Shallow water offshore structures – foundation stability: an essential part of design work

When evaluating the feasibility of shallow water offshore concepts attention has to be paid to foundation stability from the very beginning. If the designers involved are naval architecture orientated and used to working mainly with floating structures and not soil properties, they may not pay sufficient attention the latter area. As a consequence, civil engineers are often called on to provide the solutions. Civil engineers, however, commonly do not fully understand the “offshore” features of the concepts. Both the aforementioned cases may lead to end results that are not satisfactory.

This article mainly concentrates on Gravity Base Structures (GBS) for two reasons. Firstly, the problematic for other structures such as Jackets and jack-ups is very much different from Gravity Base structures and should be the topic of a separate article. Secondly, Wellquip Ltd is actively involved with shallow water GBS concepts and the topic is very current.

To fully understand the importance of foundation stability already at the earliest stage of concept design it is instructive to refer to Det Norske Veritas (DNV) Classification Note 30.4. It states that ‘Requirements for foundation stability are often the most decisive factor for the determination of the foundation area, foundation embedment (i.e. skirt penetration depth) and submerged weight for a structure with a gravity type foundation. It is therefore essential in an optimal design process to heavily emphasize foundation stability calculations’.

This means that foundation stability has to be taken into consideration already when determining the main dimensions and weights of the concept and continuously thereafter as the design work proceeds.

Calculating foundation stability in conceptual phase

There are several possibilities for the very first conceptual evaluation of foundation stability, some of which can be loaded from the Internet (Hansen’s Bearing-Capacity Equations). The American Petroleum Institute (API) has also issued a method in API RP 2A WSD and DNV has a system in their aforementioned Classification Note and also in their standard DNV-OS-J101.

The equations look very simple, but working with them raises many questions and the support of either the issuing institute or of a professional civil engineer is often needed. The equations normally simply provide the actual bearing stress together with the safe bearing capacity.

The results can also be presented in the form of a foundation stability envelope for horizontal and vertical load combinations inside which the foundation is stable. Figure 1 depicts this kind of envelope for one eccentricity of loading. The eccentricity also defines the effective area on the seabed to be used in the calculations.

The lower straight line indicates the sliding capacity. The envelope was developed for a WQ GBS concept based on the DNV method in Classification Note 30.4 with a computer program developed for the purpose; first to test the program itself and thereafter to test the feasibility of the actual concept.

It is to be noted, that the aforementioned consideration is generally only to determine the basic dimensions of the concept and to provide some confidence to continue. Even DNV states in their Classification Note that ‘for gravity foundations with relatively small areas, e.g. mudmat foundations for temporary supports of jackets or foundations for small subsea structures, bearing capacity formulae may be acceptable’. Further analysis is required depending on the concept type and especially on its design life.

Figure 1. An envelope for safe horizontal FH and vertical FV force combinations (inside boundaries) for a given eccentricity of the load. The straight line describes the sliding capacity. As can be seen from the graph it can also be a significant factor for low FV values

Picture 1. A scaled model of a WMCS module

Plaxis 3D used to confirm analysis results

An example of such further problems, which cannot be solved with conceptual evaluation analysis, is lateral displacement under loading. Advanced Finite Element programs can be used for this purpose. In the spring of 2013 a Wellquip GBS concept (See image on pg. 33) was tested at the Tampere Technical University in Finland at their laboratory with a scaled model. The results were confirmed with Plaxis 3D. A photo of the scaled model can be seen in Picture 1. Figure 2 shows the model test results as verified by Plaxis 3D.

The analysis indicates that Plaxis 3D can be fully trusted. It seems that model testing is not necessary for future concepts, especially, when it is difficult to simulate the actual full-scale situation accurately enough with the scaled model.

Cyclic loading can cause failures

The second problematic area is cyclic (wave) loading and its influence on the shear strength of the soil, which in combination with the number of cycles can lead to failure. DNV generally requires cyclic (wave) loading to be studied, but the guidance provided for the analysis is somewhat unclear and several cross references are provided to documents, which in turn refer to others.

As such it is difficult to find a definitive source that would provide a complete understanding of the matter. It, nevertheless, seems to be an item worth studying in more detail in order to clarify whether in-house analysis is in general possible.

Another issue with cyclic loading is that the displacements caused by cyclic loading are not reversible. A kind of hysteresis phenomenon takes place throughout the design life. The maximum deviation from a location is, of course, of interest to engineers when designing e.g. a safe piping connection from the top of GBS to the bottom of a well. It has become clear that the deviation caused by long term wave action cannot be evaluated with Plaxis 3D.

Figure 2. Comparison of model testing and Plaxis 3D results

Special analyses required to study liquefaction

Another important phenomenon that cannot be evaluated with Plaxis 3D is liquefaction caused by earthquakes or vibrations that result from ice/structure/soil interaction. Consolidation and scouring are also interesting items, which may appear fairly minor, but could have severe effects. A lot of guidance is provided in the literature with regards how these special analyses should be performed.

Model tests only provide part of the answer

There has been much discussion about shallow water model testing in basins. In model testing the scaled model lies at the bottom of the basin and a moving ice field places loads on the model. This allows the seabed pressure distribution and displacements to be measured.

There are some institutes and laboratories around the world that have the facilities required for this kind of testing, but it seems that they produce more qualitative than quantitative results. Real engineering, on the other hand, produces quantitative results and makes the use of finite element models and advanced analysis necessary.

Summary

Foundation design is an essential part of offshore shallow water structure design work. It needs to be analysed at the very beginning when determining the main dimensions of the concept and considered until the end of design work to guarantee the safe operation of the unit over its entire lifetime. At the conceptual phase it is important that designers are familiar with the systems used to evaluate the soil bearing capacity and the feasibility of the new concept in general. For later phases a more detailed analysis using highly developed software programs and professional assistance are needed.

Foundation design of offshore structures is a challenging and interesting part of offshore design and project managers should have a broad general understanding of this design area in order to guarantee good end results.

The original text was published in our 2/2014 Top Engineer magazine