Offshore construction design codes, such as API and DNV, provide guidance for gravity based design They are formulated for single foundation sand contain only general guidance for many smaller subsea construction application in use today. It is commercially attractive to Contractors and Operators alike to install structures with gravity base rather than piled foundations w here horizontal applied loads are low The normal layout of these structures is to have two or four mudmats placed on the edges or comers of the structure Analysing these structural layouts for foundation stability is actually more complex than single foundations. Drawing on experience with the design of a variety of subsea foundations, the authors address some of the important issues that have arisen recently and suggest methods and approaches for dealing with them. These include
Design basis and geotechnical design methodology,
Selection of soil parameters if limited data are available,
Load distribution and limit state for structures with multiple mudmats
Bearing capacity and skirt design issues,
Cyclic loading and influence on shear strength,
Foundation stability under impact loads
The Intent of the paper is to prevent delays in the design and review process, thus reducing costs, by encouraging the use of geotechnical design methodology as a basis for technical communication, and outlining some of the practical design methods that can be adopted for small subsea gravity structures
The various codes of practice (for example DNV, 1995, API, 2000) provide design guidance for gravity base structures. While the principles are also applicable to smaller subsea applications such as protection structures, riser bases, pipeline end manifolds etc, there are a number of geotechnical issues that arise which are particular to these smaller, less sensitive structures. Because they are smaller does not mean that the geotechnical issues are necessarily simpler. This paper sets out to highlight and propose solutions to some of the design issues related to the foundations of such structures
Foundation design should never be separated from the overall design process except that it may require a geotechnical engineer to perform the work. The structure and foundation act together, not apart. To pass loads from one to the other without consideration of the overall behaviour of the system may not provide the best solution. This is particularly true for some subsea structures In which two or more separate bearing are as are involved
This paper seeks to face some practical issues related to certain areas of gravity base designs that are particularly important for small subsea structures The main steps of the foundation design process for small structures are considered to be
Agree design basis,
Define soil parameters,
Agree design loads and serviceability limitations,
Perform foundation design analysis for limit states,
Confirm stability and service ability,
Achieve independent verification
Aspects of these steps will be addressed in this paper. No attempt has been made to guide the reader through all aspects of design-this would be far too extensive Rather, certain topics have been chosen that are unusual or contentious and not dealt within the codes.
