SS: LPG & LNG Offshore Operations: Numerical simulations to optimize offshore offloading operations
Authors: Voogt Arjan J. , Brugts Hielke 
Published: July 2010

In the past seven years, joint industry project ‘Offloading Operability’ has been executed. The project developed a tool ‘Shuttle’ that can reliably simulate complex offloading operations, including those of future operations by LNG carriers loading at floating LNG production installations. The tool allows simulation of side by side moored vessels as well as in tandem configuration. Significant research was done to correctly simulate the wind and current shielding, wave resonance in between two vessels in close proximity and damping on the complicated natural modes of motions. The validation tests and full scale reality checks executed showed that the tool is capable to predict typical behavior and associated mooring loads of vessels during offloading operations.

The paper discusses choices made in the simulation model, marine operations involved and feedback from mooring masters. Results of these simulations are compared to model tests experiments. The chosen simulation methods allow a reliable evaluation of critical weather conditions and the related downtime figures. Safety of the operation and possible contact problems can be evaluated during normal operation as well as during special safety cases.

Introduction

Environmental considerations limit allowance to flare gas. Combined with rising energy demand, transportation of LNG is considered around the world. Escalating construction costs and high local opposition to onshore LNG facilities coupled with geopolitical issues are key drivers to offshore offloading operations. Considering offloading operations in an early stage of the design increases the safety and reliability of gas transfer.

Over the years, mooring masters gained significant experience in offloading of oil and gas from offshore terminals like Floating (Production) Storage and Offloading (F(P)SO) vessels to transport vessels and back to land based receiving terminals. With the increasing transportation of LPG and LNG, side by side offloading gained significant interest. The side by side operation is a proven methodology for ship lightering and is successfully implemented for the first few permanently moored structures in relatively benign waters. Computer programs are developed to study and increase availability of safe and reliable offloading operations.

Offloading from one vessel to another involves operational and hydrodynamic challenges and requires focus on both man and mathematics. The weather limits are affected by experience of mooring masters and by assistance from tugs to reduce the hawser loads and relative motions. But loads on bollards and fairleads are also driven by complicated hydrodynamics and proper design of the mooring configuration (both between the vessels and to the seabed). Offloading operations have large impact on the design and operation of offshore terminals, because:–possible weather downtime of the offloading operation affects the overall economic performance of the terminal–the choice of the offloading system affects capital costs as well as operational costs of the terminal significantly–safe day-to-day operation requires clear procedures and trained personnel

Finally, offloading operations have a large safety impact, because they involve by definition the operation of two or more structures in close proximity. The safety of personnel and the offshore structure, plus the possible environmental impact (pollution) has been of concern of regulating authorities as well as the industry itself.

The subject of offloading plays e.g. an important role in the investigations of the US authority MMS related to FPSOs in the Gulf of Mexico.