Mini-TLP may join Spar as workhorse in deepwater production

July 1, 1999
Attacking marginal field challenges

The face of deepwater floaters is changing, driven by oil price weakness, cost cutting, and the pursuit of early production. What has been called a mini-TLP (tension leg platform) may join the versatile Spar design as the production workhorse for deepwater.

The anthem of these new designs seems to be "less is more."

Everything from fabrication time to the number of people needed to man facilities is minimized. With the future of oil prices uncertain and the cost of deepwater exploration high, operators are demanding more from offshore facilities. The necessity to get economically marginal deepwater fields onstream has led to many innovations - including the Seastar(r) TLP concept.

In late 1995, British-Borneo spotted this opportunity and assembled a small team of contractors and suppliers to perform two deepwater projects, under the leadership of Dr. William Roach of British-Borneo. The projects were then considered to be marginal and as such, not sufficiently robust to gain sanction for development. Both projects were committed to by British-Borneo, based to a large extent on the economic advantages afforded by Atlantia's Seastar production facility. With a distinct profile and attractive versatility, Atlantia hopes the "star" of British-Borneo's Morpeth and Allegheny developments will become a popular option offshore.

A second, smaller Seastar, now being prepared for loadout from Gulf Island Fabrication in Houma, Louisiana, represents a further tweaking of that pursuit. The unit will be installed on British-Borneo's Allegheny field in Green Canyon Block 254. The second Seastar is similar to the first in size and cost, though it will be installed in twice the water depths.

Size matters

In Houma, the hull and deck components of the Allegheny Seastar production facility are nearing completion. The topside equipment, mounted on a two-level deck measuring 110 sq ft and weighing 2,900 tons, is capable of processing 25,000 b/d of oil and 42 MMcf/d of gas and treating 10,000 b/d of produced water. Production comes from four separate subsea completed wells clustered in two groups of three - one north and the other south, offset laterally from the platform by a distance of 3,000 ft.

This offset accommodates the use of steel cantenary production risers (SCR) and permits adequate clearance for the MODU to access the wells in the future. A change from the flexible riser system used on Morpeth. To use SCRs, there has to be a minimum distance from the wellhead to the tieback at the platform's riser porch. The SCR option is sometimes preferable to flexible flow lines because of the economics of installation with the export risers.

Each of the initial four wells will have an independent flow line with no commingling of fluids. Three wells from the North cluster will be tied back initially. One well drilled in the South cluster will be tied back initially with plans to drill one more well in the South cluster for a total of five wells tied back, leaving one spare slot.

Flow assurance is a chief concern for British-Borneo in this development, says Glen Lochte, British-Borneo Subsea Engineering Manager. The flowlines are insulated and the short offset minimizes heat loss under flow conditions. Chemical injection is provided downhole and at the tree. There is a specially designed manifold that ties together the 4-in. production risers near the subsea trees and allows a pig to be launched down one production riser and returned back to the platform up an adjacent production riser. Coiled tubing can be deployed from the TLP to clear blocked flowlines if required. A riser end manifold is provided at each well cluster site. Rigid jumpers connect the manifold to each satellite well 40-50 feet away. The subsea production system is controlled by direct hydraulics using a flexible umbilical. An oil export line is provided which connects into the Leviathan oil transportation system. Gas is routed to the Discovery transportation system. SCRs are used for the riser portion of the export system, based on successful use in Morpeth and overall cost effectiveness.


J. Ray McDermott has the contract directly with British-Borneo to install all the field development components except the subsea trees. Atlantia Offshore Limited was responsible for delivering the complete Seastar TLP facility on an EPCi contractual basis. This contract included British-Borneo holding only the actual TLP installation contract with Atlantia furnishing all installation engineering/ management support. This helped minimize the required staffing on part of British-Borneo. The foundation piles were preinstalled in the fall of 1998. The tendons will be installed by McDermott Each tendon is naturally buoyant is stabbed/connected into each foundation pile and supported vertically using buoyancy modules located at the top of each tendon. Once hull mating is complete the buoyancy modules will be removed. There will be six 28-in. diameter tendons, two for each pontoon. Once the tendons are in place, the hull will be mobilized and installed by McDermott. After it is lifted and set in the water the hull will be ballasted down, connected to the tendons, and deballasted to pretension the tendons. At this point the hull will be ready to receive the SCRs. McDermott will install the risers and suspend them from the hull before the topsides are mobilized. Once the topsides are in place the export lines will be hooked up.


The steel risers were used on Allegheny rather than the more expensive flexible risers installed on Morpeth because of their lower cost in this water depth. Allegheny is located in 3,300 ft of water, almost twice as deep as Morpeth, which lies in 1,700 ft water depth. The deeper water and longer offset of wells made the use of the more rigid SCRs possible.

McDermott will pull in the risers and hook them up to the hull before the topsides are set (mobilized). Once the topsides are in place, the export lines will be hooked up. Allseas in April/May of this year installed the twin 12-in. export pipelines. The oil export pipeline is owned and operated by Leviathan Gas Pipeline Co. and the gas line is owned and operated by Discovery Pipeline Systems. Allseas contracted with British-Borneo to provide the lines and transport the gas for a fee. Atlantia is handling the engineering, procurement, and fabrication work, as well as the hookup and load-out of the rig. The entire installation, excluding the pre-installation of pilings and tendons, should take no more than 10 days.

Because Allegheny is the second Seastar TLP, the time required for both design and construction was reduced compared to the Morpeth project. Working from the original Morpeth design, Atlanta was able to design and construct Allegheny Seastar for roughly the same cost as the Morpeth Seastar, although it will be installed in water almost twice as deep. David Snell of Atlantia said the $86 million price tag (excluding installation) was approximately the same as a result of offsetting differences in topsides facility requirements and tendons. There will be no water injection facilities on Allegheny as there were on Morpeth and the throughput on Allegheny will be less. These differences save weight and cost on the topsides. The longer tendons on Allegheny are also of a greater diameter, 28 in. as opposed to 26 in., on Morpeth. The less expensive topsides and the more expensive tendons more or less evened out the cost of designing and building the two TLPs.

While Morpeth took 13 months for Gulf Island to fabricate, Allegheny took only 12. Atlantia has no alliance with Gulf Island, although the fabrication yard has built both the Seastar hulls. Snell said the design plans for the hull are extremely specific, so that welders, accustomed to working with tubular structures, can efficiently assemble the stiffened plate structure.

At the same time, shipyards, accustomed to welding plates, can also fabricate this detailed design. The point is that Atlantia created a flexible, scalable design that can be put out for bids to a number of fabrication facilities. In the current market, there is an excess of yard capacity, giving Atlantia the upper hand in negotiations. With the detailed design work done on Seastar, Atlantia can cast a wide net for bids. By the same token, almost any yard available could be used for construction in a tight market.

Future applications

Snell said, the hull design scales well, and can be readily adapted larger or smaller payloads. The design is such that efficiency, in terms of cost versus deckload, remains favorable.

The concept, as designed, would be viable out to 7,000-ft water depths. Beyond that depth, Snell said some modifications to the stationkeeping would be required. Atlantia sees the Seastar as a niche design focusing on what are considered marginal fields off the continental shelf. These fields don't warrant the more expensive option of a four-leg TLP, spar, or a moored semisubmersable, but could still produce a profitable return if development costs are kept low.