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Research Papers: Offshore Technology

“Bigfoot” Direct Vertical Access Semisubmersible Model Tests and Comparison With Numerical Predictions

[+] Author and Article Information
Stergios Liapis

Shell International Exploration and Production,
200 N. Dairy Ashford,
Houston, TX 77079
e-mail: Stergios.Liapis@shell.com

Yile Li

Shell International Exploration and Production,
200 N. Dairy Ashford,
Houston, TX 77079
e-mail: Yile.Li@shell.com

Haining Lu

State Key Laboratory of Ocean Engineering,
Shanghai Jiao Tong University,
1954 Huashan Road,
Shanghai 200030, China
e-mail: haining@sjtu.edu.cn

Tao Peng

State Key Laboratory of Ocean Engineering,
Shanghai Jiao Tong University,
1954 Huashan Road,
Shanghai 200030, China
e-mail: pengtao@sjtu.edu.cn

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received June 10, 2015; final manuscript received November 13, 2015; published online June 3, 2016. Assoc. Editor: Ron Riggs.

J. Offshore Mech. Arct. Eng 138(5), 051301 (Jun 03, 2016) (13 pages) Paper No: OMAE-15-1043; doi: 10.1115/1.4032561 History: Received June 10, 2015; Revised November 13, 2015

The Bigfoot direct vertical access (DVA) semisubmersible is a novel floating drilling and production host that provides an attractive alternative to the spar. This concept utilizes heave plates (big feet) that improve the motion characteristics of a semisubmersible in all mild environments (Southeast Asia, West Africa, and Brazil). Bigfoot offers riser-friendly motions that enable top-tensioned risers, which is often a project requirement. This floater works in all water depths, in particular ultra-deepwater (5000 + ft) where a tension leg platform (TLP) is not an option, supports top-tensioned risers, and enables drilling and workover operations. The Bigfoot has several advantages over a spar. These include: (1) quayside topsides integration. This eliminates offshore topsides integration, a significant issue for all spar projects in terms of cost, safety, and schedule, (2) a more open deck layout compared to a spar, and (3) no fabrication location restrictions as it can be built by many yards worldwide potentially offering local content to a project. Model tests were undertaken at the Shanghai Jiao Tong University (SJTU) Offshore Basin to assess the dynamic response of the Bigfoot in waves, swell, wind, and current. Five mild non-Gulf of Mexico (GOM) environments were considered. In all the cases, the floater motions are an order of magnitude smaller than those of a conventional semisubmersible for similar deck payload, thus enabling drilling operations and top-tensioned production risers. In a parallel effort, a cosmos numerical model of the Bigfoot was developed for coupled motion analysis. The experimental results and the cosmos numerical predictions are in close agreement. In addition to measuring global motions, two heave plates were instrumented with load cells to measure forces and moments. The force measurements from the model tests are in good agreement with numerical predictions using computational fluid dynamics (CFD).

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References

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Figures

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Fig. 1

The Bigfoot DVA semisubmersible

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Fig. 2

Existing DVA solutions for different water depths

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Fig. 3

Wet tow of the Perdido spar platform

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Fig. 4

Topsides heavy lift installation for a spar platform

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Fig. 5

Deck layout of a spar platform versus a semisubmersible

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Fig. 6

The Bigfoot DVA semisubmersible hull and heave plates

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Fig. 7

The Bigfoot heave RAO

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Fig. 8

Bigfoot dimensions. Top: side view and bottom: top view.

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Fig. 9

Bigfoot mooring system. Top: side view and bottom: top view.

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Fig. 10

The SJTU Offshore Basin

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Fig. 11

Schematic of the SJTU facility

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Fig. 12

The Bigfoot semimodel

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Fig. 13

Schematic of the mooring and the top-tension risers in prototype scale

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Fig. 15

Heave plate force/moment instrumentation

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Fig. 16

Detailed sketch of the heave plate force/moment instrumentation

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Fig. 17

Mooring lines used in the experiments

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Fig. 18

Panel model of the Bigfoot

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Fig. 19

Morison-type line members to simulate viscous drag on heave plates

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Fig. 20

Bigfoot current only tests. Platform tilt in current direction: 5 deg (top) and 10 deg (bottom).

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Fig. 21

Vertical loads on heave plate segments in current. Semitilt = 10 deg.

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Fig. 22

Bigfoot in 1000 year Malaysia environment: heading = 0 deg and draft = 90 ft

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Fig. 23

Comparison of surge SD: heading = 0 deg and draft = 95 ft

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Fig. 24

Comparison of heave SD: heading = 0 deg and draft = 95 ft

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Fig. 25

Comparison of pitch SD: heading = 0 deg and draft = 95 ft

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Fig. 26

Comparison of tensioner stroke SD: heading = 0 deg and draft = 95 ft

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Fig. 27

Vorticity contours around a plate in vertical oscillation

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Fig. 28

Relative velocity at heave plates in waves of various wave heights

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Fig. 29

Vertical loads on the heave plate segments: comparison between CFD and experiments

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Fig. 30

Wave exciting force with and without heave plates

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Fig. 31

Heave added mass with and without heave plates

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Fig. 32

Pitch added mass with and without heave plates

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Fig. 33

Heave damping with and without heave plates

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Fig. 34

Heave RAOs with and without heave plates

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Fig. 35

Pitch RAOs with and without heave plates

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Fig. 36

SCR fatigue life calculations

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Fig. 37

Structural design of the heave plates

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Fig. 38

Finite-element mesh

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