An Evaluation of Strength, Fatigue and Operational Performance of Dry Tree Semisubmersible Riser Tensioning Equipment

Abstract

One key decision operators make when planning field developments is whether the production system will use only subsea equipment or will also include dry trees. Although dry tree floating platforms have primarily utilized SPAR and TLP hull forms, semisubmersible options are being developed and qualified for the Gulf of Mexico. A key component of the dry tree semisubmersible is the riser tensioning system, which must be carefully matched to hull motion characteristics and arrangements. The SPAR platform utilizes self-supporting buoyancy cans or long stroke riser tensioners in a protected location within the SPAR centerwell and in combination with a keel guide. The TLP utilizes short stroke riser tensioners with or without a keel guide and in a location open to interaction with waves and currents. A dry tree semisubmersible is likely to use long stroke riser tensioners in an open wellbay configuration either with or without a keel guide. This paper uses computer analysis and motions calibrated to model test data to evaluate the strength and operational performance of a riser tensioning system for a specific dry tree semisubmersible configuration. The system uses existing riser tensioning equipment in an arrangement that is open to wave and current interaction. The paper compares arrangements with and without a keel guide and presents differences in strength, stroke range and cost. The semisubmersible hull form includes columns arranged in pairs at each corner of the platform and the paper investigates the influence of the paired-column arrangement on wave loading on the risers and supporting structures. The results of the paper indicate what modifications are necessary, if any, for riser tensioning equipment integration. The information and results presented in this paper are applicable to operators evaluating dry tree interfaces on proposed new floater concepts in addition to equipment suppliers planning to provide or qualify riser tensioning equipment for semisubmersibles. The results will also contribute to further development of a dry tree semisubmersible option for Gulf of Mexico projects, which will provide cost and execution plan improvements compared to existing options for deepwater.

Tension variations of hydro-pneumatic riser tensioner and implications for dry-tree interface in semisubmersible

“…Zeng et al (2013) proposed a deep-draft DTS comprising a two-axis symmetrical hull with draft in the range of 100 ft (30.48 m) to 155 ft (47.24 m) to accommodate the tensioner stroke in the range of 35 ft (10.67 m) to 45 ft (13.72 m). Poll et al (2013) designed a Paired-Column Semisubmersible Hull (PC-Semi) with draft of 175 ft (53.34 m) to accommodate the tensioner stroke of 28 ft (8.53 m). In addition, Bian and Xiang (2013) also suggested a DTS hull with the draft of 150 ft (45.72 m) to support the tensioner stroke within 35 ft (10.67 m).…”

VIM Model Testing and VIM Induced Mooring Fatigue of a Dry Paired-Column Semisubmersible Platform

“…The study results are highlighted in Ref [15]. Poll et al (2013) further presented the results of a riser tensioner with a regular stroke of 8.5m (28ft) on the PC Semi using Central GoM environmental conditions including 1, 000-year hurricanes. Major design parameters including maximum riser stroke ranges, minimum airgaps, maximum offsets/rotations and maximum accelerations in the Central GoM region for 100-year and 1, 000-year hurricanes were examined and validated.…”