Controlling Fatigue Damage during Deepwater Installation of Inline Components

Abstract

The exploitation of ultradeepwater fields often implies the use of large-diameter export pipelines. The installation of such large-diameter and long-distance export pipelines is commonly accomplished by applying J and S lay technology. In the case of complex and large fields, early export pipelines include inline valves on T/Y branches, with the relevant protection or foundation structures. When the resulting huge and heavy bulky items are to be carefully transferred from the firing line (FL) through the stinger to the touchdown point in the depths, a step out beyond the normal pipe lay is required. It involves the real-time control of operational parameters with a strict link to weather bulletins. This resolution is principally bound to the endorsement of the overall structural integrity of the as-laid pipeline and inline assembly while meeting the stringent installation tolerances (location, verticality, etc.) imposed by the circumstances. The use of large inline assemblies may cause additional challenges to the lay equipment, which is significantly impacted by heavy weight and geometrical or stiffness discontinuities. Nevertheless, for their installation, the occurrence and persistence of mild weather conditions at the right times during the lay campaign are of major concern. The limit sea states defined at the design stage for the pipe-lay dynamic allowance from encountered environmental load effects, superimposed on the static setting, may not be enough during operations. It is sometimes necessary to quantify the accumulated fatigue damage in real time immediately after the sea states are encountered and to anticipate the expected accumulation of fatigue damage in the next few days. This is needed because of the long-lasting deployment of the assembly onto the seabed. The rational and extensive use of weather forecasts, real-time monitoring, and numerical modeling of the structural response in smart combinations is a must. In this paper, a new engineering tool for the real-time prediction of the accumulated fatigue damage, based on actual sea states encountered during operation, is described.