Conceptual choice mooring system

    Overview

The conceptual phase of the mooring system is a very important phase indeed. It uses a vast amount of input data and has varies boundary conditions which it uses to come a number of possible mooring systems. These then will be awarded points via a multi criteria analysis (MCA), as to obtain the solution best tuned to the situation at hand. An overview of this process is presented below.

    Boundary conditions

The boundary conditions consist of the hard restrictions considering the mooring system. Those hard restrictions can include for example:

Soil and seafloor conditions: Hard rock will decrease the number of possible anchor types. Suction piles won’t work in hard bottoms.

Water depth: The choice between anchor chains and lines depends on the water depth. In large water depths is the usage of chains uncommon or even impossible. 

Space limitations: The limited available sea floor The footprint of a catenary mooring system is much larger than the footprint of a taut leg system.

Vessel limitations: Anchors and anchor lines have to be installed with special vessels. In this boundary condition is the availability of those vessels taken into account. 

International regulations: The region in which the spar is going to be installed may reduce the number of options.

Client wishes: The client may have special preferences for some mooring types or may not want to use other types of mooring systems.


    All possible mooring options

This block includes all options with regards to mooring systems. This ‘all options’ will be reduced by all further blocks so there will be 1 option left. So ‘All options’ I very abstract and will include all possible and impossible system known.

    Client consultations

The first step in deducting of the total number of possibilities is a client meeting. The client has some expectations and wishes about the mooring system. The designers are limited by the boundary conditions. The purpose of the client meeting is bringing the designer and the client into line.
Most of the ‘all options’ will be gone over the board. 

    Result (1)

After the first selection a number of options are scrapped. At this phase only the feasible and realistic options are left over. 

    Multi criteria analysis

To come to the best option, a multi criteria analysis is used. The remaining options are judged on several aspects. Input for the MCA is the options, the criteria and the importance of each criterion. Output from the MCA is the best option for the spar mooring system. 

    Criteria

The success of the MCA depends on the definition of the criteria. Criteria will be split in several main groups considering technical, financial, environmental, time etc aspects.
Technical criteria may include criteria about complexness of the system. Financial criteria may consist for example of OPEX and CAPEX. Environmental criteria may include damage to the sea bottom, removal and decommissioning.

Once all of these have been undertaken, it is time to move on to the design phase, or some theoretical work which will help the designer determine the mooring stiffness K.