DeepStar reaches deep for semi design

10,000-ft vessel ready for ultra-deepwater

Working at water depths of 8,000-10,000 ft is undeniably challenging. And in the ultra-deepwater prospects in the Gulf of Mexico, the oil and gas industry expects a high percentage of the prospects of today and tomorrow to be high-pressure/high-temperature wells, with pressures exceeding 10 ksi and temperatures close to or exceeding 250° F. The combination of HP/HT and ultra-deepwater conditions can be extremely challenging in designing wells and facilities and can add substantially to development costs. One of the objectives of DeepStar’s deepwater structures initiative was to design viable production options for deepwater.

Over the past five years, the DeepStar program has funded a series of studies to identify technology gaps in deepwater HP/HT production facilities and to assess the effort required to close the gaps.

These studies included both subsea developments (i.e., wet tree concepts) and direct access wells (i.e., dry tree concepts).

KBR performed the first DeepStar study in 2002. The KBR study assumed a large HP/HT oil field in 10,000 ft water depth and focused on a dry tree classic spar concept as the solution.

Technip completed the second DeepStar study in 2004, assuming a medium sized HP/HT oil field in 10,000 ft of water. The Technip study focused on the truss spar concept as the production solution.

Mid way through 2004, DeepStar funded a third ultra-deepwater HP/HT study set at the same working water depth. Aker Kværner and Stress Engineering Inc. carried out this study, which focused on a wet tree semisubmersible concept. Aker Kværner and Aker Marine Contractors carried out the hull and mooring design and installation evaluations, while Stress Engineering Inc. performed the riser design and coupled analysis.

Design concerns

According to Magne Nygard, vice president deepwater technology at Aker Kværner, the topside weight and the need to contend with high vertical weights from risers and moorings determine the size of the semi. The DeepStar semi has a total topside weight of about 17,000 tons and carries risers that are hung off from the underwater pontoons representing a suspended load of 14,500 tons, creating a total payload of 31,500 tons. The steel weight for the hull required to carry this payload is 14,000 tons. This results in a payload-to-hull ratio of 2:1, one indicator of the high efficiency of the semi concept, Nygard says.

The motions of the semi largely affect riser response, but due to the magnitude of the mentioned riser loads in 10,000 ft of water, considerable interaction occurs. The overall response of the risers is important, especially at the touchdown region. This concerns both the fatigue exposure and the extreme motions of the risers, which earlier proved to be difficult when examining conventional semisubmersible designs. The solution to this problem, Nygard says, is to extend the draft of the semi compared to a conventional semi, creating what is referred to as a deep draft semi. Increasing the draft in the range of 30-50% beyond that of a conventional semi creates the needed improvement in the response. “This does not change the basic design of the semi,” Nygard says. “We simply add some more steel and ballast water.”

Vortex induced vibration (VIV) of the risers, in some cases, can sometimes play a more important role in the fatigue design than the motions of the semi, Nygard says. The existence of VIV drives the need for VIV suppressing devices on the risers. According to Nygard, VIV suppressing devices are necessary for any of the three development options investigated because VIV occurs regardless of the type of floater the riser connects to.

The large Loop Currents in the GoM drive the mooring system design, Nygard says. The large number of risers provides a current load nearly in the same order as the current load on the hull.

Deep submerged currents are high currents that typically reach all the way to the ocean bottom at very deepwater locations near the Sigsbee Escarpment. Such strong submerged currents can require VIV suppression of the risers in the form of strakes all the way to the bottom. Adding strakes increases the drag load that is transferred from the riser to the floater and drives the mooring cost.

Aker Kværner�s deep draft semi features a two-level deck.Click here to enlarge image

Aker Kværner based its deep draft semi design on a taut polyester mooring system, with chain at the top and bottom and a suction pile as anchor. The mooring arrangement is in four groups of three lines.

With the mooring system and the assistance of chain jacks, the operator can slightly reposition the semi over the field life, which leads to shift of the touchdown point along the riser. Each movement restarts the fatigue clock, Nygard explains.

Installing risers at this water depth is a demanding task. The weight of the risers at 10,000 ft is very large. The limiting factor is pipe-in-pipe risers, where installation weight can exceed the capacity of available installation vessels.

The mooring is arranged in four groups of three lines.Click here to enlarge image

The study considered the impact of a high integrity pressure protection system, also known as HIPPS, in reducing the riser wall thickness and therefore the feasibility of riser installation. HIPPS is another research project DeepStar is funding, Nygard adds.

Production advantages

There are many advantages of applying a semisubmersible for deepwater in the GoM, Nygard says. Some of the advantages include:

At-shore or at quayside erection, completion, and commissioning in the yard, eliminating the need for costly offshore lifts that bring schedule risks with them
An efficient hull steel to topside payload-to-steel ratio
With the right focus in the design, the motions can be optimized for steel catenary risers
An inherently safe design based on “safety by distance” and separation of areas. A semisubmersible offers a large deck area and good physical spacing between living quarters and the hazardous zones, like the riser landing area and the production area with high-pressure hydrocarbon containing vessels or systems
Ease of reuse, as the platform is not strongly site dependent or water-depth dependent. The semi can be brought back to quayside for a retrofit and even dry-docked without having to carry out heavy lift operations, and the topside does not have to be removed. The semi requires only a simple fabrication process that uses stiffened plate construction, as is the case for ship structures, which many yards can provide.

The DeepStar 10,000-ft semi study concluded that the semi is an attractive candidate for ultra-deepwater production. The only technology gap identified in the study is the industry’s inability to install large pipe-in-pipe risers at this water depth, Nygard says. If additional funding is made available, DeepStar might well address this technology shortcoming in the future.

Magne K. Nygard is vice president of the deepwater technology business unit of Aker Kværner. He holds an MSc from University of California, Berkeley, and a PhD in applied mechanics from The Norwegian Institute of Technology. Nygard has a background in research and offshore classification and has been an adjoin professor in applied mathematics at the University of Oslo for eight years. His current focus is to advance Aker Kværner’s ability to design and deliver deepwater floaters. [email protected].

The offshore industry has pushed through the 7,000-ft water depth production barrier. DeepStar, with the objective of addressing operators’ future deepwater business needs, has in recent years included a focus on developing technology to enable cost-effective and reliable 10,000-ft water depth production and transportation.

In addition to funding the production semisubmersible study, DeepStar has undertaken detailed investigations into system design for a TLP and spar for 10,000-ft water depth. The JIP has also funded studies evaluating moored and dynamically positioned FPSOs for the GoM as well as use of subsea tiebacks and subsea processing.

Having several viable alternatives is important to an operator when evaluating field development options. The 10,000-ft water depth concept studies provide DeepStar participants a basis for early phase alternate evaluation and provide the vendor community a platform upon which to continue their product development.