FY20/21: Offshore Engineering and Analysis Station Keeping and Structures Assignment B.
Thornton
DUE: 23:00 Ma rch 18, 2021 Stationkeeping and Structures Assignment This individual assignment is worth 10% of your final grade. Submit your answers and supporting material via Turnitin on blackboard by the deadline. Submission format: Maximum 4 page PDF report and supplementary material PDF report: Your report should consist of: o Method: Equations used, description of any assumptions and their justification o Results: Figures and tables that illustrate your main results o Discussion: Description of your results, their implications and your recommendations Supplementary material: Separate file(s) with working annotated codes or spreadsheets used in your calculations. This doesn’t count towards your page limit. Marking criteria: The PDF report is marked based on how you use the concepts taught in the module to complete the assignment. Only aspects explained in the PDF report get marks. “See code for method” in the PDF report isn’t a satisfactory explanation and will get no marks. You can organize the document as you see fit. If you use values from design regulations, course notes or papers, please cite them. Inidicative marks for each task are shown as [20 marks]. Category Classification (Fail) 0-40 Unsatisf actory (3rd) 40-50 Adequat e (2.2) 50- 60 Fair (2.1) 60-70 Good (1st) 70-100 Outstand ing Methods Appropriate choice and correct implementation of method. Description of assumptions, their justification and/ or implications Analysis and interpretation Documentation of results. Relevance of analysis and discussion Presentation Structure, Language and communication, Figures, tables, Referencing Late penalty is 10% per day. For exceptional circumstances please see special considerations: http://www.southampton.ac.uk/quality/assessment/special_considerations.page FY20/21: Offshore Engineering and Analysis Station Keeping and Structures Assignment B.
Thornton
DUE: 23:00 Ma rch 18, 2021 Consider a floating platform with 20m diameter that is pretensioned using four identical steel mooring chains, each of length 800m with a mass of 300kg/m in air. The lines are evenly distributed, attached to the outside of the platform at the waterline. Each line is pretensioned to 1200kN along the line at the platform. The water depth h is 100m. The other ends are fixed using drag anchors. Task 1: Determine the horizontal pretension on a line, and the horizontal distance to its anchor. Plot the catenary shape for the pretensioned condition. [2 marks] Task 2: Consider a 100 year maximum environmental load of 3000kN aligned with one of the mooring lines. What’s the excursion from the pretensioned equilibrium point Plot the catenary of the loaded line and its opposing (relieved) pair. Will the loaded anchor get lifted [4 marks] Task 3: Plot the platform’s watch circle diameter as it experiences a mean environmental load of 500kN from all angles (assume torque to be zero). If you have assumed constant pretension, comment of how the watch circle will change if an iterative solver was used. [7 marks] Task 4: For the 100 year load condition in Task 2, use Gobat’s simple method to estimate the dynamic tension in the most loaded line:* The equivalent diameter of the chain is D=0.20 m. Assume a line vertical drag coefficient is C’d=0.75, unloaded added mass Mo=26000 kg, added mass of lifted line per unit length Ma= -5 kg/m. The root mean squared (rms) vertical dynamics have values = 1.3 m/s2 and || = 2.3 m 2/s2 respectively with a period of 11.4s. Use a standard SN-curve for steel to determine the cumulative damage ratio for ~50 years operation, assuming the maximum dynamic load condition occurs for 5 weeks during that period, and the rms dynamic load _ = 0.06 with a 3s period at all other times. Which component causes more damage What other factors could impact line survivability [7 marks] *NB: If you did not complete task 2, make sensible assumptions for the mean line tension and catenary length, S, and use those in task 4. [Total out of 20 marks] = = (0 + ) (1 + ) + 1 2 ′ hD ||
