Not Logged In, Login,

Wednesday, October 17, 2018

CORROSION, DESIGN OF TANKERS AND EXPERIENCE WITH DOUBLE HULLS DISCUSSED AT THE TANKER STRUCTURE CO-OPERATIVE FORUM IN TOKYO

A meeting of the Tanker Structure Co-Operative Forum (TSCF) took place in Tokyo last week to discuss a few high priority issues related to tanker structure. A total of 9 papers were presented at the Tokyo meeting covering 3 major themes.

Below is a copy of the full report including the outcome and follow-up of the issues raised by INTERTANKO.  Also available on are copies of the papers presented at the Forum.

Participation - More than 120 participants, representing 80 organisations from 20 countries. The large majority of the tanker operators’ representatives present were INTERTANKO members. Oil companies attending - ExxonMobil, Chevron Shipping and Shell. INTERTANKO and OCIMF were the only two representatives of the trade organisations. Class Societies attending - NK (hosting the meeting), ABS, DNV, LR, BV, GL. There were representatives from 15 shipyards, all from the Far East except Odense Shipyard.

Scope of the meeting - To discuss a few high priority issues related to tanker structure.

Background - TSCF (Tanker Structure Co-operative Forum) is a group of 27 companies (7 Class Societies, 4 Oil Majors and 16 tanker operators). TSCF was established in 1983 with the aim of its members sharing important information and experience related to the structure of their tankers. The second aim was to develop corrective actions to overcome the problems experienced and to publish guidelines that could assist the tanker community at large. In the 1990s TSCF issued three important guidelines on ballast tank coating, guidelines for inspection and maintenance, and a manual on tanker structure. The latter identified a number of structural damages and gave methods of repairs and corrections for new buildings.

In almost 18 years of existence, TSCF has organised only three meetings with shipbuilders: in 1987 in the U.S when corrosion on ballast tanks and coating of ballast tanks was an important topic, in 1992 in London when the double hull structure was the meeting theme and in 2000 in Tokyo.

It may be of interest to mention that the TSCF has no secretariat or fixed administration. The work is decided by a steering committee and carried out by smaller working groups.

Proceedings - A total of 9 papers were presented at the Tokyo meeting covering three major themes:

  • Corrosion in ballast tanks and cargo tanks
  • Design of tankers
  • Experience with double hulls

The papers were made available and circulated by e-mail 2 months in advance. TSCF then invited participants to send written comments and questions one month before the meeting to give the authors time to reflect and give proper answers.

INTERTANKO participation - Dragos Rauta and Bjarne Thygesen represented INTERTANKO. Bjarne is a member of ISTEC and covers in particular the issues of corrosion and policy with regard to shipyards. Bjarne has been a member of the TSCF Steering Committee for more than 10 years as a representative for Gotaas-Larsen.

INTERTANKO participation was determined by the importance of this meeting, by the high level of knowledge and also because INTERTANKO sees TSCF as a possible vehicle for determining quality standards that could be accepted by the whole industry. High standards and proper procedures to be promoted are highlighted below.

INTERTANKO was one of the very few of the attendees who had comments and questions, and the only participant that had comments and questions on all the items discussed. INTERTANKO was also one of the most active in the discussions during the meetings in Tokyo.

The issues raised by INTERTANKO and the outcome/follow-up were:

Longer Period of Guarantee for new buildings - This is the first time that a longer guarantee was requested in a public meeting and this was expressed by INTERTANKO. It was a good opportunity because shipyards were present.

INTERTANKO suggested 5 years’ guarantee and furthermore that shipbuilders attend the first Class special survey. INTERTANKO also raised the Class’ position on this matter, because Class approves the designs as well as acting as the supervisors of the quality of the work done by builders.

There was no immediate response from the shipyards but the issue has now been made public and will keep occurring in future discussions. It was agreed that there should be separate arrangements for hull and coating on the one hand and for , onboard equipment on the other.

Individual owners have managed already to get 3 or even 5 years’ guarantee for coating. This should become the standard.  LNG/LPG tankers have already received a 3-year guarantee for the hull. Oil tanker owners should then ask for a similar length of guarantee, but all should aim for a guarantee up to the first special survey. The age of five years seems a natural cut-off point. From this age the ship’s condition is increasingly dependent on the owner’s maintenance standard.

INTERTANKO members are advised to press yards for longer guarantees in their private negotiations, up to 5 years for hull and coating. We need this collective effort even if we do not always succeed. This would accelerate the process by which such a guarantee would become the standard.

Class Societies’ representatives said that they have standard new building contracts that stipulate a 3-year guarantee. They repeatedly said that it is up to the owner to negotiate and get it approved. It was clear that Class intended to leave the promotional work on this issue with the owners. The positive aspect could be that Class would not oppose it.

Since the presentations given by the shipyards all stressed the high quality of their products, INTERTANKO took the opportunity to invite them to consider a longer guarantee as a positive move to raise the minimum standard and as an opportunity to get a better feedback on their products. They should not see the longer guarantee as a liability. This was appreciated by the audience.

Other parties’ significant comments on this issue: the Hyundai representative considered that 1 year was sufficient for any hidden building error to emerge and a more than 1-year guarantee brought liability into the picture. Ulf Tweita of Bergesen turned the issue around very well - if Class approved the ship for a 20 years’ life expectancy as designed and built by shipyards, why should a guarantee not be granted for 5 or even 10 years?

IACS negative tolerance for steel plate thickness - At INTERTANKO’s initiative, TSCF agreed to suggest to IACS to remove the negative tolerance of -0.3 mm accepted for steel plate thickness. It has been noted that steel mills are fully equipped to roll steel plates at the required thickness, i.e. zero deviation. Thus there is no need for an acceptable tolerance, especially a negative tolerance. It has been reported that, in some cases, steel mills purposely produce steel plates with the - 0.3 mm accepted tolerance in order to save costs.

Tank coating

It was a common opinion that a one-coat ballast tank paint system was not as good as two coats with stripe coats. Although not all the ships had the same experience, accelerated corrosion in cargo tanks of crude carriers has been discussed at length. Several observations and studies have concluded that the steel corrosion rate at +40°C was double that at +23°C.

The TSCF working group was scheduled to be brought up-to-date on , TMPC steel at their closed meeting on 20 October.

The Stena Tankers, which are often held up as shining examples of good owners and good maintenance, were built to a standard in excess of the Class rules at the time 25 -30 years ago. The running cost with large crews, 30-40 strong, was in the range of USD 8000. (What that included was not stated.) By doing maintenance with the ships in service it was possible to reduce the time at drydock and the cost of the drydock.

During the paint guide session it was mentioned that INTERTANKO had contributed helpful data from the membership who had responded to the TSCF's questionnaire.

Use of High Tensile Steel (HTS) - A variety of opinions. Shipyards have noticed that owners ask for the use of HTS to be limited to 35%, but they wished to use more (Hyundai). Odense reported the use of 70% of HTS on a double hull VLCC built for AP Moller with good results. However, they did say that in certain locations of the hull known to have concentrated stresses they would recommend the use of mild steel. Odense has also increased the thickness of the HTS in areas more exposed to corrosion.

There was no clear agreement, but it was recognised that:

  • HTS is simpler to produce and thus cheaper
  • HTS as such is not a lower quality steel

However, calculations to determine the thickness of HTS steel need to be supplemented with a prudent margin for corrosion, should mild steel not be used.

A good comment was made by Ulf Tweita of Bergesen that years ago, when the use of HTS was contemplated, it was agreed (by Class Societies) that there were three essential conditions which needed to be fulfilled for the use of HTS to be safe, i.e.: perfect detail design, perfect workmanship, optimised tolerances. Since experience showed that none of these had been fulfilled it was better to adopt a more prudent approach.

Feedback to shipyards/Information to Owners during the building stage - None of the shipyards present at the meeting could give a positive response to whether they had a systematic approach to follow up the ships they built, at least until the first Special Survey. Owners’ experience shows that some shipyards are interested and, if invited, come onboard, but some other shipyards require persuasion. Again, INTERTANKO’s view that builders should be onboard at the first Special Survey was expressed.

It was also agreed that there was a need to revisit the tri-partite relationship between Class/Yard/Owner and that the owner needed to gain more access to the relevant information exchanged between Class and shipyard. Some owners had managed to get this in their contracts but not without effort. This should become a standard.

Fatigue Analysis - This was an important matter to which most of the discussions referred. Although fatigue analysis has been used for some time by Class and offered as an additional service, the results were contradictory. Ships designed to a 20 years’ life expectancy have experienced serious structural problems only a few years after delivery. A remark was made that there is too much theoretical optimisation for assessment of the fatigue life of double hulls as the experience with these tankers is very limited. It was recommended to have a more conservative approach in the design of double hulls and improve the methodology step by step based on feedback from actual experience.

INTERTANKO’s comment was that there is by far too much choice in the conditions for assessing the fatigue life. IACS should develop one single model that could be applied.

TSCF Guidelines on Tanker Structure - This TSCF booklet provides guidelines on:

  • Survey Preparation
  • Survey Execution
  • Data Analysis
  • Maintenance and Repairs

INTERTANKO suggested that such guidelines should be reviewed and considered to be implemented as the acceptable standard throughout the industry, similar to the way in which ISGOTT is accepted as the industry guide for cargo and ship operations.

TSCF will publish a new book next year on Guidelines for Inspection and Repairs containing experience with double hulls.

Click here for a copy of all papers presented at the meeting and the meeting report. Please note that the document is 5.9 MB and will require some time to download.