Hennes de Ridder

The Full Probabilistic Design Method of the Storm Surge Barrier Near the Port of Rotterdam, The Netherlands

After the storm surge disaster in 1953, which caused more than 1800 casualties in the Southwestern part of The Netherlands, a large dyke-strengthening and coastline-shortening programme was agreed upon and laid down by law. Work on the first projects commenced in the early sixties of the last century and the last phase of the programme was planned to start in 1990 and comprised of the dyke-strengthening programme in the Rhine Delta upstream from Rotterdam. This large project encountered growing public resistance as the required safety standards were established at the expense of both social and cultural values as well as ecological values. A feasibility study was started to ensure the required safety requirements of a storm surge barrier. The outcome was positive and the project was started in 1990 and was completed in 1996. In 1987, six contractors were invited to tender for the design and construction of a storm surge barrier, with only four “demand” specifications: (1) Reduction of the design water level in Rotterdam by 1.6 metres. (2) Reduction of the design water level 25 km (15 miles) upstream by 0.6 meter. (3) Lifetime of 100 years. (4) No obstacles to navigation. This set of requirements pertained to failure criteria. Based on this set of requirements, a full probabilistic method was adopted for the design of the storm surge barrier. A breakdown was made, starting from the basic probabilities of failure. The breakdown was based on failure trees with parallel and serial connected components and elements. In that way the design engineers were provided with centrally distributed failure criteria. This full probabilistic method, however, did not appeared to be adequate for several reasons. After a few months the full probabilistic design method was changed into a semi probabilistic method. Nevertheless, for the assessment of the load cases, a probabilistic approach was used, but for the design work on components and elements a traditional method introducing partial safety factors was used. Throughout the design period it was very difficult to prove that the actual designed system as well as, the designed sub systems and designed components met with the basic failure requirements. In order to avoid discussions, the designers embraced higher limits for their dimensioning calculations, resulting in a safer and more reliable storm surge barrier than was initially required.Copyright

The Top Down Risk Management System of the Offshore Operations of the Ekofisk Protective Barrier

The Ekofisk Field in the centre of the North Sea is a very important junction in the oil producing and transporting North Sea network. In 1987, the wave loads on the Ekofisk Installations had become unacceptable, due to a significant seabed subsidence. In order to cope with the increased wave loads, it was decided to build a Protective Barrier around the central Ekofisk Storage Tank. This barrier was to be installed in two separate half units, which were to be brought to the Ekofisk Field in floating condition. After installation, the two halves were to be structurally connected. The paper deals with the overall risk management of the total offshore operations, including tow-out, installation, coupling and completion. The operations were governed by conflicting requirements with respect to relevant aspects like stability, strength, stiffness, weight, geometry and were perceived to be extremely risky due to the marginal resistance against the expected load cases. The operations were weather dependent, thus dominated by changing stochastic boundary conditions. Most of the processes were irreversible, non-linear, and determined by a large number of variables. A top down risk management system was developed for control purposes. The system provided a continuous insight right from the very start of the project till the end of the project in the percentage of failure in the most relevant failure modes as a function of tow out date. The system allowed for strategic, tactical and operational interventions in case critical criteria were exceeded. This in particular makes that the approach—even 10 years after completion of the project—is still a new development in risk management and an interesting basis of further research on control of the design, construction and installation of complex Civil Engineering and other Systems.Copyright