aip for kvaerner deepwater dry tree semi approval in ...€¦ · 1.2 introduction of kvaerner...

AIP FOR KVAERNER DEEPWATER DRY TREE SEMI

Approval in Principle of

Kvaerner Deepwater Dry Tree

Semi RPSEA

Report No.: 15U5O2I-10, Rev. 2

Document No.: 15U5O2I-10

Date: 2014-12-30

DNV GL – Report No. 15U5O2I-10, Rev. 2 – www.dnvgl.com Page ii

Table of Contents

ABBREVATIONS ........................................................................................................................... 1

CONCLUSIVE SUMMARY ................................................................................................................ 2

1 INTRODUCTION .............................................................................................................. 3

1.1 Approval in Principle Process 3

1.2 Introduction of Kvaerner Deepwater Dry Tree Semi Design Concept 6

1.3 Applicable Rules, Regulations and Standards 8

2 INTRODUCTION .............................................................................................................. 9

3 CONCLUSIONS AND WAY FORWARD ............................................................................... 11 Appendix A Drawing List Appendix B Conceptual HAZID Report Appendix C Comments and Responses

DNV GL – Report No. 15U5O2I-10, Rev. 2 – www.dnvgl.com Page 1

ABBREVATIONS

AiP: Approval in Principle

API: American Petroleum Institute

DNV GL: Legacy Det Norske Veritas and legacy Germanischer Lloyds have been merged as of September

12th, 2013, DNVGL is the new brand. Legal entity remains the same. This project contact was signed in

October, 2012 between Det Norske Veritas USA (Inc.) and RPSEA.

DWDTS: Deepwater Dry Tree Semi

HAZID: Hazard Identification

HOE: Houston Offshore Engineering

KFD: Kvaerner Field Development

RPSEA: Research Partnership to Secure Energy for America

SIMOP: Simultaneous Operation of Drilling and Production

TLP: Tension Leg Platform

VIM: Vortex Induced Motion


CONCLUSIVE SUMMARY

DNV GL has been requested by RPSEA to perform an “Approval in Principle”, AiP, for Kvaerner Deepwater

Dry Tree Semi concept design.

The AiP review has been based upon the assessment of the overall feasibility of the concept, detailed

calculations, and drawings that will be verified at the later design stages. With all identified issues being

addressed and improvements as recommended being carried out; the risks identified for the concept can be

reduced to an acceptable level that will enable the concept feasibility.


1 INTRODUCTION

1.1 Approval in Principle Process

DNV GL has been requested by RPSEA to perform an “Approval in Principle”, AiP, of the design of Kvaerner

Deepwater Dry Tree Semi concept.

Approval in Principle (AiP) is a service offered by DNV GL to carry out an independent assessment of a

concept within an agreed requirement framework. The aim of AiP assessment is to confirm that the design is

feasible and that there are no insurmountable obstacles (“showstoppers”) that would prevent the concept

being realized.

AiP is typically carried out at an early stage of a project to confirm its feasibility towards the project team,

company management, external investors or future regulators.

It should be noted that the Approval in Principle statement does not constitute classification of the design to

DNV GL Rules for Classification. AiP will typically identify a number of areas that will need to be addressed

during detail design in order to prepare the design for final Classification Approval.

Figure below shows the methodology of the Approval in Principle process.

Figure 1-1 Approval in Principle process

1.1.1 Scope

In order to evaluate the dry tree semi concept design with respect to AiP, design documentation related to

the following disciplines has been reviewed:


• Global performance

• Mooring

• Riser tensioning assembly (typically not class scope but included in this evaluation due to its

criticality for this concept)

• Hull structural strength

• Stability

• High level system and safety

The objective of this review is to familiarize with the design concept, identify any showstoppers or major

shortcomings with respect to compliance with the defined regulatory framework and the specified design

standards. Some limited independent calculation has been carried out as needed.

1.1.2 Initial Technology Assessment

The concept is broken down into key technology elements and these are assessed with regards to degree of

novelty. The areas identified with degree of novelty were the focus of the AiP.

1.1.3 Conceptual HAZID

In order to systematically assess the novelty of the design, maturity of technology applied and feasibility of

the concept, conceptual HAZID workshops were carried out. First an internal workshop was carried out,

which was attended by DNV GL personnel covering all disciplines. Concerns, novelty, mitigations, etc. for

each element/discipline were thoroughly discussed. A second workshop was arranged to include the lead

engineers of the designer (KFD), where the design philosophy was explained, engineering study performed

was elaborated and all concerns raised by DNV GL’s internal workshop were discussed/clarified. Finally a

workshop attended by all industry subject matter specialists was arranged. . The objectives of the

conceptual HAZID workshops are to:

Identify and describe challenges and critical elements in the design concept

Identify specific risks related to this particular project and how they are addressed in the design

Provide systematic input to the Approval in Principle (AiP) Process

The assessment is qualitative. Results of the workshop are included in Appendix B.


Attendees of the final workshop are listed as follows:

Session Attendees Company

KFD

Concept

Jenny Lu DNV GL

Heather Davis DNV GL

Reza Mostofi DNV GL

Robert Gordon DNV GL

Lihua Wang Statoil

Oddgeir Dalane Statoil

Wei Ma Chevron

Ming-Yao Lee Chevron

Amal C. Phadke Conoco Philips

Gail Baxter Marathon Oil

Bill Head RPSEA

Aifeng Yao Shell

Sam Ryu ExxonMobil

John Murray BP

Petruska, David J BP

Ricky Thethi 2H Offshore

David Garrett Stress Engineering

Jack Zeng Kvaerner

Leiv Wanvik Kvaerner

Wan Wu Kvaerner

Knut Pedersen Kvaerner

John Koos MHD Offshore Group


1.1.4 Review Safety Assessment and Technical Studies

The review of safety assessment and technical studies is also part of the AiP process. Any major hazards or

obstacles to the project identified through the review are to be properly addressed.

1.1.5 Assess Finalized Concept

Some comments/concerns raised in the AiP process require the designers to re-visit certain aspects of the

design and produce some additional engineering studies, evaluations, etc. Some of the comments related to

the detailed design are expected to be addressed at the later design stage.

A statement of Approval in Principle is issued once all pending comments/concerns are properly addressed

and no major show-stoppers are identified.

1.2 Introduction of Kvaerner Deepwater Dry Tree Semi Design

Concept

Table below provides main dimensions of the design. Figures below show general arrangement of this design

concept. Further details of the design basis are found in KFD’s “Dry Tree Semi Conceptual Design Report”,

Doc. No. KFD-RP-ZZZ-0001, Section 4. Lists of document that have been received and reviewed are

presented in Appendix A.

DWDTS Hull and Mooring Configuration

Hull Dimensions

Water depth (ft) 8,000

Draft (ft) 145

Main column c-c distance (ft) 236

Main column width (ft) 72

Column height (ft) 215

Pontoon height (ft) 35

Pontoon width (ft) 67

Load Balance

Topside weight (st) 34,600

Hull weight (with fluid) (st) 46,404

Riser loads (st) 17,740

Mooring loads (st) 4,137

Ballast water (st) 40,906

Total platform displacement (st) 143,787

Mooring

Number of mooring lines 16

Chain size/diameter (in) 6.0

Polyester rope size/diameter (in) 10.75

Table 1-1 Key Design Data


Figure 1-2 General Arrangement


1.3 Applicable Rules, Regulations and Standards

The assessment has been carried out with respect to the requested class notations as follows:

OI Column-stabilized Drilling and Production Unit, POSMOOR

The applicable rules are given in below offshore service specifications and the Design Basis established for

the project:

• DNV-OSS-101 Rules for Classification of Offshore Drilling and Support Units, October 2012 edition

• DNV-OSS-102 Rules for Classification of Floating Production, Storage and Loading Units, October

2012 edition


2 INTRODUCTION

The AiP review has been based upon assessment of the overall feasibility of the concept, focusing on

identified critical elements for this concept. The Approval in Principal is mostly based on document review;

where some limited independent analysis had been performed, when found necessary.

The following engineering disciplines have been evaluated:

• Global performance

• Mooring

• Production riser system

• Floating stability

• Structure

• Marine systems

• Safety systems

• E&I systems

• Production systems

• Geotechnical

• Construction, Transportation and Installation

• Operation

Document review was followed up by extensive discussions and meetings to clarify the questions/concerns

raised by DNV GL. These comments are attached in Appendix C.

The system design is still very preliminary and lacks specific design details. Some important safety design

principles had been discussed but they are similar to what has been applied to existing production units.

Assuming all systems are properly designed following applicable design standards, there should not be any

unique challenge in this concept that will prevent the design from being feasible. Preliminary safety related

comments have been provided to KFD as advice based on review of the general arrangement drawings.

All comments have been discussed and clarified; some additional engineering work was performed by KFD to

address the concerns raised. All critical comments have been resolved. Some comments still remain open.

These are in the nature of documentation or further engineering, not considered as show-stoppers to the

feasibility of the concept.

All critical issues/areas of concerns are registered in the Conceptual HAZID matrix in Appendix B. Such have

been further discussed at various workshops including the final workshop, additional concerns and

comments raised by subject matter specialists were registered and followed up after the workshop.

The ‘Initial risk ranking’ was assessed by DNV GL based on the likelihood of a failure and consequence of the

failure before all the design documents were thoroughly reviewed and clarified. The ‘Final risk ranking’ was

assessed by DNV GL after thoroughly reviewing all design documents and going through

discussions/clarifications/workshops, reviewing additional engineering work that were carried out and

http://exchange.dnv.com/publishing/downloadPDF.asp?url=http://exchange.dnv.com/publishing/codes/docs/2014-10/OSS-101.pdf

http://exchange.dnv.com/publishing/downloadPDF.asp?url=http://exchange.dnv.com/publishing/codes/docs/2014-10/OSS-102.pdf


therefore achieve better understanding of the design concept. The likelihood of failure has been reduced in

some cases; consequence of failure mostly remains the same unless design changes have been made or

mitigations are implemented to lower the consequences. Reference is made to the definition in the

Conceptual HAZID Matrix in Appendix B. As can been seen in the Conceptual HAZID matrix, all high risk

items have been addressed; remaining items are mostly at low risk with a few at medium risk level. Further

actions are suggested for the items identified with medium risks. Other low risk items will still need to

properly follow relevant classification rules, statutory requirements, and other international

standards/practice for design, construction, installation, and operation etc. like any other design. All critical

elements identified for this concept before the beginning of the project have been fully addressed (i.e.

Technology Qualification of the riser tensioner system, VIM, comprehensive engineering work and

documentation, e.g. airgap and structural design). Details are presented in Appendix B.


3 CONCLUSIONS AND WAY FORWARD

As shown in the conceptual HAZID matrix, all critical elements of this design concept have been properly

addressed. There are no remaining issues that will prevent the concept from being feasible. Provided that all

comments/concerns raised will be addressed properly at a later design stage and the design properly follows

recognized design standards, classification rules and statutory requirements, the concept should be feasible

to be further developed into projects.

Most of the items registered in the conceptual HAZID matrix are considered as low risk assuming that proper

design will be carried out following applicable classification rules, statutory requirements and other

international standards.

A couple of items ranked as medium risk require additional attention at project phase:

− Compression in lower part of riser (potentially up to 3000’ of riser under compression during the worst

of 1000-year wave cycle, with 680 kips at the lowest riser joint). KFD provided calculation to

demonstrate there is no buckling of riser joint under such compression; such should be validated with

more advanced analysis at the later design stage. Alternatively, mitigations have been considered. As

such, the compression is not considered to impact the feasibility of the design.

KFD Mitigations:

1. Increase top tension from 1.3 to 1.5. Such can be done for storm condition only - to be included in

operational procedure. However riser strength, tensioner capacity, and payload capacity of the unit

will need to be checked.

2. Optimize of the design to reduce heave RAO.

3. Adjust with initial position of tensioner to balance down stroke and up stroke.

4. Increase capacity of tensioners to accommodate higher strokes.

− The supporting structure for the riser tensioning system - further engineering work is required to

develop the details to withstand high impact load due to bottom out/top up condition.

− SIMOP (simultaneous operation of drilling and production) should be further studied including

consideration of tensioner system installation at offshore and focus on additional risks comparing to TLPs

/ spars.

− Sensitivity to wave periods: current design is based on single seastates as defined in API 2INT-MET.

Variations of period or contour seastates should be evaluated at later design stage.

− Airgap - Negative airgap (-1' to -12') have been concluded for some part of deck bottom under 1000-

year hurricane. This is acceptable provided that wave slamming is properly accounted for in the design.

Structure and risers (anything below cellar deck) should be designed for wave impact under 1000-year

wave.

DNV GL – Report No. 15U5O2I-10, Rev. 2 – www.dnvgl.com A-1

APPENDIX A

Drawing List

Drawing No. Rev. DNV

GL No. Title

NA 9 Preliminary VIM test program

RPS-KFD-SP-ZZZ-00002 B 8 HULL VIM MODEL TEST SPECIFICATION

RPS-KFD-RP-ZZZ-0001 A 7 DRY TREE SEMI CONCEPTUAL DESIGN REPORT

1045131-KFD-N-XG-0001 A 1 SEMI-SUBMERSIBLE EAST ELEVATION LOOKING

WEST

1045131-KFD-N-XG-0002 A 2 SEMI-SUBMERSIBLE SOUTH ELEVATION LOOKING

NORTH

1045131-KFD-N-XG-0003 A 3 SEMI-SUBMERSIBLE MAIN STRUCTURE /

SCANTLING COLUMN DECK PLANS - SHEET 1


SCANTLING NODE BOTTOM PLATE PLAN


SCANTLING RING FRAMES - COLUMNS


SCANTLING RING FRAMES - NODES & PONTOONS

DNV GL – Report No. 15U5O2I-10, Rev. 2 – www.dnvgl.com B-1

APPENDIX B

Conceptual HAZID Report

Action at later Design Stage Additional Follow-up 4/10/2014 Session

Later design1 Global Performance

1.1 Rigid Body Motion

Somewhat different range of motions from traditional semi

Excessive motions and excessive riser responses

Md O M L 1. KFD performed sensitivity study of heave RAO wrt different tensioner stiffness. Heave amplitude increases by about 4.5%, min airgap increase about 7.5% in 1000-yr hurricane if the tensioner stiffness change from 15 kip/ft to 25 kips/ft (a 67% increase), which slightly increases the corresponding maximum von Mises stress on the riser.2. KFD documentated that 0 and 45are most critical ones in terms of offset, heave and heel, which are the governing parameters for the riser performance.3. The total heave damping ratio is about 2% of critical damping and Cd = 4 for vertical direction at 100-yr and 1000-yr hurricane conditions, results calibrated against model test results.

OMNI direction considered for current, wind, waves - should consider additional cases (eg. no wind case with max wave) in order to not take advantge of offset for tensioner stroke

1.2 VIM Prone to VIM Potential risers/mooring system failure due to movement outside their design limits

Md S M L Model Test conducted and witnessed by DNV. Results look good. Both with (2 different heights) and without strakes on columns are tested.

1.3 Airgap Required to meet the airgap design criteria: 5' under100 year and 0' under 1000 year

Structural failure due to wave impact loads in 1000 year condition.

Mj O H M Structure / risers (anything below cellar deck) to be designed for wave impact under 1000yr wave.

Negative airgap -1' to -12'. Wave slamming force has been considered in structural design for both Morrison members and stiffened shell plates. DNV independent analysis concluded similar results.

To avoid negative airgap, deck box needs to be increased by 10 - 15 ft. For Morrison members on deckbox such as the dropdown structure and wellbay, particle velocity of 15m/s is assumed with Cs factor to be 5.5. For stiffened shell, both local slamming and global slamming are considered. At the conceptual design stage, the slamming pressures are selected from previous similar projects. Local slamming pressure used to check plate and stiffener is 450KN/m^2 and the global slamming pressure used to check girders is 255KN/m^2. Maximum vertical extent of slamming is assumed to be 2.6m.

1.4 Sensitivity to deckloads

Flexibility on the limit of deck loads and different throughput: could be more vulnerable than a conventional semi?

Potential resizing Mn S L L KFD has done some sensitivity study on varying the topside weight from 35,000 st to 40,000 st or even 50,000 st while maintaining the same hull draft. For different deck loads, slight change of column and pontoon size provide more buoyancy. The heave natural periods of the different cases remains around 20s. Thus, the heave motion is expected to be similar.

Stability will be effected by increasing deck load therefore sizing of columns and pontoons required to change; all other conditions (eg. Integration, transportation, installation) need to be assessed. Should not impact the concept but may result in reconfiguation.

Sensitivity to riser payloads

125ksi used for inner and outer riser

can be covered in design phase

High strength steel may need to be utilized for keel joint or redesign the keel joint;

Kvaerner Dry Tree Semi Concept

Comments/NotesID Critical Issues Concerns Failure mode / Consequence

Initial risk rankingFinal risk rank-ingCons. Likeli-

hoodRisk Rank





hoodRisk Rank

1.5 Sensitivity to riser tensioner stiffness

Has different number of riser cases been fully considered in design?

Excessive motions due to different riser vertical restoring

Md O M L Initial risers and all-risers cases are documented.

1.6 Validity of model tests

Model the model verification to validate scaling from model test results to design - possible truncation effects, accurate damping level etc.

Inaccurate motions and responses due to incorrect model test/analytical results

Md O M L Model correlation report included all critical elements for the comparison and indicated that the analytical results are reliable.

KFD states:The mooring and risers were truncated due to the limit of water depth in the wave basin, but they were modelled with the same stiffness as the un-truncated ones. The same with the lumped riser. The lumped riser stiffness equals to the total stiffness of three risers. As long as the mooring and riser stiffness are the same, the floater motion will be similar. These methods are commonly used in wave basin model test and accepted by the industry. The truncated model under-estimated the damping due to shorter length, which means the model test results are conservative. When we do the analytical correlation, we used un-truncated mooring and risers. And each riser is modelled individually. The damping ratio in the analytical model is smaller compared to the model test (as shown in the correlation report), which means we are even more conservative. The correlated motion results are consistent with the model test results.

In addition, based on field measurement results, it is generally believed that our analytical simulation with

1.7 3rd party independent verification /analysis

Lack of validation of the lower (fatigue) seastates since model test only validates the extreme (high) seastate

Inaccurate motions and responses due to incorrect analysis results

Md O M L Fatigue including single event (also considering keel joint) needs to be further evaluated

10yr, 100yr, 1000yr were validated by model tested. DNV GL did independent strength verification on Riser analysis (1000yr).

1.8 Accidental conditions

Higher tensioner strokes due to compartment flooding

Riser and tensioner failure

Mj S M L The tensioner stroke for 10-yr winter storm with 1-compartment damage (10-yr WS TDM) is much smaller compared to 100-yr Hurricane intact case.The resulting riser stress is much smaller compared to 100-yr Hurricane case. With the same allowable stress, the utilisation for 10-yr WS TDM is much smaller than the 100-yr Hurricane case. Therefore,10-yr WS TDM is not the governing case for riser design.





hoodRisk Rank

1.9 Sensitivity to design wave periods

This concept is more sensitive to selection of design wave periods than wet tree Semis. Care should be taken on how to handle the above in design. Robustness need to be demonstrated.

Excessive motions and riser responses due to wave excitation

Md O M L Sensitivity to wave periods to be - contour line seastates

100yr and 1000yr hurricane based on single seastates as defined by API-2INT MET.

2 Mooring System

2.1 Fiber ropes and TTR

Higher loads on the riser systems due to changes in elasticity of fiber ropes due to long-term effects

Failure/overstress of riser system(s)

Mj S M L TTR is more sensitive to offset than SCR. In the design, mean offset is based on lower stiffness; Dynamic analysis is based on higher stiffness. Loop current may last a period of time, the long duration caused creeping should be considered - Proper installation by pre-stretching.

2.2 Accidental conditions

Higher riser strokes due to mooring line failure

Failure/overstress of riser system

Mj R M L 2-line damage case may be checked.

One line damaged analysed. Latest API-RP17B, recommends 2-line broken under 10yr as a robustness check for flexible pipe design.

2.3 Mooring fatigue Mooring system failure, especially top chain segment.

Failure/overstress of riser system

Mj S M L More detailed study

The mooring system fatigue analysis was carried out for all the components including the chain with a safety factor of 10. Sea state fatigue and VIM fatigue are all analysed. The results shown in the report are the combined fatigue life. The minimum fatigue life is 28 yrs.

3 Production Riser System

3.1 Riser Tensioner system

1. Higher Strokes (35'' under 100 yr and bottom out/top up under 1000 yr hurricane)

2. Tensioner get stuck

Damage to the piston, top plate of the tensioner and potential damage to deck structures.

Mj O H M More detailed design

- Tensioner bottom out/top up under 1000-year wave. Similar philosophy is used on Spar and other in-service FPU. - Bottom out supported by riser support structure, will not hit tensioner barrel. - Tensioner roller is conventional. Roller can withstand normal tear and wear under applicable design conditions. - TQ of riser tensioner completed.





hoodRisk Rank

3.2 Riser Tensioner system

1. Interaction of various tensioners, are they fully independent? 2. Torsion of tensioner -any mechanism to prevent rotation?3. Eccentric loading-impact on synchronicity in case of one tensioner failure?

Damage to riser Mj S M L Proper design documentation

KFD states: Non-linearity of the tensioners is also accounted for (if there is any) in the design. However, due to using relative large volume of tensioner accumulators, there is almost no non-linearity for the proposed tensioner design. This has been verified by engineering analysis and model test verification.1. Each tensioner is fully independent from the others. A bigger wellbay spacing used in this Dry Tree Semi design will avoid interference of jumpers between adjacent risers.2. The centralizing system of the riser tensioner has been designed with the riser spool joint (tension joint) to resist any rotational motion of the riser and tensioner.3. When one cylinder was offline, a review of the upper and lower centralizer loads indicated that no significant change in loading took place, as verified during the model test. The geometry of the five cylinders around the load ring was sufficient to share the slightly eccentric loading on the riser and not to affect the tensioner performance and loading on the centralizers.

3.3 Riser and riser tensioner system

Exposed to direct wave loading (comp. to spar)

Riser damage or failure

Md S M L Detailed riser tensioner design

The RAM style riser tensioner head and rod are located at an elevation without any wave impact even at 1000-yr hurricane condition. The riser tensioner barrel with a wall thickness 1.5 inch is designed to withstand the expected wave slamming.Note that the riser tensioner arrangement on the DWDTS will be similar or better comparing to a TLP riser tensioner arrangement.

3.4 Riser interference Evaluate possible riser/riser interference

Failure of riser system & strakes

Mj S M L Further engineering and documentation.

No difference from other TTR design This will impact the top tension factor (1.3 assumed currently)





hoodRisk Rank

3.5 Loads on riser 1. compression in riser under 1000year/top up case.

2. Accuracy of the riser evaluation that should account for the presence of columns.

Buckling of risers; potential fatigue of risers due to higher stress range

Mj O H M More advanced analysis to investigate impact of compressive loads in riser or mitigations to be implemented.

1. compression in lower part of riser (680kips max tension; potentially up to 3000' of riser under compression during part of 1000-year wave cycle)2. Without the protection frame, the loads on risers are similar to TLP risers. Offbody kinematics due to the disturbed wave should be properly captured.

KFD Mitigations:1. Increase top tension from 1.3 to 1.5. Such can be done for storm condition only - to be included in operational procedure. Howevera) check riser strengthb) check tensioner capacityc) check payload capacity of the unit - 12x250kips increase2. Re-configuration of the design to reduce heave RAO.3. Adjust with initial position of tensioner to balance down stroke and up stroke.4. Increase capacity of tensioners to accomodate higher strokes.

3.6 Damping qualification/stick-slip simulations (keel guide)

Accurate evaluation of damping and riser responses for TTR design (stick/slip effects)

Overstress of the risers/components.

Mj S M L Since the floating system displacement is significantly larger than the friction force at riser keel joint/guide, it is not expected that the friction force will have major impact on global motion and riser performance as validated during wave basin model test.

3.7 Access to tensioner system

Access for Operation, maintainance, Repair of the riser system

Higher downtime/ cost due to SIMOPS

Mn S L L KFD to review the IMR plan wrt longer strokes, production vs. drilling

According to KFD: Regular, no new features, design life is > 20yrs.

3.8 Operation How can the tools go through keel guides

Higher downtime/cost Mn S L L The riser keel guide opening will be designed with sufficient diameter to run the riser components through. No special tool is required to run through the keel guide for TTRs. If there is a need to lower some other subsea equipment in the well bay area, there are two spare slots available

4 Floating Stability

4.1 Design standards Industry standards for conventional semi used. Operational limit on heeling angle?

stability problem Mj S M L Site-specific wind loads to be calculated.

USCG requires site-specific, min 100knots for production unit. 120knots for intact stability was used in design. TTR stiffness was not taken into account for stability analysis. 50knots was used for damage stability.

4.2 Pre-service stability

For deep draft semi, stability could be more critical during quay side integration and installation than in-place condition

stability problem Md S M L More detaild analysis and verification to be performed

Study report includes transition phase, stability has sufficient margin to meet the MODU rule requirment.

5 Structure





hoodRisk Rank

5.1 Hull Deeper draft - higher hydrostatic pressure

Structural failure due to overstress

Mj S M L More detailed design documentation

Preliminary scantling check indicate most structure are in compliance with DNV Offshore codes.

5.2 Riser support structures/Keel guides

High impact load due to bottom out/top up.

Overstress in riser support structure and potential damage into the deck/hull

Md O M M 3rd party review of detailed calculation and structural connection design.

Supporting structure should properly designed to withstand the impact loads.

Kvaerner states: Dynamic impact load is at maximum with bottom out condition, however, this impact is a very quick impulse load and the riser can’t be stretched during this short period. Based on our calculation, the impact load is much less than our riser tensioning load at bottom out condition. Well bay structure and keel guide frame are not overstressed under our conservatively applied riser tensioning loads. We will do the detailed connection check at later design stage.

5.3 Topside/Deck Higher tensioner loads to support than conventional semi, ref. 3.3

Structural failure due to overloading

Md O M L Proper engineering design and documentation

6 Marine systems6.1 Ballast systems Sufficient ballast

capacity for in-place as well as integration/installation phase? Redundancy?

Mj S M L Capacity, redundancy and rate of filling as required for safe operation should be documented.

Active ballast only required when changing risers or topside. For semi, insallation period can be prolonged or one may use other pumps (e.g. fire pumps) for temp. ballasting function as long as they are blind off during operation. - Ballasting pumps needs to be self-priming.

6.2 Bilge systems Sufficient bilge capacity for in-place as well as integration/installation phase? Redundancy?

Mj S M L More detailed design for bilge and drainage system should be submitted. Independent bilge system, independent drainage system etc required.

7 Safety system7.1 Hazardous areas New haz. Area due to

hydrocarbon API RP505 shall be used.

Md S M L Proper design documentation

7.2 Lifesaving appliance

Mj S M L Proper design documentation

7.3 Escape Make sure escape routes is clear or well protected from well bay and other areas affected accidental loads.


7.4 fire/gas Mj S M L Proper design documentation

All these are important safety issues. But they are no different from existing production units. Assuming these safety aspects are properly designed following applicable design standards, likelihood of failure is low.





hoodRisk Rank

7.5 ESD Mj S M L Proper design documentation

7.6 Well bay Potentially higher explosion pressure


blast wall required, impact on weight and CG? Wind profile.

8 E&I8.1 Regular deign

issuesMj S M L Proper design

documentation 9 SIMOP

9.1 Production/drilling New risks due to production/drilling together, probably bigger fire pump capacity.

Md S M M Further study Larger stroke, results in potential issues (e.g. more sagging in jumpers, riser clashing) of the jumpers, needs to be accounted for in design. . Space for jumpers from risers - responsibility/interface.

Additional study in the future:1. Include consideration of installation of tensioner system offshore2. Focus on additional risk comparing to TLPs / Spars

10 Geotechnical10.1 Seabed riser c-c

spacingswelling of seabed due to soil/temp. effects from production risers

Md R L L

11 Construction, Transportation and Installation11.1 Deep Draft Availability for

quayside integration, stability during installation,

Md O M L Further documentation

Preliminary documentation provided indicates sufficient margin.

11.2 Installation procedure

Any special requirements on ballast system due to installation process? Redundancy?

Md O M L Further documentation

DWDTS ballast system is no difference to any other conventional deep draft semi, many of which are still in operation now.

11.3 Transportation

12 Operation

Notes:

N: Not documentedY Sufficiently documented for this phase by analysis/model test or both P Partially documentedI IncidentalMn MinorMd ModerateMj MajorR RemoteS SeldomO OccasionalL Likely

Initial risk ranking: The risk ranking based on likelihood of a failure and consequence of the failure, which was assessed before reviewing design documents.

Final risk ranking: after reviewing design documents, discussion with the designer and the threat assessment, better understanding of the design concept and work performed was achieved and likelihood of failure has been reduced in some cases, consequency of failure remains the same. The final risk ranking is then evaluated.

Likely High

Occasional Medium

Seldom

Remote Low

Incidental Minor Moderate Major

Consequence of Failure

Like

lihoo

d of

Fai

lure

DNV GL – Report No. 15U5O2I-10, Rev. 2 – www.dnvgl.com C-1

APPENDIX C

Comments and Responses


Ver-Com

no.: Description:

Category

*)

Status

**)

1

Please confirm that the model will be connected to a

tow carriage using a system of linear springs.

REPLY: Kvaerner confirms the use of linear springs on

the four mooring lines.

TQ C

2

Section 3.3 – Measurements calls for measurements

of mooring line tensions and drag and lift force on the

hull. Since in most of the tests, the model will be free

to move, it is not possible to directly measure the

forces acting on the hull. Rather, the forces can be

deduced by the reactions from the mooring line

tensions.

REPLY: Kvaerner is measuring the force reactions

from the model in the mooring lines.

A

3

How will the number of VIM oscillations be assured to

be sufficient to provide stable statistics of A/D? The

tow tank length is 168m. The natural sway period of

the model will be approximately 26s (Prototype sway

period is 212s. Model is Froude scaled. Time ~

sqrt(L).). Tow speeds are from 0.17 to 0.76 m/s. At

the highest tow speed (Vr=18), there will be less

than 8 resonant oscillations. With half the velocity (Vr

= 9) the number of oscillations will still only be 16. Is

this enough to get stable A/D statistics?

REPLY: Using the Froude scaled speed (VMS = VFS

/sqrt(60) scale 1:60) we will achieve model speeds

the range 0.053 – 0.307 m/s (Ur 18/ heading 45).

With a sway period of ~212sFS/27.3sMS this will

ensure about 19 cycles for Ur 18, which is sufficient

for a statistical analysis if the signal is periodically.

TQ C

DNV Project Title: DNV Project Job No:

Ultra-Deepwater Dry Tree Semi-submersible for Drilling and

Production in the Gulf of Mexico

PP055893

Document title: Prepared

by:

Date: Sign: Document. No.:

Hull VIM Model Test

Specification

RGOR 04/15/20

13

RPS-KFD-SP-ZZZ-00002

Verified by: Date: Sign: Document

rev.

VHAN 04/15/20

13

C (For

Approval)

VERIFICATION COMMENTS:


*) NC = Non-Conformance TQ=Technical Query A=Advice (need not be clarified) **) O = Open C = Closed (requires a reference) CN-(Closed with note)

DNV Project Title:

PRSEA Project 10121-4405-02

Document Title:

Hull VIM Model Test Correlation Analysis

Report

DNV Project No.:

PP055893

Document Number:

RPS-KFD-RP-ZZZ-0003, Rev A

Ver-Com

no.: Description:

Category

*)

Status

**)

1.

Effect of Strakes –Figure 3-8 shows that at a Ur of

4.5, the VIM for the small strake case is lower than

the large strake case. Please comment on this in the

report.

Kvaerner response: at Ur = 4.5, for the test case

with small strakes, VIM response is not locked-in yet,

the corresponding VIM amplitude may be lower than

that for the test case with bigger strakes, which may

be already or close to VIM lock-in. On the other

hand, we still investigate the strake effects on VIM

suppression and did not use any model test result

with strakes for our current Dry Tree Semi design.

Therefore, we suggest putting the test cases with

strakes aside at this stage.

DNV GL: Closed

TQ C



DNV Project Title:


Document Title:

Dry Tree Semi Conceptual Design Report,

Deepwater Dry Tree Semi Development -

Stability Analysis Clarification

DNV Project No.:

PP055893

Document Number:

RPS-KFD-RP-ZZZ-0001 Rev. A

Ver-Com

no.: Description:

Category

*)

Status

**)

1.

The documents have been reviewed for compliance

with DNV-OS-C301 corresponding to the IMO MODU

Code Chapter 3. It is to be noted that no independent

calculations have been carried out by DNV GL in

connection with this review. It is highly recommended

that such be conducted in the later design phase.

KFD response: Noted.

TQ A

2.

It is observed that the IMO MODU CODE requirements

of Ch. 3.3.1.2, 3.4.3.2 and 3.4.4.2 related to

immersion of unprotected and weathertight openings

have not been documented. It is not clear whether

openings have been accounted for in the calculations.

KFD response: The CODE in Ch 3.3.1.2 has been

considered in the stability calculation as shown on Page

11 of Kvaerner’s “Deepwater Dry Tree Semi

Development - Stability Analysis” (dated December 19,

2013). The criterion that area ratio should be larger

than 1.3 is consistent with this code. For Code in Ch

3.4.3.3 and 3.4.4.2 regarding the watertight and

weathertight openings (such as chain lock and

ventilation pile openings), these are considered during

design and analysis of the DWDTS. In general, we

design the DWDTS stability as we designed and

delivered other Semi-submersible projects complied

with all applicable regulatory codes and requirements.

DNVGL: It is understood that the GZ curve will not be

terminated by any unprotected openings in the

relevant calculation range, and no weathertight

opening will be submerged prior to the first intercept of

the GZ curve and the wind heeling moment curve.

TQ CN

3. It is observed that a wind speed of 122 knots has been

applied for the In-place Severe Storm Condition. TQ A


Ver-Com

no.: Description:

Category

*)

Status

**)

4.

It is not clear to us why the Hull light weight and

Topside weight are different in the loading conditions

presented on page 10 of the Stability Analysis. It is

also observed that the VCG of the In-shore Tow

Condition is different in the two documents.

KFD response: In this study, we used the hull weight

during topside Integration for the In-shore Tow

stability analysis. Practically, the same weight condition

for both In-shore Tow and Wet Tow should be checked.

If we adjust the hull weight to be consistent for both

In-shore Tow and Wet Tow cases, the center of gravity

will be increased slightly. Thus, the AVCG margin and

GM values will be slightly reduced as shown in the table

below. However, since the DWDTS was designed to

float on the pontoon elevation with sufficient AVCG

margin during In-shore Tow, after adjustment of the

hull weight for In-shore Tow condition the DWDTS still

satisfies applicable requirements.

VCG (from

keel) AVCG Margin GM

(ft) (ft) (ft)

In-shore Tow 140.21 42.50 183.31

Transitioning 134.16 6.96 6.30

There is a typo for the VCG of the In-shore Tow

condition in the conceptual study report. This will be

corrected for the final version of the conceptual study

report.

DNVGL: Please update and clarify in the next revision.

TQ CN

5.

On page 11(Stability Analysis Clarification) it is

observed that the Governing Criteria for the Transition

is GM>0.164. Kindly be informed that a GM of no less

than 0.3 m/ 1 ft should be considered for any

transitory conditions.

KFD response: There are no specific requirements on

GM values, other than positive, on either IMO MODU

Code or DNV-OS-C301. The GM>0.164 ft criterion is

from USCG CFR46 Ch174.040: “Each unit must be

designed to have at least 50mm of positive metacentric

height in the upright equilibrium position”. We

appreciate your suggestion on the GM values.

TQ C


Ver-Com

no.: Description:

Category

*)

Status

**)

Practically, we design the DWDTS with a target of GM

> 3ft values at all conditions.

6.

It is observed that two compartments flooding has

been accounted for which is beyond the minimum IMO

MODU Code requirement.

TQ A

7.

It is observed that In-shore Tow has been checked for

70 knots wind speed and no damage stability.

KFD response: The In-shore Tow is considered as a

short period temporary condition with specific

restriction to avoid any damage of the hull

compartments. In the IMO and DNV rules, there are no

specific requirements for the damage stability on

temporary conditions. Therefore, the damage stability for In-shore Tow is not considered for this study.

DNVGL: Kindly note that we in general consider any

kind of towing as transit conditions while temporary

conditions are limited to transient conditions during

change of draught, ref. DNV-OS-C301 Ch. 1 Sec. 1

[4.2.19].red for this study. This can be further

discussed and resolved in the next phase.

TQ CN

8.

It is not clear whether your criterion “Area Ratio > 2.0”

presented on page 12 is the criteria of IMO MODU Code

3.4.3.3.

KFD response: This criterion should have been stated

as “Moment ratio > 2.0”, which is based on IMO MODU

code 3.4.3.3 “… the righting moment curve should

reach a value of at least twice the wind heeling

moment curve…” We will update it accordingly.

DNVGL: Please update in the next phase.

TQ CN



DNV Project Title:


Document Title:

Dry Tree Semi Conceptual Design Report

DNV Project No.:

PP055893

Document Number:

RPS-KFD-RP-ZZZ-0001

Ver-Com

no.: Description:

Category

*)

Status

**)

1.

Section 4

To satisfy the requirements of AiP, the following

safety aspects should be addressed:

- General Layout and topside arrangement

- Hazardous Area Plan

- Area Safety Chart

- F & G Design Philosophy

- ESD Design Philosophy

- Ventilation design Philosophy

- Ventilation Ducting Design Philosophy

- Active/passive fire protection design

Philosophy

- Power management philosophy

- Lifesaving arrangement (evacuation study)

A high level Lifesaving arrangement is indicated in

Appendix A, which appears to be not in line with the

recognized standards, in case south side is impaired.

Due to simultaneous operation of drilling and production,

deck spaces appear to be limited and congested.

Therefore, the above safety philosophy document shall

be in place prior to further project development.

KFD response: With respect to the life safety

arrangement in the event that the south side is

impaired, our intention is to have alternative survival

craft located on the north end of the facility (either on

the north face of the facility or along the east or west

sides of that face). The current configuration is intended

to be in-line with items identified in the study equipment

list even though we have identified the need for

additional craft. Due to the layout requirements for

safety in the next phase of development additional

survival craft will be identified and located as required to

provide for the safety arrangement (in the north sections

of the facility).

With respect to deck congestion due to the SIMOPS

TQ O


Ver-Com

no.: Description:

Category

*)

Status

**)

functions of drilling and production - a more detailed

evaluation of SIMOPS challenges and mitigation has

been identified and is planned for the DWDTS concept.

However, in the interim we do not see the congestion

level as significantly different from that of other floating

concepts that also employ full drilling facilities. In

general we have sought to provide maximum

segregation between the processing facilities on the

north section of the platform and the drilling support

functions to the south area of the facility. While

development is still preliminary we envision fire/blast

walls segregating the manifold from the processing area

to the north and from the wellbay to the south.

The current DWDTS design is still at the concept level.

The detailed listing of safety aspects that have been

identified (lifesaving arrangement, area safety chart,

hazardous area plan, F&G design philosophy, ESD design

philosophy, active/passive fire protection design

philosophy, etc.) will all be addressed in detail in the

next phases of the concept development. We agree that

all of these design philosophies should be established

and reviewed in more detail. The project has just not yet

reached that phase of design development.

It should be noted, with respect to ventilation design

philosophy (as well as ventilation ducting design

philosophy) that a key feature of the DWDTS design is

the open wellbay area, which provides for a high level of

natural ventilation across the width of the facility.

DNVGL: Comments to be followed up in the next phase.

System design is still preliminary with limited

information. Comments are given as advice at this stage,

not considered as barriers to feasibility.

2.

Section 4.6

With higher vertical motions for a dry tree semi the

susceptibility to obtain SCR compression in the TD area

has to be controlled and checked out. What has been

done so far?

KFD response: SCR compression is not included in the

scope of work of this conceptual study, but it should be

noted that SCR were considered as a field proven

technology and are being used for field development

with a deep draft semi. Furthermore, to satisfy TTR

performance, the DWDTS heave motion has been

TQ C


Ver-Com

no.: Description:

Category

*)

Status

**)

reduced noticeably and is smaller than that of a typical

deep draft semi. Therefore, this should not be an issue.

3.

Section 5.1 Description, Page 37

It is noted that the design of the tensioner system is (or

has been) conducted by MHD Offshore Group. Design

documentation of the MHD Offshore group requires to be

reviewed.

KFD response: Due to concern of the riser tensioner IP

design, detail tensioner design information will not be

provided in the public domain, but has been provided to

DNV TQ group for technology qualification process.

DNVGL: refer to TQ report.

TQ CN

4.

Section 5.2 Tensioner Configuration, Page 38

“An additional benefit of having 6 cylinders in a tensioner

is the increase in system redundancy if a cylinder fails.

Since an instantaneous failure of one cylinder only

reduces the riser tensioning capacity by 16.6 % as

opposed to 25% in a 4-cylinder system.”

The proposed design appears to require balancing by de-

activating another tensioner in case of loss of a

tensioner. Therefore, using a 4-tensioner system does

not seem to satisfy the redundancy requirement. Hence,

a minimum of six tensioners in the system is not so

much a benefit but a necessity. Please clarify.

KFD response: If there are only 4 cylinders, when one

cylinder fails, the opposite one will also be de-activated.

Therefore, for 4-cylinder tensioner system, the capacity

will reduce by 50%. While for 6-cylinder tensioner

system, the capacity will only reduce by 16.6% when

one cylinder fails, or 25% when one cylinder fails and

the opposite cylinder is also de-activated. It is

extremely important to support a heavy riser for ultra-

deep water development.

DNV GL: Comment closed with the note that the current

6-cylinder tensioner design is the minimum requirement

for providing system redundancy in case of the loss of a

single cylinder.

TQ CN

5.


“The tensioner system is designed to have sufficient

capacity after one cylinder failing and allow for manually

TQ CN


Ver-Com

no.: Description:

Category

*)

Status

**)

taking one cylinder (opposite to the failed cylinder)

offline, but increasing the pressure in the remaining 4

cylinders to continuously operate at the required null

riser tension..”

Is the stated pressure increase in the remaining

tensioners carried out manually? Or does the system

dynamically adjust for the tensioner loss by tensioner

stroke? If the adjustment is performed manually as

seems to be the case, how quickly should the

adjustment take place? What happens if there is any

delay in performing pressure adjustment?

KFD response: In operating condition, it will be manually

turned off to balance the system. In Hurricane condition,

if one cylinder down, the remaining cylinders will

continue work during the storm condition, which is the

base case for tensioner design as other dry tree systems

in service. This design condition has been tested during

the model test for this project.

DNV GL: Comment closed with the note that the

required response time in manually de-activating a

cylinder (in case the opposite cylinder fails) and also

adjusting the pressure in the remaining cylinders need to

be determined and documented in next design phase.

6.


“The tensioner system is designed to have sufficient

capacity after one cylinder failing and allow for manually

taking one cylinder (opposite to the failed cylinder)

offline, but increasing the pressure in the remaining 4

cylinders to continuously operate at the required null

riser tension..”

Is the de-activation of the opposite cylinder (after loss of

a tensioner) an immediate requirement? What happens

to the tensioner system if there is a delay in de-

activation or while the de-activation is carried out?

KFD response: In operating condition, the opposite

cylinder will be manually de-activated. In Hurricane

condition, if one cylinder down, the remaining cylinders

will continue work. Both cases have been tested in the

model test for this project. The test results showed no

significant change of side load on the upper and lower

centralizers for both cases attributed to proper design of

the riser tensioner components and support structure.

DNV GL: Comment closed with re-emphasizing the note

to TQ #5.

TQ CN


Ver-Com

no.: Description:

Category

*)

Status

**)

Establishing immediacy (or otherwise) of manual re-

adjustment of pressure in the remaining cylinders needs

to be evaluated at next design phase. In other words,

‘fail-safe’ nature of a cylinder and the adequacy of

mitigation against such a failure will need to be

established.

7.

Section 5.3 Primary Components, Page 38

It is recommended to include a schematic of a single

tensioner in this section of the document identifying all

the components.


design, this information will not be provided in the public

domain, but has been provided to DNV TQ group for

technology qualification process.


TQ CN

8.


It is recommended to include a brief description of the

functionality of the identified components.


design, this will not be provided in the public domain,

but has been provided to DNV TQ group for technology

qualification process.


TQ CN

9.


How is the sealing between the cylinder and the rod for a

tensioner accomplished?


design, this information will not be provided in the public

domain, but has been provided to DNV TQ group for

technology qualification process.


TQ CN

10.

Section 5.4 Layout, Page 39

Do the tensioner systems (12 in figure 5.2 arrangement)

act independent of each other? What, if any, is the

impact of failure, malfunction, or tensioner loss in any

tensioner system on the overall configuration?

KFD response: Yes, each tensioner system works

independently from each other. Failure or malfunction of

TQ CN


Ver-Com

no.: Description:

Category

*)

Status

**)

one tensioner system will not affect other tensioner

systems.

DNVGL: Further documentation is expected for next

phase.

11.

Section 6.1

The report has provided only the end results without

providing any documentation of methodologies,

calculations and assumptions etc. Please provide such.

KFD response: The detailed description of the stability

analysis will be provided in a separate document.

DNVGL: closed based on further documentation

received.

TQ C

12.

Section 6.2.3.

MOORING SYSTEM PROPERTIES – “The minimum breaking

load (MBL) of mooring chain has taken into account for

annual corrosion allowance of 0.4 mm.” Assume it is

0.4mm/year. Please clarify.

KFD response: Yes, it is assumed the chain corrosion is

0.4mm/year based on API 2SK and design basis.

TQ C

13.

Section 6.2.5.

MOORING SYSTEM FATIGUE ANALYSIS – “The combined

spectrum analysis method with dual narrow-banded

correction factor has been used for the wind/wave

induced fatigue analysis.” Should cite reference for the

method used.

KFD response: This method is based on API-RP-2SK.

Citation will be added in the report: “The combined

spectrum analysis method with dual narrow-banded

correction factor has been used for the wind/wave

induced fatigue analysis. [1]”

TQ C

14.

Section 6.2.5.

MOORING SYSTEM FATIGUE ANALYSIS – “VIM fatigue analysis

is based on the correlated results of the DWDTS VIM

model test for this project.” No details given on VIM

responses or how VIM induced fatigue has been

determined. Please clarify how much of the fatigue

damage is due to VIM. Also, what basis is used for

representing the current bins?

TQ C


Ver-Com

no.: Description:

Category

*)

Status

**)

KFD response: The co-related VIM response design

curves are listed in Figure 10-5 of the report. Max

tension variation is determined by moving the vessel

transversally according to the proposed VIM design

curves.

The detailed data of VIM fatigue damage will be provided

in a separate file.

Met-ocean data for calculating VIM is not available in the

design basis for this project. The occurrence frequency

of eddy current and the corresponding amplitude are

based on a typical metocean data in the central region of

GoM as used in the DeepStar project. The total

percentage of eddy current occurrence is approximate

27% in a year. At this stage, the selected metocean

conditions of eddy current should give a reasonable

foundation for the VIM fatigue calculation. The eddy

current metocean data include 23 bins. Each bin runs

for 24 directions (15 degrees apart).

DNVGL: acceptable for concept phase.Further study

based on site specific data in the actual project phase.

15.

Section 6.3.3.

RAO CALCULATION – The calculation of the heave response

is critical to the feasibility of this concept. There is no

discussion regarding the resonant heave performance,

such should be provided. For example, how is the low

heave peak RAO of 0.8 achieved? How is the viscous

damping modeled? If the viscous damping is significant,

then the heave RAO curves are seastate dependent and

should be presented for representative seastates.

KFD response: RAO reported in the conceptual study

report is the inverse of the time domain results for 100-

yr hurricane wave case. In the time domain motion

calculation, the viscous damping has been modeled as

nonlinear hydrodynamic drag through Morison model.

The drag coefficients have been correlated with the wave

basin model test results.

TQ C

16.

Section 6.3.4.

GLOBAL MOTION – The report should include all rigid body

motion responses of the, not just heave.

KFD response: The detailed motion response results will

be provided in an attachment.

TQ CN


Ver-Com

no.: Description:

Category

*)

Status

**)

DNVGL: To be included in updated report in the next

phase.

17.

Section 6.3.4.3

Air Gap – “An asymmetric factor is used to account for

non-linearity in the incoming waves.” Please clarify what

asymmetric factor was used? What is the basis?

KFD response: The use of asymmetric factor is based on

DNV-RP-C205. With consideration of previous project

experience, the recommended value 1.2 was used.

TQ C

18.

Section 6.2.5.2 & Table 6-7

Please specify what level of OPB effects has been

included in the fatigue estimates?

KFD response: In modern mooring fairlead design, the

fairlead can be rotated freely within a design range that

is in general larger than the maximum yaw angle, thus

OPB effects are negligible.

TQ C

19.

1. Section 8.2.1

Operating condition should be checked, because

allowable stress is lower (0.6*Fy).

KFD response: The interface structure is designed to

withstand the forces during riser tensioner bottom-out

and top-up under survival conditions. In the operating

condition, there are no bottom-out and top-up. The force

will be much smaller than the survival conditions.

Therefore, operating condition is not the governing case

at all.

DNVGL: Comment is referring to str. overall, not just

interface. Comment closed based on further discussion

with designer. Better documentation expected in the

next phase.

TQ CN

*) NC = Non-Conformance TQ=Technical Query A=Advice (need not be clarified)

**) O = Open C = Closed (requires a reference) CN-(Closed with note)


DNV Project Title: RPSEA Ultra-Deepwater Dry Tree System for Drilling and Production

in the Gulf of Mexico

Document Title:

Dry Tree Semi Conceptual Design Report by

Kvaerner Field Development Inc.

DNV Project No.:

PP055893

Document Number:

RPS-KFD-RP-ZZZ-0001

Ver-Com

no.:

Description: Category

*)

Status

**)

1. Weight of internal tubing inconsistent:

Table 3-4: 35.4 vs. Table 7-1: 33.7

Please clarify – will update.

33.7 is the old number in which the tubing is 5.5”OD and

0.65” Wall Thickness. Then we realized in the design

basis the wall thickness is 0.689”, which increases the

unit dry weight from 33.7 lb/ft to 35.4 lb/ft. So 35.4 is

the correct number. The number 33.7 in Table 7-1 should

be 35.4 and will be updated in the final version of the

study report.

DNV GL (11-14-2014): Comment closed per clarification

above. Please ensure the final version is updated

accordingly.

TQ C

2. Page 80

“Experience also indicates that riser fatigue performance

is not a concern for the proposed top tensioned risers.”

Please clarify the experience referred to herein. Is it from

the field or simply based on past design? From the

lessons learned in the field, a fatigue study along with a

thorough modal analysis of the TTRs is strongly

recommended.

Fatigue needs to be addressed at design stage, marine

growth should also be accounted for.

In a past project with top-tensioned risers, fatigue

analysis was performed on similar production riser and

the analysis results showed fatigue was not a concern.

In this dry tree semi design, tensioner only bottoms out

or tops up in 1000 hurricane survival conditions, which

TQ CN


Ver-Com

no.:


*)

Status

**)

happens at a very low probability. Most of the time, the

top tension varies in a small range because the long

stroke tensioner has a low stiffness. Under this condition,

the stress range in the riser will be small, which is similar

to the top-tensioned risers supported through buoyancy

cans. Therefore we state that fatigue is not a concern for

the top tensioned riser in our design.

DNV GL (11-14-2014): The explanation provided means

the stress range for ‘most of the time’ is ‘probably’ small.

Whilst the argument has its merits, it cannot be regarded

as a substitute for actual fatigue damage evaluation.

Comment will be closed upon receiving fatigue results.

This is expected to be documented at next design phase.

3. Has marine growth been considered? If not, please

explain why. Please also estimate the potential impact on

riser design.

The marine growth was not considered. First, marine

growth will affect slightly the hydrodynamic force and

increase the weight of the riser. But marine growth is

concentrated near the sea surface. The water depth in the

design is 8000 ft, which is much longer than the marine-

growth portion. A small portion of marine growth on the

riser is not expected to affect the riser global

performance significantly.

Secondly, for this ultra-deep water top-tensioned riser

with relative high riser pretension and stiffness, the effect

of marine growth on riser displacement and stress will not

be significant and could be considered at the project

detail design phase.

Third, marine growth build up can be controlled by

routine cleaning as needed, using anti-fouling materials,

etc.

So marine growth was not considered in the analysis for

this conceptual study.

DNV GL (11-14-2014): Appreciate the clarification.

However, the technical inquiry is not solely related to

weights and stresses which we agree would have limited

consequences. The issue of marine growth on material

TQ CN


Ver-Com

no.:


*)

Status

**)

integrity should also be addressed – whether marine

growth will impact material characteristics. Such can be

addressed at next design phase.

4. As per API RP 2RD

Several tensioner failure conditions should generally be

examined, including one where there is reduced capacity

and one where there is total collapse of the tensioning

system.

Tensioner failure analysis is recommended.

Under 1000year condition, when the tensioner tops up,

lower part of the riser is under compression – is that a

concern? DNV will discuss internally to investigate the

relevant design criteria.

Under the extreme conditions, loss of one cylinder will not

cause a problem. In case one cylinder is damaged and

has lost its capacity, its opposite cylinder will be

deactivated. The remaining cylinders will be enhanced to

meet the working requirement.

This has been proved by tensioner model test.

Loss of one cylinder in survival conditions and the total

collapse of the tensioning system are not design criteria.

DNV GL (11-14-2014): Advisory comment. Appreciate the

explanation that there is redundancy in the number of

tensioners and two opposite tensioners can be de-

activated without any loss of performance.

A

5. Independent Analysis by DNV:

(See Appendix)

Please re-check the maximum stress in the inner casing

of the production riser in 1000-year hurricane event

(7.3.3.1 Governing Heel Case)

Stress on inner casing and tubing could be under-

estimated.

(1) To obtain the shared effective tensions for three

tubes is not straightforward. One assumption is that

these three tubes have the same axial strain. By

solving the following equations, one can get effective

TQ O


Ver-Com

no.:


*)

Status

**)

tensions and wall tensions for three tubes.

333333

222222

111111

eooiiw

eooiiw

eooiiw

TApApT

TApApT

TApApT

+−=+−=

+−=

( ) ( ) ( ) zwww

eeee

oioi

EA

T

EA

T

EA

T

TTTT

pppp

ε===

++===

3

3

2

2

1

1

321

3221,

where

Tw1, Tw2 and Tw3: wall tension of outer casing, inner

casing and tubing respectively;

p1i, p2i and p3i: internal pressure of outer casing,

inner casing and tubing respectively;

p1o, p2o and p3o: external pressure of outer casing,


Te1, Te2 and Te3: effective tension of outer casing,


A1i, A2i and A3i: internal cross section area of outer

casing, inner casing and tubing respectively;

A1o, A2o and A3o: external cross section area of

outer casing, inner casing and tubing respectively.

Te : effective tension of the production riser;

(EA)1, (EA)2 and (EA)3: axial stiffness of outer

casing, inner casing and tubing respectively;

z: axial strain of the production riser.

(2) The maximum effective tension is always at the load

ring position on the riser. It depends on the tensioner

stroke which depends on the vessel motion. The

maximum curvature also depends on the vessel

motion. In addition, it depends on the environmental

conditions like current.

5. Con DNV GL (11-14-2014): Appreciate the explanation.

However, the above does not address the specific

technical inquiry. We are not in agreement with the

argument advanced towards (what might be considered)

indeterminacy of load-sharing between casing strings.

The technical inquiry relates to a load case where the


Ver-Com

no.:


*)

Status

**)

outer casing stresses from KFD and DNVGL analyses are

in agreement but there is noticeable variation between

the respective results on both inner casing and the

tubing. (See table 1-3 in the appendix). This discrepancy

and its implications should be explained. This is not

considered as a barrier to feasibility but should be

properly addressed at later design stage.


About DNV GL Driven by our purpose of safeguarding life, property and the environment, DNV GL enables organizations to

advance the safety and sustainability of their business. We provide classification and technical assurance

along with software and independent expert advisory services to the maritime, oil and gas, and energy

industries. We also provide certification services to customers across a wide range of industries. Operating in

more than 100 countries, our 16,000 professionals are dedicated to helping our customers make the world

safer, smarter and greener.

aip for kvaerner deepwater dry tree semi approval in ...€¦ · 1.2 introduction of kvaerner...

Documents