ISO 19905-3:2021 download free.Petroleum and natural gas industries — Site-specific assessment of mobile offshore units — Part 3: Floating units.
The International Standards on offshore structures prepared by TC 67 (i.e. ISO 19900. the ISO 19901 series, ISO 19902, ISO 19903, ISO 19904-1, the ISO 19905 series, ISO 19906) constitute a common basis addressing design requirements and assessments of all types of offshore structures used by the petroleum and natural gas industries worldwide.
NOTE These are sometimes referred to as the ISO 19900 series on offshore structures.
Through their application, the intention is to achieve adequate structural integrity and performance based on reliability levels appropriate for manned and unmanned offshore structures, whatever the nature or combination of the materials used.
Structural integrity is an overall concept comprising: models for describing actions; structural analyses; design or assessment rules; safety elements; workmanship; quality management; and national requirements, all of which are mutually dependent. The modification of any of these elements in isolation can cause an imbalance or inconsistency, with possible impact on the reliability inherent in the offshore structure. The implications involved in modifying one element, therefore, are considered in relation to all the elements and the overall reliability of the offshore structure.
The International Standards on offshore structures prepared by TC 67 are intended to provide latitude in the choice of structural configurations, materials and techniques and to allow for innovative solutions. Sound engineering judgement is, therefore, necessary in the use of these documents.
ISO 19905-3 states the general principles and basic requirements for the site-specific assessment of mobile floating units. The technical information used in the assessment primarily resides in documents referenced herein. ISO 19905-3 is intended to be used for assessment and not for design.
Site-specific assessment is normally carried out when an existing mobile floating unit is to be installed at a specific site. The assessment is not intended to provide a full evaluation of the unit; it is assumed that aspects not addressed herein have been addressed at the design stage using other practices and standards.
The purpose of the site-specific assessment is to demonstrate the adequacy of the mobile floating unit, its stationkeeping system and any connected systems for the applicable assessment situations and defined limit states, taking into account the consequences of failure. The results of a site-specific assessment should be appropriately recorded and communicated to those persons required to know or act on the conclusions and recommendations. Alternative approaches to the site-specific assessment can be used provided that they have been shown to give a level of reliability equivalent, or superior, to that implicit in this document.
1 Scope
ISO 19905-3 specifies requirements and recommendations for the site-specific assessment of mobile floating units for use in the petroleum and natural gas industries. It addresses the installed phase, at a specific site, of manned non-evacuated, manned evacuated and unmanned mobile floating units.
ISO 19905-3 addresses mobile floating units that are monohull (e.g. ship-shaped vessels or barges); column-stabilized, commonly referred to as semi-submersibles; or other hull forms (e.g. cylindrical? conical shaped). It is not applicable to tension leg platforms. Stationkeeping can he provided by a mooring system, a thruster assisted mooring system, or dynamic positioning. The function of the unit can be broad, including drilling, floatel, tender assist, etc. In situations where hydrocarbons are being produced, there can be additional requirements.
ISO 19905-3 does not address all site considerations, and certain specific locations can require additional assessment.
ISO 19905-3 is applicable only to mobile floating units that are structurally sound and adequately maintained, which is normally demonstrated through holding a valid RCS classification certificate.
ISO 19905-3 does not address design, transportation to and from site, or installation and removal from site.
ISO 19905-3 sets out the requirements for site-specific assessments, but generally relies on other documents to supply the details of how the assessments are to be undertaken, In general:
— ISO 19901-7 is referenced for the assessment of the stationkeeping system;
— ISO 19904-1 is referenced to determine the effects of the metocean actions on the unit;
— ISO 19906 Is referenced for arctic and cold regions;
— the hull structure and air gap are assessed by use of a comparison between the site-specific metocean conditions and its design conditions, as set out in the RCS approved operations manual;
— ISO 13624-1 and ISO/TR 13624.2111 are referenced for the assessment of the marine drilling riser of mobile floating drilling units. Equivalent alternative methodologies can be used;
— I MCA M 220 is referenced for developing an activity specific operating guidelines. Agreed alternative methodologies can be used.
8.2.2 Monohull
ISO 19905-3 contains no specific requirements for freeboard or the explicit assessment of the structure and permanent equipment on a monohull unit for the effects of green water lithe unit is classed by its RCS for unrestricted ocean service, lithe unit is classed for restricted ocean service, the hull and deck equipment shall be assessed to demonstrate that there will be no significant green water damage in wave conditions of up to and including the ULS extreme storm (see £4).
DP monohull units shall be assessed to the same criteria, unless it can be shown, through a risk-based approach, that reduced metocean conditions are suitable (e.g. through moving off location). Plans to move DP monohull units off location shall be documented. It shall be demonstrated that the unit can be relocated safely within the constraints of the alert system. In tropical revolving storm areas, the reduced storm should be no less severe than the ULS sudden hurricane.
8.2.3 SemI-submersible
ISO 19905-3 contains no specific requirements that the air gap ci a semi-submersible unit be explicitly assessed lithe unit
— is classed by its RCS for wave conditions equal to or exceeding the ULS extreme storm at the site, and
— has been reviewed following the methodology set out in the latest rules and guidelines of the applicable RCS for metocean conditions up to the ULS extreme storm at the site. Roth the survival and operating conditions shall be assessed.
NOTE Air gap can be affected by the wave steepness, vessel motions, draft, trim, stability, and other factors.
If this condition is not fulfilled, the adequacy of the air gap shall be assessed following the methodology set out in the latest rules and guidelines clan applicable RCS for metocean conditions up to the following:
a. the ULS extreme storm at the site in the survival condition;
b. the metocean limits for the operating conditions.
If negative air gap exists, the relevant structure and permanent equipnwnt shall be assessed using appropriate hydrostatic and hydrodynamic loading. Alternatively, a risk-based approach may be used, as agreed between unit owner and operator, to demonstrate that no waves conditions up to and including that of the ULS extreme storm can have significant consequences.
A management of change procedure shall be developed.
An example of implementation of such a procedure includes the temporary operation of other adjacent vessels that are not specifically identified within the ASOG (e.g. SIMOPS, diver support operations, pipelay, etc.).
NOTE 2 It is possible that a mobile floating unit can be in conformity with the requirements set out in this document, but is not suitable for the specific activities that it is Intended to carry out (e.g. have insufficient up-time).
NOTE 3 Iso 13624-1, supplemented with additional information for deep water operations from ISO/TR 13624.2,111 gives valuable information on developing the limits of service for risers and the methods for developing the inputs to an activity specific operating guideline document, While the documents are written for marine drilling risers, they contain useful information and techniques that can be used to assess and develop the required documents for other systems and activities (e.g. pipelay vessels, tender assist drilling vessels, etc.).
NOTE 4 InstallatIon of integrated real-time mooring line tension and weather monitoring systems can help develop better understanding and calibrate performance of the unit against mooring analysis predictions and offsets (stationkeeping).
11 Confirmation of compatibility between analysis and as-installed condition
Data on the as-installed configuration shall be collected and compared to the site-specific assessment configuration to ensure that any differences do not materially change the site-specific assessment conclusions.
In the case of significant differences between the assessed and as-installed conditions, the site-specific assessment and ASOG shall be updated accordingly. Alternatively, the operating limits shall be reduced to a level consistent with safe operation of the unit and its equipment, and a suitable ASOG developed. This should be carried out before activity specific operations are commenced.
If necessary, and where practical, the location of the anchors, including embedment, should be verified after installation, and tensioning records maintained. The mooring test load should satisfy
ISO 19901-7:2013, 10.4.6.