DNV Gives Thumbs Up for Hoegh's Unique LNG FPSO Concept

After completing an extensive front-end engineering design project, Höegh LNG received Approval in Principle from DNV for its unique LNG FPSO concept, representing a first for the Höegh organization and a significant advance for the LNG industry.

According to Conn Fagan, DNV Business Development Manager for offshore LNG projects, the project represented a significant challenge and opportunity for DNV. "Some aspects in the Höegh LNG design have been previously classed, but since there are no LNG FPSOs currently in operation the combination and application of the various technologies has yet to be proven in practice," said Fagan. "Our work on this project has helped to document the feasibility of such projects and to develop our expertise in the next-generation of LNG technology."

Øivin Iversen, Vice President, Project Development at Höegh LNG, says that DNV and Höegh LNG maintained an open and cooperative dialogue throughout the process. "Because we are doing something different it was vital that we involved DNV at an early stage," he said. "We have welcomed their comments regarding safety issues and some of the more unique features of the design and worked with them to expand their notations where appropriate."

In 2007, Höegh LNG began a six month pre-FEED project to develop an LNG Floating Production, Storage and Offloading (LNG FPSO) unit. The project was expanded to a full-scale FEED in cooperation with DNV, Chicago Bridge & Iron (CB&I) and Daewoo Shipbuilding & Marine Engineering (DSME) among other suppliers in 2008, producing a highly detailed design. Following an extensive review, the project was subsequently awarded an Approval in Principle from DNV earlier this year.

Due to the congested nature of floating gas processing units, managing the potential risk of fire and explosion was a critical part of the project. "DNV carried out initial safety studies which included fire and explosion assessments of hazards caused by potential leakages in the gas pre-treatment and liquefaction systems," said Fagan. "The results confirm that the Höegh LNG FPSO design is well thought out and addresses the hazards associated with its operation."

Fagan also notes that novel GTT double row tank layout for the containment system proposed for the Höegh FPSO is effective in addressing the challenge of sloshing in membrane tank designs. "Together with GTT, DNV carried out a number of sloshing assessments at potential operation sites selected by the project, which produced good results," he said. "The reduced tank dimensions of the double row design gives a favorable shift in sloshing resonance frequency and reduces the impact pressure intensity compared to equivalent single row designs. In addition, the double row concept facilitates design of topsides support."

A further challenge for proposed LNG FPSOs is the means of transferring LNG from the FPSO to LNG carriers for export. The Höegh LNG selected a marinized loading arm design which can be used in a side-by-side configuration. DNV has already carried out extensive work in qualification of the major side-by-side loading arrangements currently being proposed in the industry, including marinized loading arms. DNV is also at present engaged in the ongoing qualification of a large number of tandem offloading solutions.

Øivind Iversen said that while most of the topside components on Höegh LNG FPSO are proven solutions, how they work together is unique. "There are other LNG FPSO concept vessels out there, but none of them have been taken this far," he said. "By investing time and resources in detailed engineering, Höegh LNG has demonstrated a genuine commitment to this market concept."