Crude Oil or Product Tanker Calculations

Survey Documentation -  Determining Fullness of Pipelines between vessels and shore tanks

Introduction

The purpose of this section is to recommend procedures for determining the fill condition of pipeline systems used for the transfer of liquid before and after the liquid is loaded onto or discharged from marine vessels.

Scope

This section is intended to cover crude oil and petroleum products that are liquid at atmospheric pressure and ambient temperature.  They apply at both loadings and discharges of cargos, before and after transfers.  They include recommendations on how pipeline fullness should be determined.  They include descriptions of methods, procedures, calculations, and examples. 

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While this section is intended to describe the methods and procedures available for determining the fill condition of pipeline systems, the procedures described do not necessarily determine the precise volume of liquids in the designated pipeline systems. 

This section is not meant to recommend equipment or to describe equipment in detail.  The guideline is only intended to describe the expected end result of using properly installed and functioning equipment. While this section includes descriptions of common line fill verification methods, it does not recommend anyone method as preferred.  The responsibility for documenting the effectiveness of any of the methods described belongs to those responsible for the terminal or installation where it is applied.

Significance and Use

When custody of petroleum liquids is transferred to or from marine vessels, accuracy in the measurement of quantities transferred is affected by the contents and integrity of the shore and vessel pipelines and by shore and vessel tank measurements.  Pipelines that contain air or vapour, when assumed to be filled with liquid, require procedures that either eliminate or determine the volume of air or vapour contained in the pipeline.  This can be accomplished by one of the following means:

1. Displacing the air or vapour with liquid.

2. Displacing the entire line contents

3. Quantifying the volume of any air or vapour contained in the line with the liquid.

The following methods, listed alphabetically, are recommended for line fill determination.  Local limitations, product characteristics, and specific designs may restrict the best selection and use of a procedure and may affect the accuracy of the procedure selected.

1. High-point bleed-valve method (or sight-glass method).

2. Internal circulation method.

3. Line displacement method.

4. Line press method (or line pack method)

5. Pigging method.

The high-point bleed-valve method (or sight-glass method), the internal circulation method, and the line displacement method are all intended to fill the designated pipeline system with liquid.  The pigging method is intended to entirely displace the contents of the designated pipeline system.  The line press method (or line pack method) is intended to define the fill condition of the designated pipeline system.

Only the line displacement method requires the presence of the vessel.  All other methods provide an opportunity for any correction required without affecting the vessel's transferred volume.

All the procedures described and recommended here shall be performed in accordance with the following:

1. The safety requirements specified in the following:

   1. The International Safety Guide for Oil Tankers and Terminals (lSGOTT).

   2. API Recommended Practice 2003.

   3. The Inert Flue Gas Safety Guide.

2. Applicable state and federal regulations.

3. Other specific requirements made by owners, operators and state and federal authorities.

Safety and Health Considerations

Due consideration should be given to applicable safety and health procedures.  Considerations should include but are not limited to potential electrostatic and other fire and explosion hazards; considerations regarding potential personnel exposure (such as exposure limits, hazard communication, training, and associated protective clothing and equipment requirements, and work practices); and potential explosive and toxic hazards associated with a cargo tank's atmosphere.  The physical characteristics of the cargo and existing operational conditions should be evaluated, and applicable international, federal, state, and local regulations should be observed.  Safety procedures designated by the employer, the vessel operator, and other concerned parties should also be observed.  The International Safety Guide/or Oil Tankers and Terminals and appropriate Oil Companies International Marine Forum (OCIMF) and API publications should be consulted for additional safety information.

CAUTIONARY NOTE: Petroleum vapours and associated substances, including hydrogen sulphide vapours from "sour" crude, also may involve potential toxicity.  Petroleum vapours with high concentrations of hydrogen sulphide may cause unconsciousness or death.  During and after the opening of the gauge hatch, stand so that vapour inhalation is minimized.

CAUTIONARY NOTE: Harmful vapours or oxygen deficiency cannot be detected safely by smell, visual inspection, or judgement.  Appropriate precautions should be used for protection against toxic vapours or oxygen deficiency. Procedures should be developed to provide for appropriate exposure monitoring, personal protective equipment, and emergency rescue precautions.  When it is necessary, personnel should have suitable respiratory protection before entering the gauge site and during the gauging procedure.  

Sections of text taken from API - MPMS Chapter 17 Marine Measurement