Shell Australia. LPGC Cargo Operations - Guidance to LPGC Master/Ship Owners/Operators. Shell Australia. Marine OPS_PRE_ Report. Approved.

Transcription

1 Shell Australia LPGC Cargo Operations - Guidance to LPGC Master/Ship Owners/Operators Originating Company Shell Australia Department Document Number Marine OPS_PRE_ Revision Number 1.0 Document Type Document Status Report Approved Publish Date 18/01/2018 Cyclical Review Date 18/01/2020 Safety Critical Content Technical Reviewer TA 2 or SME Process Owner Security Classification Export Classification Number true Head Of Marine Head Of Marine Unrestricted Non-US content - Non Controlled

2 Ver. Version Update Description Date Changed BCD Development Roles Authors, Reviewers, Approvers 1.0 Written by Shell Shipping & Maritime (Report No.SSTO th April 2017) 1/17/2018 Joseph Pereira Jonathan Wilson Reviewer (SME) Approver (PO) Click here to enter a date. Click here to enter a date. Click here to enter a date. Doc Number: OPS_PRE_ Unrestricted Page 2 of 7

3 Contents 1 Abbreviations Disclaimer Objective Problem statement Providing a Safe Terminal Operability Criteria for Side by Side Loading Operational Risk Management Analysis of cargo planning philosophies Guidance to Ship Master/Ship Owners/Operators Summary Conclusions... 7 Doc Number: OPS_PRE_ Unrestricted Page 3 of 7

4 1 Abbreviations DEMIST FLNG g GM k LPGC MLA T FSE KG Decision making support tool Floating LNG Gravitation acceleration Fluid metacentric height Radius of gyration LPG Carrier Marine loading arm Natural roll period Free Surface Effect Height of centre of gravity Doc Number: OPS_PRE_ Unrestricted Page 4 of 7

5 2 Disclaimer This guidance has been delivered to Master/Ship Owners/Operators for information purposes only and upon the express understanding that it will be used as an aid for planning cargo operations for LPGC at the Prelude FLNG facility. No party shall have any right of action against Shell Shipping & Maritime Technology & Innovation, a division of Shell International Trading and Shipping Company Limited or its affiliates, shareholders or subsidiaries and their respective officers, employees or agents (together Shell ) or any other person in relation to the accuracy or completeness of the information contained in this document or any other written or oral information made available to it in connection with the subject matter of this guidance note. Under no circumstances will Shell be responsible for any costs, loss, damage or expenses caused by reliance on the information or advice in this guidance document or howsoever provided. In furnishing this document, Shell undertakes no obligation to provide the recipient with access to any additional information or to update this document or additional information or to correct any inaccuracies therein which may become apparent. 3 Objective This Technical note aims to provide LPGC Master/Ship Owners/Operators with guidance on planning LPGC cargo/ballast handling operations at the Prelude FLNG, based on the analysis of various cargo planning philosophies for LPGC alongside Prelude FLNG, with the aim of minimizing vessel motions, subsequent roll period and roll angle. The natural roll period and angle of a LPGC are key motions that could lead to exceedance of LPGC operating criteria. 4 Problem statement A LPGC is most likely to roll when the wave encounter period is close to the natural roll period of the vessel. For Prelude FLNG location it has been determined through side by side dynamic motion analysis underpinned by 39 years of hind cast Met ocean data that LPGCs in 70k to 84k range are more sensitive to rolling when the natural roll period lies between 12.5 to 15.5 seconds. The range between 13.0 and 14.0 seconds is considered to be more sensitive. Realistically at some stage during a loading/de-ballasting operation it is expected that the natural roll period of a LPGC will transit this range. 5 Providing a Safe Terminal The Master of each visiting LPGC is responsible for the stability and safety of his or her ship, whilst the terminal has a role to provide a safe berth. This section presents the assessments and measures undertaken by terminal to minimise the exposure to sensitive natural roll period range. This section is further divided into three subsections: Operability criteria for side by side loading Operational risk management Analysis of various cargo planning philosophies Doc Number: OPS_PRE_ Unrestricted Page 5 of 7

6 5.1 Operability Criteria for Side by Side Loading Limiting criteria have been defined for side by side operations of LPGC alongside Prelude FLNG. These limitations include criteria for: - Safe operability of tugs Absolute and relative vessel motions Fender and mooring loads Operational envelope for loading arms The above limitations are an integral part of the terminal s operating procedures. 5.2 Operational Risk Management Although the Met-ocean conditions at Prelude FLNG are considered to be relatively benign, the wave climate is complex and both wind and multiple swell waves can be present. This may lead to instances where operability criteria, referred to in section 3.1, might be exceeded. To manage this risk, the Met-ocean conditions and motions of both the Prelude FLNG and the LPGC will be assessed before every offloading operation and the carrier will not be berthed unless a sufficient weather window is forecast to complete the transfer operation within the operability criteria. A tool, DEMIST has been developed to assist in forecasting an operable berthing and offtake window for side by side moored carriers. Using forecast and real-time Met-ocean data, the tool will assist in identifying the headings that will enable a complete loading operation to be completed without exceeding the set criteria. Where necessary the equilibrium heading of the Prelude FLNG could be changed by utilising the Prelude FLNG thrusters to maintain a heading within permissible heading sectors to meet the operating criteria. 5.3 Analysis of cargo planning philosophies Shell, together with Babcock International Group, have conducted extensive studies into alternative ballasting philosophies for loading a generic 84k LPGC alongside the Prelude FLNG, to determine the most pragmatic way to manage the GM and roll natural periods. Natural roll period of the vessel is inversely proportional to square root of GM as illustrated by the following formulae: T- Natural roll period k-radius of gyration g- Gravitation acceleration GM-Fluid metacentric height FSE-Free surface effect The analysis shows that in order to keep the roll period outside the sensitive range cargo/ballast operations should be planned in order to maintain as high a GM as possible. For the typical LPGC carrier it was achieved by Doc Number: OPS_PRE_ Unrestricted Page 6 of 7

7 De-ballasting the top side hopper ballast tanks as early as possible once cargo loading has commenced, bringing the ballast to sea level within the ballast trunk connecting the upper and lower ballast spaces thus minimizing the KG (Height of centre of gravity) and FSE (Free surface effect). Maintaining the double bottom ballast for a long as possible during cargo loading ensuring the manifold remains in the MLA operational envelope. De-ballasting only one set of double bottom tanks at a time thus minimising FSE. Departing the terminal with at least one set of double bottom ballast tanks full to sea level (in the trunk) subject to the vessels own stability criteria and MLA operating envelope. 6 Guidance to Ship Master/Ship Owners/Operators Each LPGC differs in structure and equipment, and each vessel should plan their cargo/ballast handling utilising the advice contained herein to minimize exposure to 12.5 to 15.5 seconds range whilst always remaining within their certified stress and stability limits. It is the master s responsibility to manage the GM of his/her vessel to minimise the exposure to the sensitive range, but it is envisaged that above cargo planning philosophy may assist him/her in doing so. LPGCs having loading instruments with capability to calculate natural roll period should use the loading instrument to plan cargo/ballast operations that minimise exposure to above sensitive ranges. LPGCs having loading instruments without capability to calculate natural roll period may prefer to seek guidance from their Classification society. 7 Summary Conclusions a) The Master/Ship Owners/Operators of each visiting LPGC is responsible for the safety of their own vessel. b) The terminal operating procedures contain environmental limitsfor LPGC manoeuvring, berthing, alongside cargo operations and for un-berthing. The environmental operating limits have been established based on safe limits for tug operations, mooring line loads, marine loading arm envelopes, and absolute and relative roll motions of LPGC and Prelude FLNG. c) The Met-ocean conditions and motions of both the Prelude FLNG and the LPGC will be assessed before every offloading operation and the carrier will not be berthed unless a sufficient weather window is forecast to complete the transfer operation within the operability criteria. d) Real-time monitoring of Prelude FLNG and LPGC motions will alert the operators should motions exceed safe mooring limits, for example, due to unexpected vessel behaviour. A roll motion criterion of 3-degrees single amplitude has been established for a LPGC moored alongside e) Prelude FLNG has undertaken extensive studies into alternative ballasting philosophies for loading a generic 84k LPGC alongside the Prelude FLNG, to determine the most pragmatic way to manage the GM and roll natural periods. f) This document provides guidance to LPGC Master/Ship Owners/Operators to assist in planning cargo operations to manage the GM and roll natural periods. Doc Number: OPS_PRE_ Unrestricted Page 7 of 7