Prediction of Spar Responses: Model Tests vs. Analysis

Abstract

The DeepStar Program has sponsored a series of tasks to evaluate the current industry capability in predicting the responses of deepwater theme structures (FPSO, TLP and SPAR). In its Phase IV program, model tests of the FPSO, TLP and SPAR were conducted, and in the Phase V program, engineering companies as well as test basins were invited to evaluate the correlations between the tests and analyses. This paper presents an overview of DeepStar's effort with emphasis on Spar responses. It covers the following areas of response predictions:Model test setup and test conditionsThe state-of-art analytical tools available to the industryEvaluation approachKey results of comparisons - tests vs. analysesSensitivities and uncertainties in predicting Spar responsesAssessment of current industry capabilitiesAreas of future effort Introduction Model tests were conducted November 2000 in the new offshore basin of the Maritime Research Institute Netherlands (MARIN). The objective of the tests was to assess global responses of floaters, with emphasis on the effect of mooring line and riser dynamics in deeper water. These tests included a TLP, Spar and FPSO. During the early part of last year, industry participants of the DeepStar Program were contracted by Det Norske Veritas (DNV) to analytically model the tests using their in-house programs. This paper presents results of that effort for the spar platform conducted by the authors. Modeling particulars and applied computer programs are presented. Test results and predictions are tabulated and shown graphically. Overall, there is good to excellent agreement between predictions and test measurements. Model Tests A brief description of the applied test facility, model and test particulars are given herein. Details may be found in references 1 and 2. Test Facility The MARIN deepwater test basin is 10.5m deep and 45m × 36m in cross section. Multi-flap wave generators on two sides can produce long-crested or short crested waves up to 0.4m with periods of 0.3 to 3.0 seconds, model scale. Current can be generated over the full basin depth with a vertical profile. Wind spectra are produced with electrical fans on a free moving and position-controlled platform. Spar Model Spar tests were conducted with length scale factor 1:87. This simulated a water depth of 913.5m. The spar model was a "classic" spar which is a vertical steel cylinder supporting topside facilities and a centerwell for top tensioned risers. The full-scale hull depth was 214.9m with a 198.1m draft and 37.2m diameter. The deck length, width and height were 65m, 65m, and 27.1m, respectively, with a vertical cg 241.7m above the spar keel. Mass properties, including adjustments to KG and GM for entrained centerwell water are given in Table 1. A sketch of the spar with mooring and risers is given in Figure 1. Table 1 Spar Mass Properties (Available in full paper)

Experimental and numerical investigations on the dynamic characteristics of a spar-type offshore wind turbine under irregular waves

“…In addition, an experiment using a 1:87 scaled model of a classic petroleum spar platform was conducted to confirm the predicted results. The study showed similar and excellent agreement between predictions and test measurements [23].…”

Flow Induced Motions on Multi Column Floaters

“…Evaluation of vortex induced motion (VIM) in offshore floating structures has been largely been carried out for spars. Classic spar, truss spar and cell spar hulls are all known to exhibit VIV (VIM) [1], [2], [6], [7], [8], [9], [10], [12], [13], [14], [15], [16], [19], [27], [28], [29]. [31], [32],…”

Mooring truncation design of a deepwater SPAR

“…The relationships between main mooring parameters and the truncation factor were put forward, static characteristics of truncated SPAR mooring systems were unchanged. Coupled analysis to the whole system in the time domain were also conducted, attempting to let the global response and mooring tensions of truncation results be consistent with full depth and MARIN test results from Steen et al (2004). After detailed comparison, results of the two truncation system were found correct, and the truncation design was proved to be reasonable and reliable.…”

“…Research on a SPAR mooring system working in 913 m water depth was conducted by Wichers and Devlin (2004) and Steen et al (2004). Further truncation design of the same SPAR was carried out in this paper with the water depth reduced to 736 m and 552 m respectively.…”