AB The International Marine Contractors Association

Acrylic Plastic Viewports

www.imca-int.com

AODC 030 November 1984

AODC 030 – November 1984 Page 1

Acrylic Plastic Viewports

Acrylic Plastic Viewports have been in satisfactory use for a number of years.However, a recent development, testing them by polarised light has shown that thereare various misconceptions about them.

This Guidance Note, which has been prepared following advice from DnV seeks tocorrect these misconceptions.

1 CERTIFICATION

1.1 The date of examination, a serial number and the certifying surveyorsidentification should always be marked indelibly on the circumference of theviewport. This can be by engraving or indelible ink or similar but it must notbe by the use of metal stamps, as this could cause cracking.

1.2 Viewports which are not marked or properly documented should not be used.

2 DETERIORATION

2.1 Acrylic Plastic will start to decompose after a certain period. As it decomposesit gradually turns yellow. This can be demonstrated by holding a white sheetof paper behind it. Slight yellowness is, however, acceptable.

2.2 Acrylic Plastic exposed to direct sunlight decomposes more quickly andshould be examined at least annually for signs of deterioration. However,experience has shown that 10 years is normally a safe working life for anacrylic plastic viewport. After 10 years service a viewport should be replaced,even if it appears not to have deteriorated.

2.3 It is very important that regular visual inspections are carried out to determineif there has been any mechanical damage as evidenced by chips, deepscratches or crazing. If any significant damage is seen then expert adviceshould be sought (for example from a manufacturer) to establish whetherreplacement is needed.

3 YIELDING

3.1 All acrylic plastics yield continuously even when not under pressure. Theeffect of pressure is to increase the rate of yield.

3.2 Any acrylic plastic viewport which has been subjected to pressure will have aslight permanent deformation and will, therefore, contain a certain level ofinbuilt stress.

Page 2 AODC 030 – November 1984

4 POLARISED LIGHT

4.1 Polarised light will show up areas of different stress as different colours.

4.2 As explained in 3.2 any acrylic plastic viewport will show areas of stress whenexamined by polarised light. This does not necessarily indicate a fault.

4.3 It takes a very skilled and experienced specialist to interpret the level of stressshown by polarised light.

4.4 The removal of inbuilt stress is a complex process involving removal andspecialised heat treatment, which should only be undertaken by a companyspecialising in such work. Viewports will almost certainly need recertificationafter such treatment.

5 CONCLUSIONS

5.1 Acrylic Plastic viewports should be properly certified and marked prior to use.

5.2 Acrylic Plastic viewports should be replaced after 10 years service.

5.3 Examination of an acrylic plastic viewport by polarised light, after the viewporthas been in service, is of very limited use as all such viewports will showevidence of inbuilt stress.

5.4 The use of polarised light to check that acrylic plastic viewports are correctlymounted and evenly prestressed is not a simple matter and should not be usedas a principal test. It may be used as a supplementary test but indications ofstress in the viewport are normal and should not be regarded as reason forreplacement, unless there are other indications, or unless expert adviceconfirms a fault.

5.5 When mounting an acrylic plastic viewport it is important to check that thehousing is the correct dimension and that the viewport and any sealing gasketsor 'O' rings are properly seated and of the correct size.

Oversized 'O' rings to take up wear or minor dimensional variations,particularly if the 'O' ring locates in a groove in the viewport, can causeundesirable stresses in the acrylic material.