Design Tool for Green Sea, Wave Impact, and Structural Response on Bow and Deck Structures
Authors: Hellan Ø. , Hermundstad O.A. , Stansberg C.T. 
Published: October 2008

In the offshore industry, there has been a growing attention around damages on bow and deck structures on FPSOs and floating platforms caused by wave impact in steep storm wave conditions. A standard design tool taking properly into account these effects still does not seem to have been established, although there are various research and development activities around in the industry. This is because the problem involves strongly non-linear wave-structure interaction effects that are hard to describe accurately by present theoretical models. The development of fully 3D nonlinear tools is expected to still take some time, and simplified models are considered for present engineering applications.

One such simplified approach is discussed in the present paper. This is based on a combination of presently available theory, with the aim to end up with a practical and robust tool taking into account the most essential physical effects. A discussion of the main physical mechanisms considered important for the prediction of events with serious damages is given. This includes such as non-linear effects in steep and energetic random waves combined with wave diffraction and vessel motion, non-linear water propagation on deck with resulting impact load pressure distributions, and finally, the structural integrity assessment. As a part of this analysis, the prediction of impact events is also included. Empirical calibration against model tests in regular and irregular waves is included as an essential part of the approach. The ideas are applied in the development of an engineering design tool.

Introduction

Background.

In recent years, a number of incidents with damage on bow and deck structures have been reported from floating production systems at sea, due to wave impact in steep wave conditions. Several incidents of damages due to green water on deck have been reported /1/, and an FPSO operating in the North Sea west of Scotland experienced heavy bow slam with damages to the bow structure /2/.

Significant research activities have already been established on different aspects of this subject. Examples found among the extensive amount of works published in the literature include such as green water modelling /3/-/7/, slamming force modelling /8/-/9/, fully non-linear flow and wave diffraction models /10/-/11/, and coupling between hydrodynamic loads and structural responses (hydroelastic effects) /12/-/14/. This includes experimental works as well as basic theoretical modelling. Since parts of the problems are considerably complex and highly non-linear, this includes, to some extent, long-term research studies, from which significant developments are expected to be achieved after some time. Such studies are important steps for future design models.

So far, however, a consistent design tool combining the complete chain of different load and response effects in a generally accepted way does not seem to be available today, although various empirical methods are in use on different elements in the procedure.