Taking TLPs to new depths

Tension-leg platforms (TLPs) have been used in water depths of up to 1,524 m (5,000 ft) and are generally considered viable for depths of up to 1,829 m (6,000 ft). But the technological limits of the tendon system and high costs often are named as the obstacles for deploying TLPs in deeper water.

A more detailed study conducted by FloaTEC, however, has shown that the 1,829-m mark is neither a technical nor economic limit. Increased costs of mobile drilling units have made deepwater TLPs comparatively more attractive, and existing tendon technology has been found to be sufficient for water depths of up to 2,134 m (7,000 ft).

TLPs are used as permanently installed drilling and production platforms. The concept evolved as an alternative to jacket-based fixed platforms for water depths too deep for bottom-founded structures. They are moored to the seabed using several vertical tension legs, or tendons. The hull generates sufficient buoyancy to keep these tendons under tension in addition to supporting the payload.

Tendons are pipes made from high-strength steel and are sufficiently rigid to restrain vertical platform motions. As a result, vertical motions of TLPs are smaller than those of other floating platforms. This unique feature makes them not only well suited to support various types of production risers but also provides an ideal drilling platform. Following the first use of TLPs offshore in 1984, a total of 25 have been put into operation to serve as production and drilling platforms. Two more are scheduled to be installed in the period from 2013 to 2014.

The deepest TLP is located on the Magnolia field in a water depth of 1,425 m (4,675 ft). This record is, however, about to be broken by the FloaTECdesigned and Chevron-operated Big Foot TLP. This platform is planned for installation early in 2014 in the Gulf of Mexico (GoM) in a water depth of 1,580 m (5,185 ft). Figure 1 shows the progression of the TLP water depth record over time.

FIGURE 1. The water depth record for TLPs has progressed steadily over the past three decades.

The question as to how much deeper TLPs can be deployed is as old as the TLP concept itself. Fundamentally there is no physical restriction preventing them from operating in 3,000 m (10,000 ft) or more. However, there are obstacles that make ultra-deep water – i.e. water deeper than 1,524 m – a bigger challenge for TLPs than for other floater concepts.

Tendon challenge

Tendons must be strong enough to restrain the platform’s vertical motions. They also have to be stiff enough to keep the platform’s heave, pitch, and roll natural periods below the wave energy range, i.e. below about 5 seconds. When these natural periods become too high, dynamic tendon loads increase. As a result, tendon strength and fatigue life become affected. Earlier TLPs in shallow water were designed with natural periods between 2 seconds and 3 seconds. The more recent designs for deeper water allow periods up to about 4.5 seconds. However, keeping the heave, pitch, and roll natural periods below the predominant wave energy range is one of the fundamental design principles for all TLPs. As the water becomes deeper it becomes more difficult to keep the platform’s natural periods low, a consequence of the tendons’ increasing length. As tendons get longer, their overall axial stiffness diminishes proportionally. The reduced tendon stiffness, in turn, leads to higher natural periods. To compensate for the effect of the increased length, more stiffness has to be built into the tendon system, which can be achieved by increasing the cross-sectional areas of the individual tendons or simply by adding more. Either way, the total amount of steel required for the tendons has to increase significantly with water depth (Figure 2).

Higher hydrostatic pressure is another challenge for tendons in deeper water. To prevent tendon pipe collapse, wall thickness has to be increased. As a result, its submerged weight increases. The overall weight penalty is moderate, however, since only the tendon bottom sections need to be thickened. Tendons for a water depth of 2,134 m can still achieve an overall buoyancy ratio of about 80%.

For high axial stiffness and high collapse resistance, deepwater tendons should have a large diameter together with a high thickness-to-diameter ratio. The limits for these two parameters are set by fabrication technology. Currently, tendon pipes can be produced to a maximum diameter of 60 in. and a maximum wall thickness of 1.7 in. Tendon connectors, however, are currently only available to a 44-in. diameter. So unless a welded one-piece construction of the tendon is considered, the available connector size limits the largest usable tendon diameter to 44 in.

For a conventional tendon design, the currently available pipe and connector sizes are sufficient for water depths of more than 2,134 m. According to suppliers of pipes and connectors, increasing the pipe wall thickness to 1.8 in. and the tendon connector size to 48 in. is feasible. Such increases would push the practical limit for conventional steel tendons further into deeper water.

Hull challenge

The knock-on effect on the hull due to larger tendons is small. For a given payload, the required hull displacement increase corresponding to a water depth increase from 1,524 m to 1,981 m (6,500 ft) is only about 3%. On the other hand, the required tendon steel increase to achieve this depth change is about 50%. That is because tendons are 80% to 90% buoyant and, hence, only a small portion of the hull’s buoyancy is used to support the tendon’s submerged weight. The remaining hull buoyancy is used for tendon tensioning and to support the platform weight and the payload.

It is a higher payload that often requires the hull size of TLPs to increase in deeper water. The payload increase is typically a result of higher riser and drilling loads. Often these effects are compounded by high reservoir pressures found in some deepwater fields. Also, deepwater developments tend to be located in remote areas where operators have an increased incentive to tie production from several fields into a single platform.

Although an increased hull size does not directly impose a water depth limit, it can become a limiting factor when its dimensions or weight exceed the capacity of a specific fabrication yard or transportation vessel.

Finding suitable tendon attachment locations on the hull can become difficult too. Tendons should be arranged in a way that leads to uniform tendon-loading and a high pitch-and-roll stiffness for the platform. Also, clear access to the tendon from above is required for installation. With an increasing number of tendons and increasing tendon spacing, suitable attachment points are more difficult to find. Pontoon extensions on the hull can help to mitigate this problem.

Installation challenge

The preferred installation method for TLPs is preinstalling the tendons. Preinstallation means that the tendons are already anchored to the seafloor and ready to receive the vessel when it arrives at the installation site. This method minimizes the risk of exposure to severe weather for the free-floating platform.

During the preinstallation stage the tendons must be kept upright and tensioned, which is achieved by temporary buoyancy tanks attached to the top ends. As the tendons become longer and heavier, these tanks have to become larger. As the tanks grow larger, they require the tendons to be spaced farther apart. For large tendon spacing, the tendon arrangement on the hull becomes difficult, as mentioned above.

The alternative to preinstalled tendons is a co-installation of the hull and tendons. This method avoids the use of costly temporary buoyancy modules. The downside of co-installation is that the overall duration of the installation campaign can become very long, increasing risk exposure.

Another installation challenge is the anchoring of the tendons to the seabed, conventionally done by driven foundation piles. Since the pile driving is done by means of a hydraulic hammer, such foundation design is limited by the operating depth of underwater hammers. Currently available equipment has been used in water depths up to 2,027 m (6,650 ft). According to hammer suppliers, existing designs can be adapted to work in depths beyond 2,134 m.

Economic challenge

FIGURE 2. The biggest economic challenge for deepwater TLPs is the progressive increase of the required tendon size with water depth. A TLP in 1,524 m of water and with a 30,000-short ton payload requires 23,000 short tons of steel for the tendon system, while a TLP in 2,134 m of water and with the same payload requires about 37,000 short tons of steel.

The biggest economic challenge for deepwater TLPs is the progressive increase of the required tendon size with water depth. While a TLP in a water depth of 1,524 m with a 30,000-short ton payload requires approximately 23,000 short tons of steel for the tendon system, a TLP in a water depth of 2,134 m with the same payload requires about 37,000 short tons. Considering a fabrication cost of US $6,000 per ton, these two tendon systems would amount to approximately $140 million and $220 million, respectively.

Since the hull size of a TLP for a given payload is not very sensitive to the water depth, the resulting cost penalty is modest. The hull steel weights for two TLPs in water depths of 1,524 m and 2,134 m, each with a 30,000-short ton payload, are about 28,000 short tons and 29,000 short tons, respectively. It is worth noting that for 2,134 m water depth the required tendon steel exceeds the amount of steel needed for the hull. Approximate hull costs are in the order of $280 million and $290 million, respectively, based on an assumed fabrication cost of $10,000 per ton.

Installation costs of TLPs also are important cost contributors. They, too, increase in deeper water. However, the incremental cost penalty in deeper water is not necessarily more than that of other offshore structures. A significant portion of the total installation cost is related to vessel mobilization and rigging, which is largely independent of water depth.

Installation costs of offshore platforms are highly dependent on a wide range of project-specific factors. Therefore, their economic impact on a specific TLP project has to be evaluated on a case-by-case basis.

No longer cost-prohibitive

TLPs are technically feasible for water depths up to 2,134 m without using new technology. Their cost, however, increases significantly with water depth due to their large tendon systems. For a TLP in 2,134 m of water and with a 30,000-short ton payload, the cost of the tendon system can exceed $200 million. The increase over a tendon system for a water depth of 1,524 m and with an equal payload is about 60%.

Previously, TLPs were considered uneconomical in ultra-deep water due to this tendon cost penalty. However, the recent surge in overall field development costs has shifted the economic water depth limit of TLPs toward deeper water.

Currently, the average drilling cost for a single deep-water well is reported to be between $70 million and $100 million when drilled by a mobile unit. For an entire program of 10 to 15 wells, the drilling costs are likely to exceed $1 billion. A similar amount needs to be budgeted for a 30,000-ton topside.

For today’s ultra-deepwater developments, TLP tendons should not be considered the significant commercial penalty they once were. Although high, tendon costs pale in comparison to other expenditures, especially drilling costs. With the option to drill wells from a TLP, the cost of the entire tendon system would be recovered after just three wells were drilled.

Further helping to offset the tendon cost penalty are improved well maintenance and better reservoir recovery, both of which are benefits of the TLP’s dry completion option. A holistic approach is needed to evaluate the viability of TLPs for a given field development, but they should not be ruled out for water depths greater than 1,829 m.