Use of Computational Fluid Dynamics to Investigate the Impact of Cold Spots on Subsea Insulation Performance
Authors: Taxy Stéphane , Lebreton Eric 
Published: October 2008

Hydrate management strategy of most of the latest deepwater developments relies mainly on prevention because of the inherent difficulty to deploy remediation procedures. For subsea developments relying on passive insulation, Christmas trees, manifolds, flowlines and risers are designed with thermal insulation which provides sufficient time to implement on adequate operational procedure (depressurisation / methanol flushing or crude oil displacement with dead oil). The later strategy aims at eliminating the hydrate formation risk in the production network for extended shut-down periods and facilitating production restart procedures. During the commissioning activity on the field, the thermal performance of the production system can be tested and compared to the thermal performance specification through specific procedures and measurements. Components with degraded insulation performance have measurable effects even when they have a limited surface area. When it is feasible, corrective actions can be implemented on the system to improve their insulation performance, the main difficulty being to locate the cold spots with accuracy.

This paper presents the application of Computational Fluid Dynamics (CFD) as a tool to evaluate the impact of cold spots and convection heat transfers on the thermal performance of typical deepwater field of West Africa. The thermal modelling approach during the detailed design, the results of the thermal tests during the commissioning and investigation of potential cold spots through thermal modelling are developed.

Introduction

The typical deepwater field of West Africa is a stand alone developed following a subsea wellhead scheme. The water depth is 1400 m. Production lines are gathered in flowlines and risers.

Flow assurance is of paramount importance for the deepwater fields. Operators are facing the combined challenge of deep water, a large field, and the risks of hydrate, wax, sand control, Barite scale and slugging regime. This paper will concentrate on the thermal management aspect and more specifically on the Computational Fluid Dynamics simulations (CFD) performed to model the insulation assembly of the riser tower.

Hydrate management

The hydrate management strategy of the field relies on a conventional strategy employing passive thermal insulation and inhibition with methanol to operate outside the hydrate formation envelope during steady state and transient operations.

As a reference for system design, a hydrate dissociation value of 20°C was selected. This included uncertainties within the hydrate predictive model and an early estimation of the field operating envelope.

One of the requirements set for the riser thermal insulation performance was as follows:16 hours cool-down time for liquid filled sections of risers from 51°C to 20°C.4 hours cool-down time for gas filled sections of risers from 51°C to 20°C. To fulfil these thermal requirements, sealine and riser bundles have been selected. With this concept, the traditional syntactic material used for buoyancy was also used as thermal insulation.

Full-scale insulation tests

The novelty of the selected insulation concepts required extensive qualification tests. Risks associated with this innovative development were mitigated through material qualification and full scale bench tests of insulated equipment. Findings made through equipment testing provided a better understanding of heat transfer mechanisms in complex geometry, helped to identify performance limits and allowing design modifications within project frame.