Det Norske Veritas is placing emphasis on anticipated changes to traditional LNG supply chain patterns as it continues to develop its technical competence in the LNG field
The emergence of the LNG sector as a dominant force in the world shipping industry is bound to affect traditional LNG supply chain patterns. These expected changes include an increase in offshore gas production; a growing volume of spot market trading; an increase in the size of LNG carriers; and the appearance of the first offshore loading and discharge terminals in the not-too-distant future.
"All these developments require either new or modified technologies or new evaluations," points out Wouter Pastoor, senior engineer in DNV's Hydrodynamics & Structures Department.
"We at DNV believe that for these changes to be accommodated and implemented safely and for the same high level of class services to be maintained in the LNG sector, then improved technical competence is required of all class societies."
As part of its own development efforts, DNV is concentrating on building its level of expertise with membrane containment systems to the same high degree of understanding it has, through long experience, of Moss spherical tanks.
Within this initiative, cargo sloshing in membrane tanks has been a topic of special focus for DNV. LNG sloshing in partially filled tanks induces both fatigue and high loads upon the containment system, the hull structure and the pump tower in the tank.
The DNV work on membrane tank sloshing is also relevant to the changes about to impact the industry in general, for a number of reasons, i.e.
(a) the present membrane tank filling restrictions would be compromised in the case of liquefied storage of LNG at a floating offshore installation, where fill levels will vary widely;
(b) as spot trading increases, shipowners want to increase their trading flexibility by being able to operate with partially filled tanks, thus signalling the need for a reduction in the upper filling restriction;
(c) larger LNG carriers imply changes in tank dimensions, ship motions and, thus, sloshing characteristics in the tanks;
(d) despite the advantage of remoteness from densely populated areas, offshore locations entail ships having to cope with more severe environmental conditions when loading or discharging.
The experimental sloshing R&D programme drawn up by DNV seeks to tackle these issues head-on. The key question is how to determine the maximum sloshing loads over a ship's lifetime, at the same time as accommodating the evaluation of new designs, such as large LNG carriers; lower filling levels; localised impacts; and the effect of changes in ship speed and sea states.
Another factor that DNV has had to confront in its research work is the viability of scaling model sloshing tests up to full scale. The use of a 'comparative approach', in which actual measurements of sloshing forces onboard a conventional size LNG carrier are scaled up, has proved reliable, but only up to a point.
When LNG carrier sizes go beyond 200-230,000 cu m in size, the results from the comparative approach start to become doubtful. A more careful analysis is required and, in this respect, DNV has carried out two series of model tests in order to determine how best to scale sloshing model test results to full-scale absolute values.
"These tests have clearly shown that sloshing impact pressures increase significantly for lower filling rates," warns Wouter Pastoor. "In addition, even for lower sea states these impacts are still larger than the largest loads measured for filling levels within current filling restrictions."
The sloshing test programme is continuing. Since the first two series, DNV has carried out, or is scheduling, studies covering aspects like sloshing-induced fatigue loading, pump tower loading, structural analyses and the numerical simulation of sloshing. In addition, the class society is working on the development of full-scale measurement equipment to monitor sloshing pressures and containment structural responses.
DNV complements its classification activities with the provision of consultancy services to the LNG industry. In this respect, the society's competence on risk analyses and the direct calculation of loads and responses of the ship and tank structures have gained particular recognition.
DNV conducts wave load and finite element analyses to determine global and local responses and, thus, ultimate and fatigue limit states of the LNG hull structure, cargo tanks, tank supports, etc. In addition, thermal stress analyses can be carried out.
The risk of LNG carrier collisions or groundings is another ongoing concern that shipowners continuously strive to manage in the best way possible. DNV is able to simulate collisions with advanced nonlinear finite element analyses and to determine the amount of damage and, thus, the consequence of such accidents.
Reproduction kindly permitted by Lloyd's List