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Day 2Paper No. 3

Opening the Operability Envelope Tug Support forOffshore LNG

Gary Dockerty, Lamnalco Limited, United Arab Emirates

SYNOPSIS: Assisting LNG carriers at offshore floating LNG terminals, presents a challenge to the tug industry.These facilities will require the agility and responses of a ship assist tug with the sea keeping capability of theirmuch larger cousins operating in the offshore industry. This paper provides a review of some of the researchcarried out to date into resolving the challenges this concept and environment presents for tugs, crews andoperators.

INTRODUCTIONMuch of the proven gas reserves in the world are found inoffshore fields, perhaps as much as one-thirdi, or 60 trillionm3. Gas that is found offshore generally has to be broughtonshore for its value to be realised. Thus, in all of the manyoffshore gas fields, whether non-associated or gasassociated with oil production, it is processed to the extentnecessary to pipe it to shore, where it can be processed toconsumer specification and, ultimately, sold into the gasdistribution network. But, what happens if your reserves aretoo far offshore or the route too challenging to make apipeline economically viable? What then?

Offshore Floating Liquefied Natural Gas (FLNG)production and storage has been promoted by the oil andgas industry as one of the leading technology developmentareas for the future for some years. Paper studies foroffshore LNG production have been performed, in varyingdegrees of detail, since the late 1970sii. However, it was inthe mid 1990s that substantial work backed up with specificexperimental development was performed. Thesedevelopments were motivated by a sustained world-wide risein natural gas consumption, exceeding the growth of otherfuels. Its current share of total primary energy consumptionis over 25% and the increase in global gas consumption in2004 exceeded the 10-year average rate and rose in everyregion.iii

The obvious imperatives for this research and developmenthave been the perceived need to process and monetiseassociated, stranded and in some cases marginal gasreserves for various economic and environmental reasons.There was a time when LNG only played a speculative role inthe development of offshore fields but more recentlyinternational accords, like the Kyoto protocol, coupled withthe economic advantages of unlocking stranded gasreserves, has provided a renewed impetus to the concept.

Many of the most promising targets lie a considerabledistance offshore in deep water where the weather can beanything but benign. Concepts to exploit these finds havefaced many technological challenges particularly in theareas of mooring and cargo transfer of LNG carriers to afloating structure; in reaching a point where it is generallyaccepted that the concept is feasible; albeit that some of thetechnologies are as yet unproven in a shipboard or floatingapplication.

Despite this concentration on the subject, one area thathas received, until recently, very little meaningful attention indeveloping these various concepts has been that of shipassist. Clearly, where the floating production unit is stationedpermanently offshore and relies on a standard LNG carrier(LNGC) to export the LNG produced, an understanding ofthe limitations this places on a support fleet has to bedeveloped and a methodology has to be established ifberthing and un-berthing is to be carried out safely, as wellas providing a safe working and living platform for the crews.

Floating LNG terminals are being proposed in open sealocations where generally the conditions are much moresevere than close to the coast at a jetty or in shallow waterwhere most receiving terminals or gravity based structures(GBS) will be situated. It is currently difficult to predict howtugs and their crews will perform in these conditions andtherefore almost impossible to establish a quantifiable safeoperating envelope.

In defining what is regarded as severe in the context ofship assist in open waters, its worth summarising thegeneral observations of tug behaviour from the MARIN pilotstudy. The tests, it noted, confirmed the motions of theassisting tugs in waves are significant, even in waveconditions that are considered mild for the berthing andoffloading of LNG carriers assisted by tugs. What resulted

Figure 1 The Disputed Greater Sunrise Field offshore NWAustralia

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were repeated slack tow line events and frequent high towline peak load events in the pull mode coupled with very highpeak fender loads in the push mode. The worst casesoccurred in waves as low as Hs 1.9m and as high as Hs3.0m and Tp 8.3s when the tug was operating on theunshielded side of the LNGC.

Major challenges are thus presented to tug designers,equipment suppliers, tug operators and LNG carrier ownersand charterers. As one leading tug designer commentedrecently, tug boats have not generally been perceived as atopic worthy of significant research but the changingdemands on the towing industry over the last decade or sohas created a need to examine the nature of tugperformance through a fresh set of eyes.iv

OPTIONS FOR OFFSHORE DEVELOPMENTAs in the oil trade, export from FLNG terminals to visitingexport tankers can be achieved in one of two ways, either ina tandem mooring or side by side arrangement. Until suchtime as a cryogenic subsea pipeline suitable for the lowcarriage temperatures of LNG (minus -163oC) is developed,loading from a Single Point Mooring (SPM) stationed closeby is not an option.

LNG project developers and floating systems contractors,together with cargo custody and fluid transfer systemspecialists, have for several years being developing andpresenting an array of options for tandem off-take for FLNG.

Some of these concepts or parts thereof, particularly thefluid transfer arrangements, have been approved byclassification societies in principle. Nonetheless, the oil andgas industry has always exhibited a certain amount ofconservatism towards utilising these technologies and asVan de Valk and Watson (2005)v pointed out in a recentstudy, stern to bow schemes are less attractive in view of thenovel LNG transfer technologies.

In the same study it was concluded that side by sidemooring of a LNGC to a weathervaning barge in a severemulti-directional wave climate is not feasible; as operationalexperience suggests it results in insufficient uptime. AsMARIN stated following their pilot studyvi prior to initiating theSAFE TUG JIP last year, and presented at OMAE in June,the requirements for the uptime of new offshore terminals forLNG are extremely high at between 95-99%. In the waveclimates associated with the majority of proposed offshorelocations, it is generally accepted that the currentdevelopment of tug technologies and modes of operation donot permit these demanding levels of uptime with sufficientconfidence.

Some in our audience today may take exception to thisassertion, but the fact remains that the mode of operation isunproven and until such time as developers are able topresent what can be called quantitative evidence ofperformance to meet these requirements, such schemes willremain no more than a basis for design. However, for ratherbenign metocean conditions, Van de Valk and Watsonconclude, the option is the preferred solution because it isbased on the use of proven equipment and procedures.

Figure 2 The sort of weather Offshore LNG can expect

Figure 3 Schematic showing export tanker loading from aSingle Buoy Mooring

Figure 4 LNGC tandem moored at an offshore LNG facility(courtesy of SBM Inc.)

Figure 5 Side by Side Mooring configuration (courtesy of SBM Inc.)

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Despite these stated limitations, the concensus ofpotential developers is that the technology would work, andthe view of some developers is that the economics probablywould work as wellvii. As Onassis and Hurdle (2004)viii

indicated in their recent paper, FLNG facilities areincreasingly being seen as a viable way of retrieving LNGfrom remote locations. Certainly, had it not been for thesovereignty dispute surrounding the Greater Sunrise projectoff the coast of NW Australia we may well have been closerto FLNG being a reality than we are today.

HURDLES TO EXECUTIONResearch into the behaviour of tethered tugs in severewave climates has received little serious attention. Floatingoil production systems have always relied on tandemmooring solutions to their custody transfer challenges insevere wave climates and dynamic positioning or tautmoorings in place of conventional marine support and sideby side berthing/loading. This has only required the servicesof an OSV/standby vessel to mitigate the effects offishtailing on occasion and has provided no incentive tostudy the subject further.

In meeting such demanding uptime requirements as thosestated above, tug designers have had to embark on areassessment of the nature of tug performance; in much thesame way that escorting brought a new set of requirements,and a similar reassessment in predicting with a high degreeof accuracy the actual performance achievable under a newset of parameters.

Nonetheless, there are a number of hurdles to makingfloating LNG a reality in more severe wave climates inaddition to accurately defining the basic calm waterbehavioural characteristics of the tugs themselves.

Tug Design OptimisationThe motions of floating vessels in waves have exercised theminds of designers for many decades. Modernhydrodynamics has identified and proven several ways inwhich motion response of floating vessels can besignificantly improved. Amongst them the addition of deepskegs; appropriately sited bilge keels; the judicious use ofwater line sponsons and flares together with, in the case oftugs, close attention to the optimisation of the towing point.Many modern tug