WP 4 - Systematic parameter variation
Progress status November 2011
The first version of the CIV Artic vessel was model tested in MARINTEK's Towing Tank for documentation of calm water performance and speed loss in waves. The calm water performance of the vessel was not satisfactory and actions were taken to find ways to optimize the hull design. For a vessel with ice breaker capabilities, large headboxes above the propulsion pod units are required for strength purposes. Using the experience of designers of SXT OSV and Aker Arctic, it was stated that there could be a large potential in resistance reduction by optimizing the headbox design. Since the headboxes are quite large it was found that a modification of the entire aft ship was needed to achieve smooth hull lines and not compromise to much on the ice breaker capabilities. A CFD-study using Star CCM+ was introduced on the calm water performance and concluded that the headboxes were a significant contributor to the total resistance of the vessel. By reducing the headbox resistance by 50% one could achieve a total reduction on the power consumption by 15%.
In parallel to the optimization of the aft hull a numerical parametric study of the CIV Arctic vessel was calculated in VERES. The study looked at the length parameter's influence on operability in Barents Sea regions. The parametric study consisted of a length-span of up to 60m longer and 20m shorter than the first version which has a Lpp of 109.3m. The study included operability comparisons for typical transit speed comfort criteria and operational criteria for selected typical operational tasks the CIV Arctic vessel was intended to do. The latter was a result of input from "WP 2 - Intervention philosophy and tasks" and several workshops with the industry, i.e. crane manufacturers and ROV operators. Also experienced personnel from STATOIL were present on several workshops to give guidance to select adequate tasks and thereby choosing relevant criteria for comparing the various designs. A decision from one workshop was that the vessel also should have satisfactory seakeeping characteristics in more southern regions than in the Barents Sea region and it was decided to extend the operational area with a "Southern route" of the CIV Arctic vessel to Haltenbaken, the southernmost point. Metocean input was received from met.no with help from STATOIL and used as a basis for the study.
The numerical study concluded that the length of the original vessel, i.e. with Lpp=109.3m, was found to be a good compromise between length, the increased cost of building longer vessels and it's seakeeping characteristics. One important reason to this conclusion is that the waves are generally shorter in the Barents Sea region than in the Norwegian Sea and thereby less influenced by the normally positive effect of increasing length. I.e., a length increase in Norwegian Sea areas would probably have shown a larger advantage with respect to operability by increasing the vessel length. Then a modified hull model version of the original design was manufactured and model tested to document seakeeping- and station keeping characteristics in the MARINTEK Ocean Basin. The final test in open water was then a documentation of calm water performance in the MARINTEK Towing Tank. The results corresponded well with the CFD-analysis and resulted in a 14% reduction in break power at the design speed 15 knots compared to the original design.
Then the hull model was shipped to Finland for ice performance testing in the Aker Arctic ice model tank. Special interest was made to the bulbous bow which was optimized for calm water performance which generally speaking is non-optimum for ice breaking. Though, by forwardly trimming of the vessel so that the bulbous bow was submerged below the free surface and the ice, the vessel showed satisfactory performance in ice. Since the vessel also was designed to break ice with the stern first, this was documented to be a good alternative to the bow first approach.