In depth: Offshore LNG Units

Dubai will soon host the 125 000m3 capcity Golar Freeze, a floating, storage and regasification unit (FSRU) for Dubai Supply Authority (DUSUP) and Shell in Dubai. Hamworthy is delivering the regasification skids for this vessel in September 2009. The system is being installed on the 1977-built LNG carrier that is being converted into an FSRU before being time chartered by Dubai Supply Authority (DUSUP) for 10 years, with options to extend for up to another five years.

After its delivery to DUSUP in the second quarter of 2010, Golar Freeze will be permanently moored alongside a purpose-built jetty within Jebel Ali port. The FSRU will be capable of storing 125 000m3 of LNG and delivering up to 14 million m3/day (about 3 MTA) of regasified LNG to DUSUP for delivery into Dubai’s gas network. Shell is acting as advisor on the project.

DNV Class

Conn Fagan, business development manager, Offshore LNG, DNV Energy lifts the lid on how class rules are being formulated for the next generation of LNG FPSOs and FSRUs.

Many factors are contributing to the current trend to develop and deploy offshore units for production and for import of LNG. Key among these are the desire to monetise remote gas fields, to avoid onshore safety and permitting issues and to ensure diversification of energy supply. Whatever the reason, these new applications provide a number of challenges when it comes to finding and applying relevant standards and predicting regulatory requirements.

Different Applications

Offshore gas deposits are unfortunately often inconveniently located when it comes to being able to get to market. Whereas oil, with a little processing, can be readily stored and transported, natural gas can be more problematic, particularly where pipeline connection is either not technically or economically feasible. The solution being most evaluated at present is to liquefy the gas offshore to facilitate its transportation by ship to market. There are a number of variations in the concepts being proposed. These are related to scale of production, selected liquefaction process, extent of pre-treatment of gas, and limitations on the composition of feed gas to name some.

At the other end of the value chain, at the market end, the transported LNG needs to be re-gasified to be made available for end-users. Carrying out such an operation offshore has several advantages compared to use of existing or new construction of land-based terminals. The offshore solutions have ranged from equipping the LNG carriers with their own regasification plant so that they can temporarily connect and discharge gas to a shore pipeline (Shuttle Regasification Vessels) or permanently locating a storage vessel with regasification plant which is regularly replenished by shuttle tankers. The permanently moored units may either be moored offshore or moored inshore at a jetty.

What all of these concepts have in common is that they lie within a grey area with regard to applicability of existing legislation and technical codes and standards. This is an area that DNV has been looking into in order to bring clarity to our clients.

An Acceptable Approach

Classification has long been used as a widely recognised means of verifying that an acceptable level of safety has been achieved in design, construction and operation of maritime vessels and floating offshore installations.

Classification is typically used as a means also of demonstrating compliance with both maritime and offshore legislation.

Traditionally vessels primarily engaged in transportation of LNG have been defined as ships and follow a maritime compliance regime and ship-classification procedures. Vessels engaged primarily in processing of hydrocarbons have been defined as offshore installations and follow an offshore compliance regime and associated offshore-class procedures.

Thus vessels used as Shuttle Regasification Vessels have followed a shipping regime and vessels acting as LNG FPSOs will follow an offshore regime.

In between these applications are the various forms of Floating Storage and Regasification Vessels (FSRUs), inshore and offshore. In this case, partially dependent on authority acceptance, either a ship approach or an offshore approach may be used as seen fit.

However, from the DNV perspective the important issue is the level of safety achieved rather than definition as ship or offshore unit. DNV have therefore developed requirements applicable to the various applications and aim at ensuring that an acceptable level of safety is achieved in all cases.

Classification Rules

For ship-regime vessels, DNV use as basis their rules which cover gas carriers and have supplemented these with requirements related to the operation as a regasification vessel. Specifically DNV have developed a guidance document entitled Classification Note 61.3 Regasification Vessels.

For offshore-regime vessels, DNV bases its approach on design as an FPSO and adds considerations based on the fact that the hydrocarbon being processed results in LNG. The relevant DNV document is then DNV OSS 309 Verification, Certification and Classification of Gas Export and Receiving Terminals, together with underlying Offshore Standards which address the technical details.

However whichever approach is adopted learnings from the second regime will also need to be accounted for.

For example most LNG FPSOs will adopt a ship-shaped structure and use a containment system based on maritime application so that design methods and requirements applicable for such may form the basis for the offshore application.

Similarly a ship carrying out regasification operations may adopt offshore practice for design of the position mooring system and adopt typical offshore requirements for process safety and shutdown systems.

In either case however the pecularities of the specific application will need to be taken account of in considering the suitability of using requirements not originally developed for such a new application.

Devil in the Details

Classification Rules will also typically make reference to international codes and standards when it comes to detailed design of systems or components on the vessel to be classified. When dealing with a concept which has never been built before it may well be the case that a code or standard used on a similar existing concept will be used as a basis.

Here it is important to make some assessment of the applicability of individual requirements. It may well be that existing codes and standards have either explicit or, more problematically, implicit assumptions which may not
be wholly appropriate for the new application.

An example of this would be the assumption of defined filling levels in vessels for marine transportation of LNG, which have not anticipated sloshing effects due to the intermediate filling levels experienced in offshore terminals.

Similarly the source and potential increased frequency of leakage of gas may indicate an increased need for detection and automatic action. Other codes make assumptions about available separation of main functions which may not be practicable in an offshore application.

One important means of ensuring that design, construction, and in-service requirements adequately address a particular and perhaps novel application is the use of risk assessment as an integral part of
the DNV rules related to these LNG vessels.

Risk Assessment

Classification Societies have long realised that it is extremely difficult to formulate purely prescriptive Rules which will address all types of vessels and their applications. Where designs remain relatively static over time, as has been the case in some parts of the maritime industry, prescriptive Rules will be adequate. However in many offshore-related projects where key parameters will be very site- and project- specific there has been a long tradition of use of risk analysis. Similarly for novel concepts and in projects where key design features will change from project to project then an approach based on a combination of prescriptive and risk-based requirements will be the sensible way to proceed.

For LNG FPSO and FSRU projects therefore, comprehensive risk assessments have been carried out to define specific requirements related to layout, escape, fire protection, venting and flaring arrangements, explosion prevention and protection.

The industry is rapidly developing new concepts to address new offshore LNG applications. DNV is providing assistance to our clients to ensure that this can be done within an appropriate rule framework which will lead to safe and regulatory compliant designs.

About the expert:

Conn Fagan is responsible within DNV for business development within the area of offshore LNG terminals both for regasification and for liquefaction. He has been responsible for developing DNV standards and services with respect to classification of both FSRUs and LNG FPSOs and is closely involved with classification of the first FSRUs to be deployed and the first LNG FPSOs being designed.