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Ptil Webinar – 2. juni 2020

Per R. Nystrøm - IKM Ocean Design AS

UTVIKLING I STØRRELSE PÅ TRÅLUTSTYR

NCS Trawl Development Study

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Trawling on NCS - Trends

Over the last 40 - 50 years, the fishing

industry has been through large structural

changes where no. of fishing vessels has

declined.

Instead, fishing vessels has increased in

size, with more personnel, larger equipment,

more storage capacity and more

industrialised.

Modern trawl vessels use the latest

technology and sensors for controlling and tuning the trawl gear.

Trawling on NCS

Beam Trawling

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Trawl gear

VMS records and research

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VMS records and research

AIS data available

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Using the AIS data together with vessel databases

Trawl gear size vs Vessel Engine BHP

http://thyboron-trawldoor.dk/products/clumps/

https://www.rock.fo

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Catch data available from Fiskeridirektoratet (fiskeridir.no)

Timeline for Trawl Gear and Load Development (Pipelines)

DNV 1976 & 1981

DNV 1996 & DNV Guideline 13 (1997)

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Timeline for Trawl Gear and Load Development (Pipelines, cont’d)

DNV RP-F111 (2006)

Timeline for Trawl Gear and Load Development (Pipelines, cont’d)

DNV RP-F111 (2010, 2014 & 2019)

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Timeline for Trawl Gear and Load Development (Subsea Structures)NORSOK U-CR-001, U-001 and U-002

NORSOK U-002 Subsea Structures and Piping System

NORSOK U-002, started as NORSOK standard U-CR-001 in its Rev. 01 in 1995, Ref. [15] and was renamed to U-002 in its Rev.02 in 1998.

NORSOK U-002 defines the minimum requirements for subsea structures and piping systems (template and satellite structures, manifold and riser base structures, protection structures, piping modules).

In Rev.02, NORSOK U-002 gave the following recommendations wrt design for overtrawlability (Sect. 4.2.5):

Timeline for Trawl Gear and Load Development (Subsea Structures)NORSOK U-CR-001, U-001 and U-002

NORSOK U-001 Subsea Production Systems

The NORSOK U-001 standard, had a major revision update in 2002 when it was re-issued as Rev.03. In addition to replacing the Rev. 02 of the U-001 standard, it also incorporated:

The above NORSOK U-002 Subsea Structures and Piping System, Rev. 2 [14]

NORSOK U-006, Subsea Production Control Umbilicals, Rev. 2

NORSOK U-007, Subsea Intervention, Rev. 2.

The new revision of the NORSOK U-001 standard is further based on ISO 13628 [18], Petroleum and natural gas industries – Design and operation of subsea production systems series of standards, and includes specific national requirements and recommendations that are not covered by ISO 13628. This NORSOK standard shall therefore be read in conjunction with all parts of ISO 13628.

Snagging shall be considered as an abnormal operation (PLS), while impact and frictional loads caused by passing fishing gear shall be regarded as normal operation (ULS). This applies unless the frequency of trawling allows it to be considered as a PLS condition. Specimen loads for a typical North Sea location are given in 5.1 (see Fig. 5.19).

Model tests may be used to document smallerloads. Loads from beam trawls shall, in addition, be considered for areas where such equipment is used.

A model test shall investigate the overtrawlability of the

structure and quantify the trawl loads to which it may be

subjected. The model test shall as a minimum simulate the

following:

• trawl gear type (otter/cotesi, beam etc.);• trawl speed;• water depth;• friction on seabed and structure;• length;• stiffness and angle of warp lines;• minimum breaking strength of warp lines;• bobbins and ground ropes.

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Timeline for Trawl Gear and Load Development (Subsea Structures)

Another change in the new standard, is that the minimum trawl speed has been reduced from 3.0 m/s to 2.8 m/s. The new revision

also includes requirements for a truncated pyramid and trapezoid shaped structures to assist trawl and trawl wire deflection.

During the same period, DNV Guideline 13 gave a maximum trawl board weight of 3500 kg in 1997, DNV RP-F111 in 2006 gave

an increased maximum trawl board weight to 5000 kg but also introduced the clump weight load of maximum 9000 kg, and in 2014

these loads were adjusted to 7400 kg and 8000 kg respectively. For the trawl load, this constitutes an increase of more than 110%

and in addition the clump weight load has been introduced.

Some reflections to the above:

Has NORSOK U-001 been able to capture the development of bottom trawl gear over the last period?

NORSOK U-001 has very few references and it is hence very challenging to track sources for information.

The model Tests requirements seems to be as they were in 2002. Due to the introduction of new type bottom trawl gear and larger masses, it

may be recommended to revisit the test requirements and detailing.

12" rigid flowline - Using Screening Analysis based on Trawl Loads from DNVGL RP-F111

Trawl Board

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12" rigid flowline - Using Screening Analysis based on Trawl Loads from DNVGL RP-F111

Clump Weight

12" rigid flowline - DNVGL RP-F111 vs Detailed Clump Weight simulations in SIMLA

Storyboard for 7T CW Pullover for on-bottom 12" pipe

Lat. Displacement & BM for 12" on-bottom Case, 7T CW

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Potential for Trawling within Platform Safety Zone

Findings

The trawl fleet is being modernized, industrialized, i.e. larger vessels, larger enginesand gradually larger trawl equipment being used

Use of GPS and sensors -> good position control and tuning of gear.

AIS data availability enables improved precision in defining location specific trawlfrequency, type of fishery, trawl gear used etc.

The catch data published by Fiskeridirektoratet indicates negligible use of double and triple trawl over the last 5-6 years. Their reporting should be further assessed.

Analysis tools are seen able to reproduce trawl tests. Such tools can, whencalibrated against tests, be used as a cost effective alternative to optimise design loads for both subsea pipelines, cables and structures.

Further trawl tests and publication of such test data is recommended.

Detailed trawl pullover analysis indicates that assuming maximum trawl velocity is not allways conservative.

The DNVGL-RP-F111 guideline appear to be more up to date and more scientific in its trawl load definition compared with NORSOK U-001.

Several incident with trawl gear within safety zones have occured over the years. DNVGL-RP-F111 gives good guidance in how to estimate this potential threat, however it seems to be seldomly considered during design of new facilities.

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Note

‘’Due to a good cooperation and mutual understanding between the "long term" fishing industry and the "short term" offshore oil and gas industry, both has been able to develop and grow alongside each other with minimal conflicts’’

Continuing the above policy and principles is considered important also for the future success of offshore petroleum field developments.

The same principles are considered important for new type "short term" industries that may also interfere with traditional fisheries on NCS, e.g. offshore wind farms.

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