Influence of Sea State in Evaluating the Accelerations at Critical Points of a Semisubmersible
Authors: Nunes Renata Grabowsky , Nabuco Bruna , Nascimento Paula Teixeira 
Published: June 2014

In current offshore operations it has been noticed the importance to evaluate the accelerations submitted by a semisubmersible especially during towing operation. The effect of these accelerations on the integrity of a platform structure is extremely important and it should be taken into account by means of a motion analysis.

Aiming at obtaining an evaluation of the influence of the sea state in the accelerations at critical points of a semisubmersible, a case study is presented considering different values of significant wave height (Hs) and of mean zero up-crossing period (Tz) in the definition of each sea state for an analysis of the behavior of a semisubmersible unit in waves.

Additionally, considering that one of the most challenging steps in a towing operation analysis is to determine which sea state that will occur during the operation, a study has been done in order to verify which sea states that comprises with the criteria of maximum accelerations of that specific floating unit. Throughout this study an abacus of allowable wave height can be developed in function of the wave period, mainly when the criteria of maximum accelerations are known. From these criteria, for different drafts and unit headings, it is possible to obtain the maximum significant wave heights so that the accelerations will not be higher than the established criteria. According to the sea state during the towing operation the abacus could be used to ensure the structure integrity. Also, it could be used previously based on forecast sea state of the scheduled days.

Being aware of the influence of the sea state for a platform structure makes the abacus a useful and significant tool in order to analyze a towing operation since it represents the most critical sea state. Indeed, through the abacus technique it is possible to establish the best route in accordance with the wave characteristics of each sea state.

Introduction

The influence of the sea state parameters is studied to assess their impact on the acceleration responses of a semisubmersible during a towing operation. It has been verified, based on Froude number, that the floating unit velocity during towing operation is small enough so that the forward speed effects can be neglected when evaluating acceleration responses. The software Wadam, typically used to calculate wave-structure interactions, has been used to perform global response analysis for the unit with no forward speed.

This study comprises of acceleration transfer functions which have been calculated from the response amplitude operators (RAO) generated by Wadam and then combined with several sea states.

Considering the challenges to determine the sea state during the tow route operation an alternative solution is described. The purpose is to check if the sea state is covered by the criteria avoiding critical occurrences of acceleration on the platform structure. There are two options for this alternative solution: using forecasted sea data or assessing the sea state during the operation.

Motion Analysis

The transfer functions - responses of a body in harmonic waves over a range of frequencies - of longitudinal, transversal and vertical accelerations for a specific point will be defined based on the coordinates of this point and the RAO motions of the floating unit (surge, sway, heave, roll, pitch and yaw).