

A vessel such as the S-7000 can save as much as 6-10 weeks and $40-60 million on a typical Spar-type installation. However, to optimize life of field economics, the vessel's capabilities need to be incorporated prior to finalizing the field development design.

Saipem considers the South Atlantic in general, and the Gulf of Mexico in particular, as the premier region for developing integrated design approaches for the optimal extraction of petroleum resources in deepwater.

Authors

Eric Smith is Manager of Market Analysis for Saipem SpA, based in Milan, Italy. He has 15 years of experience in the offshore construction market. He is a graduate of Georgia Institute of Technology (B.ChE) and Tulane University (MBA).

Steven Davey joined Saipem this year as Resident Manager in Houston. He attended the University of Manchester and has an executive MBA from Harvard University. He has 30 years of experience in the offshore arena, much of it with Stena Offshore.

Copyright 1997 Oil & Gas Journal. All Rights Reserved.

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