Onboard Screening of Forecast Weather During Installation

Abstract

Pipelines and umbilical are key elements in subsea infrastructure for energy transport. Offshore installation of long pipelines and umbilicals on uneven seabeds in deep waters are critical operations. Uncertainties in weather conditions and currents add to the challenge. During the planning of an installation, many detailed static and dynamic analyses are performed in order to determine the weather conditions under which the operation may take place: the operational weather window. Weather windows may either be established for the whole operation, or different weather windows may be established for different phases of the operation, e.g., depending on water depth, bottom conditions or prevailing current along the route. To allow these weather windows to be established, representative conditions must be chosen for each part of the route. If the weather deteriorates during the operation so that forecasted weather conditions are not within the weather window, the operation must be halted until the weather conditions improve. Considering the day rates for laying barges, the increased costs may be significant. A delay may also be detrimental to the total project. An alternative is to base the decision to proceed or halt on updated analyses at the present location. The correct water depth, soil characteristics, local seafloor geometry and possible free spans in the laid pipeline may thus all be included in the analyses along with the most up-to-date weather and current forecasts. The most correct estimate of the response will then be available as a basis for the decision to continue or halt the operation. Other “what if” scenarios may also be investigated; e.g., allowing the operator to explore the effect of changed operation or deviations from the predicted weather. Estimates of fatigue damage rates may be used to evaluate continued operation in deteriorating conditions. A system for onboard screening of forecast weather condition is presented along with examples from a prototype application. Using knowledge of the system, the extent of non-linear simulations is limited to where they will give the most contribution to the extreme response in the forecasted conditions.