Enabling All-Electric Subsea Control Systems without Compromising Safety - A Case Study Comparing Functional Safety Systems Using Springs or Batteries

Abstract

The electrification of subsea control and production systems is essential to achieve the low carbon emission and high productivity targets that the offshore industry is facing now. But how to electrify these controls without compromising safety? This paper provides a comparison of the design strategies using mechanical springs or electric batteries to implement safety functions in control systems, considering as a case study the development of the new subsea electric actuator for small-bore valves SVA R2 from Bosch Rexroth. Traditionally, to control process valves with safety functions, hydraulic actuators have been applied with field-proven springs to bring the valve to a specific position (open or closed, depending on the application) in case of a safety-related failure such as a loss of power supply. However, the first electric subsea actuators developed with fail-safe springs turned out to be too large, heavy, inefficient, and costly due to their complex electro-mechanic system for broad adoption by the subsea industry. Thus, new electric subsea actuators [2] were developed using electric batteries to reduce overall size, but this added new risks: now the safety control has to detect a dangerous situation by itself and actively drive the system to the safe position. But, at the OTC 2021, a new electric subsea actuator for small bore valves was presented which achieves Safety Integrity Level SIL 3 using field-proven springs as compact as a hydraulic actuator (OTC-31083-MS) [17]. Following the functional safety methodology acc. to the IEC 61508, IEC 61511 and ISO 13849, this work provides a comparison between a control architecture with mechanical springs and another with electric batteries, considering the whole product life cycle (design, manufacture, installation, commissioning, and operation). An evaluation of the failure modes and failure mechanisms of existing subsea equipment, from field data available in the OREDA@Cloud, gives a proper assessment of the risks associated with implementing the new functional safety architecture. Costs of engineering, production and operation are taken into account to find out the most technical and economically feasible solution. This case study is a reference for the design and qualification of new subsea electric actuators with functional safety requirements using mechanical springs or electric batteries. It provides technical guidelines and risk assessment for designers and users of subsea control systems such as machine builders, EPCI companies, operators, service providers or certification bodies. If electrification is a prerequisite for a low carbon future, safety is a prerequisite for the adoption of all-electric control systems. This paper shows how to electrify subsea production and process systems by using mechanical springs or electric batteries without compromising safety.

Digital Redundance for Compact Subsea Electro-Hydrostatic Actuators Using Sensor Fusion

“…The focal physical system modeled in this study is an electro-hydrostatic Subsea Valve Actuator (SVA) depicted in Figure 1. The SVA is engineered for the operation of rotary small-bore valves at water depths reaching up to 4,000 meters, designed concisely to match the size and weight of traditional subsea hydraulic actuators [7]. It not only accommodates a standard electric interface but also seamlessly integrates essential components, including electric drives and controls.…”

“…The Subsea Valve Actuator (SVA) illustrated in Figure 1 and detailed in Figure 2 is designed to operate rotary small-bore valves at depths of up to 4,000 meters [7]. The application software, housing the Digital Twin, is tailored to meet various normative, proprietary, and customer requirements, enabling the actuator to perform multiple functions.…”

“…This shift offers multiple advantages including reduced installation (CAPEX) and operation costs (OPEX), quicker system response, increased energy efficiency, reduced umbilical cable diameter (due to the elimination of hydraulic lines), enhanced operational flexibility, and environmentally sustainable design [1] [2]. Electro-hydrostatic actuators (EHAs) are integral to this approach, amalgamating the benefits listed above with those of traditional electro-hydraulic systems, such as compactness, robustness, high power density, high load capacity, and effective overload protection with fail-safe functions performed by springs [3] [4].…”

“…A significant challenge in implementing EHAsself-contained control systems with numerous componentsis the requirement to accommodate these within the space that is traditionally occupied by subsea hydraulic actuators. Figure 1 illustrates the various modules comprising a rotary electrohydrostatic subsea valve actuator, designed to fit within the dimensions of a conventional hydraulic cylinder [4]. The limited space in these intricate systems restricts the inclusion of redundant components, calling for alternative approaches to improve system reliability.…”

“…The limited space in these intricate systems restricts the inclusion of redundant components, calling for alternative approaches to improve system reliability. Orth et al [7] and Placido Neto et al [8] highlight a significant challenge in applying EHA technology to replace hydraulic Subsea Valve Actuators (SVAs) where the new designs must fit into the space occupied by conventional units. This spatial limitation can restrict system redundancies, necessitating alternative approaches to bolster reliability.…”