The Naval Architect: July/Aug 2018
The International Association of Classification Societies (IACS) first published its Common Structural Rules for bulk carriers (CSR-BC) and Common Structural Rules for oil tankers (CSR-OT) in 2005, formally adopting them in 2006. Because the two ship types broadly shared many characteristics – e.g. loads, ship motions and accelerations and hull girder strengths – there were obvious benefits to harmonising them.
Philippe Baumans, who is currently halfway through a three-year tenure as the Chair of IACS’ Hull Panel, has long been involved with the development of CSR. He explains: “We didn’t want to reinvent the wheel and so looked to harmonise areas of commonality. We aimed to accommodate small differences and when it was really specific – such as the different bulkheads for dry and liquid cargoes – these would be covered in separate parts.”
In part, the CSR BC&OT adopted in 2013 anticipated IMO’s introduction of the overarching Goal-Based Standards (GBS) framework for ship design and construction, and in 2016 this compliance of CSR with GBS was confirmed by the IMO’s Maritime Safety Committee at MSC96. However, this now means that all bulkers and tankers over 150m length constructed after 1 July 2016 now also need to comply with the GBS requirements in SOLAS, any changes to the CSR affecting GBS compliance must be verified by the IMO.
IMO’s initial auditing process raised some non-conformities and observations which IACS were obliged to address; among these were some of the assumptions underlying the probability distribution model for a ship’s response to waves. The technical background for these is extensively detailed on the IACS website and the corrective actions in relation to the non-conformities were completed in the first half of 2017. As a facet of this work, the IACS Hull Panel decided to form a dedicated project team to take a fresh look at the wave loads definitions that form the basis for these models.
Probability model
Equivalent Design Waves (EDW) were devised as a practical means of modelling the waves used in computing the stresses a vessel will be subjected to across a 25-year lifespan operating in the North Atlantic, which is considered to reflect the appropriate operating conditions in designing these vessels. The North Atlantic is represented by a two parameter Pierson Moskowitz wave spectrum S(ω)=fct (Hs, Tz, ω), where ω is the angular wave-frequency.
While IMO’s GBS requirements aren’t specific in relation to what constitutes ‘North Atlantic’, the 2000-published IACS Recommendation 34 for Standard Wave Data stipulates a four-zone region (see Figure 1), which has assumed de facto status. The data recorded for this area by BMT’s Global Wave Statistics had shown that regular waves fell within a narrow-band spectrum and this data is typically recorded in a matrix which discretises the wave height (Hs) and the mean zero up-crossing wave period (Tz). From this it has been possible to calculate the long-term probabilities of the ship responses.
However, this probability model was based on the assumption that waves were of equal heading probability and could come from any direction, something the IMO’s GBS auditors didn’t agree with. Baumans explains: “They said that it was more probable that the waves come from certain directions, but the difficulty is that a non-equal heading probability model doesn’t exist.”
The solution was to consider the situations in which a ship’s master might be compelled to adjust course in response to the undesirable effect of waves. It was determined that a change in direction most frequently occurred when one of three conditions were met:
1. When likely to exceed a given roll angle.
2. When the freeboard at the forward part of the ship became too small.
3. When the propeller is no longer fully immersed.
Baumans explains: “We created a probability model for 3D situations for 22 bulk carriers and 27 oil tankers of different sizes, to calculate what happened under those conditions. As this was quite burdensome it was determined we could achieve with good accuracy the same results with a 1D probability model.”
The results indicated that some ship headings, notably a head or following sea, had a far higher probability of occurrence than others. But what was really needed was how this impacted on the design loads and scantling compared to the earlier uniform model for CSR.
“When we calculated with this new EDW we found that some factors increase, while other factors decrease. There was an increase of between 4% and 4.5% for the vertical shear force, vertical wave-bending moments amidships, and surge and pitch accelerations in head or following sea conditions. In beam sea conditions, there is less pressure at the waterline amidships and a decrease in heave and roll accelerations of 4% to 4.5%, while with transverse or oblique sea conditions there are less horizontal wave bending moments (2.5-3.0%).
“So we decided that where there was an increase in wave parameters we would introduce a coefficient of 1.05 in the CSR for amplifying the long-term values of the pressure from accelerations and hull girder stresses. When there was a decrease no corrections were made for precautionary reasons, but we applied this coefficient in head sea and following sea cases, on the vertical wave-bending moment, on the shear force and the accelerations. So this coefficient was introduced into the rules on 1 January 2018.”
