Ocean current variability in relation to offshore engineering

By Rune Yttervik
PhD Thesis (24th February, 2005)

The offshore oil and gas activity has up until recently taken place mainly on the continental shelves around the world. During the last few years, however, the industry has moved past the continental shelf edge and down the continental slope towards increasingly deeper waters. In deep water locations, marine structures may span large spaces, marine operations may become more complicated and require longer time for completion and the effect of the surface waves is diminished. Therefore, the spatial and temporal variability of the current is expected to become more important in design and planning than before. The purpose of the work has been to shed some light on the issues raised by two questions that are often encountered when a marine structure is to be designed or a marine operation is to be planned :

• What is the current flow like in the area?
• How is the structure/operation affected by this current?

Analysis of a set of observed data that were recorded close to the seabed on the continental slope west of Norway (HF in the Figure) are presented. The data suggest that some strong and abrupt current events (changes in flow speed of ~0.4~m/s in just a couple of hours) were caused by motions of the deep pycnocline¹, driven by variations in the surface wind field. This conjecture is partly supported by numerical simulations of an idealised continental slope and a two-layer ocean. The data also contains an event during which the flow direction at the sea bed changed very rapidly (within a few minutes) from down-slope to up-slope flow. The change in speed during this event was as high as 0.5 m/s.

Another data set, collected at the locations OL-I and OL-II, has been analyzed in order to illustrate the spatial variation in the current that can sometimes be found. The flow in the upper layer was found to be virtually decoupled from the flow in the lower layer. This is either caused by bottom topography, stratification or both.

High variability, and increased importance, of the current presents new requirements to the way that the current should be modelled by the offshore engineer. For instance, it is necessary to consider which type of operation/structure that is to be carried out or installed before selecting design current conditions. Reliable methods for obtaining design current conditions for a given deep water location have yet to be developed, and only a brief discussion of the topic is given in this thesis.

It is shown, through calculations of VIV-response and simulations of typical marine operations, that the variability of the current will sometimes have a significant effect on the response/operation.

¹Pycnocline = density surface between water masses. The pycnocline between two water masses of different density is defined by the maximum of the density gradient.