As metocean information is used throughout the lifecycle of a development, the Metocean Team at ABPmer consider it appropriate to plan this at a very early stage and to take a ‘whole life’ approach towards its acquisition, management and use. In this article, Robin Stephens, Head of Metocean consultancy at ABPmer outlines key considerations when using metocean information.
Plan storage, version control and sharing
On large, multi-disciplinary offshore developments, there may be many different parties using metocean information for various aspects of the programme. Knowledge of site-specific metocean conditions that builds during the project lifecycle should be captured and made available to all. This is particularly important for remote locations where there may be little pre-existing knowledge or observation of conditions, and no past precedent for extreme metocean parameter values for use in engineering design. It is important therefore to rigorously plan for storage of data and information, and have a very clear protocol for version control in place together with effective dissemination of updates to project metocean criteria.
Know the limitations of the data you use
The figure below shows, conceptually, how estimates of extreme metocean parameter values, and their associated confidence levels, will evolve throughout the project life cycle. The figure is for illustrative purposes and has no absolute values of defined units.
With the evolution of metocean criteria from the initial estimates with relative low confidence, through the various intermediate stages and ultimately to final values with high confidence, it is vital that the end user takes note of any ‘fitness for purpose’ statements within the metocean reports.
A ‘fitness for purpose’ statement should make clear any limitations on the data usage. Specifically, it should clearly state whether the results are suitable for design work (FEED onwards) or limited to preliminary assessments (e.g. feasibility or concept selection). Very early stage preliminary assessments should not be used for final design work. An intentional conservatism at early stages may partially mitigate against potential misuse of the results but cannot guarantee suitability.
Understand that metocean criteria is not cast in stone
It is often the case that major early-stage decisions on project feasibility and concept selection are informed by metocean criteria that have been prepared rapidly and on a low budget, using readily available datasets and publications. It is prudent to select sensibly conservative criteria values during these early stages, when understanding and confidence of metocean conditions is limited.
As a project develops and investment is sanctioned, budget will normally become available for conducting detailed site-specific metocean measurements and location-specific numerical model simulations. This allows enhancements to be made in criteria value estimation and corresponding increases in their associated confidence levels. Whilst, often, criteria refinements will result in reduced extreme value estimates, there will be circumstances in which criteria need to be revised upwards.
Major engineering schemes will often have multi-decadal design lives. It is interesting to consider factors that may cause our estimates of extreme metocean criteria to change over such time periods. There may be physical phenomena such as long-term evolution of bedforms and coastal alignment causing changes in local wave severity and current flow. The occurrence of large rogue storm events may impact the ‘upper tails’ of the probability distributions of winds, waves, currents and water levels. Particular physical processes or characteristics may become apparent that were not anticipated at the design stage.
Climate change, and long-term natural variability, may only become evident and quantifiable once the facility has been designed and constructed. Other factors are the continuing technological advances in our ability to measure and model physical processes. Thus, we should never consider metocean criteria to be ‘cast in stone’, and should allow for their periodic re-evaluation throughout the lifetime of an engineering scheme using latest data, information, knowledge, analytical procedures and technological advances.
Invest early – it pays off
The engineering implications of changes in design (and operating) criteria values can have major cost and safety impact. For this reason, it is highly advantageous to place strong emphasis on metocean work as early as possible in the development of a project, such that criteria estimates are reasonably stable and reliable by the time that preliminary design takes place. Early project investment in competent metocean work will result in positive impacts throughout the project lifetime!
Prepared by Robin Stephens, Head of Metocean Consultancy, ABPmer
+44 (0) 2380 711 879