On the rapid discharge of subsea accumulators: remarks on the normed design method and proposal of improvement

Abstract

This study is focused on the design of the subsea accumulators currently used to deliver a pressurized fluid whose function is to actuate safety operations on a subsea well. API 16D, whose specifications regulate the design method of a bank of accumulators, was revised, discussed, and resumed in terms of a nomogram whose engineering value is useful for designers. An additional nomogram was derived allowing to perform the verification process which is suggested by the norm, in order to verify the fulfillment of the actuations requested to the accumulators. This verification phase caught the interest of the Authors: API 16D assumes for the design that the Functional Volume Requirement (FVR)the sum of the quantities of pressurized fluid to be delivered for different sequenced-in-time functions -is delivered at the Minimum Operating Pressure (MOP) requested by the last actuation. This hypothesis simplifies the design, but the result cannot fulfill the previous functions because of the pressure lack (energy content in the pressurized propelling gas) which can happen. The use of the two nomograms proposed simplifies the application of the design procedure and allow to discuss main sensitivities of the variables involved (FVRs & MOPs), evidencing which variables deserve more attention and accuracy. In particular, the use of the second nomogram permits to verify that the right volumes of fluid during the actuations are delivered above a minimum pressure level, so guaranteeing the fulfillment of the function. It is based on a thermofluid-dynamic discharge modeling of the accumulators. The paper outlines a design direction which minimizes the number of accumulators and opens the way to a different design approach, based on a fully physical discharge process description. 240

An Integrated FTA-FMEA Model for Risk Analysis of Engineering Systems: A Case Study of Subsea Blowout Preventers

“…Future research can be directed towards the effects of double and multiple failures, the comparative difference obtained when different importance measures are used, and linking the software interphases of the proposed integration scheme. Also, the normed design of subsea accumulators for actuation and control of BOPs may sometimes not be safe (see reference [35]). Thus, consideration of the actuation of the BOP and its design in reliability analysis can be an interesting subject for research.…”

Transient mathematical prediction and experimental verification of the charging characteristics of subsea accumulators

“…The low energy storage density, minimum operating pressure (MOP), and functional volume requirements (FRV) of subsea accumulators have been studied by some scholars. Roberto et al [7][8] studied the characteristics of the rapid discharge of subsea accumulators, and improved design method and theoretical model are presented based on the FRV and MOP of each actuation. The optimum precharge pressure estimated by the modified method is higher than the one estimated by the API 16D method.…”