MOOROPT-2

A MIMOSA TOOLBOX program. Performs costs minimization of mooring and riser line systems. Variables may be; line segment length, diameter, pre-tension or distance to anchor, buoyancy, line direction. Constraints to be satisfied may be; safety factor requirements for maximum and minimum tension, curvature radius, line angle at seabed, offset.

Purpose

MOOROPT-2 is a toolbox program for MIMOSA. The purpose of the program is to find values of design variables that give minimum system cost while satisfying a specified set of constraints.

The program carries out conceptual design of both mooring lines and riser lines. Analysis methods are limited to the ones available in MIMOSA. In practice this means that for riser lines with buoyancy, only quasi-static analysis can be selected with the present version, and the influence of bending stiffness is neglected. This can to some extent be accounted for when specifying tension constraints and curvature radius constraints.

Cost parameters

• Mooring lines
• Cost per kg for mooring line segments
• Riser lines

Two cost specification alternatives:

1. Riser cost as function of maximum offset, linear and nonlinear term
2. Cost per m of riser line material plus cost per kg of buoyancy material

Design variables

One can select one or more of the following design parameters as variables:

1. Line direction
2. Line pretension or distance to anchor
3. Segment length
4. Segment diameter for mooring line segments
5. Submerged weight (buoyancy volume) for riser line segments
6. Net force of concentrated loads (buoys or clump weights)

Constraints

One or more of the following nine constraints are allowed:

1. Tension maxima in mooring line segments
2. Tension minima in mooring line segments
3. Mooring line slope at anchor, maxima
4. Mooring line slope at anchor, range
5. Tension maxima, riser line segments
6. Minimum curvature radius, riser line segments
7. Riser slope at lower end, maxima
8. Riser slope range at lower end
9. Maximum offset of specified point on vessel.

References and further information:

• Fylling, I. J., Yttervik, R.,Olsen, G.: "Optimised design of riser and mooring including partial load factors and fatigue life requirements" 15th Floating Production systems conference, IBC, London, December 2000.
• Fylling, I. J., Kleiven, G.: "Simultaneous Optimisation of Mooring and Riser Systems for Floating Production Vessels", Offshore Mechanics and Arctic Engineering Symposium, OMAE'2000, New Orleans, Feb. 2000.
• Fylling, Ivar: "Automated optimization of deepwater mooring system design", Offshore Mechanics and Arctic Engineering Symposium OMAE’97, Yokohama, April 1997.
• Fylling, Ivar: "Optimization of deepwater mooring systems",Offshore Mediterranean Conference, Ravenna, March 1997.

Email contact: Div. Offshore Structures