SIGTTO-Recommendation for the installation of cargo strainer on LNG carrier

RECOMMENDATIONS FOR THE INSTALLATION OF

CARGO STRAINERS ON LNG CARRIERS

1. Introduction

Until relatively recently cargo strainers have not been fitted or considered necessary on LNG carriers. LNG beingessentially a "clean" cargo, filtration has been generally limited to the installation of strainers in the loading lines atthe loading terminal principally during the initial commissioning of the plant and build-up period of the project.Avoidance of contamination on the ships has relied on good housekeeping and visual inspection during constructionand subsequent maintenance periods. Incidents have occurred, however, where gross solid matter has been dischargedinto receiving terminal facilities to the extent that terminal operators now require ships to provide some form of cargostraining as a protection against serious contamination or damage to their terminal installations. At the same time,export and import terminals have recognised the part they can play in maintaining the cleanliness of cargo transfer inprotecting downstream units and in safeguarding their own interests.

2. General

The purpose of this document is to provide recommendations, based on current experience, for the type, disposition,performance and usage of LNG shipboard cargo strainers. On the assumption that existing ships in service havealready adopted philosophies and arrangements mutually acceptable within their projects, these recommendations areintended as a guide for newbuildings or existing ships presently not employed in project trading.

3. Philosophies

In current LNG trading, two broad philosophies on cargo transfer straining have emerged. One is that the ship acceptsthat a cargo delivered to it by an export terminal will be clean and, by a primary emphasis on strainers in its owndischarge, it in turn ensures a clean cargo to the receiving terminal. In a strict application of this philosophy, the shipwould not require to provide strainers during loading and any strainers on the shore side at discharge, additional to theship's strainers, would be entirely at the discretion of the receiving terminal. However, a shore export strainer cannotprotect against debris picked up in any piping and the loading arms downstream of the strainer and removing cleanshipboard strainers before loading and replacing them for discharge is an unwanted work load. Thus, ships operatingwithin this philosophy commonly have strainers which may be, and are, left in place during loading.

The alternative philosophy takes the rationale that it is in the interests of the ship, by fitting strainers during loading,to ensure that it receives a clean cargo and, likewise, in the interests of the import terminal, by means of shore sidestrainers, to ensure that it receives a clean cargo. Thus, for the ship the prime requirement is that it strains the cargo asit is loaded.

Here again, however, as an additional precaution and to minimise work load, dual-flow strainers maybe provided andbe left in place during discharge. Alternatively, some ships operating within this philosophy provide easilyinterchanged single-flow strainers separately for discharge.

Recognising the energy adsorption and consequent additional boil-off resulting from any restriction deliberatelyplaced in the way of cargo flow, two regimes of strainer mesh are generally accepted - a finer mesh for periods whengeneral contamination is more likely, as at commissioning into service or after maintenance, and a less fine mesh forperiods when the normal routine cleanliness of ships and terminals can be expected.

Strainer requirements in both philosophies are thus virtually identical in that ships generally provide strainer facilitiessuitable for both loading and discharge flow and at two levels of strainer fineness.