Methodology to Define Design Motion Criteria for Performance of Floating LNG Process Facilities
Authors: Terashima Takeshi , Shimada Kiyoshi , Ichimaru Yoshikazu , Orimo Yohei 
Published: May 2011

This paper proposes a generalized methodology to determine motion criteria for required performance of process facilities using the Abadi Floating LNG (Abadi FLNG) as a sample. The Abadi FLNG is planned to be installed in the Indonesian territory of the Arafura Sea by INPEX Masela, Ltd.

Although existing FPSOs all have its own motion criteria fed into topside equipment design, it is known that there is no codified nor comprehensive approach as to how those motion criteria can be specified. Those circumstances could lead to misunderstanding of information among owner, contractors and vendors, and potentially unnecessary cost increase. One of the conventional methods is that the design motion criteria are given as single maximum motion amplitude in the extreme condition for operation. Usually 1 year or 10 year cyclonic condition is adopted as the extreme condition for operation where motion is calculated under assumed wave heading angle with short term prediction manner. In this approach, the wave heading has to be conservatively assumed with some fixed angle, as there is no concurrent environmental data, such as wind, wave, and current, which should be utilized. In addition, the common method to present associated period is not established.

For the application to FLNG case, there are some motion sensitive process facilities, such as cryogenic heat exchangers and acid gas absorbers, whose performances degrade with given motion criterion, and the degree of degradation varies with concurrent motion period. To establish the comprehensive motion criteria, FLNG wave heading analysis followed by motion analysis were performed for 10 years time-series concurrent wind, wave, swell and current data including extreme and non-extreme conditions. Calculated motions were incorporated into scatter diagram, which is a table of occurrence probability for motion amplitude and period to develop motion envelope of amplitude and period. This envelope was adopted as design motion criteria of motion sensitive process facilities.

This envelope approach to define performace of process facilities due to motion criteria is recommended to other FPSO and FLNG as a generalized approach which has great benefit for clear understanding among owner, contractors and vendors while minimizing unnecessary margins which could have tremendous cost impact particularly for those with motion sensitive floaters.

Introduction

There are more than 100 FPSOs in operation in the world today. Those FPSOs apply their own design motion criteria to their facilities, however there is no standard method to define those motion criteria.

There are two types of design motion criteria. One is design motion criteria for extreme condition, which is applied to all structural design on FPSO. Another one is design motion criteria for operation condition. Under such condition, all topside process facilities are designed to satisfy its full performance. As process facilities performance can be affected by relatively small motion amplitude, both dynamic motion and static inclination are included in design motion criteria for operation condition.

In this paper, only dynamic motion of design motion criteria for operation condition are discussed. As for static inclination, equipment fabrication tolerance, equipment installation tolerance, module frame fabrication tolerance, module frame deflection, hull main deck construction tolerance, hull main deck deflection and ballast water control margin are considered.