Studies clear TLP cost, depth limit misconceptions

Next generation technology ready for new deepwater regions

Progression of TLP deployment depths.

Authors:
John Chianisis the founder and Managing Partner of Han-Padron Associates´ Houston office, which has been responsible for detailed design and fabrication support of the Auger, Mars, Ram/Powell, and Ursa TLP hulls, and is now responsible for detailed design of various structures on the Marlin TLP project. Chianis has nearly 20 years of engineering experience in deepwater platform design and related advancements. He participated in his first TLP project in 1979. He is currently the Structural Design Manager for the Marlin TLP project, and was recently appointed to the ISO WG5 Panel 2 floating structures committee.
Philip Pollis an engineer for Han-Padron Associates specializing in the development and application of automated finite element analysis procedures for TLP design projects. He has over eight years of experience developing and applying engineering analysis methods in the fields of aerospace, mechanical, and offshore engineering.

Recent successes of tension leg platforms in the Gulf of Mexico are leading to continued expansion of this deepwater concept, both domestically and worldwide. The tension leg platform (TLP) offers numerous advantages over other floating production concepts for specific ranges of payload and water depth. This article discusses concepts of the design and implementation of TLP technology which have contributed to the system´s success. Future trends which promote continued streamlining of the entire process, and recent work expanding the range of applicability of the TLP to water depths of 6000 ft are presented.

The tension leg platform is a floating structure which is vertically moored to the ocean floor using high strength steel pipe referred to as tendons. The tendons are designed to hold the TLP to a draft greater than its free floating displacement such that the tendons are always in tension. The axial stiffness of the tendon pipe provides restraint against vertical motions, and the tendon pretension provides lateral restraint.

The world´s fleet of seven installed TLPs range in water depth from approximately 500 ft to 3214 ft. Since 1990, five TLPs have either been installed, or are currently in a state of detailed design and fabrication for the Gulf of Mexico. This accelerated pace has resulted in the rapid development of more efficient design, analysis, and fabrication procedures.

The TLP concept offers significant advantages over other floating production systems and this has led to a considerable increase in the number of planned installations. For operations, the TLP provides dry trees, very small motions in the offshore environment, and expandability for future production. The importance of planning for expansion has been demonstrated by the outstanding production in deepwater Gulf of Mexico. Shell, a leader in the implementation of TLP technology, has experienced higher than anticipated production rates for both the Auger and Mars TLPs. Both platforms are being expanded to accommodate the increased production.

Due to the maturity of the concept, the TLP allows aggressive schedules for earlier first oil and reduced risk for improved project economics.

Water depth, cost

Recently, the TLP has had success in correcting several misconceptions regarding the concept. A common misconception is that TLPs are limited to water depths of 3,000 ft. The overall project economics for a particular field will actually depend on many factors in addition to water depth.

These include topsides payload, time to first oil, and risk factors associated with concept selection. Several examples exist in which the TLP has proven to be the best solution for water depths beyond the preconceived range. The Ursa project (Shell) resulted in a massive TLP (90,000 tons of displacement) in approximately 4,000 ft of water.

In addition, feasibility studies performed for another project, examined a smaller TLP in 6,000 ft of water. Hydrodynamic analyses were performed along with tendon sizing and preliminary structural design to evaluate the deeper installation. Results of the work were very encouraging, demonstrating that for the smaller topsides facilities, a technically feasible tendon system could be developed even for a significant increase in water depth.

A second misconception regarding the TLP concept is that the system is expensive, especially for smaller payloads. Recent experience however, demonstrates that for a given field size and water depth, the TLP cost is competitive with other systems for both large and small payloads. In other words, a conventional TLP designed for smaller production requirements is competitive in cost with other production concepts.

Schedule compression

One significant development in maturing the TLP concept has been the ability to compress the schedule from start of design to end of fabrication. Using Auger (the first Gulf of Mexico TLP of the 1990´s) as a reference, total schedule has been reduced by more than half. For the Mars TLP project, 13 months were saved from the original design schedule.

The savings resulted from an aggressive hull design schedule coupled with a design-while-build approach. As design and analysis procedures become even more automated, and designer and fabricator continue to work closely together in an integrated design environment, this trend in schedule compression is expected to continue into the future.

Past successes in schedule reduction for the TLP have been enabled by automation of the design, analysis, and drafting process. Shell, together with Han-Padron Associates (HPA), have developed an integrated software system in which structural information is stored in a central database and is accessed by other programs for design, finite element analysis, hydrodynamic analysis, and drafting.

The driving philosophy behind this process development is to remove the redundant specification of geometric and other fabrication-related data to speed up the design process while improving quality.

The system has actual project experience, having been successfully implemented on the Ursa TLP project. Every piece of stiffened plate structure in the TLP is stored in the data base. This includes plates, stiffeners, girders, brackets, end connections and details, etc. After the global size parameters have been defined and preliminary scantling is designed based on applicable codes, the initial database can be generated in just a few hours.

When the initial database is complete, hydrodynamic and structural models for global analysis are generated automatically from the geometric data. As the detailed design effort progresses, the database is refined to account for changes in the design. At any point in the design process, the database can be used to generate global or detailed local finite element models for analysis, weight data for weight management, and material data for ordering steel.

When the detailed design is completed for a given assembly, shop drawings are generated automatically using the geometry, material, welding, and tracking information stored in the database. The shop drawing is completed using traditional computer drafting techniques and then transferred electronically to the fabricator.

As more deepwater fields are developed, and the emphasis for aggressive schedules continues, advancements in design process automation are expected to continue.

Structural team

Another trend which has resulted in significant improvements in structural design, with a corresponding reduction in cost and schedule, is the integration of the design engineers and fabricators into a common team.

On recent TLP projects, several fabrication yard personnel have worked closely with engineers and drafters at the design office. The fabricator provides input on the design, which significantly improves the constructability of the structure without affecting performance. A formal procedure is established by which constructability suggestions are submitted, reviewed, and incorporated into the project.

The result of the collaboration between designer and fabricator is standardization of components, simplification of details, and a structure which is easier and faster to fabricate overall. The integration process will be further enhanced by continued improvements in communications, network connectivity, and contracting strategy.

It should be noted that the development work related to automated design and analysis, and the integration of design and fabrication, is actually independent of the particular structure being designed. Han-Padron Associates has developed the procedures and associated software which are applicable to a wide range of floating drilling and production concepts.

Concept trends

Different design variations have been considered and evaluated for future TLPs. Typical configuration variations for marginal fields are three-column and other mini-TLP concepts. The goal of these concepts is to incorporate the improved motion characteristics of the tendon system into smaller platforms which are simpler (and faster) to fabricate for marginal fields.

For deeper water, variations of the traditional tendon system are critical to maintaining the TLP´s reduced motion advantage.

The TLP relies on a low natural period to maintain reduced vertical motions when subjected to the typical design wave spectrum. As water depth increases, the tendon stiffness must be increased in a cost effective manner to maintain the low natural period. Alternate tendon concepts include the use of high stiffness materials such as composites and other high performance materials, stepped tendons (non-uniform dimensions from mud line to hull), cables, and others. Again, these efforts to develop tendon technology for deeper water are being pushed by the virtue of the TLP´s favorable motion characteristics.

Fleet expansion

The worldwide fleet of TLPs has more than doubled since early 1990. Given the advantages of the TLP concept, and the existing and future technology developments related to the design, analysis, and fabrication procedures used to implement the concept, it can be expected that the TLP will continue to play a major role in deepwater into the next century. Looking at the offshore deepwater fields slated for development in the near future, many areas are ideal candidates for tension leg platform technology. In particular, several Gulf of Mexico deepwater fields remain to be developed. In addition, water depths off the coast of West Africa, an up and coming region for offshore activity, is ideal for existing TLP technology.