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Research Papers: Piper and Riser Technology

Review of Steel Lazy Wave Riser Concepts for the North Sea

[+] Author and Article Information
Airindy Felisita

Department of Mechanical Engineering
and Materials Science,
University of Stavanger,
Stavanger N-4036, Norway
e-mail: airindy.felisita@uis.no

Ove Tobias Gudmestad

Department of Mechanical Engineering
and Materials Science,
University of Stavanger,
Stavanger N-4036, Norway
e-mail: ove.t.gudmestad@uis.no

Daniel Karunakaran

Department of Mechanical Engineering
and Materials Science,
University of Stavanger,
Stavanger N-4036, Norway
e-mail: daniel.karunakaran@uis.no

Lars Olav Martinsen

DEA E&P Norge AS,
Løkkeveien 103,
Stavanger 4007, Norway
e-mail: LarsOlav.Martinsen@dea-group.com

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received May 21, 2015; final manuscript received September 15, 2016; published online October 20, 2016. Assoc. Editor: Robert Seah.

J. Offshore Mech. Arct. Eng 139(1), 011702 (Oct 20, 2016) (15 pages) Paper No: OMAE-15-1040; doi: 10.1115/1.4034822 History: Received May 21, 2015; Revised September 15, 2016

A steel riser has benefits over a flexible riser in terms of pressure rating and cost. A steel riser in lazy wave configuration (steel lazy wave riser (SLWR)) is often considered as a good alternative solution for harsh environments where large floater excursions take place. The SLWR configuration is achieved by introducing buoyancy modules into a steel catenary riser (SCR). The buoyancy modules act as a damper and isolate the floater motions from the critical touchdown area. Hence, the SLWR generally has better overall performance than an SCR configuration. This paper attempts to analyze the correlation between the geometric shapes of the SLWR configuration with its capability to absorb the dynamic loadings. For deepwater cases, the behavior of the bottom part of the riser is correlated with the velocity experiences at the riser's hang-off location. Hence, the riser's performance is analyzed by comparing the velocity at the riser's hang-off with the velocity at the sag, hog, and near touchdown. The geometric shape of an SLWR is represented by its arch height, which is the vertical distance between the lowest point at the sag and the highest point at the hog bend of a riser. The results show that there is a correlation between the arch height of an SLWR with the riser's strength and wave-induced fatigue performance. SLWR configurations with higher arch generally have greater capability to absorb the dynamic loadings, as indicated by the lower velocities along the riser, which leads to lower stress utilizations and lower fatigue damage.

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References

Figures

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Fig. 1

Different riser configurations with uplift. Picture modified from Ref. [2].

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Fig. 2

Plan view of the SLWR model. The term MG refers to marine growth.

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Fig. 3

Side view of the SLWR model

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Fig. 4

The base case riser and variation group A. Variation group A uses a similar type of buoyancy module to the base case, but with different lengths of buoyancy section (L2). The base case riser and variation group A are oil-filled.

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Fig. 5

The base case riser and variation group B. Variation group B uses similar arrangements to group A, but with different hydrocarbon content. Risers no. 6–10 are gas-filled; hence, the arch shape is higher and more narrow.

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Fig. 6

The base case riser and variation group C. Variation group C uses different types of buoyancy module, but with similar length of buoyancy section (L2) as the base case. The base case riser and variation group C are oil-filled.

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Fig. 7

The base case riser and variation group D. Variation group D uses similar arrangements to group C, but with different hydrocarbon content. Risers no. 15–19 are gas-filled; hence, the arch shape is higher and narrower.

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Fig. 8

The base case riser and variation group E. Riser variation E: gas-filled risers with similar lazy wave configuration to the risers in variation A.

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Fig. 9

Riser variation group F: gas-filled risers with similar lazy wave configuration to the risers in variation C

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Fig. 10

A typical RAO of a semisubmersible platform used on the Norwegian Continental Shelf. This RAO is for wave direction 0 deg (following the global x-axis as shown in Fig. 2). The boxes indicate the applied wave periods where the observations on the velocities along the riser were carried out.

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Fig. 11

Time history record of vertical velocity at the hang-off position, compared with the time history of effective tension and bend moment at the sag and hog bends of the base case riser configuration

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Fig. 12

Sea-state blocks used in fatigue analysis

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Fig. 13

Velocity ratios for motions in wave period of 5 s. The graphs show that the motions in low wave period are mostly transferred into vertical motions, as indicated by the significantly higher ratio for Z-velocity.

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Fig. 14

Velocity ratios for motions in horizontal direction (global x-axis) at sag bend, hog bend, and touchdown zone

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Fig. 15

Velocity ratios for motions in vertical direction (global z-axis) at sag bend, hog bend, and touchdown zone

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Fig. 16

Comparison of the differences for lazy wave risers with different arch heights. Insert: a heave-dominated motion at sag bend from a horizontal motion at hang-off point. Picture taken from Ref. [15] with modification.

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Fig. 17

A typical trend of riser's effective tension from dynamic analysis

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Fig. 18

A comparison of riser's arch height with the minimum tension for different riser variations

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Fig. 19

The utilization criteria along the riser's length for various lazy wave riser configurations

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Fig. 20

Typical fatigue damage along the riser's length for a steel lazy wave riser

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Fig. 21

Fatigue damage at touchdown in comparison with the riser's arch heights

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