Simultaneous optimisation of riser- and mooring systems for floating production units, as presented in MARINTEK Review, April 2000, is now approaching a stage of development that allows utilisation by engineering companies. Current development aims at preparing a web interface to allow monitoring and modification of the optimisation process. Participating companies: Norsk Hydro, Statoil, APL, and Halliburton.

New design requirements included

Proposed new design codes involve partial load factors, allowing in some cases a higher degree of utilisation of the line capacity. Depending upon environment conditions and floater- and mooring system properties, the fatigue load may become more critical than the extreme load conditions. Fatigue response analyses and fatigue life requirements have been included.

Additional features to be included are; minimum clearance to obstructions, pipelines or other lines, and maximum vertical pretension force, related to disconnectable systems.

Fatigue life constraint

Figure 1. Influence from environment data condensation on fatigue life prediction

In the case of a fatigue life constraint, a representative range of sea states: Wave directions, wave heights, wave periods, wind- and current conditions must be covered. Representing a typical Hs – Tp scatter diagram with one wind condition and one current condition per wave state, in eight direction sectors, may typically require 1500 sea states.

A procedure to account for irregular loads, as well as bi-modal spectra is used. In order to increase the speed of the optimisation process, the number of sea states can be condensed. Figure 1 shows that the number of sea states can be reduced from 1400 to less than 100 without affecting the fatigue life prediction too strongly.

Results for an 8-line chain-wire system is shown as example in Table 1. The initial system has 12% too high extreme loads, and the calculated fatigue life is 160 years instead of the 300 specified. The extreme offset after line breakage is 115 m and unacceptable. The final system satisfies all requirements, has slightly shorter lines, larger diameters, and is 11.6% more expensive.

Variable	Initial	Final	Min.	Max.
Pretension (kN)	3000	3346	1500	6000
Length lower chain segment (m)	400	363	200	800
Diameter, lower segm. (mm)	128	146	102	154
Length upper wire segm. (m)	2000	1900	1000	4000
Diameter, upper segm. 2 (mm)	144	159	116	173

Selected constraints	Initial	Final	Requirement
Max. tension utilization	1.12	1.0	1
Max offset	115	88	90
Calculated fatigue life	160	300	300
Cost	58.6	67.8

Table 1. Example with fatigue life constraint.

Key contraints	Initial	Final	Requirement
Max. tension utilization	0.907	1.0	1.0
Tension max, riser (kN)	24400	2500	2500
Radius min, riser (kN)	233	80.0	80
Slope max, riser (deg)	67.4	77.2	89
Slope range, riser (deg)	10.8	8.4	12.0

Mooring line cost	60.4	42.4
Riser line cost	75.7	77.5
Total cost	136.1	120.0

Table 2. Riser and mooring optimisation.

Mooring and riser system with line directions as variables

Figure 2 shows an example of mooring and riser line optimisation including mooring line directions as variables. The optimisation algorithm is able to ‘detect’ the benefit of grouping the lines, as shown in Figure 2B. The line directions are specified to be symmetric about both the transverse and the longitudinal axes.All of the constraints, except riser line tension, are satisfied for the initial system. In the final system all of the constraints are satisfied, the lengths of the lower mooring line segment and the upper riser segment have shrunk to their lower limits, indicating a potential for further cost reduction.
The cost summary shows that a considerable saving on mooring line cost has been obtained, while the riser cost has increased.

Figure 2. Riser and mooring optimisation.

MARINTEK contact: Ivar j. Fylling

(Article in MARINTEK Review 1-2001)

Published January 26, 2005