Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Mooring system optimization for floating platforms in operational mode
Alexander S. Bolshev, Sergey A. Frolov, Ekaterina V. Shonina
Peter the Great St. Petersburg Polytechnic University
St. Petersburg, Russia

ABSTRACT

and on the structure (Bolshev, Frolov and Shonina, 2021). The specified
parameters will differ for different modes.
For example, when the platform is operating at the drilling point, hardconstraints on displacements must be provided. If the weather conditions
deteriorate, the horizontal displacements should not exceed thepermissible values to avoid equipment damage. Constraint satisfaction
within the specified limits ensures the normal operation of the platform.
Horizontal displacement occurs due to the movement of the structure
under the effect of external loads. This parameter can be adjusted bychanging the tension of the lines or by changing their location relative tothe vector of external forces. This parameter must be controlled during
drilling operations because excessive displacement can result inequipment damage and oil spillage. The importance of this parameter has
led to the solution for optimization problem, the ultimate goal of whichis to minimize displacements (Sabziyan et al., 2014; Carbono et al., 2005;
Monteiro et al, 2010).
However, consideration of the optimization problem in order tominimize only the movement of the structure can lead to a number oferrors. For example, to achieve reliable station-keeping of a structure, thelines tension is increased. This can lead to deterioration in the safety
factor. The displacement of the structure needs to be limited, but
reduction to a minimum is not necessary. Other characteristics of thelines should also be taken into account.
The maximum horizontal displacement should be checked for themaximum storm that the offshore structure can outride and at which
drilling and extraction is possible. This avoids downtime and does not
require transition the structure to survival mode. Typically, platform
displacement is limited to 6-8% of the sea depth. This value depends on
several factors such as water depth and riser flexibility. A lower value istaken for greater sea depths, and a larger value is taken for shallow sea
depths.
As for external loads that determine the possibility of normal operationof the floating platform, there are no definite requirements for the choiceof the maximum allowable storm or movement algorithm along thevariants of weather conditions. Recommendations are defined by anumber of regulatory documents. For the operational condition, theenvironmental condition shall represent a condition when a riser isconnected (DNV-OS-E301, 2021). The level of environmental loads inoperational mode is determined in accordance with the operational
requirements and constraints on the behavior of the MODU/FOP, which

In this article the mooring system of an offshore floating anchoredstructure is reviewed. A criterion and procedure for optimizing theanchor mooring system which ensures the requirements for safe andcontinuous operation of the structure, taking into account the imposed
restrictions is proposed. The optimal mooring system should also provide
the widest range of external loads, under which it is possible to continue
operating and not transition platform to survival mode. The presented
methodology is implemented in a specialized software package thatallows determining all categories of external loads, simulate the structure
dynamics under the action of loads, and modify the parameters of anchor
mooring systems.

KEY WORDS: Offshore structure; mooring system; multi-objectiveoptimization; penalty function method; semi-submersible platform;
tension force; external load.

INTRODUCTION
Semi-submersible platforms are in active use in the development of oiland gas fields. A necessary part of this process is prospecting andexploratory drilling in various water areas. The purpose of such drillingis a preliminary assessment of deposits types and sizes, as well as theirgeneral characteristics.
The design of the structure or its positioning in the water area should be
carried out taking into account the requirements of safe operation.
Anchor lines analysis is performed to evaluate parameters such as
maximum line tensions, angular and linear displacements that can occur
under various external conditions. The maximum values of theseparameters must be less than the permissible ones to ensure the safety ofthe structure. The efficiency of the platform over the entire period of
operation depends on the time of drilling or extraction. Weather related
downtime leads to inevitable economic losses. The operation duration
under various external loads is ensured by the competent design ofplatform mooring systems. The design of the optimal anchor system for
operational mode must be carried out for a wide range of external loads.
To find the optimal variant it is necessary to analyze which parameters
affect the safety of the structure, which parameters can be selected as
criteria, and also take into account the constraints imposed on the lines754