Dynamic Nodal Analysis

Introduction

Engineering teams face numerous challenges during fields turnaround and increasing capacity.

The promotion of integration, communication between disciplines is very instrumental to adequately assess the performance of process facilities under normal and abnormal operation, and more importantly is to preclude the risk of un-necessary cost.

The traditional approach is basically assessing and modelling every domain separately, and the outputs from reservoir needs to be inputted to the domain of flowline and process facilities.

This workflow has mainly resulted in a key problem- process simulator represent only one point in time which in return fail to update the process modelling if reservoir performance changed.

Due to the increase of awareness and maturation level in engineering firm, the concept of IAM (Integrated Asset Modeling) has started to dominate the upstream industry and flow assurance assessment such as PIPESIM & MAXIMUS application.

Be that as it may, the two-latter software are limited in process equipment and transient analysis. This article primarily focuses on the utilisation of HYSYS Dynamics along with VBA to build the reservoir model and its Inflow Performance Relationship and rigorously predict the production of the gas, oil and water in response to bottom hole pressure change due to abnormal event may occur at surface facilities.

Basics

IPR: The inflow performance relationship or IPR is defined as the functional relationship between the production rate and the bottom hole flowing pressure.

OPR: The components of the producing system that are downstream of the selected node are included in the outflow sections. The main components of an outflow section are:

• Production Tubing.
• Wellhead.
• Choke.
• Flowline.
• Separator.

A comprehensive understanding of all these components is required when constructing the outflow performance curves

Technical Discussion

Steady State Approach: Based on the steady state HMB, the analysis has considered the total flow from production manifold with no consideration of the actual reservoir performance using HEM method.

HEM method under steady state approach has reported an orifice area requirement of 119.63 cm² which is largely more than existing orifice area. To satisfy the requirements, the system should employ extra two (2) T orifice in addition to the existing T orifice PSV.

Dynamic Nodal Analysis Approach: The relation between bottom hole pressure and reservoir production has been considered in the dynamics to assess the adequacy of existing T orifice PSV against total blocked outlet case. It was found that the transient pressure rises has exceed the allowable over pressure of 15.18 barg and reached 20.4 barg.

A sensitivity cases has been performed considering various orifice sizes for additional PSV, to determine the adequate size that avoid an overpressure case. The results of the sensitivity case per tested orifice were plotted in following figure to demonstrate the transient pressure rise.

It was evident from above results that the proposed Q orifice is not adequate where the transient pressure has exceeded the MAWP of the vessel, while R orifice is deemed adequate

Given the mathematical link between the surface facilities behaviour and the IPR (Inflow Performance Relationship) of reservoir, the blocked outlet case had a detrimental impact on the production profile where the production rates have decreased.

Relieving Load Modelling

The relief loads from existing PSV (T orifice), and the proposed PSV (R orifice) are figured below.

The blue dotted area demonstrates an overshoot in gas and oil relieving flow. This transient behaviour is attributed to the rises in water level during nearly eight (8) minutes.

During the rises of water, the accumulated volume inside three phase separator which contains oil and gas is continuously packed due to the expansive push from rising water.

At the time the gas pressure reaches the set point, the rate of relieve is proportionally depending on driving force (∆P=relieving pressure-flare back pressure), and in inverse proportion with resistance (i.e. orifice area). In principal, the following rule governing the case.

Rate α (Driving Force/Resistance). 

When water level hits the outlet gas nozzle, the system will therefore reach the settle out conditions. At the settle out condition, the corresponding relieving loads from existing T orifice PSV + R orifice PSV are as follows:

• Q_gas =61.1 MMSCFD
• Q_oil = 25,065 bbl/d
• Q_w = 80,471 bbl/d

Additional Challenges

Although the dynamic nodal analysis has determined the new orifice size of R, notwithstanding; the field operation team were quite adamant to the similarity where the new PSV should be the same size of existing one to ease up the maintenance, calibration and improve the redundancy in the system.

The dynamic nodal analysis has re-examined the system using T orifice, the following figure depicts the relieving load

It was observed the relieving load from the T orifice is much more and the existing FKOD will not handle the load. The following table presents a comparative analysis between the recommended outcomes from Dynamic Nodal Analysis and operators’ preferences

	Existing T Orifice + New R Orifice	Existing T Orifice + New T Orifice
	Dynamic Nodal Analysis outcomes	Operators preference
Gas Load	61.1 MMSCFD	380 MMSCFD
Oil Load	25,065 bbl/d	938,631 bbl/d
Water Load	80,471 bbl/d	173,490 bbl/d

Conclusions and Recommendations

Steady State Approach

Dynamic Nodal Analysis Approach

Steady state approach has concluded inadequacy of existing T orifice and recommended to install two additional PSV with T orifice. The inlet piping size was found adequate and within the criteria of 3% of PSV set point. The check of momentum (ρν²) and Mach number across the main flare header was found within the design limit. Acoustic induced vibration analysis was established based on steady state and the change in velocity across the PSV was found reasonable and within the acceptable range of LOF as per EI guideline.

Dynamic nodal analysis has concluded an inadequacy of the existing PSV but recommended to install one PSV with R orifice. Maximum gas rate overshoot under dynamic nodal is 200 MMSCFD, whereas having three PSVs (recommendation) have resulted in 380 MMSCFD. Dynamic nodal analysis has examined the overshoots of gas, oil & water under three (3) T orifice and was found that the system is acoustically induced through vibration. Thus, the overconservative outcomes obtained from steady state approach will impinge on the integrity of the system and potential piping failure through vibration. It is highly recommended to engage the dynamic analysis for the brownfield facilities to ensure save operation, minimum CAPEX and maintain high integrity during field life cycle. Advantage of introducing Dynamic Nodal Analysis has positively reflected on the CAPEX saving 66%.