Oil and gas

WaDeck JIP

Airgap and Wave in Deck Loads for Semi-submersibles

Airgap and Wave in Deck Loads for Semi-submersibles

The possibility of negative airgap and subsequent wave in deck loading on semi-submersibles in storm conditions has attracted significant attention during the last year. Many regulators have expressed concerns and raised questions with regards to airgap assessment and potential wave in deck. Whereas the clearance between the still water and the underside of the deck is an expensive design parameter, the consequences of wave impact with the deck can be severe. Accurate assessment of the airgap and the associated deck loading under wave slamming are difficult and associated with significant uncertainty.
In order to help the industry ensure safety / consistency and address concerns from the regulatory bodies, in September 2016, DNV GL issued two Offshore Technical Guidance documents (OTG 13 and OTG 14) to supplement the existing Offshore Standards and Recommended Practices in the analysis of airgap and possible wave in deck loading. Provisional instructions have been issued by DNV GL, PSA and NMD based on these OTGs to ensure safe operation of MOUs in harsh environment for the coming winter season.

However due to limited time and data, the OTG documents still have room for further improvement, e.g.:
•    Further study and calibration based on different semi-submersible design configurations to establish more accurate coefficients/formulae (instead of enveloping all semi-submersibles in one simple format)
•    Provide more simplified and pragmatic design approach, e.g. simplified pseudo static load levels which can be used to evaluate structural integrity instead of peak loads and time evolution of pressures
•    Provide guidance on assessing overall integrity due to negative airgap (e.g. stability, global structural integrity, riser integrity, etc.) in addition to local impact loads
•    Consider metocean conditions in different geographic regions, including tropical storm regions
•    Consider differences between drilling and production semi-submersibles

BENEFITS TO PARTICIPANTS
•    Achieve an industry practice that accounts for interests of all stakeholders
•    Establish basis to address concerns of regulators
•    Further calibrate factors, parameters based on experiences, test results, etc. to improve accuracy and cost efficiency
•    Achieve pragmatic design methodology
•    Gain insight to the technical background of rules and guidelines

SCOPE OF WORK
The main objectives of the JIP are to improve and document the guidance presently included in OTG 13 and OTG 14 based on the areas for improvement as described above. Depending on the number of participants and feedback from the participants, it is possible that the JIP may be divided in two phases. It is proposed that the 1st phase of the JIP will carry out five distinct work packages with detailed scope dependent on the secured budget:

bullets - 2 - pic




DELIVERABLES
Reports detailing the findings from each work package and a separate report outlining the main conclusions and recommendations.

SCHEDULE
It is intended that the JIP will start during the first quarter of 2017 and have a duration of one year.

PARTICIPATION FEE
Proposed participation fee: 
Oil companies:
75 k€ payable over two financial years
Others:
35 k€ payable over two financial years
 
The JIP will commence when there is firm commitment from participants corresponding to a sum of 350 k€ in participation fees.

The possibility of negative airgap and subsequent wave in deck loading on semi-submersibles in storm conditions has attracted significant attention during the last year. Many regulators have expressed concerns and raised questions with regards to airgap assessment and potential wave in deck. Whereas the clearance between the still water and the underside of the deck is an expensive design parameter, the consequences of wave impact with the deck can be severe. Accurate assessment of the airgap and the associated deck loading under wave slamming are difficult and associated with significant uncertainty.
In order to help the industry ensure safety / consistency and address concerns from the regulatory bodies, in September 2016, DNV GL issued two Offshore Technical Guidance documents (OTG 13 and OTG 14) to supplement the existing Offshore Standards and Recommended Practices in the analysis of airgap and possible wave in deck loading. Provisional instructions have been issued by DNV GL, PSA and NMD based on these OTGs to ensure safe operation of MOUs in harsh environment for the coming winter season.

However due to limited time and data, the OTG documents still have room for further improvement, e.g.:
•    Further study and calibration based on different semi-submersible design configurations to establish more accurate coefficients/formulae (instead of enveloping all semi-submersibles in one simple format)
•    Provide more simplified and pragmatic design approach, e.g. simplified pseudo static load levels which can be used to evaluate structural integrity instead of peak loads and time evolution of pressures
•    Provide guidance on assessing overall integrity due to negative airgap (e.g. stability, global structural integrity, riser integrity, etc.) in addition to local impact loads
•    Consider metocean conditions in different geographic regions, including tropical storm regions
•    Consider differences between drilling and production semi-submersibles

BENEFITS TO PARTICIPANTS
•    Achieve an industry practice that accounts for interests of all stakeholders
•    Establish basis to address concerns of regulators
•    Further calibrate factors, parameters based on experiences, test results, etc. to improve accuracy and cost efficiency
•    Achieve pragmatic design methodology
•    Gain insight to the technical background of rules and guidelines

SCOPE OF WORK
The main objectives of the JIP are to improve and document the guidance presently included in OTG 13 and OTG 14 based on the areas for improvement as described above. Depending on the number of participants and feedback from the participants, it is possible that the JIP may be divided in two phases. It is proposed that the 1st phase of the JIP will carry out five distinct work packages with detailed scope dependent on the secured budget:

bullets - 2 - pic




DELIVERABLES
Reports detailing the findings from each work package and a separate report outlining the main conclusions and recommendations.

SCHEDULE
It is intended that the JIP will start during the first quarter of 2017 and have a duration of one year.

PARTICIPATION FEE
Proposed participation fee: 
Oil companies:
75 k€ payable over two financial years
Others:
35 k€ payable over two financial years
 
The JIP will commence when there is firm commitment from participants corresponding to a sum of 350 k€ in participation fees.