STRUCTURAL INTEGRITY MANAGEMENT OF FIXED OFFSHORE STRUCTURES

Introduction

API RP 2SIM provides guidance on SIM of existing fixed offshore structures used for the drilling, development, production, and storage of hydrocarbons in offshore areas. However, the general principles of SIM apply to any structure.

1. SIM is the process for demonstrating a structure’s fitness-for-purpose over its entire life, and managing the effects of deterioration, damage, changes in loading and accidental overloading.
2. The SIM process consists of four elements: data, evaluation, strategy, and program.
3. The SIM process provides the opportunity for owners/operators to adopt risk principles for developing SIM strategies.
4. A risk-based approach recognizes that higher risk platforms may warrant more frequent, and more focused, inspection than lower risk platforms.
5. During the development of an inspection strategy, the platform risk category may be used for setting survey intervals and work scopes as part of a risk-based SIM strategy.
6. It is important to note that surveys alone do not guarantee structural integrity.

Definitions

 

Anomaly

An in-service survey measurement, which is outside the threshold considered acceptable from the design or most recent fitness-for-purpose assessment.

Assessment Initiators

Changes in platform condition or operating experience, such as storms, which require an existing platform to undergo an assessment to demonstrate fitness-for-purpose.

Collapse

The ultimate load bearing capacity of the platform, at which the jacket structure or deck columns are no longer able to support vertical loads.

Condition Assessment

The process of gathering the information on the platform's present condition needed in order to perform a fitness-for-purpose assessment.

Consequence

The adverse effects of an extreme event, such as metocean, seismic, ice or accidental, on personnel, the environment, or property.

Damage Tolerance

The quantity of deterioration or damage that a structure can withstand without failing.

Design Level Analysis

A fitness-for-purpose analysis of a platform using linear-elastic methods with an appropriate safety margin, similar to the analysis methods used for new platform designs.

Design Life

The planned time period from initial installation or reuse until permanent decommissioning, which may include extensions justified through the SIM process.

Deterioration

The reduction in the ability of a component to provide its intended purpose.

Exposure Category

The classification used to categorize the platform consequence of failure based on the consideration of life safety, environmental pollution, and business disruption.

Inspection

The visit to the platform for purposes of collecting data required in evaluating its structural integrity for continued operation.

Survey

A specific visual or non-destructive examination of one or more platform components.

Fitness for Purpose

A demonstration that an existing structure has adequate strength to resist the imposed assessment loads

Mechanical Damage

A defect type that includes dents, bows, gouges, holes and separated or severed members.

Mitigations

Platform strengthening, modification or repairs and/or operational procedures that reduce loads, increase capacities, or reduce the exposure category.

DATA

Up-to-date platform information is required for the SIM process.

Information on the original design, fabrication and installation, structural analyses, in-service inspections, engineering evaluations, structural assessments, modifications, strengthening, repairs, and operational incidents, all constitute parts of the SIM knowledge base.

SIM data falls into two broad categories: platform characteristic data, and platform condition data.

Characteristic Data

The platform’s characteristic data is the baseline data that represents the structure at installation.

The characteristic data includes :

1. General Platform Data
2. Design Data
3. Fabrication data
4. Installation data

Condition Data

The platform condition data represents the changes to the characteristic data that may occur during the life of the platform.

The condition data includes the following :

1. in-service inspection data;
2. damage evaluation data;
3. corrosion protection data;
4. strengthening/modification/repair (SMR) data;
5. platform modifications;
6. condition monitoring data;
7. operational incident data

EVALUATION

SIM evaluation is the method of applying engineering to assess the impact that new data has on the fitness-for-purpose and the SIM strategy for the platform.

Evaluation is routinely performed throughout the life of a platform. As additional data is collected, an evaluation should be performed by a competent structural engineer.

The evaluation should consider all relevant SIM data for the platform and similar platforms, where appropriate.

 

Recommendations for the performing data evaluation are provided in the following sections :

1. Factors to consider
2. The risk of platform failure
3. The platform exposure category
4. The likelihood of platform failure
5. The requirement for platform assessment

In many instances much of this data will not be available; however missing data may impact the evaluation, strategy and program for the on-going SIM of the platform. Where characteristic data are not available, or are inaccurate, surveys of the structure and facilities should be considered to collect the necessary information.

A fitness-for-purpose assessment of the structure shall be performed if the engineering evaluation of relevant SIM data determines that an assessment initiator, has been triggered.

Damage Evaluation Process

The objective of the damage evaluation process is to determine whether damage is potentially significant to the structural integrity of the platform using the fitness-for-purpose assessment.

Determining the effects of damage on the strength of individual components, and used the results to examine the need for risk reduction.

It is important to recognize that not all damage is structurally significant. Light corrosion or minor bowing of a member are examples of damage that may not be structurally significant to the affected components.

A robust platform may be fit-for-purpose throughout its remaining life, even if one or more of its structural components (members and/or joints) have structural damage.

 

Degradation Mechanisms that may reduce the structural capacity of the platform :

1. Dropped objects, result in mechanical impact damage to structural components and can cause gouging, denting, bowing, holing of members, and also cause cracks at the end connections of the member and/or member severance and/or joint deformation or tearing. Examples: tubular components (e.g. drill strings and piles), drill collars, scaffold poles, and from activities at link bridges, cranes, drilling derricks, etc.
2. Vessel Collision, result in damage to structural components in the upper part of the jacket around the splash zone: gouging, denting, and bowing of members. Can be prevented by protective structures such as boat fenders, protection frames, etc.

 

Corrosion :

1. Uniform corrosion of the underwater structural components may result from a failure of the corrosion protection system or through premature depletion of the system due to excessive debris in contact with the structure. In the splash zone and above-water locations of the structure, similar corrosion damage can result from failure of the protective coating systems.
2. Localized corrosion may develop at areas on the structure not adequately protected by the CP system, for example, dense conductor arrays or appurtenance connections, in particular inside conductor guides and bolted clamps. Localized corrosion may also develop due to galvanic (bimetallic) corrosion, for example, caissons housing stainless steel pump/strainer components.

Strategy

A significant commitment to ongoing in-service inspection with the goal of reducing the possibility of major repairs (clamps, member replacements) in the future. This approach relies on early detection of damage and defects with prompt implementation of relatively inexpensive repairs and preventive measures. Early detection of defects typically requires greater use of nondestructive testing (NDT) techniques.

Minimization of in-service inspection scope where adequate measures have been taken to reduce the risk of damage, defects, or deterioration that would require major repair efforts in the future. This approach assumes that in-service inspection without the use of NDT techniques will be able to detect damage, defects, or deterioration before structural integrity is threatened. This approach may be appropriate for robust structures that are tolerant to damage and overload.

Inspection Strategy :

1. Above-water inspections,
2. Post-installation baseline below-water inspections,
3. Routine below-water risk-based, or exposure-based inspections
4. For non-routine special inspections.