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)
(2.2) 50-
Methods Appropriate choice
and correct
implementation of
method. Description
of assumptions, their
justification and/ or

Analysis and
Documentation of
results. Relevance of
analysis and discussion

Presentation Structure,
Language and
Figures, tables,

Late penalty is 10% per day. For exceptional circumstances please see special considerations:

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]
∆ =
− ℎ

= (0 +

) (1 + ∆) +
′∆hD ||